Micro Thermal Mechanical Coupling Analysis Of Hardened Bearing Steel GCr15 In High Speed Precision Machining

High speed machining is a large plastic deformation process with high temperature,high pressure and high strain rate,which belongs to a typical thermal-mechanical coupling problem.The part machining is a process of changing the surface and near-surface properties of the material,what the most concerned about in this process is not the whole material properties,but the material near surface(or sub-surface)response characteristics.In precision cutting,the cutting edge condition,workpiece surface state and tool-chip micro contact characteristics have a great influence on the cutting characteristics because of the thin thickness of the cutting layer,these also directly affect the tool life and the machined surface quality.In this thesis,on the basis of the absence of a preset cutting separation layer and considering the influences of the cutting edge radius,material elastic-plastic deformation,thermal mechanical coupling,two-dimensional and three-dimensional models of high speed precision cutting of hardened bearing steel GCr15 are established,and by establishing the cutting tool model with restricted rake face length,the effects of the cutting edge blunt or not,the rake angle and the length of the rake face on the chip shape,cutting force,cutting temperature and energy dissipation during the cutting process are analyzed.At the same time,using W-M fractal function,the fractal rough cutting model of cutting tool surface and fractal rough cutting model of workpiece surface are established respectively combined with experimental results of workpiece and tool surface roughness data in relevant literature.Furthermore,the frictional contact characteristics between the tool and chip rough surface are analyzed dynamically from the point of view of microscopic thermodynamics and energy.The results show that:Under high speed cutting,regardless of whether the cutting edge is blunt circle or not,the chips are serrated,but the chip curl of the blunt circular tool is more serious,and the corresponding average value stress on the surface of the workpiece has been relatively large.Chip shape,stress and temperature distribution,energy dissipation and so on are closely related to the rake angle and the rake face length,the chip morphology coincides with the experimental results in relevant literatures.With the increase of rake angle,the chip may invade the workpiece,in the actual situation it is shown as the extrusion slippage between the chip and raw workpiece surface,for the hardened material,when the chip is extruded to a certain extent,it will break open.Therefore,the chip breaker must be designed reasonably for the large rake angle of tool.When the rake face length is less than the cutting depth,the tool surface "hot spot" appears at the corner of the rake face and the chamfer;if not,the maximum temperature of the tool surface appears at a certain distance from the cutting edge on the rake face.Compared to the ideal rake face of cutting tool,due to the existence of the random asperities on the rake face of the actual rough tool,the chip crimp degree is bigger,the tool surface produces more local "hot spots",the surface temperature fluctuations more intense,the distribution of the corresponding surface localized high stress points is more extensive and the stress value is greater.These local high thermal-stress points can easily cause the tool surface wear,thereby reducing tool life.Compared to the ideal flat workpiece,the fractal rough surface on the upper of the workpiece increases the chip formation rate,resulting in increasement of the strain rate and the maximum temperature of the cutting layer,the chip internal shear instability increased.In high speed cutting,the chip breaks into pieces,and the fracture is at the bottom of the fractal rough surface.The results of the study have some theoretical significance for the design of the tool and the improvement of the surface quality.A good foundation is established for further understanding of the chip breakage and the micro thermal-mechanical mechanism of the rough surface,and to explore the friction and wear of the tool-chip rough surface and the essential characteristics of the cutting.