Preparation of superfine TiH2 powder with high energy ball milling


High energy ball milling is an effective way of producing superfine powders and is a kind of new preparation way for powder materials. As titanium sponge that has high purity will be soft and has tenacity under normal temperature and pressure, it will be very difficult to produce titanium powder by directly smashing titanium sponge. Titanium hydride that appears to be loose shape will generate after flexile titanium sponge particles react with hydrogen. Due to the hydrogen brittleness of titanium, it will be easy to smash and will be shattered to nanocrystalline by mechanical milling in a short time. In the preparation of titanium alloy with powder metallurgy, by replacing titanium powder with hydrogenated titanium powder, the superfine hydrogenated titanium powder that generated in this process will have very important influence on the subsequent forming and sintering technology, which can refine grains as well as improve compact density and product performance. Besides, hydrogen that exists in titanium hydride can be smoothly removed in the subsequent sintering process.

 a. The selection of ball mill mechanism and technological parameter

In the high energy ball milling process, grinding balls that have same weight will shock the materials at the centripetal acceleration more than dozens of times of the gravitational acceleration, thus the forces of grinding balls that are acted on the materials will increase dozens of times and will greatly improve the impact crushing capacity. Micro forging process can be called as the first stage of metal powder ball milling, in which the particles will be deformed as well as will be shocked and squashed repeatedly by grinding balls. However, the brittle powders usually have no micro forging process; the second stage is fracture process, in which the crack will produce, extend and the particles will rupture, besides, the particle size will continuously decrease; in the final stage after a certain ball milling time, as ball mill is not enough to provide larger fragmentation energy, cold welding and fragmentation will reach a balance, thus it will generate balanced and agglomerated particles, of which the particle size is also the ultimate particle size of smashing.

hydrogenated titanium powdersBall mill process is a very complicated process and can be influenced by a lot of factors that mainly include the revolving speed of ball mill cylinder, ball mill time, ball mill medium, ball mill atmosphere, process control agent and the size of grinding balls. Different ball mill results will occur under different ball mill conditions. Media-to-material ratio is also one of important parameters in the ball mill process, as when it increases, the grinding ball quantity and the collision frequency will both increase, thus it will transfer more energy to powder and reduce ball mill time.

b. The influence of ball mill time on the particle size and morphology of TiH2 powder

Ball mill time will have very large influence on ball mill results. In the ball mill process, the particle size will decrease sharply in a short time firstly and will gradually decrease with the extension of time. The powder morphology will gradually change from irregular shape to equiaxed shape and finally appear to be glomerate flocculence. Powder agglomeration is mainly caused by fine particles and the rapid increase of surface free energy. Therefore, by reuniting the small particle powders, the surface energy can be reduced.

There is a particle size limit in the ball mill of brittle materials. When reaching the limit value, the powder particle size will not reduce anymore with further ball mill. At this time, the energy that provided by ball mill may change the thermodynamic state of powders and lead to alloying. When the particle size reaches to limit value, the particle size will not become fine anymore. On the contrary, the particle size may increase with further ball mill. This is because in the ball mill process, powder particles will pass welding and rupture process and when the two processes reach a balance, the particle size will also become stable and reach limit value. In this experiment, after observing the TiH2powders that respectively generated in the ball mill time of 60min and 30min under scanning electron microscope, the result shows that powder will have agglomeration at the ball mill time of 60min. Besides, when the ball mill time is too long, the pollution level of powder will increase due to long time impact and friction between ball mill medium and powder.

c. The influence of other ball mill conditions

High energy ball milling is an effective way of producing superfine powder materials. In the ball mill process, process control agent is generally added, which can reduce the cold welding that occurred between powder particles, inhibit powder agglomeration and visco-wall, reduce the powder surface activity and agglomeration as well as obtain finer powders. Ball mill is generally divided into dry grinding and wet grinding. When using wet grinding process, after adding THi2 and steel ball (the grinding medium) in the ball mill pot, heptane will be added in the free space for ball mill. By adding heptane, it can effectively reduce metal oxidation, prevent the reaggregation and growth of particles and reduce the composition segregation. If the ball mill pot is not added with heptane and use dry grinding under highly pure argon protection, powders will rapidly change the color when the ball mill pot is opened, which means that powders have been partially oxidized.

By carrying out ball mill test under normal temperature, THi2 may be resolved in the ball mill process after a period of time of ball mill under wet grinding due to the fierce collision and impact between grinding balls and powders. Some experiments have shown that by having ball mill with aluminum powder and titanium hydride instead of titanium, titanium hydride will dehydrogenize in the high energy ball milling process. This experiment is carried out under wet grinding, as the ball mill pot is filled with heptanes, it can be seen that a small amount of bubbles are adsorbed on the ball mill pot wall, this could be some hydrogen that was separated out by titanium hydride in the ball mill process.

The conclusions are as follows: a. as titanium hydride is brittle and easy to crush into fine particles, it can be made into nanoscale superfine powders by high energy ball milling; b. ball mill time will have large influence on the particle size of THi2 powders. By using titanium hydride powder that has the particle size of 40m as raw materials and grind it in 60min with planetary high energy ball milling, the superfine titanium hydride powder that has the particle size of 0.1m can be obtained; c. in the ball mill process, with the extension of ball mill time, the powder particle morphology will change from irregular shape to equiaxed shape and finally appear to be glomerate flocculence.