The detailed study of surface morphology evolution in copper under moving pulsed laser ablation considering thermal-fluid-solid coupling effects

Abstract

Nanosecond lasers are widely used in surface modification of materials, and research into their ablation mechanisms remains ongoing. In this study, copper was selected as the research object, and a finite element model (FEM) coupling heat transfer and fluid flow was established using the level set method. The nanosecond laser ablation process under movement was studied in detail. The effects of thermal accumulation from multi-pulse moving lasers on the morphology of ablation craters were analyzed, along with the variation patterns of crater morphology under different average powers, pulse frequencies, and scanning speeds. The results indicate that (a) the increase in average power and the decrease in pulse frequency lead to higher energy density, resulting in a smaller recast layer, a larger heat-affected zone (HAZ), deeper craters, larger crater radii, and higher crater rims. Additionally, subsequent pulses have an increasing impact on the morphology of previously formed craters, potentially leading to crater tilt defects. (b) In operating conditions (OC) 1, 2, 7, and 8, copper has already melted and vaporized by the end of the first pulse. During solidification, the crater size gradually decreases, and small rims form on both sides of the crater due to Marangoni forces and recoil pressure. The solidification process lasts 11.3 times longer than the melting process. (c) The ablation threshold of copper ranges between 12.73 and 19.10 J/cm². A multiple linear regression fit yielded a relationship between the crater depth and energy density as well as the overlap rate: h_d = -8.78 + 0.68ψ + 0.60δ, which accurately predicts crater depth. This provides a theoretical foundation for predicting the crater morphology in moving pulsed laser ablation (PLA) and optimizing laser processing parameters.