Numerical Research on Magnetic Field, Temperature Field and Flow Field During Melting and Directionally Solidifying TiAl
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tstanding properties of TiAl alloys, for instance, their high specific strength, low density, excellent corrosion and creep resistance, as well as good oxidation resistance at higher temperatures,[1,2] researchers regard it as the most promising light material to replace the Ni-based high-temperature alloys partly in the range of 873 K to 1273 K (600 °C to 1000 °C).[3,4] Therefore, TiAl alloys are the materials with the most potential for reciprocating and rotating components used at elevated temperatures, such as aero engine blades, car turbo-charge blades and engine valves.[5,6] Alloying is one of the most effective methods to improve
RUIRUN CHEN, YAOHUA YANG, XUE GONG, JINGJIE GUO, YANQING SU, HONGSHENG DING, and HENGZHI FU are with the School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. Contact e-mail: [email protected] Manuscript submitted April 11, 2017. Article published online August 21, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
the properties of TiAl alloys but have a higher requirement for melting[7,8] and could result in segregation. Moreover, TiAl alloys can react with the mold during the melting process because of their high activity, which obviously results in decreasing of properties.[9–11] Therefore, to satisfy the requirement of alloying and processing of TiAl alloys, a new method for obtaining homogeneous and low-contamination large-scale ingots with a directionally solidified microstructure and superior properties is needed. The directionally solidified gas turbine blades manufactured by the Bridgman process have been widely used in the aeronautical and energy industries.[12] Saari et al.[13] found that the directional solidification technique is a promising manufacturing method for processing TiAl alloys. Magnetic fields have been widely used in metallurgical processing to improve the energy efficiency and purity of melt.[14–18] Based on Faraday’s law of electromagnetic induction and Maxwell’s equation, the cold crucible directional solidification (CCDS) technique is an effective and new method for purity and superior properties of TiAl processing.[19,20] The schematic diagram of CCDS is presented in Figure 1. The EMCC is installed VOLUME 48B, DECEMBER 2017—3345
in a vacuum cavity connecting the high-frequency current and cold water. Then, a primer and feeding rod with the same composition are put into the EMCC at the appropriate position and connected with the pulling and feeding system, respectively. Increasing the power on coils progressively, the primer will be melted until a steady meniscus is formed. After that, the directionally solidified ingot can continuously grow by turning on the pulling and feeding system. The coils of the EMCC with AC motivate an induction electromagnetic field inside the crucible, as a consequence generating heating energy for melting because of ohmic losses[21] and kinetic energy for stirring as well as the ‘‘soft contact’’ of melt results from the Lorenz force.[22,23] During the directional solidification process, the
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