Optimized helical drilling of deep high-quality microholes in stainless steel using ultrafast laser and real-time depth monitoring

High-quality microholes in thick materials are essential for advanced applications such as cooling holes for tools and turbine blades, production of spinnerets, and lubrication holes for mold making. However, achieving sufficient drilling depth in thick materials remains challenging due to decreasing or stagnating depth progress and long drilling times, especially during helical drilling. While deep holes can be obtained at high fluence levels, this often results in damage near the hole inlet. Real-time insight into the current drilling depth is required to optimize the drilling process. In this work, depth-monitored helical drilling of microholes was carried out using a machining setup consisting only of industrial components. An ultrafast laser with a wavelength of 1030 nm, a pulse duration of 0.9 ps, and pulse energy up to 1.5 mJ was used for the experiments. The drilling depth was monitored online during the drilling process using optical coherence tomography. Strategies such as beam rotation via helical motion, energy ramping, pushing process limits (e.g., repetition rate and energy ramp slope), and focal position shifting were explored to optimize the drilling process. The optimized strategy enabled high-quality microholes with an aspect ratio of 13 and a low taper angle of 0.8° to be drilled in 4 mm thick stainless-steel samples with short drilling times of 60 s/hole.