Creep behavior of high-pressure die-cast AlSi10MnMg aluminum alloy

Abstract

High-pressure die-cast (HPDC) AlSi10MnMg (AA365) has been widely used in the manufacturing of automotive components because it has excellent castability, is lightweight, and possesses good mechanical strength. In particular, this alloy must be capable of withstanding long-term exposure to high temperatures in order for these automobile parts to be useable. To investigate the viscoelastic behavior of HPDC AA365 with the aim of improving the long-term structural performance at high temperatures, we performed a creep test in the temperature range of 373–573 K with a constant applied stress of 64–217 MPa. A micro-hardness test of the HPDC AA365 alloy, performed after the creep test, yielded hardness values of 64.0 HV and 49.9 HV for the gauge and grip sections, respectively. The observed grain-size differences between the sections may have led to this difference in micro-hardness. Moreover, both values of micro-hardness were relatively low compared with the value of 93.5 HV for HPDC AA365 before the creep test. This was because of recovery and recrystallization. The creep tests also showed that the minimum creep rate increased with increasing applied stress for each temperature, causing a decrease in the time to failure. Based on the true stress exponent (n) on a scale of 1 to 90, various creep mechanisms for HPDC AA365 were revealed, including the Harper-Dorn, dislocation, and power-law breakdown mechanisms. The specific creep mechanism depended on temperature and applied stress. Furthermore, owing to the presence of the threshold stress, the apparent creep activation energy (259.08 kJ/mol) was substantially higher than that of aluminum (Al) self-diffusion (Qd = 143 kJ/mol). The existence of the threshold stress can also be demonstrated by a high n value that does not correspond to the n of pure Al under certain conditions. Failure in these alloys can be caused by void coalescence, cracked-brittle Si particles, and intermetallic phases that served to initiate and propagate the crack. Our results demonstrated that the fracture mechanism changed from transgranular to intergranular fracture in response to the applied stress and temperature. Our findings can improve the applications of high-pressure die-cast aluminum alloys.