Fabrication and Microstructure Optimization of TiAl Castings Using a Combined Melting/Pouring/Heat Treatment Device
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pyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00525-z
Abstract In this paper, a vacuum melting furnace which combines melting, pouring, mold preheating and heat treatment was designed and built, for the purpose of fabrication and microstructure optimization of TiAl castings. The apparatus provides the accurate control of pouring, mold preheating and subsequent heat treatment with various cooling rates, which makes the complete process from casting to final heat treatment can be carried out in one equipment and avoids the sample transfer process from casting to heat treatment device. The performance of the furnace was evaluated by casting the Ti–48Al–2Cr–2Nb and high-Nb-containing TiAl sheet and rod components. The X-ray detection images and scanning electron microscope results showed that the cavity and segregation were
eliminated obviously. Significantly, an optimal nearly lamellar microstructure and fine fully lamellar microstructure were obtained, respectively. In addition, the transmission electron microscopy results show the lamellar spacing size after moving out of the heat treatment chamber is thinner than that cooling in the chamber. The results are of interest to researchers devoted to technical innovations and modifications for TiAl investment casting at the industrial scale.
Introduction
320neo equipped with forged TiAl turbine blades of TNM alloy completed its maiden flight in 2014.2
Titanium aluminum, the intermetallic compounds of titanium and aluminum, possesses the advantages of low density (* 4.0 g/cm3), corrosion resistance and excellent balanced mechanical properties at elevated temperatures.1,2 Over the past two decades, many efforts have been delivered on the engineering application of this kind of lightweight high-temperature structural intermetallics in aerospace and auto industries. Mitsubishi developed a levitation melting/counter-gravity casting technique to produce TiAl turbocharger.3 A parallel work on the preparation of automotive valve was processed in IMR (Institute of Metals Research, China) using CaO crucible.4 Ti–48Al–2Cr–2Nb (at.%) castings produced by General Electric have been successfully applied to low-pressure turbine blades in GEnxTM engine.5 In addition, the Airbus
International Journal of Metalcasting
Keywords: TiAl alloys, investment casting, solidification, heat treatment, microstructure
However, the process for casting TiAl components is not an easy task, mainly due to the strong chemical activity of the molten metal, poor fluidity and high viscosity in the molten state.6 High superheating is beneficial to increase the liquid fluidity and avoid casting defects.4,7 However, the significant higher superheating will increase the probability of a metal–crucible interaction and oxygen contamination.4 If this happens, embrittlement of the cast alloy8 and oxide formation can be expected,9 which is detrimental to the mechanical properties of the finished component.10 On the other hand, using raw materials based on the commercially pure element
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