A method for preparing TiAl alloy ingots by vacuum induction melting

Abstract

The present invention discloses a method for preparing TiAl alloy ingots by vacuum induction melting, belonging to the field of titanium-aluminum alloy melting. By subjecting the metal to induction melting, flipping the alloy ingot for melting, and during the process of power reduction, by cooling the molten metal temperature below the melting point by 10 °C and holding for 30 min, the electromagnetic stirring of the melt can be maintained for a longer time, thereby improving the uniformity of the metal composition. Subsequently, the stirring magnetic field of the alloy melt is slowly reduced to 10 °C above the α phase transformation point at a speed of 10 kW / min and held for 30 min. Using 10 °C above the α phase transformation point for single-phase region heat preservation heat treatment will not cause rapid growth of α grains. Then, furnace cooling is carried out to obtain a fine fully lamellar structure with better comprehensive mechanical properties at room temperature and more uniform composition. Then, the power is reduced to the lowest at a speed of 80 kW / min, and the ingot is cooled with the furnace, ultimately achieving a reduction in the macrosegregation of the ingot.

Description

Technical Field

[0001] The present invention belongs to the field of melting of titanium aluminide alloys, and particularly relates to a method for preparing TiAl alloy ingots by vacuum induction melting with controllable macrosegregation and higher melting efficiency. Background Art

[0002] As a new type of lightweight high-temperature structural material, TiAl alloy has unique performance advantages in many aspects, such as lower density, higher specific strength and specific modulus, good oxidation resistance and creep properties, and excellent fatigue properties. It is a candidate high-temperature structural material in fields such as contemporary aerospace industry and civil industry, and has important potential for engineering applications. The engineering application of TiAl alloy is directly related to the macro-morphology and defects of the ingot. To obtain structural components with excellent properties, precise control of macrosegregation is a necessary condition.

[0003] The vacuum consumable electrode skull furnace used in Patent CN202310294669.5 has a low vacuum degree, cannot effectively remove volatile harmful impurities in the melt, is easily contaminated by elements such as O, N, and H, and the purity of the ingot is low. The graphite crucible used is prone to introducing impurities, prone to segregation, and has coarse grains. In Patent CN202111039002.8, under higher power, as the melting time progresses, the element loss is more serious. And during long-term high-power operation, the temperature of the circulating water will continuously rise until the equipment alarms, which is likely to cause damage to the equipment itself. This significantly limits the time of electromagnetic stirring and directly affects the macro-uniformity of the ingot. The macrosegregation in TiAl alloy will affect the subsequent use of the alloy. Summary of the Invention

[0004] In order to solve the problem of macrosegregation of titanium aluminide alloy ingots, the purpose of the present invention is to provide a new method for vacuum induction melting of TiAl alloy.

[0005] The specific technical solution adopted by the present invention is as follows: A method for preparing TiAl alloy ingots by vacuum induction melting, under a protective atmosphere, using vacuum induction melting, specifically including the following steps:

[0006] Step 1: Weigh the raw materials according to the composition of the TiAl alloy.

[0007] Step 2: Slowly increase the power to the maximum power, heat the metal raw materials placed in the water-cooled copper crucible. After the raw materials in the crucible are completely melted, keep warm for 10 min, then reduce the power at a rate of 60 kW / min, and let the ingot cool down with the water-cooled copper crucible. After the power drops to the lowest, turn off the heating power supply. After the ingot cools, take it out to obtain a primary ingot.

[0008] Step 3: Flip the primary ingot and place it in a water-cooled copper crucible, and remelt it according to Step 2 to obtain a secondary ingot;

[0009] Step 4: Flip the secondary ingot again and place it in a water-cooled copper crucible, and remelt it again. The process is as follows: Slowly increase the power to the maximum power. After the secondary ingot in the crucible is completely melted, keep it warm for 10 min, reduce the power at a rate of 30 kW / min to lower the temperature of the molten metal below the melting point by 10 °C, then keep it warm for 30 min, then reduce the power at a rate of 10 kW / min to above 10 °C of the α phase transformation point, keep it warm for 30 min, reduce the power at a rate of 80 kW / min, and let the ingot cool down with the water-cooled copper crucible. After the power drops to the lowest, turn off the heating power supply. After the ingot cools, take it out to obtain a TiAl alloy ingot with uniform structure and reduced macroscopic segregation.

[0010] Further, the TiAl alloy is Ti(43 - 48)Al(1 - 13)Nb.

[0011] Further, the TiAl alloy is Ti45Al8Nb.

[0012] Specifically, when preparing the ingredients, high-purity sponge Ti, high-purity Al blocks, and Nb-Al master alloy particles are used as metal raw materials.

[0013] Further, the protective atmosphere is high-purity argon, and the vacuum degree is maintained at 0.05 Mpa.

[0014] Further, slowly increasing the power to the maximum power means slowly increasing the power to the maximum power at a rate of 60 kW / min.

[0015] Compared with the prior art, the present invention provides a method for induction skull melting of TiAl alloy. During the vacuum induction melting process, the coupling effect of the magnetic field and temperature field in the water-cooled copper crucible is utilized to accurately control the temperature of the melt during the melting process. When the melt temperature is 10 °C below the melting point, it is kept warm for the first time. At this time, the temperature of the equipment cooling water will not continue to rise, and long-term heat preservation will not damage the equipment, and the element burning loss is relatively low. It can increase the time of electromagnetic stirring and significantly improve the macroscopic uniformity. And it is kept warm for the second time in the single-phase region 10 °C above the α phase transformation point. After that, it is rapidly cooled and solidified under the action of water cooling, and a structure with more uniform composition can be obtained. The present invention combines the melting process with heat treatment by designing a two-step heat preservation process during the melting process, and finally realizes a significant reduction in the macroscopic segregation of the ingot and improves the melting efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0017] Figure 1Among them, (a, b) is the Ti45Al8Nb alloy prepared in Example 1, and (c, d) is the macroscopic morphology diagram of the Ti45Al8Nb alloy prepared in Example 2.

[0018] Figure 2 Among them, (a, b) is the Ti45Al8Nb alloy prepared in Example 1, and (c, d) is the SEM-BSE microstructure diagram of the Ti45Al8Nb alloy prepared in Example 2.

[0019] Figure 3 It is the ICP composition detection result diagram of the Ti45Al8Nb alloy prepared in Example 2.

[0020] Figure 4 It is the optical microscope image of the Ti45Al8Nb alloy prepared in Example 1.

[0021] Figure 5 It is the optical microscope image of the Ti45Al8Nb alloy prepared in Example 3.

[0022] Figure 6 It is the ICP composition detection result diagram of the Ti45Al8Nb alloy prepared in Example 3.

[0023] Figure 7 It is the optical microscope image of the Ti45Al8Nb alloy prepared in Example 4.

[0024] Figure 8 It is the ICP composition detection result diagram of the Ti45Al8Nb alloy prepared in Example 5.

[0025] Figure 9 It is the process flow chart for preparing TiAl alloy ingots by vacuum induction melting of the present invention. Specific embodiments

[0026] The technical solution of the present invention will be further described below according to specific embodiments. The protection scope of the present invention is not limited to the following embodiments. These embodiments are listed for illustrative purposes only and do not limit the present invention in any way.

[0027] The present invention heats metal raw materials by induction melting, flips and melts alloy ingots, and during the process of reducing the power of vacuum induction skull melting, the central temperature of the melt is detected by an embedded thermocouple. When the melt temperature is below the melting point of the TiAl alloy and 10 °C below the melting point without meeting the supercooling degree, it is insulated for 30 min. At this time, the temperature reduction of the melt by the cooling water and the temperature increase of the melt by electromagnetic induction reach equilibrium, and the temperature of the equipment circulating water will not continue to rise, and it can continue to operate safely. At this time, the melt has not solidified, and the electromagnetic stirring of the melt can continue for a longer time, thereby improving the uniformity of metal components. Then, by turning the power knob, the stirring magnetic field of the alloy melt is slowly reduced at a speed of 10 kW / min to 10 °C above the α phase transformation point and insulated for 30 min. Using 10 °C above the α phase transformation point for single-phase region heat treatment is because this temperature is only slightly higher than the α transformation temperature, so it will not cause the rapid growth of α grains. However, the diffusion rate of Nb is very slow by conventional methods, and the electromagnetic stirring of induction melting can promote this process. Subsequently, furnace cooling is carried out to obtain a fully lamellar structure. The fine fully lamellar structure has better comprehensive mechanical properties at room temperature and more uniform composition. Select a power reduction speed of 80 kW / min to reduce the power to the lowest level, and let the ingot cool with the furnace. At this time, it is most beneficial to control the macroscopic shape segregation of the alloy. Generally speaking, the fully lamellar structure of the TiAl alloy is obtained by insulating in the α single-phase region and furnace cooling. When insulating in the α single-phase region, an α single-phase structure is formed, which destroys the α2 / γ lamellar structure in the original as-cast structure. If the insulation temperature is too high, the α grains are easily rapidly grown, resulting in a larger size of the α2 / γ lamellar clusters after cooling, thereby reducing the room temperature mechanical properties of the alloy.

[0028] Example 1

[0029] Combined with Figure 9 , a method for melting TiAl alloy with precisely controllable composition according to this embodiment includes the following steps:

[0030] (1) According to the atomic ratio of Ti: 0.47, Al: 0.45, Nb: 0.08, accurately weigh 8022 g of high-purity sponge Ti, 3459.6 g of high-purity Al block, and 3518.4 g of Nb-Al master alloy particles respectively, with a total mass of 15000 g of metal raw materials.

[0031] (2) Before the start of melting, place the metal raw material particles in beakers respectively, pour in high-purity anhydrous ethanol; place the beakers in an ultrasonic cleaner to thoroughly clean the materials, and wait for drying after cleaning before preparing for melting.

[0032] (3) Before melting, the inside of the copper crucible needs to be cleaned with alcohol to ensure it is free of impurities. Stack the materials according to specific requirements. Based on the temperature field and magnetic field analysis of the skull furnace, place the low-melting-point elements at the lower part of the crucible and then place the high-melting-point elements at the top, which can maximize the utilization of the heating power.

[0033] (4) Turn on the low-pressure cooling water, dryer, mechanical pump, and argon gas cylinder switches in sequence; close the hatch and lock the latch; turn on the mechanical pump and the large and small Roots pumps to evacuate the air, then turn on the diffusion pump and preheat it to 280 °C, and pump until the vacuum degree reaches 5 10 -3 Pa; turn off the diffusion pump, open the inflation valve, and fill in argon gas to keep the vacuum degree of the melting chamber at 0.05 Mpa.

[0034] (5) Turn on the cooling water system and heating power supply, slowly increase the power to the maximum power (360 KW), heat the metal raw materials placed in the water-cooled copper crucible. After the raw materials in the crucible are completely melted, keep it warm for 10 min, reduce the power at a rate of 60 kW / min, let the ingot cool down with the water-cooled copper crucible. After the power drops to the lowest level, turn off the heating power supply. After the ingot cools down, take it out to obtain the primary ingot.

[0035] (6) Flip the above primary ingot and place it back into the water-cooled copper crucible. Turn on the low-pressure cooling water, dryer, mechanical pump, and argon gas cylinder switches in sequence; close the hatch and lock the latch; turn on the mechanical pump and the large and small Roots pumps to evacuate the air, then turn on the diffusion pump and preheat it to 280 °C, and pump until the vacuum degree reaches 5 10 -3 Pa; turn off the diffusion pump, open the inflation valve, and fill in argon gas to keep the vacuum degree of the melting chamber at 0.05 Mpa; turn on the cooling water system and heating power supply, slowly increase the power to the maximum power, heat the primary ingot placed in the water-cooled copper crucible. After the primary ingot in the crucible is completely melted, keep it warm for 10 min, reduce the power at a rate of 60 kW / min, let the ingot cool down with the water-cooled copper crucible. After the power drops to the lowest level, turn off the heating power supply. After the ingot cools down, take it out to obtain the secondary ingot.

[0036] (7) Flip the above secondary ingot again and place it into the water-cooled copper crucible. Turn on the low-pressure cooling water, dryer, mechanical pump, and argon gas cylinder switches in sequence; close the hatch and lock the latch; turn on the mechanical pump and the large and small Roots pumps to evacuate the air, then turn on the diffusion pump and preheat it to 280 °C, and pump until the vacuum degree reaches 5 10 -3Pa; Close the diffusion pump, open the gas charging valve, and charge argon to keep the vacuum degree of the melting chamber at 0.05 Mpa; Turn on the cooling water system and the heating power supply, slowly increase the power to the maximum power. After the secondary ingot in the crucible is completely melted, keep it warm for 10 min, reduce the power at a rate of 30 kW / min to lower the temperature of the molten metal 10 °C below the melting point, i.e., 1570 °C (measure the temperature using an embedded thermocouple), then keep it warm for 30 min, and then reduce the power at a rate of 10 kW / min to 10 °C above the α phase transition point, i.e., 1310 °C, and keep it warm for 30 min. In order to achieve rapid solidification, refine the grains, and improve the surface quality of the ingot, reduce the power at a rate of 80 kW / min, and let the ingot cool down with the water-cooled copper crucible. After the power is reduced to the lowest, turn off the heating power supply. After the ingot cools down, take it out. Obtain the TiAl alloy ingot, and its macroscopic morphology photos are as shown in Figure 1 the (a) and (b) in Figure 2 the (a) and (b) of which are shown. The average value of the composition detection results of this alloy ingot is shown in Table 1, and the optical microscope images are as shown in Figure 4 .

[0037] Table 1 Composition of Ti45Al8Nb after secondary feeding

[0038] Alloying element Ti Al Nb At(%) 47.07 44.99 7.94 wt(%) 53.59 28.87 17.54

[0039] Example 2

[0040] For a TiAl alloy melting method with precisely controllable composition described in this example, other steps are the same as those in Example 1. The difference is that in step (1), according to the atomic ratio of Ti: 0.47, Al: 0.45, Nb: 0.08, accurately weigh 10696 g of high-purity sponge Ti, 4612.8 g of high-purity Al block, and 4691.2 g of Nb-Al master alloy particles, and the total mass of the metal raw materials is 20000 g. Finally, obtain a TiAl alloy with a precise composition, and its macroscopic morphology photos are as shown in Figure 1 the (c) and (d) in Figure 2 the (c) and (d) of which are shown. The average value of the composition detection results is shown in Table 2. The fluctuations of the composition detection results of aluminum and niobium at multiple positions of this alloy are as shown in Figure 3 .

[0041] Table 2 Composition of Ti45Al8Nb after secondary feeding

[0042] Alloying element Ti Al Nb At(%) 47.17 44.91 7.92 wt(%) 53.70 28.81 17.49

[0043] Example 3

[0044] A TiAl alloy melting method described in this embodiment is the same as that in Embodiment 1 in other steps, except that: in step (7), the above secondary ingot is turned over and then placed back into the water-cooled copper crucible. Then, the low-pressure cooling water, dryer, mechanical pump, and argon gas cylinder switches are turned on in sequence; the hatch is covered and the lock is tightened; the mechanical pump and the large and small Roots pumps are turned on to evacuate the air. Then, the diffusion pump is turned on and preheated to 280 °C until the vacuum degree reaches 5 10 -3 Pa; the diffusion pump is turned off, the inflation valve is opened, and argon gas is filled to keep the vacuum degree of the melting chamber at 0.05 Mpa; the cooling water system and the heating power supply are turned on, and the power is slowly increased to the maximum power to heat the secondary ingot placed in the water-cooled copper crucible. After the secondary ingot in the crucible is completely melted, it is kept warm for 10 min, and the power is reduced at a rate of 80 kW / min, and the ingot is cooled with the water-cooled copper crucible. After the power is reduced to the lowest level, the heating power supply is turned off. After the ingot is cooled, it is taken out to obtain a TiAl alloy ingot. The optical micrograph is as Figure 5 shown, and the composition detection results of aluminum and niobium at multiple positions of the alloy fluctuate as Figure 6 shown.

[0045] Embodiment 4

[0046] A TiAl alloy melting method described in this embodiment is the same as that in Embodiment 1 in other steps, except that: in step (7), the above secondary ingot is turned over again and placed in the water-cooled copper crucible. Then, the low-pressure cooling water, dryer, mechanical pump, and argon gas cylinder switches are turned on in sequence; the hatch is covered and the lock is tightened; the mechanical pump and the large and small Roots pumps are turned on to evacuate the air. Then, the diffusion pump is turned on and preheated to 280 °C until the vacuum degree reaches 5 10 -3 Pa; the diffusion pump is turned off, the inflation valve is opened, and argon gas is filled to keep the vacuum degree of the melting chamber at 0.05 Mpa; the cooling water system and the heating power supply are turned on, and the power is slowly increased to the maximum power. After the secondary ingot in the crucible is completely melted, it is kept warm for 10 min, and the power is reduced at a rate of 30 kW / min to lower the temperature of the molten metal 10 °C below the melting point, that is, 1570 °C, and then it is kept warm for 30 min. Then, the power is reduced at a rate of 80 kW / min, and the ingot is cooled with the water-cooled copper crucible. After the power is reduced to the lowest level, the heating power supply is turned off. After the ingot is cooled, it is taken out to obtain a TiAl alloy ingot. The optical micrograph is as Figure 7 shown.

[0047] Embodiment 5

[0048] A TiAl alloy melting method according to this embodiment is the same as that of Embodiment 1 in other steps, except that: in step (7), the above-mentioned secondary ingot is turned over again and placed in a water-cooled copper crucible, and the low-pressure cooling water, dryer, mechanical pump and argon gas cylinder switches are sequentially turned on; the hatch is closed and the lock is tightened; the mechanical pump and the large and small Roots pumps are turned on to evacuate the air, and then the diffusion pump is turned on and preheated to 280 °C, and the vacuum is pumped to 10 -3 Pa; the diffusion pump is closed, the inflation valve is opened, and argon gas is filled to keep the vacuum degree of the melting chamber at 0.05 Mpa; the cooling water system and the heating power supply are turned on, and the power is slowly increased to the maximum power. After the secondary ingot in the crucible is completely melted, keep it warm for 10 min, reduce the power at a rate of 30 kW / min to 10 °C above the α phase transformation point, that is, 1310 °C, keep it warm for 30 min, reduce the power at a rate of 80 kW / min, and let the ingot cool down with the water-cooled copper crucible. After the power is reduced to the lowest, the heating power supply is turned off. After the ingot is cooled, it is taken out. Let the ingot cool down with the water-cooled copper crucible. After the power is reduced to the lowest, the heating power supply is turned off. After the ingot is cooled, it is taken out to obtain a TiAl alloy ingot. The composition detection results of aluminum and niobium at multiple positions of the ingot fluctuate as Figure 8 shown.

[0049] The present invention accurately controls the melt temperature, and controlling the temperature in a stable temperature range is more conducive to obtaining the desired grain size and uniform structure. This new and efficient vacuum induction skull melting process for TiAl alloys can reduce macrosegregation, improve the uniformity of the ingot, and improve the melting efficiency.

Claims

1. A method for preparing a TiAl alloy ingot by vacuum induction melting, wherein the TiAl alloy is Ti(43-48)Al(1-13)Nb, characterized in that: Under a protective atmosphere, vacuum induction melting is used, which specifically includes the following steps: Step 1: Prepare ingredients according to the TiAl alloy composition; Step 2: Slowly increase the power to the highest power, heat the metal raw material placed in the water-cooled copper crucible, and keep it warm for 10 minutes after the raw material in the crucible is completely melted. Reduce the power at a rate of 60 kW / min, and let the ingot cool down along with the water-cooled copper crucible. Turn off the heating power after the power drops to the lowest power, and take out the ingot after it cools down to obtain a primary ingot; Step 3: turning over the primary ingot and placing it in a water-cooled copper crucible, and re-smelting it according to step 2 to obtain a secondary ingot; Step 4: The secondary ingot is turned over again and placed in a water-cooled copper crucible for smelting again. The process is: slowly increase the power to the maximum power, and after the secondary ingot in the crucible is completely melted, keep warm for 10 minutes, reduce the power at a rate of 30 kW / min to reduce the metal liquid temperature to 10°C below the melting point, and then keep warm for 30 minutes, and then reduce the power at a rate of 10 kW / min to 10°C above the α phase transformation point, keep warm for 30 minutes, and reduce the power at a rate of 80 kW / min to allow the ingot to cool down with the water-cooled copper crucible. After the power drops to the minimum, turn off the heating power supply, and take out the ingot after it is cooled to obtain a TiAl alloy ingot with uniform structure and reduced macro-segregation.

2. The method according to claim 1, characterized in that TiAl alloy is Ti45Al8Nb.

3. The method according to claim 1, characterized in that When the TiAl alloy is Ti(43-48)Al(1-13)Nb, high-purity sponge Ti, high-purity Al blocks and Nb-Al master alloy particles are used as metal raw materials.

4. The method according to claim 1, characterized in that The protective atmosphere was high-purity argon, and the vacuum degree was maintained at 0.05Mpa.

5. The method according to claim 1, characterized in that Slowly increasing the power to the maximum power means slowly increasing the power to the maximum power at a rate of 60 kW / min.

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