The effect of Mg/Si mass ratio on the microstructure and mechanical properties of Al-Mg-Si cast aluminum alloys under sub-rapid solidification conditions was investigated. This study utilized four different Mg/Si ratios: 2.83, 1.91, 1.73, and 1.53. To analyze the evolution of the microstructure, particularly the second phase, various techniques were employed: optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and electron backscatter diffraction (EBSD). Additionally, thermodynamic calculations were performed using the Thermal-calc software to further understand the microstructural changes. Results show that as the Mg/Si ratio decreases from 2.83 to 1.53, α-Al grains become more uniformly distributed. Meanwhile, the morphology of the Mg₂Si phases changes from skeletal to short stick shapes with a decreasing aspect ratio. An as-cast Al-Mg-Si alloy with a Mg/Si ratio of 1.53 exhibits high strength, achieving an ultimate tensile strength (UTS) of 320.6 MPa and a yield strength (YS) of 249.9 MPa. The cast alloy with a Mg/Si ratio of 2.83 exhibits the highest elongation, reaching 5.31%. This superior elongation is attributed to the uniform distribution of Mg₂Si phases, which possess a long skeletal shape. Conversely, the alloy with a Mg/Si ratio of 1.53 demonstrates the lowest elongation, primarily due to the central concentration of Mg₂Si phases, which are characterized by their short stick shapes.

In the present work, microalloying is put forward to improve the microstructure and tensile properties of A356.2-0.15Fe (wt.%) alloy by the co-addition of trace Mn and Mg. A suitable Mn/Fe mass ratio of 0.5 is obtained for alloys with 0.15Fe. The yield strength, ultimate tensile strength, and elongation of the A356.2-0.15Fe alloy with an Mn/Fe ratio of 0.5 and containing 0.42 wt.% Mg is 179 MPa, 286 MPa, and 9.1%, respectively, which is acceptable for automotive wheel hub applications. Optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and electron-probe microanalyzer (EPMA) methods are used to characterize the microstructure of the alloys. The results indicate that Mn addition promotes the transformation of the acicular β-Al₅FeSi phase to the block-shaped α-Al(Fe, Mn)Si phase. The ratio of length/width of the Fe-rich phase in the alloy is reduced by 78.8% with an Mn/Fe ratio of 0.5 and containing 0.35 wt.% Mg, compared with that of the alloy without Mn addition and containing 0.35 wt.% Mg. The addition of Mg reduces the secondary dendrite arm spacing (SDAS) from 26.1 μm to 20.9 μm. The volume fraction of the precipitated Mg₂Si phase in the alloy containing 0.42 wt.% Mg increases by 60% compared with that in the alloy containing 0.35 wt.% Mg. The morphology transformation of the Fe-rich phase, the reduction of SDAS, and the increase in volume fraction of precipitated Mg₂Si phase comprehensively contribute to the improvement of A356.2-0.15Fe alloy. The microstructure evolution mechanism and the effect of microstructure on tensile properties are analyzed and discussed.

In the present work, the effect of heat treatment on the cyclic deformation behavior of as-extruded ZA81M magnesium alloy was investigated. Two heat treatment conditions were applied to the as-extruded ZA81M alloy: a solution treatment (T4, 653 K for 40 h and quenched with 298 K water) and a solution treatment plus artificial aging (T6, 348 K for 32 h (pre-aging at low temperature) and 453 K for 8 h (the second aging) and quenched with 353 K water). The results showed that the fine second phase precipitated after the aging treatment, the tensile yield strength of the T6-treated specimens increased, and the stress amplitude of T6-treated specimens was always higher than that of T4-treated specimens. The T6-treated specimens had a higher total strain energy density and a shorter fatigue life at a strain amplitude of 0.4%, and a lower total strain energy density and a longer fatigue life at a strain amplitude of 0.8%, compared to the T4-treated specimens. All fatigue cracks of the T4 and T6 ZA81M alloy were initiated at the second phase or along the grain boundary and propagated perpendicular to the loading direction.