Multiscale Simulation On Crack Propagation Behaviour Of WC-Co Cemented Carbide

WC-Co cemented carbide is widely used in cutting tools,mining tools,wear-resistant mechanical parts,molds and other fields due to excellent properties such as high hardness,high strength,high toughness and good wear resistance.In engineering applications,due to the combined effects of thermal stress,electrochemical corrosion,abrasive wear and impact load,the main failure modes of cemented carbide are complex and diverse,one of which is the fracture caused by crack propagation.In order to ensure the safety,reliability and utilization of cemented carbide,a more comprehensive and in-depth understanding of the mechanism of its crack propagation behavior is required.Therefore,in this paper,the influence of multiple scales is considered from the perspectives of nano,micro and macro,the crack growth behavior of WC-Co cemented carbide is simulated by using numerical simulation technology,and the fracture characteristics are studied and analyzed.The molecular dynamics method on the nanoscale is used to construct the crack propagation and fracture simulation model of WC/Co interface,WC/WC interface,WC grains and Co phase.Data results are analyzed such as the curve between the bonding force and the opening displacement(T-S curve),atomic distribution state,and fracture dissipation energy at each interface during the fracture process.The results show that the peak value of the bonding force of the WC grain fracture model is the largest,the fracture initiation is the most difficult and the energy expended is the most,which is mainly concentrated in the stage before the onset of fracture.The peak value of the bonding force of Co is second only to the WC grains and the WC/WC interface,however,the crack passivation effect occurs during the fracture process,which hinders the crack propagation,therefore the energy is mainly consumed in this stage.The crack propagates most easily at the WC/Co interface,which consumes the least energy.On the mesoscopic scale,the microstructural parameters such as WC average grain size and distribution,Co phase volume fraction,adjacency of WC-Co cemented carbide are obtained by image processing technology.Aiming at the microstructure parameters,a Voronoi diagram-based modeling method for the microstructure of WC-Co cemented carbide is proposed.The perturbation method is introduced to establish the initial Voronoi diagram with the characteristics of the average grain size of WC.Because some cemented carbides have large grain size differences and large size spans,a unit based on modifying the local distribution density of Voronoi seed points to control the grain size is proposed.The extraction method and the selection method are combined to establish a grain size distribution control model.A multi-objective optimization function with two parameters is established because of the uncontrollable error of the Co phase volume fraction and adjacency in the model,and optimized by multi-objective genetic algorithm NSGA-Ⅱ.The results show that the error of WC grain size distribution can be controlled below 6%,however,other parameters are not controllable.The errors of WC average grain size,Co phase volume fraction and adjacency are all controlled below 3% after NSGA-II optimization.The model characterizes the randomness of the real microstructure of cemented carbide,verifying the feasibility of the modeling method,and providing an error-controlled microstructure model for the finite element simulation of WC-Co cemented carbide crack growth.Based on the cohesive zone model,the parameters of the finite element cohesion unit are obtained by fitting the T-S curve obtained from the molecular dynamics simulation,therefore,the nanoscale-macroscopic combination of molecular dynamics and finite element is realized.A single-interface CT fracture finite element model is constructed.The results of simulation show that the parameters such as WC/Co interface,WC/WC interface,WC grains,Co phase stress distribution,stress intensity factor are similar in general trend,but also different,which proves the cohesion model integrated with molecular dynamics T-S curve can characterize the properties of the four interfaces at the macroscopic scale.The constructed WC-Co microstructure model is embedded into the CT tensile fracture model,so the composite interface CT fracture finite element model is established.The characteristics of stress distribution,stress intensity factor,fracture toughness and fracture loss energy during the fracture process of the composite interface are analyzed.The results show that the model can simulate the propagation of cracks in WC-Co cemented carbide at each interface,in addition,each interface still has the characteristics of the single interface fracture model.Seven groups of CT fracture finite element models of WC-Co cemented carbide composite interface with different WC grain proportions are constructed.The simulation results show that the fracture toughness firstly increases and then decreases with the increase of the coarse WC content.Compared with the experimental results,the error is less than 7.9%,which proves that the composite interface model is feasible in predicting the fracture toughness of WC-Co cemented carbide.