COMPARATIVE AGING BEHAVIOR OF COMMERCIAL AND HIGH-PURITY AL-ZN-MG ALLOY

An investigation of three Al-5. 9Zn-1. 4Mg alloys of different purity (99. 7, 99. 9, 99. 99% Al) 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 the alloy produced from 99. 9% Al can be used to obtain a combination of properties comparable to those of the highest purity alloy by preaging the alloy of lower purity at a 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 was also tried and found to increase the initial aging rate and zone density and to improve properties on subsequent aging. Tear tests indicate that the energy of crack propagation in the commercial purity alloy is higher than in 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.