Effect of post-weld natural aging on mechanical and microstructural properties of friction stir welded 6063-T4 aluminium alloy

•Effect of natural aging on friction stir welds of 6063-T4 AA is studied.•Welding process parameters significantly influence the aging process.•Accelerated aging occurs for process parameters resulting in higher temperatures.•Strength increases, ductility decreases, and stress serrations increase with aging.•Mg–Si co-clusters forming during the aging process promote stress-serrations.

Influence of natural aging on mechanical and microstructural properties of friction stir welded 6063-T4 aluminium alloy plates was investigated through mechanical testing, X-ray diffraction studies, and transmission electron microscopy, for aging times up to 8640 h. Mg–Si co-clusters formed during the natural aging process resulted in an increase in strength, decrease in ductility, and occurrence of serrated plastic flow. Hardness increase from aging was fastest in welds obtained at higher tool rotational speeds due to greater amount of “quenched-in” vacancies from higher peak stir zone temperatures. Peak broadening analyses and classical Williamson–Hall plots were used to investigate the effect of friction stir welding and post weld natural aging on microstrain in different weld regions. Higher microstrain was found in stir zone as well as heat affected zone as compared to that for base metal, albeit for different reasons.