Abstract: Obtaining zero springback and good post-form performance simultaneously is an ultimate pursuit in metal sheet forming. The stress-relaxation ageing (SRA) behavior and mechanical properties of a commercial 2219 aluminum alloy largely pre-deformed (LPD) by 80 % have been systematically investigated. The stress relaxation ratio of the LPD alloy reaches approximately 94 % regardless of the initial stress (50-350 MPa) after ageing for 12 h at 140 °C. This relaxation ratio is about 2.9 and 1.8 times that in the T4 tempered alloy (27.6 % under 50 MPa and 31.5 % under 150 MPa) and T3 tempered alloy (37.6 % under 50 MPa and 51.2 % under 150 MPa), respectively. The microstructures, comprised of GP zones/θ' precipitates plus dislocation tangles, and tensile properties in the stress-relaxation-aged LPD alloys remain basically invariant with different initial stresses, as is vital importance for property consistency at different locations of the formed part. Under the same SRA condition, the LPD alloy has an increase of 150-230 MPa in yield strength relative to T3/T4 tempered alloy and obtains a uniform elongation of about 8 %. A simple dislocation-based constitutive model accurately describing stress relaxation enhanced by the high dislocation density is established and embedded in the finite element package through a user subroutine. Simulations and experimental verifications show the LPD alloy sheet parts exhibit a nearly zero springback (< 5 %) after unloading in contrast to the springback larger than 65 % in the T3/T4 alloy sheet parts under the same condition. Our findings demonstrate the high-dislocation-density-enhanced SRA response enables a high-performance springback-free age forming of Al alloy sheet.

Key words: Age forming, Stress relaxation, Dislocation density, Springback effect, Microstructure evolution