Microstructural Characterisation and High-cycle Fatigue Behaviour of Semisolid Al-Si-Cu Alloy

This study presents an experimental investigation of the high-cycle fatigue behaviour of an Al-Si-Cu alloy. The test specimens were fabricated via semisolid metal processing, namely thixoforming. High-cycle fatigue testing was conducted on an hourglass specimen subjected to T6 heat treatment with a stress ratio of R = 0.1 and a uniform-amplitude load of 20 Hz up to the final failure of the specimen. Result showed the S-N curves for the T6 heat-treated thixoforming alloy specimens improved by 50 MPa fatigue strength at 106 cycles compared with that of the T6 heat-treated as-cast specimens. SEM examination of the fracture surface reveals that fatigue damage was governed by porosity, which served as the main cause for the initiation of cracking in the as-received alloy. By contrast, fatigue failure in the thixoformed alloy was governed by the inclusion of impurities such as carbon and an oxide film. Therefore, this study confirmed that the thixoformed T6 alloy has 19% greater resistance to fatigue failure and 0.25 quality index (QT) as compared with the as-received T6.