Microstructure and mechanical properties of friction stir welds on unmodified and P-modified Al-Mg2Si-Si alloys

Welded joints formed by friction stir welding (FSW) consist of three distinct zones: a base material zone (BMZ), a thermo-mechanically affected zone (TMAZ), and a weld nugget (WN). Primary Mg2Si phases are identified as equiaxed crystals and polygonal particles in unmodified and P-modified Al-Mg2Si-Si alloys in the BMZ, respectively. In the WN, the equiaxed primary Mg2Si crystals in the unmodified alloys are transformed to significantly smaller polygonal/irregular particles; the corners of the polygonal primary Mg2Si particles in the modified alloys become smoother and smaller. The segregation of the primary Mg2Si phase is reduced for both the unmodified and modified alloys in the WN. Both unmodified and modified alloys have a lower solidus temperature in the WN. The ultimate tensile strengths (UTSs) of the welded joints are enhanced by 5% and 8% for the unmodified and modified alloys, respectively, in comparison with the parent material. The UTS of the welded joints in the modified alloy is ∼20% higher than in the unmodified alloy.