Achieving ultra-low surface roughness on aluminum alloys via picosecond laser milling–polishing

Producing high-quality milled surfaces on metals, particularly aluminum alloys, using laser-based techniques remains a considerable processing challenge, especially when ultra-low surface roughness is required. In this study, a novel laser milling–polishing (LMP) process employing a 1064 nm picosecond laser is proposed by integrating laser milling and laser polishing into a single processing strategy. Under optimized conditions with a laser fluence of 45.69 J/cm2 and 425 scanning passes, the surface roughness of the aluminum alloy is significantly reduced from 1.33 μm for the untreated surface to 90 nm after LMP treatment. Surface characterization indicates an increase in the oxygen content together with enhanced microhardness after laser treatment, indicating significant modification of the near-surface layer. Based on the observed evolution of the surface morphology, composition, and mechanical properties, a plausible mechanism for ultra-smooth surface formation is proposed. The results suggest that progressive reduction in effective energy density during multi-pass scanning induces a transition from ablation-dominated removal to shallow micro-melting and rapid resolidification, promoting material redistribution and surface leveling. The proposed LMP approach, therefore, provides an effective route toward high-precision laser finishing of aluminum alloys and other thermally sensitive metals.