A method of controlling the depth of a variable cross-section titanium alloy VAR electrode pool

Abstract

The present application belongs to the technical field of metal material preparation, and relates to a method for controlling the VAR electrode molten pool depth of variable cross-section titanium alloy, which changes the existing equal cross-section electrode into an inverted round table variable cross-section electrode, changes the heating area of the molten pool by changing the cross-section area of the electrode, and reasonably matches the smelting parameters according to the relationship formula of the molten pool depth, the electrode diameter and the current intensity, so as to accurately control the shallow U-shaped molten pool on the basis of maintaining a large smelting rate, make the positive segregation elements overall directional diffusion to the ingot head, effectively reduce the segregation degree of the large-size titanium alloy ingot, improve the ingot organization uniformity, improve the ingot surface quality, reduce the amount of skinning, and improve the yield.

Description

Technical Field

[0001] This invention belongs to the field of metal material preparation technology, and relates to the preparation of titanium alloys, specifically to a method for controlling the molten pool depth of a variable cross-section titanium alloy VAR electrode. Background Technology

[0002] Vacuum consumable arc melting (VAR) has been widely used in titanium alloy smelting due to its low cost and ability to mass-produce large-sized ingots. In VAR melting, the consumable electrode and the water-cooled crucible are connected to the negative and positive terminals of a power source, respectively. Under a certain voltage, the rarefied gas between the consumable electrode and the water-cooled crucible is excited, releasing thermionic electrons. The consumable electrode is continuously melted into droplets by these high-energy thermionic electrons, which then fall onto the surface of the chilled copper crucible and solidify. Therefore, the electrode, as the primary heat source, has a decisive influence on the VAR process. Currently, most VAR electrodes used for titanium alloys are circular, square, or hexagonal with a uniform cross-section. Furthermore, to maximize production volume, manufacturers tend to use a larger crucible ratio (the ratio of electrode diameter to crucible diameter).

[0003] In the aerospace and nuclear fields, titanium alloys operate under extremely demanding conditions, placing higher requirements on the microstructure and compositional uniformity of titanium alloy ingots. VAR process parameters directly influence the temperature and concentration field distributions and the depth of the molten pool during melting, significantly impacting ingot microstructure and macroscopic segregation. Generally, at higher melting rates, greater heat input leads to a deeper V-shaped molten pool. This deep V-shaped pool has a smaller temperature gradient, resulting in a slower cooling rate in the ingot core, causing segregating elements to accumulate axially in the core. Furthermore, this temperature field promotes the radial growth of columnar crystals from the ingot surface towards the axis, tilting towards the ingot head and forming equiaxed crystal structures in the core. This difference in as-cast microstructure negatively affects the uniformity of the subsequent forging microstructure. However, at lower melting rates, less heat input results in a shallower U-shaped molten pool. This shallow U-shaped pool has a larger temperature gradient, leading to a faster cooling rate in the ingot core, making axial core segregation less likely. This temperature field can change the growth direction of columnar crystals from radial to axial, enabling the ingot to acquire an axially oriented columnar crystal structure, resulting in a fully columnar crystal structure and significantly improving the uniformity of the ingot structure. Therefore, in the VAR melting of titanium alloys prone to segregation, the melting rate is often reduced to decrease the degree of segregation, improve the uniformity of the structure, and thus improve the quality of the ingot. For example, Chinese patent (publication number: CN112501449A, publication date: March 16, 2021) obtained high-quality ingots by reducing the melting rate to 2-5 kg / min. However, significantly reducing the melting rate will significantly reduce production efficiency, leading to a several-fold increase in ingot melting time, poor edge treatment of the molten pool, poor surface quality of the ingot, increased peeling, and reduced yield. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for controlling the depth of the molten pool of a variable cross-section titanium alloy VAR electrode. By changing the cross-sectional area, the heating area of ​​the molten pool is changed, and the melting parameters are reasonably matched according to the relationship between the molten pool depth, electrode diameter and current intensity. While maintaining a large melting rate, the shallow U-shaped molten pool is precisely controlled, the surface quality of the ingot is improved, the amount of peeling is reduced, and the yield is increased.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This method for controlling the molten pool depth of a VAR electrode for variable cross-section titanium alloys involves replacing the existing constant cross-section electrode with a variable cross-section electrode during the smelting of finished titanium alloys containing easily segregating elements. The smelting parameters are rationally configured based on the molten pool depth, electrode diameter, and current intensity to obtain a stable molten pool while ensuring the smelting rate. The variable cross-section electrode is in the shape of an inverted frustum. The crucible ratio of the variable cross-section electrode continuously varies from bottom to top between 0.4 and 0.9.

[0007] Furthermore, the relationship between the molten pool depth and the diameter of the variable cross-section electrode and the current intensity is as follows:

[0008]

[0009] In equation (1), h p Indicates the depth of the molten pool; D e J represents the electrode diameter; J represents the current intensity.

[0010] Furthermore, the titanium alloy containing easily segregating elements is a titanium alloy containing at least Fe, Cr and Cu elements.

[0011] Furthermore, the formation process of the variable cross-section electrode is as follows: the primary ingot obtained from the first melting is placed in a frustum-shaped crucible for a second melting, and after cooling and solidification, a frustum-shaped secondary ingot is obtained. The secondary ingot is then inverted to serve as a variable cross-section electrode for the third melting.

[0012] Furthermore, the instantaneous length h of the variable cross-section electrode e The calculation formula is as follows (2), where the instantaneous weight m of the variable cross-section electrode is... e The calculation formula is as follows (3):

[0013]

[0014]

[0015] In equation (2), h0 is the initial length of the variable cross-section VAR electrode, h eD1 is the instantaneous length of the variable cross-section VAR electrode, D2 is the diameter of the upper base of the variable cross-section VAR electrode, and D3 is the diameter of the lower base of the variable cross-section electrode. e The instantaneous bottom diameter of the variable cross-section electrode is given by equation (3); where m is the instantaneous bottom diameter of the electrode. e ρ represents the instantaneous weight of the variable cross-section electrode, which is read in real time by the VAR device; ρ represents the density of the variable cross-section electrode.

[0016] Furthermore, the instantaneous weight m of the variable cross-section electrode is realized through a programmable logic controller. e Automatic matching with current intensity J.

[0017] Furthermore, during the finished product smelting process, the process parameters of the arc initiation stage, the stabilization smelting stage, and the feeding stage are all controlled by a programmable logic controller.

[0018] Furthermore, the arc stabilization current range is 5–15A, and the arc stabilization period range is 3–12s.

[0019] Compared with existing technologies, the technical solution provided by this invention has the following beneficial effects: This invention employs an inverted frustum-shaped variable cross-section electrode and rationally matches melting parameters through theoretical formulas. Without reducing the current intensity and thus the melting rate, it achieves a stable shallow U-shaped molten pool at a relatively high melting rate. This allows positively segregated elements to diffuse directionally to the ingot head, effectively reducing the segregation degree of large-size titanium alloy ingots. Simultaneously, the higher melting rate ensures good edge coverage of the molten pool, resulting in higher ingot surface quality, reduced peeling, and improved yield. Attached Figure Description

[0020] The accompanying drawings are incorporated in and form part of this specification, and together with the description serve to explain the principles of the invention.

[0021] Figure 1 This is a schematic diagram of secondary smelting and finished product smelting provided in Embodiment 1 of the present invention;

[0022] Figure 2 Numerical simulation diagram of the molten pool morphology during the stable smelting period provided in Embodiment 1 of the present invention;

[0023] Figure 3 This is a surface structure diagram of a Φ720mm Ti5331 alloy ingot in Embodiment 1 of the present invention;

[0024] Figure 4 This is a surface structure diagram of the Φ720mm Ti5331 alloy ingot in Comparative Example 1 of the present invention;

[0025] Figure 5 This is a low-magnification microstructure image of the Ti1023 longitudinal sample section after air burning in Example 2 of the present invention;

[0026] Figure 6 This is a low-magnification microstructure image of the Ti1023 longitudinal sample piece from Comparative Example 2 of this invention, obtained by air burning. Detailed Implementation

[0027] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of methods consistent with some aspects of the invention as detailed in the appended claims.

[0028] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0029] Example 1

[0030] This embodiment provides a method for controlling the molten pool depth of a variable cross-section VAR electrode for Ti-5Al-3Zr-3V-1Cr (Ti5331) alloy. Ti5331 alloy is mainly used in the nuclear industry, and Cr is the primary positive segregating element. According to numerical simulation results, the ideal molten pool depth for a 5-ton Φ720mm Ti5331 alloy ingot is approximately 510mm. The specific smelting process is as follows:

[0031] S1. Place a 5000kg, 520mm diameter ingot obtained from a single melting process into a frustum-shaped crucible with a bottom diameter of 640mm and a top diameter of 580mm. Set the crucible inlet water temperature to 18℃ and the melting station inlet water flow rate to 1000L / min. When the pre-vacuum is ≤5.0Pa and the leakage rate is ≤1.5Pa / min, weld the variable cross-section electrode to the auxiliary electrode. After cooling for 45 minutes, open the furnace to clean the weld bead. After cleaning, reseal the furnace and re-evacuate the vacuum.

[0032] S2. When the pre-vacuum is ≤2.0 Pa and the leakage rate is ≤1.0 Pa / min, secondary melting begins. Since the secondary melting uses a constant cross-section electrode, the stable melting stage is the same as the conventional process, using a constant input current and melting rate. A schematic diagram of the secondary melting of Ti5331 alloy is shown below. Figure 1 As shown, the cooling time after melting is ≥6h.

[0033] S3. After cooling and exiting the furnace, measure the diameters D1 and D2 of the two bottom surfaces of the variable cross-section electrode obtained from the secondary melting and the electrode height h0 and input them into the VAR equipment.

[0034] S4. Invert the variable cross-section electrode obtained from the secondary melting and place it into a Φ720mm crucible. Set the crucible water inlet temperature to 18℃ and the melting station water inlet flow rate to 1000L / min. When the pre-vacuum is ≤5.0Pa and the leakage rate is ≤1.5Pa / min, weld the variable cross-section electrode to the auxiliary electrode. After cooling for 45 minutes, open the furnace to clean the weld bead. After cleaning, reseal the furnace and draw a vacuum.

[0035] S5. When the pre-vacuum is ≤2.0Pa and the leakage rate is ≤1.0Pa / min, the finished product melting begins. The arc initiation stage, stabilization melting stage, and feeding stage are all controlled by PLC to achieve automatic matching of various parameters and control the molten pool depth between 470 and 530mm. The control ranges of each parameter are as follows: melting voltage range is 25-30V, melting current range is 10-20kA, stabilization current range is 5-10A AC, stabilization period range is 6-10s, and melting rate range is 15-20kg / min; a schematic diagram of the Ti5331 alloy variable cross-section electrode finished product melting is shown below. Figure 1 As shown, the cooling time after melting is ≥6h.

[0036] The numerical simulation diagram of the molten pool morphology during the stable melting period in this embodiment is shown below. Figure 2 As shown, the molten pool is shallow U-shaped, and the surface of the ingot after melting is as follows: Figure 3 As shown, the surface quality of the ingot is very high. The Φ720mm Ti5331 alloy ingot melted in this embodiment was sawn longitudinally to obtain slices. Ten longitudinal samples were taken from the edge, R / 2, and center of the slices to analyze the overall compositional uniformity of the ingot. The results are shown in Table 1.

[0037] Table 1. Cr content (wt.%) in different longitudinal sections of the ingot from Example 1

[0038]

[0039]

[0040] Comparative Example 1

[0041] This comparative example provides a method for preparing Ti5331 alloy using low-rate VAR (Vacuum Atom Reduction). The electrode used is a cylindrical electrode. Compared with Example 1, the single and double melting processes are standard operating procedures in the industry and do not significantly affect the final product quality; therefore, they are not described in detail here. The specific steps of the three-stage melting process are as follows:

[0042] Step 1) Load a 5000kg, Φ640mm secondary VAR melting Ti5331 alloy ingot electrode into a Φ720mm crucible. Set the crucible water inlet temperature to 18℃ and the melting station water flow rate to 1000L / min. When the pre-vacuum is ≤5.0Pa and the leakage rate is ≤1.5Pa / min, weld the variable cross-section electrode to the auxiliary electrode. After cooling for 45min, open the furnace to clean the weld bead. After cleaning, reseal the furnace and re-evacuate the vacuum.

[0043] Step 2) When the pre-vacuum is ≤1.0Pa and the leakage rate is ≤0.6Pa / min, start the arc ignition and melting, and control the parameters within the following ranges: melting voltage range is 20~25V, melting current range is 6~10kA, arc stabilization current range is 5~10A AC, arc stabilization period range is 3~7s, and melting rate range is 5~8kg / min.

[0044] Step 3) When the remaining weight of the consumable electrode is 200kg, feeding begins. In the early stage of feeding, when the current is greater than 8kA, the current decreases at a rate of 0.5kA / min. In the middle stage of feeding, when the current is between 6 and 8kA, the current decreases at a rate of 0.1kA / min. In the final stage of feeding, the current decreases at a rate of 0.4kA / min. The cooling time after melting is ≥6h.

[0045] Elemental analysis of the ingot after smelting revealed that although a low melting rate could control Cr segregation to some extent, the molten pool did not reach the edges well due to the low melting rate. Therefore, compared to Example 1, the surface quality of the ingot was poor. Figure 4 As shown.

[0046] Example 2

[0047] This embodiment provides a method for controlling the molten pool depth of a variable cross-section VAR electrode for Ti1023 alloy. Ti1023 alloy is mainly used in the aerospace industry, and its primary positive segregating element is Fe. Based on numerical simulation results, the ideal molten pool depth for a 5-ton Φ720mm Ti1023 alloy ingot is approximately 420mm. The specific smelting process is as follows:

[0048] S1. Place a 5000kg, 520mm diameter ingot obtained from a single melting process into a frustum-shaped crucible with a bottom diameter of 640mm and a top diameter of 580mm. Set the crucible inlet water temperature to 18℃ and the melting station inlet water flow rate to 1000L / min. When the pre-vacuum is ≤5.0Pa and the leakage rate is ≤1.5Pa / min, weld the variable cross-section electrode to the auxiliary electrode. After cooling for 45 minutes, open the furnace to clean the weld bead. After cleaning, reseal the furnace and re-evacuate the vacuum.

[0049] S2. When the pre-vacuum is ≤1.0Pa and the leakage rate is ≤0.6Pa / min, the secondary melting begins. Since the secondary melting uses a uniform cross-section electrode, the stable melting stage is the same as the conventional process. A constant input current and melting rate are used for melting. After melting, the cooling time is ≥6h.

[0050] S3. After cooling and exiting the furnace, measure the diameters D1 and D2 of the two bottom surfaces of the variable cross-section electrode obtained from the secondary melting and the electrode height h0 and input them into the VAR equipment.

[0051] S4. Invert the variable cross-section electrode obtained from the secondary melting process and place it into a Φ720mm crucible. Set the crucible inlet water temperature to 18℃ and the melting station inlet water flow rate to 1000L / min. When the pre-vacuum is ≤5.0Pa and the leakage rate is ≤1.5Pa / min, weld the variable cross-section electrode to the auxiliary electrode. After cooling for 45 minutes, open the furnace to clean the weld beads. After cleaning, reseal the furnace and re-evacuate the vacuum.

[0052] S5. When the pre-vacuum is ≤1.0Pa and the leakage rate is ≤0.6Pa / min, the finished product melting begins. The arc ignition stage, stabilization melting stage, and feeding stage are all controlled by PLC to achieve automatic matching of various parameters, controlling the molten pool depth between 380 and 450mm. The control ranges of each parameter are as follows: melting voltage range is 28–30V, melting current range is 8–12kA, stabilization current range is 8–13A AC, stabilization period range is 3–7s, and melting rate range is 10–15kg / min. The cooling time after melting is ≥6h.

[0053] The Φ720mm Ti1023 alloy ingot obtained in this embodiment was longitudinally sawn into sections. Ten longitudinal samples were taken from the edge, radius (R / 2), and center of each section to analyze the overall compositional uniformity of the ingot. The results are shown in Table 2. A no-fire test was performed on the longitudinal sample section, and no defects such as β spots due to β-stabilizing element segregation were found. Figure 5 As shown.

[0054] Table 2. Fe content (wt.%) in different longitudinal sections of the ingot in Example 2

[0055]

[0056] Comparative Example 2

[0057] This comparative example provides a method for preparing Ti1023 alloy using high-rate VAR (Vacuum Atom Reduction). The electrode used is a cylindrical electrode. Compared with Example 2, the single and double melting processes are standard operating procedures in the industry and do not significantly affect the final product quality. Therefore, they are not described in detail here. The specific steps of the three-stage melting process are as follows:

[0058] Step 1) Place the 5000kg, Φ640mm secondary VAR melting ingot obtained from the first melting into a Φ720mm crucible. Set the crucible inlet water temperature to 18℃ and the melting station inlet water flow rate to 1000L / min. When the pre-vacuum is ≤5.0Pa and the leakage rate is ≤1.5Pa / min, weld the variable cross-section electrode to the auxiliary electrode. After cooling for 45 minutes, open the furnace to clean the weld bead. After cleaning, reseal the furnace and re-evacuate the vacuum.

[0059] Step 2) When the pre-vacuum is ≤1.0Pa and the leakage rate is ≤0.6Pa / min, start the arc ignition and melting, and control the parameters within the following ranges: melting voltage range is 28~30V, melting current range is 8~12kA, arc stabilization current range is 8~13A AC, arc stabilization period range is 3~7s, and melting rate range is 10~15kg / min.

[0060] Step 3) When the remaining weight of the consumable electrode is 200kg, feeding begins. In the early stage of feeding, when the current is greater than 8kA, the current decreases at a rate of 0.5kA / min. In the middle stage of feeding, when the current is between 6 and 8kA, the current decreases at a rate of 0.1kA / min. In the final stage of feeding, the current decreases at a rate of 0.4kA / min. The cooling time after melting is ≥6h.

[0061] Elemental analysis of the ingot after smelting revealed that while a high melting rate improves the surface quality of the ingot, the deeper molten pool due to the higher melting rate also increases the tendency for segregation. Therefore, compared to Example 2, a dry-firing inspection of the longitudinal sample revealed defects such as β spots caused by the segregation of β-stabilizing elements. Figure 6 As shown.

[0062] As can be seen from the above embodiments and comparative examples, the method for controlling the molten pool depth of a variable cross-section titanium alloy VAR electrode provided by the present invention can obtain a shallow U-shaped molten pool while maintaining a high melting rate, and control the ingot deviation of easily segregated elements within 2000 ppm, effectively reducing the degree of ingot segregation. At the same time, it ensures the surface quality of the molten pool, reduces the amount of peeling, and improves the yield, making it suitable for industrial production.

[0063] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention.

[0064] It should be understood that the present invention is not limited to the content already described above, and various modifications and changes can be made without departing from its scope. The scope of the present invention is limited only by the appended claims.

Claims

1. A method for controlling the molten pool depth of a variable cross-section titanium alloy VAR electrode, characterized in that, When smelting finished titanium alloys containing easily segregating elements, by replacing the existing constant cross-section electrode with a variable cross-section electrode and rationally configuring the smelting parameters according to the molten pool depth, electrode diameter and current intensity, a stable molten pool can be obtained while ensuring the smelting rate; the variable cross-section electrode is in the shape of an inverted frustum; the crucible ratio of the variable cross-section electrode changes continuously from bottom to top between 0.4 and 0.

9. The relationship between the molten pool depth and the diameter of the variable cross-section electrode and the current intensity is as follows: (1) In equation (1), h p This indicates the depth of the molten pool, in meters (m). D e The instantaneous bottom diameter of the variable cross-section electrode is expressed in meters (m). J represents current intensity, measured in kA.

2. The method for controlling the molten pool depth of a variable cross-section titanium alloy VAR electrode according to claim 1, characterized in that, The titanium alloy containing easily segregating elements is a titanium alloy containing at least Fe, Cr and Cu elements.

3. The method for controlling the molten pool depth of a variable cross-section titanium alloy VAR electrode according to claim 1, characterized in that, The process of forming the variable cross-section electrode is as follows: the primary ingot obtained from the first melting is placed into a frustum-shaped crucible for a second melting, and after cooling and solidification, a frustum-shaped secondary ingot is obtained. The secondary ingot is then inverted to serve as a variable cross-section electrode for the third melting.

4. The method for controlling the molten pool depth of a variable cross-section titanium alloy VAR electrode according to claim 1, characterized in that, The instantaneous length h of the variable cross-section electrode e The calculation formula is as follows (2), where the instantaneous weight m of the variable cross-section electrode is... e The calculation formula is as follows (3): (2) (3) In equation (2), h0 is the initial length of the variable cross-section VAR electrode, h e D1 is the instantaneous length of the variable cross-section VAR electrode, D2 is the diameter of the upper base of the variable cross-section VAR electrode, and D3 is the diameter of the lower base of the variable cross-section electrode. e The instantaneous bottom diameter of the variable cross-section electrode is given by equation (3); where m is the instantaneous bottom diameter of the electrode. e ρ represents the instantaneous weight of the variable cross-section electrode, which is read in real time by the VAR device; ρ represents the density of the variable cross-section electrode.

5. The method for controlling the molten pool depth of a variable cross-section titanium alloy VAR electrode according to claim 4, characterized in that, The instantaneous weight m of the variable cross-section electrode is realized through a programmable logic controller. e Automatic matching with current intensity J.

6. The method for controlling the molten pool depth of a variable cross-section titanium alloy VAR electrode according to claim 1, characterized in that, During the finished product smelting process, the process parameters of the arc initiation stage, the stabilization smelting stage, and the feeding stage are all controlled by a programmable logic controller.

7. The method for controlling the molten pool depth of a variable cross-section titanium alloy VAR electrode according to claim 1, characterized in that, The arc stabilization current range is 5~15A, and the arc stabilization period range is 3~12s.

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