New WC Based Nanocomposite Tool Materials And Their Cutting Performance

High performance machining(HPM) is one of the advanced manufacturing technologies and is going to be the mainstream due to its advantages.The tool material is indispensable and more important for high performance machining.It's well known that the requirements of tool material for HPM are more stringent than traditional processing in mechanical properties and reliability to tool materials.Currently,there are more comprehensive types of tool materials,such as PCBN,PCD,ceramics,cermet, cemented carbide and coated tool materials,etal,each of which has different peoperties and is suitable for machining the difinite workpiece materials.However,there is no tool material that the mechanical properties is between cemented carbide tool material and ceramic tool materials,which is suitable for high performance machining cast iron. For example,the TiC(N) based cermet which is suitable for high performance machining steel.In this paper,a novel WC based nanocomposite tool material is developed that is suitable for high performance machining cast iron and expand the variety of tool material.In this paper,the design theory for nanocomposite tool materials,the composition of the tool material,the toughening and strengthing mechanisms,hot pressing technology,mechanical properties,microstructure,fracture mechanisms,tribological behaviors and the cutting performance of the tool material are investigated.A design methodology for nanocomposite tool material(NTM) is developed based on interface-enhanced theory.An intensity model is developed based on interfacial debonding and interfacial fracture mechanisms.Meanwhile,the effect of reinforcing phase content on grain boundariy strength is discussed also.Theoretical frame for the design of NTM is established.The critical volume content of reinforcing phase is determinated based on interface-enhanced theory and toughening mechanism of residual thermal stresses in nanocomposite tool material. Grain diameter rate between the matrix and reinforcing grain is determined based on the maximum loading when interface fractured Densification model is built based on the interface toughening mechanism.Based on thermo-dynamics theory,the composition system for the tool material is decided as tungsten carbide matrix,two reinforcing phases-zirconia and alumina.Chemical compatability between the phases in the tool material is considered based on chemical comparability theory when designing the tool materials.Meanwhile, the chemical compatability between the tool material and the workpiece material must be considered.An appropriate matching relation between the tool materials and the workpiece is determined.Fabrication technique of NTM is studied and a dispersion experiments is carried out to investigate the dispersivity of nano additives.The factors for influencing the dispersion processing are also explored.The dispersion experiments indicated that a good quality,re-agglomerated composite powder can be obtained by the optimization of the molecular weight and concentration of dispersant,especially the adjustment of the media temperature is the most important.Results show that 100℃is the best temperature to achieve a good dispersion mixture.The densification mechanism of the nanocomposite tool materials is studied and the sintering parameters,for example,the sintering temperature and soaking time,are optimized.The optimal results show that the best mechanical properties of WZ 10A are obtained under the condition of the sintering temperature 1600℃,soaking time 30min and pressure 32MPa～35MPa.The mechanical properties are:density 12.8g/cm³, Vicker's hardness 19.8GPa,bending strength 996MPa,fracture toughness 9.2MPa.m^1/2 and heat conductivity coefficient 38W/(m·K),respectively.Strengthening and toughening mechanisms for NTM were thoroughly studied. Results show that the main strengthening and toughening mechanisms of the materials include the effect of grain fining and grain boundary strengthening caused by nano-scale particles,residual stress toughening and the phase transition toughening effect caused.Additionally,the multi-scale effect,the dispersion strengthening effect and nano-grain pinning effect are emphasized on the toughening and strengthening the nanocomposite tool materials.Microstructure of WC matrix nanocomposite tool material is studied.By TEM and SEM,it reveals that grain boundaries have a large proportion in nanocmposite tool materials and inter granular microstructure is the main structure.Meanwhile,a few small intra granular structures are involved in the tool materials.In this paper,much effort is paid on the microcrack propagation and fracture mechanism of the nanocmposite tool material.Results reveal that sintering parameters have great influence on the fracture mode and fracture mechanisms.It shows that intergranular fracture is the main fracture mode when sintering temperature is lower than 1600℃while intragranular fracture appeared when sintering temperature is higher than 1650℃in WC based nanocomposite tool materials.Three fractal fracture models(including intergranular fracture,intragranular fracture and the mixture fracture of intergranular and intragranular) are proposed based on the fractal theory.Analysis and calculation is accomplished to fractal fracture models based on the principle of energy dissipation.Results show that energy consumption of intergranular fracture is the most while intragranular fracture is the least.Preparation technology can be optimized based on the combination of fracture fractal models and microstructure analysis.Cutting performance of WC based nanocomposite tool materials in machining cast irons and nodular cast iron are studied.The relationship between cutting parameters and cutting forces or cutting temperature is discussed.Results show that cutting force and cutting temperature of WZ10A nanocomposite tool materials is smaller than YG8 cemented carbide tool material.WZ10A tool material is more suitable for cutting nodular cast iron in the cutting speed of 200m/min to 300m/min and the tool life is prolonged more than 60%～125%.Wear mechanisms and failure mechanisms of the WZ1 0A tool materials are studied based on the wear morphologies of the tool.