Microstructure and mechanical properties of high strength die-casting Al–Mg–Si–Mn alloy

Die casting Al–Mg–Si–Mn alloy, with good strength and toughness, has a bright prospect for the mass production of automotive security components. Effects of magnesium content and age treatment on fatigue and mechanical properties of AlMgxSi2Mn (x = 5.7–7.2%) alloys are studied to broaden their application and improve mechanical properties further. Tensile and fatigue tests were conducted on as-cast and age treated (250 °C/1 h) AlMg5.5Si2Mn samples. Microstructure and fracture surface of those alloys are analyzed by SEM, XRD, EDXS and optical microscope (OM). Results indicated that as the magnesium content varied from 5.7% to 7.2%, yield strength and hardness increased 11% and 9%, while elongation decreased dramatically from 8.31% to 4.52%, respectively. Ultimate tensile strength and yield strength of AlMg5.5Si2Mn alloy increased 14% and 29% respectively, after the age treatment. Fatigue limits increased from 57 MPa to 75 MPa with enhanced magnesium contents. Surface pores and plastic deformation are dominantly responsible for the failure. Microstructure of AlMgxSi2Mn alloy consists of α-Al, Mg2Si, β-Al3Mg2 and a small amount of Al15(Fe,Mn)3Si2 particles distributed along the grain boundaries.