Effects of Zn Addition on the Natural Ageing Behavior and Bake Hardening Response of a Pre-Aged Al-Mg-Si-Cu Alloy

Al-Mg-Si(-Cu) alloys are widely used in automotive body panels because of their excellent combined performance of high strength-to-weight ratio, good formability and corrosion resistance. Zn additions to Al-Mg-Si(-Cu) alloys have been tested and shown to effectively affect the precipitation microstructure and enhance the age-hardening response. The present study investigates the natural ageing (NA) behavior and bake hardening response in the pre-aged Al-0.9Mg-0.8Si-0.2Cu (mass fraction, %) and Al-0.9Mg-0.8Si-0.2Cu-0.6Zn (mass fraction, %) alloys. The results are compared to clarify the effect of Zn addition. During NA after pre-ageing at 80 degrees C for 15 min (PA), cluster growth is the dominant process in the Zn-free and Zn-added alloys. Some Zn atoms are partitioned into the clusters under PA+NA condition. Partitioning of Zn may change the stability of clusters, increasing the growth rate of clusters. The yield strength of the two alloys increases with the increasing NA time. The smaller cluster spacing and larger cluster shear modulus lead to the higher yield strength in the Zn-added alloy during NA after PA. The prolonged NA inhibits the transformation of clusters to GP zones and beta '' phases during bake hardening (BH) treatment at 170 degrees C for 30 min in the Zn-free and Zn-added alloys, resulting in the lower BH response. The Zn does not significantly partition into clusters or precipitates, and the majority of Zn remains in the Al matrix during BH treatment, prompting the transformation from solute clusters to GP zones and beta '' phases. As a result, the yield strength of the Zn-added alloy after PA+NA+BH treatment is higher than that of the Zn-free alloy.