Thermodynamic Behavior And Phase Analysis Of Hydride Decomposition In Vacuum Sintering Of TiH 2 -Al-V Powder System

Titanium and titanium alloys have been widely used in aerospace,petro-chemical,bio-medical,automotive and sports industries owing to their excellent characteristics,including low density,high specific strength,high temperature resistance,good corrosion resistance,good mechanical properties and excellent bio-inert.However,the high production cost and long processing cycle restrict its large-scale application.Looking for simple preparation process of titanium alloy and reducing the cost of titanium become research hotspots at home and abroad.Powder metallurgy method has the advantages of near net shape and short process flow.So it’s a good way to improve the production efficiency of titanium alloys.In this paper,the Ti-6A1-4V alloy was chosen as the research system.Using TiH₂ as raw material,in order to reduce cost of production.In this study,the thermal analysis and thermodynamic study on the dehydrogenation behavior of titanium hydride were carried out.The TiH-2 and TiH₂-AI-V compacts were respectively sintered at 400℃,500℃,600℃,750℃and 1200℃.After sintering at different temperatures,the component of TiHx and other alloys were investigated during the hydrogen evolution process.The size and density of samples sintered at different temperatures were analyzed.Analyzing the microstructure of Ti-6A1-4V alloy sintered at 1200℃.Through thermodynamic calculation,the titanium hydride dehydrogenation thermodynamics Gibbs free energy calculation formula AG,was get.The calculation shows that when the temperature is 300℃,the dehydrogenation reaction can be carried out smoothly with the hydrogen partial pressure under 7.432×10-2Pa.With the increase of temperature,the vacuum degree of the reaction is lower and lower.With the increase of the hydrogen partial pressure in the furnace,the dehydrogenation temperature of TiH₂ is lower and lower.In this paper,through TG thermal analysis test,the dehydrogenation behavior of titanium hydride during heating was studied.The results show that the titanium hydride dehydrogenation behavior is a multi-stage reaction.Under argon atmosphere,the TiH₂-Al-V dehydrogenation temperature is 400℃.It decomposed a lot between 440℃ and 530℃,the end dehydrogenation temperature is about 660℃.The addition of Al-V alloy does not change the quality of titanium hydride dehydrogenation.But it decreased the start temperature and end temperature of dehydrogenation,and reduced the dehydrogenation rate of titanium hydride.The temperature span of dehydrogenation was prolonged.The vacuum sintering experiments show that dehydrogenation of titanium hydride begins at 300℃,and at 400～420℃a large amount of dehydrogenation occur.The dehydrogenation is finished at about 600℃.The dehydrogenation temperature of titanium hydride was verified by actual sintering experiment and thermodynamic calculation.In this study,the main component of the raw material TiH₂ powder is TiH 1.924.After sintering at 400℃,TiH1.924 Changed to TiH1.5 and a little α-Ti.After sintering at 500℃,TiH1.924 disappeared,α-Ti become the main phase.TiH1.5 continued to decompose into TiH.After sintering at 600℃,TiH1.5 disappeared,leaving a little of TiH.With decomposed into TiH0.71,TiH disappeared after sintering at 750℃.TiH0.71 disappeared after sintering at 1200℃.Finally,the dehydrogenation of titanium hydrided to α-Ti.The XRD results of sintered TiH₂-Al-V samples show that the dehydrogenation process of titanium hydride in TiH₂-Al-V samples are the same as that of the hydrogenated titanium in TiH₂ samples before 750℃.After 750℃,β-Ti begins to appear in the samples.After sintering at 600℃,the phases of Al-V alloy are Al3V and V5Al8.The mass fraction of Al in the alloy is reduced,which indicates that Al is a kind of a phase stabilizer of titanium alloy,and it begins to dissolve into Ti.After sintering at 750℃,with the disappeare of Al3V and V5Al8,a new Al6V phase appeared.The mass fraction of V in the alloy is reduced,which indicates that V is a kind of β phase stabilizer of titanium alloy,and it begins to dissolve into Ti.The transformation of a phase to β phase began to occur,and the β-Ti started to form.In the process of sintering,Al and V were dissolved into α-Ti to form substitutional solid solution,resulting in the shift and broadening of the XRD peak of α-Ti phase.The metallurgical structure of Ti-6Al-4V alloy sintered at 1200℃ is typical basket-weave microstructure.There are some α-Ti twin crystals in the alloy structure.The twin grain boundaries are straight lines under microscope.The result of energy spectrum analysis shows that the mass fraction of the alloy obtained at 1200℃ is consistent with the Ti-6Al-4V alloy.After sintering at 1200℃,the distribution of Ti and Al in the alloys is relatively uniform,and the V elements in the α/β phase boundaries are significantly enriched.It illustrates that the V element is a β phase stable element.V element enhances the stability of β phase and keeps it to room temperature without eutectoid decomposition.The size of the samples decreased with the increase of sintering temperature.The samples shrinkage rate increased with the increasing of sintering temperature.But after sintering at 400℃,samples’ longitudinal size increased.The shrinkage rate and longitudinal shrinkage of Ti-6A1-4V alloy obtained at 1200℃ are larger than that of pure titanium.The relative density of pure titanium is more than 98%and the hydrogen content is 30ppm.The relative density of Ti-6A1-4V alloy reaches 99.7%and its hydrogen content is 18ppm.