Fabrication And Cutting Performance Of Two-dimensional Gradient Ceramic Cutting Tool Based On Fatigue Crack Growth Resistance

With the rapid development of modern manufacturing,all kinds of components with high hardness are used abundantly,and the most surfaces of the components are discontinuous.It is difficult to turn intermittently hardened materials under the high speed in the traditional cutting technology.Tool materials are becoming the bottleneck,which restrict the extensive application of the hardened steel turning technology,so the development of cutting tools with high performance suitable for high-speed intermittent cutting is of great significance to promote the progress of modern manufacturing.Based on failure mechanism of ceramic tools under the intermittent turning hardened steel 20CrMnTi,material system and structure of the two-dimensional gradient nano-composite ceramic tool have been determined.Furthermore,the graded structure and sintering process have been optimized.In the end,two kinds of two-dimensional gradient ceramic cutting tool suitable for intermittent turning have been developed successfully,and their cutting performances have been studied.In intermittent turning tests,the main failure mechanism of ceramic cutting tools was thermal-mechanical fatigue damage.Under the high turning speed,the thermal fatigue is the main failure mechanism to control the service life of ceramic tools.Furthermore,the worn tools were mainly identified as fracture in the tool-chip interface region.And under the high feed rate,the main failure mechanism of ceramic cutting tools was mechanical fatigue due to the relatively higher impact force.So on the rake face,the shell-like fatigue fracture was observed.Based on the cutting simulation,the low cycle fatigue simulation was made.The results showed that in the intermittent cutting process the rake face of cutting tools was prone to crack.So tool performances meeting the demand of intermittent turning hardened steel were put forward,as follows.Under the high speed,the cutting tools should have good abrasion resistance and thermal fatigue resistance.On the other hand,in addition to the high hardness,the cutting tools should have the high toughness and strength under the high feed rate.According to the intermittent cutting hardened steel test and finite element simulation results,the content of raw material of each layer of the gradient ceramic tools material was preliminarily determined.And macro to micro design model of the gradient tool suitable for discontinuous cutting hardened steel were constructed.The finite element simulation was used to simulate the three point bending fatigue of ceramic tool materials.The relation between the fatigue life and gradient ceramic material parameters was studied.The structure size of two-dimensional gradient tool suitable for turning under the high feed rate was determined.Based the heat transfer analysis,the structure of two-dimensional gradient tool suitable for turning under the high speed was optimized.By powder filling and two-step hot pressing sintering,one-dimensional and two-dimensional gradient tool material were developed.The gradient structure parameters of the one-dimensional gradient tool material were optimized.It could be concluded that the five-layer graded nano-composites with a thickness of the surface layer of 0.085 mm had the optimum comprehensive mechanical properties.The most appropriate sintering temperature of the first stage of the gradient tool materials was 1700 ℃,the second stage of sintering temperature and holding time were 1450 ℃ and 60 min respectively.Two-dimensional gradient tool materials GAWTN51-20 and GAWTN51-45 with the optimal comprehensive mechanical properties were sintered.The bending strength,surface hardness and fracture toughness of GAWTN51-45 were 1005 MPa,23.20 GPa and 10.56 MPa-m1/2,respectively.The water quench experiment method was used in thermal shock resistance test of homogeneous,one dimensional gradient,two-dimensional gradient materials.The critical thermal shock temperature difference of one dimensional gradient material GAWTN51 and two-dimensional gradient material GAWTN51-20 were both 600℃,and higher than that of homogeneous ceramics AWT.An indention-quenching method was adopted to investigate the thermal fatigue resistance of two-dimensional graded ceramic tool materials.The crack propagation rate of two-dimensional gradient materials was slightly lower than that of one dimensional gradient material,and significantly lower than that of homogeneous ceramic tool material.Thermal fatigue crack propagation along the gradient direction on two-dimensional gradient tool material was restrained.The cutting performance of the two-dimensional gradient nano-composite ceramic cutting tool was studied when turning hardened steel 20CrMnTi intermittently under different parameters.Under the different feed rate,the life of two-dimensional gradient ceramic cutting tool was more than that of one dimensional gradient ceramic tool and the corresponding homogeneous ceramic tool.When the feed rate was higher,the advantage was more obvious.Two-dimensional gradient ceramic tool could inhibit the initiation and propagation of cracks on the rake face significantly.So the damage of the rake was suppressed.In addition,under the different cutting speed,the two-dimensional gradient ceramic cutting tool had a relative longer life expectancy than one dimensional gradient ceramic tool and the correspondinghomogeneous ceramic tool.The two-dimensional gradient tool had the relatively higher thermal fatigue resistance,so the thermal damage in the tool-chip contact area decreased under the high speed.