Effects of solidification structure on short fatigue crack growth in Al–7%Si–0.4%Mg alloy castings

Crack initiation and successive short fatigue crack behavior of the peak-aged Al–7%Si–0.4%Mg alloy castings were examined using rectangular smooth bar specimens under the axial loading condition. Tests were performed for a permanent mold cast alloy with ordinary dendrite structure and a semi-liquid die-cast alloy with fine effective grain structure. In the alloy containing few casting defects, fatigue cracks preferentially initiated at the Si particle/matrix interface in the inter-dendritic region and grew tracing eutectic cell boundaries to form the meshy crack growth path. In the ordinary dendrite structure, grain boundaries acted as effective obstruction to the growing fatigue crack. On the other hand, the effective grain of the semi-liquid die-cast was a moderate obstruction to the crack growth. Short crack growth behavior in the semi-liquid die-cast was comparable to that of the long crack. Fine effective grain structure is considered to promote homogeneous cyclic deformation at the fatigue crack tip in spite of short crack length and the corresponding small plastic zone size.