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研究采用由国晶辉公司由CVD法生产的ZnSe多晶作为工件,工件直径和厚度分别为42 mm和5 mm。ZnSe晶体材料性能如表1所示。在超精密车床(IL300,Inolite)上进行了槽切和端面车削实验。在生产加工中选用大圆弧半径刀具会与非球面、衍射面等功能表面发生严重干涉,无法加工指定面型且刀具磨损严重[20],因此文中选用小圆弧半径刀具进行。为确定ZnSe晶体脆塑转变深度,分别采用两把不同前角的天然单晶金刚石刀具以分析刀具前角对脆塑转变深度的影响,ZnSe晶体材料特性如表2所示[21]。首先将工件装夹在弹性夹具上,随后弹性夹具通过真空吸附固定于空气主轴。通过万分表反馈调节工件径向圆跳动,确保切削工程平稳,车削实验装置及工件局部放大分别如图1(a)和1(b)所示。
Material ZnSe Grain size (typical) /μm 70 Yang’s modulus /GPa 79 Hardness /GPa 1.2 Fracture toughness/MPa·m0.5 0.9 Poisson ratio 0.28 Density /g·cm−3 5.3 Table 1. Material properties of ZnSe crystal[21]
No. Parameters Values 1 Nose radius/mm 1.15 (tool Ⅰ), 1.12 (tool Ⅱ) 2 Rank angle/(°) −25 (tool Ⅰ), 0 (tool Ⅱ) 3 Clearance angle/(°) 10 (tool Ⅰ, Ⅱ) 4 Clearance angle/(°) 10 (tool Ⅰ, Ⅱ) 5 Cutting speed/m·s−1 8.3×10−3 (PCT), 2.88 (FTT) 6 Depth of cut/μm 0-2 (PCT), 3 (FTT) 7 Feed rate/μm·rev−1 0.5, 1, 1.5 and 2 (FTT) Table 2. Cutting tool parameters and experimental condition
Figure 1. Experimental step-ups and schematic of cutting experiments. (a) the ultra-precion lathe; (b) enlarged view of workpiece; (c) the machined groove of plunge-cutting test; (d) schematic of machined surface for face-turning test
槽切实验(Plunge-cutting test, PCT)如图1(c)所示,刀具以恒定切削速度在长度为2 mm的切削路径上切削深度由0 nm线性增加到2 μm。已有研究表明[9],切削速度对脆塑转变深度的影响最小。考虑到机床稳定性,将切削速度设置为500 mm/min (8.3×10−3 m/s),分别进行5次试验以排除偶然因素干扰。进给率对工件表面和亚表面损伤有极大影响[21],相较而言切削深度的影响可以忽略不计[18]。因此采用单因素实验法进行端面车削实验(Face-turning test, FTT)以确定进给率对工件表面质量及材料去除机理的影响。在工件边沿至工件回转中心依次设置四个切削区域其进给率分别为0.5、1.0、1.5、2.0 μm/rev,其中每个切削区域宽度为3 mm,切削区域间距均为1 mm,切削区域分布如图1(d)所示。为排除线速度对实验结果的影响,通过调节主轴转速以保持所有切削区域线速度恒定。考虑机床超精密切削稳定性须控制主轴转速低于5000 RPM即最内侧切削区域主轴转速,因此最终确定切削线速度为2.8 m/s。超精密切削实验均在24 ℃、54%湿度和标准大气压下进行,刀具参数及实验条件如表2所示。
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采用场发射扫描电子显微镜(FESEM,FEI Nova NanoSEM 450)观察已加工表面和切屑形貌。工件三维形貌信息采用白光干涉仪(WykoNT 9100,Vecco)测量,通过分析工件表面轮廓信息以量化评估工件表面质量。为揭示ZnSe晶体材料去除机制,主要针对不同进给率下ZnSe晶体已加工表面和切屑进行物相结构分析。因此选用激光共聚焦拉曼光谱仪(HR 800, HORIBA)进行无损检测,其激光输出功率为50 mW,波长为532 nm,曝光时间为5 s,选用100倍物镜观测,激光光斑聚焦尺寸约为1 μm。
Study on the material removal mechanism of ZnSe crystal via ultra-precision diamond turning
doi: 10.3788/IRLA20200403
- Received Date: 2020-10-22
- Rev Recd Date: 2020-11-12
- Publish Date: 2021-06-30
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Key words:
- ZnSe crystal /
- ductile-brittle transition depth /
- ductile regime machining /
- surface defects formation /
- material removal mechanism
Abstract: ZnSe crystal has been widely used in infrared imaging and laser systems. However, as a typical soft-brittle material, the material removal mechanism in ultra-precision diamond turning process has not been clarified, it is still challenging to obtain nano-smoothed surface. In the study, the effect of tool ranke angle on ductile-brittle transition depth of ZnSe crystal have been investigated through novel plunge-cutting tests. The ductile regime machining model was revealed by comparing the maximum undeformed chip thickness and ductile-brittle transition depth. With the aid of FESEM, white light interferometer, and Raman spectrometer, the effect of feed rate on surface roughness, surface quality, phase transition and subsurface damage were systematically investigated. The surface defects formation mechanism was proposed. Furthermore, the material removal mechanism of ZnSe crystal in ultra-precision diamond turning process have been revealed.