Microstructural characteristics, mechanical properties, and corrosion behavior of FSW dissimilar joints between Al-Zn-Mg-(Sc-Zr) and Al-Mg-Si alloys

Friction stir welding (FSW) was used to fabricate a dissimilar joint (Joint 2#) between plates of the new Al-Zn-Mg-Sc-Zr alloy and the Al-Mg-Si alloy. For comparison, a dissimilar joint between conventional Al-Zn-Mg and Al-Mg-Si alloys was also fabricated using the same FSW procedure (Joint 1#). The mechanical properties, corrosion behavior, and microstructural characteristics of different regions within these two dissimilar FSW joints were then comparatively investigated. Both dissimilar FSW joints exhibited their lowest mechanical strength in the heat-affected zone (HAZ) on the Al-Mg-Si side. In the weld nugget zone (WNZ), granular α-AlFeMnSi phases with a body-centered cubic (BCC) structure were predominantly distributed. During the salt spray corrosion tests, Joint 1# and Joint 2# exhibited average weight-loss rates of 0.0181 g/(m 2 ·h) and 0.0124 g/(m 2 ·h), respectively. Compared to each region on the Al-Zn-Mg side of Joint 1#, the corresponding regions on the Al-Zn-Mg-Sc-Zr side of Joint 2# exhibit superior corrosion resistance. The improved corrosion resistance in the HAZ results from the pinning effect of Al 3 (Sc,Zr) particles, which promotes a high density of finely dispersed subgrain boundaries and subgrain-boundary precipitates (SGBPs). Such a microstructure effectively reduces the segregation of anodic, Zn/Mg-rich phases at the grain boundaries, thereby promoting a more homogeneous distribution of electrochemical potential throughout the alloy.