Influences of the microstructure, micro defects and the stress concentration factor on fatigue characteristics were investigated for a JIS-AC4B alloy containing 6.79%Si, 2.93%Cu, 0.17%Mg and 0.59%Fe, and for an iron free Al-Si-Cu-Mg alloy. Solidification microstructures consist of dendritic α phase, eutectic Si, Al2Cu and Mg2Si phases in both alloy specimens and a few gas and shrinkage porosities appear in every specimen; while Fe compound modified by Mn appears among the dendrites in the AC4B alloy. Rotating bending fatigue tests were carried out on specimens with notches of 2, 1, 0.3 and 0.1 mm radius. Both AC4B and Al-Si-Cu-Mg alloys show the same fatigue sequence when the notch size is larger than 1 mm, indicating that the gas and shrinkage porosities act as the origins of cracking and thus govern the fatigue characteristics. Contrarily, when the notch radius becomes smaller than 0.3 mm, so that the stress concentration factor becomes larger than 2.4, the AC4B alloy has a higher fatigue strength than the Al-Si-Cu-Mg alloy, indicating that Fe-compounds may retard crack propagation.
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