Welding of Nonferrous Metals

After talking about Welding of Ferrouse Metals, now Piping & Fabrication will talk about Welding of Nonferrous Metals.

Aluminum. Aluminum and aluminum alloys have high thermal conductivity, high coefficients of thermal expansion, and high fluidity in the molten state. The predominant welding methods used for joining them are GMAW and GTAW, both manually and in automatic modes. Joint designs are much like those used for ferritic metals, except that the included angles are usually 60 to 75°, increasing to 90 or 110° for welding overhead. The root pass may be welded against a permanent aluminum backing strip or removable stainless-steel backup or with an open butt or consumable insert. Joint cleanliness is very important, so oil, grease, and dirt must be removed. For heavy oxide, wire brushing or chemical cleaning may be required. Preheating is normally not needed but may be required when the mass of the parts is large enough to conduct the heat of welding away from the joint faster than it can be supplied by the arc. Depending on the welding process used, as the weld thickness increases from about 1⁄4 to 1 in (19 to 25 mm), a preheat of 200 to 600°F (95 to 316°C) may be required. Since the properties and tempers of certain alloys may be affected, care should be exercised when preheat is applied.

Shielding gases are usually helium or argon. For critical applications and heavier sections a mixture of 75 percent helium, 25 percent argon is recommended. Heat treatment after welding is not required. It is important to remember that the annealing effect of the heat of welding can reduce the strength level of cold-worked and heat-treatable alloys. In this case the allowable stress value for the material in the annealed condition should be used for design. An exception to this can be made in the case of heat-treatable materials when the finished weldment is subjected to the same heat treatment which produced the original temper and both the base metal and weld joint are similarly affected.

Aluminum and aluminum alloys are suitable for service temperatures down to -452°F (-269°C). See B31.3 for information on this subject. Copper and Copper Alloys. Although copper and copper alloys can be welded by other processes, GTAW welding is commonly applicable for all-position welding of most copper and copper alloys. GMAW with pulsed current can also be used for some alloys. Shielding gases may be argon, helium, or mixtures thereof. Argon is preferred for walls to 1⁄8 in (3 mm), but a 75 percent helium, 25 percent argon mixture is most often used for heavier walls and weld positions other than flat (1G). Like aluminum, the coppers have high thermal conductivity and high coefficient of thermal expansion. Accordingly, preheating is recommended to compensate for heat loss at the joint due to the metal mass and to reduce distortion. Welding current should not be used to compensate for heat loss. The degree of preheat is a function of alloy, welding process, and metal mass. More heat input is needed for the pure coppers, with decreasing amounts needed as the alloy content increases. Preheat should increase with wall thickness, from about 200°F (95°C) for 1⁄4-in (6 mm) wall increasing to 750°F (400°C) minimum for walls 5/8 in (16 mm) and over. Surface cleanliness is very important, and some alloys require a chemical cleaning to remove oxides. Copper-nickel alloys are susceptible to hot cracking if sulfur is present.

The heat of welding will soften the HAZ of cold-worked material, and it will be weaker than the base metal. When precipitation-hardenable alloys are used, it is recommended that welding be done on base metal in the annealed condition and the entire weldment be given the precipitation-hardening heat treatment. For detailed information refer to the Welding Handbook,16 the Metals Handbook,20 or contact the Copper Development Association. Many coppers are suitable for services down to -325°F (-199°C), See ASME B31.3