Mechanisms Research On High Speed Grinding Of Cemented Carbide With Vitrified Diamond Wheels

Cemented carbide is a kind of hard and brittle composite material which is a hybridof hard carbide and soft metal phase, and different from soft metal materials and hardand brittle ceramic material. The combination of hard particles in a relative ductilematrix leads to different materials micro-structure. It leads to different grindingperformance. Due to their excellent mechanical properties, such as high strength, highhardness and high wear resistance, cement carbide materials have been widely used inthe engineering fields where high wear resistance and thermal stability are required.Although the preeminent mechanical properties of cement carbides facilitate theirwide applications, they also render a challenging problem in grinding. The machiningefficiency and processing quality can not be considered at the same time. This hasthus limited the widespread applications of advanced engineering ceramics. Thisthesis project thus aimed at developing a high speed grinding technology forefficiency machining of cement carbides and investigating the associated removalmechanisms. Up to now, only a few research has been reported to investigatemechanisms for grinding of cemented carbides in high speed regime with vitrifieddiamond wheels. Therefore, the grinding mechanisms are explored in this thesis forhigh speed grinding of cemented carbides with vitrified diamond wheels at thegrinding speed of up to120m/s. The main work of this thesis can be summarized asfollows:1. Five kinds of cemented carbides with unique features in their microstructuresand mechanical properties are grinding in high speed regime with vitrified diamondwheel. The horizontal and vertical grinding forces are measured in different grindingparameters (e.g. peripheral wheel speed, depth of cut and workpiece velocity) duringgrinding. Effects of the various grinding parameters, the maximum undeformed chipthickness, the cutting length and physical-mechanical properties of materials on thespecific grinding forces, grinding force ratio, the average grinding force per grain andspecific grinding energy were investigated. The specific grinding forces and thespecific energy not only relate to the maximum undeformed chip thickness and thecutting length but also to physical-mechanical properties of materials and removal modes. The regression model on specific grinding forces and specific energy areestablished according to the relationship of the specific grinding forces and thespecific energy with the maximum undeformed chip thickness and the cutting length,which uses least square method fitting experimental data of high speed grinding.Through the analysis of the distribution mechanism of the grinding energy shows thatthe grinding energy expended is mainly associated with sliding and ductile plowing.A nearly proportional relationship is obtained between the consumed power per unitwidth (Pm) and the plowed face areas generated by all cutting points per unit width(Sw). Compared to conventional grinding, it is found that high speed grinding canobviously reduce grinding forces and the average grinding forces per grain butincrease force ratio and specific grinding energy.2. During the experiments, temperature distributions along the workpiece surfaceare measured with a foil thermocouple and the energy partition to the workpiece wasestimated using a temperature matching method. The influences of the grindingconditions, including wheel speed, depth of cut, workpiece velocity, and differentkinds of cemented carbides, on the temperatures and energy partitions wereinvestigated. The heat source acting on the workpiece surface is assumed to have aparabolic heat flux distribution is used to simulate the temperature field, for the heatflux density between the grinding wheel and the workpiece is related to the maximumundeformed chip thickness. The study then carries out three-dimensional finiteelement simulation to investigate the transient temperature field induced by thethermal loading to the workpiece. The simulation demonstrates that more convincingresults are obtained using the parabolic heat-flux distribution than the triangularheat-flux distribution. The simulation results is analyzed and compared to the testresults, the simulated result was verified. According to the simulation results oftemperature distribution, we can predict the heat affected grade of the workpiece. Inall cases, the maximum temperature rise at the grinding zone in wet grinding is lessthan185℃when down grinding five kinds of cemented carbides.Under all grindingparameters applied, the energy partitions to the workpiece in wet grinding, obtainedby the temperature matching method varied approximately from2.4to14%.Compared to conventional grinding, it is found that high speed grinding can reduceenergy partition but increase grinding temperature.3. Grinding vibration acceleration signals are measured under different grinding parameters during grinding. The surface roughness values are measured and themorphological features of ground workpiece surfaces are examined. Thecharacteristics of grinding vibration and surface morphology are researched. Amethod which is the combination of trispectrum and the autoregressive model used toevaluate the relationship between the vibration and the surface roughness.Mechanisms for high speed grinding of cemented carbides with vitrified diamondwheel are elucidated. High speed grinding induces an increase in the vibrationamplitude remarkably and cracks are generated on the ground surface, which havebad influences on the ground surface quality. It is found that grinding conditions, theremoval mode, microstructures and mechanical properties of the cemented carbideshave a significant influence on the ground surfaces. Surface roughness decrease withthe increasing of the peripheral wheel speed, while increase with a larger depth of cutor a faster workpiece velocity or a bigger tungsten carbide particles. The results offitting experimental data of surface roughness using least square method show thatthe workpiece velocity more influence on surface roughness compared to depth of cutand peripheral wheel speed. Surface roughness increases linearly with the maximumundeformed chip thickness. Through contrasting the surface roughness of five kindsof cemented carbides materials after grinding, pointed out that high speed grinding ismore advantageous to brittle material to reduce the surface roughness value. Theobservations on the ground morphology of cemented carbides demonstrate that theground surfaces are generated by the combined removal modes of brittle and ductile.Microscopic examination of the ground surfaces by a digital and video microscopesystem also revealed that material removal occurred mainly by ductile flow whilegrinding the cemented tungsten carbide and by brittle fracture while grinding thecemented titanium-tungsten carbide.