Effects of zinc and magnesium on the properties of Al-Si-Cu die-casting alloys

Abstract

When Zn and Mg were contained together in die-casting aluminum alloys by the content equivalent to ADC 12, the effects of the both metals on the mechanical properties and corrosion resistance at room temperature and the effects on the change of mechanical properties after tempering (T5 treatment) were investigated.The following results were obtained.(1) The effects on the mechanical properties at room temperature were the same in die-cast specimens and mold cast specimens. Zn and Mg contents had little effects on tensile strength, but yield strength (0.2%), elongation, hardness, and impact value were affected by Mg content more remarkably as compared with Zn. Yield strength (0.2%) and hardness were higher for higher Mg content (0.30%), while elongation and impact were higher for lower Mg content (0.08%). On the other hand, yield strength (0.2%) was almost constant for the increase of Zn content, but hardness slightly increased with the increase of Zn. Furthermore, elongation and impact value had also the same tendency as above.(2) The suitable tempering (T5 treatment) conditions of the specimens (having chemical components used in these experiments) would be at 150°C and for 8hr in the air atmosphere. The effects on mechanical properties were more sensitive after T5 treatment was performed. Especially, yield strength (0.2%) and hardness of higher Mg content specimens were substantially increased. The effects of T5 treatment on tensile strength were not observed for any contents of Mg. Elongation and impact value had the same tendencies in almost uniform decreasing with the increase of tempering time for any contents of Mg. However, they were always higher for lower Mg content (0.08%) than higher one (0.30%).(3) In salt water (3% NaCl) immersion test, the specimens, which had been cast by permanent molds, showed weight loss directly proportional to immersion time. Mg content had almost no effects, but Zn content had slight effects on corrosion resistance. On the other hand, in die-cast specimens, which had undergone salt spray (5±1wt.%) test at 35°C, yield strength (0.2%) even after 2 months period test was not different from that value at room temperature. However, the effects of spraying time appeared on tensile strength, elongation, and impact value. The above effects were more remarkable in Zn content as compared with Mg in all specimens.(4) In specimens of higher Mg content (0.30%), the presence of S compound (Al2CuMg) was microscopically observed. It was supposed that the above compound had the very effects on mechanical properties and other properties after ageing and T5 treatment.