Inhibition of Abnormal Grain Growth in Stir Zone via In‑Situ Intermetallic Particle Formation During Friction Stir Welding of AA6061

Abnormal grain growth(AGG)has been widely observed in many friction stir welded(FSWed)joints during post-weld heat treatment(PWHT).The coarse grain structure not only reduces the strength of the joint but also limits its usage in superplastic forming.Several methods have been reported in previous studies to inhibit AGG,but all of them can only mitigate AGG.Complete inhibition of AGG was not achieved.In the current research,AGG was widely observed during the PWHT of friction stir welded AA6061.Multi-pass FSW enhanced the thermal stability of the as-welded grain structure but did not eliminate the occurrence of AGG.A new welding method was developed with the ball-milled Al–Ti powder mixture introduced into the stir zone and proved effective in inhibiting AGG in FSWed AA6061 during PWHT.The adoption of 5-pass FSW with an alternate rotation mode succeeded in producing an AGG-free sample.Microscopic characterizations conducted in the stir zone showed an evolution of Al–Ti powder mixture into different particle formations and Al3Ti new phase.Quantitative analysis of the second phase particles(SPPs)in the stir zone confirmed the increases in both particle number and average size.The quantitative results fit well with Humphreys’grain growth model,which theoretically explains the mechanism for AGG inhibition,i.e.,the significantly enhanced particle pinning effect.