Rotating bending fatigue tests were conducted using A6061 and Al-Si eutectic alloys to investigate fatigue strengths and small fatigue crack growth behavior. Both alloys were fabricated by continuous casting and extrusion, and the effect of fabrication processes on the fatigue properties was investigated. The fatigue strengths of continuously-casted and extruded A6061 alloys were nearly comparable. The specimens, which were sampled parallel to the extrusion direction, exhibited higher fatigue crack initiation resistance than the specimens sampled perpendicular to the extrusion direction. However, the fatigue crack growth resistance was higher in the specimens sampled perpendicular to the extrusion direction. The differences in the crack initiation and growth resistances were attributed to the different microstructures depending on the extrusion direction. The fatigue strength of the extruded Al-Si eutectic alloy was higher than that of continuously-casted alloy, which could be attributed to the higher fatigue crack initiation resistance of the extruded alloy. The continuously-casted Al-Si eutectic alloy had typical dendritic microstructure, in which eutectic Si dispersed heterogeneously. The extrusion process homogenized the dispersion of eutectic Si in the microstructure, which resulted in the higher fatigue crack initiation resistance.
