Interfacial Reaction Research Of BT20Titanium Alloy By Sand Mould Casting And Investment Casting

In present work, BT20titanium alloy has been induction heated under vacuumcondition and casted under gravity. The reactions between the interface of the meltand three casting molds have been analyzed using XRD, SEM and nanoindention,respectively. The effects of the wall thickness on the interface reactions have beeninvestigated, and the mechanisms and influencing factors of the interface reactionhave been discussed. Furthermore, the variation of the interface reaction layers of theβ phase region in BT20alloy under furnace cooling, β phase air cooling, dual phaseair cooling, and dual phase furnace cooling have been discussed.XRD analysis indicates the emergence of TiO, TiO₂, CaTiSiO6phase at thesurface of the casting mold, a mutual diffusion reaction have occured at the interfaceof the melt and casting mold, the variation trend of O and Ti is very significant. Theinterfacial zone is composed by outermost sintered layer, O-stabilised alpha case andthe layer, in which interstitial elements were concentrated. Moreover the positioneffect of interfacial reaction is non-significant.Scouring action of molten liquid will activate the atoms of the coating. Mouldmaterials will influence the extent of diffusion of element. Under the same thickness,the ZrO₂ceremic shell has a largest diffusion degree, followed by zircon, the zirconmold with ZrO₂coating has the smallest diffusion degree. The extent of diffusion ofelement became wane with the thickness decreasing from30mm to10mm. The nanoindention test indicates the tensile module, hardness, tensile load is larger than that ofmatrix, the scale of indention is well corresponded with the degree of diffusion ofelement at the interface.Both β-phase region and α+β-phase region has non-significant effect on thecomposition of the brittle α layer, when the matrix micrstructure transits at thedifferent phase regions. But the quantity and the dimension of atomic-rich region ofsolute increases compared with that of as-cast sample. The diffusion of O hasoccurred in different phase region. The degree of oxygen diffusion is largest amongthe β phase region under furnace cooling, and the oxygen concentration is highest atthe interface when the dual phase region is cooled under air with largest tensilemodule, hardness and tensile load. The value of content of each element in reactionlayer not only depends on the solubility of metal lattice, but also on the extent ofself-diffusion of element, degree of supersaturation of vacancy, the dimension ofprecipitated phase and segregation region in different phase region.