Numerical Prediction and Response-Surface Optimization of Narrow-Opening Micro-Pits Produced by Oblique Dual-Beam Femtosecond Laser Irradiation on Polyamide 6

Microstructures with narrow openings and enlarged internal volumes are highly desirable for durable superlubricating surfaces, as they enable increased lubricant storage and improved lubrication stability. However, conventional laser texturing typically produces tapered features with wider openings due to the intrinsic Gaussian energy distribution of laser beams, limiting the achievable internal volume. To overcome this limitation, this study investigates an oblique dual-beam femtosecond laser ablation strategy for polyamide 6 (PA6) using a mechanism-informed optimization framework integrating COMSOL-based multiphysics simulations with response surface methodology (RSM). The effects of pulse energy, incident angle, and beam separation on micro-pit morphology are systematically analyzed. Pulse energy governs ablation intensity and feature scaling; increasing incident angle suppresses depth while promoting lateral expansion; and reduced beam separation enhances depth through local energy overlap. Quadratic regression models for pit depth, inner width, outer width, and ablated area are established via a central composite design and exhibit strong predictive capability. Multi-objective optimization identifies an effective processing window of 30–40 μJ pulse energy, 20–40° incident angle, and −12 to 12 μm beam separation, enabling uniform micro-pits with narrow openings and depths of 70–80 μm. Overall, this work provides a concise and predictive framework for oblique dual-beam laser texturing for predicting and guiding the future fabrication of narrow-opening, enlarged-interior microstructures.