Dehydrogening Character, Compactibility And Sintering Performance Of TiH₂ Powder

In this paper compactibility,dehydrogenation and sintering behaviors of TiH₂ have been studied systematically in order to establish a sound foundation for manufacturing high performance Ti-based products with TiH₂ powders as raw materials.The results of pressing experiment show that,though the compactibility of pure TiH₂ powders is too poor to obtain a fine compact,high quality compacts can be pressed as a good binder is added into the powders.With stearic acid,octadecanol, octadecyl amine,adamantane and their intermixtures as candidate binders,the three binders addition ways are used as blending with TiH₂ powders,being coated on the surface of a pressing mold's cavity and adopting the former two ways simultaneously. As the first way is used the results reveal that intermixture binders show better effect on improving compactibility than single constituent one do,and among them a mixture of octadecanol and adamantane becomes the best.A green compact with relative density over 86%is produced when 0.4wt.%the best intermixture binder is added.It is found that among the there binder addition ways,the second one is the best, by which not only the pollution of a compact from a binder is notably reduced,but also a green compact with high relative density and good surface quality can be fabricated.However the third way shows less advantage than the other two ones. Moreover,it is also found that the function between relative density of a TiH₂ green compact and pressing force is exponential relationship and a reasonable pressing force for TiH₂ powder compacts is proposed as 600～700MPa.Through TG and DSC testing,dehydrogening experiment in vacuum of TiH₂ powders and microscopic structure and phase constituent inspectation of TiH₂ powder samples quenched from high temperatures as well as dehydrogenation kinetics calculation,it is revealed that dehydrogenation process from TiH₂ to Ti as temperature increases acts as following:TiH₂→TiH_1.5→Ti(with H in solution)→Ti and the activation energy for each step is 240 kJ/mol,190 kJ/mol,140-150 kJ/mol respectively. TiH₂ decomposes in vacuum more easily than in a high purity argon atmosphere.The dehydrogenation temperature range for both TiH₂ powders and its green compact is almost the same,but their main behavior difference lays in the temperature of the maximum dehydrogening speed with green compact's higher by 100℃.The microscopic structure of the quenched sample from 600℃～700℃consist of TiH_1.5 needle distributed onα-Ti with orientation in random.When temperature is above 800℃,H only can solid soluted in Ti.TiH₂ powder dehydrogenation kinetics calculation shows that model-free method is more suitable than model-fitting method and that the mechanism of TiH₂→TiH_1.5 is three dimensions phase boundary reaction and the mechanism of TiH_1.5→Ti is chemical reaction.The results of sintering experiment with hydrogen atmosphere at low temperature and vacuum atmosphere at high temperature and sintering temperature between 1150℃～1350℃reveal that the sintering ability of TiH₂ powders is good.With sintering temperature increasing,sample density increases rapidly at first and then obviously more slowly as sintering temperature is over 1250℃,but the grain size grows more and more quickly at the same time,and pore size transforms from big to small as sintering temperature below 1300℃and to big gain as sintering temperature over 1300℃.Moreover,it is also found that the size of raw powders has significant influence on microscopic structure of sintered samples.The smaller the particle size, the finer the grain size,and vice versa.In order to produce products with both high density and fine grain size sintering temperature should be blow 1250℃and it is better dehydrogening in Ar or H₂ atmoshere.