Effect of post-weld heat treatment on AA6351 friction stir welds

Abstract

Al-Mg-Si (6xxx) alloys have superior properties to other aluminium alloys, making them suitable for aerospace and defence applications. However, their weldments are susceptible to various issues that affect their microstructural changes, mechanical properties, and corrosion resistance. The use of friction stir welding has been proven to be an effective solution for these problems, and this study aimed to investigate the microstructural changes, mechanical properties, and pitting potential (Epit) of AA6351 alloy friction stir welds. The present study used AA6351 with a thickness of 6mm alloy plates for welding and characterized the welds' microstructure, pitting corrosion resistance, and mechanical properties of hardness and tensile strength. Post-weld heat treatment (PWHT) for solutionizing at 523°C for 0.5hrs followed by 8 hrs at 177⁰C (STA-solutionizing treatment with ageing) was applied to improve the welds' corrosion resistance and mechanical properties. A comparison between the base metal and weldment before and after PWHT showed that the ageing treatment of the welds improved their mechanical properties and pitting corrosion resistance. This treatment caused re-precipitation and redistribution of precipitates in the welds’ grain-refined stir zone, restoring their mechanical properties and pitting corrosion resistance. Therefore, this study established that post-weld solutionizing at 523°C for 0.5 hrs and then ageing at 177⁰C for 8hrs can restore the overall mechanical and pitting corrosion behaviour of AA6351 alloy friction stir welds.