Microstructure and mechanical properties of friction stir welded dissimilar Mg alloys of ZK60-AZ31

Dissimilar friction stir welding (FSW) of ZK60 and AZ31 magnesium alloys was studied. Two kinds of material arrangements, with ZK60 or AZ31 in advancing side (AS), were applied. The results showed that the dissimilar plates with both material arrangements were joined successfully. No obvious defect was observed. Microstructure and texture variation across the joints were examined by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The grains in stir zone (SZ) were significantly refined during FSW (∼8.7 μm in AZ31 side and ∼2.7 μm in ZK60 side). The second phases in ZK60 side were broken up, which contributed to grain refinement. Two-dimensional (2D) hardness maps across the normal direction (ND)-transverse direction (TD) sections in the joints were obtained. Clear boundary was observed between SZ and AZ31 base metal, but it was less clear between SZ and ZK60 base metal. The mixture of materials, grain refinement and broken up of particles were responsible for the sharp increasing of hardness between SZ and AZ31 base metal. The transverse ultimate tensile strength (UTS) of the joints was 78-82% of AZ31 base metal. The joints consistently fractured in the transition region of the AZ31 side. The sudden hardness decrease from SZ to AZ31 base metal might result in deformation incompatibility at the interface, which is considered as the main cause for the failing in the AZ31 side. In addition, the activation of twinning in SZ-side (with c-axis parallel to TD) and slip in the region adjacent to the SZ-side (with c-axis at ∼45° angle to TD) aggregated the deformation incompatibility in the transition region of the AZ31 side. As a result, the break of the specimens started at the boundary of transition zone (TZ)/SZ. Many extension twins were observed in AZ31-SZ-side after fracture. Although the ZK60-SZ-side presented the similar texture with the AZ31-SZ-side, much less extension twins were observed in the former because of the hindering effects by the refined grains and secondary particles.