Improving formability and retaining dislocation hardening of heavily cold-worked Al alloy by fast heating and fast deformation

Abstract A certain amount of aluminum alloy sheets are supplied in heavily cold-worked (HCW) state, e.g. H temper. Cold deformation can greatly improve the strength while significantly degrade the ductility of metals, leading to a low formability of cold-worked alloy. A fast heating and fast deformation (FHFD) method is developed to enhance the formability without appreciable sacrifice of dislocation hardening in HCW AA2219 aluminum alloy. The FHFD incorporates heating to 100–350 °C by induction heating and holding for less than 5 s before tensile deformation at 0.1s-1 to reduce the processing time at elevated temperature. The microstructure evolution of HCW alloy during FHFD and subsequent artificial aging is observed in detail by transmission electron microscopy. Compared with room temperature tests, the yield strength is reduced by ∼150MPa while the elongation to fracture is increased by ∼3 times at 200 °C. The obtained strength-ductility synergy after post-aging at 120 °C is much better than that in the T8 alloy, as is ascribed to a high dislocation density and a bimodal distribution of θ'/θ" precipitates. Although deformation above 250 °C could further improve the formability, it results in a considerable loss of both strength and ductility in the deformed and post-aged alloy, due to the substantial precipitate coarsening and dislocation recovery during deformation. Constitutive modeling based on the dislocation density can accurately describe the deformation behavior during FHFD and suggests the dynamic recovery determines the uniform deformability. Our findings reveal that FHFD at appropriate temperature can effectively improve the formability and prevent the dislocation recovery as well as precipitate coarsening in HCW alloy. The new process has the potential to form HCW alloy parts and simultaneously acquire excellent performance.