Investigation on machining performance of soft-brittle KDP crystals with surface micro-defects in the ball-end milling repairing process

Micro ball-end milling technology is an excellent repair strategy for economically removing surface micro-defects of large-scale KDP optics in the high-power laser devices. However, it is an urgent challenge for defective KDP crystals to achieve ultra-smooth repaired surface in ductile mode cutting, because pre-existing surface defects with different geometry and dimensions can affect machining performance during the reparative process by changing the uncut chip thickness (UCT). Therefore, the influence of various surface micro-defects on the machining performance is systematically investigated from the perspectives of the UCT, tensile stress, cutting force, and surface quality in this work. Interestingly, the plastic scratches and fracture pits could transform the brittle mode cutting into a ductile removal process by increasing the UCT, showing a decrease in the brittle fractures, the maximum tensile stress and the fluctuation of cutting force, indicating that the ductile removal ability can be improved. While, the effect of the surface protuberances on the repair quality is opposite. Besides, although the convex scratches and the cracks could cause a slight increase in the UCT, it has almost no impact effect on the material removal mode. More interestingly, for the fracture pits with specific dimensions, when it is not completely removed, the growth behavior of defect dimensions occurs during the reparative process due to the action of mechanical forces, reducing the surface quality. Consequently, the above work provides significant theoretical importance and engineering application value to formulate corresponding repair strategies for various surface defects to obtain the ultra-smooth surface of optical components.