Both the gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) processes are used for welding aluminum castings. In general, welding aluminum alloy castings requires a technique similar to that used on aluminum sheet and other wrought products. However, many castings are susceptible to thermal strains and cracks because of intricate design and varying section thicknesses. In highly stressed structures, castings depend on heat treatment for strengthWelding tends to destroy the effect of the initial heat treatment. In these cases, welding is not recommended unless it is possible for the casting or assembly to be heat-treated again after welding, when the loss in strength can largely be restored.

Preparation for Welding

Before welding, castings should be cleaned carefully with a wire brush and an appropriate solvent to remove every trace of oil, grease and dirt. When welds are to be made in sections heavier than approximately 5 mm (3/16 in.), the edges should be beveled at an angle of about 45″. When preparing defective areas for welding, any unsoundness or dross must be completely melted or cut away before proceeding with the weld. When two or more pieces are to be assembled, or if a broken piece is to be welded, the parts should be held by a fixture and clamped in the correct position for preheating and welding. The clamps should be attached in a way that will permit free expansion of the casting during heating, otherwise stresses may develop which will result in excessive distortion or cracks.

Preheating

Prior to welding a casting that is large or intricate in design, it should be preheated slowly and uniformly in a furnace to avoid thermal stresses and facilitate development of the required temperature for welding. A temperature of 370 to 425°C (700 to 800°F) is generally sufficient for preheating. If the casting is small, or if the weld is near the edge and in a thin walled section, an experienced welder can often do the necessary preheating with an oxyfuel gas torch applied in the region of the weld. After welding, the casting should be cooled slowly and uniformly to room temperature to reduce the danger of excessive stresses and possible cracks.

Welding Precautions

Surface defects and small holes in aluminum castings can be repaired by welding after the part is correctly prepared and preheated. However, when working with assemblies or broken castings, there are several points to consider during welding. The individual parts should first be tack-welded into place, and actual welding should begin at the center and proceed toward the end. When any difference exists in the thickness of the sections being joined, the GTAW welder must carefully distribute the heat from the torch in order to avoid melting the lighter section while bringing the heavier section up to welding temperature. A similar precaution must be taken with sheet and casting assemblies, and welders may require a little experience to develop the proper technique.

Choice of Welding Rod

When welding castings of the non-heat-treatable aluminum alloys or assemblies involving such castings, consisting of welding rod Al-S%Si or A1-4%Cu, 3%Si is generally used. However, in the case of castings requiring subsequent heat treatment, a welding rod of the same alloy as the casting should be used. The size of the rod best suited for the job will, of course, depend to some extent on the thickness of the metal being welded, but in general, a rod 1.6 to 2.4 mm (1/16 in. to 3/32 in.) diameter will be satisfactory.

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