Research On Fatigue Behavior And Corrosion Properties Of High-toughness Die Cast Al-Mg-Si-Mn Alloy

High pressure die casting (HPDC) is an efficient near-net shape manufacturing process, which can be used to produce the structural components with complicated structures and thin walls. However, the air pores resulted form the high filling rate deteriorate the ductility and fatigue resistance of the die castings, therefore, die cast components fail to be used as high-duty structural components. Hence, high vacuum die casting technology and high performance die cast alloys were developed to solve those problems. It have been extensively studied that the performance of Al-Si, Al-Si-Mg and Al-Si-Cu die cast alloys with good formability. However, the research on Al-Mg-Si alloys, which can be used to produce high-duty structural component, was rarely reported. On the other hand, AlMg5Si2Mn possess good application prospect and several advantages, such as the good ductility, corrosion resistance and recyclability. Moreover, it is suitable to be applied in the mass-production of thin-wall die cast parts which possess low porosity level. According to the discussion above, microstructures, static mechanical behavior, fatigue and corrosion properties of HPDC AlMg5Si2Mn are systematically studied in this paper.The microstructure and mechanical properties of die cast AlMg5Si2Mn alloy are studied in details. The effects of magnesium content, aging and vacuum assistance on the mechanical properties are preferentially studied. Results indicate that the microstructure of HPDC AlMg5Si2Mn alloy mainly contains:a-Al grains,[Al+Mg2Si] eutectic region and Al15(Fe, Mn)3Si2phase. Mechanical properties of die cast specimens are superior to those of permanent mold cast (PM) specimens. Numerous cleavage steps are observed in the tensile fracture surface. As the magnesium content decreased from5.5%to7.2%, tensile strength is fluctuated around320MPa and the yield strength is improved from189MPa to220MPa, while the elongation decreased form8.31%to4.5%. The age treatment under250℃enhances the yield strength and Brinell hardness, while the elongation is decreased with the increased aging time. In addition, the vacuum assistance just slightly improved the mechanical properties of HPDC AlMg5Si2Mn alloy.Fatigue life and crack growth behavior of HPDC AlMg5Si2Mn alloy are firstly studied in this research. Effects of magnesium content, stress ratio, aging and corrosive environment on the fatigue life are systematically investigated. Crack initiation and propagation mechanisms in different testing parameters are explored. Fatigue tests results indicate that the fatigue limits increased from54MPa to75MPa as the magnesium content from4.5%to7.2%. Influence of the age treatment on the fatigue strength of the alloy is insignificant. Fatigue lives obtained under the stress ratio of-1are much longer than those under stress ratio of0. Most of the fatigue cracks initiated from surface of sub-surface pores, oxide inclusions and severe ductile deformation bands. Fatigue lives of HPDC specimens are superior to those of PM specimens. In addition, the slope and y-intersept of the crack growth rate curves are m and lgC where crack growth rate exponent m=5.756～5.874and coefficient C=2.421×10-10～4.285×10-9. It indicates that crack growth rate of HPDC AlMg5Si2Mn is lower than that of die cast Al-Si-Mg alloys. Vacuum assistance slightly decreased the crack growth rate while its effect on the fatigue life is insignificant.Corrosion resistance of HPDC and PM AlMg5Si2Mn alloys are deeply studied and the corrosion mechanisms and the effect of microstructure on the corrosion resistance are also discussed. Electrochemical corrosion tests results indicate that corrosion and pitting potential of HPDC and PM samples are-1220mV,-690mV and-1250mV,-760mV respectively. The maximum corroded depth and weight loss of HPDC and PM samples are130μm、62mg/cm3and365μm、220mg/cm3respectively, which indicates the great inter-granular corrosion tendency of AlMg5Si2Mn alloy. Corrosion resistance of HPDC samples is better than that of PM samples, in addition, numerous pits and Si-rich regions are observed around the grain boundaries.According to the structural fatigue test result of die cast AlMg5Si2Mn rear sub-frame, the critical positions of that component is capable of bearing over200,000cycles under the testing stress without fatigue failure. Fatigue resistance of all testing positions satisfy corresponding requirements. In addition, Rm, R0.2and elongation of the specimens sectioned from the component are260MPa、150MPa and6.2%which satisfy the mechanical properties requirement (Rm≥240MPa, R0.2≥145MPa and elongation≥6%). Therefore, die cast AlMg5Si2Mn alloy can be used to produce large size high-duty automotive structural components.