Study On The Powder Boronizing Of TC4Titanium Alloys At Low Temperature

TC4(Ti-6Al-4V) was the most widely used in titanium alloys due to the characteristics of low density, high ratio of strength to weight, high corrosion resistance, non-magnetic and so on. However, the low hardness, poor wear resistance limited its wider application. Currently, the solid powder boronizing as a simple and effective method of surface modification has been successfully used to improve the wear resistance of the titanium alloy. The existing power boronizing technology of TC4alloy is mostly carried out in1000～1200℃, which is over the β-transus temperature, and can lead to great change of lattice, large deformation of the work-piece and high brittleness of the diffusion layers.In this paper, the solid power boronizing and the RE-boronizing on TC4alloy at low temperature, the morphology and structure of the diffusion layers, the effect of adding rare earth in boronizing on the diffusion layers, the catalyzing mechanism of rare earth were systematically studied. The morphology, properties and changes in the matrix of diffusion layers were comparatively studied when TC4alloy was RE-boronized at low temperature and at high temperature. In addition the β phase transformation in the process of powder boronizing was investigated.(1) To improve the surface properties of TC4titanium alloy by forming boron-titanium compounds layer on the surface of TC4, the method of solid powder boronizing at low temperature was studied. By means of optical microscopy, X-ray diffraction, micro-hardness tester, friction and wear testing machines and other equipments, the morphology, structure and properties of boride layer on the surface was analyzed. The observation of microstructure revealed the diffusion layer was composed of the outer surface boron-titanium compounds and the transition region. The whiskers combined closely with the TC4matrix. In addition the formation of biphasic boron-titanium compounds was analyzed with thermodynamic principles. At950℃for5h to40h, the thickness of diffusion layer ranged from5.3μm to15.4μm. The hardness value of the diffusion layer ranged from2500HV0.05to1250HV0.05 which was five more times than the matrix (36OHV0.05～39OHV0.05)-The results of friction and wear experiment showed that the friction coefficient of the diffusion layer ranged from0.2to0.3, and the friction coefficient of the transition region was0.4, both less than the friction coefficient of the matrix (about0.5), so the wear resistance had been significantly enhanced.(2) RE-boronizing on TC4alloy at low temperature was studied. The optimum content of rare earth in boronizing agent was determined, and the effect of rare earth on morphology was investigated. The order from the surface to inner of the diffusion layer is TiB2â†'TiBâ†'transition layer according to the results of SEM. The hardness and wear resistance of diffusion layers were comparatively studied both for the boronizing and RE-boronizing. The results showed that the optimum content of rare-earth was5%. After adding rare earth the depth of boron-titanium layer increased, the micro-hardness increasing about25%in range of31OOHV0.05～175OHV0.05, and the friction coefficient of the diffusion layer remained in0.2～0.3, which improved the wear resistance.(3) The study on the catalyzing mechanism of rare earth showed that with the unique electronic structure, the rare earth can shorten the cycle of powder boronizing and reduce the treatment temperature. Active rare-earth atoms can be formed and completed the inward diffusion according to the "bivacancy" and "short-range diffusion" mechanisms. The catalytic effect of rare earth is considered the result of interaction of "active center-surface effect" mechanism and "lattice distortion-air mass channel" mechanism.(4) The diffusion layers were compact, uniform and less porous, and the hardness gradient and brittleness were improved to a certain extent in comparison with the case of boronizing at high temperature (above β-transus temperature). The TC4matrix became equiaxial structure with good function. Low-temperature boronizing can effectively prevent the formation of coarse (3phase at the high-temperature.(5) The transition of α phase and β phase was discussed when the TC4alloy was cooled. The precipitation of α phase is a process of nucleation and growth. During the slow cooling of TC4alloy from the β phase region, α phase formed crystal first, then bunched and expand to the intragranular, whileβ-phase formed reticulated layer between sheet α phase, forming widmanstatten structure; while during the slow cooling from the (α+β) phase region, α precitates nucleated along the crystal boundary of β phase and on the interface of a phase, polymerized into the α equiaxed structure, and formed the equiaxed structure with -transformed beta.