Evolution of Strengthening Precipitates During Friction Stir Welding of Al-Zn and Al-Cu Alloys

This research paper explores the impact of rotational speed on precipitate behaviour and mechanical properties of naturally aged FSW dissimilar joints. The outcome of this work showed that the rotational speed affects the mechanical behaviour of naturally aged FSW joints due to variations in microstructure. Severe plastic deformation resulted in the formation of fine equiaxed grains in the stir zone (SZ). The grain size increased from SZ to heat-affected zone (HAZ) on AS and RS due to uneven heat input and deformation. The grain size and extent of precipitate coarsening increased with rotational speed. Intermetallic particles were observed in the form of MgZn₂, Cu₅Zn₈, and Al₂CuMg, resulting in enhanced SZ microhardness. The lowest hardness was observed at the TMAZ/HAZ interface towards AS owing to the coagulation of the Al₂Cu phase. Peak microhardness of 183.8 Hv was found in the SZ of the joints at 900 rpm. The ultimate tensile strength was reduced from 288 ± 12.3 MPa to 242 ± 9.8 MPa with increased rotational speed from 900 to 1120 rpm. The lowest microhardness zone was the fracture location for both welds during the tensile test.