Effect Of Hydrogen On Casting Character And Investment Casting Interface Reaction Of TiAl Alloy

Ti Al alloy has high specific strength, good high temperature properties, creep resistance and oxidation resistance, with the potential of lightweight high temperature structural materials in the aerospace industry, but the brittleness at room temperature and poor machinability limit its further applications. Precision casting as an ideal net-shape forming technology is able to solve the poor workability and reduce the production cost, but its poor mould filling properties and the interface reaction with moulds limits the applications of casting.Thermal hydrogen treatment technology taking hydrogen as a kind of temporary alloying element, has been proven to improve the performance of high temperature deformation and modify the microstructures of the alloy, but there are limited studies on the casting process. The Ti Al alloy was melted in arc-melting furnace under hydrogen atmosphere, then poured into graphite mold and casting yttrium oxide ceramic shell, respectively. Effects of hydrogen on cast character of Ti Al alloy and interface reaction of investment casting were studied.This paper has studied the effect of hydrogen on fluidity of Ti Al alloy through the spiral graphite mould. With the increase of hydrogen partial pressure in arc-melting furnace, the fluidity of Ti Al alloy increases. At the hydrogen partial pressure of 15 k Pa, the fluidity of Ti Al alloy has increased by 209% than that at hydrogen partial pressure of 0 k Pa. When hydrogen partial pressure is more than 15 kpa, the fluidity increasing trend is steady. Hydrogen reduces the bonding effects between atoms of Ti Al alloy, reduces the alloy melt viscosity and surface tension, at the same time reduces the liquidus, relatively increased the degree of superheat, narrowed the crystallization temperature range.This paper has studied the effect of hydrogen on complex casting of Ti Al alloy through thin perforated graphite mold. With the increase of hydrogen partial pressure, the volume fraction filled increases. When hydrogen partial pressure was 20 k Pa, the volume fraction has increased by about 25%. Due to the low deformability of graphite mould and turbulence of alloy, there are a small amount of cracks and pores in castings. hydrogen had no significant effect on the number and distribution of the castings’ crack and pores.This paper has studied the effect of hydrogen on the interface reaction of Ti Al alloy cast by yttrium oxide ceramic shell. Interface reaction generated oxide Ti3 O, Ti O, Ti O2 and Al2O3, and new phase Y2(Ti2O7), Y3Al2(Al O4)3. The diffusion of oxygen and yttrium elements in reaction layer is thin. Hydrogen and oxygen atoms in the interface reaction to generate the water molecules, the reaction layer formed micromorphology of pore on the surface, reduce the oxygen content and diffusion trend in the reaction layer. With the increase of hydrogen partial pressure, oxygen content in the reaction layer surface reduced. When hydrogen partial pressure of 7.5 k Pa, oxygen content decreased by 34% in reaction layer surface than hydrogen partial pressure of 0 k Pa.