Thermo-mechanical and low cycle fatigue behavior of aluminum alloy cylinder heads

In this study, out-of-phase thermo-mechanical fatigue (OP-TMF), room and high temperature low cycle fatigue (RT-, HT-LCF) tests are performed on die cast aluminum-silicon alloy, A356.0 which has been widely used in diesel engine cylinder heads. According to realistic loading condition of cylinder heads, the temperature varies between 50°C to a maximum temperature which is considered as 200 and 250°C with no dwell time. In TMF tests, constraint factor, defined as mechanical strain to thermal strain ratio, is constant as 125%. For LCF tests, strain amplitude is considered as mechanical strain amplitude of mid-life in TMF tests with a constant temperature varying as 25, 200 and 250°C. Fatigue test results show that plastic strain increases during cycles at a constant mechanical strain amplitude until the specimen fails. LCF results show that hardening happens at room temperature for A356.0 alloy and softening occurs at high temperatures as it is also observed in TMF tests. The radial and longitudinal temperature gradients during TMF tests are almost 2 and 3°C, respectively. Comparing stress-strain hysteresis loop, TMF tests have higher tensile stress at minimum temperature because of out-of-phase loading condition (maximum temperature occurs within compressive regimes and minimum temperature happens at tensile loads) than the tensile stress at LCF cases. In general, fatigue lifetime of TMF tests is less than LCF tests due to severe conditions and the temperature deviation from a minimum value to maximum one.