AC4CHアルミニウム合金レオキャスト材およびスクイズキャスト材の引裂靭性

抄録

Cast plates of the AC4CH aluminum alloy with different thickness were fabricated by rheocasting and squeeze casting. Tear tests were performed for the as cast and heat treated products (T5 and T6) and effects of solidified structure and heat treatment on the unit crack propagation energy (UE_p) were examined. Increase of the cooling rate (corresponding to the decrease in plate thickness) resulted in refined solidified structure and enlarged UE_p values for as cast samples. Both spheroidized eutectic Si particles and age-hardened α-Al matrix by T6 treatment were effective to increase UE_p for both rheocast and squeeze cast samples. UE_p of the squeeze cast was larger than that of the rheocast. Observation of crack growth path and fracture surface revealed that the tear toughness of the present cast alloy was controlled by distribution of eutectic solidified region in the cast structure, which provided a preferential crack growth path. The discontinuous distribution for the squeeze cast is considered to be beneficial for increase in crack growth resistance rather than the continuous arrangement of the network-like eutectic region for the rheocast. Relatively small UE_p was obtained for the T6 treated rheocast sample collected from the 6 mm thickness plate. This was caused by the fact that the volume fraction of the eutectic solidified region in the sample was larger than others under the present experimental condition.