Research status and prospect of ceramic crucibles for induction melting of titanium alloys

Abstract

<p indent="0mm">Titanium alloys possess high specific strength, superior heat resistance, excellent corrosion resistance, outstanding physiological compatibility, etc., and thus being considered as the most ideal lightweight structural materials as well as potent biomedical materials. Generally, Ti alloys are manufactured via the technique of ingot metallurgy; however, it suffers from critical issues of high energy consumption and high cost, which desperately need to be well addressed. According to previous research, the crucible vacuum induction melting technology has high superheat and strong electromagnetic stirring, with which titanium alloy ingots with uniform composition can be achieved without applying multiple remelting. The crucible vacuum induction melting not only benefits the removal of low melting point impurities, but also significantly improves the energy consumption efficiency as the forced water cooling becomes not indispensable during the processing. As a result, the crucible vacuum induction melting has been the most potential method to overcome the problems of high energy consumption and low material yield existed in conventional melting technology. However, titanium alloys can react with almost all the refractories because of their high chemical activity, resulting in the pollution of titanium ingot and thus seriously restricting the development and application of this technology in titanium alloy production. For this reason, preparation special crucibles with corrosion resistance to titanium liquid is crucial to realizing low-cost, high-quality, and high-efficiency smelting of titanium alloys. Herein, we provide a comprehensive review of the research status of ceramic crucibles for titanium alloys including the selection of materials (thermodynamic criterion, corrosion resistance mechanism, composition optimization) and the promising processes to improve the durability of these crucibles. Usually, the thermodynamic criterion is used as the first-step to pick ceramic materials with high chemical stability, which involves a comparison of the free energies of formation of ceramic materials with their corresponding titanium compounds. It is necessary to further evaluate the interface reaction mechanism between candidate ceramic materials and titanium melts under induction melting conditions to compensate for the neglect of disregarding physical and chemical erosion in thermodynamic approach. Based on a systematically comparison of the interface reaction mechanism between titanium melts and various common refractory materials involving oxide ceramics (Y₂O₃ and CaO), zirconate ceramics (BaZrO₃ and (Ca,Sr,Ba)ZrO₃) and non-oxide ceramics, AlN and TiN show promising properties for melting titanium alloys due to their relatively smaller oxygen contamination. In addition to corrosion resistance of ceramics against titanium melts, the thermal shock resistance is the other critical criterion to realize a long service life of the crucibles under repeated induction melting conditions. Scholars attempted many methods, such as porosity optimization, doping modification, surface coating, to solve the imbalance between the corrosion resistance of titanium liquid and thermal shock resistance of materials. Currently, the crucibles can only be prepared at laboratory size, how to realize the production of large-size crucibles with balancing the cost, corrosion resistance and thermal shock resistance is full of serious challenges. On the basis of the recent research progress of Huazhong University of Science and Technology, innovative thinking in terms of the future development of ceramic crucibles for titanium alloy induction melting is suggested: (1) the optimization of refractories: how to reap the advantages of computer technology to speed up the development of new crucible materials; (2) higher demands for thermal shock resistance and high temperature strength: figuring out the relationship between the preparation parameters and the surface quality of the crucibles; (3) the preparation of the special crucibles with large size, high durability: addition of ultra-high temperature reinforcements being a promising route for the fabrication of industrial grade crucibles for titanium alloys induction melting.