| Powder metallurgy is an important means to achieve low cost preparation and integrated molding of titanium alloy.However,current powder metallurgy titanium alloys still face the problems of excessive oxygen content,insufficient density,and coarse structure,which leads to deterioration of plasticity and fatigue properties.In this study,hydrogenated titanium sponge and AlV40 master alloy particles were used as raw materials to prepare the powder raw materials of the alloy by synchronous mechanical milling,and the passivation reaction of the powder was inhibited.With the use of atmosphere-protected thermomechanical consolidation equipment,low oxygen content and Fully dense powder metallurgy TC4 titanium alloy has been successfully prepared.The dehydrogenation behavior,deoxidation mechanism and alloying process of TiH2 powder compact during sintering and the microstructures and mechanical properties of powder metallurgy TC4 titanium alloy were systematically studied.The powder raw materials of TiH2 and AlV40 master alloy prepared by synchronous mechanical ball milling have an average particle size of 2.4 ?m,and the fine AlV40 particles are uniformly distributed.The dehydrogenation and deoxidation behavior of TiH2 based powder cold compacts were studied by means of thermogravimetry and mass spectrometry combined with TG-MS.It was found that a large amount of adsorbed water vapor was released during the heating period of 50?to 300?,and the release of H2 and H2O was simultaneously performed during the temperature rise after 350?;at the two temperatures of 400? and 650?,the release of H2 and H2O peaked at the same time,and reached the highest peak at 650?.The weight loss during the temperature rise of the sample is mainly caused by the release of H2,and the trend of the two curves of the rate of change of weight and the release of H2 are completely consistent.Hydrogen evolution is an endothermic reaction,and the endothermic peak position coincides with the peak position of hydrogen release.This indicates that TiH2 undergoes an endothermic dehydrogenation reaction during the sintering process,and dehydrogenation causes the release of H2O,resulting in deoxidation and purifying the surface of the powder particles.The mechanism of chemical reaction of deoxidation during dehydrogenation was studied.First,the powder was analyzed by XPS photoelectron spectroscopy,it was found that a layer of nanometer thickness TiO2 oxide film existed on the surface of the TiH2 powder particles;further,by means of the calculation of the standard free energy change of the chemical reaction,it is proved that the reaction is not H2 but the solid solution[H]atom,that is,the essence of the purification reduction reaction is:TiO2+4[H]=Ti+2H2O(g).Dehydrogenation can produce a large amount of solid solution[H]atoms on the surface of the powder particles,and after the temperature rises into the ? phase,the hydrogen is completely present in the solid solution[H]atomic state,which provides a guarantee for the continuous purification and reduction reaction.The micro structure and mechanical properties of powder metallurgy TC4 titanium alloy bars prepared by hot extrusion consolidation process were studied.The extruded alloy has an oxygen content of only 0.26 wt.%and a density close to 100%,and the microstructure is a fine and uniform Wei's structure with an average crystal size of only 18 ?m.After vacuum dehydrogenation and solution aging treatment(950?/20min/WQ+580?/6h/FC),the micro structure of the TC4 titanium alloy is transformed into a special basket structure consisting of fine profiling ? grains,?sheets of 2?4 ?m width,and ?-transformed structures.Among them,a high-density ultrafine-structured needle-like a phase precipitated in the sheet-like ?-transformed structure.This special basket structure breaks the limitations of the strength-plasticity of conventional powder metallurgy titanium alloys.The tensile yield strength is as high as 1250 MPa and the elongation at break reaches 12.5%.The precipitation of high-density ultra-fine structure acicular ?-phase in ?-transformed structure strengthens the ?-phase matrix and induces back stress strengthening and significant dislocation strengthening of micron-sized a lath during stretching.In addition,reasonably controlled oxygen content and high density are also important guarantees for obtaining high strength and high plasticity.Compared with the above results,the annealing treatment(900?/20min+1?/min colling to 700?+2h/FC)resulted in spheroiddization and broadening of the alpha lamellar and no ?-transformed structure.The average free path of dislocation motion during stretching is larger,and the dislocation strengthening effect is weaker.The yield strength of the micro structure was reduced to 1065 MPa and the elongation was increased to 13.8%.In addition,it was found that the PM-TC4 titanium alloy prepared by using the passivated TiH2 powder as raw material has an oxygen content of up to 0.45 wt.%,the solid solution strengthening effect of oxygen is remarkable,and the yield strength of the alloy is as high as 1460 MPa,but the elongation is only 6%. |
PDF Full Text Request