On the Impact Properties and Fracture Mechanisms of A356.2-Type Cast Alloys

Abstract

A study was carried out to investigate the effects of iron content, porosity and solidification rate on the impact properties and fracture behavior of A356.2-type alloys. The results show that impact properties improve with increase in solidification rate and decrease in Fe content. Unmodified A356.2 alloys show linear correlations, while modified A356.2 alloys show logarithmic correlations at all solidification rates (R 2 > 0.95 in all cases). Impact properties obtained at the highest solidification rate are far superior to those obtained at other solidification rates. The β-Al5FeSi intermetallic deteriorates impact properties significantly, the effect being most apparent within 10–50 μm β-platelet sizes in A356.2 alloy. Fairly good correlations between porosity and impact properties are obtained. Strontium is effective in improving impact energy, even at high Fe levels. A good inverse relation is obtained between average crack speed and impact energy, highest crack speeds being observed in unmodified samples obtained at highest Fe contents and lowest solidification rates. Impact testing is sensitive to variations in microstructure or casting defects. Impact energy–strength plots show exponential relationships, whereas impact energy–ductility plots display linear relationships for all alloys, modified or not, regardless of the alloy composition. In A356.2 alloys, cracks initiate mainly through the fracture of Si particles or their debonding from the Al matrix, while crack propagation occurs through the coalescence of fractured Si particles, except when β-Al5FeSi platelets are present, in which case the latter take precedence.