Lei Hongqin, Cheng Jian, Yang Dinghuai, Zhao Linjie, Chen Mingjun, Wang Jinghe, Liu Qi, Ding Wenyu, Chen Guang
State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin 150001, China.
School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China.
Materials (Basel). 2022 Oct 22;15(21):7407. doi: 10.3390/ma15217407.
When serving in extremely high-power laser conditions, KHPO (KDP) surfaces are susceptible to incur laser damage points (also known as defects). Using micro-ball end milling cutters to repair and remove the pre-existing damage points on the flawed KDP crystal surface is the most effective method to control the growth of laser damage points on KDP crystal surfaces and prolong their service life. However, there are various forms of micro-defects (such as pits, scratches and brittle fractures) around the laser damage points on KDP crystal surfaces which possess remarkable effects on the micro-milling repair process and consequently deteriorate the repair quality. In this work, combined with nano-indentation experiments, elastic-plastic mechanics and fracture mechanics theory, a constitutive model considering the anisotropic property of KDP crystals and a three-dimensional (3D) finite element model (FEM) were established to simulate the cutting force and surface topography involved in the ball-end milling repairing of flawed KDP crystal surfaces. Besides, the micro-milling experiments were conducted to evaluate the change of cutting force and machined surface quality in the presence of micro-defects with various feed rates. The results show that micro-defects would induce the fluctuation of cutting force and a change of the undeformed cutting thickness (UCT) in the process of repairing the damage points on the crystal surface, which would lead to the brittle-ductile transition (BDT) and affect the machined surface quality. The machined surface quality was found to be deteriorated by the pre-existing micro-defects when the UCT was small (the UCT was less than 375 nm). On the contrary, brittle mode cutting in the local area can be transformed into ductile mode cutting, resulting in an improvement of repaired surface quality that is exhibited by the cutting force and microtopography. This work has great theoretical significance and engineering practical value for the promotion and application of micro-milling repairing technology in the practical manufacturing and operation of KDP optics applied to high-power laser systems.
在极高功率激光条件下工作时，磷酸二氢钾（KDP）晶体表面容易出现激光损伤点（也称为缺陷）。使用微型球头铣刀修复并去除有缺陷的KDP晶体表面预先存在的损伤点，是控制KDP晶体表面激光损伤点生长并延长其使用寿命的最有效方法。然而，KDP晶体表面激光损伤点周围存在各种形式的微缺陷（如凹坑、划痕和脆性断裂），这些微缺陷对微铣削修复过程有显著影响，进而降低修复质量。在这项工作中，结合纳米压痕实验、弹塑性力学和断裂力学理论，建立了考虑KDP晶体各向异性特性的本构模型和三维（3D）有限元模型（FEM），以模拟有缺陷的KDP晶体表面球头铣削修复过程中的切削力和表面形貌。此外，进行了微铣削实验，以评估在不同进给速度下存在微缺陷时切削力和加工表面质量的变化。结果表明，微缺陷会在修复晶体表面损伤点的过程中引起切削力波动和未变形切削厚度（UCT）变化，这会导致脆塑转变（BDT）并影响加工表面质量。当UCT较小时（UCT小于375 nm），预先存在的微缺陷会使加工表面质量恶化。相反，局部区域的脆性模式切削可转变为延性模式切削，从而使切削力和微观形貌所显示的修复表面质量得到改善。这项工作对于微铣削修复技术在应用于高功率激光系统的KDP光学元件实际制造和运行中的推广应用具有重要的理论意义和工程实用价值。
