Research On Single Point Diamond Machining Performance Of Titanium Alloy Based On Cryogenic Technology

TC4 titanium alloy is a kind of material with excellent physico-mechanical properties,biocompatibility,and corrosion resistance.However,TC4 titanium alloy is also characterized by low thermal conductivity,small elastic modulus,large processing viscosity,large elastic recovery,high temperature at the cutting zone and profound tool wear during processing,which make it difficult to obtain a high-quality machined surface.Nowadays,there is increasing demand for titanium alloy components with nanoscale surface roughness and micro-nano structured functional surfaces.Lacking of investigation of the ultra-precision machining technology of titanium alloy,especially under the cryogenic processing condition,has led to the conflict between the low-level processing techniques and the actual needs in China.Single-point diamond turning is a superior ultra-precision processing technology,which can be used to manufacture titanium alloy workpieces with sub-micron-level shape accuracy and nanoscale surface roughness.During the turning process of titanium alloy,the cooling and lubrication method,the action site of the lubricant,the cooling source temperature,and the cutting speed will significantly affect the machining performance,which will affect the tool life and the quality of the machined surface.In order to gain a comprehensive understanding of the effects of these factors on the machining performance of single-point diamond tuning of titanium alloy,and explore their mechanisms of action,our team conduct a series of cutting experiments based on cryogenic machining technology.The results of our experiments are listed as follows.Abaqus,a finite element analysis software,was used to simulate the cutting process of titanium alloy at conventional scale and micro-nanoscale respectively.The results show that both cryogenic nitrogen wind(CW)and combination of minimum quantity lubrication(MQL)and cryogenic nitrogen wind(CW+MQL)can effectively reduce the cutting temperature and cutting force.Compared with other cooling methods in experiments,CW+MQL demonstrated the highest cooling efficiency.A set of cryogenic nitrogen gas generating device using liquid nitrogen as refrigerant has been developed.Nitrogen gas generated from this device meets the requirement of ultra-precision machining experiments and helps in creating a continuous and stable nitrogen-protected cryogenic machining environment for the experiments of titanium alloy.When the CW nozzle and the MQL nozzle are pointed at the rake face simultaneously,the best surface quality can be obtained,indicating that high cooling efficiency and lubricity are essential for improving the processing performance of titanium alloy.Considering the ploughing effect,elastic recovery,plastic lateral flow and thermal softening effect of the machining process systematically,we found that the elastic recovery caused by the ploughing effect and the plastic lateral flow caused by the thermal softening effect,together form the M-shaped structural feature of the micro-surface of the machined titanium alloy.Based on this M-shaped feature,the elastoplastic deformation mechanisms of titanium alloy turning under different processing conditions were studied and the results appear as shown below: lowering the processing temperature can reduce elastic recovery and limit plastic lateral flow;At the spindle speed of 50 rpm,heat generation and thermal softening effect were low and elastic recovery became the key factor affecting the processing quality,and the surface roughness achieved either under the condition of CW or CW+MQL was significantly lower than those from MQL method,indicating that the cryogenic processing condition can significantly reduce the elastic recovery and improve the material removal rate;at the speed of 2000 rpm,the obtained surface roughness was low because the high thermal softening effect reduced the shear strength of the material and made it easier to cut.However,the high thermal softening effect would promote plastic lateral flow at the cutting edge,which is not conducive to obtaining a high-quality machined surface.The good lubrication effect provided by MQL can isolate the tool from the chip,reduce the bonding force between them,and significantly reduce the high friction coefficient originated from adhesion of the materials on the tool surface and the high strain hardening effect caused by the built-up edge,thereby reducing the surface hardness of the machined surface.Cryogenic processing conditions weakened the effect of the lubricant,leading to an increase in the bonding force between the tool and the alloy workpiece.The increasing bonding force would enlarge the size of the built-up edge and produce a stronger plowing effect,thereby increasing elastic recovery and further exacerbating the friction effect.In addition,the cryogenic machining technology would enhance work hardening and increase the surface hardness of the machined surface,resulting in higher friction and shear forces.Experimental results show that three methods are in the descending order of degree of tool wear: CW+MQL,MQL,CW.Distinctions among the tool wear degrees were resulted from the effective lubricating film produced by MQL and the built-up edge from CW cooling method,which were able to provide protection for the cutting edge of the tool.