Why is it difficult to weld aluminum?

 　　Before emphasizing the different welding processes used to connect aluminum, it is important to understand some of the inherent difficulties of welding aluminum. One challenging area is filler metal. Firstly, some aluminum alloys cannot be welded without filler materials. If there is no filler metal during welding, alloys such as 6061 will undergo solidification cracking. In addition, the correct filling material must be selected. For example, welding 6061 alloy with 6061 filler metal can lead to welding failure. On the contrary, when welding 5356 substrate, 4043 or 6061 aluminum filler metal should be used. Another challenge for aluminum filler metal is feeding. If a mechanical wire feeding process is used, it is likely to require a special drive system. This is because the strength of aluminum columns is lower than that of steel, and without the use of special wire drive systems such as push-pull guns, aluminum is likely to bend and tangle. This is especially true for thinner aluminum filler metals (i.e. 0.8 mm or 1 mm in diameter).

　　Aluminum also has higher thermal conductivity than steel. The heat generated when starting the welding process on aluminum is dispersed faster than when welding iron based alloys. Therefore, complete penetration may not occur before the weld is carried out far from the beginning. This is called a cold start. When welding aluminum, attention must be paid not to cold start. Another result of increasing thermal conductivity is larger craters. When reaching the end of the weld, there is more heat present than at the beginning. This heat is well dispersed in aluminum and can form a large crater. Aluminum is easily affected by shrinkage and cracking, therefore, shrinkage should be filled to avoid failure at the end of welding.

　　Aluminum also requires different pre weld and post weld treatments. Aluminum forms an oxide layer with a melting temperature higher than the actual aluminum itself. To avoid the occurrence of unmelted aluminum oxide particles in the weld seam, oxide removal processes such as wire brushing or chemical cleaning should be used before welding. Several aluminum alloys, such as 6061-T6, undergo artificial aging to increase their strength. The heat generated by welding destroys the benefits of artificial aging, and a significant reduction in strength will be found in the Heat-affected zone. Therefore, for such alloys, manual aging after welding may be required.