The combined effect of pre-aging and Mg/Si ratio on the natural aging and bake hardening in Al-Mg-Si-0.7Cu alloys

The combined influences of pre-aging treatment (PA) and changing Mg/Si ratio on the natural aging (NA) effect and bake hardening response for a series of 6000 series alloys with excess Mg and Cu content was studied. The results showed that PA provides a clear improvement in alloy hardness after paint baking whereas a rising excess Mg (from 1.5 to 3.0) decreases this effect. The hardness in T4P temper however was generally relatively low, due to the combined effect of PA and a high Mg/Si ratio. The strong interaction energy between Cu and Mg was responsible for facilitating the clustering and precipitation processes inducing this behavior. During NA, Mg-Cu and Mg-Mg clusters were found in addition to the Mg-Si(-Cu) clusters, with the total cluster number density decreasing in excess-Mg alloys, resulting in reduced T4P temper hardness. After bake hardening, the disordered β″ precipitates, containing substructures of “low density cylinders” (LDC) and Cu sub-unit clusters, and the lath-like QP2 phases are responsible for the excellent hardening response regardless of Mg/Si ratio effects. The alloy with a Mg/Si ratio of 1.5 exhibits a low strength in T4P temper (134 MP) and very high strength (274 MPa), coupled with a good ductility (14%) after paint baking. This alloy has great potential for automotive application. •Cu addition can strengthen NA hardening and improve the bake hardening response of the alloys.•The combination of Mg/Si ratio and PA can produce a lower hardnness in T4P temper while higher hardness after aging.•The disordered β" and QP2 phases are responsible for hardening for Mg-rich high Cu alloys.