Effect of grain size on the microstructure and mechanical properties of friction stir welded non-combustive magnesium alloys

The novel fine-grained and coarse-grained non-combustive AMX602 (Mg–6% Al–0.5% Mn–2% Ca) magnesium plates with average grain sizes of 1.06 and 6.14 μm respectively were subjected to the friction stir welding. The effect of the initial grain size on the microstructure and mechanical properties of the joints were studied. The results showed that the grains growth occurred in the stir zone of the fine-grained specimens after welding. However, grain refinement occurred in the coarse-grained counterpart. The texture evolution of the fine and coarse grained materials joints had similar trends but with different texture intensities. It is suggested that the initial grain size affects the texture intensity through the slip inhomogeneity caused by different amount of grain boundaries. The microhardness of the fine-grained material joints was lower than that of the base material, whereas it became higher in the coarse-grained specimens. The results of the tensile test indicated that the grain size and grain orientation of the joints had an effect on the mechanical properties and fracture locations of the joints. Although both the fine-grained and coarse-grained material joints had similar yield strengths, the fine-grained specimen had a higher ultimate tensile strength of 261.6 MPa and the coarse-grained specimen had a relatively lower one of 203.6 MPa.