Balancing surface quality and machining efficiency in recycling process achieved by micro ball-end milling of flawed potassium dihydrogen phosphate optics

In the inertial confinement fusion experiments, under high-power laser irradiation , the dimension of micro-flaws on the potassium dihydrogen phosphate (KDP) surface can expand dramatically, leading to the failure of optical components. Conical mitigation pits (CMP) processed by a ball-end cutter are the preferred microstructures to eliminate surface flaws and recycle flawed KDP optics. However, due to the presence of numerous surface flaws, and deliquescent nature of soft-brittle KDP optics, obtaining high-quality and high-efficiency CMP surfaces in dry milling poses an urgent challenge. In this work, based on the finite element model , the optimal cutting edge radius ( R e ) of a ball-end cutter is determined to be 1.8 μm in accordance with the maximum tensile stress , stress distribution morphology, and cutting force. Moreover, using a ball-end cutter with the optimal R e , CMP preparation experiments are conducted to establish the initial population of non-dominated sorting genetic algorithm (NSGA-II). The prediction models of processing time ( t ) and surface roughness ( Sa ) based on the NSGA-II are developed, with average relative errors reach 2.5 % and 10.4 %, respectively. Besides, considering the demand of the recycling process under various working conditions, the quality- and efficiency priority solutions are obtained by fast non-dominated sorting algorithm and elite strategy. Under the premise of ensuring the optimal CMP surface quality, the recommended layer milling allowance, feed speed, spindle speed , and tool mark interval are 0.5 μm, 0.4 mm/s ∼ 0.9 mm/s, 4.3 × 10 4 r/min ∼4.6 × 10 4 r/min and 5 μm for acceptable repair efficiency. Compared with previous reparative processes, the optimal combination of micro-milling parameters obtained by machine learning reduces the average t for a single conical mitigation pit (with a Sa less than 30 nm) from 360 s to 278 s, achieving a reduction of 23 %. This work can provide technical supports and engineering application values for achieving a high-quality recycling process and increasing the laser damage resistance of flawed KDP optics.