Revealing the microstructure evolution, deformation behavior and tensile properties of AZ31/AM60 dissimilar Mg alloy joints under different welding technologies

In this work, friction stir welding (FSW) and tungsten inert gas welding (TIG) were applied to prepare two kinds of dissimilar AZ31/AM60 Mg joints. The relationship between mechanical properties, deformation behavior, and microstructure characteristics of both joints was investigated. Microstructural characterization demonstrated that a significant grain orientation fluctuation occurred in the weld zone (WZ) of the FSW joint, but this was not obvious for the TIG joint. Compared with the base metals, the grain size of the FSW joint slightly decreased, but the grain of the TIG joint obviously coarsened. Locally strong texture appeared in the WZ of the FSW joint with a symmetrical distribution, while a random texture character occurred in the TIG joint. Furthermore, the WZ of the TIG joint possessed more and larger second phases than the FSW joint. Tensile tests indicated that both joints had equivalent yield strength (similar to 120 MPa) and ultimate tensile strength (similar to 255 MPa), but the elongation (9.0 %) of the FSW joint was lower than that of the TIG joint (12.1%). The similar strength originated from the higher Schmid factors (SFs) of basal slip and extension twinning in the WZ side of the FSW joint and the larger grain size and lower dislocation degree in the heat-affected zone of the TIG joint. The decreased elongation in the FSW joint was due to the apparent inhomogeneous deformation caused by significant SFs (basal slip) fluctuation, while the weaker SFs fluctuation and notable twinning behavior in the TIG joint contributed to the relatively higher elongation.