Preparation, Microstructure And Properties Of Ultrafine Grain Gradient Cemented Carbide For High Speed Machining Of Titanium Alloy

Titanium alloys（Ti-alloy）are widely used in aerospace due to the high specific strength,corrosion resistance and good high temperature properties.But the Ti-alloy also has high chemical activity,low thermal conductivity and elastic modulus,so that it is a kind of typical difficult-to-machine materials.Nowadays the coated tools with ultrafine grained cemented carbide substrate are usually used for the Ti-alloy machining.Due to the difference in thermal expansion coefficient between coating and substrate,the micro cracks are inevitably formed in the coating and extend into the substrate,and might finally cause insert failure.Thus,the cutting efficiency is lower in the traditional titanium alloy processing,because both cutting speed and tool life are lower.Functionally gradient cemented carbide（FGCC）with surface zone enriched in binder phase which can prevent propagation of micro cracks into the bulk,is used as an ideal substrate for coated tools.In order to get a dense body,FGCC is mainly prepared by a two-step sintering.But this method causes coarse grain,low performance and complex process.If the ultrafine grain FGCC with surface zone enriched in binder phase which is combined the ultrafine cemented carbide and gradient cemented carbide could be fabricated and used as substrates for coated tools,it would be able to fulfill the requirements of Ti-alloy high speed cutting for the properties of coated tools,and the performance of coated tool will be greatly improved.After the best pre-sintering method for preparing ultrafine grain FGCC was established in this study,the ultrafine grain FGCC was prepared by a two-step method which was combined low-pressure pre-sintering and gradient sintering.The effect of cubic phase on the gradient formation,grain growth and properties for ultrafine grain FGCC was studied by adding different cubic carbides.And on this basis,the influence of particle size and sintering pressure on microstructure and properties of ultrafine grain FGCC was studied by further optimizing of alloy composition and sintering technology,and the preparation of ultrafine grain FGCC was realized by one-step sintering method.The coated tools with ultrafine grain FGCC substrate was finally prepared,and compared the performance of high speed machining Ti-alloy with domestic and international tools.The results show as follows:（1）Ultrafine grain gradient cemented carbide with surface zone enriched in binder phase can be achieved by two-step sintering method which is combined low-pressure pre-sintering and gradient sintering.After low-pressure pre-sintering,a thinner gradient layer can be formed,ultrafine grains are obtained and the binder phase become more uniformly distributed among the grains,which can shorten the diffusion distance and increase the diffusion paths and diffusion efficiency of atoms,thus the thicker gradient layer can be formed after gradient sintering.Different cubic phase has great influence on the gradient formation and grain growth of ultrafine grain FGCC.The thickness of gradient layer depends on the content of cubic phase elements and the diffusing distance of cubic phase elements with the slowest diffusion rate.The thicker gradient layer can be obtained by only adding the cubic phase Ti（C,N）,and the thickness of gradient layer decreased by adding other cubic phases（W,Ti）C and（Ta,Nb）C.But the abnormal grain growth of WC can be inhibited,and the cubic phase with "core-rim" structure can be eliminated by adding the cubic phase（Ta,Nb）C.（2）The gradient formation of ultrafine grain FGCC was affected not only by the diffusion,but also by the dissolution rate of cubic phase in the process of the gradient sintering.With the temperature increasing,the dissolution rate of the cubic phase increased,thus the gradient layer thickness increase.However,much of coarse grains caused by abnormal growth were observed when the temperature reached to a value in the Co12Ti4 alloy.Consequently,the atoms diffusion was blocked and the growth of the gradient layer thickness increased less when the temperature was increased.The gradient layer thickness of the Co12Ti4 and the Co12Ta6 alloy vary linearly with the square root of the time.Co12Ti4 and Co12Ta6 alloy can obtain a better comprehensive properties at 1400℃ for 60 min,the gradient layer thickness,grain size,hardness and transverse rupture strength are 54 μm,0.44 μm,HV₃₀ 1904,3375 MPa and 42μm,0.35 μm,HV₃₀ 1897,3337 MPa,respertively.（3）Through refining the powder particle size of cubic phase and WC and increasing sintering pressure,the dissolution rate of cubic phase in the binder phase was accelerated and the diffusion paths of atoms for the gradient formation was increased significantly,the ultrafine grain FGCC with thicker gradient layer was obtained by one-step sintering method.The growth of WC grain could be inhibited,the amount of abnormal growth WC grains could be reduced and the transverse rupture strength of alloy could be improved by refining cubic carbide powder particle size.Although the gradient layer thickness of alloys can be increased significantly by refining the WC powder particle size,the WC grain could be grown up obviously,the number of abnormal growth WC grains could be increased and the transverse rupture strength of alloy could be reduced.In the experiment of preparing ultrafine grain FGCC by one-step sintering,when the sintering pressure was 9 MPa,the nanoscale Ti（N,C）and（Ta,Nb）C powder was added,the gradient layer thickness of Co12Ti4 and Co12Ta6 alloy was 36 μm and 23 μm,the mean WC grain size was 0.28 μm and 0.27 μm,respectively.In addition,the thickness of gradient layer could be further improved by decreasing the content of（Ta,Nb）C and increasing the content of Co.（4）Compared the results of high speed cutting Ti-alloy between coated tools with ultrafine grain FGCC substrate and YBG202 and GC4225 coated tools with different substrate,the performance of coated tools can be improved significantly by using the ultrafine grain FGCC substrate.The flank wear of Co10Ti3-ACT coated tool is only 50%and 30%of YBG202 and GC4225 under the same cutting stroke,respectively.The Co10Ti3-ACT coated tool has the similar flank wear compared with the GC1105 coated tool which is produced by Sandvick for Ti-alloy cutting.The results show that the performance of the ColOTi3-ACT coated tool reached the international levels.