Microstructural Analysis on the High-temperature Mechanical and Electrochemical Behaviors of AlSi10MnMg die-cast alloy for Automobile Applications

Abstract

High-pressure die-casting (HPDC) AlSi10MnMg (AA365) has been used widely in the manufacturing of automotive components because it has excellent castability, good mechanical property, and lightweight. Particularly, this alloy must be capable of enduring exposure to high temperatures and corrosive environments in the long term with a view to using these automobile parts efficiently. Therefore, the high-temperature tensile property, the viscoelastic behavior, and the corrosion characteristic of AA365 alloys were examined with the aim of certifying with reliability and safety in various industries. At first, the excellent mechanical property of HPDC AA365 alloy was revealed through the comparison of as-cast and T4/T6 heat treated AA365 alloys due to the significant refinement of grain size. Then the high-temperature tensile test was conducted in the range of temperatures from 273 K to 737 K. The tensile and yield strengths of this alloy were declined from 299 MPa to 10 MPa, but the tensile strain was increased from 12.4 % to 44.6 %, as the temperature increased. However, the yield strength unusually increased from 146 MPa at 373 K to 181 MPa at 473 K due to the strengthening effect that intermetallic compounds block the movement of dislocations. Additionally, the creep test of HPDC AA365 alloy was performed in the temperature range of 373 K to 573 K with a constant applied stress of 64 to 217 MPa. Firstly, the micro-hardness test before the creep test showed 93.5 HV, whereas it decreased to 64.0 HV and 49.9 HV for the gauge and grip sections, respectively, after the creep test. The recovery and recrystallization caused a decrease in the hardness after the test. Furthermore, the difference in the grain size led to the discrepancy of micro-hardness in the gauge and grip sections. Based on the minimum creep strain rate through the creep test, true stress exponents (n) were presented as 1 to 90.7, indicating various creep mechanisms, such as the Harper-Dorn, dislocation, and power-law breakdown (PLB). Considering the high n (22.4 to 90.7) and the apparent activation energy (259.08 kJ/mol), the threshold stress of AA365 alloy could be proved on certain experimental conditions, indicating the high creep resistance with a comparison of other Al alloys and Al-based composites. All failures of the alloys were stimulated by micro-void, cracked-brittle intermetallics and Si particles. In addition to the characteristic of the alloy under high temperatures, the electrochemical property of AA365 alloy was investigated with the aim of understanding the effect of T4 (813 K for 8hours) heat treatment (ST) on the electrochemical properties of AA365 alloy with various cooling media, i.e., furnace (E), air (A), forced air (FA), and water (WQ). In terms of the microstructure, the amount and size of intermetallics and eutectic Si strongly depend on the cooling rate after heat treatment, predicting that ST-WQ AA365 alloy would be less harmful to the corrosion with few intermetallics and eutectic-Si particles by a rapid cooling rate. By conducting the immersion test in 3.5 wt.% NaCl at first, it was found that ST-WQ AA365 alloy showed 1.66 mpy of the lowest corrosion rates and 1.40 of a low fitting factor rather than the values of the alloys cooled at other media. Through the cyclic potentiodynamic polarization (CPDP) and electrochemical impedance spectroscopy (EIS) tests in 3.5 wt.% NaCl, ST-WQ AA365 alloy had the lowest corrosion current density (0.394 x 10-6 Acm-2) and the highest value of corrosion potential (-707.8 mVSCE), and this also showed a larger value of impedance more than the as-cast and other AA365 alloys cooled with furnace and air after T4 heat treatment. Consequently, this research could prove the excellent high-temperature mechanical property of HPDC AA365 alloy and recognize the effect of the cooling rate after T4 heat treatment on AA365 alloys, authenticating the superior corrosion resistance of the alloys. Based on the results of this study, the excellence of this alloy's parts can contribute to improving the reliability and safety of application in various industries.