Injection Molding Of Titanium Alloy

Titanium alloy has wonderful properties and is widely used in many industries, but it is not easy to be machined. Metal Injection Molding (MIM) is considered as a promising shaping technique. An investigation was started to test whether injection molding of titanium alloy can result in products with good mechanical properties. It had two parts including using hydrogenation-dehydrogenation (HDH) titanium powder and hydride titanium powder individually to make MIM Ti-6Al-4V alloy. A large amount of experimental and theoretical analyses were conducted by optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDXA), X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA) and differential thermal analysis (DTA).The process of making MIM Ti-6Al-4V alloy using HDH titanium powder was studied. An oil-wax-polyolefine binder system with powder loading 55vol% was injection molded. The optimal molding parameters for the feedstock are as following: temperature for molding 152+2℃, temperature of mould 38℃, molding pressure 65%. Two steps of debinding were used in the study. Firstly, green parts were dipped into the solvent CH2C12 for 4 hours with the debinding rate up to 58%~60%. And then themal debinding began in Ar atmosphere with the following process: 2℃/min 200℃ (30min) 2℃/min 400℃ (90min) 2℃/min 600℃ (60min) 5℃/min 720 ℃ (90min) , cooling in furnace. During the process of sintering, high properties were obtained in the high vacuum of 10-3Pa with the temperature of 1250℃ for 3h. Tensile strength of the sample with low impurity contents is 819MPa and elongation percentage is 7%, which is up to the international levels of MIM Ti-6Al-4V alloy.Theoretical analyses of dehydrogenation/debinding process in making MIM Ti-6Al-4V alloy by using hydride titanium powder were studied. Dehydrogenation reaction starts around 300℃ when the pressure of hydrogen is down to 10-2Pa. The DTA-TGA of binder and hydride Ti-6Al-4V powder respectively show that the decomposition temperature of binder is between 172℃ to 513℃ and the dehydrogenationtemperature of hydride Ti-6Al-4V powder is between 350℃ to 700℃. Therefore, when themal debinding process is taken in 10-2Pa under 720 ℃, hydrogen can be detached from the parts. In the experiment because of the limitation of vacuum debinding condition, another debinding/ dehydrogenation-sintering process was used. That is, parts were themal debinded in Ar atmosphere under the temperature of 720 ℃, and then were dehydrogenated-sintered in the vacuum of 10-3Pa. The specimens had the hydrogen content only with 0.014wt%, which is in the standard domain of interstitial elements of Ti-6Al-4V alloy.