Study On The Preparation,Microstructure,and Properties Of Ultrafine/Nanocrystalline WC-Co Cemented Carbide

Ultrafine/nanocrystalline WC-Co carbide has become one of the best candidate materials for high-speed and precision machining due to its ultra-high hardness,strength,wear resistance,and good fracture toughness.At the moment,advancements in the processing of integrated circuit board microdrills have been developed,medical dental drills,precision molds,and difficult-to-process tool materials.The preparation process of cemented carbide is mainly affected by the original powder and sintering method.To regulate the development of WC grains during the sintering process,nanocomposite powder is commonly used to make ultra-fine or nanocrystalline cemented carbide.Yet,because of the nanopowder tiny size effect,high surface activity,and great sintering driving force,it is very simple to develop WC grains during the sintering process,and it is difficult to obtain high-density cemented carbide.Therefore,it is of great significance to select a suitable sintering technology to prepare high-strength and tough cemented carbide.In this paper,WC-10Co cemented carbide was studied,and nanocrystalline WC-10Co cemented carbide was prepared by oscillating pressure assisted sintering（HOP）and oscillating pressure sintering forging（OSF）.By comparing HP and CSF,the link between the preparation procedure,microstructure,and mechanical characteristics was established.Finally,the mechanism of the material toughening was also investigated.The following conclusions were drawn from the experimental results.:（1）HOP can increase the density of WC-10Co carbide,inhibit the growth of WC grains,accelerate the flow of Co binder,and significantly improve the mechanical properties of the alloy.Hardness and fracture toughness of HOP samples are always greater than those of HP samples at the same sintering temperature.The results of temperature experiments showed that the HOP samples had the best performance at1250°C,with hardness and fracture toughness of 2120 kg/mm² and 15.3 MPa·m^1/2,respectively,and a grain size of 330 nm.Also,the effect of different frequencies at the solid phase sintering temperature（1200°C）was investigated.Increasing the sintering frequency helps to accelerate densification and obtain higher density.Excellent performance WC-10Co carbide with hardness and fracture toughness of 1913 kg/mm²and 13.11 MPa·m^1/2,respectively,and grain size of 225 nm was prepared at 20 Hz.The value of n reflects the degree of influence of the densification process of the sintering parameters,2<n<3,the densification process may be dislocation-controlled grain boundary slip,and n≥3,the dislocation-slip plastic deformation is dominant.This indicates that certain oscillatory sintering parameters help promote sintering densification.（2）The results of the sintered forging experiments showed that the samples prepared by OSF had higher deformation and greater strain rate,as well as an excellent combination of hardness and fracture toughness than the specimens prepared by CSF.The hardness of the sample prepared by OSF at 1125°C was 1911 kg/mm2,and the fracture toughness was 17.52 MPa·m1/2.Unlike CSF,OSF has outstanding synergistic hardening and toughening properties.The excellent mechanical properties of OSF samples can be attributed to the better microstructure produced by OSF,including higher relative density,finer grains（180 nm）,and more uniform microstructure.（3）The sintered forging process was explored to study OSF-1125（40±10 MPa）and CSF-1125（50 MPa）cemented carbide samples for strength and toughness analysis.The fracture toughness of the OSF and CSF samples were 17.52 MPa·m^1/2 and 13.55MPa·m^1/2 at 1125°C,respectively,and the fracture toughness of the OSF-1125 samples was improved by more than 30%compared with the samples prepared by the CSF-1125method.The improved toughness of OSF-1125 is due to the smaller Co free range and WC grain adjacency resulting from the homogeneous microstructure.The EBSD results show that the OSF specimens have more fcc-structured Co phases and more∑2 low-energy grain boundaries compared to the CSF.In turn,a synergistic effect of fine grain strengthening,interfacial strengthening,and solid solution strengthening（increased fcc-Co phase content）is achieved.