Numerical Simulation Of Thermal Residual Stresses For Graded Cemented Carbides

In the present study, macrostress in functionally graded cemented carbides was investigated by finite element method, yet microstress in homogeneous cemented carbides was analyzed by finite element unit model and compared to the results of Eshelby theoretical model. To prove the accuracy of the models, the relationship between properties and microstructure for graded cemented carbides were studied by using SEM, XRD and EDAX, following conclusions are obtained:(1) Graded cemented carbides have been developed by pre-sintering carbon-deficient compacts and subsequently carburizing. The three-layered structure of the cemented carbides was formed after the carburization, the outer layer which is WC rich, the middle layer which is Co-rich and the inner layer which is as-sintered microstructure. The graded structure has significant advance compared to conventional cemented carbides.(2) The elastic constitutive relations of graded cemented carbides were developed by the definition of elastic constraint factor and modified mixed law, namely the formation of Young's modulus for composite materials according to simple volume fraction and the thermal expansion coefficient and passion ratio were obtained by simple mixed law. The model can be achieved by the finite element software of MSC. Marc2005. The calculated results show when the temperature drops from initial stress-free temperature of 800°C to room temperature, compressive stress generates in the surface zone and tensile stress in the Co-rich zone, and the nominal cobalt content should be above 15wt.% in order to obtain compressive stress of 500-700MPa to prevent thermal cracking occurrences.(3) The microstress in homogenous cemented carbides was ananlyzed by finite element unit cell model which their material parameters are varied with temperature. The calculated results show that the maximum tensile stress in the reinforcement phase Co is 1400 MPa, the maximum compressive stress in matrix WC is 1064 MPa; When the Co phase is non-ideal ellipsoidal shape (here is cylindrical shape), there is large equivalent stress (565 MPa) and plastic flow generates in stress concentration place.(4) The constitutive relation of elastoplastic deformation of functionally graded cemented carbides is developed by introducing plastic constraint factor. Compared to other researchers not long ago, the constitutive relation is temperature-dependent, volume fraction dependent and could be achieved in the finite element sofeware of MSC.Marc2005, having more objective rationality. The calculated results show in elastoplastic deformation (compared to elastic-only), the maximum axial stress is 750MPa and the maximum equivalent stress is 600MPa, namely decreases by 70%.(5) The elastoplastic constitutive relation for cemented carbides after denitriding was developed also by constraint factors. The calculated results show tensile stresses generates in the surface two-phase zone and gradually transfer to compressive stress in the inner layer; the maximum hydrostatic tensile stress in the two-phase zone is 140MPa and the hydrostatic compressive stress in the centre is 120MPa.(6) The thermal residual macrostress for monolayer and multilayer of homogenous and graded cemented carbides were investigated also by finite element method. When the temperature drops from 800℃to 0℃for TiC/Al₂O₃/TiN composite coating with cemented carbide substrate, there is tensile stress in TiN coating. There is compressive stress in cemented carbide substrate. Furthermore there is tensile stress in TiC coating and the stress in Al₂O₃ coating is smaller than other coating layers.(7) The macrostress in the surface zone of cemented carbides after carburing was measured by XRD method and the results have a good agreement with the calculated results.