Investigation Of TiH₂ Powder Injection Molding

In this paper, with low-cost TiH₂ powder as raw material an new kind of clean and efficient binders was designed at first, and then each step of TiH₂ powder injection molding was investigated and main attension was paid into debinding and sintering. Finally sintered pure titanium products with full density and good-performance was successfully fabricated.During designing binders used titanium product manufaction by metal injection molding, how to reduce the impurity contamination as low as possible during debinding and improve powder loading and debinding efficiency were carefully considered. A new kind of efficient and clean waxed-based binders had been developed for TiH₂ powder injection molding by experiments. The binders mainly consist of (68～77%)Parafin Wax,(5～10%)Liquid Parafin Wax,(6～10%) M,(5～10%) (EVA+PP) and 5%SA. The miscibility, rheological, thermal and debinding properties of newly designed binders had been evaluated systematically. Experimental results indicate that:there is good miscibility among all the constituents and uniform mixing can be obtained between the binders and TiH₂ powders even in the case of with a powder loading as high as over 70vol%; the optimized binders shows extremely low viscosity and very good rheological property stability with temperature, such as a viscosity and viscosity fluid energy E are of 0.02625Pa.s and 29.05kJ.mol^-1 at 160°C for a shear rate of 1250s^-1; the binder can be debonded efficiently by normal heptane solvent debinding and binder loss ratio reaches 72.01% and 79.21% for 3 and 6 hours respectively at 50°C. Compared to common binder only consisting of paraffin wax and polymer, the binder loss ratio can be increased by 20%～30% under the same debinding conditions. Debond brown compacts reveal less flaws and good shape-holding capability; it was also found that because the binders with polymer content as low as 7vol% can provides injected compacts with good strength, carbon conten as impurity of Brown compacts was increased by around 0.02wt% in comparison with raw powder's, and its oxygen and nitrogen content were slightly decreased by 0.11wt% and 0.22wt% respectively.Injected compacts with the newly developed binder were then solvent and thermally debond respectively. The results reveal solvent debinding process could be divided into two stages; in the initial stage dissolution of binder into the solvent was the key process control link, and temperature is the main factor in improving solvent debinding rate; in the second stage diffusion of the solvent binder in normal heptane became the key control link, and increasing the volume ratio of normal heptane solvent to injected compacts as well as strenthening convection in normal heptane solvent were effective ways to improving solvent debinding rate. In a heptane environment, an optimized solvent debinding process was obtained by experiments, that is of solvent debinding for 5h at 40°C with a binder loss ratio of 76.37%.It was found that thermal debinding process and parameters were determined by both thermal decomposition properties of the binder constituent and the dehydrogenation properties of TiH₂ powder.Through optimization of thermal debinding parameters, defect-free brown compacts with good strength and good shape-holding capability cound be obtained.The effects of Sintering parameters such as temperature, sintering time and sintering atmosphere on the densification and microstructure of as-sintered titanium products were investigated sysmatically. The results shew that full dengsity sintered titanium products could be manufacture with TiH₂ powders as raw materials by MIM. With sintering temperature and time increase, their porosity was reduced and the density increased markably. Through the optimized sintering process, the relative density of sintered product reached 98.96%, and its oxygen and carbon contens were reduced and increased by about 0.06wt% and 0.07wt% respectively in comparison with raw powders, and with nitrogen content 0.0005wt% and Hydrogen content 0.0001wt%. The mechanical properties of sintered product were:σ0.2=264MPa,σb=325MPa,δ=4.43%.