Microstructure evolution of large-scale titanium slab ingot based on CAFE method during EBCHM
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The purpose of this work is, based on CAFE method, to study the microstructure evolution and optimize the quality of the large-scale titanium slab ingot during EBCHM. The nucleation parameters of the microstructure simulation of titanium ingot are determined based on one of the actual experimental results. For the determined parameters, our theoretical results are agreement with other experimental results. The effects of pouring temperature and pulling speed on the microstructure are presented based on CAFE method. The quantitative analyses of the simulated results show that with the pulling speed increasing, the number of grains decreases, whereas the mean grain radius increases under identical thermal condition; with the pouring temperature increasing, the mean grain radius increases under the given pulling speed. Our results are very important to obtain the optimal structure of the ingots by controlling pulling speed and pouring temperature.
I. INTRODUCTION
The ability of the electron beam cold hearth melting (EBCHM) method, removing the low- and high-density inclusions and solving the problem of inhomogeneity of structure and composition, is higher than vacuum arc remelting (VAR).1–4 The EBCHM process is used extensively for the fining of titanium ingot.5,6 To make high-quality titanium strip coil, a single EBCHM for the large-scale rectangular titanium ingot, which can be directly rolled applying the existing equipment of rolling steel, has the potential for significant cost reductions, due to leave out the operations of forging and stamping.7 One of important advantages of EBCHM is the improvement in ingot quality because of the fact that the long residence time of the molten titanium in the melting/ refining cold-hearths also greatly reduces the incidence of high-density inclusions, which submerge into the bottom face of the cold-hearth and are thus incorporated into the solid skull.8 Furthermore, a wider range of feedstock like scraps can be used while a compacted electrode need be fabricated in VAR. There is also a greater flexibility in the ingot form (cylinder or slab) in terms of the cross-section geometry of the water-cooled crystallizer and the ingot length depending on the adequate continuously withdrawing operations.9 Because of the energy of the heating source which can be controlled independently and the
Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.174
casting conditions (the speed of melting and the pouring temperature of molten in the crystallizer) which are feasible to change, the EBCHM offers considerable potential with regard to the control of metallurgical microstructure of titanium. The microstructure of castings is of great importance due to its role on mechanical properties. Though the potential of the EB process, the microstructure of largescale titanium slab ingot produced from this technology method have not been established to any great extent.10 Hence, the task of this work is to study the microstruct
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