Studies on Al-Mg2Si-Cu alloys

Abstract

The effects of addition of Cu (0.65.5%) to Al-1.58wt%Mg2Si alloys were studied by hardness measure-ments, tensile tests, and transmisson electron microscopy. It is known that the addition of Cu to Al-Mg2Si alloys has some effects on age hardening, mechanical properties, and corrosion resistance of the alloys. This paper describes in details the effects of Cu on age hardening and mechanical properties of the alloys.The results obtained were summarized as follows:(1) The isothermal ageing curves of the alloys containing less than 1.5% of Cu were different from those containing more than 2%, of Cu. The former represented 1 stage ageing curves and the latter represented 2 stage ones. In general, peak hardness (maximum hardness) was higher with the increase in copper content. However, the peak hardness of the alloys containing 2% of Cu was lower than that of the alloys containing 1.5% of Cu. The time required to reach the peak hardness was longer with the increase in copper content when it was less than 1.5%; however, it was shorter with the increase in copper content when it was more than 2%.2) It was observed by transmission electron microscopy that the precipitation hardening system of Al Mg2Si-Cu alloys had a transition point at 2% of Cu content. It was assumed that the system was composed of Al-CuAl2 and Al-Mg2Si when Cu was 1.5% or less; and was composed of Al-Al2CuMg, Al-Q (Quarternary phase), and Al-CuAl2 when Cu was 2% or more.3) The results of tensile tests at room and higher temperatures showed that the tensile strength was higher with the increase in copper content, but proof stress of the alloys containing 1.2 or 2% of Cu was lower than that of the alloys containing 0.6% of Cu. These facts would mainly be due to the difference of precipitates formed during the ageing.