Macrostructure, microstructure and mechanical properties of bobbin tool friction stir welded ZK60 Mg alloy joints

Friction stir welding based on bobbin tool was studied for the joining of ZK60 Mg alloy. Sound joints with the ultimate tensile strength reaching 91.0–94.9% of that of the parent metal can be obtained under the appropriate welding parameters. The welding causes dynamic segregation of Zr-rich precipitates in the stirred zone followed by development of a bimodal recrystallized grain structure with distributed bands of 0.6–1.5 μm ultra-fine grains and 4.4–9.5 μm coarser equiaxed grains. Extensive softening occurred in the stirred zone owing to dissolution of the Mg-Zn based precipitate particles. With the increase in tool rotational speed, the average grain size in the stirred zone increases while the tensile properties are not declined as the enhanced complicated type of plasticized metal flow at the high tool rotational speed produces relatively randomized {0001} textures. The joint produced at the 600 rpm tool rotational speed exhibits the weakest isotropy of {0001} texture and the best combination of strength and ductility. The randomized {0001} textures alleviated the inhomogeneity of plastic deformation during the tensile test, which hindered the joint fracturing with low elongation.