Fatigue crack growth behavior in semi-liquid die-cast Al-7%Si-0.4%Mg alloys with fine effective grain structure

Structural control was performed on Al-7%Si-0.4%Mg alloy castings by three different fabrication routes: sand mold cast and HIP, permanent mold cast and semi-liquid die-cast. In contrast to the ordinary dendrite structure in the sand and permanent mold cast alloys, the microstructure of the semi-liquid die-cast alloy was characterized by colonies consisting of single or several dendrite cells. Crystallographic misorientation among these colonies is large and they can be regarded as ‘effective grains’. Fatigue crack growth tests were performed using CT specimens at a stress ratio of 0.1 and effects of microstructure were examined. Difference in the overall growth rates was not evident in three samples. However, crack growth path and fatigue fracture surface of the semi-liquid die-cast alloys was different from those in the others. The fatigue crack grew straight even at small ΔK levels. Large area of the fatigue fracture surface was covered with striations. These characteristic features are correlated with the relationship between the plastic zone size at the crack tip and the effective grain size in the semi-liquid die-cast alloys.