Study On Preparing Low Oxygen Content Titanium Alloy Powder By Metallothermic Reduction And Its Forming Process

Titanium alloys have excellent properties such as low density,high strength,and strong corrosion resistance,which have been widely used in fields such as aerospace,national defense and military,petrochemical,energy,and biomedicine.The traditional titanium metal preparation process uses up-graded titanium slag（UGS）as raw material to prepare sponge titanium through the Kroll method.This process requires processes such as chlorination,magnesium reduction,and vacuum distillation,with a long process and high energy consumption,resulting in high production costs for titanium alloys.The chlorination process requires the use of chlorine gas,which is prone to the production of toxic substances.Based on above discussion,if UGS can be directly used to prepare titanium alloy powder,avoiding the highly polluting chlorination process,it will effectively shorten the existing process flow and achieve a short and clean preparation of titanium alloy powder.Therefore,this article proposes a method of metal gradient reduction（magnesium reduction-heat treatment-magnesium/calcium deoxidation）,which directly prepares low oxygen titanium alloy powder from UGS or metal oxides,and uses the one-step reduction of UGS to compact and sinter titanium alloy powder,achieving a short process preparation of low oxygen titanium alloy powder and titanium alloy.The main research content and results are as follows:（1）The phase evolution laws of magnesium powder reduction of TiO₂under argon atmosphere,H₂ separate reduction of TiO₂,and H₂ assisted reduction of TiO₂ by magnesium powder were systematically explained.The differences in reducing products under Ar/H₂ atmosphere was explored,and the auxiliary phase transformation strengthening deoxygenation mechanism of hydrogen in the process of reducing TiO₂ with magnesium powder was clarified.The differences in the reduction products under Ar/H₂ atmosphere were clarified,and the auxiliary phase transformation strengthening deoxygenation effect of hydrogen gas in the process of reducing TiO₂ by magnesium powder was explained.The process of reducing TiO₂ by Mg powder in Ar atmosphere can be described as TiO₂→（Ti₄O₇,Ti₃O₅,Ti₂O₃,Mg Ti₂O₅,Mg₂TiO₄,TiO₂）→（Ti₃O,Ti₆O）→α-Ti.H₂ cannot reduce TiO₂,but it has a promoting effect on the reduction reaction,and the final product of reduction is Ti₃O₅ powder.The thermodynamic calculation results indicate that H element has a strengthening effect on deoxidation during the reduction of TiO₂ by magnesium powder.The intervention of H₂ can transform the Ti-O solid solution into the Ti-O-H solid solution.The thermodynamic calculation results show that the Gibbs free energy of the formation of the Ti-O solid solution is less than the Gibbs free energy of the formation of the Ti-O-H solid solution.Magnesium cannot react with the Ti-O solid solution within a certain temperature range,but can react with the Ti-O-H solid solution to remove the oxygen element in the solid solution to achieve the deoxidation.The synchrotron radiation XRD results of the reduction of nano-TiO₂ by Mg powder show that the introduction of H element can makeα-Ti（Ti-O solid solution）converted intoβ-Ti（Ti-O-H solid solution）.At 750°C for 4 hours,the oxygen content of TiO₂products reduced by magnesium powder in Ar and H₂ atmospheres is 4.48wt.%and 1.98 wt.%,respectively.The reduction powder in Ar and H₂atmospheres requires a heat treatment temperature of 1100°C and 1300°C,respectively,to sealing the pores in the reduced powder.After 6 hours of hydrogen assisted deoxygenation treatment of magnesium powder,the oxygen content in the powder is 1.12 wt.%and 0.051 wt.%,respectively.From an experimental perspective,the phase evolution strengthening deoxygenation effect of hydrogen was demonstrated.（2）Low oxygen（O<0.1 wt.%）deoxygenated titanium alloy powder was directly prepared using UGS as raw material using the method of“hydrogen assisted magnesium reduction-heat treatment-hydrogen assisted magnesium deoxygenation”,achieving a short process and clean preparation from UGS to low oxygen titanium alloy powder.A short process method of“hydrogen assisted magnesium reduction-heat treatment-hydrogen assisted magnesium powder deoxidation”was proposed to prepare low oxygen titanium alloy powder using UGS as raw material.β-phase stable elements such as Fe,Cr,Ni,etc.in UGS can not only be used as alloying elements,but also can reduce the adhesion of Ti-O solid solution.The morphology,elemental content,and phase evolution of powder products during the preparation process were studied.The oxygen content differences of titanium alloy powder products prepared by reducing UGS with magnesium powder under different atmospheres were compared.The oxygen content in titanium alloy powder products reduced by magnesium under H₂ and Ar atmospheres was about 0.96 wt.%（about 1 wt.%）and 4.12wt.%（about 4 wt.%）,respectively.After deoxidation,the oxygen content in the powder was 0.032 wt.%and 0.94 wt.%,respectively.（3）The titanium alloy powder（with an oxygen content of about 1 wt.%）prepared by hydrogen assisted reduction of UGS was compacted and sintered to prepare low porosity and high strength titanium alloy products,achieving a short process preparation from UGS to titanium alloy products.Titanium alloys were prepared using a short process of“static pressure-sintering”using powders reduced by UGS as raw materials.A powder with an average particle size of 4.70μm and an oxygen content of approximately 1 wt.%.was obtained by hydrogen assisted magnesium reduction of UGS.Elements such as Fe and Cr in UGS can lead to the formation ofα-Ti,Ti₅Si₃,Ti-Fe,and the laves-C₁₄ phase formed by Ti with the high melting point elements Cr which at a sintering temperature of1000°C generates 6.7%of the liquid phase inside the alloy,which leads to liquid phase sintering and is beneficial for sintering densification.High melting point elements Fe,Cr,etc.are beneficial for improving the mechanical properties of the alloy,while the precipitation of Ti₅Si₃ phase is not conducive to the mechanical properties of the alloy.The effects of sintering temperature,powder forming method,Si element,etc.on the phase changes,porosity,and mechanical properties of the alloy during sintering preparation process was investigated.The alloy powder was compacted using axial single compression,and the alloy strength was 1563 MPa at 1100°C.The alloy powder was compacted using cold isostatic pressing technology at 350 MPa,and the compressive strength of the alloy was significantly improved.The compressive strength of 1563 MPa obtained from axial single compression increased to 1886.95 MPa.Na OH solution can remove Si element from UGS.After desilication,the Si element content in the alloy decreased from1.51 wt.%to 0.18 wt.%.The Ti₅Si₃ phase in the alloy was removed,and the compressive strength of the alloy is improved.At 1100°C,the compressive strength of the alloy increased from 1886.95 MPa before desilication to 2083.6 MPa.Micro CT data shows that the porosity of sintered alloys at 1100°C decreased from 0.14%before desilication to 0.04%after desilication.（4）Low oxygen dense titanium alloy powder was directly prepared from metal oxides using the method of“Mg powder reduction-heat treatment-Ca particle deoxygenation”under Ar atmosphere.An alloy powder with an oxygen content of 2.47 wt.%can be obtained in the first step of Mg reduction,and an alloy powder with an oxygen content of 1.65 wt.%,and 0.055 wt.%can be obtained in the second step of deoxidation using Mg powders and Ca particles,respectively.The thermodynamic calculation results indicate that the affinity between V-Al and Ti Al are greater than those between Al Mg.Magnesium reduces three mixtures,and the reduced V element is first incorporated into the Al element before further diffusing into the Ti element.The thermodynamic calculation of preparing Ti-48Al-2Cr-2Nb alloy powder by reducing TiO₂,Al₂O₃,Nb₂O₅,and Cr₂O₃ with Mg powder shows that the equilibrium composition of the system is Ti₃Al,Ti Al,and a small amount of Al Nb Ti,Al₂O₃.The first step of Mg reduction in Ar atmosphere can obtain alloy powder with an oxygen content of 1.9 wt.%.The second step of Mg powder,and Ca particle deoxidation can obtain deoxygenated alloy powder with an oxygen content about 1.6 wt.%and below 0.07 wt.%,respectively.Al element is partially lost during reduction leaching,so the amount of Al₂O₃ added to the raw material should be excessive by 10%.