Study On Strengthening And Toughening Of WC-6Co Cemented Carbide Doped With Graphene And Nano-alumina Particles

WC-6Co cemented carbide has good mechanical properties,but due to the combined effects of binder phase and grain size,the alloy cannot have high hardness,high strength and high toughness at the same time.Therefore,how to improve itsstrength and toughness while maintaining high hardness is the core subject on WC-6Co cemented carbide currently.Although the preparation of ultra-fine/coarse-grained cemented carbide and doped grain growth inhibitors,rare earth elements,etc.had improved some of the properties,there were still technical problems of difficulty in process control and insignificant improvement in performance.Therefore,based on the strengthening mechanism of various additives on the cemented carbide and the effect of carbon content on the alloy structure and properties,this paper selected carbon materials as additives to explore its effect as a recarburizer and reinforcement on the mechanical properties of WC-6Co cemented carbide.It provided new ideas for strengthening and toughening of low-cobalt WC-based cemented carbide.In this paper,WC-6Co cemented carbide samples with different contents of multi-layer graphene were prepared by low-pressure sintering,and then characterized and analyzed from the aspects of microstructure,densification,thermal conductivity,and mechanical properties.The results showed that graphene existed in the form of reinforcing phases in the Co phase,WC/WC interface and WC/Co interface,and the alloy grains were refined to varying degrees.At the same time,adding graphene was conducive to the densification of the alloy,and it significantly improved the thermal conductivity of the alloy.When the graphene content was 0.05 wt.%,the graphene distribution was the most uniform,so the grain refining effect was the most obvious and the degree of densification was the highest.When the graphene content exceeded 0.05wt.%,the powder began to agglomerate,then the grain refinement effect and the improvement of densification of the alloy were affected to a certain extent.With the increase of graphene content,the hardness of the cemented carbide decreased slightly,the transverse fracture strength first increased and then decreased,and the fracture toughness was improved to varying degrees.When the graphene content was 0.05 wt.%,the transverse fracture strength reached 2321.2 MPa,which was an increase of 35.3%compared to the matrix;the fracture toughness increased to 13.1 MPa·m^1/2,which was an increase of 31.0%compared to the matrix.Comprehensive consideration,the cemented carbide containing 0.05 wt.%graphene had relatively excellent comprehensive properties.In addition to graphene itself as a reinforcing phase,increased densification and graphene-induced fine-grain strengthening were the reasons for the strengthening and toughening of the alloy.However,in view of the adverse effect of multi-layer graphene on the hardness of cemented carbide,this paper then prepared GO/Al₂O₃composite particles by colloidal processing and added them to WC-6Co cemented carbide.It was observed by TEM that the GO/Al₂O₃composite particles were coated inside the WC grains and were tightly combined with the WC grain interfaces through their own plastic deformation.Through XRD and EDS detection,GO/Al₂O₃composite particles were dissolved in the cobalt phase to cause lattice distortion of the cobalt phase.When the content was 0.05 wt.%,the cobalt phase dissolved more W atoms,which improved the strengthening effect of solid solution strengthening.Just like the graphene,GO/Al₂O₃composite particles were beneficial to improve the grain refinement and densification of the alloy,and the degree was related to the dispersion state of GO/Al₂O₃composite particles.When the GO/Al₂O₃composite particles were 0.05 wt.%,the dispersion of GO/Al₂O₃composite particles was the most uniform,so the alloy grain refinement was the most obvious,and the degree of densification was the highest.The hardness,strength and fracture toughness of WC-6Co cemented carbide showed a consistent trend of first increasing and then decreasing with the increase of the amount of GO/Al₂O₃composite particles,and the optimum was achieved when the content of GO/Al₂O₃composite particles was 0.05 wt.%.The hardness was 2021 HV₃₀,which was 3.0%higher than the matrix;the transverse fracture strength was 2480.4 MPa,which was 121.3%higher than the matrix;the toughness was11.5 MPa·m^1/2which was 14.5%higher than the matrix.In addition to the increased densification of the alloy,the strengthening and toughening mechanisms induced by GO/Al₂O₃composite particles included grain refinement,dislocation strengthening,solid solution strengthening and dispersion strengthening.Although the hardness,strength and toughness of the alloy had been improved,the powder agglomeration phenomenon that occurred when the content of GO/Al₂O₃composite particles was large（>0.1 wt.%）cannot be avoided.Therefore,in the end,this article adopted ultrasonic-freeze-dried powder mixing technology to improve on the basis of mechanical ball milling.By comparing the mechanical ball milling powder mixing,it was found that the ultrasonic-freeze-drying powder mixing process can effectively solve the agglomeration problem of GO/Al₂O₃composite particles to obtain fine and uniform powder particles.Especially for the alloy with 0.15 wt.%GO/Al₂O₃composite particles,after the ultrasonic-freeze-drying powder mixing process,the composite particles were evenly distributed among the WC grains to make the grain refinement effect more significant,and the relative density of the alloy was relatively high.The strength of the alloy was increased to 2723.6 MPa,which was about 600MPa higher than that of ordinary mechanical ball milled alloy,and its hardness and toughness were also improved accordingly.For the alloy with 0.2 wt.%GO/Al₂O₃composite particles,although the agglomeration phenomenon was reduced after the ultrasonic-freeze-drying powder mixing process,a good dispersion effect was still not achieved.Therefore,the strength and hardness were only slightly improved compared to the alloy after mechanical ball milling,but the fracture toughness was reduced.