Microstructure, mechanical properties, and corrosion resistance of friction stir welded Mg-Al-Zn alloy thick plate joints

Mg-Al-Zn (AZ31) of 10-mm thick plates were subjected to friction stir welding (FSW) using a conventional tool (CT) and bobbin tool (BT), respectively. The microstructure, room temperature tensile properties, dynamic compressive mechanical properties, and corrosion resistance of the base metal (BM) and various regions of the friction stir welded joints were investigated systematically. The mean grain size, dislocation density, and precipitate distribution in the various regions of the friction stir welded joints were various due to different thermal cycle and plastic deformation. Compared with the CT, a relatively more homogeneous microstructure, a larger number of high-density dislocations and precipitates were produced in the various regions of the joint prepared by the BT due to higher thermal cycle and strain rate. As a result, the friction stir welded joint prepared by the BT exhibits excellent tensile properties and dynamic compressive mechanical properties, and outstanding corrosion resistances compared to those of the CT. In addition to slight changes in tensile properties and dynamic compressive mechanical properties, the corrosion resistance of the joint prepared by the BT was significantly improved compared to the BM. FSW with BT was suggested to be the optimal method to weld AZ31 alloy thick plate.