MECHANICAL PROPERTIES OF Aluminium-Magnesium-Silicon ALLOYS FOR AUTOMOTIVE AND AEROSPACE APPLICATIONS: A DENSITY FUNCTIONAL THEORY-BASED STUDY

Abstract

Aluminum and its alloys are utilized in various purposes, including aircraft skin, cookware, building cladding, train carriages, and electrical lines. This is attributable to their advantages, which encompass low density (physical density of 2.7g/cm3, which is approximately a third that of steel), non-corrosivity, formability, good thermal and electrical conductivity, and availability. Moreover, its non-corrosivity diminishes with alloying. Several studies, mostly experimental, have been done on aluminum magnesium-silicon (Al-Mg-Si) alloys (6xxx series). Mechanical properties, especially strength and ductility, have been explored in those earlier studies. However, other mechanical properties such as bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, Pugh’s ratio, creep, and resilience have not been explored extensively. While this study touched on ductility and hardness, it also explored the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, Pugh’s ratio, and yield strength of Al-Mg-Si alloys. The main objective of this study was to determine the alloy composition that could yield stronger, harder, and more ductile materials that are appropriate for both aerospace and automotive industries by making use of density functional theory (DFT) calculations. The modeling of the structures was done using an aluminum cell as the starting structure, whose crystallographic information file was downloaded from the Crystallography.net website. It was then transferred to Burai software, where the unit cell was visualized and then transformed into 3 × 3 × 3 supercells containing 108 atoms, after which the supercells were alloyed with the appropriate number of Mg and Si atoms. Nine structures of Al-Mg-Si alloys with different percentages of Al, Mg, and Si were investigated. Structural optimization of the alloyed supercells was done as a preliminary to the study. The variable-cell relaxation was done using the Brodyden-Fletcher Goldfarb-Shanno (BFGS) algorithm. The stress-strain method was employed in the calculation of elastic stiffness constants, from which mechanical properties were obtained. The elastic constants were calculated using Density Functional Theory (DFT) with the Perdew-Burke-Ernzerhof for Solids (PBESOL) functional, as implemented in the Quantum Espresso software. This work has conclusively demonstrated that the Si/Mg ratio is a pivotal determinant of the mechanical properties of Al-Mg-Si alloys. The optimal parameters identified in this study include a density of 2762 kg/m³, a bulk modulus of 83.3 GPa, a shear modulus of 34.4 GPa, a Vickers hardness of 2.79 GPa, a Poisson’s ratio of 0.413, a Pugh’s ratio of 5.42, and a yield strength of 8.38 GPa. The ideal Si/Mg ratio for the majority of characteristics is 4.5. The alloys exhibiting these optimal features are suitable for industrial applications that necessitate such characteristics, including aircraft skins and mining equipment, particularly those with maximum hardness and yield strength. Their superior ductility enables their application in the fabrication of motor vehicle components and rail carriages. The alloys' low density renders them appropriate for manufacturing airplane components, as they enhance load capacity by minimizing part weight.