Surface chemistry investigation of an additively manufactured Al-Mg-Si-Zr alloy: Studies from experiments and first-principles simulation

Abstract

The surface chemistry of additively manufactured aluminum alloys plays a critical role in corrosion resistance and joining with external materials. In this work, the near-surface elemental composition of a laser powder bed fusion (PBF-LB/M) processed Al-Mg-Si-Zr alloy was characterized by glow discharge optical emission spectroscopy GDOES depth profiling. The measurements reveal pronounced Mg enrichment within the near-surface region extending to approximately 10 μm, consistent with the characteristic scale of surface roughness, together with an increase in oxygen concentration, whereas Al, Si, and Zr approach stable bulk-like levels at greater depths. To understand this observation on the atomic scale, first-principles calculations based on density functional theory (DFT) were performed on low-index Al surfaces. The calculated results show a strong thermodynamic driving force for Mg surface segregation, with diffusion energy differences ranging from approximately -0.30 eV to -0.41 eV, while Zr exhibits a pronounced preference for remaining in the bulk matrix. Vacancy migration calculations further demonstrate that full structural relaxation substantially reduces the migration barrier for Mg to below that of Si, which makes Mg diffusion kinetically highly favorable. Moreover, the presence of adsorbed surface oxygen dramatically promotes the tendency of Mg to diffuse toward the surface, which lowers the diffusion energy difference of Mg to as much as -3.0 eV. This promotes the formation of a locally reconstructed Mg-O-Al coordinated precursor structure accompanied by localized electron transfer shown by an electron localization function analysis.