Ageing characteristics of some Al-Zn-Mg alloy

A study has been made of the ageing characteristics of some Al-6% Zn-3% Mg alloys made from Indian commercial aluminium, with a view to determining the particular characteristics imparted to such alloys by the impurities present in indigenous aluminium, viz. high silicon and iron contents. It has been observed that (i) the hardness vs. ageing time curves show double peaks in the lower temperature range, (ii) the peak hardness is maximum at an ageing temperature of 100°C, and (iii) at the optimum ageing temperature of 100°C, it is possible to increase the hardness of these cast alloys to about 175 VPHN from about 80 VPHN. The mechanism of age hardening has been studied by metallographic examination which indicates that hardening may be due to sub-microscopic precipitation within the grains. (Dr. Y.N. Trehan, Scientist, National Metallurgical Laboratory, Jamshedpur)