Dissimilar friction stir welding of AA2198 and AA7475: Effect of solution treatment and aging on the microstructure and mechanical strength

A post-weld heat treatment consisting of solution treatment and subsequent aging (STA) is widely applied to aluminum joints fabricated by friction stir welding (FSW) to improve the mechanical strength via precipitation hardening. In this study, aerospace aluminum alloys of AA2198 and AA7475 were FSWed in similar and dissimilar states. Differential scanning calorimetry (DSC) was used to trace the precipitation strengthening during the aging of welded specimens. The post-weld aging procedures were designed based on the DSC outputs. Accordingly, welded sheets were solution treated at 480 degrees C and 540 degrees C for 10-90 min, air-cooled and aged at 155 degrees C and 170 degrees C for 2-40 h, respectively. Optical micrographs revealed that due to the faster kinetics of the recrystallization, higher homogenizing temperature led to nucleation of the finer grains from highly stress localized points in the stir zone (SZ) and TMAZ by faster growth rate. Higher time and temperature of the solutiontreatment eventuated in accumulation of Cu-enriched intermetallic phases in the grain boundaries at SZ and TMAZ of AA7475, attenuation of the grains adhesion and failure of the sample. Hardness test results showed that the hardness increased in AA7475 alloy while decreasing in AA2198 alloy in the as-welded state. Post weld heat treatment enhanced the hardness in AA2198 and reduced it in AA7475. However, it had no significant effect on the grain size.