共晶Siを含むAl-Si合金の連続鋳造・押出し方向が疲労強度に及ぼす影響

抄録

To investigate the effect of continuous casting direction and extrusive direction on fatigue strength of Al-Si eutectic alloys which have fine eutectic Si, tension-compression fatigue tests which have been developed to evaluate fatigue properties of extruded bars by small size specimen, and rotational bending fatigue tests were carried out. The results show that the fatigue strength of the extruded bar with homogenizing distribution of eutectic Si and another metallic compound is higher than that of continuous casting bar. The specific fatigue strength, which is defined as the fatigue limit, σ_w, normalized by tensile strength, σ_B, is 0.44 for the specimen sampled in the longitudinal direction of extrusion, that is higher than powder metallurgy aluminum. The fatigue fracture mode of continuous casted bar is shear type caused by the dendrite morphology which has inhomogeneous distribution of eutectic Si and another metallic compound, and the fatigue strength depends on the direction of continuous casting. In case of extruded bars, caused by homogenized distribution of eutectic Si and another metallic compound, the fatigue fracture is tensile type and the fatigue strength dependence on the direction of extrusion is smaller than in case of continuous casted bars. The developed fatigue test method using small size specimen which has 7mm length is able to evaluate the fatigue properties vertical to the extrusive direction using high strength extruded bar with small diameter.