Study On Technology Of Investment Cast For Titanium Alloy Using Low Cost Oxide Ceramic Mould Shell

A novel low cost investment cast technology was developed for titanium alloy and was investigated in this thesis. The properties of binder, face coat slurry and mould shell were studied in detail, which help determine the technology of preparing mould shell including drying and baking technologies. The interface reaction between titanium alloys and the developed low cost oxide ceramic mould shell were studied. Microstructures and mechanical properties of the cast titanium alloys were studied as well. Numerical simulations of melt filling and solidification processes were carried out for the investment cast of titanium alloy, including designing pouring system, optimizing process parameters of melting and pouring, establishing theoretical basis for practical casting process. The large-size thin-wall complex-shape components were cast successfully by centrifugal investment cast. The quality and property tests of the casts verified that the newly developed low cost oxide ceramic mould shell is good enough to produce high quality titanium alloy cast in centrifugal force field.Size-grade distribution was investigated on the refractory powder of the low cost investment face coat slurry, and the rheological properties of slurry were studied. The results showed that the viscosity of slurry increased with increasing S/L ratio. A lowest slurry viscosity was found when the fine powder was 40% at the same S/L ratio. Better rheological properties of slurry were reached when S/L ratio was 2.4 and the content of fine powder was 40%.The drying time of mould shell was shortened with rising ambient temperature, but prolonged with increasing ambient humidity, particularly when ambient humidity was more than 70% the drying time was prolonged remarkably. Increasing air flow velocity could evidently shorten drying time, but an air flow velocity more than 5m/s did not change drying time much.A smallest bending strength of mould shell was obtained when the powder of slurry was composed of full coarse powder, and increasing the content of fine powder increased the bending strength. The bending strength of mould shell increased with increasing baking time and rising baking temperature as well.Based on the above results, the processing technology of preparing novel low cost oxide ceramic mould shell was determined to be: ambient temperature, 25℃-30℃; ambient humidity, 40%-50%; air flow velocity, 2.5m/s-3m/s; and drying time, 8-10 hours. The mould shell baking is with the following steps: holding for 30min at 150℃, holding for 1hour at 600℃followed by holding for 2-3hours at 950℃.The chemical activities of various elements in ternary titanium alloy system were computed on the basis of Miedema model and Kohler formula, and their activity coefficients were calculated. The simulation results showed that the increasing Al content results in an increase of activity coefficient of Al and a decrease of activity coefficients of Ti and Zr. Increasing Zr content increased the activity coefficient of Ti and reduced the activity coefficient of Al. It was also found that the activity coefficient of Al decreased with rising temperature and that the activity coefficients of Zr and Ti increased with rising temperature.The interface reaction between titanium alloys and the low cost oxide ceramic mould shell was studied. The results showed that the powder size grade distribution and optimized S/L contributed to weakening the interface reaction to obtain regular interface with an optimum state at an S/L of 2.4 and the content of fine powder of 40%. XRD and XPS analysis on the surface of cast titanium alloys showed that the surface of cast titanium alloy was composed of Al₂O₃,SiO₂ and little amount of Ti-Si compound. The results of interface reaction investigation and analysis of XRD and XPS confirmed that adding alloy elements into Ti alloy decreased the activity coefficient of Ti.To understand the characteristics of investment casting of large-size thin-wall complex-shape component, different runner systems were design and the cast filling time, temperature field and viscosity field were simulated during the processes of cast filling and solidification of titanium alloy.The processing technology of preparing wax patterns was investigated. The high surface finish wax patterns could be prepared using current technology, with an average roughness of about 1.04μm. The large-size thin-wall complex-shape titanium alloys casts were successfully cast in centrifugal force field using a water-cooling crucible vacuum induction skull melt (ISM) furnace. The experimental results showed that with the currently developed technology of preparing low cost oxide ceramic mould shell, ISM and centrifugal pouring technology, the good quality large thin-wall complex titanium alloy cast could be cast successfully.