Improving the mechanical properties of a friction stir welded AZ31 joint by adjusting the microstructure and texture

Softening caused by an inhomogeneous fibre texture distribution during friction stir welding (FSW) Mg joints can deteriorate the joint performance. This study aims to understand the effect of microstructure and texture distribution in joints on the deformation behaviour and mechanical properties of FSW Mg alloys. Characterizations of two joint types were compared: the as-received joint (fabricated by the optimum welding parameters) and the LHH joint (obtained with a lower rotation rate-higher welding speed and extra heat input). In contrast with the as-received joint, the grains in most regions of the LHH joint were coarser and more uniform in size. Furthermore, a joint with a weaker texture was obtained by LHH welding. The tensile results showed that the ultimate tensile strength and yield strength of the LHH joint slightly increased, but the elongation was largely improved, close to that of the AZ31 base material. Furthermore, the LHH joint showed more stable mechanical properties. The better mechanical performance of the LHH joint was mainly due to the reduced texture near the sides of the nugget zone, leading to hard orientation with decreased Schmid factor (SF) values of basal slip and extension twinning. The higher elongation was mainly attributed to decreased localized strain caused by more similar SF values, coarser grains with uniform size in most regions and a lower stress concentration at the LHH joint.