The Effects of Casting Defects Microstructure on the Mechanical Properties of Die Cast AM50 Magnesium and 356 Aluminum

摘要

The increased utilization of magnesium by theauto industry is a key factor in meeting current and future needsto reduce vehicle weight. This paper describes one aspect of aninvestigation to use magnesium for chassis components; inparticular wheels and control arms. A modified die castingprocess, utilizing AMSO magnesium, was employed to fabricateprototype wheels and control arms. Test specimens for roomtemperature tensile and uniaxial fatigue R=-l) testing weremachined from these prototype components and, for comparisonpurposes from 356 aluminum wheels. Fatigue test specimenswere characterized by x-ray radiography and SEM fractographybefore and after fatigue testing, respectively. Casting defects,mostly shrink and gas porosity, resulted in a wide distribution inthe tensile and fatigue properties. Samples exhibiting the "best"tensile properties obtained from these AMSO magnesiumprototype components had yield strengths of 15.5 ksi, tensilestrengths equal 32.5 ksi and elongations of 21%. Infinite fatiguelives ( > 10~7 cycles) were observed for stress amplitudes of +-8ksi. The effect of the size and density of various casting defectson the fatigue properties was characterized. In addition, thelocation in the component from which the fatigue samples wereremoved was important in influencing the fatigue life. Statisticalanalysis of the fatigue lives at a particular stress level shows thata log-normal distribution is appropriate. An important aspect ofthis investigation, was verification of the concept that fatiguetesting is a better indicator of casting quality than either x-rayinspection or metallographic examination.