An optimum process window to preferable microstructure distribution and improved macroscopic property for friction stir–assisted incremental aluminum alloy sheet forming

Friction stir–assisted incremental sheet forming (FS-ISF) of aluminum alloy enables increased formability, solute precipitation strengthening, and stable microstructure with fine grains by dynamic recrystallization. However, higher rotating speed may destroy the surface quality of the forming part since process parameters have contradictory effects on macroscopic and microscopic results. To solve this dilemma, microstructure characteristics and macroscopic differences in FS-ISF experiments combining rotating speed and step-down increment on AA2024 and AA6061 are revealed. And the effects of parameter combinations on key macro- and micro performance indicators including forming depth, surface roughness, mechanical properties, solute precipitation, and grain sizes are comprehensively analyzed to propose an optimum process window for achieving better macro-micro indicators at the same time by both raising rotating speed (S≥4500RPM) and decreasing step-down increment (∆d≤0.4mm). Finally, the applicability of the proposed process window for preferable microstructure distributions and improved macroscopic properties is proved by supplementary FS-ISF experiments.