In depth analysis of microstructure and mechanical properties of joints in thick plate friction stir welding of high–strength Al–Zn–Mg–Cu alloy

This study investigated the microstructure and mechanical performance of 8 mm-thick Al–Zn–Mg–Cu (AZMC) alloy joints fabricated by friction stir welding (FSW) under various combinations of tool rotational speeds (RSp, 400, 800, 1200 rpm) and welding speeds (WSp, 50, 100, 150 mm/min), both with and without post-weld heat treatment (PWHT). In the as-welded condition, both grain size and precipitate size in the nugget zone (NZ) increased with heat input, ranging from 1.46 μm and 0.27 μm to 2.45 μm and 0.73 μm, respectively. Meanwhile, the microhardness of NZ increased with both RSp (from 139.8 HV at 400 rpm to 152.1 HV at 1200 rpm) and WSp (from 132.8 HV at 50 mm/min to 140.2 HV at 150 mm/min), primarily due to the competition between grain boundary, precipitation, and solid solution strengthening mechanisms. PWHT induced abnormal grain growth (AGG) at low and medium heat input, which was suppressed under high heat input; the estimated Zener–Hollomon threshold for AGG was 4.7 × 10¹¹–6.6 × 10¹¹ s⁻¹. The 1200 (RSp)–100 (WSp) condition delivered the best weld-direction (WD) performance (σ_YS = 610 MPa, σ_UTS = 662 MPa, δ_EL = 9.5 %), reaching up to 97.9 % of base material (BM)’s properties, primarily due to the retention of ultrafine grains after PWHT. In the transverse direction (TD), 400–100 yielded the highest strength (σ_YS = 591 MPa, σ_UTS = 654 MPa; 94.0 % and 96.9 % of BM), while 400–150 showed the best ductility (δ_EL = 10.2 %, 107.4 % of BM). The plasticity variation in the TD under different parameters is largely attributed to the uniformity and grain size differences between the NZ and TMAZ. These results highlight the critical role of optimized FSW parameters and PWHT in enhancing joint performance for high-strength thick Al alloys.