Effect Of Hydrogen On The Microstructure And Composition Of The Interfacial Reaction Layer Of Titanium Casting

Due to the titanium alloy in molten state is extremely active, can be easily react with the mold during investment casting process. Recent research reveals that hydrogen can refine the microstructure and purify the molten titanium alloy. So in this study, the thermohydrogen process is used to treat the Ti-6Al-4V castings, and under laboratory conditions the effect of hydrogen containing atmosphere on the interfacial reaction between titanium alloy melt and Al2O3 mold is investigated. The chemical composition, microstructure and micro hardness of the interfacial reaction layer are examined to research the hydrogen-involved interfacial reaction, and the mechanism of hydrogen induced in restraining interfacial reaction of titanium alloy melt and Al2O3 mold is discussed. An effective methods on controlling the interfacial reaction is proposed.The results show that, after thermohydrogen process, the oxygen concentration and the micro hardness along the interface layer in Ti-6Al-4V castings is decreased with increasing hydrogen content and treatment temperature. The microstructure is refined and significantly improved the performance of TC4 alloy castings.The variation trend of the element content and micro hardness under different hydrogen partial pressure proved that the melt hydrogenation can effectively restrain the interfacial reaction between titanium alloy melt and Al2O3 mold, and decrease the reaction distance. When the hydrogen percentage is 10%(10 kPa), the thickness of interfacial reaction layer between pure titanium and Al2O3 mold is decreased from 200μm without hydrogen to 55μm. When the hydrogen partial pressure is 20 kPa, the reaction distance between Ti-6Al-4V and Al2O3 mold is decreased from 130μm to 50μm, and reduce the melting current a little bit, the thickness of interfacial reaction layer can be reduced to 30μm. By comparing the SEM micrograph of different hydrogen partial pressure, it can be seen that after melt hydrogenation the diffusion of ceramic phase is restrained. Less ceramic phase can diffuse into the titanium alloy melt, so the content of each element in the interfacial reaction layer fluctuates within a smaller range.Beside the oxygen concentration in the interface decreases significantly.The mechanism of hydrogen induced in restraining interfacial reaction between titanium alloy melt and Al2O3 mold is proposed: the oxygen atoms coming from the decomposition of ceramic phase react with the hydrogen atoms dissolved in the alloy melt form water molecules. The exist of water molecules restrains the diffusion of ceramic phase. It also hinders the substitution reaction between the titanium alloy and the surface layer of the mold. At the same time hydrogenation refines the microstructure of titanium, so it will take more energy for the atoms diffuse from the surface to the inside during solid-state phase transformation processes.