Study On Surface Integrity In Ultrasonic Elliptical Vibration Cutting Of Pure Iron

Pure iron curved thin-walled parts which require high precision and surface integrity,are important components in precision physics experiments.It still has many challenges about ultra-precision machining.On the one hand,pure iron is a kind of ferrous metals,so traditional machining methods like grinding,lapping and polishing are all difficult to meet demand due to low processing efficiency,surface contamination,and poor process stability.On the other hand,diamond turning can effectively improve the machining efficiency and accuracy,but diamond tool is severely worn when machining ferrous metals,which also makes it difficult to apply.Therefore,ultrasonic elliptical vibration cutting(UEVC),a new ultra-precision cutting technology,was investigated in detail in this paper.UEVC can reduce diamond tool wear effectively,so as to meet the ultra-precision machining of ferrous metals.We mainly focus on the characteristics of the cutting deformation zone,the surface morphology and the microstructure and mechanical properties of surface layer.Based on the theory of UEVC,combined with tool cutting trajectory,tool geometry,cutting parameters and ultrasonic vibrator characteristics,a mathematical model of surface geometric residual height was established.By using Matlab software,the three-dimensional surface topography was simulated,and the influence of ultrasonic vibration frequency,amplitude,cutting speed,feed rate and tool radius on the surface topography was also analyzed.Further considering the effect of tool edge wear and trajectory offset on the surface residual height,mathematical algorithms for geometric residual height were given.Experimental results shows that the minimum surface roughness Sa of ultrasonic elliptical vibration cutting is less than 10 nm.As the cutting speed and feed rate increase,the surface roughness Sa becomes larger,so we suggested that select the cutting parameters according to surface requirements and processing efficiency.Diamond tool flank wear caused squeezing friction on the machined surface in the cutting direction when cutting distance was up to 3 km,and the surface topography became unevenness.Based on the dynamic mechanical properties of pure iron and the characteristics of ultrasonic elliptical vibration cutting,a finite element model of pure iron ultrasonic elliptical vibration cutting was established in Abaqus,then some experiments was carried out.By comparing with conventional precision turning,the characteristics of the cutting deformation zone were analyzed.In the first deformation zone,the machining stress of the workpiece changes periodically with the periodic contact between tool and chip.The degree of plastic deformation of the chip is smaller,the thickness of the chip is thinner,and the temperature in the cutting zone is greatly reduced.In the second deformation zone,reversal friction between tool and chip promotes chip outflow.In the third deformation zone,the machining stress is lower and the thickness of the stress layer is thinner.Experimental studies showed that the chip fromed by UEVC was thinner than that fromed by conventional precision turning,and the bottom surface of chip was smoother,only with some slight folds.Filamentary fractures were found at chip boundary.The average cutting force is less than 0.1 N,the depth of cut,feed rate and cutting speed are all increase the cutting force including principle force and thrust force,of which the depth of cut effect is the most significant.Based on the relevant experimental technology for surface integrity,the microstructure and mechanical properties of surface layer by ultrasonic elliptical vibration cutting was studied,including hardening,machining residual stress and microstructure.Experimental results showed that the surface of pure iron by ultrasonic elliptical vibration cutting was hardened obviously,but the thickness of the hardened layer was thinner than that of conventional precision turning.The residual stress of ultrasonic elliptical vibration cutting surface was compressive stress state,but the conventional precision turning surface was mainly residual tensile stress state.Grains deformation along the cutting direction was found in machining affected layer of pure iron by ultrasonic elliptical vibration cutting,and the thickness of deformation layer was very thin,about 2 ~ 3 ?m.However,the deformation layer thickness was also up to 20 ?m in conventional precision turning.