碘化铯晶体的力学性能和超精密车削研究

Mechanical Properties and Ultra-precision Turning of Cesium Iodide Crystal

目的研究碘化铯(CsI)晶体(110)晶面的力学性能和以及车削参数对超精密车削表面粗糙度的影响。方法分别采用纳米压痕和霍普金森压杆(SHPB)试验,获得并分析CsI晶体(110)晶面在准静态下和高应变率下的力学性能。采用单点金刚石车削(SPDT)的方法在不同的方向和车削参数对晶体进行超精密加工,并使用白光干涉仪、测力仪和红外热像仪分别测量超精密车削过程中已加工表面的粗糙度Ra、切削力和切削温度。结果CsI晶体在压痕过程中主要发生塑性变形,且无明显的脆性裂纹,其(110)晶面的维氏硬度约为100 MPa。当应变率从6000 s^(–1)提高8000 s^(–1)时,晶体的屈服强度提高了7 MPa。在试验中,沿着270°方向车削,可以获得Ra为20 nm以下的表面粗糙度。沿着该方向使用10°前角的金刚石车刀、转速为2000 r/min、进给速度为4μm/r、切削深度为2μm时,可以获得最好的表面质量,平均表面粗糙度Ra为8.53 nm,最大表面粗糙度Ra为11.02 nm。结论CsI晶体具有较强的塑性,且硬度低,高应变率下,材料的强度和硬度明显提高。通过提高转速即切削速度,增大超精密车削过程中的材料应变率,改善了软塑性材料的可加工性,使CsI晶体的表面粗糙度降低了80%。结合优选的车削方向、刀具前角、进给速度和切削深度等其他车削参数,获得了Ra在10 nm以下的光滑表面。

This work aims to reveal the mechanical properties of CsI(cesium iodide)crystal on(110)plane and the influence of turning parameters on surface roughness in an ultra-precision turning process.Firstly,the mechanical properties under quasi-static and high strain rates were obtained and analyzed by nano-indentation and split Hopkinson pressure bar(SHPB)experiments,respectively.Then,single point diamond turning(SPDT)experiments on crystal were conducted in different cutting orientations with different turning parameters.Meanwhile,white light interferometer,dynamometer and infrared thermal imager were utilized to measure the machined surface roughness,cutting force,and cutting temperature during the ultra-precision turning process,respectively.Experimental results show that CsI crystal mainly undergoes plastic deformation during the indentation process without obvious brittle cracks and the elastic recovery coefficient is 0.03.The Vickers hardness of(110)crystal plane is about 100 MPa.These results indicate that CsI crystal is one kind of typical soft and ductile materials.In SPHB experiments,when strain rate increases from 6000 s^(–1)to 8000 s^(–1),the yield strength of crystal increases by 7 MPa,which proves that the hardness and strength of this material can be improved by high strain rate.In the turning experiments,overall surface roughness below 20 nm was obtained by turning along the orientation of 270°,while along the turning orientations of 0°,90°,180°,surface roughness Ra of some positions on the machined crystal reached 80 nm.The results of response surface experiment along this direction indicate that the best surface quality can be obtained when the tool rake angle is 10°,rotational speed is 2000 r/min,feed rate is 4μm/r,and the depth of cut is 2μm.The mean surface roughness is Ra 8.53 nm,and the maximum surface roughness is Ra 11.02 nm.When utilizing a 0°rake angle tool,and the rotational speed is 3000 r/min,feed rate is 6μm/r,and the depth of cut is 6μm,a finish surface can also be obtained.The mean surface roughness is Ra 9.07 nm,and the maximum surface roughness is Ra 17.36 nm.The negative rake angle tool is not suitable for turning the CsI crystal.As for the influence of turning parameters,the surface roughness is mainly affected by the rotational speed(cutting speed)in the experiments,and higher cutting speed is beneficial for better machined surface.However,the impact of feed rate and depth of cut on the surface roughness is not significant.The machining efficiency can be raised by relative high feed rate and large depth of cut in the rough machining process.Through above experiments and analysis,it can be concluded that the CsI crystal is a kind of ductile material with low hardness.The strength and the hardness increase significantly under high strain rate.Increased cutting speed and strain rate in the ultra-precision turning process improved the machinability of soft and ductile materials,and the surface roughness of the CsI crystal was reduced by 80%.A smooth surface below Ra 10 nm was obtained by combining with other optimized turning parameters,such as the turning orientation,tool rake angle,feed rate and depth of cut.