| Polycrystalline diamond compact(PDC)is synthesized by the polycrystalline diamond(PCD)and cement carbide(YG)through the ultra-high pressure and high temperature technology.As its excellent properties such as high hardness,great strength and toughness,the PCD tool is widely used in the aerospace,integrated circuit,semiconductor,automotive,and wood processing industries.In recent years,the cemented carbide tool has been replaced gradually by PCD tools in many industry fields as rapid development of the cubic press synthesis technology,so that the machining requirement of the precision and ultra-precision machining could be satisfied by PCD tools.In this research,the removal mechanism of diamond in PCD surface machined by EDG is studied to achieve the great machining efficiency and surface quality,which is affected by the metamorphic layer of PCD surface during the EDG process.The specific content is as follows:(1)Design of plasma diagnostic system of EDG.The plasma diagnostic system of EDG is designed to adapt the instantaneous and high frequency impulse characteristics of EDG.The plasma diagnostic system is consist of collimating lens module,optical fiber and spectrometer,which the weak optical signal of EDG could be detected and analyzed in different machining time points and spaces of plasma channel in real-time.It is represented that the spectrum intensity is mainly affected by the slit size of spectrometer.The spectrum profile is different in different region of plasma channel with continuous impulse condition in 2000?s.The plasma characteristics could be represented by the spectrum profile that in different machining time points and region of plasma channel.(2)Study on the plasma characteristics of EDG machining PCD.The discharge energy is translated into heat energy by the impact between electrons in plasma channel and PCD positive electrode surface during the EDG process.So that the discharge process of EDG could be represented by the plasma characteristics in plasma channel.It is represented that the plasma in plasma channel is contained by electrode elements such as Cu,C,Co,W and dielectric elements such as H,O.The plasma temperature is about 8000K-8300K which is calculated by the line pair method.The temperature of PCD electrode surface is higher than both the center region of plasma channel in axial direction and the surface of tool electrode,which respectively are 8120K-8235K,8080K-8170K and 8110K-8205K.The electron density in plasma channel is about 2.5·1016cm-1-5.6·1016cm-1 which the PCD positive electrode surface is about 6 times higher than Copper negative electrode surface.The plasma characteristic is affected by processing parameters of EDG.The removal amount of C atoms in PCD is increased with increasing peak current and the electrode wear amount is increased with impulse width increasing.The electron density in plasma channel is increased with breakdown voltage.The materials removal rate is improved with increasing the rotation speed of tool electrode as the cooling rate is improved when the rotation speed is higher than 600 rad/min.(3)Study on the formation mechanism,structural and properties of metamorphic layer on PCD surface in EDG machining.The formation mechanism,structure and properties of metamorphic layer on PCD surface are studied experimentally by the discharge behavior and plasma temperature characteristics in EDG machining PCD process.The results indicate that the discharge craters in the PCD layer are mostly concentrated in the Co-concentration area.There is no recast layer on the diamond surface in the PCD layer,but the interior of discharge craters is covered by the recast layer which is mainly consisted of Cobalt.The heat effect caused by discharge energy is the main affect factor to the formation of metamorphic layer and the graphitization of diamonds.The degree of crystal defect is increased with increasing discharge energy.The graphene with 3-5 layers existed on diamond surface in rough machining condition and the layer of graphene is increased with decreasing discharge energy.The graphite defect and the disorder degree of the graphite crystal is increased with increasing discharge energy.The graphene structure with 3-5 layers C atoms layer is obtained through rough processing conditions and the amount of layers of graphene is increased with decreasing discharge energy.The thickness of the metamorphic layer on PCD surface is increased with increasing the discharge energy and the surface roughness is positive correlation with the thickness of metamorphic layer.The residual tensile stress is about 2.84-3.89GPa in heat erosion area which is higher than the PCD surface in non-discharge area.The value of residual tensile stress is about 1420-1730MPa which is lower than the tensile strength than diamonds.The materials removal mechanism of PCD in electrical discharge machining is first proposed based on the plasma temperature and mechanical behavior during the process of electrical discharge machining in this research:the diamond phase in PCD layer are transformed to the graphite phase firstly,then the graphite phase is removed as the gaseous state with intense heat effect.(4)Study on processing rule and material removal mechanism of EDDG machining PCD.The processing rule and material removal mechanism of EDDG is proposed to achieve great machining efficiency and surface quality in electrical discharge machining PCD process in this part.The results indicate that the proportion of grinding behavior of EDDG is improved with increasing the grinding wheel particle size.The rotation speed of the grinding wheel and the discharge energy are superimposed that lead to the increasing of material removal rate while increasing the abrasive wear behavior of diamond grinding wheel electrode and the breakage and shedding of the abrasive grains.The rotation speed of the diamond grinding wheel is not inversely proportional to the surface roughness but insensitive to Vicker's hardness of PCD.The heat effect of discharge energy is decreased by the grinding behavior as the machining characteristic of continuous in time in EDDG.The metamorphic layer on PCD surface is decreased by the grinding behavior in EDDG which the content of metamorphic layer is relatively the lowest when the grinding wheel particle size is 28?m than other machining conditions.It means that the grinding behavior and discharge behavior during the EDDG process are balanced dynamically so that to the best state in EDDG machining PCD could be achieved.(5)Study on optimization of processing parameters in EDDG machining PCD.The orthogonal test results indicate that the optimal parameter combination for different processing property indexes are difference.The optimal parameter combination to obtain the great surface roughness Ra is that discharge pulse width 10?s,breakdown voltage 225V,peak current 15A,grinding wheel particle size 14?m,electrode rotation speed 16m/s.The optimum parameter combination for material removal rate MRR is that grinding wheel speed 20m/s,discharge pulse width 20?s,peak current 25A,grinding wheel particle size 14?m and breakdown voltage 200V.The optimal parameter combination for the relative electrode loss rate RTW is that grinding wheel speed 14m/s,discharge pulse width 10?s,peak current 10A,grinding wheel size 40?m and breakdown voltage 150V. |
PDF Full Text Request