Diffusion Sintering Fabrication And Cutting Performance Of Functional Gradient Cermet Composite Cutting Tools

Aimed at the contradiction between hardness and toughness of homogeneous cutting tool materials, as well as the thin coating that was unable to resharp for coated cutting tools and easy to delaminate for laminated cutting tools, a new type of functional gradient cermet composite cutting tool materials were fabricated. To investgated the cutting tool materials which were suitable for cutting conditions of difficult-to-machine materials, high hardness and chemical stability and other advantages of ceramic phase were used. The phase composition, distribution of microstructure and properties,gradient formation mechanism,mechanical property at room temperature,tribological properties and cutting performance were investigated systematically. The mechanical properties and microstructures of the gradient cermet composite cutting tool materials had a gradient distribution, which was characterized by high surface hardness and good wear resistance, high toughness of subsurface layer and high bonding strength and excellent overall mechanical properties. The developed GC-A gradient functional cermet cutting inserts exhibited a good cutting performance when turning 17-4PH martensitic stainless steels, and had the value of application promotion. Therefore, this work had a wide application prospect and an important theoretical and practical significance.According to the performance requirements of high speed cutting tools, the design principle of functional gradient cermet composite tool materials was put forward, and a kind of functional gradient cermet cutting tool materials component system with good matching performance was designed and a one-step sintering preparation process was developed. Before sintering, the functional gradient cermet tool materials were comprised of surface layer and substrate, while during sintering a high toughness subsurface layer was formed based on element diffusion driving and metal phase redistributing mechanisms, therefore after sintering the gradient structure was comprised of surface layer, subsurface layer and substrate. Gradient cermet composites with different anti-failure properties had a hard and wear resistant surface layer, a tough and anti-surface peeling off subsurface layer, and a strong and anti-fracture overall. The formation mechanism of the gradient microstructure for the cermet composite tool materials was revealed, that was, the redistribution mechanism of metal phase driven by the elements diffusion, and the evolution mechanism model of the gradient microstructure for the functional gradient cermet composite tool materials was established. TiB2-TiC and Ti(C,N) composite powder were used as surface layer and substrate composition, respectively. Ni, Mo and VC were used as metal phase, the second phase and surface layer addition phase, respectively. The vaccum hot pressing sintering and postprocessing were used, and the functional gradient cermet composite tool materals were fabricatied.Effect of surface layer characteristic, metal phase content and sintering processes on microstructure and mechanical properties were investigated. The results showed that the optimum surface VC content is 4 wt% within the range of the parameters of this study. The addition of VC to the surface layer could reduce the surface layer sintering temperature, improve the surface density and promote the matching between the surface layer and the substrate. The optimum thickness of the surface layer was about 230 ?m.The thickness of the surface layer mainly affected the residual thermal stress in the surface layer, and then affected the flexural strength of the gradient cermet tool materials, while the effect on the hardness was not significant. Ni content in the surface,Ni/Co ratio and Ni content in the substrate was optimized, respectively. The results showed that Ni is more suitable metal phase than Co binder in the gradient cermet composite tool materials. The optimum Ni content in the surface layer was 6 wt%, and the optimum Ni content in the substrate was 12 wt%. The content of metal phase had a significant effect on the core-shell structure of Ti (C, N) grains in the substrate. The optimum core shell area ratio k was 1.20-1.23. The sintering processes of functional gradient cermet cutting tool materials were optimized. The functional gradient cermet composite tool materials had the best comprehensive mechanical properties under the conditions of heating rate of 30 ?/min, sintering temperature of 1500 ?, holding time of 40 min and sintering pressure of 32 MPa. The flexural strength, surface hardness,substrate hardness and fracture toughness of optimal cutting tool materials were 1520±66 MPa, 27.28±0.55 GPa, 21.63±0.51 GPa and 7.04±0.55 MP·m1/2, respectively.The friction characteristics and wear mechanism of gradient functional cermet composite tool materials were studied. Compared with Ti(C,N) -based cermet TC, the friction coefficient of gradient cermet tool material GC was higher, but the wear rate was lower, and the main wear mechanism was different. The friction behavior and wear resistance of GC and TC were studied by friction and wear experiments sliding against 440C stainless steel ball, Al2O3 ball and Si3N4 ball. When sliding against 440C stainless steel, the friction coefficient had no obvious relationships with the sliding speed, but the friction coefficient is obviously affected by the sliding speed when sliding against Al2O3 ball and Si3N4 ball. The relationship between the wear rate and the sliding speed or load was studied. The experimental results showed that the laws of different friction materials were different. The friction and wear process was obviously affected by various factors such as material properties, crystal structure, and experimental conditions. In most cases, the wear resistance of GC was significantly higher than that of TC. In most sliding conditions, GC wear rate was slightly higher than TC. The wear mechanism of the gradient function cermet GC when sliding against different materials was studied. The experimental results showed that the main wear mechanism of GC was adhesion wear when sliding against 440C stainless steel but adhesion loss of GC was less than TC, while the main wear mechanism of GC was abrasive wear when sliding aginst Al2O3 and Si3N4 ball but grinding crack of GC was slight. In a word, GC exhibited good wear resistance and suitable for cutting tool materials.A set of dry press molds was designed and the rapid prototyping of molds was realized by stereolithographic 3D printing technology. Three gradient cermet composite cutting tools with different gradient area were fabricated via dry-pressing molding along with hot pressing sintering. Those were GC-A only with a gradient on rake face, GC-B only with a gradient on flank face and GC-C with a gradient on all face. The cutting performance of three gradient cermet composite cutting inserts was studied and evaluated comprehensively. Compared with the commercial Ti(C,N) cermet tool CX50,GC-A tool exhibited a longer tool life and higher surface quality when cutting the stainless steels under the same conditions. The tool life of GC-A was 89 min when cutting 17-4PH stainless steel at 150 m/min and it was 21 min when cutting 321 stainless steel at 100 m/min. During turning 17-4PH stainless steel at a lower speed, the insert showed the flank wear failure, the main wear mechanism was abrasive wear and groove wear. When cutting speed was high, the insert showed the breakage failure.When turning 321 stainless steel at a lower cutting speed, the main failure mode of GC-A was flank wear. When the turning speed was high, the rack face of GC-A was damaged and the flank face was peeled off. The main reason was that adhensive wear between tool and workpiece was serious resulting in the breakage of cutting tool.Compared with the GC-A, the tool life of GC-B was only 35 min under the same cutting conditions, while the tool life of GC-C was 85 min during turning 17-4PH stainless steels. GC-B showed the tool tip breakage, while GC-C showed the flank wear without groove wear.