Changes in texture and microstructure of friction stir welded Mg alloy during post-rolling and their effects on mechanical properties

Abstract

A strong and complicated microtexture usually forms in stir zone (SZ) of friction stir welded (FSW) Mg alloys, which significantly influences the tensile properties of the joint due to the strong anisotropic mechanical properties of Mg alloys. This study aims to study the evolution of microstructure and texture in SZ during post-rolling of FSW AZ31 alloy and its effect on the tensile properties and fracture of the FSW joint. A detailed characterization of microstructure and texture in SZ and especially in SZ/base material (BM) interface was conducted on the FSW-undeformed and FSW-rolled specimens. Schmid factor (SF) changes for extension twinning and basal slip for transverse tensile tests were analyzed and discussed. It confirmed that the c-axis in SZ-center was parallel to welding direction (WD), but tilted towards transverse direction (TD) with the area moving to SZ-side. After subsequent rolling (by ∼2.5% strain), a large number of extension twins were found in SZ-side, but it was less in SZ-center. The production of extension twins rotated the grains in SZ-side and affected the transverse tensile properties significantly. The electron backscatter diffraction (EBSD) analyses showed that the SF of the twins was decreased compared with the matrix for both extension twinning and basal slip for the transverse tensile tests. So, the extension twins generated by post-rolling could cause texture strengthening for the transverse tensile tests. Moreover, the introduction of twin boundaries subdivided the grains, which also increased the tensile properties of the FSW-rolled specimens. The effect of post-rolling on the tensile properties and fracture was studied. It showed that post-rolling increased the transverse yield strength (YS) of the FSW AZ31 alloy, and the YS was increased further with the increase of rolling strain. The YS was raised from ∼87MPa of the FSW-undeformed specimen to ∼115MPa for the FSW-R-2.5 and ∼171MPa for the FSW-R-7 specimens. Because most extension twins were generated in SZ-side, the strength increase was most significant in SZ-side, which reduced the plastic inhomogeneity between SZ-side and SZ-center of the post-rolled specimens. Fracture occurred in SZ-side of the FSW-undeformed specimens because most grains in there were favorably oriented for extension twinning and basal slip. But it occurred in BM for all the FSW-rolled specimens. This was mainly attributed to the texture strengthening and grain subdivision strengthening by the extension twins generated in SZ-side during post-rolling of the FSW specimens. So, it suggested that subsequent rolling was effective to increase the joint strength of FSW AZ31 alloy.