Abstract
An investigation on three (99.7%, 99.9%, 99.99%) different purity Al-5.9 wt%Zn-1.4 wt%Mg alloys was carried out to optimize the aluminum purity level which will be adequate for developing tensile properties. Resistometric studies corroborated by tensile, tear and hardness tests were used to study zone formation and precipitation behavior. The results show that 99.9% aluminum based alloy can be utilized to produce combination of properties comparable to alloy based with 99.99% aluminum by preaging the impure alloy at relatively lower temperature. Quench clustering, initial aging rate and the peak height are all reduced due to the presence of impurities. The quenched-in clusters appear to act as centers for growth on isothermal aging. Preaging with ultrasonic stirring has also been tried which is found to increase the initial aging rate, zone density and improve properties on subsequent aging. Tear tests indicate that the energy of crack propagation in the commerical purity alloy is higher than the high purity alloy, whereas both the tear strength and the crack initiation energy of the high purity alloy are superior to commercial purity alloy.
