Research In Mutual Influences Between Substances During Preparation Of Ti-W Alloy Via Molten Salt Electro-Deoxidation

Titanium alloys have many good properties, such as high specific strength, good corrosion resistance, low heat conductivity coefficient and wide temperature scope. Among them, TiW alloy with excellent characteristics of high strength, corrosion resistance, light quality, good chemical stability and good toughness, has been widely used in biology, micro-electronics, aircraft and spacecraft components manufacturing fields, and so on. At present, the main methods used for preparing TiW alloy are powder metallurgy, high temperature vacuum melting and mechanical alloying method, but these processes all have a high production cost, a complex preparation process and a low product purity, thus largely limiting the application of titanium tungsten alloy. In 2000, Professors Chen G.Z. and Fray D.J. in the university of Cambridge first proposed molten salt electro-deoxidation method, also known as FFC Cambridge process, by which spongy titanium was successfully prepared. In contrast to conventional methods, the electro-deoxidation method is relatively simple, inexpensive, energy efficient and environmentally friendly. Nonetheless, the extremely low electrolysis efficiency is considered a major challenge in optimizing the method for industrial applications. So, further research needs to be conducted.In this paper, TiW alloy was prepared by FFC method process, and the sintering experiment parameters of TiO₂-WO₃ pellets were explored. Studies about the interaction effect between substances and influence factors for the enhancement of TiO₂-WO₃ electrolysis efficiency during FFC Cambridge Process were conducted. Besides, researches about the influence of blending ratio towards electrolysis were carried on. Additionally, by comparing experimental results of TiW system with those of Ti Zr system electrolyzed in the same conditions, the deep analyses of the influence of mixed metal oxides reduction potential difference size on electrolysis were made. The following show the main contents of research and work:During the course of experiments, TiO₂-WO₃ pellets after pressing or sintering were easily split or broken due to the great difference of density and expansion coefficient between raw material WO₃ and TiO₂. Therefore, experimental parameters of the binding agent, pressing pressure and sintering temperature are optimized, with distilled water as the binding agent, 15 Mpa as pressing pressure and held on for 30 s and middle temperature district of tubular resistance furnace at 900℃ used for sintering samples. The problem of WO₃ pulverization during electrolysis process was solved by gradually improving cathode assembly, ultimately choosing the molybdenum rods-molybdenum basket to support the cathode samples and collect products. In addition, advice about adding cathode holes on the electrolytic device cover to accept more cathode rods was given for the purpose of electrolyzing multiple samples in an electrolysis experiment, which makes for reducing energy consumption and improving the efficiency of experiments.TiO₂, WO₃ and TiO₂-WO₃ were electrolyzed under the same conditions, and the results further confirmed the existence of interaction between substances during electrolysis process, leading to the promotion of TiO₂-WO₃ electrolysis efficiency. And it showed that the reduction of WO₃ is prior to that of TiO₂ during the electrolysis process. To explain why, a series of articles were consulted and the thermodynamic calculation was made, and results indicated that the reduction sequence of mixed metal oxides during the electrolysis process was mainly determined by the value of metal oxide decomposition voltage. The lower the value of metal oxide decomposition voltage is, the thermodynamically easier and kinetically faster the reduction of metal oxide would be. And thus it preferentially completed reduction within a shorter time.As for the enhancement of TiO₂-WO₃ electrolysis efficiency during FFC Cambridge Process, The following three points are described principally: 1) During the sintering process, TiO₂-WO₃ particles turned to be obviously refined in comparison to that of each pure TiO₂ or WO₃ due to the separation by the other kind of particles that avoided the same particles from growing up into larger ones. As a result, it shortened the solid phase transmission distance of O^2-, increased the specific surface area and porosity, which were advantageous to the electrolysis. 2) The early stage of electrolysis was mainly the reduction of WO₃ since the decomposition voltage of WO₃ is lower than that of TiO₂. During the electro-reduction process of WO₃, the existence of TiO₂ had hindered sample from forming a closed surface, which made the WO₃ inside and outside of TiO₂-WO₃ happen to reduce at the same time. In addition, O^2- removed from inner WO₃ can combine with inner TiO₂, which decreased the concentration of O^2- quickly and promoted the deoxidation of inner WO₃ smoothly, so the reduction rate of WO₃ in TiO₂-WO₃ is faster than that of pure WO₃. 3) The growth and decomposition process of CaTiO₃ in TiO₂-WO₃ was faster than that of CaTiO₃ in TiO₂ due to the co-reduction of WO₃, which accelerated the growth and decomposition of CaTiO₃ and promoted the rate of electrolysis. Meanwhile, the rapid reduction of WO₃ in TiO₂-WO₃ increased the sample conductivity, specific surface area and porosity, thereby promoting the reduction of TiO₂. However, Ti reduced on the surface combined with W forming dense TiW alloy surface, hinding the transportation of molten salt and O^2-, so that the reduction rate of TiO₂ is less than that of WO₃.TiO₂-WO₃ in different proportion were electrolyzed under the same condition in order to study the effect of mixing ratio on electrolysis. The results showed that the extent of particle refinement, specific surface area and porosity of sample decreased with the increase of WO₃ in TiO₂-WO₃, therefore, electrolysis efficient gradually becomes poor.TiO₂ and ZrO₂ and TiO^2-ZrO₂（mole ratio of 1:1） were electrolyzed under the same condition, and the experimental results were compared with those of TiW system under the same condition so as to study the influence of mixed metal oxides reduction potential difference size on electrolysis. The results showed that the electrolysis efficiency of TiW system with larger mixed metal oxides reduction potential difference size is better than that of TiZr system, indicating that the greater the difference size of mixed metal oxide reduction potential is, the better electrolysis efficiency is. In the meantime, it was found that the reduction of TiO₂ in the early stage of electrolysis hindered the reduction of ZrO₂, but on the whole, the electrolytic efficiency of mixed metal oxides is still better than that of each pure metal oxide.