Defocus-induced energy redistribution and surface morphology control in multimode laser cleaning of rusted carbon steel

ABSTRACT Defocus is generally regarded as an alignment error in laser cleaning, but it can also regulate the spatial energy input of multimode pulsed lasers. In this study, a defocus-corrected multimode beam model was established by representing a flat-top laser spot as a regular array of Gaussian sub-beams with uniformly partitioned pulse energy. Laser cleaning experiments were conducted on naturally rusted 45# carbon steel at different defocus distances, and the surface response was characterized by optical microscopy, areal surface roughness measurements, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The calculations showed that defocusing broadened the effective irradiation region, reduced the peak fluence, and smoothed the edge fluence gradient. The measured cleaning-track widths at defocus distances of 0-36 mm were bounded by the effective widths predicted using iso-fluence levels of 1.0 and 1.1 J/cm 2 , constraining the operational rust-removal threshold to approximately 1.0-1.1 J/cm 2 under the present conditions. Under focused irradiation, the calculated peak fluence reached approximately 4.40 J/cm 2 , causing pronounced substrate remelting and resolidification. At D= 12 mm, the peak fluence decreased to approximately 3.42 J/cm 2 ; the original grinding traces were retained, and no oxygen was detected at the selected EDS point, indicating effective non-remelted cleaning in the examined region. Cross-sectional SEM and EDS mapping revealed a sharp oxide-removal boundary under focused irradiation and nonuniform residual oxides near the low-fluence track edge at D= 18 mm. Appropriate defocus therefore improves pulse-overlap continuity and balances oxide removal against substrate protection.