Microstructure and tensile properties of friction stir welded AZ31B magnesium alloy

The microstructural change in AZ31B-H24 magnesium (Mg) alloy after friction stir welding (FSW) was examined. The effects of tool rotational speed and welding speed on the microstructure and tensile properties were evaluated. The grain size was observed to increase after FSW, resulting in a drop of microhardness across the welded region from about 70 HV in the base metal to about 50 HV at the center of the stir zone. The obtained Hall–Petch type relationship showed a strong grain size dependence of the hardness. The aspect ratio and fractal dimension of the grains decreased towards the center of the stir zone. The welding speed had a significant effect on the microstructure, with larger grains at a lower welding speed. The yield strength and ultimate tensile strength increased with increasing welding speed due to a lower heat input. A lower rotational speed of 500 rpm led to higher yield strength than a higher rotational speed of 1000 rpm. The friction stir welded joints were observed to fail mostly at the boundary between the weld nugget and thermomechanically affected zone at the advancing side. Fracture surfaces showed a mixture of cleavage-like and dimple-like characteristics.