Using silica sol as a binder for titanium investment casting is very attractive due to its good stability and reasonable cost as compared with yttrium sol and zirconium sol. However, the mechanism of interface reaction in the related system remains unclear. In this investigation, the interface reaction between Y_2O_3-SiO_2(YSi) shell mold and titanium alloys was studied. A group of shell molds were prepared by using Y_2O_3 sand and silica sol with different contents of SiO_2. Ti-6Al-4V alloy was cast under vacuum by gravity casting through cold crucible induction melting(CCIM) method. Scanning electron microscopy(SEM) and energy dispersive x-ray spectroscopy(EDS) were employed to characterize the micromorphology and composition of the reaction area, respectively X-ray photoelectron spectroscopy(XPS) was used to confirm the valence state of relevant elements. White ligh interferometer(WLI) was used to obtain the surface topography of Y-Si shells. The results show that the thickness of reaction layers is below 3 μm when the SiO_2 content of silica sol is below 20 wt.%. Whereas, when the SiO_2 content increases to 25 wt.%, the thickness of the reaction layer increases sharply to about 15 μm. There is a good balance between chemical inertness and mechanical performance when the SiO_2 content is between 15 and 20 wt.%. Moreover, it was found that the distribution of SiO_2 and the roughness at the surface of the shell are the key factors that determine the level of reaction.
The chemical reaction between mold material and titanium melt during investment casting was studied intensively. However, the in fluence of residue gas in ceramic mold on interface reaction remains unclear. In this investigation, the effect of residue gas in Y2O3-silica sol shell mold on interface reaction during Ti-6A1-4V alloy investment casting was investigated. Two groups of shell molds were prepared by adding different kinds of pore formers, i.e., spherical starch particles or nylon fibers, respectively. Ti-6Al-4V alloy was cast under vacuum by gravity casting through cold crucible induction melting (CCIM) method. Porosity of different shell molds was measured based on Archimedean method. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were employed to characterize the micromorphology and composition of the reaction area, respectively. White light interferometer (WLI) was used to obtain the surface topography of the shell mold. The results show that the direct chemical reaction is very weak for all specimens. The release of residue gas in closed pores is the key factor influencing surface defects. However, open pores make nearly no difference on the interface reaction.
The forming process and mechanism of the reaction of Ti–6Al–4V investment casting made by Zr(CH_3COO)_2-Y_2O_3 shell in vacuum casting was studied.Statistic was manipulated to study the distributions and types of the reaction layers. The morphology and composition of the reaction layers were tested using field emission scanning electron microscopy(FESEM) combined with energy-dispersive spectroscopy(EDS). Phase of the reaction layers was characterized by X-ray diffraction(XRD).Composition of the shell reaction zone was measured by X-ray fluorescence(XRF) method. The results suggest the reaction contains oxidation and element evaporation, and the melt reacts little with the shell but mainly with the remnant gas. The reaction layers contain three types due to different forming stages: the titanium oxidation film, the concretion film and the shell reaction zone. The interfacial temperature and pressure affect the reaction mechanism and degree, leading to three types of concretion films that differ in thickness, content and color.
The pH value and viscosity of Y_2O_3–SiO_2 (Y–Si) slurry made by Y_2O_3 powders and silica sol for the face coat of Ti–6Al–4V investment casting were measured. The thermal behavior of the shell made by the Y–Si face coat system was investigated by differential scanning calorimeter(DSC), thermal gravimetric(TG) analysis combined with mass spectrometry(MS), and the phase transformations were determined by X-ray diffraction(XRD). Hot strength, residual strength, linear expansion coefficient, and wearing resistance performance of the shell were also tested. The microstructure and elements distribution of the interaction layer were studied by scanning electron microscope(SEM) and energy-dispersive spectrometer(EDS), respectively. The microhardness tester was applied for the microhardness. The results showed that the slurry was stable for at least 60 h. A very small amount of YZrO_3 was formed below 1050 °C and Y_2 SiO_5was formed around 1450 °C. The shell made by Y–Si system had good mechanical property which could reduce cracks during the procedure of dewaxing and inclusions during pouring.Some Al volatilized from the melt, permeated the surface of the face coat shell, and formed the black reaction layer,which blocked the permeation of O so that O penetration was limited to 5 lm. The depth of Si penetration was about 60 lm. The hard layer was also around 60 lm.
