Investigation Of Milling Of TiH₂Powders And Its Inlfuence On Microstructure Evolution Of Sintered Ti From The Milled Powders

Using titanium hydride as raw material, ultrafine titanium could be prepared byhigh energy ball milling and adding graphite respectively. The influence of differentmilling condition on powder size and powder morphology had been studied, and theeffect of milling condition and C content on the evolution of microstructures wasdiscussed as well.The speed of fixed planetary ball milling was200r/min, using polyethyleneglycol4000as PCA（process control agent）, and the spherical tank was flushed with highpure argon （99.99%）, followed by evacuation. The study on effect of TiH₂powderfrom different milling condition showed that the powder had high activity after milledso that it could react with oxygen and generate TiO₂in air, thus while sampling invacuum glove box filled with argon could protected power from being contaminated.TiH₂didn’t decomposed in any milling condition.The milling process of TiH₂powder can be divided into three steps: The powdersize sharply decreased at the early stage of milling. The size did not change obviouslyin the middle stage and during this period what mainly happened is powderhomogenization. In final stages, the size increased because the cold welding wasbetter than crushing effect. The most appropriate time of ball milling was10hours.After30min, the size of the powder declined abruptly from30μm to5μm because ofnano-powder produced. After10hours, all powders were broken into nanometer level,and the size was about500nm and surface of the powder was smooth without reuniting.Optimum ball to powder ratio was20:1and it was more important than the time of ballmilling.DSC results showed that TiH₂powder dehydrogenation temperature decreasedwith increasing milling time. Termination temperature of TiH₂→TiH1.5was from480°C down to435°C after15h milling and temperature of TiH1.5→αTi was reducedfrom550°C to535°C.The results of pressing experiment showed that the green density increased withthe increasing of milling time and ball to powder ratio within10h during milling.After15h, the longer the ball to powder ratio, the smaller the green density was. Thehighest density appeared when the ball to weight ratio was20:1and the milling timewas10h, but the density was2.42g/cm³, which was very low. According to DSCcurves of PEG and the color change of the sample surface after debinded, optimum debinding process was determined as follows：The sintering temperature studies had shown that the optimum sinteringtemperature was1050°C. Sintered density was4.5g/cm³and shrinkage was46.8%,The microstructure observation showed that microstructure with less pore was uniformand the grain size was below5μm at1050°C. Because of liquid’s emergence over1050°C, the liquid-phase sintering resulted in grain growth, some up to20μm. At1250°C, flake-axis grain by liquid tension severely deformed distortions changed intoflocculent. The best sintering process was:Study of the milling process, on the sintering organizational impact, revealed that:the sintered samples pressed by power milled10h has the highest density, which wasup to4.62g/cm³. After milled5h, lamellar equiaxed grain size sharply dropped from40μm to10μm; sintered grain size of sample milled10h maintained at below5μm, andmilled15h sample grains began to coarse. When Ball to powder ratio was20:1,organization of sample is the most uniform, and grain size was below5μm.After adding graphite, C reacted with Ti to generate granules of TiC by diffusionmechanism, which size was about500nm and mainly distributed at the grainboundaries of αTi. The TiC effectively inhibit grain growth, refined grain, andprepared ultrafine grained titanium with grain size of about3μm. With the increasingof graphite content, TiC volume fraction and size increased. Fe element in theorganization mainly distributed in the holes.As Ball to powder ratio increasing, The αTi grain size remained unchanged and TiCparticle size increased slightly. After milled5h, the microstructure was still the bulkysheet αTi and no TiC particles appeared. As milling time increasing, αTi refined andthe shape from flake gradual over to the irregular shape, and finally into granular. TiCcontents increased significantly with the increase of milling time.