Microstructure evolution and strengthening mechanisms in friction-stir welded Al-Mg-Sc alloy

The present work was undertaken to evaluate the effect of second-phase particles on microstructure evolution during friction-stir welding (FSW). To this end, Al-6.0Mg-0.35Mn-0.2Sc-0.1Zr alloy containing insoluble nanoscale Al-3(Sc,Zr) dispersoids was used as a program material. A particular emphasis was given to an establishment of a microstructure-strength relationship in the welded material. It was found that pinning effect of the Al-3(Sc,Zr) precipitates efficiently suppressed grain-boundary migration and promoted a concentration of slip activity in heavily-stressed near-grain-boundary regions. This gave rise to a preferential development of deformation-induced boundaries in the latter areas. Due to a complex character of slip at the grain boundaries, the newly evolved grains had nearly random crystallographic orientations. As a result, the final microstructure evolved in stir zone was characterized by relatively-fine grain size, poorly-developed texture and large fraction of high-angle boundaries. The retention of the coherent dispersoids as well as considerable grain refinement occurring during FSW resulted in substantial material hardening in the stir zone.