Titanium alloys are widely used in aerospace, shipbuilding, petrochemical, medical, transportation and other fields due to their high specific strength, good heat resistance, low-temperature toughness, low-temperature superconductivity, and corrosion resistance. Electron Beam Cold Hearth Melting (EBCHM) is one of the main equipment for melting and casting titanium and titanium alloys. It is a metallurgical technology that combines electron beam and industrial cold bed for melting under high vacuum and high temperature. Electron beam melting refers to a vacuum melting technology that converts the kinetic energy of a high-speed electron beam into thermal energy as a heat source for material melting under high vacuum. Electron beam melting technology has the characteristics of controllable melting temperature and speed, less restriction on raw material quality and shape, high product quality, diverse specifications, high energy utilization rate, and no environmental pollution. EBCHM technology has long been used to eliminate serious metallurgical defects such as high-density and low-density inclusions in titanium materials, and to improve the quality of titanium and titanium alloys used in aerospace. The American aviation standard has included cold bed furnace melting as a mandatory melting technology for titanium alloy materials used in aviation rotating and structural components. The difference between EBCHM and other melting methods is that it uses a cold bed to separate the three processes of melting, refining, and crystallization. The scanning power and pattern in the crystallizer area have a significant impact on the fluidity of the metal liquid in the crystallizer and subsequent solidification.
At present, researchers have studied the power and graphic energy distribution of the No. 4 gun in a 3200 kW electron beam cold bed furnace, and prepared ingots with macroscopic defects, tight internal structure, and no defects such as segregation, inclusions, and pores in the composition. The influence of different electron gun powers and scanning frequencies on the surface cracking of flat ingots, as well as the mechanism of the effect of different ingot pulling speeds on porosity, were studied using a 3200 kW electron beam cold bed furnace. The main defects of EB cast titanium ingots include surface tensile cracking, internal cracks, subcutaneous pores, cold insulation, folding, slag inclusion, oxidation, and element segregation. Folding mainly occurs on the non overflow side of the blank, and the milling and grinding of the EB blank cannot completely remove the influence of folding. After rolling, peeling occurs on the surface of the titanium coil, which seriously affects the quality of the titanium coil. This study conducted experiments on four EB furnaces with different crystallizer widths, electron gun power distribution within the crystallizer, and crystallizer liquid level, in order to obtain the influence of crystallizer width, electron gun power within the crystallizer, and crystallizer liquid level on the non overflow side folding layer of the blank, in order to improve the quality of EB furnace casting blanks.

　Graphic and textual content
Using a 3200 kW electron beam cold bed melting furnace with 4 guns from the United States, the main features of the furnace are a "C" - shaped melting and casting area composed of a melting cold bed, a refining cold bed, and a crystallizer, as shown in Figure 1. Four electron guns are used for melting and refining in the casting area. The schematic diagram of the melting principle is shown in Figure 2. The raw material used is 0-grade sponge titanium from the same manufacturer with similar impurity elements. The composition is shown in Table 1, and the hardness (HBW) is 95-97. Casting was carried out using crystallizer 1 (thickness 240 mm, width 1080 mm) and crystallizer 2 (thickness 240 mm, width 1280 mm), respectively. The power settings of the electron gun scanned by the crystallizer are shown in Table 2. 9 TA1 blanks were melted and cast separately. During the melting process, the first 1-3 blocks were melted and cast at a low liquid level (-20 mm below the overflow port), 4-6 blocks were melted and cast at a medium liquid level (-10 mm below the overflow port), and 7-9 blocks were melted and cast at a high liquid level; The schematic diagram of the 4th gun and crystallizer in the EB furnace with 4 guns is shown in Figure 3. Under the same conditions of other processes and operations (such as the power and pattern size of other electron guns, the return water flow rate of the crystallizer, and the pulling speed of the ingot), analyze the number and formation reasons of non overflow side folds in the blank.

Figure 1: The "C" - shaped structure of the EB furnace with four guns
1. Refining cooling bed 2. Smelting cooling bed 3. Crystallizer 4. Non overflow side

Figure 2 Schematic diagram of melting principle
1. Inlet 2.1 electron gun 3.2 electron gun 4.3 electron gun
5.4 Electron gun 6. Casting billet 7. Crystallizer 8. Refining cooling bed 9. Coarse refining cooling bed
Table 1 Chemical Composition of Grade 0 Sponge Titanium (%)

Table 2 Electron gun power scanned inside the crystallizer

Figure 3: Schematic diagram of gun 4 and crystallizer in EB furnace
1. Crystallizer 2. Overflow device 3. Electron gun 4. Non overflow side 5. Titanium liquid
The number and trend of folded layers in the casting blank of crystallizer 1 are shown in Figure 4. It can be seen that as the power of the electron gun in the crystallizer increases, the number of folds gradually decreases, and there is no significant decrease when the power is (350 ± 10) kW and (380 ± 10) kW; When the power is the same, the liquid level and overflow port remain flat with fewer folds. The number and trend of folded layers in the casting blank of crystallizer No.2 are shown in Figure 5. It can be seen that as the power of the electron gun in the crystallizer increases, the number of folds gradually decreases, and there is no significant decrease after the power reaches (380 ± 10) kW and (410 ± 10) kW; When the power is the same, the liquid level and overflow port remain flat with fewer folds.

Figure 4: Blank Folding at Different Power and Liquid Levels in Crystallizer No.1
Quantity change trend

Figure 5: Blank Folding at Different Powers and Liquid Levels in Crystallizer No.2
Quantity change trend

Figure 6: Blank Folding Layers of Crystallizers 1 and 2 with Different Power and Liquid Levels
Quantity change trend
Due to the angle θ between the 4th gun of the EB furnace and the non overflow side of the crystallizer (see Figure 3), the electron beam cannot be directly directed onto the titanium liquid near the non overflow side, which prevents timely melting of the shell and replenishment of new titanium liquid energy; In addition, the high cooling intensity and rapid temperature drop of the upper part increase the viscosity and surface tension of the titanium liquid on the non overflow side. After testing, the θ of crystallizer 1 is 24 °, and the θ of crystallizer 2 is 21 °. When the temperature of the new titanium liquid is low, the fluidity is poor and the electron beam does not scan the shell for a long time, resulting in continuous folding layers, and severe folding layers will extend inward. It is necessary to increase the milling amount to remove the folding layers, otherwise it will cause edge double skin during hot rolling, seriously reducing the quality of the coil.
The flow of titanium liquid and the formation of folds at the edge of the crystallizer are shown in Figure 7. Due to the uneven inner wall of the crystallizer, the titanium liquid forms a wetting contact with the inner wall of the crystallizer, forming a "U" - shaped liquid surface, as shown in Figure 7a. The titanium liquid attached to the inner wall solidifies and forms a very thin "shell"; When pulling down the ingot, due to the untimely filling of titanium liquid and the influence of surface tension, the shell will not continuously detach from the inner wall of the crystallizer, forming a meniscus, as shown in Figure 7b; When new titanium liquid is added to the edge of the crystallizer, the new titanium liquid forms a "U" - shaped liquid surface with the inner wall of the crystallizer, repeatedly forming "fish scale patterns" on the surface of the blank. The formation of folds is related to the viscosity and surface tension of titanium liquid. The viscosity and surface tension of liquid metal affect the fluidity, filling, solidification, and forming process of the melt in the mold, while viscosity and surface tension are related to the temperature of the titanium liquid. The viscosity and surface tension of liquid metal decrease with increasing temperature. During melting and casting, the temperature of the titanium liquid at the edge is low, resulting in high viscosity, reduced fluidity, and increased surface tension. When the electron beam fails to melt the edge of the shell, a clear interface is easily formed between the new titanium liquid and the shell, which remains on the surface of the casting and forms a folded layer, as shown in Figures 7c and 7d.

Figure 7 Schematic diagram of the formation process of the blank folding layer
Table 3 Power Density in Crystallizers at Different Liquid Levels

(a) Crystallizer No.1,
1.37 W/mm2

(b) Crystallizer No.2,
1.21 W/mm2
Figure 8 Non overflow side folding situation of high-level blank
Research Conclusion
(1) The lower the liquid level in the crystallizer, the more folds on the non overflow side. When the liquid level is level with the overflow port, there are fewer folds on the non overflow side; At the same liquid level, as the power of the electron gun in the crystallizer increases, the number of folds decreases and * tends to stabilize.
(2) The non overflow side folding layer is caused by the large angle θ between the normal of the horizontal section of the crystallizer and the electron gun, which increases the blind spot of the electron beam scanning at low liquid levels, resulting in fast cooling speed of the titanium liquid and poor edge fluidity.
(3) When the width of the crystallizer is 1080 mm, the liquid level is level with the overflow port, and the liquid level power density is 1.37 W/mm2, there are fewer folds in the blank, and the relative melting and casting energy consumption is *; When the width of the crystallizer is 1280 mm, the liquid level is level with the overflow port, and the liquid level power density is 1.21 W/mm2, there are fewer folds in the blank, and the relative melting and casting energy consumption is *.