Effect of Pre-Aging Treatment on Bake-Hardenability of Al-8.0Zn-2.5Mg-2.0Cu Alloy Sheet Fabricated by Twin-Roll Casting Process

Abstract

The effects of natural and pre-aging on the microstructure and mechanical properties of Al-8.0Zn-2.5Mg-2.0Cu aluminum alloy sheet fabricated by twin roll casting were investigated. Immediately after solution treatment, the Al-8.0Zn-2.5Mg-2.0Cu alloy sheet had a yield strength of 173 MPa and elongation of 19.5%. The hardness of the Al-8.0Zn-2.5Mg-2.0Cu alloy sheet increased sharply for 48 hours with natural aging and then increased gradually for up to 336 hours. After 1 week of natural aging, the yield strength was 371 MPa and the elongation was 12.5%. The yield strength after bake hardening treatment at 180 degrees C for 30 minutes was 519 MPa immediately after solution treatment, 534 MPa after natural aging. The fine GP zones generated by natural aging caused local deformation in the aluminum matrix, which reduced the elongation. In addition, they were mostly dissolved during the bake hardening treatment, and some GP zones served as the nuclei of the metastable eta'. Pre-aging at 120 degrees C for 10 minutes minimized the decrease in elongation by natural aging, and improved the yield strength after baking hardening. The elongation before bake hardening treatment increased to 21%, and the yield strength after bake hardening treatment increased to 573 MPa.