Notch size effects in the fatigue characteristics of Al-Si-Cu-Mg cast alloy

Influences of the solidification microstructure and the stress concentration factor on the fatigue characteristics were investigated for the JIS-AC4B alloy containing 6.79%Si, 2.93%Cu, 0.17%Mg and 0.59%Fe, and also for a iron tree AC4B-type Al-Si-Cu-Mg alloy. Solidification microstructures of specimens consist of dendritic alpha, eutectic Si, Al₂Cu and Mg₂Si phases in both alloys, and furthermore, Fe compound appears among the dendrites in AC4B alloy. However, a few gas and shrinkage porosities appear in every specimen. The rotating bending fatigue tests were carried out on specimens with notches of 2, 1, 0.3 and 0.1 mm radius. Both of AC4B alloy and iron free AC4B type alloy specimens show the same fatigue sequence with increasing notch size larger than 1 mm, indicating that the gas and shrinkage porosities act as the origin of cracking and govern the fatigue characteristics. However, when the notch radius becomes smaller than 0.3 mm, that is, the stress concentration factor becomes larger than 2.4, AC4B alloy has higher fatigue strength than Al-Si-Cu-Mg alloy, which indicates the Fe-compounds retard the crack propagation.