94 Welding Brazing And Soldering 941 Welding

Welding is the process of uniting parts by either heating, applying pressure, or both. When heat is used to weld aluminum (as is usually the case), it reduces the strength of all tempers other than annealed material, and this must be taken into account where strength is a consideration [13].

Aluminum's affinity for oxygen, which quickly forms a thin, hard oxide surface film, has much to do with the welding process. This oxide is nearly as hard as diamonds, attested to by the fact that aluminum oxide grit is often used for grinding. It has a much higher melting point than aluminum itself [3725°F (2050°C), versus 1220°F (660°C)], so trying to weld aluminum without first removing the oxide melts the base metal long before the oxide. The oxide is also chemically stable; fluxes to remove it require corrosive substances that can damage the base metal unless they are fully removed after welding. Finally, the oxide is an electrical insulator and porous enough to retain moisture. For all these reasons, the base metal must be carefully cleaned and wire brushed immediately before welding, and the welding process must remove and prevent reformation of the oxide film during welding.

The metal in the vicinity of a weld can be considered as two zones: the weld bead itself, a casting composed of a mixture of the filler and the base metal, and the heat affected zone (HAZ) in the base metal outside the weld bead. The extent of the HAZ is a function of the thickness and geometry of the joint, the welding process, the welding procedure, and preheat and interpass temperatures, but rarely exceeds 1 in. (25 mm) from the centerline of the weld. The strength of the metal near a weld is graphed in Fig. 9.4. Smaller welds and higher welding speeds tend to have a smaller HAZ. As the base metal and filler metal cool after freezing, if the joint is restrained from contracting and its strength at the elevated temperature is insufficient, hot cracking may occur.

The magnitude of the strength reduction from welding varies: for non-heat-treatable alloys, welding reduces the strength to that of the annealed (O) temper of the alloy; for heat-treatable alloys, the reduced strength is slightly greater than that of the solution heat treated but not artificially aged temper (T4) of the alloy. Minimum strengths across groove-welded aluminum alloys are given in Table 9.26. These strengths are the same as those required to qualify a welder or weld procedure in accordance with the American Welding Society (AWS)

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