Reaction between Ti–6Al–4V and Y₂O₃–SiO₂ based face shell for investment casting

The pH value and viscosity of Y₂O₃–SiO₂ (Y–Si) slurry made by Y₂O₃ 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 YZrO₃ was formed below 1050 °C and Y₂SiO₅ was 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 μm. The depth of Si penetration was about 60 μm. The hard layer was also around 60 μm.