Macro-scale Compositional Inhomogeneity in Friction Stir Processed Mg–Al–Zn Cast Alloy and Its Effect on Mechanical Property

It has long been considered that compositional homogeneity would not be altered on the macro-scale by solute segregation during friction stir processing (FSP) without additives. Yet in this study, for the first time, macro-scale compositional inhomogeneity was found to be induced by FSP in AZ91. Using micro X-ray fluorescence spectrometry, three regions that different from conventional microstructural zones were clearly divided from the processed material, i.e., R1 with obvious enrichment of Al and Zn, R2 with significant depletion of Al and Zn, R3 with a similar composition to base material. R1 was found almost identical to the stir zone, while R2 occupied only a part of thermo-mechanically affected zone where grain structure was refined, and the evolution of R2 should have a strong correlation with grain refinement in thermo-mechanically affected zone. The underlying mechanisms of this newly discovered phenomenon could be attributed to dissolution of β-Mg17(Al, Zn)12 in R2 and accelerated segregation of Al and Zn from R2 to R1 driven by the reduction in their chemical potentials in R1 caused by higher temperature in this region during FSP. The local mechanical property in regions of different composition was evaluated by micro-hardness. Local softening was observed in R2, which can be mainly attributed to significantly reduced solid solution strengthening resulting from the depletion of Al and Zn.