Study On Oxide Ceramic Shell Mold Fabrication For The Investment Casting Of Titanium Alloy

Investment casting is an advanced molding process,which overcomes the disadvantages of lowmetal utilization, large amount of machine and high production costs in traditional plastic processing,and produces high-quality precision castings. However, using investment casting for titanium alloy,because of the high temperature chemical activity of titanium liquid, Ti can easily has chemicalreaction with the shell mold and the molten titanium has strong impact on the shell. Meanwhile, Theface layer’s surface roughness of the shell will directly affects the surface quality of titanium castings.This requires that the shells using for titanium investment casting have high chemical stability,strength, and a lower surface roughness.In a sense, the titanium investment casting is also the study ofthe shell mold used.This topic first considers the face layer of the shell’s using performance,craft and economy, andselects the shell materials,Namely each layer’s refractory powder, binder and sand materials. Anddetermine the slurry of face layer’s pulping process with a lot of basic tests: Slurry is prepared inaccordance with the order of zirconium sol→additives→ZrO2powder, and the stirring speed iscontrolled at30～50rpm/min,finally the ZrO2slurry is obtained with the viscosity of about30～40sand the suspension rate of approximately80%. In addition, the powder to liquid ratio has a greatimpact on the performance of the slurry and shell. Comprehensively considering the viscosity of theslurry,the temperature strength of the shell mold and the surface roughness, the powder to liquid ratioshould be controlled within5.0～6.0g/ml.In order to improve the suspension of the ZrO2water-based slurry, the test obtains a kind ofsuspending system: namely per1L of ZrO2water-based slurry was added each2ml of20g/L of theCMC,10g/L of PAMA,20g/L of sodium bentonite aqueous, increasing the suspension of originalwater-based slurry of zirconia from about78%to92%,and to prevent the precipitate compaction ofthe slurry effectively, to improve the life of the slurry and to improve the uniformity of theshell,increasing the retention time of the slurry from18days to36days.Meanwhile,because theadding amount only accounts0.002～0.003wt%of the ZrO2slurry, the shell has limited contact withTi liquid and the pollution layer of titanium alloy castings is Relatively thin.Trough the TG-DSC curve analysis and roasting process’s impacts on the shell high temperatureresidual strength,surface roughness and permeability, the test determines the baking process of theshell mold: After preheating30min at600℃, roast1.5².5h at the temperature of1000¹050℃. Withthis roasting process, the ZrO2shell does not have a significant volume expansion, and the phases arestable tetragonal before and after the roasting. After the roasting, the high temperature residual strength of the shell is about5MPa, and the surface roughness value maintains at1.2～1.6μm, whichmeets the pouring conditions of titanium alloy. The ZrO2shells using for pouring titanium andtitanium alloy have uniform thickness, smooth inner surface, and don’t have the disadvantages ofvisible cracks and coarse. The ZrO₂shells after pouring titanium solution do not form a new phaseboth in the face layer and the back layer.The shell mold obtained trough the above pulping and shell roasting technology pours TA2bodycastings, there is no casting defects in the surface and internal, and the surface roughness Ra=6.01μm,the surface pollution layer’s thickness is uniform and less than200μm, and pollution elements focusesonly within the contamination layer in the surface200μm, and do not penetrate into the interior of thetitanium castings.The TA2casting’s temperature tensile strength σb=745MPa, elongation rate δ=4.5%,and has no obvious casting defects.