Part II: CHAPTER 7 Continued
METALS IDENTIFICATION Part II
7-13. TOOL STEELS
a. General. Steels used for making tools, punches, and dies are perhaps the hardest, strongest, and toughest steels used in industry. In general, tool steels are medium to high carbon steels with specific elements included in different amounts to provide special characteristics. A spark test shows a moderately large volume of white sparks having many fine, repeating bursts.
b. Carbon is provided in tool steel to help harden the steel for cutting and wear resistance. Other elements are added to provide greater toughness or strength. In some cases, elements are added to retain the size and shape of the tool during its heat treat hardening operation, or to make the hardening operation safer and to provide red hardness so that the tool retains its hardness and strength when it becomes extremely hot. Iron is the predominant element in the composition of tool steels. Other elements added include chromium, cobalt, manganese, molybdenum, nickel, tungsten, and vanadium. The tool or die steels are designed for special purposes that are dependent upon composition. Certain tool steels are made for producing die blocks; some are made for producing molds, others for hot working, and others for high-speed cutting application.
c. Another way to classify tool steels is according to the type of quench required to harden the steel. The most severe quench after heating is the water quench (water-hardening steels). A less severe quench is the oil quench, obtained by cooling the tool steel in oil baths (oil-hardening steels). The least drastic quench is cooling in air (air-hardening steels).
d. Tool steels and dies can also be classified according to the work that is to be done by the tool. This is based on class numbers.
(1) Class I steels are used to make tools that work by a shearing or cutting actions, such as cutoff dies, shearing dies, blanking dies, and trimming dies.
(2) Class II steels are used to make tools that produce the desired shape of the part by causing the material being worked, either hot or cold, to flow under tension. This includes drawing dies, forming dies, reducing dies, forging dies, plastic molds, and die cast molding dies.
(3) Class III steels are used to make tools that act upon the material being worked by partially or wholly reforming it without changing the actual dimensions. This includes bending dies, folding dies, and twisting dies.
(4) Class IV steels are used to make dies that work under heavy pressure and that produce a flow of metal or other material caressing it into the desired form. This includes crimping dies, embossing dies, heading dies, extrusion dies, and staking dies.
e. Steels in the tool steels group have a carbon content ranging from 0.83 to 1.55 percent. They are rarely welded by arc welding because of the excessive hardness produced in the fusion zone of the base metal. If arc welding must be done, either mild steel or stainless steel electrodes can be used.
f. Uniformly high preheating temperatures (up to 1000°F (583°C)) must be used when welding tool steels.
g. In general, the same precautions should be taken as those required for welding high carbon steels (para 6-12). The welding flare should be adjusted to carburizing to prevent the burning out of carbon in the weld metal. The welding should be done as quickly as possible, taking care not to overheat the molten metal. After welding, the steel should be heat treated to restore its original properties.
h. Drill rods can be used as filler rods because their high carbon content compares closely with that of tool steels.
i. A flux suitable for welding cast iron should be used in small quantities to protect the puddle of high carbon steel and to remove oxides in the weld metal.
j. Welding Technique. When welding tool steels, the following techniques should be kept in mind:
(1) If the parts to be welded are small, they should be annealed or softened before welding. The edges should then be preheated up to 1000°F (538°C), depending on the carbon content and thickness of the plate. Welding should be done with either a mild steel or high strength electrode.
(2) High carbon electrodes should not be used for welding tool steels. The carbon picked up from the base metal by the filler metal will cause the weld to become glass hard, whereas the mild steel weld metal can absorb additional carbon without becoming excessively hard. The welded part should then be heat treated to restore its original properties.
(3) When welding with stainless steel electrodes, the edge of the plate should be preheated to prevent the formation of hard zones in the base metal. The weld metal should be deposited in small string beads to keep the heat input to a minimum. In general, the application procedure is the same as that required for medium and high carbon steels.
k. There are four types of die steels that are weld repairable. These are water-hardening dies, oil-hardening dies, air-hardening dies, and hot work tools. High-speed tools can also be repaired.
7-14. HIGH HARDNESS ALLOY STEELS
a. General. A large number and variety of obtain high strength, high hardness, corrosion alloy steels have been developed to resistance, and other special properties. Most of these steels depend on a special heat treatment process in order to develop the desired characteristic in the finished state. Alloy steels have greater strength and durability than other carbon steels, and a given strength is secured with less material weight.
b. High hardness alloy steels include the following:
(1) Chromium alloy steels. Chromium is used as an alloying element in carbon steels to increase hardenability, corrosion resistance, and shock resistance, and gives high strength with little loss in ductility. Chromium in large amounts shortens the spark stream to one half that of the same steel without chromium, but does not affect the stream's brightness.
(2) Nickel alloy steels. Nickel increases the toughness, strength, and ductility of steels, and lowers the hardening temperature so that an oil quench, rather than a water quench, is used for hardening. The nickel spark has a short, sharply defined dash of brilliant light just before the fork.
(3) High chromium-nickel alloy (stainless) steels. These high alloy steels cover a wide range of compositions. Their stainless, corrosion, and heat resistant properties vary with the alloy content, and are due to the formation of a very thin oxide film which forms on the surface of the metal. Sparks are straw colored near the grinding wheel, and white near the end of the streak. There is a medium volume of streaks which have a moderate number of forked bursts.
(4) Manganese alloy steels. Manganese is used in steel to produce greater toughness, wear resistance, easier hot rolling, and forging. An increase in manganese content decreases the weldability of steel. Steels containing manganese produce a spark similar to a carbon spark. A moderate increase in manganese increases the volume of the spark stream and the intensity of the bursts. A steel containing more than a normal amount of manganese will produce a spark similar to a high carbon steel with a lower manganese content.
(5) Molybdenum alloy steels. Molybdenum increases hardenability, which is the depth of hardening possible through heat treatment. The impact fatigue property of the steel is improved with up to 0.60 percent molybdenum. Above 0.60 percent molybdenum, the impact fatigue proper is impaired. Wear resistance is improved with molybdenum content above about 0.75 percent. Molybdenum is sometimes combined with chromium, tungsten, or vanadium to obtain desired properties. Steels containing this element produce a characteristic spark with a detached arrowhead similar to that of wrought iron, which can be seen even in fairly strong carbon bursts. Molybdenum alloy steels contain either nickel and/or chromium.
(6) Titanium and columbium (niobium) alloy steels. These elements are used as additional alloying agents in low carbon content, corrosion resistant steels. They support resistance to intergranular corrosion after the metal is subjected to high temperatures for a prolonged period of time.
(7) Tungsten alloy steels. Tungsten, as an alloying element in tool steel, tends to produce a fine, dense grain when used in relatively small quantities. When used in larger quantities, from 17 to 20 percent, and in combination with other alloys, tungsten produces a steel that retains its hardness at high temperatures. This element is usually used in combination with chromium or other alloying agents. In a spark test, tungsten will show a dull red color in the spark stream near the wheel. It also shortens the spark stream and decreases the size of or completely eliminates the carbon burst. A tungsten steel containing about 10 percent tungsten causes short, curved, orange spear points at the end of the carrier lines. Still lower tungsten content causes small, white bursts to appear at the end of the spear petit. Carrier lines may be from dull red to orange, depending on the other elements present, providing the tungsten content is not too high.
(8) Vanadium alloy steels. Vanadium is used to help control grain size. It tends to increase hardenability and causes marked secondary hardness, yet resists tempering. It is added to steel during manufacture to remove oxygen. Alloy steels containing vanadium produce sparks with detached arrowheads at the end of the carrier line similar to those produced by molybdenum steels.
(9) Silicon alloy steels. Silicon is added to steel to obtain greater hardenability and corrosion resistance. It is often used with manganese to obtain a strong, tough steel.
(10) High speed tool steels. These steels are usually special alloy compositions designed for cutting tools. The carbon content ranges from 0.70 to 0.80 percent. They are difficult to weld, except by the furnace induction method. A spark test will show a few long, forked spades which are red near the wheel, and straw colored near the end of the spark stream.
c. Many of these steels can be welded with a heavy coated electrode of the shielded arc type, whose composition is similar to that of the base metal. Low carbon electrodes can also be used with some steels. Stainless steel electrodes are effective where preheating is not feasible or desirable. Heat treated steels should be preheated, if possible, in order to minimize the formation of hard zones, or layers, in the base metal adjacent to the weld. The molten metal should not be overheated, and the welding heat should be controlled by depositing the metal in narrow string beads. In many cases, the procedures for welding medium carbon steels (para 7-11) and high carbon steels (para 7-12) can be used in the welding of alloy steels.
7-15. HIGH YIELD STRENGTH, LOW ALLOY STRUCTURAL STEELS
a. General. High yield strength, low alloy structural steels (constructional alloy steels) are special steels that are tempered to obtain extreme toughness and durability. The special alloys and general makeup of these steels require special treatment to obtain satisfactory weldments. These steels are special, low-carbon steels containing specific, small amounts of alloying elements. They are quenched and tempered to obtain a yield strength of 90,000 to 100,000 psi (620,550 to 689,500 kPa) and a tensile strength of 100,000 to 140,000 psi (689,500 to 965,300 kPa), depending upon size and shape. Structural members fabricated from these high strength steels may have smaller cross-sectional areas than common structural steels and still have equal strength. These steels are also more corrosion and abrasion resistant than other steels. In a spark test, these alloys produce a spark very similar to low carbon steels.
b. Welding Technique. Reliable welding of high yield strength, low alloy structural steels can be performed by using the following guidelines:
To prevent underbead cracking, only low hydrogen electrodes should be used when welding high yield strength, low alloy structural steels.
(1) Correct electrodes. Hydrogen is the number one enemy of sound welds in alloy steels; therefore, use only low hydrogen (MIL-E-18038 or MIL-E-22200/1) electrodes to prevent underbead cracking. Underbead cracking is caused by hydrogen picked up in the electrode coating, released into the arc, and absorbed by the molten metal.
(2) Moisture control of electrodes. If the electrodes are in an airtight container, place them, immediately upon opening the container, in a ventilated holding oven set at 250 to 300°F (121 to 149°C). In the event that the electrodes are not in an airtight container, put them in a ventilated baking oven and bake for 1-1/4 hours at 800°F (427°C). Baked electrodes should, while still warm, be placed in the holding oven until used. Electrodes must be kept dry to eliminate absorption of hydrogen. Testing for moisture should be in accordance with MIL-E-22200.
Moisture stabilizer NSN 3439-00-400-0090 is an ideal holding oven for field use (MIL-M-45558).
c. Low Hydrogen Electrode Selection. Electrodes are identified by classification numbers which are always marked on the electrode containers. For low hydrogen coatings, the last two nunbers of the classification should be 15, 16, or 18. Electrodes of 5/32 and 1/8 in. (4.0 and 3.2 mm) in diameter are the most commonly used, since they are more adaptable to all types of welding of this type steel. Table 7-14 lists electrodes used to weld high yield strength, low alloy structural steels. Table 7-15 is a list of electrodes currently established in the Army supply system.
d. Selecting Wire-Flux and Wire-Gas Combinations. Wire electrodes for submerged arc and gas-shielded arc welding are not classified according to strength. Welding wire and wire-flux combinations used for steels to be stress relieved should contain no more than 0.05 recent vanadium. Weld metal with more than 0.05 percent vanadium may brittle if stress relieved. When using either the submerged arc or gas metal-arc welding processes to weld high yield strength, low alloy structural steels to lower strength steels the wire-flux and wire-gas combination should be the same as that recommended for the lower strength steels.
e. Preheating. For welding plates under 1.0 in. (25.4 mm) thick, above 50°F (10°C) is not required except to remove surface moisture metal. Table 7-16 contains suggested preheating temperatures.
f. Welding Heat.
(1) General. It is important to avoid excessive heat concentration in order to allow the weld area to cool quickly. Either the heat input nomograph or the heat input calculator can be used to determine the heat input into the weld.
(2) Heat input nomograph. To use the heat input nomograph (fig. 7-9), find the volts value in column 1 and draw a line to the amps value in column 3. From the point where this line intersects colunm 2, draw another line to the in./min value in column 5. Read the heat units at the point where this second line intersects column 4. The heat units represent thousands of joules per inch. For example, at 20 volts and 300 amps, the line intersects column 2 at the value 6. At 12 in./min, the heat input is determined as 30 heat units, or 30,000 joules/in.
(3) Heat input calculator. The heat input calculator can be made by copying the pattern printed on the inside of the back cover of this manual onto plastic, light cardboard, or other suitable material and cutting out the pieces. If no suitable material is available, the calculator may be assembled by cutting the pattern out of the back cover. After the two pieces are cut out, a hole is punched in the center of each. They are then assembled using a paper fastener, or some similar device, which will allow the pieces to rotate. To determine welding heat input using the calculator, rotate until the value on the volts scale is aligned directly opposite the value on the speed (in./min) scale. The value on the amps scale will then be aligned directly opposite the calculated value for heat units. As with the nomograph, heat units represent thousands of joules per inch.
(4) Maximum heat input. Check the heat input value obtained from the nomograph or calculator against the suggested maximums in tables 7-17 and 7-18. If the calculated value is too high, adjust the amperes, travel speed, or preheat temperature until the calculated heat input is within the proper range. (The tables are applicable only to single-arc, shielded metal-arc, submerged arc, gas tungsten-arc, flux-cored arc, and gas metal-arc processes. They are not applicable to multiple-arc or electroslag welding, or other high heat input vertical-welding processes, since welds made by these in the "T-1" steels should be heat treated by quenching and tempering.) For welding conditions exceeding the range of the nomograph or calculator, the heat input can be calculated using the following formula:
g. Welding Process. Reliable welding of high yield strength, low alloy structural steel can be per formal by choosing an electrode with low hydrogen content or selecting the proper wire-flux or wire gas combination when using the submerged arc or gas metal arc processes. Use a straight stringer bead whenever possible. Avoid using the weave pattern; however, if needed, it must be restricted to a partial weave pattern. Best results are obtained by a slight circular motion of the electrode with the weave area never exceeding two elect-rode diameters. Never use a full weave pattern. The partial weave pattern should not exceed twice the diameter of the electrode. Skip weld as practical. Peening of the weld is sometimes recommended to relieve stresses while cooling larger pieces. Fillet welds should be smooth and correctly contoured. Avoid toe cracks and undercutting. Electrodes used for fillet welds should be of lower strength than those used for butt welding. Air-hammer peening of fillet welds can help to prevent cracks, especially if the welds are to be stress relieved. A soft steel wire pedestal can help to absorb shrinkage forces. Butter welding in the toe area before actual fillet welding strengths the area where a toe crack may start. A bead is laid in the toe area, then ground off prior to the actual fillet welding. This butter weld bead must be located so that the toe of the fillet will be laid directly over it during actual fillet welding. Because of the additional material involved in fillet welding, the cooling rate is increased and heat inputs may be extended about 25 percent.
7-16. CAST IRON
a. General. A cast iron is an alloy of iron, carbon, and silicon, in which the amount of carbon is usually more than 1.7 percent and less than 4.5 percent.
(1) The most widely used type of cast iron is known as gray iron. Gray iron has a variety of compositions, but is usually such that it is primarily perlite with many graphite flakes dispersed throughout.
(2) There are also alloy cast irons which contain small amounts of chromium, nickel, molybdenum, copper, or other elements added to provide specific properties.
(3) Another alloy iron is austenitic cast iron, which is modified by additions of nickel and other elements to reduce the transformation temperature so that the structure is austenitic at room or normal temperatures. Austenitic cast irons have a high degree of corrosion resistance.
(4) In white cast iron, almost all the carbon is in the combined form. This provides a cast iron with higher hardness, which is used for abrasion resistance.
(5) Malleable cast iron is made by giving white cast iron a special annealing heat treatment to change the structure of the carbon in the iron. The structure is changed to perlitic or ferritic, which increases its ductility.
(6) Nodular iron and ductile cast iron are made by the addition of magnesium or aluminum which will either tie up the carbon in a combined state or will give the free carbon a spherical or nodular shape, rather than the normal flake shape in gray cast iron. This structure provides a greater degree of ductility or malleability of the casting.
(7) Cast irons are widely used in agricultural equipment; on machine tools as bases, brackets, and covers; for pipe fittings and cast iron pipe; and for automobile engine blocks, heads, manifolds, and water preps. Cast iron is rarely used in structural work except for compression members. It is widely used in construction machinery for counterweights and in other applications for which weight is required.
b. Gray cast iron has low ductility and therefore will not expand or stretch to any considerable extent before breaking or cracking. Because of this characteristic, preheating is necessary when cast iron is welded by the oxyacetylene welding process. It can, however, be welded with the metal-arc process without preheating if the welding heat is carefully controlled. This can be accomplished by welding only short lengths of the joint at a time and allowing these sections to cool. By this procedure, the heat of welding is confined to a small area, and the danger of cracking the casting is eliminated. Large castings with complicated sections, such as motor blocks, can be welded without dismantling or preheating. Special electrodes designed for this purpose are usually desirable. Ductile cast irons, such as malleable iron, ductile iron, and nodular iron, can be successfully welded. For best results, these types of cast irons should be welded in the annealed condition.
c. Welding is used to salvage new iron castings, to repair castings that have failed in service, and to join castings to each other or to steel parts in manufacturing operations. Table 7-19 shows the welding processes that can be used for welding cast, malleable, and nodular irons. The selection of the welding process and the welding filler metals depends on the type of weld properties desired and the service life that is expected. For example, when using the shielded metal arc welding process, different types of filler metal can be used. The filler metal will have an effect on the color match of the weld compared to the base material. The color match can be a determining factor, specifically in the salvage or repair of castings, where a difference of color would not be acceptable.
d. No matter which of the welding processes is selected, certain preparatory steps should be made. It is important to determine the exact type of cast iron to be welded, whether it is gray cast iron or a malleable or ductile type. If exact information is not known, it is best to assume that it is gray cast iron with little or no ductility. In general, it is not recommended to weld repair gray iron castings that are subject to heating and cooling in normal service, especially when heating and cooling vary over a range of temperatures exceeding 400°F (204°C). Unless cast iron is used as the filler material, the weld metal and base metal may have different coefficients of expansion and contraction. This will contribute to internal stresses which cannot be withstood by gray cast iron. Repair of these types of castings can be made, but the reliability and service life on such repairs cannot be predicted with accuracy.
e. Preparation for Welding.
(1) In preparing the casting for welding, it is necessary to remove all surface materials to completely clean the casting in the area of the weld. This means removing paint, grease, oil, and other foreign material from the weld zone. It is desirable to heat the weld area for a short time to remove entrapped gas from the weld zone of the base metal. The skin or high silicon surface should also be removed adjacent to the weld area on both the face and root side. The edges of a joint should be chipped out or ground to form a 60° angle or bevel. Where grooves are involved, a V groove from a 60-90° included angle should be used. The V should extend approximately 1/8 in. (3.2 mm) from the bottom of the crack. A small hole should be drilled at each end of the crack to keep it from spreading. Complete penetration welds should always be used, since a crack or defect not completely removed may quickly reappear under service conditions.
(2) Preheating is desirable for welding cast irons with any of the welding processes. It can be reduced when using extremely ductile filler metal. Preheating will reduce the thermal gradient between the weld and the remainder of the cast iron. Preheat temperatures should be related to the welding process, the filler metal type, the mass, and the complexity of the casting. Preheating can be done by any of the normal methods. Torch heating is normally used for relatively small castings weighing 30.0 lb (13.6 kg) or less. Larger parts may be furnace preheated, and in some cases, temporary furnaces are built around the part rather than taking the part to a furnace. In this way, the parts can be maintained at a high interpass temperature in the temporary furnace during welding. Preheating should be general, since it helps to improve the ductility of the material and will spread shrinkage stresses over a large area to avoid critical stresses at any one point. Preheating tends to help soften the area adjacent to the weld; it assists in degassing the casting, and this in turn reduces the possibility of porosity of the deposited weld metal; and it increases welding speed.
(3) Slow cooling or post heating improves the machinability of the heat-affected zone in the cast iron adjacent to the weld. The post cooling should be as slow as possible. This can be done by covering the casting with insulating materials to keep the air or breezes from it.
f. Welding Technique.
(a) Cast iron can be welded with a coated steel electrode, but this method should be used as an emergency measure only. When using a steel electrode, the contraction of the steel weld metal, the carbon picked up from the cast iron by the weld metal, and the hardness of the weld metal caused by rapid cooling must be considered. Steel shrinks more than cast iron when ceded from a molten to a solid state. When a steel electrode is used, this uneven shrinkage will cause strains at the joint after welding. When a large quantity of filler metal is applied to the joint, the cast iron may crack just back of the line of fusion unless preventive steps are taken. To overcome these difficulties, the prepared joint should be welded by depositing the weld metal in short string beads, 0.75 to 1.0 in. long (19.0 to 25.4 mm). These are made intermittently and, in some cases, by the backstep and skip procedure. To avoid hard spots, the arc should be struck in the V, and not on the surface of the base metal. Each short length of weld metal applied to the joint should be lightly peened while hot with a small ball peen hammer, and allowed to cool before additional weld metal is applied. The peening action forges the metal and relieves the cooling strains.
(b) The electrodes used should be 1/8 in. (3.2 mm) in diameter to prevent excessive welding heat. Welding should be done with reverse polarity. Weaving of the electrode should be held to a minimum. Each weld metal deposit should be thoroughly cleaned before additional metal is added.
(c) Cast iron electrodes must be used where subsequent machining of the welded joint is required. Stainless steel electrodes are used when machining of the weld is not required. The procedure for making welds with these electrodes is the same as that outlined for welding with mild steel electrodes. Stainless steel electrodes provide excellent fusion between the filler and base metals. Great care must be taken to avoid cracking in the weld, contracts approximately 50 percent more than because stainless steel expands and mild steel in equal changes of temperature.
(2) Arc Welding.
(a) The shielded metal arc welding process can be utilized for welding cast iron. There are four types of filler metals that may be used: cast iron covered electrodes; covered copper base alloy electrodes; covered nickel base alloy electrodes; and mild steel covered electrodes. There are reasons for using each of the different specific types of electrodes, which include the machinability of the deposit, the color match of the deposit, the strength of the deposit, and the ductility of the final weld.
(b) When arc welding with the cast iron electrodes (ECI), preheat to between 250 and 800°F (121 and 425°C), depending on the size and complexity of the casting and the need to machine the deposit and adjacent areas. The higher degree of heating, the easier it will be to machine the weld deposit. In general, it is best to use small-size electrodes and a relatively 1ow current setting. A medium arc length should be used, and, if at all possible, welding should be done in the flat position. Wandering or skip welding procedure should be used, and peening will help reduce stresses and will minimize distortion. Slow cooling after welding is recommended. These electrodes provide an excellent color match cm gray iron. The strength of the weld will equal the strength of the base metal. There are two types of copper-base electrodes: the copper tin alloy and the copper aluminum types. The copper zinc alloys cannot be used for arc welding electrodes because of the low boiling temperature of zinc. Zinc will volatilize in the arc and will cause weld metal porosity.
(c) When the copper base electrodes are used, a preheat of 250 to 400°F (121 to 204°C) is recommended. Small electrodes and low current should be used. The arc should be directed against the deposited metal or puddle to avoid penetration and mixing the base metal with the weld metal. Slow cooling is recommended after welding. The copper-base electrodes do not provide a good color match.
(d) There are three types of nickel electrodes used for welding cast iron. These electrodes can be used without preheat; however, heating to 100°F (38°C) is recommended. These electrodes can be used in all positions; however, the flat position is recommended. The welding slag should be removed between passes. The nickel and nickel iron deposits are extremely ductile and will not become brittle with the carbon pickup. The hardness of the heat-affected zone can be minimized by reducing penetration into the cast iron base metal. The technique mentioned above, playing the arc on the puddle rather than on the base metal, will help minimize dilution. Slow cooling and, if necessary, postheating will improve machinability of the heat-affected zone. The nickel-base electrodes do not provide a close color match.
(e) Copper nickel type electrodes cane in two grades. Either of these electrodes can be used in the same manner as the nickel or nickel iron electrode with about the same technique and results. The deposits of these electrodes do not provide a color match.
(f) Mild steel electrodes are not recommended for welding cast iron if the deposit is to be machined. The mild steel deposit will pick up sufficient carbon to make a high-carbon deposit, which is impossible to machine. Additionally, the mild steel deposit will have a reduced level of ductility as a result of increased carbon content. This type of electrode should be used only for small repairs and should not be used when machining is required. Minimum preheat is possible for small repair jobs. Small electrodes at low current are recommended to minimize dilution and to avoid the concentration of shrinkage stresses. Short welds using a wandering sequence should be used, and the weld should be peened as quickly as possible after welding. The mild steel electrode deposit provides a fair color match.
(3) Carbon-arc welding of cast iron. Iron castings may be welded with a carbon arc, a cast iron rod, and a cast iron welding flux. The joint should be preheated by moving the carbon electrodes along the surface. This prevents too-rapid cooling after welding. The molten puddle of metal can be worked with the carbon electrode so as to move any slag or oxides that are formed to the surface. Welds made with the carbon arc cool more slowly and are not as hard as those made with the metal arc and a cast iron electrode. The welds are machinable.
(4) Oxyfuel gas welding. The oxyfuel gas process is often used for welding cast iron. Most of the fuel gases can be used. The flame should be neutral to slightly reducing. Flux should be used. Two types of filler metals are available: the cast iron rods and the copper zinc rods. Welds made with the proper cast iron electrode will be as strong as the base metal. Good color match is provided by all of these welding reds. The optimum welding procedure should be used with regard to joint preparation, preheat, and post heat. The copper zinc rods produce braze welds. There are two classifications: a manganese bronze and a low-fuming bronze. The deposited bronze has relatively high ductility but will not provide a color match.
(5) Brazing and braze welding.
(a) Brazing is used for joining cast iron to cast iron and steels. In these cases, the joint design must be selected for brazing so that capillary attraction causes the filler metal to flow between closely fitting parts. The torch method is normally used. In addition, the carbon arc, the twin carbon arc, the gas tungsten arc, and the plasma arc can all be used as sources of heat. Two brazing filler metal alloys are normally used; both are copper zinc alloys. Braze welding can also be used to join cast iron. In braze welding, the filler metal is not drawn into the joint by capillary attraction. This is sometimes called bronze welding. The filler material having a liquidous above 850°F (454°C) should be used. Braze welding will not provide a color match.
(b) Braze welding can also be accomplished by the shielded metal arc and the gas metal arc welding processes. High temperature preheating is not usually required for braze welding unless the part is extremely heavy or complex in geometry. The bronze weld metal deposit has extremely high ductility, which compensates for the lack of ductility of the cast iron. The heat of the arc is sufficient to bring the surface of the cast iron up to a temperature at which the copper base filler metal alloy will make a bond to the cast iron. Since there is little or no intermixing of the materials, the zone adjacent to the weld in the base metal is not appreciably hardened. The weld and adjacent area are machinable after the weld is completed. In general, a 200°F (93°C) preheat is sufficient for most application. The cooling rate is not extremely critical and a stress relief heat treatment is not usually required. This type of welding is commonly used for repair welding of automotive parts, agricultural implement parts, and even automotive engine blocks and heads. It can only be used when the absence of color match is not objectionable.
(6) Gas metal arc welding. The gas metal arc welding process can be used for making welds between malleable iron and carbon steels. Several types of electrode wires can be used, including:
(a) Mild steel using 75% argon + 25% CO2 for shielding.
(b) Nickel copper using 100% argon for shielding.
(c) Silicon bronze using 50% argon + 50% helium for shielding.
In all cases, small diameter electrode wire should be used at low current. With the mild steel electrode wire, the Argon-CO2 shielding gas mixture issued to minimize penetration. In the case of the nickel base filler metal and the Copper base filler metal, the deposited filler metal is extremely ductile. The mild steel provides a fair color match. A higher preheat is usually required to reduce residual stresses and cracking tendencies.
(7) Flux-cored arc welding. This process has recently been used for welding cast irons. The more successful application has been using a nickel base flux-cored wire. This electrode wire is normally operated with CO2 shielding gas, but when lower mechanical properties are not objectionable, it can be operated without external shielding gas. The minimum preheat temperatures can be used. The technique should minimize penetration into the cast iron base metal. Postheating is normally not required. A color match is not obtained.
(8) Studding. Cracks in large castings are sometimes repaired by studding (fig. 7-10). In this process, the fracture is removed by grinding a V groove. Holes are drilled and tapped at an angle on each side of the groove, and studs are screwed into these holes for a distance equal to the diameter of the studs, with the upper ends projecting approximately 1/4 in. (6.4 mm) above the cast iron surface. The studs should be seal welded in place by one or two beads around each stud, and then tied together by weld metal beads. Welds should be made in short lengths, and each length peened while hot to prevent high stresses or cracking upon cooling. Each bead should be allowed to cool and be thoroughly cleaned before additional metal is deposited. If the studding method cannot be applied, the edges of the joint should be chipped out or machined with a round-nosed tool to form a U groove into which the weld metal should be deposited.
(9) Other welding processes can be used for cast iron. Thermit welding has been used for repairing certain types of cast iron machine tool parts. Soldering can be used for joining cast iron, and is sometimes used for repairing small defects in small castings. Flash welding can also be used for welding cast iron.
Section IV. GENERAL DESCRIPTION AND WELDABILITY OF NONFERROUS METALS
7-17. ALUMINUM WELDING
a. General. Aluminum is a lightweight, soft, low strength metal which can easily be cast, forged, machined, formed and welded. Unless alloyed with specific elements, it is suitable only in low temperature applications. Aluminum is light gray to silver in color, very bright when polished, and dull when oxidized. A fracture in aluminum sections shows a smooth, bright structure. Aluminum gives off no sparks in a spark test, and does not show red prior to melting. A heavy film of white oxide forms instantly on the molten surface. Its combination of light weight and high strength make aluminum the second most popular metal that is welded. Aluminum and aluminum alloys can be satisfactorily welded by metal-arc, carbon-arc, and other arc welding processes. The principal advantage of using arc welding processes is that a highly concentrated heating zone is obtained with the arc. For this reason, excessive expansion and distortion of the metal are prevented.
b. Alloys. Many alloys of aluminum have been developed. It is important to know which alloy is to be welded. A system of four-digit numbers has been developed by the Aluminum Association, Inc., to designate the various wrought aluminum alloy types. This system of alloy groups, shown by table 7-20, is as follows:
(1) 1XXX series. These are aluminums of 99 percent or higher purity which are used primarily in the electrical and chemical industries.
(2) 2XXX series. Copper is the principal alloy in this group, which provides extremely high strength when properly heat treated. These alloys do not produce as good corrosion resistance and are often clad with pure aluminum or special-alloy aluminum. These alloys are used in the aircraft industry.
(3) 3XXX series. Manganese is the major alloying element in this group, which is non-heat-treatable. Manganese content is limited to about 1.5 percent. These alloys have moderate strength and are easily worked.
(4) 4XXX series. Silicon is the major alloying element in this group. It can be added in sufficient quantities to substantially reduce the melting point and is used for brazing alloys and welding electrodes. Most of the alloys in this group are non-heat-treatable.
(5) 5XXX series. Magnesium is the major alloying element of this group, which are alloys of medium strength. They possess good welding characteristics and good resistance to corrosion, but the amount of cold work should be limited.
(6) 6XXX series. Alloys in this group contain silicon and magnesium, which make them heat treatable. These alloys possess medium strength and good corrosion resistance.
(7) 7XXX series. Zinc is the major alloying element in this group. Magnesium is also included in most of these alloys. Together, they form a heat-treatable alloy of very high strength, which is used for aircraft frames.
c. Welding Aluminum Alloys. Aluminum possesses a number of properties that make welding it different than the welding of steels. These are: aluminum oxide surface coating; high thermal conductivity; high thermal expansion coefficient; low melting temperature; and the absence of color change as temperature approaches the melting point. The normal metallurgical factors that apply to other metals apply to aluminum as well.
(1) Aluminum is an active metal which reacts with oxygen in the air to produce a hard, thin film of aluminum oxide on the surface. The melting point of aluminum oxide is approximately 3600°F (1982°C) which is almost three times the melting point of pure aluminum (1220°F (660°C)). In addition, this aluminum oxide film absorbs moisture from the air, particularly as it becomes thicker. Moisture is a source of hydrogen, which causes porosity in aluminum welds. Hydrogen may also come from oil, paint, and dirt in the weld area. It also comes from the oxide and foreign materials on the electrode or filler wire, as well as from the base metal. Hydrogen will enter the weld pool and is soluble in molten aluminum. As the aluminum solidifies, it will retain much less hydrogen. The hydrogen is rejected during solidification. With a rapid cooling rate, free hydrogen is retained within the weld and will cause porosity. Porosity will decrease weld strength and ductility, depending on the amount.
Aluminum and aluminum alloys should not be cleaned with caustic soda or cleaners with a pH above 10, as they may react chemically.
(a) The aluminum oxide film must be removed prior to welding. If it is not completely removed, small particles of unmelted oxide will be trapped in the weld pool and will cause a reduction in ductility, lack of fusion, and possibly weld cracking.
(b) The aluminum oxide can be removed by mechanical, chemical, or electrical means. Mechanical removal involves scraping with a sharp tool, sandpaper, wire brush (stainless steel), filing, or any other mechanical method. Chemical removal can be done in two ways. One is by use of cleaning solutions, either the etching types or the nonetching types. The nonetching types should be used only when starting with relatively clean parts, and are used in conjunction with other solvent cleaners. For better cleaning, the etching type solutions are recommended, but must be used with care. When dipping is employed, hot and cold rinsing is highly recommended. The etching type solutions are alkaline solutions. The time in the solution must be controlled so that too much etching does not occur.
(c) Chemical cleaning includes the use of welding fluxes. Fluxes are used for gas welding, brazing, and soldering. The coating on covered aluminum electrodes also maintains fluxes for cleaning the base metal. Whenever etch cleaning or flux cleaning is used, the flux and alkaline etching materials must be completely removed from the weld area to avoid future corrosion.
(d) The electrical oxide removal system uses cathodic bombardment. Cathodic bombardment occurs during the half cycle of alternating current gas tungsten arc welding when the electrode is positive (reverse polarity). This is an electrical phenomenon that actually blasts away the oxide coating to produce a clean surface. This is one of the reasons why AC gas tungsten arc welding is so popular for welding aluminum.
(e) Since aluminum is so active chemically, the oxide film will immediately start to reform. The time of buildup is not extremely fast, but welds should be made after aluminum is cleaned within at least 8 hours for quality welding. If a longer time period occurs, the quality of the weld will decrease.
(2) Aluminum has a high thermal conductivity and low melting temperature. It conducts heat three to five times as fast as steel, depending on the specific alloy. More heat must be put into the aluminum, even though the melting temperature of aluminum is less than half that of steel. Because of the high thermal conductivity, preheat is often used for welding thicker sections. If the temperature is too high or the time period is too long, weld joint strength in both heat-treated and work-hardened alloys may be diminished. The preheat for aluminum should not exceed 400°F (204°C), and the parts should not be held at that temperature longer than necessary. Because of the high heat conductivity, procedures should utilize higher speed welding processes using high heat input. Both the gas tungsten arc and the gas metal arc processes supply this requirement. The high heat conductivity of aluminum can be helpful, since the weld will solidify very quickly if heat is conducted away from the weld extremely fast. Along with surface tension, this helps hold the weld metal in position and makes all-position welding with gas tungsten arc and gas metal arc welding practical.
(3) The thermal expansion of aluminum is twice that of steel. In addition, aluminum welds decrease about 6 percent in volume when solidifying from the molten state. This change in dimension may cause distortion and cracking.
(4) The final reason aluminum is different from steels when welding is that it does not exhibit color as it approaches its melting temperature until it is raised above the melting point, at which time it will glow a dull red. When soldering or brazing aluminum with a torch, flux is used. The flux will melt as the temperature of the base metal approaches the temperature required. The flux dries out first, and melts as the base metal reaches the correct working temperature. When torch welding with oxyacetylene or oxyhydrogen, the surface of the base metal will melt first and assume a characteristic wet and shiny appearance. (This aids in knowing when welding temperatures are reached.) When welding with gas tungsten arc or gas metal arc, color is not as important, because the weld is completed before the adjoining area melts.
d. Metal-Arc Welding of Aluminum.
(1) Plate welding. Because of the difficulty of controlling the arc, butt and fillet welds are difficult to produce in plates less than 1/8 in. (3.2 mm) thick. When welding plate heavier than 1/8 in. (3.2 mm), a joint prepared with a 20 degree bevel will have strength equal to a weld made by the oxyacetylene process. This weld may be porous and unsuitable for liquid-or gas-tight joints. Metal-arc welding is, however, particularly suitable for heavy material and is used on plates up to 2-1/2 in. (63.5 mm) thick.
(2) Current and polarity settings. The current and polarity settings will vary with each manufacturer's type of electrodes. The polarity to be used should be determined by trial on the joints to be made.
(3) Plate edge preparation. In general, the design of welded joints for aluminum is quite consistent with that for steel joints. However, because of the higher fluidity of aluminum under the welding arc, some important general principles should be kept in mind. With the lighter gauges of aluminum sheet, less groove spacing is advantageous when weld dilution is not a factor. The controlling factor is joint preparation. A specially designed V groove that is applicable to aluminum is shown in A, figure 7-11. This type of joint is excellent where welding can be done from one side only and where a smooth, penetrating bead is desired. The effectiveness of this particular design depends upon surface tension, and should be applied on all material over 1/8 in. (3.2 mm) thick. The bottom of the special V groove must be wide enough to contain the root pass completely. This requires adding a relatively large amount of filler alloy to fill the groove. Excellent control of the penetration and sound root pass welds are obtained. This edge preparation can be employed for welding in all positions. It eliminates difficulties due to burn-through or over-penetration in the overheat and horizontal welding positions. It is applicable to all weldable base alloys and all filler alloys.
e. Gas Metal-Arc (MIG) Welding (GMAW).
(1) General. This fast, adaptable process is used with direct current re-verse polarity and an inert gas to weld heavier thicknesses of aluminum alloys, in any position, from 1/016 in. (1.6 mm) to several inches thick. TM 5-3431-211-15 describes the operation of a typical MIG welding set.
(2) Shielding gas. Precautions should be taken to ensure the gas shield is extremely efficient. Welding grade argon, helium, or a mixture of these gases is used for aluminum welding. Argon produces a smother and more stable arc than helium. At a specific current and arc length, helium provides deeper penetration and a hotter arc than argon. Arc voltage is higher with helium, and a given change in arc length results in a greater change in arc voltage. The bead profile and penetration pattern of aluminum welds made with argon and helium differ. With argon, the bead profile is narrower and more convex than helium. The penetration pattern shows a deep central section. Helium results in a flatter, wider bead, and has a broader under-bead penetration pattern. A mixture of approximately 75 percent helium and 25 percent argon provides the advantages of both shielding gases with none of the undesirable characteristics of either. Penetration pattern and bead contour show the characteristics of both gases. Arc stability is comparable to argon. The angle of the gun or torch is more critical when welding aluminum with inert shielding gas. A 30° leading travel angle is recommended. The electrode wire tip should be oversize for aluminum. Table 7-21 provides welding procedure schedules for gas metal-arc welding of aluminum.
(3) Welding technique. The electrode wire must be clean. The arc is struck with the electrode wire protruding about 1/2 in. (12.7 mm) from the cup. A frequently used technique is to strike the arc approximately 1.0 in. (25.4 mm) ahead of the beginning of the weld and then quickly bring the arc to the weld starting point, reverse the direction of travel, and proceed with normal welding. Alternatively, the arc may be struck outside the weld groove on a starting tab. When finishing or terminating the weld, a similar practice may be followed by reversing the direction of welding, and simultaneously increasing the speed of welding to taper the width of the molten pool prior to breaking the arc. This helps to avert craters and crater cracking. Runoff tabs are commonly used. Having established the arc, the welder moves the electrode along the joint while maintaining a 70 to 85 degree forehand angle relative to the work. A string bead technique is normally preferred. Care should be taken that the forehand angle is not changed or increased as the end of the weld is approached. Arc travel speed controls the bead size. When welding aluminum with this process, it is must important that high travel speeds be maintained. When welding uniform thicknesses, the electrode to work angle should be equal on both sides of the weld. When welding in the horizontal position, best results are obtained by pointing the gun slightly upward. When welding thick plates to thin plates, it is helpful to direct the arc toward the heavier section. A slight backhand angle is sometimes helpful when welding thin sections to thick sections. The root pass of a joint usually requires a short arc to provide the desired penetration. Slightly longer arcs and higher arc voltages may be used on subsequent passes.
The wire feeding equipment for aluminum welding must be in good adjustment for efficient wire feeding. Use nylon type liners in cable assemblies. Proper drive rolls must be selected for the aluminum wire and for the size of the electrode wire. It is more difficult to push extremely small diameter aluminum wires through long gun cable assemblies than steel wires. For this reason, the spool gun or the newly developed guns which contain a linear feed motor are used for the small diameter electrode wires. Water-cooled guns are required except for low-current welding. Both the constant current (CC) power source with matching voltage sensing wire feeder and the constant voltage (CV) power source with constant speed wire feeder are used for welding aluminum. In addition, the constant speed wire feeder is sometimes used with the constant current power source. In general, the CV system is preferred when welding on thin material and using all diameter electrode wire. It provides better arc starting and regulation. The CC system is preferred when welding thick material using larger electrode wires. The weld quality seems better with this system. The constant current power source with a moderate drop of 15 to 20 volts per 100 amperes and a constant speed wire feeder provide the most stable power input to the weld and the highest weld quality.
(4) Joint design. Edges may be prepared for welding by sawing, machining, rotary planing, routing or arc cutting. Acceptable joint designs are shown in figure 7-12.
f. Gas Tungsten-Arc (TIG) Welding (GTAW).
(1) The gas tungsten arc welding process is used for welding the thinner sections of aluminum and aluminum alloys. There are several precautions that should be mentioned with respect to using this process.
(a) Alternating current is recommended for general-purpose work since it provides the half-cycle of cleaning action. Table 7-22 provides welding procedure schedules for using the process on different thicknesses to produce different welds. AC welding, usually with high frequency, is widely used with manual and automatic applications. Procedures should be followed closely and special attention given to the type of tungsten electrode, size of welding nozzle, gas type, and gas flow rates. When manual welding, the arc length should be kept short and equal to the diameter of the electrode. The tungsten electrode should not protrude too far beyond the end of the nozzle. The tungsten electrode should be kept clean. If it does accidentally touch the molten metal, it must be redressed.
(b) Welding power sources designed for the gas tungsten arc welding process should be used. The newer equipment provides for programming, pre-and post-flow of shielding gas, and pulsing.
(c) For automatic or machine welding, direct current electrode negative (straight polarity) can be used. Cleaning must be extremely efficient, since there is no cathodic bombardment to assist. When dc electrode negative is used, extremely deep penetration and high speeds can be obtained. Table 7-23 lists welding procedure schedules for dc electrode negative welding.
(d) The shielding gases are argon, helium, or a mixture of the two. Argon is used at a lower flow rate. Helium increases penetration, but a higher flow rate is required. When filler wire is used, it must be clean. Oxide not removed from the filler wire may include moisture that will produce polarity in the weld deposit.
(2) Alternating current.
(a) Characteristics of process. The welding of aluminum by the gas tungsten-arc welding process using alternating current produces an oxide cleaning action. Argon shielding gas is used. Better results are obtained when welding aluminum with alternating current by using equipment designed to produce a balanced wave or equal current in both directions. Unbalance will result in loss of power and a reduction in the cleaning action of the arc. Characteristics of a stable arc are the absence of snapping or cracking, smooth arc starting, and attraction of added filler metal to the weld puddle rather than a tendency to repulsion. A stable arc results in fewer tungsten inclusions.
(b) Welding technique. For manual welding of aluminum with ac, the electrode holder is held in one hand and filler rod, if used, in the other. An initial arc is struck on a starting block to heat the electrode. The arc is then broken and reignited in the joint. This technique reduces the tendency for tungsten inclusions at the start of the weld. The arc is held at the starting point until the metal liquefies and a weld pool is established. The establishment and maintenance of a suitable weld pool is important, and welding must not proceed ahead of the puddle. If filler metal is required, it may be added to the front or leading edge of the pool but to one side of the center line. Both hands are moved in unison with a slight backward and forward motion along the joint. The tungsten electrode should not touch the filler rod. The hot end of the filler rod should not be withdrawn from the argon shield. A short arc length must be maintained to obtain sufficient penetration and avoid undercutting, excessive width of the weld bead, and consequent loss of penetration control and weld contour. One rule is to use an arc length approximately equal to the diameter of the tungsten electrode. When the arc is broken, shrinkage cracks may occur in the weld crater, resulting in a defective weld. This defect can be prevented by gradually lengthening the arc while adding filler metal to the crater. Then, quickly break and restrike the arc several times while adding additional filler metal to the crater, or use a foot control to reduce the current at the end of the weld. Tacking before welding is helpful in controlling distortion. Tack welds should be of ample size and strength and should be chipped out or tapered at the ends before welding over.
(c) Joint design. The joint designs shown in figure 7-11 are applicable to the gas tungsten-arc welding process with minor exceptions. Inexperienced welders who cannot maintain a very short arc may require a wider edge preparation, included angle, or joint spacing. Joints may be fused with this process without the addition of filler metal if the base metal alloy also makes a satisfactory filler alloy. Edge and corner welds are rapidly made without addition of filler metal and have a good appearance, but a very close fit is essential.
(3) Direct current straight polarity.
(a) Charcteristics of process. This process, using helium and thoriated tungsten electrodes is advantageous for many automatic welding operations, especially in the welding of heavy sections. Since there is less tendency to heat the electrode, smaller electrodes can be used for a given welding current. This will contribute to keeping the weld bead narrow. The use of direct current straight polarity (dcsp) provides a greater heat input than can be obtained with ac current. Greater heat is developed in the weld pool, which is consequently deeper and narrower.
(b) Welding techniques. A high frequency current should be used to initiate the arc. Touch starting will contaminate the tungsten electrode. It is not necessary to form a puddle as in ac welding, since melting occurs the instant the arc is struck. Care should be taken to strike the arc within the weld area to prevent undesirable marking of the material. Standard techniques such as runoff tabs and foot operated heat controls are used. These are helpful in preventing or filling craters, for adjusting the current as the work heats, and to adjust for a change in section thickness. In dcsp welding, the torch is moved steadily forward. The filler wire is fed evenly into the leading edge of the weld puddle, or laid on the joint and melted as the arc roves forward. In all cases, the crater should be filled to a point above the weld bead to eliminate crater cracks. The fillet size can be controlled by varying filler wire size. DCSP is adaptable to repair work. Preheat is not required even for heavy sections, and the heat affected zone will be smaller with less distortion.
(c) Joint designs. The joint designs shown in figure 7-11 are applicable to the automatic gas tungsten-arc dcsp welding process with minor exceptions. For manual dcsp, the concentrated heat of the arc gives excellent root fusion. Root face can be thicker, grooves narrower, and build up can be easily controlled by varying filler wire size and travel speed.
g. Square Wave Alternating Current Welding (TIG).
(1) General. Square wave gas tungsten-arc welding with alternating current differs frozen conventional balanced wave gas tungsten-arc welding in the type of wave from used. With a square wave, the time of current flow in either direction is adjustable from 20 to 1. In square wave gas tungsten-arc welding, there are the advantages of surface cleaning produced by positive ionic bombardment during the reversed polarity cycle, along with greater weld depth to width ratio produced by the straight polarity cycle. Sufficient aluminum surface cleaning action has been obtained with a setting of approximately 10 percent dcrp. Penetration equal to regular dcsp welding can be obtained with 90 percent dcsp current.
(2) Welding technique. It is necessary to have either superimposed high frequency or high open circuit voltage, because the arc is extinguished every half cycle as the current decays toward zero, and must be restarted each tire. Precision shaped thoriated tungsten electrodes should be used with this process. Argon, helium, or a combination of the two should be used as shielding gas, depending on the application to be used.
(3) Joint design. Square wave alternating current welding offers substantial savings over conventional alternating current balanced wave gas tungsten arc welding in weld joint preparation. Smaller V grooves, U grooves, and a thicker root face can be used. A greater depth to width weld ratio is conducive to less weldment distortion, along with favorable welding residual stress distribution and less use of filler wire. With Some slight modification, the same joint designs can be used as in dcsp gas tungsten-arc welding (fig. 7-11).
h. Shielded Metal-Arc Welding. In the shielded metal-arc welding process, a heavy dipped or extruded flux coated electrode is used with dcrp. The electrodes are covered similarly to conventional steel electrodes. The flux coating provides a gaseous shield around the arc and molten aluminum puddle, and chemically combines and removes the aluminum oxide, forming a slag. When welding aluminum, the process is rather limited due to arc spatter, erratic arc control, limitations on thin material, and the corrosive action of the flux if it is not removed properly.
i. Shielded Carbon-Arc Welding. The shielded carbon-arc welding process can be used in joining aluminum. It requires flux and produces welds of the same appearance, soundness, and structure as those produced by either oxyacetylene or oxyhydrogen welding. Shielded carbon-arc welding is done both manually and automatically. A carbon arc is used as a source of heat while filler metal is supplied from a separate filler rod. Flux must be removed after welding; otherwise severe corrosion will result. Manual shielded carbon-arc welding is usually limited to a thickness of less than 3/8 in. (9.5 mm), accomplished by the same method used for manual carbon arc welding of other material. Joint preparation is similar to that used for gas welding. A flux covered rod is used.
j. Atomic Hydrogen Welding. This welding process consists of maintaining an arc between two tungsten electrodes in an atmosphere of hydrogen gas. The process can be either manual or automatic with procedures and techniques closely related to those used in oxyacetylene welding. Since the hydrogen shield surrounding the base metal excludes oxygen, smaller amounts of flux are required to combine or remove aluminum oxide. Visibility is increased, there are fewer flux inclusions, and a very sound metal is deposited.
k. Stud Welding.
(1) Aluminum stud welding may be accomplished with conventional arc stud welding equipment, using either the capacitor discharge or drawn arc capacitor discharge techniques. The conventional arc stud welding process may be used to weld aluminum studs 3/16 to 3/4 in. (4.7 to 19.0 mm) diameter. The aluminum stud welding gun is modified slightly by the addition of a special adapter for the control of the high purity shielding gases used during the welding cycle. An added accessory control for controlling the plunging of the stud at the completion of the weld cycle adds materially to the quality of weld and reduces spatter loss. Reverse polarity is used, with the electrode gun positive and the workpiece negative. A small cylindrical or cone shaped projection on the end of the aluminum stud initiates the arc and helps establish the longer arc length required for aluminum welding.
(2) The unshielded capacitor discharge or drawn arc capacitor discharge stud welding processes are used with aluminum studs 1/16 to 1/4 in. (1.6 to 6.4 mm) diameter. Capacitor discharge welding uses a low voltage electrostatic storage system, in which the weld energy is stored at a low voltage in capacitors with high capacitance as a power source. In the capacitor discharge stud welding process, a small tip or projection on the end of the stud is used for arc initiation. The drawn arc capacitor discharge stud welding process uses a stud with a pointed or slightly rounded end. It does not require a serrated tip or projection on the end of the stud for arc initiation. In both cases, the weld cycle is similar to the conventional stud welding process. However, use of the projection on the base of the stud provides the most consistent welding. The short arcing time of the capacitor discharge process limits the melting so that shallow penetration of the workpiece results. The minimum aluminum work thickness considered practical is 0.032 in. (0.800 mm).
l. Electron Beam Welding. Electron beam welding is a fusion joining process in which the workpiece is bombarded with a dense stream of high velocity electrons, and virtually all of the kinetic energy of the electrons is transformed into heat upon impact. Electron beam welding usually takes place in an evacuated chamber. The chamber size is the limiting factor on the weldment size. Conventional arc and gas heating melt little more than the surface. Further penetration comes solely by conduction of heat in all directions from this molten surface spot. The fusion zone widens as it depends. The electron beam is capable of such intense local heating that it almost instantly vaporizes a hole through the entire joint thickness. The walls of this hole are molten, and as the hole is moved along the joint, more metal on the advancing side of the hole is melted. This flaws around the bore of the hole and solidifies along the rear side of the hole to make the weld. The intensity of the beam can be diminished to give a partial penetration with the same narrow configuration. Electron beam welding is generally applicable to edge, butt, fillet, melt-thru lap, and spot welds. Filler metal is rarely used except for surfacing.
m. Resistance Welding.
(1) General. The resistance welding processes (spot, seam, and flash welding) are important in fabricating aluminum alloys. These processes are especially useful in joining the high strength heat treatable alloys, which are difficult to join by fusion welding, but can be joined by the resistance welding process with practically no loss in strength. The natural oxide coating on aluminum has a rather high and erratic electrical resistance. To obtain spot or seam welds of the highest strength and consistency, it is usually necessary to reduce this oxide coating prior to welding.
(2) Spot welding. Welds of uniformly high strength and good appearance depend upon a consistently low surface resistance between the workplaces. For most applications, some cleaning operations are necessary before spot or seam welding aluminum. Surface preparation for welding generally consists of removal of grease, oil, dirt, or identification markings, and reduction and improvement of consistency of the oxide film on the aluminum surface. Satisfactory performance of spot welds in service depends to a great extent upon joint design. Spot welds should always be designed to carry shear loads. However, when tension or combined loadings may be expected, special tests should be conducted to determine the actual strength of the joint under service loading. The strength of spot welds in direct tension may vary from 20 to 90 percent of the shear strength.
(3) Seam welding. Seam welding of aluminum and its alloys is very similar to spot welding, except that the electrodes are replaced by wheels. The spots made by a seam welding machine can be overlapped to form a gas or liquid tight joint. By adjusting the timing, the seam welding machine can produce uniformly spaced spot welds equal in quality to those produced on a regular spot welding machine, and at a faster rate. This procedure is called roll spot or intermittent seam welding.
(4) Flash welding. All aluminum alloys may be joined by the flash welding process. This process is particularly adapted to making butt or miter joints between two parts of similar cross section. It has been adapted to joining aluminum to copper in the form of bars and tubing. The joints so produced fail outside of the weld area when tension loads are applied.
n. Gas welding. Gas welding has been done on aluminum using both oxyacetylene and oxyhydrogen flames. In either case, an absolutely neutral flame is required. Flux is used as well as a filler rod. The process also is not too popular because of low heat input and the need to remove flux.
o. Electroslag welding. Electroslag welding is used for joining pure aluminum, but is not successful for welding the aluminum alloys. Submerged arc welding has been used in some countries where inert gas is not available.
p. Other processes. Most of the solid state welding processes, including friction welding, ultrasonic welding, and cold welding are used for aluminums. Aluminum can also be joined by soldering and brazing. Brazing can be accomplished by most brazing methods. A high silicon alloy filler material is used.
7-18. BRASS AND BRONZE WELDING
a. General. Brass and bronze are alloys of copper. Brass has zinc, and bronze has tin as the major alloying elements. However, some bronze metals contain more zinc than tin, and some contain zinc and no tin at all. High brasses contain from 20 to 45 percent zinc. Tensile strength, hardness, and ductility increase as the percentage of zinc increases. These metals are suitable for both hot and cold working.
b. Metal-Arc Welding. Brasses and bronzes can be successfully welded by the metal-arc process. The electrode used should be of the shielded arc type with straight polarity (electrode positive). Brasses can be welded with phosphor bronze, aluminum bronze, or silicon bronze electrodes, depending on the base metal composition and the service required. Backing plates of matching metal or copper should be used. High welding current should not be used for welding copper-zinc alloys (brasses), otherwise the zinc content will be volatilized. All welding should be done in the flat position. If possible, the weld metal should be deposited with a weave approximately three times the width of the electrode.
c. Carbon-Arc Welding. This method can be used to weld brasses and bronzes with filler reds of approximately the same composition as the base metal. In this process, welding is accomplished in much the same way the bronze is bonded to steel. The metal in the carbon arc is superheated, and this very hot metal is alloyed to the base metal in the joint.
d. Oxyacetylene Welding. The low brasses are readily jointed by oxyacetylene welding. This process is particularly suited for piping because it can be done in all welding positions. Silicon copper welding rods or one of the brass welding rods may be used. For oxyacetylene welding of the high brasses, low-fuming welding rods are used. These low-fuming rods have composition similar to many of the high brasses. A flux is required, and the torch flame should be adjusted to a slightly oxidizing flame to assist in controlling fuming. Preheating and an auxiliary heat source may also be necessary. The welding procedures for copper are also suitable for the brasses.
e. Gas Metal Arc Welding. Gas metal arc welding is recommended for joining large phosphor bronze fabrications and thick sections. Direct current, electrode positive, and argon shielding are normally used. The molten weld pool should be kept small and the travel speed rather high. Stringer beads should be used. Hot peening of each layer will reduce welding stresses and the likelihood of cracking.
f. Gas Tungsten Arc Welding. Gas tungsten arc welding is used primarily for repair of castings and joining of phosphor bronze sheet. As with gas metal arc welding, hot peening of each layer of weld metal is beneficial. Either stabilized ac or direct current, electrode negative can be used with helium or argon shielding. The metal should be preheated to the 350 to 400°F (177 to 204°C) range, and the travel speed should be as fast as practical.
g. Shielded Metal Arc Welding. Phosphor bronze covered electrodes are available for joining bronzes of similar compositions. These electrodes are designed for use with direct current, electrode positive. Filler metal should be deposited as stringer beads for best weld joint mechanical properties. Postweld annealing at 900°F (482°C) is not always necessary, but is desirable for maximum ductility, particularly if the weld metal is to be cold worked. Moisture, both on the work and in the electrode coverings, must be strictly avoided. Baking the electrodes at 250 to 300°F (121 to 149°C) before use may be necessary to reduce moisture in the covering to an acceptable level.
7-19. COPPER WELDING
a. General. Copper and copper-base alloys have specific properties which make them widely used. Their high electrical conductivity makes them widely used in the electrical industries, and corrosion resistance of certain alloys makes them very useful in the process industries. Copper alloys are also widely used for friction or bearing applications. Copper can be welded satisfactorily with either bare or coated electrodes. The oxygen free copper can be welded with more uniform results than the oxygen bearing copper, which tends to become brittle when welded. Due to the high thermal conductivity of copper, the welding currents are higher than those required for steel, and preheating of the base metal is necessary. Copper shares some of the characteristics of aluminum, but is weldable. Attention should be given to its properties that make the welding of copper and copper alloys different from the welding of carbon steels. Copper alloys possess properties that require special attention when welding. These are:
(1) High thermal conductivity.
(2) High thermal expansion coefficient.
(3) Relatively low melting point.
(4) Hot short or brittle at elevated temperatures.
(5) Very fluid molten metal.
(6) High electrical conductivity.
(7) Strength due to cold working.
Copper has the highest thermal conductivity of all commercial metals, and the comments made concerning thermal conductivity of aluminum apply to copper, to an even greater degree.
Copper has a relatively high coefficient of thermal expansion, approximately 50 percent higher than carbon steel, but lower than aluminum.
The melting point of the different copper alloys varies over a relatively wide ranger but is at least 1000°F (538°C) lower than carbon steel. Some of the copper alloys are hot short. This means that they become brittle at high temperatures, because some of the alloying elements form oxides and other compounds at the grain boundaries, embrittling the material.
Copper does not exhibit heat colors like steel, and when it melts it is relatively fluid. This is essentially the result of the high preheat normally used for heavier sections. Copper has the highest electrical conductivity of any of the commercial metals. This is a definite problem in the resistance welding processes.
All of the copper alloys derive their strength from cold working. The heat of welding will anneal the copper in the heat-affected area adjacent to the weld, and reduce the strength provided by cold working. This must be considered when welding high-strength joints.
There are three basic groups of copper designations. The first is the oxygen-free type which has a copper analysis of 99.95 percent or higher. The second subgroup are the tough pitch coppers which have a copper composition of 99.88 percent or higher and some high copper alloys which have 96.00 percent or more copper.
The oxygen-free high-conductivity copper contains no oxygen and is not subject to grain boundary migration. Adequate gas coverage should he used to avoid oxygen of the air caning into contact with the molten metal. Welds should be made as quickly as possible, since too much heat or slow welding can contribute to oxidation. The deoxidized coppers are preferred because of their freedom from embrittlement by hydrogen. Hydrogen embrittlement occurs when copper oxide is exposed to a reducing gas at high temperature. The hydrogen reduces the copper oxide to copper and water vapor. The entrapped high temperature water vapor or steam can create sufficient pressure to cause cracking. In common with all copper welding, preheat should be used and can run from 250 to 1000°F (121 to 538°C), depending on the mass involved.
The tough pitch electrolytic copper is difficult to weld because of the presence of copper oxide within the material. During welding, the copper oxide will migrate to the grain boundaries at high temperatures, which reduces ductility and tensile strength. The gas-shielded processes are recommended since the welding area is more localized and the copper oxide is less able to migrate in appreciable quantities.
The third copper subgroup is the high-copper alloys which may contain deoxidizers such as phosphorus. The copper silicon filler wires are used with this material. The preheat temperatures needed to make the weld quickly apply to all three grades.
c. Gas Metal-Arc (MIG) Welding (GMAW).
(1) The gas metal arc welding process is used for welding thicker materials. It is faster, has a higher deposition rate, and usually results in less distortion. It can produce high-quality welds in all positions. It uses direct current, electrode positive. The CV type power source is recommended.
(2) Metal-arc welding of copper differs from steel welding as indicated below:
(a) Greater root openings are required.
(b) Tight joints should be avoided in light sections.
(c) Larger groove angles are required, particularly in heavy sections, in order to avoid excessive undercutting, slag inclusions, and porosity. More frequent tack welds should be used.
(d) Higher preheat and interpass temperatures are required (800°F (427°C) for copper, 700°F (371°C) for beryllium copper).
(e) Higher currents are required for a given size electrode or plate thickness.
(3) Most copper and copper alloy coated electrodes are designed for use with reverse (electrode positive) polarity. Electrodes for use with alternating currents are available.
(4) Peening is used to reduce stresses in the joints. Flat-nosed tools are used for this purpose. Numerous moderate blows should be used, because vigorous blows could cause crystallizations or other defects in the joint.
d. Gas Tungsten-Arc (TIG) Welding (GTAW).
Never use a flux containing fluoride when welding copper or copper alloys.
(1) Copper can be successfully welded by the gas tungsten-arc welding process. The weldability of each copper alloy group by this process depends upon the alloying elements used. For this reason, no one set of welding conditions will cover all groups.
(2) Direct current straight polarity is generally used for welding most copper alloys. However, high frequency alternating current or direct current reverse polarity is used for beryllium copper or copper alloy sheets less than 0.05 in. (0.13 cm) thick.
(3) For some copper alloys, a flux is recommended. However, a flux containing fluoride should never be used since the arc will vaporize the fluoride and irritate the lungs of the operator.
e. Carbon-Arc Welding.
(1) This process for copper welding is most satisfactory for oxygen-free copper, although it can be used for welding oxygen-bearing copper up to 3/8 in. (9.5 mm) in thickness. The root opening for thinner material should be 3/16 in. (4.8 mm), and 3/8 in. (9.5 mm) for heavier material. The electrode should be graphite type carbon, sharpened to a long tapered point at least equal to the size of the welding rod. Phosphor bronze welding rods are used most frequently in this process.
(2) The arc should be sharp and directed entirely on the weld metal, even at the start. If possible, all carbon-arc welding should be done in the flat welding position or on a moderate slope.
7-20. MAGNESIUM WELDING
a. General. Magnesium is a white, very lightweight, machinable, corrosion resistant, high strength metal. It can be alloyed with small quantities of other metals, such as aluminum, manganese, zinc and zirconium, to obtain desired properties. It can be welded by most of the welding processes used in the metal working trades. Because this metal oxidizes rapidly when heated to its melting point in air, a protective inert gas shield must be provided in arc welding to prevent destructive oxidation.
b. Magnesium possesses properties that make welding it different from the welding of steels. Many of these are the same as for aluminum. These are:
(1) Magnesium oxide surface coating which increases with an increase in temperature.
(2) High thermal conductivity.
(3) Relatively high thermal expansion coefficient.
(4) Relatively low melting temperature.
(5) The absence of color change as temperature approaches the melting point.
The normal metallurgical factors that apply to other metals apply to magnesium as well.
c. The welds produced between similar alloys will develop the full strength of the base metals; however, the strength of the heat-affected zone may be reduced slightly. In all magnesium alloys, the solidification range increases and the melting point and the thermal expansion decrease as the alloy content increases. Aluminum added as an alloy up to 10 percent improves weldability, since it tends to refine the weld grain structure. Zinc of more than 1 percent increases hot shortness, which can result in weld cracking. The high zinc alloys are not recommended for arc welding because of their cracking tendencies. Magnesium, containing small amounts of thorium, possesses excellent welding qualities and freedom from cracking Weldments of these alloys do not require stress relieving. Certain magnesium alloys are subject to stress corrosion. Weldments subjected to corrosive attack over a period of time may crack adjacent to welds if the residual stresses are not removed. Stress relieving is required for weldments intended for this type of service.
d. Cleaning. An oil coating or chrome pickle finish is usually provided on magnesium alloys for surface protection during shipment and storage. This oil, along with other foreign matter and metallic oxides, must be removed from the surface prior to welding. Chemical cleaning is preferred, because it is faster and more uniform in its action. Mechanical cleaning can be utilized if chemical cleaning facilities are not available. A final bright chrome pickle finish is recommended for parts that are to be arc welded. The various methods for cleaning magnesium are described below.
The vapors from some chlorinated solvents (e.g., carbon tetrachloride, trichloroethylene, and perchloroethylene) break down under the ultraviolet radiation of an electric arc and form a toxic gas. Avoid welding where such vapors are present. These solvents vaporize easily, and prolonged inhalation of the vapor can be hazardous. These organic vapors should be removed from the work area before welding begins.
(1) Grease should be removed by the vapor degreasing system in which trichloroethylene is utilized or with a hot alkaline cleaning compound. Grease may also be removed by dipping small parts in dry cleaning solvent or mineral spirits paint thinner.
Dry cleaning solvent and mineral spirits paint thinner are highly flammable. Do not clean parts near an open flame or in a smoking area. Dry cleaning solvent and mineral spirits paint thinner evaporate quickly and have a defatting effect on the skin. When used without protective gloves, these chemicals may cause irritation or cracking of the skin. Cleaning operations should be performed only in well ventilated areas.
(2) Mechanical cleaning can be done satisfactorily with 160 and 240 grit aluminum oxide abrasive cloth, stainless steel wool, or by wire brushing.
Precleaning and postcleaning acids used in magnesium welding and brazing are highly toxic and corrosive. Goggles, rubber gloves, and rubber aprons should be worn when handling the acids and solutions. Do not inhale fumes and mists. When spilled on the body or clothing, wash immediately with large quantities of cold water, and seek medical attention. Never pour water into acid when preparing solution; instead, pour acid into water. Always mix acid and water slowly. Cleaning operations should be performed only in well ventilated areas.
(3) Immediately after the grease, oil, and other foreign materials have been removed from the surface, the metal should be dipped for 3 minutes in a hot solution with the following composition:
The bath should be operated at 70°F (21°C). The work should be removed from the solution, thoroughly rinsed with hot water, and air dried. The welding rod should also be cleaned to obtain the best results.
e. Joint Preparation. Edges that are to be welded must be smooth and free of loose pieces and cavities that might contain contaminating agents, such as oil or oxides. Joint preparations for arc welding various gauges of magnesium are shown in figure 7-13.
f. Safety Precautions.
Magnesium can ignite and burn when heated in the open atmosphere.
(1) Goggles, gloves, and other equipment designed to protect the eyes and skin of the welder must be worn.
(2) The possibility of fire caused by welding magnesium metal is very remote. The temperature of initial fusion must be reached before solid magnesium metal ignites. Sustained burning occurs only if this temperature is maintained. Finely divided magnesium particles such as grinding dust, filings, shavings, borings, and chips present a fire hazard. They ignite readily if proper precautions are not taken. Magnesium scrap of this type is not common to welding operations. If a magnesium fire does start, it can be extinguished with dry sand, dry powdered soapstone, or dry cast iron chips. The preferred extinguishing agents for magnesium fires are graphite base powders.
g. Gas Tungsten-Arc (TIG) Welding (GTAW) of Magnesium.
(1) Because of its rapid oxidation when magnesium is heated to its melting point, an inert gas (argon or helium) is used to shield metal during arc welding. This process requires no flux and permits high welding speeds, with sound welds of high strength.
(2) Direct current machines of the constant current type operating on straight polarity (electrode positive) and alternating current machines are used with a high frequency current superimposed on the welding current. Both alternating and direct current machines are used for thin gauge material. However, because of better penetrating power, alternating current machines are used on material over 3/16 in. (4.8 mm) thick. Helium is considered more practical than argon for use with direct current reverse polarity. However, three times as much helium by volume as argon is required for a given amount of welding. Argon is used with alternating current.
(3) The tungsten electrodes are held in a water cooled torch equipped with required electrical cables and an inlet and nozzle for the inert gas.
(4) The two magnesium alloys, in the form of sheet, plate, and extrusion, that are most commonly used for applications involving welding are ASTM-1A (Fed Spec QQ-M-54), which is alloyed with manganese, and ASTM-AZ31A (Fed SPec QQ-44), which is alloyed with aluminum, manganese, and zinc.
(5) In general, less preparation is required for welding with alternating current than welding with direct current because of the greater penetration obtained. Sheets up to 1/4 in. (6.4 mm) thickness may be welded from one side with a square butt joint. Sheets over 1/4 in. (6.4 mm) thickness should be welded from both sides whenever the nature of the structure permits, as sounder welds may be obtained with less warpages. For a double V joint, the included angle should extend from both sides to leave a minimum 1/16 in. (1.6 mm) root face in the center of the sheets. When welding a double V joint, the back of the first bead should be chipped out using a chipping hammer fitted with a cape chisel. Remove oxide film, dirt, and incompletely fused areas before the second bead is added. In this manner, maximum soundness is obtained.
(6) The gas should start flowing a fraction of a second before the arc is struck. The arc is struck by brushing the electrode over the surface. With alternating current, the arc should be started and stopped by means of a remote control switch. The average arc length should be about 1/8 in. (3.2 mm) when using helium and 1/16 in. (1.6 mm) when using argon. Current data and rod diameter are shown in table 7-24.
(7) When welding with alternating current, maximum penetration is obtained when the end of the electrode is held flush with or slightly below the surface of the work. The torch should be held nearly perpendicular to the surface of the work, and the welding rod added from a position as neatly parallel with the work as possible (fig. 7-14). The torch should have a slightly leading travel angle.
(8) Welding should progress in a straight line at a uniform speed. There should be no rotary or weaving motion of the rod or torch, except for larger corner joints or fillet welds. The welding rod can be fed either continuously or intermittently. Care should be taken to avoid withdrawing the heated end from the protective gaseous atmosphere during the welding operation. The cold wire filler metal should be brought in as near to horizontal as possible (on flat work). The filler wire is added to the leading edge of the weld puddle. Runoff tabs are recommended for welding any except the thinner metals. Forehand welding, in which the welding rod precedes the torch in the direction of welding, is preferred. If stops are necessary, the weld should be started about 1/2 in. (12.7 mm) back from the end of the weld when welding is resumed.
(9) Because of the high coefficient of thermal expansion and conductivity, control of distortion in the welding of magnesium requires jigging, small beads, and a properly selected welding sequence to help minimize distortion. Magnesium parts can be straightened by holding them in position with clamps and heating to 300 to 400°F (149 to 204°C). If this heating is done by local torch application, care must be taken not to overheat the metal and destroy its properties.
(10) If cracking is encountered during the welding of certain magnesium alloys, starting and stopping plates may be used to overcome this difficult. These plates consist of scrap pieces of magnesium stock butted against opposite ends of the joint to be welded as shown in A, figure 7-15. The weld is started on one of the abutting plates, continued across the junction along the joint to be welded, and stopped on the opposite abutting plate. If a V groove is used, the abutting plates should also be grooved. An alternate method is to start the weld in the middle of the joint and weld to each edge (B, fig. 7-15). Cracking may also be minimized by preheating the plate and holding the jig to 200 to 400°F (93 to 204°C) by increasing the speed of the weld.
(11) Filler reds must be of the same composition as the alloy being joined when arc welding. One exception is when welding AZ31B. In this case, grade C rod (MIL-R-6944), which produces a stronger weld metal, is used to reduce cracking.
(12) Residual stress should be relieved through heat treatment. Stress relief is essential so that lockup stresses will not cause stress corrosion cracking. The recommended stress relieving treatment for arc welding magnesium sheet is shown in table 7-25.
(13) The only cleaning required after arc welding of magnesium alloys is wire brushing to remove the slight oxide deposit on the surface. Brushing may leave traces of iron, which may cause galvanic corrosion. If necessary, clean as in b above. Arc welding smoke can be removed by immersing the parts for 1/2 to 2 minutes at 180 to 212°F (82 to 100°C), in a solution composed of 16 oz (453 g) tetrasodium pyrophosphate (Na4P2O7), 16 oz (453 g) sodium metaborate (NaBO2), and enough water to make 1 gallon (3.8 1).
(14) Welding procedure schedules for GTAW of magnesium (TIG welding) are shown in table 7-26.
h. Gas Metal-Arc (MIG) Welding of Magnesium (GMAW). The gas metal arc welding process is used for the medium to thicker sections. It is considerably faster than gas tungsten arc welding. Special high-speed gear ratios are usually required in the wire feeders since the magnesium electrode wire has an extremely high meltoff rate. The normal wire feeder and power supply used for aluminum welding is suitable for welding magnesium. Different types of arc transfer can be obtained when welding magnesium. This is primarily a matter of current level or current density and voltage setting. The short-circuiting transfer and the spray transfer are recommended. Argon is usually used for gas metal arc welding of magnesium; however, argon-helium mixtures can be used. In general, the spray transfer should be used on material 3/16 in. (4.8 mm) and thicker and the short-circuiting arc used for thinner metals. Welding procedure schedules for GMAW of magnesium (MIG welding) are shown in table 7-27.
i. Other Welding Processes. Magnesium can be welded using the resistance welding processes, including spot welding, seam welding, and flash welding. Magnesium can also be joined by brazing. Most of the different brazing techniques can be used. In all cases, brazing flux is required and the flux residue must be completely removed from the finish part. Soldering is not as effective, since the strength of the joint is relatively low. Magnesium can also be stud welded, gas welded, and plasma-arc welded.
7-21. TITANIUM WELDING
(1) Titanium is a soft, silvery white, medium strength metal with very good corrosion resistance. It has a high strength to weight ratio, and its tensile strength increases as the temperature decreases. Titanium has low impact and creep strengths. It has seizing tendencies at temperatures above 800°F (427°C).
(2) Titanium has a high affinity for oxygen and other gases at elevated temperatures, and for this reason, cannot be welded with any process that utilizes fluxes, or where heated metal is exposed to the atmosphere. Minor amounts of impurities cause titanium to become brittle.
(3) Titanium has the characteristic known as the ductile-brittle transition. This refers to a temperature at which the metal breaks in a brittle manner, rather than in a ductile fashion. The recrystallization of the metal during welding can raise the transition temperature. Contamination during the high temperate period and impurities can raise the transition temperature period and impurities can raise the transition temperature so that the material is brittle at room temperatures. If contamination occurs so that transition temperature is raised sufficiently, it will make the welding worthless. Gas contamination can occur at temperatures below the melting point of the metal. These temperatures range from 700°F (371°C) up to 1000°F (538°C).
(4) At room temperature, titanium has an impervious oxide coating that resists further reaction with air. The oxide coating melts at temperatures considerably higher than the melting point of the base metal and creates problems. The oxidized coating may enter molten weld metal and create discontinuities which greatly reduce the strength and ductility of the weld.
(5) The procedures for welding titanium and titanium alloys are similar to other metals. Some processes, such as oxyacetylene or arc welding processes using active gases, cannot be used due to the high chemical activity of titanium and its sensitivity to embrittlement by contamination. Processes that are satisfactory for welding titanium and titanium alloys include gas shielded metal-arc welding, gas tungsten arc welding, and spot, seam, flash, and pressure welding. Special procedures must be employed when using the gas shielded welding processes. These special procedures include the use of large gas nozzles and trailing shields to shield the face of the weld from air. Backing bars that provide inert gas to shield the back of the welds from air are also used. Not only the molten weld metal, but the material heated above 1000°F (538°C) by the weld must be adequately shielded in order to prevent embrittlement. All of these processes provide for shielding of the molten weld metal and heat affected zones. Prior to welding, titanium and its alloys must be free of all scale and other material that might cause weld contamination.
b. Surface Preparation.
The nitric acid used to preclean titanium for inert gas shielded arc welding is highly toxic and corrosive. Goggles, rubber gloves, and rubber aprons must be worn when handling acid and acid solutions. Do not inhale gases and mists. When spilled on the body or clothing, wash immediately with large quantities of cold water, and seek medical help. Never pour water into acid when preparing the solution; instead, pour acid into water. Always mix acid and water slowly. Perform cleaning operations only in well ventilated places.
The caustic chemicals (including sodium hydride) used to preclean titanium for inert gas shielded arc welding are highly toxic and corrosive. Goggles, rubber gloves, and rubber aprons must be worn when handling these chemicals. Do not inhale gases or mists. When spilled on the body or clothing, wash immediately with large quantities of cold water and seek medical help. Special care should be taken at all times to prevent any water from coming in contact with the molten bath or any other large amount of sodium hydride, as this will cause the formation of highly explosive hydrogen gas.
(1) Surface cleaning is important in preparing titanium and its alloys for welding. Proper surface cleaning prior to welding reduces contamination of the weld due to surface scale or other foreign materials. Small amounts of contamination can render titanium completely brittle.
(2) Several cleaning procedures are used, depending on the surface condition of the base and filler metals. Surface conditions most often encountered are as follows:
(a) Scale free (as received from the mill).
(b) Light scale (after hot forming or annealing at intermediate temperature; ie., less than 1300°F (704°C).
(c) Heavy scale (after hot forming, annealing, or forging at high temperature).
(3) Metals that are scale free can be cleaned by simple decreasing.
(4) Metals with light oxide scale should be cleaned by acid pickling. In order to minimize hydrogen pickup, pickling solutions for this operation should have a nitric acid concentration greater than 20 percent. Metals to be welded should be pickled for 1 to 20 minutes at a bath temperature from 80 to 160°F (27 to 71°C). After pickling, the parts are rinsed in hot water.
(5) Metals with a heavy scale should be cleaned with sand, grit, or vaporblasting, molten sodium hydride salt baths, or molten caustic baths. Sand, grit, or vaporblasting is preferred where applicable. Hydrogen pickup may occur with molten bath treatments, but it can be minimized by controlling the bath temperature and pickling time. Bath temperature should be held at about 750 to 850°F (399 to 454°C). Parts should not be pickled any longer than necessary to remove scale. After heavy scale is removed, the metal should be pickled as described in (4) above.
(6) Surfaces of metals that have undergone oxyacetylene flame cutting operations have a very heavy scale, and may contain microscopic cracks due to excessive contamination of the metallurgical characteristics of the alloys. The best cleaning method for flame cut surfaces is to remove the contaminated layer and any cracks by machining operations. Certain alloys can be stress relieved immediately after cutting to prevent the propagation of these cracks. This stress relief is usually made in conjunction with the cutting operation.
c. MIG or TIG Welding of titanium.
(1) General. Both the MIG and TIG welding processes are used to weld titanium and titanium alloys. They are satisfactory for manual and automatic installations. With these processes, contamination of the molten weld metals and adjacent heated zones is minimized by shielding the arc and the root of the weld with inert gases (see (2)(b)) or special backing bars (see (2)(c)). In some cases, inert gas filler welding chambers (see (3)) are used to provide the required shielding. When using the TIG welding process, a thoriated tungsten electrode should be used. The electrode size should be the smallest diameter that will carry the welding current. The electrode should be ground to a point. The electrode may extend 1-1/2 times its diameter beyond the end of the nozzle. Welding is done with direct current, electrode negative (straight polarity). Welding procedure for TIG welding titanium are shown in table 7-28. Selection of the filler metal will depend upon the titanium alloys being joined. When welding pure titanium, a pure titanium wire should be used. When welding a titanium alloy, the next lowest strength alloy should be used as a filler wire. Due to the dilution which will take place dining welding, the weld deposit will pick up the required strength. The same considerations are true when MIG welding titanium.
(a) General. Very good shielding conditions are necessary to produce arc welded joints with maximum ductility and toughness. To obtain these conditions, the amount of air or other active gases which contact the molten weld metals and. adjacent heated zones must be very low. Argon is normally used with the gas-shielded process. For thicker metal, use helium or a mixture of argon and helium. Welding grade shielding gases are generally free from contamination; however, tests can be made before welding. A simple test is to make a bead on a piece of clean scrap titanium, and notice its color. The bead should be shiny. Any discoloration of the surface indicates a contamination. Extra gas shielding provides protection for the heated solid metal next to the weld metal. This shielding is provided by special trailing gas nozzles, or by chill bars laid immediately next to the weld. Backup gas shielding should be provided to protect the underside of the weld joint. Protection of the back side of the joint can also be provided by placing chill bars in intimate contact with the backing strips. If the contact is close enough, backup shielding gas is not required. For critical applications, use an inert gas welding chamber. These can be flexible, rigid, or vacuum-purge chambers.
(b) Inert gases. Both helium and argon are used as the shielding gases. With helium as the shielding gas high welding speeds and better penetration are obtained than with argon, but the arc is more stable in argon. For open air welding operations, most welders prefer argon as the shielding gas because its density is greater than that of air. Mixtures of argon and helium are also used. With mixtures, the arc characteristics of both helium and argon are obtained. The mixtures usually vary in composition from about 20 to 80 percent argon. They are often used with the consumable electrode process. To provide adequate shielding for the face and root sides of welds, special precautions often are taken. The precautions include the use of screens and baffles (see (c) 3), trailing shields (see (c) 7), and special backing fixtures (see (c) 10) in open air welding, and the use of inert gas filler welding chambers.
(c) Open air welding.
1. In open air welding operations, the methods used to shield the face of the weld vary with joint design, welding conditions, and the thickness of the materials being joined. The most critical area in regard to the shielding is the molten weld puddle. Impurities diffuse into the molten metal very rapidly and remain in solution. The gas flowing through a standard welding torch is sufficient to shield the molten zone. Because of the low thermal conductivity of titanium, however, the molten puddle tends to be larger than most metals. For this reason and because of shielding conditions required in welding titanium, larger nozzles are used on the welding torch, with proportionally higher gas flows that are required for other metals. Chill bars often are used to limit the size of the puddle.
2. The primary sources of contamination in the molten weld puddle are turbulence in the gas flow, oxidation of hot filler reds, insufficient gas flow, small nozzles on the welding torch, and impure shielding gases. The latter three sources are easily controlled.
3. If turbulence occurs in the gas flowing from the torch, air will be inspired and contamination will result. Turbulence is generally caused by excessive amounts of gas flowing through the torch, long arc lengths, air currents blowing across the weld, and joint design. Contamination from this source can be minimized by adjusting gas flows and arc lengths, and by placing baffles alongside the welds. Baffles protect the weld from drafts and tend to retard the flow of shielding gas from the joint area. Chill bars or the clamping toes of the welding jig can serve as baffles (fig. 7-16). Baffles are especially important for making corner type welds. Additional precautions can be taken to protect the operation from drafts and turbulence. This can be achieved by erecting a canvas (or other suitable material) screen around the work area.
4. In manual welding operations with the tungsten-arc process, oxidation of the hot filler metal is a very important source of contamination. To control it, the hot end of the filler wire must be kept within the gas shield of the welding torch. Welding operators must be trained to keep the filler wire shielded when welding titanium and its alloys. Even with proper manipulation, however, contamination from this source probably cannot be eliminated completely.
5. Weld contamination which occurs in the molten weld puddle is especially hazardous. The impurities go into solution, and do not cause discoloration. Although discolored welds may have been improperly shielded while molten, weld discoloration is usually caused by contamination which occurs after the weld has solidified.
6. Most of the auxiliary equipment used on torches to weld titanium is designed to improve shielding conditions for the welds as they solidify and cool. However, if the welding heat input is low and the weld cools to temperatures below about 1200 to 1300°F (649 to 704°C) while shielded, auxiliary shielding equipment is not required. If the weld is at an excessively high temperature after it is no longer shielded by the welding torch, auxiliary shielding must be supplied.
7. Trailing shields often are used to supply auxiliary shielding. These shields extend behind the welding torch and vary considerably in size, shaper and design. They are incorporated into special cups which are used on the welding torch, or may consist only of tubes or hoses attached to the torch or manipulated by hand to direct a stream of inert gas on the welds. Figure 7-17 shows a drawing of one type of trailing shield currently in use. Important features of this shield are that the porous diffusion plate allows an even flow of gas over the shielded area. This will prevent turbulence in the gas stream. The shield fits on the torch so that a continuous gas stream between the torch and shield is obtained.
8. Baffles are also beneficial in improving shielding conditions for welds by retarding the flow of shielding gas from the joint area. Baffles may be placed alongside the weld, over the top, or at the ends of the weld. In some instances, they may actually form a chamber around the arc and molten weld puddle. Also, chill bars may be used to increase weld cooling rates and may make auxiliary shielding unnecessary.
9. Very little difficulty has been encountered in shielding the face of welds in automatic welding operations. However, considerable difficulty has been encountered in manual operations.
10. In open air welding operations, means must be provided for shielding the root or back of the welds. Backing fixtures are often used for this purpose. In one type, an auxiliary supply of inert gas is provided to shield the back of the weld. In the other, a solid or grooved backing bar fits tightly against the back of the weld and provides the required shielding. Fixtures which provide an inert gas shield are preferred, especially in manual welding operations with low welding speeds. Figure 7-18 shows backing fixtures used in butt welding heavy plate and thin sheet, respectively. Similar types of fixtures are used for other joint designs. However, the design of the fixtures varies with the design of the joints. For fillet welds on tee joints, shielding should be supplied for two sides of the weld in addition to shielding the face of the weld.
11. For some applications, it may be easier to enclose the back of the weld, as in a tank, and supply inert gas for shielding purposes. This method is necessary in welding tanks, tubes, or other enclosed structures where access to the back of the weld is not possible. In some weldments, it may be necessary to machine holes or grooves in the structures in order to provide shielding gas for the back or root of the welds.
When using weld backup tape, the weld must be allowed to cool for several minutes before attempting to remove the tape from the workpiece.
12. Use of backing fixtures such as shown in figure 7-18 can be eliminated in many cases by the use of weld backup tape. This tape consists of a center strip of heat resistant fiberglass adhered to a wider strip of aluminum foil, along with a strip of adhesive on each side of the center strip that is used to hold tape to the underside of the tack welded joint. During the welding, the fiberglass portion of the tape is in direct contact with the molten metal, preventing excessive penetration. Contamination or oxidation of the underside of the weld is prevented by the airtight seal created by the aluminum foil strip. The tape can be used on butt or corner joints (fig. 7-19) or, because of its flexibility, on curved or irregularly shaped surfaces. The surface to which the tape is applied must be clean and dry. Best results are obtained by using a root gap wide enough to allow full penetration.
13. Bend or notch toughness tests are the best methods for evaluating shielding conditions, but visual inspection of the weld surface, which is not an infallible method, is the only nondestructive means for evaluating weld quality at the present time. With this method, the presence of a heavy gray scale with a nonmetallic luster on the weld bead indicates that the weld has been contaminate badly and has low ductility. Also, the weld surface may be shiny but have different colors, ranging from grayish blue to violet to brown. This type of discoloration may be found on severely contaminated welds or may be due only to surface contamination, while the weld itself may be satisfactory. However, the quality of the weld cannot be determined without a destructive test. With good shielding procedures, weld surfaces are shiny and show no discoloration.
(3) Welding chambers.
(a) For some applications, inert gas filled welding chambers are used. The advantage of using such chambers is that good shielding may be obtained for the root and face of the weld without the use of special fixtures. Also, the surface appearance of such welds is a fairly reliable measure of shielding conditions. The use of chambers is especially advantageous when complex joints are being welded. However, chambers are not required for many applications, and their use may be limited.
(b) Welding chambers vary in size and shape, depending on their use and the size of assemblies to be welded. The inert atmospheres maybe obtained by evacuating the chamber and filling it with helium or argon, purging the chamber with inert gas, or collapsing the chamber to expel air and refilling it with an inert gas. Plastic bags have been used in this latter manner. When the atmospheres are obtained by purging or collapsing the chambers, inert gas usually is supplied through the welding torch to insure complete protection of the welds.
(4) Joint designs. Joint designs for titanium are similar to those used for other metals. For welding a thin sheet, the tungsten-arc process generally is used. With this process, butt welds may be made with or without filler rod, depending on the thickness of the joint and fitup. Special shearing procedures sometimes are used so that the root opening does not exceed 8 percent of the sheet thickness. If fitup is this good, filler rod is not required. If fitup is not this good, filler metal is added to obtain full thickness joints. In welding thicker sheets (greater than 0.09 in. (2.3 mm)), both the tungsten-arc and consumable electrode processes are used with a root opening. For welding titanium plates, bars, or forgings, both the tungsten-arc and consumable electrode processes also are used with single and double V joints. In all cases, good weld penetration may be obtained with excessive drop through. However, penetration and dropthrough are controlled more easily by the use of proper backing fixtures.
Because of the low thermal conductivity of titanium, weld beads tend to be wider than normal. However, the width of the beads is generally controlled by using short arc lengths, or by placing chill bars or the clamping toes of the jig close to the sides of the joints.
(5) Welding variables.
(a) Welding speed and current for titanium alloys depend on the process used, shielding gas, thickness of the material being welded, design of the backing fixtures, along with the spacing of chill bars or clamping bars in the welding jig. Welding speeds vary from about 3.0 to 40.0 in. (76.2 to 1016.0 mm) per minute. The highest welding speeds are obtained with the consumable electrode process. In most cases, direct current is used with straight polarity for the tungsten-arc process. Reverse polarity is used for the consumable electrode process.
(b) Arc wander has proven troublesome in some automatic welding operations. With arc wander, the arc from the tungsten or consumable electrode moves from one side of the weld joint to the other side. A straight, uniform weld bead will not be produced. Arc wander is believed to be caused by magnetic disturbances, bends in the filler wire, coatings on the filler wire, or a combination of these. Special metal shields and wire straighteners have been used to overcome arc wander, but have not been completely satisfactory. Also, constant voltage welding machines have been used in an attempt to overcome this problem. These machines also have not been completely satisfactory.
(c) In setting up arc welding operations for titanium, the welding conditions should be evaluated on the basis of weld joint properties and appearance. Radiographs will show if porosity or cracking is present in the weld joint. A simple bend test or notch toughness test will show whether or not the shielding conditions are adequate. A visual examination of the weld will show if the weld penetration and contour are satisfactory. After adequate procedures are established, careful controls are desirable to ensure that the shielding conditions are not changed.
(6) Weld defects.
(a) General. Defects in arc welded joints in titanium alloys consist mainly of porosity (see (b)) and cold cracks (see (c)). Weld penetration can be controlled by adjusting welding conditions.
(b) Porosity. Weld porosity is a major problem in arc welding titanium alloys. Although acceptable limits for porosity in arc welded joints have not been establish, porosity has been observed in tungsten-arc welds in practically all of the alloys which appear suitable for welding operations. It does not extend to the surface of the weld, but has been detected in radiographs. It usually occurs close to the fusion line of the welds. Weld porosity may be reduced by agitating the molten weld puddle and adjusting welding speeds. Also, remelting the weld will eliminate some of the porosity present after the first pass. However, the latter method of reducing weld porosity tends to increase weld contamination.
1. With adequate shielding procedures and suitable alloys, cracks should not be a problem. However, cracks have been troublesome in welding some alloys. Weld cracks are attributed to a number of causes. In commercially pure titanium, weld metal cracks are believed to be caused by excessive oxygen or nitrogen contamination. These cracks are usually observed in weld craters. In some of the alpha-beta alloys, transverse cracks in the weld metal and heat affected zones are believed to be due to the low ductility of the weld zones. However, cracks in these alloys also may be due to contamination. Cracks also have been observed in alpha-beta welds made under restraint and with high external stresses. These cracks are sometimes attributed to the hydrogen content of the alloys.
If weld cracking is due to contamination, it may be controlled by improving shielding conditions. However, repair welding on excessively contaminated welds is not practical in many cases.
2. Cracks which are caused by the low ductility of welds in alpha-beta alloys can be prevented by heat treating or stress relieving the weldment in a furnance immediately after welding. Oxyacetylene torches also have been used for this purpose. However, care must be taken so that the weldment is not overheated or excessively contaminated by the torch heating operation.
3. Cracks due to hydrogen may be prevented by vacuum annealing treatments prior to welding.
(7) Availability of welding filler wire. Most of the titanium alloys which are being used in arc welding applications are available as wire for use as welding filler metal. These alloys are listed below:
(a) Commercially pure titanium --commercially available as wire.
(b) Ti-5A1-2-1/2Sn alloy --available as wire in experimental quantities.
(c) Ti-1-1/2A1-3Mn alloy --available as wire in experimental quantities.
(d) Ti-6A1-4V alloy --available as wire in experimental quantities.
(e) There has not been a great deal of need for the other alloys as welding filler wires. However, if such a need occurs, most of these alloys also could be reduced to wire. In fact, the Ti-8Mn alloy has been furnished as welding wire to meet some requests.
d. Pressure Welding. Solid phase or pressure welding has been used to join titanium and titanium alloys. In these processes, the surfaces to be jointed are not melted. They are held together under pressure and heated to elevated temperatures (900 to 2000°F (482 to 1093°C)). One method of heating used in pressure welding is the oxyacetylene flame. With suitable pressure and upset, good welds are obtainable in the high strength alpha-beta titanium alloys. The contaminated area on the surface of the weld is displaced from the joint area by the upset, which occurs during welding. This contaminated surface is machined off after welding. Another method of heating is by heated dies. Strong lap joints are obtained with this method in commercially pure titanium and a high strength alpha-beta alloy. By heating in this manner, welds may be made in very short periods of time, and inert gas shielding may be supplied to the joint. With all of the heating methods, less than 2 minutes is required to complete the welding operation. With solid phase or pressure welding processes, it is possible to produce ductile welds in the high strength alpha-beta alloys by using temperatures which do not cause embrittlement in these alloys.
7-22. NICKEL AND MONTEL WELDING
a. General. Nickel is a hard, malleable, ductile metal. Nickel and its alloys are commonly used when corrosion resistance is required. Nickel and nickel alloys such as Monel can, in general, be welded by metal-arc and gas welding methods. Some nickel alloys are more difficult to weld due to different compositions. The operator should make trial welds with reverse polarity at several current values and select the one best suited for the work. Generally, the oxyacetylene welding methods are preferred for smaller plates. However, small plates can be welded by the metal-arc and carbon-arc processes, and large plates are most satisfactorily joined, especially if the plate is nickel clad steel.
When welding, the nickel alloys can be treated much in the same manner as austenitic stainless steels with a few exceptions. These exceptions are:
(1) The nickel alloys will acquire a surface or coating which melts at a temperature approximately 1000°F (538°C) above the melting point of the base metal.
(2) The nickel alloys are susceptible to embrittlement at welding temperatures by lead, sulfur, phosphorus, and some low-temperature metals and alloys.
(3) Weld penetration is less than expected with other metals.
When compensation is made for these three factors, the welding procedures used for the nickel alloys can he the same as those used for stainless steel. This is because the melting point, the coefficient of thermal expansion, and the thermal conductivity are similar to austenitic stainless steel.
It is necessary that each of these precautions be considered. The surface oxide should be completely removed from the joint area by grinding, abrasive blasting, machining, or by chemical means. When chemical etches are used, they must be completely removed by rinsing prior to welding. The oxide which melts at temperatures above the melting point of the base metal may enter the weld as a foreign material, or impurity, and will greatly reduce the strength and ductility of the weld. The problem of embrittlement at welding temperatures also means that the weld surface must be absolutely clean. Paints, crayon markings, grease, oil, machining lubricants, and cutting oils may all contain the ingredients which will cause embrittlement. They must be completely removed for the weld area to avoid embrittlement. It is necessary to increase the opening of groove angles and to provide adequate root openings when full-penetration welds are used. The bevel or groove angles should be increased to approximately 40 percent over those used for carbon steel.
b. Joint Design. Butt joints are preferred but corner and lap joints can be effectively welded. Beveling is not required on plates 1/16 to 1/8 in. (1.6 to 3.2 mm) thick. With thicker materials, a bevel angle of 35 to 37-1/2 degrees should be made. When welding lap joints, the weld should be made entirely with nickel electrodes if water or air tightness is required.
c. Welding Techniques.
(1) Clean all surfaces to be welded either mechanically by machine, sand-blasting, grinding, or with abrasive cloth; or chemically by pickling.
(2) Plates having U or V joints should be assembled, and if nickel clad steel, should be tacked on the steel side to prevent warping and distortion. After it is determined that the joint is even and flat, complete the weld on the steel side. Chip out and clean the nickel side and weld. If the base metal on both sides is nickel, clean out the groove on the unwelded side prior to beginning the weld on that side.
(3) If desired, the nickel side may be completed first. However, the steel side must be tacked and thoroughly cleaned and beveled (or gouged) down to the root of the nickel weld prior to welding.
(4) Lap and corner joints are successfully welded by depositing a bead of nickel metal into the root and then weaving successive beads over the root weld.
(5) The arc drawn for nickel or nickel alloy welding should be slightly shorter than that used in normal metal-arc welding. A 1/16 to 1/8 in. (1.6 to 3.2 mm) arc is a necessity.
(6) Any position weld can be accomplished that can be satisfactorily welded by normal metal-arc welding of steel.
d. Welding Methods.
(1) Almost all the welding processes can be used for welding the nickel alloys. In addition, they can be joined by brazing and soldering.
(2) Welding nickel alloys. The most popular processes for welding nickel alloys are the shielded metal arc welding process, the gas tungsten arc welding process, and the gas metal arc welding process. Process selection depends on the normal factors. When shielded metal arc welding is used the procedures are essentially the same as those used for stainless steel welding.
The welding procedure schedule for using gas tungsten arc welding (TIG) is shown by table 7-29. The welding procedure schedule for gas metal arc welding (MIG) is shown by table 7-30. The procedure information set forth on these tables will provide starting points for developing the welding procedures.
(3) No postweld heat treatment is required to maintain or restore corrosion resistance of the nickel alloys. Heat treatment is required for precipitating hardening alloys. Stress relief may be required to meet certain specifications to avoid stress corrosion cracking in applications involving hydrofluoric acid vapors or caustic solutions.