A method and apparatus for improving the composition uniformity of the end portions of a cast ingot by VAR melting

Abstract

This invention proposes a VAR melting method to improve the compositional uniformity of ingot ends, comprising the following steps: placing a base pad at the bottom of the crucible, keeping the groove of the base pad facing downwards; placing a consumable electrode inside the crucible, keeping the electrode end from contacting the base pad; melting; after the molten pool establishment stage, continuing the normal melting stage, the feeding stage, and melting until the melting is completed; this invention uses a base pad with a concave lower end, which not only causes the current to flow to the side wall of the crucible rather than the crucible base during the arc ignition and molten pool establishment stage, thus strengthening the electromagnetic stirring effect and promoting the homogenization of the molten pool composition, but also reduces the contact area between the base pad and the crucible base, more effectively reducing the cooling intensity during the arc ignition and molten pool establishment stage, increasing the molten pool depth, further promoting the homogenization of the molten pool composition, and making the composition at the bottom of the solidified ingot more uniform.

Description

Technical Field

[0001] This invention relates to the field of VAR melting technology, and in particular to a VAR melting method and melting apparatus for improving the compositional uniformity of ingot ends. Background Technology

[0002] VAR melting, or vacuum consumable arc melting, involves welding a consumable electrode to an auxiliary electrode and clamping the auxiliary electrode onto an electrode rod. In a vacuum or inert gas atmosphere, an electric arc is generated between the consumable electrode and the arc-starting material on a water-cooled copper crucible base. The heat from the electric arc melts the consumable electrode, which then enters the crucible as molten droplets, forming a molten pool. The surface of the molten pool is heated by the electric arc and remains liquid, while the bottom and surrounding areas are forcibly cooled by the crucible's cooling water, resulting in a bottom-up crystallization process. The electrode is continuously lowered at an appropriate rate to maintain the continuous arc melting process until the consumable electrode is completely melted and the melting process ends.

[0003] Electromagnetic stirring: During VAR melting, the electric arc is disturbed by the surrounding airflow and magnetic field, causing the arc to twist and deflect to varying degrees and uncontrollably. In order to make the arc controllable during the operation of the electric arc furnace, an electromagnetic coil (arc stabilizing coil) wound around the water-cooled crystallizer (copper crucible) of the electric arc furnace is used to establish a longitudinal magnetic field along the electrode axis to stabilize the arc. At the same time, it stirs the molten metal pool, which is called electromagnetic stirring. Electromagnetic stirring can make the composition of the molten metal in the pool more uniform.

[0004] In existing titanium alloy VAR melting processes, there is a significant problem of uneven composition at the ends. Besides uneven mixing during consumable electrode preparation, this is mainly caused by insufficient electromagnetic stirring of the molten pool forming the ends during melting.

[0005] The VAR melting process can be divided into three stages: arc ignition and molten pool establishment, normal melting, and head feeding. In the arc ignition and molten pool establishment stage (corresponding to the bottom of the ingot), the molten pool depth is small, meaning the distance between the upper part of the molten pool and the crucible base is small. Since the copper crucible has low resistance, the current in the molten pool does not flow towards the sidewalls of the crucible but towards the bottom of the crucible; that is, the current flows vertically out of the molten pool along the direction of the magnetic field (e.g., ...). Figure 3 Furthermore, the small arc-stabilizing current and weak magnetic field at this stage result in a small Lorentz force and minimal electromagnetic stirring, making it difficult to homogenize the molten composition. Additionally, the shallow molten pool and small volume of molten material at this stage lead to rapid cooling and solidification, which further solidifies the unevenly composed molten material, hindering homogenization. During the head-feeding stage, the melting power drops sharply, necessitating a reduction in magnetic stirring to decrease the shrinkage cavity depth, resulting in poor compositional uniformity at the ingot head. Therefore, electromagnetic stirring has a poor effect on molten pool homogenization during the arc-ignition and molten pool establishment stages (corresponding to the ingot tail) and the head-feeding stage (corresponding to the ingot head), and even after multiple melting processes, the compositional uniformity at both ends of the ingot remains poor.

[0006] In the prior art, a method for improving the solidification structure of large-size TC4 titanium alloy ingots (CN201910532447.6) uses a bottom pad to reduce the cooling intensity at the bottom of the ingot; a method for preparing NbTiTa alloy ingots for superconducting applications (CN202310573088.5) uses a bottom pad not only to prevent damage to the crucible base, but also to prevent the molten electrode from dripping onto the bottom pad and causing rapid cooling, which would lead to the formation of inclusions of high-melting-point metals. A method for preparing titanium-niobium alloy ingots (CN202210676328.X) uses a bottom pad for improved feeding during the third melting process. These technologies also describe using a bottom pad at the bottom of the crucible, but because these technologies only use conventional designs for the bottom pad, they cannot achieve the effects of changing or controlling the current direction, guiding the molten pool to stir in multiple directions, strengthening the electromagnetic stirring of the molten pool, or promoting the homogenization of the molten pool composition. Summary of the Invention

[0007] In view of the above defects, the present invention proposes a VAR melting method to improve the compositional uniformity of ingot ends, comprising the following steps:

[0008] Prepare a bottom pad with a groove at one end, place the bottom pad at the bottom of the crucible, and keep the groove of the bottom pad facing downwards;

[0009] The consumable electrode is placed in the crucible, and the consumable electrode is welded and then melted by arc ignition.

[0010] In the initial stage of establishing the molten pool: set the initial melting current to 3-8kA, voltage to 20-40V, and arc stabilization current to 3-15A. In the final stage of establishing the molten pool: set the melting current to 6-40kA, voltage to 30-40V, and arc stabilization current to 8-30A.

[0011] After the molten pool establishment stage is completed, the process transitions to the normal smelting stage and the feeding stage, until the smelting is finished.

[0012] A VAR melting device for improving the compositional uniformity of ingot ends includes: a crucible and a bottom pad inside the crucible. The inside of the crucible is used to prevent the consumable electrode to be melted. The consumable electrode and the crucible are respectively connected to a melting power source. A water-cooling jacket is installed inside the crucible to cool the molten pool inside the crucible. An electromagnetic coil is also installed outside the crucible to generate a vertical magnetic field inside the crucible. The bottom pad is made of the same material as the consumable electrode. A slot is provided below the bottom pad so that the middle position of the lower surface of the bottom pad does not contact the bottom of the crucible.

[0013] This invention uses a concave bottom pad, which not only directs the current during the arc-ignition and molten pool establishment stage to the crucible sidewall instead of the crucible base, thus enhancing the electromagnetic stirring effect and promoting the homogenization of the molten pool composition, but also reduces the contact area between the bottom pad and the crucible base, more effectively reducing the cooling intensity during the arc-ignition and molten pool establishment stage, increasing the molten pool depth, further promoting the homogenization of the molten pool composition, and making the composition of the bottom of the solidified ingot more uniform. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the structure of the present invention.

[0015] Figure 2 for Figure 1 A top view of the molten pool.

[0016] Figure 3 This is a schematic diagram of the original design.

[0017] Figure 4 This is a schematic diagram of the detection method in an example.

[0018] In the figure: crucible 10, electrode 20, unsolidified molten pool 21, solidified ingot 22, bottom pad 30, groove 31. Detailed Implementation

[0019] join Figure 1 , 2 A VAR melting method for improving compositional uniformity at the ingot ends includes the following steps:

[0020] Place the bottom pad 30 at the bottom of the crucible, keeping the bottom pad groove 31 facing downwards;

[0021] The consumable electrode is placed in the crucible 10, and the electrode 20 and the auxiliary electrode are welded together and then smelted by electricity.

[0022] In the initial stage of establishing the molten pool: set the initial melting current to 3-8kA, voltage to 20-40V, and arc stabilization current to 3-15A. In the final stage of establishing the molten pool: set the melting current to 6-40kA, voltage to 30-40V, and arc stabilization current to 8-30A.

[0023] After the molten pool establishment stage is completed, the process transitions to the normal smelting stage and the feeding stage (using conventional smelting technology) until the smelting is finished. The normal smelting stage follows the same process as the molten pool stage. For the feeding stage, specific details of the smelting process are provided; conventional processes are sufficient, or you can refer to other patented technologies.

[0024] Furthermore, the VAR melting includes at least two melting processes. During the next melting process, the ingot is inverted and placed into the furnace so that the two ends of the ingot are melted sequentially according to the establishment of the molten pool, the normal melting stage, and the feeding stage until the melting is completed.

[0025] Furthermore, the base pad is a cylinder, and the outer circle of the base pad is adapted to the inner wall of the crucible of the melting furnace. The lower end of the base pad has a recessed groove to make the thickness at the edge of the base pad greater than the thickness at the middle.

[0026] As described in the background technology introduction, Figure 3 The diagram shows that in the initial stage of smelting, because the resistivity of copper is much lower than that of titanium, the current direction (which should be the direction of negative charge movement, but for ease of understanding, this is taken as the current direction and marked with 'e', ​​the same below) passes vertically through the unsolidified molten pool 21, the solidified ingot 22, and the bottom of the crucible, and then returns from the side wall of the crucible, forming a circuit. It can be seen that the current direction passes vertically through the molten pool, and the current direction is parallel to the magnetic field direction, resulting in charged particles not being affected by the Lorentz force, that is, there is no stirring effect on the molten pool. Only the radial current direction at the bottom of the crucible is subject to the Lorentz force, but this force cannot act on the charged particles in the molten pool, so it also cannot have a stirring effect.

[0027] This invention uses a concave bottom pad. The space between the concave area and the crucible base is a vacuum or inert gas atmosphere. There is no heat-conducting medium inside the concave area; heat conduction only occurs at the edges. Because the resistance in the center of the pad is higher, the current actively avoids it and moves towards the edges, meaning the current flows towards the crucible sidewall. This causes the original vertically downward or upward (Y-direction) current direction to shift, moving from the center of the molten pool towards the crucible sidewall. Consequently, the liquid in the molten pool flows from the center towards the edge, i.e., shifts in the X-direction. This is the first direction, the shift in the X-plane. The direction of the current e is shown in the figure. Furthermore, because the magnetic field B exists vertically, the direction of the Lorentz force is in the horizontal X-plane and perpendicular to the current direction. The presence of this force F, such as… Figure 1 , 2 The diagram shows that under the action of force F, the current direction of charged particles shifts along the direction of F, which is a shift in the second direction, within the Z-plane. This creates current flow in the Z-plane, and the flow of a large number of charged particles generates stirring power, forming eddy currents within the molten pool. This promotes homogenization of the molten pool composition, resulting in uniform composition at the ingot ends. Therefore, this solution, through the design of the bottom pad structure, causes charged particles to shift in at least two directions, resulting in shifts in both the X and Z directions. This, in turn, causes omnidirectional movement within the molten pool—up, down, front, back, left, and right—ultimately achieving uniform stirring.

[0028] Furthermore, the design of the bottom pad increases the distance between the molten pool and the bottom of the crucible, raising the height of the molten pool and preventing the bottom of the molten pool from being rapidly cooled by the water cooling system at the bottom of the crucible, thus prolonging the time it remains in a liquid state and ensuring continuous stirring. The design of the groove in the middle of the bottom pad, due to its hollow interior and lack of heat transfer medium, further prolongs the time the molten pool is cooled.

[0029] This invention also proposes a VAR melting device for improving the compositional uniformity of ingot ends, comprising: a crucible and a bottom pad inside the crucible, a consumable electrode to be melted is placed inside the crucible, the consumable electrode and the crucible are respectively connected to a melting power source, a water-cooling jacket is provided inside the crucible for cooling the molten pool inside the crucible, and an electromagnetic coil is provided outside the crucible to generate a vertical magnetic field inside the crucible, the bottom pad is made of the same material as the consumable electrode, and a slot is provided below the bottom pad so that the middle position of the lower surface of the bottom pad does not contact the bottom of the crucible.

[0030] Furthermore, the base pad is a cylinder, and its outer circumference is adapted to the inner wall of the crucible for placement inside the crucible. A recessed groove is formed at the lower end of the base pad to make the thickness at the edge greater than the thickness at the center. This results in a higher resistance at the center of the base pad.

[0031] Furthermore, the material of the base pad is the same as the material of the electrode to be melted.

[0032] Furthermore, the thickness of the bottom pad is not less than 30mm, the depth of the "concave" groove is not less than 10mm, and the thickness of the thinnest part of the "concave" bottom pad is not less than 20mm. The diameter of the bottom pad is 3-20mm smaller than the inner diameter of the crucible, which facilitates its easy placement into the bottom of the crucible.

[0033] Example 1: This example provides a method for improving the compositional uniformity at the bottom of TA15 ingots smelted by VAR.

[0034] 1) The TA15 ingot referred to in this embodiment is a three-stage melting ingot. The crucible used for the first melting is φ650mm, the crucible used for the second melting is φ720mm, and the crucible used for the third melting is φ820mm.

[0035] 2) In this embodiment, the first smelting was carried out using the conventional niobium-titanium smelting method, without using the aforementioned "concave" shaped base pad.

[0036] 3) In this embodiment, before the second melting, a concave bottom pad is placed on the crucible base. The bottom pad is made of TA15 φ705mm riser, with an edge thickness of 95mm and an irregular concave pit in the middle. The deepest part of the pit has a bottom pad thickness of 85mm. Melting begins with a furnace vacuum of 1.5Pa and a leakage rate of 0.6Pa / min. The initial current is 5kA, the voltage is 28V, and the arc-stabilizing current is 6A / AC for 45s. During the final stage of establishing the molten pool, the melting current is 12-25kA, the voltage is 30-36V, and the arc-stabilizing current is 8-25A / AC for 15-40s. After the molten pool establishment stage, normal melting and feeding stages continue until the melting is complete. After the second melting, the bottom pad is sawn off and the end is flattened for use as an electrode in the third melting.

[0037] 4) Before the third melting in this embodiment, a concave bottom pad is placed on the crucible base. The bottom pad is made of TA15 φ800mm riser, with an irregular pit in the middle and machined smooth edges. The bottom pad is 130mm thick, and the thickness of the bottom pad at the deepest point of the pit is 100mm. Melting begins with a furnace vacuum of 1Pa and a leakage rate of 0.2Pa / min. The initial current is 6kA, the voltage is 28V, and the arc-stabilizing current is 8A / AC for 40s. During the final stage of establishing the molten pool, the melting current is 15-30kA, the voltage is 30-38V, and the arc-stabilizing current is 8-25A / AC for 15-30s. After the molten pool establishment stage, normal melting and feeding stages continue until the melting is completed.

[0038] 5) Inspect and remove shrinkage cavities at the head of the ingot. Cut off the bottom pad 150mm from the bottom of the ingot (including the bottom pad). Take samples from 9 points on the end face of the cut position at the head and bottom of the ingot for testing. The results are shown in the table below (mass fraction, %).

[0039]

[0040]

[0041] The detection method used in this scheme is as follows: the cut-off end face of the ingot is trimmed into a smooth plane, and then... Figure 4 The above data was obtained by taking 9 points. 1, 6, 5, and 9 are the edge detection positions, 3 is the center position, and 2, 4, 7, and 8 are the radius positions.

[0042] In this embodiment, the thickness of the base pad is 130mm, and the thickness removed after melting is 150mm. The test data shows that the composition difference is small at various points on the cross-section, that is, the composition is uniform. Especially for Cr, since this element has the lowest specific gravity, during the stage of establishing the molten pool, due to the shallow molten pool and less melt, the fluidity of this element is poor. After melting from the electrode, it falls into the molten pool and easily concentrates in a certain position. When the original melting process is used, the electromagnetic stirring is not obvious, especially in the diameter range, near the edge, such as positions 1, 6, 5, and 9, where the Cr content is low, while positions 2, 3, 4, 7, and 8 have higher Cr content. This is because the electromagnetic stirring effect in the original process is not obvious, especially in the inability to form stirring in the X and Z planes, resulting in uneven Cr composition in this plane. In this invention, by stirring in two directions, the composition of Cr is made uniform, especially in the X and Z planes, with the component deviation at the head being (2.12-2.04) / 2.12=0.03 ​​and the component deviation at the head being (2.19-2.15) / 2.19=0.018.

[0043] Example 2: This example provides a method for improving the compositional uniformity at the bottom of VAR smelting TC1 ingots.

[0044] 1) The TC1 ingot referred to in this embodiment is a three-time melting ingot. The crucible used in the first melting is φ440mm, the crucible used in the second melting is φ530mm, and the crucible used in the third melting is φ650mm.

[0045] 2) In this embodiment, the first and third smelting were carried out using conventional methods, without using the above-mentioned "concave" shaped base pad.

[0046] 3) Before the second melting in this embodiment, a concave bottom pad is placed on the crucible base. The bottom pad is made of a riser of the same grade with a diameter of φ615mm, the edge thickness is 60-65mm, and there is an irregular pit in the middle with a maximum depth of 15mm.

[0047] 4) After the furnace chamber vacuum reaches 1 Pa and the leakage rate is 0.5 Pa / min, power is supplied for melting. The initial current is 4 kA, the voltage is 25 V, and the arc-stabilizing current is 6 A / AC for 50 s. After power is supplied, argon gas is introduced until the furnace chamber pressure reaches 8000-13000 Pa. At the end of the molten pool establishment stage, the melting current is 13-20 kA, the voltage is 32-42 V, and the arc-stabilizing current is 8-20 A / AC for 15-45 s. After the molten pool establishment stage, normal melting and feeding stages continue until the melting is completed.

[0048] 5) After sawing off the bottom pad at a distance of 85mm from the bottom of the ingot, samples were taken from 9 points on the end face of the ingot at the sawing position for testing. The results are shown in the table below (mass fraction, %).

[0049] element 1 2 3 4 5 6 7 8 9 Al 2.05 2.10 2.08 2.07 2.08 2.04 2.06 2.02 2.00 Mn 1.62 1.56 1.62 1.58 1.54 1.50 1.51 1.51 1.50 Fe 0.096 0.09 0.085 0.096 0.087 0.087 0.087 0.090 0.087

[0050] Example 3: This example provides a method for improving the compositional uniformity at the bottom of VAR-melted Nb47Ti ingots.

[0051] 1) The Nb47Ti ingot referred to in this embodiment is a three-melting ingot. The crucible used in the first melting is φ365mm, the crucible used in the second melting is φ440mm, and the crucible used in the third melting is φ530mm.

[0052] 2) In this embodiment, the first smelting was carried out using conventional methods without the use of a base pad.

[0053] 3) In this embodiment, before the second melting, a concave bottom pad is placed on the crucible base. The pad's composition is the same as the electrode to be melted. The pad has a diameter of 435mm, a thickness of 80mm, and a 350mm deep recess in the center, with a depth of 1-10mm. Melting begins with a furnace vacuum of 1Pa and a leakage rate of 0.3Pa / min. The initial current is 4kA, the voltage is 25V, and the arc-stabilizing current is 5A / AC for 30s. During the final stage of establishing the molten pool, the melting current is 8-20kA, the voltage is 28-36V, and the arc-stabilizing current is 6-20A / AC for 15-30s. After the molten pool establishment stage, normal melting and feeding stages continue until the melting is complete. After the second melting, the bottom pad and the molten nodules at the ingot joint are removed by machining. The finished electrode is then used for the third melting.

[0054] 4) Before the third melting in this embodiment, a concave bottom pad is placed on the crucible base. The pad's composition is the same as the electrode to be melted. The pad has a diameter of 525mm, a thickness of 60mm, and a φ400mm concave pit in the middle with a depth ≤10mm. Melting begins with a furnace vacuum of 1Pa and a leakage rate of 0.2Pa / min. The initial current is 5kA, the voltage is 28V, and the arc-stabilizing current is 8A / AC for 40s. During the final stage of establishing the molten pool, the melting current is 12-25kA, the voltage is 30-38V, and the arc-stabilizing current is 8-25A / AC for 15-60s. After the molten pool establishment stage, normal melting and feeding stages continue until the melting is complete.

[0055] 5) Inspect and remove the shrinkage cavity at the head of the ingot. Cut off the bottom pad from 80mm from the bottom of the ingot. Take samples from 9 points on the end face of the cut position at the head and bottom of the ingot for testing. The results are shown in the table below (mass fraction, %).

[0056] Location element 1 2 3 4 5 6 7 8 9 head Ti 46.78 47.00 47.27 46.96 46.86 46.86 47.06 47.07 46.79 end Ti 47.53 47.35 47.31 47.33 47.44 47.49 47.28 47.31 47.49

[0057] The embodiments of this patent have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A VAR melting method for improving the compositional uniformity at the ends of titanium alloy ingots, characterized in that... Includes the following steps: Place the base pad at the bottom of the crucible, keeping the groove of the base pad facing downwards. Inside the crucible, place the consumable electrode to be melted. The consumable electrode and the crucible are connected to the melting power source. An electromagnetic coil is also placed outside the crucible to generate a vertical magnetic field inside the crucible. The groove at the lower end of the base pad is recessed inwards so that the thickness at the edge of the base pad is greater than the thickness at the middle. The groove at the bottom of the base pad prevents the middle of the lower surface of the base pad from contacting the bottom of the crucible. Place the consumable electrode inside the crucible, ensuring that the electrode tip does not contact the bottom pad; In the initial stage of establishing the molten pool: set the initial melting current to 3-8kA, voltage to 20-40V, and arc stabilization current to 3-15A. In the final stage of establishing the molten pool: set the melting current to 6-40kA, voltage to 30-40V, and arc stabilization current to 8-30A. After the molten pool establishment phase is completed, continue smelting during the normal smelting phase and the feeding phase until the smelting is finished.

2. The VAR melting method for improving the compositional uniformity at the end of titanium alloy ingots as described in claim 1, characterized in that: The VAR melting process includes at least two melting operations. During the two adjacent melting operations, the ingot is inverted before being placed into the furnace, so that the two ends of the ingot are melted sequentially according to the establishment of the molten pool, the normal melting stage, and the feeding stage until the melting is completed.

3. The VAR melting method for improving the compositional uniformity at the end of titanium alloy ingots as described in claim 1, characterized in that: The bottom pad is the end material formed after the ingot is cut off after melting, and the end material is machined into a groove.

4. A VAR melting apparatus for improving the compositional uniformity at the end of titanium alloy ingots, characterized in that... include: The crucible and its internal base pad are configured. The crucible contains a consumable electrode to be melted, and both the consumable electrode and the crucible are connected to a melting current. A water-cooling jacket is installed inside the crucible to cool the molten pool inside. An electromagnetic coil is installed outside the crucible to generate a vertical magnetic field inside the crucible. The base pad is made of the same material as the consumable electrode. A recessed groove is formed at the lower end of the base pad so that the thickness at the edge of the base pad is greater than the thickness at the middle, so that the middle of the lower surface of the base pad does not contact the bottom of the crucible.

5. The VAR melting apparatus for improving the compositional uniformity of titanium alloy ingot ends as described in claim 4, characterized in that: The base pad is a cylinder, and its outer diameter is adapted to the inner wall of the crucible for placing inside the crucible.

6. The VAR melting apparatus for improving the compositional uniformity of titanium alloy ingot ends as described in claim 5, characterized in that: The thickness of the bottom pad is not less than 30mm, the depth of the "concave" groove is not greater than 10mm, and the thickness of the thinnest part of the "concave" bottom pad is not less than 20mm. The diameter of the bottom pad is 3-20mm smaller than the inner diameter of the crucible, which facilitates its easy placement into the bottom of the crucible.

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