Investigations on the material flow and the influence of the resulting texture on the tensile properties of dissimilar friction stir welded ZK60/Mg–Al–Sn–Zn joints

Abstract

This work focuses on clarifying the relationship of material flow and texture evolution in friction stir welding (FSW) of magnesium alloys and the influence of the texture on the tensile properties of joints. Two types of material arrangements, with ZK60 or Mg-4.6Al-1.2Sn-0.7Zn alloys at advancing side (AS), were applied for dissimilar FSW. The material flow velocity was calculated based on the flow patterns, and the texture evolution was analyzed by electron backscatter diffraction. The results showed that the metal in the rotation layer was pushed flowing upward by the right-hand thread tool pin with clockwise rotation, and three regions with significantly different flow behavior and texture distribution were formed in stir zone (SZ). The rotation angle of the c-axis of magnesium hexagonal unit cell at each region was consistent with the material flow behavior around the tool pin approximately. The greater the flow velocity and the moving distance of deformation metal, the greater the texture intensity. Moreover, when ZK60 with superior plastic deformability was placed at AS, the relatively higher flow velocity in SZ was produced. This caused the relatively higher texture intensity, which lead to the lower tensile strength of joints. The tensile fracture of both joints with two arrangement types occurred at the interface between SZ and thermo-mechanical affected zone at AS due to the uncoordinated plastic deformation caused by the inhomogeneous severe texture distribution. This investigation can provide a new insight into FSW process optimization of magnesium alloys.