Thermal Deformation and Hole Density Effects in High Power Picosecond Laser Drilling of Stainless Steel Microhole Arrays

Abstract High-density microhole fabrication in thin metallic sheets using high power picosecond lasers is challenging because cumulative thermal loading can compromise hole geometry, surface integrity, and dimensional stability. This study experimentally investigates the effects of laser power, pulse exposure, and hole density on the quality of microhole arrays produced in 0.5 mm-thick stainless steel using a picosecond laser with a pulse duration of 10 ps. Through holes were drilled at average laser powers of 59, 134, and 178 W, with drilling times ranging from 20 to 100 ms. The influence of hole density was evaluated using area ratios of 12%, 15.8%, and 21.5%, corresponding to interhole spacing of 400, 350, and 300 µm, respectively. Hole morphology, taper, recast formation, and out-of-plane deformation were characterised using optical microscopy, scanning electron microscopy, and focus-variation microscopy. The results reveal a strong dependence of hole quality on laser energy input and array density. An intermediate drilling time of 40 ms (16,000 pulses per hole) provided the lowest taper angles, reaching approximately 2.8° at 178 W. Increasing the area ratio from 12% to 21.5% increased peak-to-valley deformation from 1.08 to 1.45 mm, accompanied by more pronounced recast formation, sidewall irregularities, and hole distortion. These findings demonstrate that thermal accumulation and sheet deformation are important process limitations in dense-array picosecond laser drilling. The results provide practical guidance for optimising laser parameters and inter-hole spacing to improve the dimensional accuracy and surface integrity of microhole arrays in thin metallic components.