Effect of friction stir welding (FSW) on mechanical properties and microstructure of Al-5.4Mg- 0.2Sc-0.1Zr sheets with ultra-fined grained (UFG) structure was studied. The UFG-sheets were produced by equal-channel angular pressing (ECAP) followed either by cold or hot rolling. FSW was found to be very effective for retaining the UFG microstructure as well as constituent coherent nano-scale dispersoids in the welded material. Despite the preservation effect, however, the essential material softening was observed in the weld zone. This was attributed to the recrystallization occurring during FSW. The joint efficiency for yield strength of the obtained friction stir welds was found to be 81% in the hot rolled condition and only 55% in the cold rolled state. The relatively low joint efficiency was associated with the recrystallization softening as well as with the formation of a specific “kissing bond” defect in the stir zone
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