Study On High Nb-TiAl Alloy For Automotive Exhaust Valve

The high Nb-TiAl alloy has some superior performances,as the low density,high specific strength,high Young's modulus,excellent oxidation resistance and creep resistance,excellent fatigue and corrosion resistance.More importantly,it still maintains these excellent properties at elevated temperature.Therefore,it is considered to be the most promising material for the development of lightweight heat-resistant structural parts used in aerospace and automotive engines.However,due to its poor ductility at room temperature and high reactivity at high temperature,the TiAl alloys are hard to be processed and manufactured,hindered the massive application of TiAl alloys from the cost consideration.In this paper,the manufacturing process of oxide ceramic mold shell for precision casting of TiAl alloy was studied.The ZrO₂-flour-Zirconium sol slurry was used as colloidal suspension and the ZrO₂ power for primary coating.The variation law of viscosity and rheological property of surface slurry were studied.Meanwhile,the problem of the mold shell cracking during dewaxing was successfully solved in this work.Moreover,the investment casting process of exhaust valve by high Nb-TiAl alloy was simulated by using the finite element casting simulation software ProCAST,and the shrinkage defects in the casting were predicted.The exhaust valves with complete shape were successfully produced,as a comparison and verification of the simulation,by the combination of investment casting and centrifugal casting.The macro-defects and microstructure of the as-cast exhaust valves were detected,the solidification path was discussed in combination with phase diagrams as well.The results showed that the viscosity of surface slurry increased with the increase of powder-liquid ratio,while,it decreased firstly and then increased with the increase of stirring time.On the contrary,it was inversely proportional to shear rate.The cracking of the mold shell during dewaxing can be sufficiently improved by the combination of reserving dewaxing channel and dewaxing in boiling water.The simulation results of the shell filling process revealed that the alloy melt flowed along the suction of the shell and then entered into the cavity under centrifugal force.It took about 3.9s to fully fill the shell cavities.However,in the actual pouring process,the upper exhaust valves of the mold shell were not fully filled,while the lower parts were fully filled.It was contrary to the simulation results.The as-cast valves had microscopic defects,such as shrinkage porosity in the stem,misrun at the edge of the head and entrapped porosity in the neck,which were basically consistent with the simulation results.The metallographic analysis results indicated that the lamellar??₂+??and equiaxed?phases,as well as the?-segregation and?-segregation,are observed in the as-cast e xhaust valves,with the grain size of 200-300?m and the interlamellar spacing of 2?m.According to the microstructure and phase transformation analysis above,the non-equi librium solidification path in the present work should be:L?L+??isomorphous????is omorphous?+??isomorphous????isomorphous&solid-state phase transformation?+??r esidual????residual?+??proeutectoid?+??residual???₂/??lamellar?+??proeutectoid?+??residual???₂/??lamellar?+?-segregation+?-segregation.