Particle-stimulated nucleation of recrystallization for grain-size control and superplasticity in an Al–Mg–Si–Cu alloy

Grain refinement of 6xxx aluminum alloys for superplasticity through particle-stimulated nucleation of recrystallization (PSN) has been a difficult task in the past due to the inhomogeneous nature of the precipitate distributions produced by traditional overaging heat treatment methods. Stretching prior to aging does not alleviate the problem. A new approach has been developed, wherein the interfaces of deformation bands caused by severe rolling were exploited as heterogeneous nucleation sites for precipitates in an Al–Mg–Si–Cu alloy. (US and International patents are pending.) This approach, combined with a two-step low–high heat treatment resulted in a homogeneous distribution of globular precipitates near 1 μm in diameter. In contrast to the globular shape, the plate-shape morphology was observed in the absence of pre-age deformation. Subsequent rolling and recrystallization resulted in a fine, uniform, equiaxed grain structure with an average grain diameter of 10 μm. The grain structure was weakly textured, statically stable, and superplastic above 500°C. A maximum strain rate sensitivity of 0.5 was achieved, with a corresponding maximum elongation of 375%.