Effects of up-quenching on age-hardening behaviors in Al-Mg-Si alloys

Abstract

Inspired by a success in applying a fluidized bed furnace to high-temperature solution treatments, the present authors have attempted to modify the age-hardening characteristics of Al-Mg-Si alloys by utilizing its superior heating rate. Two-step ageing treatments have been applied to five kinds of alloys with different Mg 2Si and excess Si contents. Heating rates for the first and the second ageing treatments have been varied as well as ageing temperature for the first ageing treatment. It has been clarified that the rapid heating to the first ageing temperature enhances strength compared to the standard T6-temper for all the alloys tested, especially for a balanced alloy with a low Mg 2Si content. It has been confirmed that small precipitates are formed in high density in such cases. In most cases, the improvements in hardness have been observed by adding the ageing treatment at 353 or 373 K for only 0.8 ks before the ordinary ageing at 450 K. We attribute this to the suppression of unfavorable clusters which ordinarily form below those temperatures during heating.