Novel Technique for Joining of Thick Section Difficult-to-Weld Aluminium Alloys

One of the “show stoppers” in fusion welding of highly alloyed aerospace aluminium alloys is their susceptibility for liquation cracking in the weld heat-affected zone. Liquation cracking is a microscopic intergranular discontinuity, which occurs under the effect of welding thermal cycle and in the presence of stresses involved with the welding process. These intergranular discontinuities are often observed in welding of thick plates and extrusions, which usually have relatively coarse elongated grains, that are generally oriented parallel to each other. Friction Stir Welding (FSW) is a low temperature non- fusion process, which produces very fine equiaxed grain structure in the weld nugget for majority of Al-alloys. It was found that bead-onplate FS welds performed on alloy, which in fusion welding is susceptible to liquation cracking, were crack free. It was therefore proposed to use FSW for grain refining of the parent material by putting a number of overlapping FS welds onto the edges of both parent plates prior to joining by fusion welding. Experimentation has shown that there was no liquation cracking after the final weld was performed. This novel welding method has been successfully proven for Electron Beam Welding (EBW) of various Al-alloys including joining of dissimilar materials. The details of experiments as well as welded coupons test results are presented.