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

Grain refinement of 6xxx aluminum alloys for superplasticity through partic le-stimulated nucleation of recrystallization (PSN) has been a difficult ta sk in the past due to the inhomogeneous nature of the precipitate distribut ions produced by traditional overaging heat treatment methods. Stretching p rior to aging does not alleviate the problem. A new approach has been devel oped, 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 mu m in diameter. In contrast to the globular shape, the plate-shape morphology was observed in the absen ce of pre-age deformation Subsequent rolling and recrystallization resulted in a fine, uniform, equiaxed grain structure with an average grain diamete r of 10 mu m The grain structure was weakly textured, statically stable, an d superplastic above 500 degrees C. A maximum strain rate sensitivity of 0. 5 was achieved, with a corresponding maximum elongation of 375%. (C) 2000 E lsevier Science S.A. All rights reserved.