Enhanced high-cycle fatigue strength of Al–12Si–4Cu-1.2Mn-T6 cast aluminum alloy at room temperature and 350 C

Abstract As a promising material for application in lightweight automobile, Al–Si alloys are subjected to complex cyclic thermo-mechanical loading when employed in engines. This study examined the high-cycle fatigue behavior of Al–12Si–4Cu-1.2Mn-T6 cast aluminum alloy at room temperature (RT) and 350 °C and evaluated the effect of different second phases by microstructure observation and nano-indentation characterization. The high-cycle fatigue strength of the studied alloy at RT and 350 °C is 125.0 MPa and 47.5 MPa, respectively, exhibiting excellent fatigue resistance compared with other heat-resistant Al alloys. Fatigue cracks originate from casting defects at RT, but nucleate from primary Al15Mn3Si2 phase on the specimen surface at 350 °C. The modulus difference between α-Al15Mn3Si2 and α-Al phases is higher at 350 °C than at RT, allowing cracks to initiate and propagate more likely along the phase interface at 350 °C. The hardness and reduced modulus of α-Al15Mn3Si2 phase are greatly higher at 350 °C than at RT, both in the suspended specimen and the fractured specimen. The significant difference in residual stress between α-Al15Mn3Si2 and α-Al phases indicates that α-Al15Mn3Si2 phase bears more stress loading than α-Al phase during fatigue process, especially at 350 °C. The findings presented in this study may shed some light on the design of high-performance heat-resistant aluminum alloys.