Friction-stir welding of ultra-fine grained sheets of Al-Mg-Sc-Zr alloy

The effect of friction-stir welding (FSW) on the microstructure and mechanical properties of Al-5.4Mg-0.2Sc-0.1Zr sheets with ultra-fine grained (UFG) microstructure was studied. The UFG material was produced by equal-channel angular pressing (ECAP) followed by either hot or cold rolling. FSW was found to be very effective for preservation of the UFG microstructure as well as the constituent coherent nano-scale dispersoids in the welded material. Despite the preservation effect, however, essential material softening was observed in the weld zone. This observation was attributed to recrystallization presumably occurring during FSW. The joint efficiency for the yield strength of the obtained friction-stir welds was found to be 85% in the hot rolled condition and only 57% in the cold rolled state. The relatively low joint efficiency was attributed to the recrystallization softening as well as to the formation of a “zigzag line” defect in the stir zone.