High cycle fatigue strength of permanent mold and rheocast aluminum 357 alloy

The high cycle fatigue resistances of aluminum–silicium–magnesium 357 alloy prepared by semi-solid forming (SSM) and conventional permanent mold casting (PM) are compared under fully reversed loading. Results, reported in S–N diagrams, show that rheocasting improves the as-cast alloy mean fatigue strength, by 36% at 107 cycles. Part of this improvement is explained by the fact that more SSM specimens are defect free than PM specimens. Comparison of the S–N diagrams also reveals that precipitation hardening slightly increases the fatigue strengths of the PM and SSM alloys, and that eutectic modification has no effect on the fatigue performance of the SSM alloy. Observation of small cracks using replicas shows the existence of crack growth decelerations at grain boundaries. No similar decelerations are observed when the crack enters a new α-Al cell within a grain. According to these results, it is proposed that in the absence of defects, the fatigue strength of aluminum alloy 357 is a function of the grain size (D) rather than of the secondary dendrite arm spacing (SDAS) or the spherical diameter of the alpha phase globules (φsph). Thus, it is concluded that the fatigue strength improvement of the SSM alloy is also related to the smaller grain of the rheocast specimens.