真空ダイカストならびに高速双ロールキャストしたアルミニウム合金の組織と引裂靭性

Abstract

Vacuum die-cast and high-speed twin-roll cast products were fabricated using a heat-treatable Al-Si-Mg base alloy (A) and a non-heat-treatable Al-Mg base alloy (B). These alloys have almost equivalent tensile strength and ductility but different solidified structure. Tear toughness tests were performed using a small-size specimen and the unit crack propagation energy, UEp, was obtained from the load-displacement curve. For the vacuum die-cast products, both A and B exhibited macroscopically plane fracture surface. Dimple formation nucleated at the globular eutectic Si particles was detected in A. In contrast to that, the fracture surface of B was covered with a lamella eutectic structure. UEp was larger in A than B. Large increase in UEp was obtained for the twin-roll cast products. Their UEp values were 1.8 and 3 times larger than those of vacuum die-cast A and B, respectively. Macroscopic appearance of the twin-roll cast products was characterized by the slanted shear-type fracture path for both A and B. In addition to that, fracture surface of B showed fine dimple fracture surface. Large increase in UEp for B is due to the morphological change of the eutectic structure from the lamellar-type to the fine and globular-type induced by the rapid cooling rate of the high-speed twin-roll casting.