Composition Optimization And Investment Casting Of Cast High Nb-TiAl Alloy

High Nb-TiAl alloys have become a development direction of TiAl alloys because of 60-100℃ higher service temperature, about 300-500 MPa higher strength and better oxidation resistance than conventional TiAl alloys. However, the inherent room temperature ductility of high Nb-TiAl alloys is lower, resulting in fabrication and processing difficulties, limiting their industrial applications. Investment casting method, as near-net forming technology, could produce complex high Nb-TiAl alloy component, with excellent surface quality. In this work, a high Nb-TiAl alloy, which is suitable for casting, was designed by alloying method. The effect of alloying elements on microstructure, mechanical and casting properties was studied. Numerical simulation method was used to optimaze the investment casting process of high Nb-TiAl alloy for blade casts. The actual casting experiments were carried out to validate the simulation results. The main conclusions and and innovations are listed as follows:(1) The effect of various alloying elements on the microstructure and mechanical properties of high Nb-TiAl alloys was studied. The experimental results showed that the addition of small amount of B (0.2%) could effectively refine the microstructure of Ti-45Al-8Nb alloy. Ti-45Al-8Nb-0.4B alloy has the best mechanical properties at room temperature. The addition of 0.5%Cr could improve the room temperature mechanical properties of Ti-45Al-8Nb-0.4B alloy because Cr is advantageous to reduce the stacking fault energy and increase deformation twins of the alloy. However, the addition of 2% Cr reduces the mechanical properties of alloy because β phase stabilizing element Cr could increase the volume fraction of brittleness B2 phase in as-cast microstructure. β phase stabilizing element Mn randomly distributes in high Nb-TiAl alloy, which increases the amount of other β phase stable elements Nb or Cr in B2 phase. The volume fraction of brittle B2 phase increases, so the mechanical properties of the alloy are reduced. The room and high temperature tensile mechanical properties of Si added alloy are lower than those of silicon-free alloy, because the silicide precipitations lead to boundary embrittlement. The B2 phase increases in 0.5Ni added alloy, which results in the decrease of room temperature tensile mechanical properties. The dot-like particles form in the colony boundary of the alloy with Y addition, which leads to boundary embrittlement and deteriorate the tensile mechanical properties of room and high temperatures.(2) The precipitated silicide phase in a high Nb-TiAl alloy with Si addition was studied. Unlike the Ti5Si3 precipitates formed in Ti-Al-Si ternary system, the silicide particles formed in this work are Nb5Si3 (designated as ε) phase and have a hexagonal D88 crystal structure. The formation of Nb5Si3 precipitates results in the reduction of Nb content, which is strong B2 phase stabilizing element. Therefore, the volume fraction of B2 phase obviously decreases in the alloy with Si addition. So, the mechanical properties of as-cast alloy with Si addition are improved. However, after heat treatments above 1000℃, the mechanical properties decrease gradually, because the presipitation of silicide particles leads to generation and propagation of microcracks. The semi-coherent interfaces of Nb5Si3 with the matrix result in formation of interface dislocations. Moreover, silicide particles further precipitate due to tensile stress, which can increase the rate of crack propagation. Si addition leads to y phase area expanded in the phase diagram.. After long-term cycling heat treatment or annealing, y phase forms at the silicide due to y phase stabilizing element Si addition, which leads to lamellar coarsening or dissolved.(3) The mold filling ability of high Nb-TiAl alloys was studied and the results showed that decreasing the liquidus and viscosity of alloys, improving interface reaction resistance between alloys and ZrO2 (Y2O3) molds, can improve the mold filling ability of alloys. For Ti-45Al-8Nb alloy, the addition of small amount (0.5%) Si, Y, Cr, B, Ni is beneficial to the mold filling ability; W, Mn, Co elememts have little effect on the mold filling ability; Mo or V is harmful to the fluidity of the alloys. Finally, the optimized composition determines to be Ti-45Al-8Nb-(0.5-0.7)Cr-0.5Si-0.4B, considering both mechanical properties and castability. Cr addition could improve the room temperature mechanical properties of the alloy because Cr is advantageous to reduce the stacking fault energy and increase deformation twins of the alloy. Si addition could improve the mold filling ability of alloy and reduce the β(B2) phase segregation. B addition could refine the microstructure and improve the mechanical properties of the alloy.(4) Finite element simulation software Procast was used to analyze the casting process and optimize the process parameters of investment casting. The optimized process paremeters of gravity casting are pouring temperature of 1650℃, the casting speed of 1 m/s, and shell mold preheat temperature of 400℃, respectively. Numerical simulation analysis results of centrifugal casting showed that the volume of shrinkage porosities in the casting is the smallest if the centrifugal rotational speed was 500 r/min. The blade components were successfully poured by vacuum induction suspended furnace with optimized process parameters. The experimental verification indicated that the simulation results are in good agreement with the experimental results. Numerical simulation is good guidance to the casting process. The experimental results showed that the internal and external quality of centrifugal casting blade is much better than that of gravity casting blade. The gravity casting blade has the misrun defects on its surface while the centrifugal casting blade has complete surface. Furthermore, the shrinkage porosities and crack defects of gravity casting are much more and dispersive than those of centrifugal casting blade.(5) Mrcrostructures of gravity and centrifugal casting blade are both fine nearly lamellar for the Ti-45Al-8Nb-0.7Cr-0.5Si-0.4B alloy. The lamellar colony size for gravity casting blade is 70 μm and the microstructures from edge to center have no significant change. The microstructure of centrifugal casting blade is finer than that of gravity casting blade which results from the fast cooling rate of centrifugal casting surface. The lamellar colony size is 50 p.m and the volume fraction of B2 phase for centrifugal casting blade reduces remarkably, which could lead to improve the mechanical properties at room and high temperature.