Study on properties and precipitation behavior of 6000 series alloys with high Mg/Si ratios and Cu contents

Abstract

A combination of high Cu and excess Mg contents has previously shown excellent effects on suppressing the natural aging (NA) and enhancing the precipitation strengthening response during artificial aging (AA) of Al-Mg-Si-Cu alloys. In this work, the precipitation behaviors of a series of 6000 alloys with high Mg/Si ratios from 1.5 to 3.0 and high Cu content levels (0.7 and 1.0 wt%) under different aging conditions were studied by Vickers hardness measurements, tensile tests, differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM) characterization. During AA, an increased content of Cu and a reduced Mg/Si ratio can promote the precipitation of fine metastable phases, and the effect of the former is more significant. A Mg/Si ratio of 1.5 is found to be the optimum composition in this study, which produces the maximum precipitation hardening response. Besides, high Mg/Si ratio can enhance the precipitation of disordered QP2 phases, which is obviously different from the conventional applied alloys strengthened by the β” phase. Thus, the effects of chemistry on the complex precipitation behavior and the properties of Mg-rich high Cu alloys have been discussed, which will be beneficial to develop new Al-Mg-Si based alloys. • The activation of clustering and precipitation increases with the Cu content but decreases with the Mg/Si ratio. • The excess Mg and high Cu results in a moderate T4 temper strength due to the balance effect between the Mg and Cu. • The combination of high Cu and excess Mg can increase the age hardening response of these alloys except for Mg/Si ratio larger than 2.2.