Study on the microstructure and corrosion of Al-Zn-Mg-Cu alloy welded joint after heat treatment

Abstract

The Al-Zn-Mg-Cu high-strength aluminum alloy is widely employed in lightweight load-bearing structures for aerospace and rail transit applications, owing to its exceptional specific strength and excellent processability. Bobbin Tool Friction Stir Welded (BT-FSW) effectively mitigates defects inherent to conventional fusion welding, including porosity and hot cracking. Nevertheless, welding-induced thermal gradients can result in heterogeneous joint microstructures, strengthening phase degradation, joint softening, property heterogeneity, and diminished corrosion resistance. The present study employed a post-weld heat treatment (PWHT) regime comprising solution treatment at 475 °C for 2 h, followed by artificial aging at 120 °C for various durations. The effects of PWHT on joint microstructure and intergranular corrosion resistance were systematically investigated by means of microstructural characterization. The results demonstrated that solution treatment promoted static recrystallization and microstructural homogenization, accompanied by complete dissolution of precipitates. Aging treatment preserved the grain morphology while inducing uniform precipitation of nano-sized η′ strengthening phases within the matrix. Concurrently, grain boundary precipitate morphology was optimized, and precipitation-free zones (PFZs) were significantly narrowed. Following the optimal aging duration of 27 h, grain boundary precipitates exhibited a dispersed, discontinuous distribution. The maximum corrosion depth in the stir zone decreased from 87.25 μm to 19.31 μm, thereby significantly enhancing corrosion resistance. This study identifies the optimal heat treatment process, providing theoretical and technical support for the engineering application of Al-Zn-Mg-Cu alloy welded components.