Microstructure and mechanical properties of friction stir welded AZ31B magnesium alloy

The microstructural change in AZ31B-H24 magnesium alloy after friction stir welding (FSW) was examined. 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 alloy to about 50 HV at the center of the stir zone. The grain size dependence of the hardness followed the Hall-Petch type relationship. The yield strength and ultimate tensile strength increased with increasing welding speed and decreasing rotational speed due to a lower heat input. Work hardening properties of FSWed Mg alloy were evaluated using two modified equations of hardening capacity and strain hardening exponent where the elastic deformation stage was excluded. A Kocks-Mecking plot was used to show different stages of strain hardening. A lower welding speed resulted in a higher fatigue strength in the high cycle region (N>105 cycles), but a lower low cycle fatigue strength.