PNNL

Abstract

Room temperature processing methods of reaching near-T6 yield and ultimate tensile strength in Al-Zn-Mg-Cu alloys provide a potentially lower cost alternative to high temperature forming and post-forming artificial aging methods. This study demonstrates a method, where a combination of pre-deformation and natural aging matched T6 strength along with ductility and strain hardening greater than the T6 values. Tensile deformation of 5% and 10% was applied to naturally aged samples at 144 hours (6 days). The mechanical properties showed stress instability, Portevin Le Chatelier (PLC) effect, at 1 day of natural aging which diminished by 6 days of natural aging. However, deforming the material at 6 days of natural aging resulted in PLC to return and remain well into 220 days of natural aging. As opposed to the common ductility decrease associated with PLC in the literature, reemergence of PLC in this study both increased the total ductility and strain hardenability, simultaneously with the strength of the samples. The return of PLC was attributed to a transition of the microstructure from GP zones in the early stages of natural aging to solute cluster type microstructure in pre-deformed and naturally aged samples, which contributed to simultaneous strength and ductility increases with further post-deformation natural aging.