Effects of defects and Fe intermetallic compounds on fatigue properties of Al-Si-Cu-Mg alloy castings (Reprinted from Journal of Japan Institute of Light Metals, vol 50, pg 116-120 2000)

In order to clarify the effects of casting microstructures on the fatigue properties of Al-9%Si-1.6%Cu-0.55%Mg alloy castings, non-degassed, degassed and HIP treated castings with different sizes of microstructures were examined. The fracture elongation and fatigue properties markedly increased with decreasing the amount of gas in the melt. Gas porosities in the castings greatly affect the crack growth as well as the crack initiation. In the castings with no porosities, Fe intermetallic compounds with a needle shape act as a crack initiation site. In the castings composed of fine microstructures with smaller aspect ratios, the existence of the compounds scatters the values of fatigue strength. In the castings composed of coarsened microstructures with larger aspect ratios, on the other hands the compounds provide not only crack initiation sites but also growth paths, resulting in the significantly decreased fatigue strength.