Improved Microstructure and Enhanced Tensile Properties of Hypoeutectic AlMg5Si2Mn Alloy Modified by Yttrium

AlMg5Si2Mn alloys are widely used in the field of automotive castings. Since the morphology, size, and distribution of the primary Al dendrite and eutectic Mg2Si have a decisive influence on the mechanical properties of the alloy, a comprehensive analysis of AlMg5Si2Mn alloys with varying Y contents was conducted using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The influence of Y on the microstructural evolution and mechanical behavior of the cast hypoeutectic AlMg5Si2Mn alloy was studied. Experimental findings indicate that the addition of Y significantly refines and alters the morphology of both primary Al and eutectic Mg2Si in AlMg5Si2Mn alloys. Specifically, in the alloy containing 0.45 wt.% Y, the primary Al undergoes a structural transformation from a coarse dendritic morphology to finer ellipsoidal grains, with a minimum secondary dendritic arm spacing (SDAS) of 18.6 ± 1.6 μm. Simultaneously, the eutectic Mg2Si morphology transitions from a coarse lamellar structure to finer worm-like, coral-like, and fibrous forms, exhibiting a reduced average length and aspect ratio (AR) of 3.1 ± 0.4. Furthermore, the AlMg5Si2Mn alloy leads to significant improvements in mechanical performance, particularly in tensile strength. The measured average ultimate tensile strength, yield strength, and elongation are 243.3 MPa, 199.0 MPa, and 8.5%, respectively, representing increases of 19.16%, 24.6%, and 203.6% compared to the Y-free alloy. The fracture mode of the alloy fracture transitioned from brittle fracture in its unmodified condition to ductile fracture characteristics.