Study On Low Interstitials Titanium Alloy Prepared By Reaction Synthesis From HDH Titanium Powder

The high cost of titanium products is the main reason that restrict its widespread application.Therefore,reducing the cost of titanium is the general trend of the development of titanium industry.Powder metallurgy is an effective method to prepare low-cost and high-performance titanium and titanium alloys.It solves the vacuum melting problem of titanium which is generally considered as a refractory metal.At the same time,its near-net-shape characteristics and microstructural advantages solve the problem of the low materials utilization rate.Low-cost hydrogenated dehydrogenation(HDH)Ti powder can be used to prepare powder metallurgy titanium alloy parts.However,due to the influence of high interstitials content,low sintered density and coarse microstructure,the titanium products prepared from HDH Ti powder can't take advantage of the powder metallurgy.In this paper,a variety of powder metallurgy processes for preparing titanium alloys with low interstitials content are established by using HDH titanium powder,and the microstructures,mechanical properties are systematically analyzed.The specific research contents and results are as follows:(1)Ti-6Al-4V alloys with low interstitials contents are successfully prepared by cold isostatic pressing and vacuum sintering from blend powder with HDH Ti powder,Al powder and Al-V alloy powder.The oxygen content is 0.07-0.15 wt.%,nitrogen content is 0.018-0.045 wt.%,hydrogen content is 0.001-0.01 wt.%,which meet the requirements of Grade F5(Ti-6Al-4V)and Grade F23(Ti-6Al-4V ELI)in ASTM B381-13 standard for titanium alloy forgings.The results show that HDH process can obtain very low interstitials content titanium powder.The increase of interstitials concentration is mainly due to the operation,transfer and storage of powder and powder compact.(2)Ti-6A1-4V alloys with about 99%relative density are obtained by vacuum sintering at 1150 0C from HDH Ti powder(D50=10?m),Al powder(D50=5?m)and Al-V alloy powder(D50=10 ?m).The tensile strength is 900?940 MPa,the yield strength is 850?900 MPa and the elongation is 14?16%,which meets the Grade F5 requirement of ASTM B381-13 standard for titanium alloy forgings.The as-sintered microstructures is near-equiaxed/short rod-like a phase with 40?70?m and intergranular ? phase.The prior ? phase increases with sintering temperature,resulting in the evolution of near equiaxed/short rod-like a phase into ?+? lamellar structure,eventually forming long and straight ? clusters,and the activation energy of ? grain growth is 518 kJ/mol.(3)Low interstitials contents Ti-23Al-17Nb alloys are prepared by HDH-low temperature diffusion process,cold isostatic pressing and vacuum sintering.The oxygen content is 0.09?0.12 wt.%,nitrogen content is 0.028?0.04 wt.%,hydrogen content is 0.002?0.01 wt.%.In the f self-propagating diffusion of the TiH2 powder,A1 powder and Al-Nb alloy powder,reactions such as TiH2?Ti+H2,Ti+Al?TiA13,Ti+A13Nb?AlNb2+ TiAl3?AlNb3+TiAl3 occur.The relative densities after sintering at 1100,1150 and 1200? by partially-alloyed powder are 95.1%,96.3%and 98.7%respectively.The microstructure of Ti-23Al-17Nb sintered at 1200 0C is homogeneous and fine three phases microstructure consisting of ?2,B2 and O phases.The mechanical properties are 941 MPa of tensile strength,862 MPa of yield strength,11.7%of elongation.(4)Low interstitials contents Ti-TiC composites are prepared by HDH-gas solid reaction,cold isostatic pressing and vacuum sintering.The oxygen content is 0.12?0.21 wt.%,nitrogen content is 0.035?0.062 wt.%,hydrogen content is 0.006?0.013 wt.%.TiH2 powder undergoes some reactions such as TiH2?TiH1.5+H2?Ti+H2,Ti+CH4?TiC+H2 when temperature is above 700? in CH4 atmosphere,and TiC particles are deposited on the powder surface.The second phase size of TiC in as-sintered materials is about 3?12?m and the TiC volume fraction is about 0-35 vol.%.Due to the pinning effect of TiC particles,? grain size is refined from 87.89 to 34.76?m.With the increasement of TiC,the material hardness is from 292 HV to 773 HV.Benefit from grain refinement,TiC second phase strengthening,C/N/O interstitials solid solution strengthening and low interstitials concentration,Ti-15 vol.%TiC obtains better comprehensive mechanical properties,about 715 MPa of tensile strength,628 MPa of yield strength,12.1%of elongation.(5)As an effective oxygen-scavenging additive,the addition of CaB6 in high oxygen content HDH Ti powder can improve the mechanical properties of pwder metallurgy titanium alloys.The appropriate addition of CaB6 can increase sintering density.The sintering relative density increases from 97.1%to 99.3%by adding 0.2 wt.%CaB6 to Ti powder.The sintering relative density increases from 97.2%to 98.8%by adding 0.1 wt.%CaB6 in Ti-6A1-4V powder.Two second phases,Ca-Ti-O ternary oxide and TiB,are formed by adding CaB6.The addition of CaB6 can refine the grain size.With the addition of 1 wt.%CaB6,a grain in Ti matrix reduces from 178?m to 36?m,and the length of ?+? lamellae in Ti-6A1-4V matrix reduces from 203 m to 38 ?m.For Ti-0.2CaB6 and Ti-6Al-4V-0.1CaB6,the better comprehensive mechanical properties are obtained,the tensile strength is 665 MPa and 944 MPa,the yield strength is 604 MPa and 903 MPa,and the elongation is 15%and 9%,respectively.Compared with Ti and Ti-6A1-4V without CaB6,the elongation increases by 8%and 5%respectively.