2007 Volume 79 Issue 12 Pages 717-725
Die-cast and high-speed twin-roll cast products were fabricated using an ADC12 alloy. Tear specimens were prepared from these cast plates for tear test and microstructure observation. The tear toughness was evaluated by using both UEp and log σmax－log λ chart. The microstructure of ADC12 alloy consisted of primary α-Al dendrites, dispersed eutectic Si particles and other secondary particles. Si particles in the die-cast product were coarse and plate-like. In contrast, they were fine and globular in the roll-cast product. The calculated UEp value of the roll cast product was six times larger than that of the die-cast product. The fracture surface of the die-cast product was relatively flat and the cleavage fracture of Si particles was clearly observed. On the other hand, for the roll-cast product, fine dimples covered the fracture surface and tear lips were also observed on the fracture surface. Difference in fracture mode, brittle and ductile, was considered to be due to a large difference in the morphology of Si particles, which occurred by rapid cooling in the roll casting process compared to the die-casting process. Refinement of the solidification structure is effective for improving the toughness of brittle cast products, such as ADC12 alloy.
