Surface-specific roughness modeling and mechanism interpretation in LPBF AlMgScZr alloy with gas-flow effects

Surface roughness in laser powder bed fusion (LPBF) is surface-dependent because top and side surfaces are formed under different thermal and geometrical conditions. In this work, 100 LPBF-fabricated AlMgScZr samples were produced using a five-factor orthogonal design to investigate surface-specific roughness and gas-flow-related position effects. Top-surface roughness was analyzed using hatch-processing parameters, whereas side-surface roughness was examined using contour-scanning parameters under a scan-then-contour strategy. Surrogate models and response maps were established to reveal parameter–roughness relationships and identify low-roughness processing regions. The top surface exhibited much higher roughness than the side surfaces, with Ra ranging from 13.74 to 44.57 μm, compared with 4.38 to 9.34 μm for the four side faces. For the top surface, roughness was mainly governed by the stability of hatch melt tracks, inter-track overlap, and local surface disturbance, with hatch laser power and scanning speed playing dominant roles. The gas-flow effect on the top surface was weak within the present range. In contrast, side-surface roughness depended mainly on the initial boundary condition before contour scanning, particle retention, remelting continuity, and contour-remelting stability. Higher gas-flow velocity generally reduced side-surface roughness by limiting particle retention, but its effect varied with face orientation relative to the gas-flow and powder-spreading directions. Cross-sectional observations showed that the sidewall was formed by stacked boundary melt pools and a final remelted boundary layer, rather than by direct exposure of regular melt-pool boundaries. These results provide a surface-specific framework for understanding and controlling LPBF roughness in AlMgScZr alloy within the investigated processing window.