Evolution of brittle-ductile transition and size effect in the micro-milling repairing process of soft-brittle KDP crystal with surface defect

Fly-cut surface defects of KDP crystals can rapidly expand with high-energy laser irradiation, severely reducing the surface quality. Repair technology based on ball-end milling can remove the pre-existing micro-defects and control their growth behavior. This work establishes a calculation model of specific cutting force (SCF) considering the defect dimension. Through a finite element model and delicately designed experiments, it is found that the surface defect can change the uncut chip thickness on the milled surface, which can lead to the evolution of brittle-ductile transition and size effect characterized by cutting force and surface morphology. When the defect depth (d_s) is less than 8 μm, the brittle mode cutting occurs, and the SCF tends to be stable. As the d_s is between 8 μm and 20 μm, the milling process turns into ductile mode cutting, and the size effect could be disregarded due to the shearing removal. Moreover, the size effect and the ploughing phenomenon are critical for the nonlinear rise of the SCF when the d_s is larger than 20 μm. Therefore, the repair strategy for KDP crystals with surface defects should be developed based on the defect dimension to avoid the size effect and brittle removal mode.