Microstructure and tensile behavior of a Bobbin friction stir welded magnesium alloy

In bobbin friction stir welding (BFSW), frictional heat input is simultaneously applied to the top and bottom surface of a workpiece by the rubbing action of the tool shoulders. Consequently, a relatively homogeneous microstructure is obtained in BFSW welds compared to the conventional FSW. In the current study, we performed BFSW bead-on-plate experiments on Mg alloy AZ31B with the goal to study evolution of microstructure and resulting tensile behavior of the welded material. We used three different combinations of intense process parameters which were intentionally chosen to be outside of the norm for BFSW i.e., tool rotational speeds >1500 rpm and constant translational speed of 10 mm/min. The BFS weldments showed the classic heat affected zone (HAZ), a small but discernible thermomechanical affected zone (TMAZ) and a large, near rectangular stir zone (SZ). The SZ consisted of a bimodal microstructure similar to the base material but some degree of grain refinement was also observed, which was more prominent on the retreating side of SZ. BFS welded specimen showed maximum of 67% and 48% joint efficiency (ratio of weld UTS to base material UTS) at room temperature and 200 °C, respectively. Fracture of specimens occurred in TMAZ/SZ interface and SZ, in room and elevated temperature tested specimens, respectively. Large number of twins were observed near fracture surface of the room temperature tensile tested specimen while coalescence of microcracks led to fracture at elevated temperature.