Study On Gelcasting Of Titanium Alloy

Titanium alloys are advanced structural materials because of low density, high strength to weight ratio, excellent elevated temperature performance and superior corrosion resistance, which have great application in aerospace and application potentials in industries of chemistry, oil, metallurgy, medical and sports. However, the major issue of high-cost limits their application in above civil fields. Therefore, it is significantly important to explore a low-cost technique for the fabrication of titanium alloy.Powder metallurgy technique could meet both high performance and low cost objectives. In order to fabricate titanium parts with large size and complex shape, gelcasting was utilized for the forming of titanium alloy in this study. To solve the oxygenation problem in powder metallurgy process, oxygen controlling in the processing of powder preparation, gelcasting and sintering were strictly operated. As for powder preparation, the low-oxygen hydride-dehydride process was studied. In gelcasting, the low-residual gel system was developed. In the processing of sintering, the effects of calcium and neodymium on the oxygen enrichment was clarified. The results are as follows:Titanium hydride and titanium powders (100mesh,200mesh,325mesh、500mesh) were prepared by hydride-dehydride process. The oxygenation in hydride, grinding and dehydride process was controlled by using high-purity hydrogen, nitrogen protection and high vacuum respectively. Besides, the powders were kept away from the air while process conversion. Therefore, the oxygen content of titanium hydride and titanium powder was controlled to be lower than1000ppm.The gelcasting parameters of titanium hydride and titanium powder were compared. The solid loading of titanium hydride and titanium powder was50vol.%and37vol.%respectively. After sintered at1300℃. the relative density of titanium hydride greenbody was95.8%and the linear shrinkage rate was22.5%. The same parameters of titanium greenbody were92.5%and20.8%.To solve the oxygenation problem in conventional gel system, the polystyrene gel system with no oxygen was developed.1) The solid loading reached50vol.%under optimized conditions:the content of dispersion was0.55wt.%and the concentration of monomer was50vol.%. The gelation time was3.5h under such conditions:the content of crosslinking agent was40vol.%, the reaction temperature was90℃and the concentration of BPO was130mmol·L-By applying the polystyrene gel system into gelcasting, the bending strength of greenbody reached24MPa.2) Low molecular mass organic gelcasting was used for gelcasting. The solid loading reached50vol.%by ball-milling. The gelation time rose with the increase of reaction temperature. The optimized temperature was in the range from40to45℃and the gelation time was controlled in the range from3to10min. By adding polystyrene (0.08g·mL-1), the bending strength of greenbody was improved to10MPa.The oxygenation and recarburization from the polystyrene gel system were0.07wt.%and0.2wt.%respectively. The oxygenation and recarburization from the low molecular mass organic gel system were0.1wt.%and0.16wt.%respectively. By using the low-residual gel system, the tensile strength of as-prepared titanium was510MPa and the elongation was6.5%.The effects of magnesium, calcium and neodymium on the densification behavior of titanium hydride powder and oxygen control were studied. The above three additives all improved the densification and the relative sintered density rose from96%to98.1%,98.2%and98.6%with the amount of0.5wt.%(Mg).0.375wt.%(CaH2) and0.5wt.%(Nd) respectively. Owing to the improvement of density, the tensile strength of the specimen containing magnesium was538MPa and the elongation was7.1%. By improving density and oxygen enrichment, the tensile strength of the specimen containing calcium was545MPa and the elongation was9.2%. The tensile strength of the specimen containing neodymium was590MPa and the elongation was10.1%.Titanium handles were developed by gelcasting and the cost was about73yuan per kilogram. Compared with conventional handles of stainless steel, copper alloy and allumen, the titanium handles have advantages of low-density, aesthetic and excellent biocompatibility. Therefore, the titanium handle has great market prospect and may expand its application in civil fields.