Multi-physics investigation on the influence of impurity elements and scanning parameters on the surface topography in laser powder bed fusion

Surface topography critically governs the mechanical performance—particularly fatigue resistance-of components fabricated via laser powder bed fusion (LPBF). To advance the understanding of how parameters govern surface formation, a comprehensive multi-physics model was developed, integrating random powder deposition (Discrete Element Method), laser-material interaction and melt pool (MP) dynamics (Finite Volume Method). The framework integrates surface-active elements with a strongly coupled laser tracing model, leading to two critical advances: first, it provides new insights into the role of surface-active elements in MP dynamics; second, it successfully reproduces, the complex formation mechanisms of varying surface topography in multi-track scans by constant surface-active elements: 1) impurity effects: explicitly modeling sulfur and oxygen interactions within the MP, and reveal these elements substantially degrade surface quality through complex thermo-physics mechanisms. Quantitative analysis reveals sulfur exerts a 2.74× stronger influence on surface peak formation than oxygen when concentrations decrease tenfold (S: 0.3 → 0.03%; O: 0.1 → 0.01%), establishing its dominance in surface topography evolution. However, at ultralow oxygen‑sulfur concentrations, surface topography converges to identical configurations. 2) scanning parameter effects: a coupled analysis of scanning parameter effects-encompassing strategy, speed, and hatch spacing-on non-uniform temperature field evolution is conducted, evaluating-i): energy absorptivity dynamics, ii) surface-induced porosity, iii) inter-track interaction mechanisms, iv) Plateau-Rayleigh instabilities and v) interaction of melt rate, melt flow velocity, inertia and surface tension on surface topography. This study reveals that localized peaks within the MP disrupt laser beam reflection, altering MP dynamics. Furthermore, irregular surface peaks and valleys contribute to the formation of various surface pore types. Critically, during multi-track process, the coupling between scanning strategy and speed generates heterogeneous thermal fields that significantly alter subsequent surface evolution. Our paper provides theoretical guides to help users of additive manufacturing optimize the topography of parts.