Titanium alloys are light in weight, and have very high tensile strength as well as toughness. With these distinctive advantages, titanium alloys are widely accepted as one of the potential future materials. However, the high cost of both raw materials and processing has limited their applica- tions. To reduce the cost, electron beam cold hearth remelting (EBCHR) has emerged as a key process in producing high-quality titanium alloy ingots and electrodes.
Compared to vacuum arc remelting (VAR), EBCHM is able to effectively consolidate both sponge and scrap material while removing unde- sired impurities and inclusions, such as low-density and high-density inclusions.Thus, EBCHM has become a possible solution for producing single-melt ingots, the application of which will signiftcantly reduce the energy and time costs of titanium alloy
plates. EBCHM also breaks through the length limitation of VAR titanium alloy ingots, and pro- duces ultralong ingots, over 10 m in length, for applications of high-quality seamless tubes.
Although EBCHM shows many advantages, the technique also faces challenges which requiring solutions. Due to the surging of the saturated vapor pressure of additive alloy elements at high temper- ature, elements like aluminum may lose 30% during the EBCHM process. Because of this serious evap- oration loss, the quality of TC4 ingots manipulated by EBCHM suffers from inhomogeneous composi- tion segregation. Even though extra aluminum has been provided in the feedstock to compensate the continuous evaporation loss on the surface of the melt, composition control is still a serious problem since none of the stirring techniques can be applied. To reduce the defects, a robust understanding of the EBCHM process, namely the evolution of tempera- ture, flow and solidiftcation during alloy casting, isurgently required. Meanwhile, a sound system for process control should be developed to homogenize the composition in the melt during EBCHM.
Unusually, unlike other casting techniques, EBCHM utilizes a high-energy density electron beam as the heat source for continuous casting of alloy ingots under vacuum condition. The electron beam is set to move following different patterns to scan the melt surface under high frequency. The beam target is subject to a high local flux, which in turn produces a strong temperature gradient in that area. By adjusting the beam-scan pattern and frequency, the temperature gradient on the molten surface can be controlled, and the evaporation of alloy elements can be restrained.In addition, elaborate control of the surface temperature and casting speed can modify the curvature of the solid– liquid interface in the mold, which is a key factor for producing ingots with a perfect crystal structure.
Recently, the ftnite element method was intro- duced to predict temperature evolution during the EBCHM process.The purpose is to declare the evolution of the solid–liquid front in the ingots, since they are directly connected with ingot quality. However, for most reported in the literature, the temperature on the surface was set as constant in order to simplify the model. Only a few attempts have been made to involve the influence of beam- pattern operation conditions on the melt surface and the effect of overflow from the cold hearth on flow evolution.In the present manuscript, an approximating beam-pattern for casting TC4 ingots, with a diameter of 260 mm, has been designed and tested by numerical modeling to discover the tem- perature evolution on the surface of the mold. To control the movement of the beam, a user-deftned function (UDF) was compiled and added to the CFD model. In addition, the evolution of the solid–liquid interface for large-scale TC4 round ingots was also investigated in different EBCHM operation condi- tions to provide details for the elaborate control ofTC4 ingot manipulation. The purpose is to restrain inhomogeneous composition segregation, and improve the quality of large-scale TC4 round ingots.
