High-niobium TiAl alloy structure regulation and control method

Abstract

The invention discloses a high-niobium TiAl alloy structure regulation and control method which comprises the following steps: carrying out long-time heat preservation on a cast ingot in an (alpha + beta) two-phase region in the smelting and solidification process of a Ti-48Al-10Nb alloy, so that elements are fully diffused, solidification segregation is reduced, segregation distribution is regulated and controlled, and alpha segregation refined grains are formed; and then the cast ingot is subjected to heat preservation and air cooling in an alpha single-phase region, finally, the temperature is increased to the ductile-brittle transition temperature or above for stress relief annealing, the comprehensive effects of segregation removal and grain refinement are achieved, and the performance of the alloy is improved.

Description

Technical Field

[0001] The invention belongs to the field of metal solidification manufacturing, and in particular relates to a method for regulating and controlling the structure of a high-niobium TiAl alloy. Background Art

[0002] Titanium-aluminum alloy is an alloy with low specific gravity and excellent high-temperature performance, making it a structural material with great application prospects in the fields of aviation, aerospace and automobile manufacturing. A high-temperature and high-performance titanium-aluminum alloy was announced in the invention patent with application number 01134629.9. In this patent, high-melting-point elements such as Nb and W were added, and a high-niobium titanium-aluminum alloy system with a composition range of Ti-(42-45)Al-(5-10)Nb was proposed, which improved the high-temperature strength and oxidation resistance of the titanium-aluminum alloy and made its service temperature reach 800℃-900℃. However, with the addition of high-melting-point elements, the segregation produced during the solidification process increased significantly, and the subsequent solid-state phase transformation β→α caused the residual β phase to transform into a hard and brittle B2 phase at room temperature due to incomplete phase transformation. When subjected to external force, it may become a crack source and a channel for crack propagation, accelerating the failure of the alloy and deteriorating the mechanical properties of the alloy. The paper "Microsegregation in high Nb containing TiAl alloy ingots beyond laboratoryscale" reports the generation mechanism of S, β, and α segregation during the solidification of Ti-45Al-(8-9)Nb-(W,B,Y) alloy and its influence on the microstructure.

[0003] Currently, the primary method for improving internal segregation in high-niobium titanium-aluminum alloys is heat treatment. According to the paper "Eliminating β-Phase Segregation in Large-Scale Cast High-Nb-Titanium-Aluminum Alloy Microstructures by Heat Treatment," holding high-niobium titanium-aluminum alloys in the α+γ two-phase region can essentially eliminate β and α segregation, but it cannot eliminate solidification segregation, nor can it eliminate the interdendritic coarsening caused by S segregation. The paper "Microstructural design and mechanical properties of a cast and heat-treated intermetallic multi-phase γ-TiAl based alloy" reports the effects of holding a Ti-44Al-4Nb-1Mo alloy in the β single-phase region on S and β segregation. Holding at 1425°C for 15 minutes completely eliminates S and β segregation, refines the lamellar clusters, and improves microstructure uniformity. However, this method involves a complex process flow, requiring the alloy casting to be cooled to room temperature and then heated again, resulting in increased costs.

[0004] The main methods for refining the grains of high-niobium titanium-aluminum alloys include: adding refiners, mechanical processing (such as forging), etc. The invention patent with application number 201410273532.2, "Method for low segregation and uniform microstructure refinement of high-niobium titanium-aluminum alloys", proposes to perform constant temperature treatment during the solidification process under high temperature conditions of 1460-1540°C to achieve the effect of eliminating S segregation and refining grains, but without long-term heat preservation, ultimately resulting in a low degree of β segregation removal.

[0005] Patent application number 201310400551.2, "A TiAl Alloy Grain Refining Method," proposes remelting coarse columnar crystals through a cyclic treatment in the solid-liquid two-phase region. The melted dendrites then serve as new nucleation sites for nucleation and growth, thereby achieving grain refinement. However, this requires repeated overheating of the alloy melt, which is complex and requires high equipment requirements. The paper "Grain refinement by low boron additions in niobium-rich TiAl-based alloys" reports the refining effect of boron on lamellar clusters and its mechanism. Adding an appropriate amount of boron to the melt forms TiB or TiB2 as heterogeneous nucleation cores, promoting grain nucleation. Furthermore, the addition of boron further increases the compositional undercooling, refining the TiAl alloy grains and reducing the structural inhomogeneity caused by sulfur segregation. However, the addition of a refiner can easily introduce foreign inclusions, and TiAl alloys are extremely sensitive to composition, which can deteriorate the mechanical properties of TiAl structural components.

[0006] In summary, in order to solve the problem of poor microstructure uniformity of high niobium titanium aluminum alloy due to segregation, it is hoped to develop a process method that has the advantages of reducing segregation, improving microstructure uniformity and refining grains, without changing the alloy phase composition, forming inclusions, and having relatively low cost. Summary of the Invention

[0007] In order to solve the problems of deterioration of alloy properties caused by segregation in high niobium titanium aluminum alloy, resulting in low material utilization and high production costs, the present invention proposes a method for removing segregation and refining grains through controlled solidification combined with subsequent heat treatment process.

[0008] The present invention specifically comprises the following steps: a method for controlling the microstructure of a high-niobium TiAl alloy, wherein the high-niobium TiAl alloy is Ti-48Al-10Nb (at.%), comprising the following steps:

[0009] Step 1: Place aluminum block, titanium sponge and niobium aluminum alloy into a water-cooled copper crucible in sequence according to alloy composition;

[0010] Step 2: Evacuate the ingot and, under protective gas, raise the heating power to the highest power at a certain temperature. After all the raw materials are melted, keep the temperature for 5 to 7 minutes. Then, reduce the power to 0 at a certain temperature-lowering power. After the ingot cools down, take it out, turn it over, and place it in a water-cooled copper crucible.

[0011] Step 3, repeat the smelting process of step 2 twice, in the cooling stage of the third smelting process, cool the alloy temperature to 1500℃~1560℃ at a cooling power of 20kW / min, enter the β single-phase region, and keep warm for 5-30min, then continue to cool to 1420℃~1490℃, enter the (α+β) two-phase region, and keep warm for 10-60min. After the insulation is completed, reduce the power to 0 at 20kW / min, wait for the ingot to cool, and then take it out;

[0012] Step 4: heat-treating the ingot.

[0013] Preferably, vacuum to 9×10 -3 Pa.

[0014] Preferably, argon is used as the protective gas.

[0015] Preferably, in step 2, the heating power and the cooling power are both 50 kW / min.

[0016] Preferably, the heat treatment process is as follows: keep the sample at 1330℃-1380℃ for 3-9h, take it out and air cool it, and then keep the sample at 900℃ for 30min after cooling it to room temperature.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] (1) The present invention achieves uniform microstructure refinement through solidification process and heat treatment control, and has simple operation, short cycle and high material utilization rate.

[0019] (2) Compared with the method of adding a refiner, the present invention is less likely to introduce foreign inclusions, the alloy solidification path does not change, and there is no problem of performance deterioration caused by foreign inclusions.

[0020] (3) Compared with the method of simply using constant temperature treatment during the solidification process to reduce segregation and improve microstructure uniformity, this method proposes to continue using heat treatment after constant temperature treatment to further eliminate B2 phase segregation, refine grains and remove residual stress inside the alloy, and then perform heat treatment above the ductile-brittle transition temperature, which can improve microstructure uniformity, significantly reduce cracking tendency, and improve alloy performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is the phase diagram of a high niobium TiAl alloy with a 10 atomic percent content.

[0022] Figure 2 DSC curve of Ti-48Al-10Nb alloy.

[0023] Figure 3 These are photos of the as-cast structure of the Ti-48Al-10Nb alloy prepared in Comparative Example 1 without controlled solidification, wherein (a) is an optical microscope photo (OM) and (b) is a scanning electron microscope photo (SEM).

[0024] Figure 4 These are photos of the as-cast structure of the Ti-48Al-10Nb sample that was kept at 1460°C for 1 hour in Example 1, where (a) is an optical microscope photo (OM) and (b) is a scanning electron microscope photo (SEM).

[0025] Figure 5 These are photos of the sample after the Ti-48Al-10Nb was kept at 1460°C for 1 hour, then kept at 1380°C for 3 hours and air-cooled, and finally kept at 900°C for 30 minutes in Example 1. (a) is an optical microscope photo (OM) and (b) is a scanning electron microscope photo (SEM).

[0026] Figure 6 These are photos of the sample after the Ti-48Al-10Nb was kept at 1460°C for 1 hour, then kept at 1380°C for 6 hours and air-cooled, and finally kept at 900°C for 30 minutes in Example 2. (a) is an optical microscope photo (OM) and (b) is a scanning electron microscope photo (SEM).

[0027] Figure 7 It is a flow chart of the method for regulating the high niobium TiAl alloy structure of the present invention. DETAILED DESCRIPTION

[0028] The present invention can reduce the structural inhomogeneity caused by S segregation by removing S segregation and regulating the distribution of B2 phase segregation under high temperature conditions of 1460°C, and can refine the grains during the controlled solidification process, reduce the stress concentration caused by the local enrichment of the B2 phase, and avoid the generation of cracks.

[0029] The present invention can significantly remove the S segregation caused by solid-liquid phase transition in the alloy by short-term heat preservation of Ti-48Al-10Nb in the β single-phase region for 5 minutes. For the Al element segregation in the Ti-48Al-10Nb alloy, due to the large diffusion coefficient of the Al element, short-term heat preservation in the β single-phase region can significantly reduce the S segregation. However, long-term heat preservation in this temperature range will cause the β phase to stabilize, making it difficult to decompose into the α phase in subsequent treatment. When the S segregation is significantly removed, the solidification path of the alloy will change from peritectic solidification between dendrites and β solidification of dendrite stems to single β solidification. This is because during high-temperature treatment, the Nb element diffuses from the dendrite stem to the inter-dendrites, and the Al element diffuses from the inter-dendrites to the dendrite stem, eliminating the metastable α phase caused by peritectic solidification due to Al segregation. p phase, thereby eliminating coarse grains and reducing grain size unevenness.

[0030] When Ti-48Al-10Nb is kept at 1460℃, i.e. in the (α+β) two-phase region, for 60 minutes, the metastable β phase in the alloy will undergo a β→α solid-state phase transformation, thereby reducing the volume fraction of the β phase in the alloy. At room temperature, the volume fraction of the hard and brittle B2 phase will be greatly reduced, and it will be transformed into more lamellar or near-lamellar structures. Moreover, part of the B2 phase transformed from the β phase at room temperature will be distributed inside the lamellar cluster, forming α segregation, which divides the lamellar cluster into multiple grains. Therefore, the structure of the high niobium titanium aluminum alloy is refined and the performance is improved.

[0031] Comparative Example 1

[0032] This comparative example is a method for not performing controlled solidification on a high-niobium TiAl alloy Ti-48Al-10Nb, comprising the following steps:

[0033] Step 1, raw material preparation: According to the Ti-48Al-10Nb ingredients, the raw materials used are: sponge titanium, aluminum block, and niobium aluminum alloy. Put the raw materials into the copper crucible in sequence, with the aluminum block at the bottom, then the sponge titanium, and finally the niobium aluminum alloy wrapped in aluminum foil on the top, and close the furnace door.

[0034] Step 2: Vacuum: Open the mechanical pump, Roots pump and main pump valve in sequence and vacuum to 1×10 -1 Pa and then open the high vacuum diffusion pump to evacuate to 9×10 -3 Pa, and argon is filled to about 520mbar as a protective gas.

[0035] Step 3, smelting: turn on the medium frequency power supply and increase the power to about 350kW at 50kW / min. After all the raw materials are melted, keep warm at this power for 7 minutes, then reduce the power to 0 at 50kW / min. After the ingot cools down, take it out, turn it over, and place it in a water-cooled copper crucible.

[0036] Step 4, remelting: Repeat the melting process of step 3 twice.

[0037] Step 5, sample cutting: Take a rod-shaped sample with a diameter of 10 mm from the middle of the ingot obtained in step 4, and cut it into a sample with a thickness of 5 mm using wire-cut electric discharge. This sample is used to observe the microsegregation and grain size of the cast state, such as Figure 3 The light microscope photo (a) and scanning electron microscope photo (b) are shown. Figure 3 From (a), we can see that the grain size is relatively coarse, about 400-500μm; Figure 3 It can be seen from (b) that the S segregation is more serious and the volume fraction of B2 phase segregation is also higher.

[0038] Example 1

[0039] Combine Figure 7 The present embodiment is a method for controlling the microstructure of a high-niobium TiAl alloy Ti-48Al-10Nb, comprising the following steps:

[0040] Step 1, raw material preparation: According to the Ti-48Al-10Nb ingredients, the raw materials used are: sponge titanium, aluminum block, and niobium aluminum alloy. Put the raw materials into the copper crucible in sequence, with the aluminum block at the bottom, then the sponge titanium, and finally the niobium aluminum alloy wrapped in aluminum foil on the top, and close the furnace door.

[0041] Step 2: Vacuum: Open the mechanical pump, Roots pump and main pump valve in sequence, and vacuum to 1×10 -1 Pa and then open the high vacuum diffusion pump to evacuate to 9×10 -3 Pa, and argon is filled to about 520mbar as a protective gas.

[0042] Step 3, smelting: Turn on the medium frequency power supply and increase the power to a maximum of about 350kW at 50kW / min for 7 minutes. After all the raw materials are melted, keep the heat at the highest power for 7 minutes, then reduce the power to 0 at 50kW / min. After the ingot cools down, take it out, turn it over, and place it in a water-cooled copper crucible;

[0043] Step 4, solidification: Repeat the smelting process of step 3 twice. During the cooling stage of the third smelting process, reduce the power by 20 kW / min, and use an infrared thermometer to measure the real-time temperature of the melt every minute. When the alloy temperature drops to 1510-1520°C and enters the β single-phase region, keep it warm for 5 minutes, then continue to cool to 1460°C: (α+β) two-phase region, keep it warm for 60 minutes. After the insulation is completed, reduce the power to 0 at 20 kW / min, wait for the ingot to cool, and then take it out.

[0044] Step 5, sample cutting: Take a rod-shaped sample with a diameter of 10 mm from the middle of the ingot obtained in step 4, and cut it into a sample with a thickness of 5 mm using wire-cut electric discharge. This sample is used to observe the microsegregation and grain size of the cast state, such as Figure 4 The light microscope photo (a) and scanning electron microscope photo (b) shown in FIG1 are compared with the sample without controlled solidification treatment in comparative example 1 ( Figure 3 ), it can be clearly seen that the S segregation is basically completely eliminated, the B2 phase segregation is significantly reduced, and the grain size is also reduced to a certain extent, from about 400-500μm to about 200μm.

[0045] Step 6, heat treatment: according to Figure 1 Phase diagram of high Nb TiAl alloy with 10 at% atomic content (cited from the literature "Microsegregation in high Nb containing TiAl alloy ingots beyond laboratory scale" and the solidification path of the alloy with nominal composition Ti-48Al-10Nb) and Figure 2 The DSC curve was used to formulate the heat treatment process. The ingot was heat treated at 1380℃ for 3 hours. After the heat treatment, the ingot was taken out and air-cooled. After cooling to room temperature, the ingot was kept at 900℃ for 30 minutes to remove the internal stress. The ingot was taken out and air-cooled. The obtained ingot was sampled and observed. Figure 5 From the optical microscope photograph (a) and the scanning electron microscope photograph (b), it can be seen that after this heat treatment, the alloy grain size is further reduced, the S segregation is completely eliminated, and the B2 phase segregation is basically completely eliminated, but a small part still remains.

[0046] Example 2

[0047] The other steps are the same as those in Example 1, except that in step 6, the temperature was kept for 3 hours instead of 6 hours. Figure 6 From the optical electron microscope photograph (a) and the scanning electron microscope photograph (b), it can be seen that the grain size is slightly smaller than that of Example 1, and the B2 phase segregation is completely eliminated.

Claims

1. A method for controlling the microstructure of a high-niobium TiAl alloy, wherein the high-niobium TiAl alloy is Ti-48Al-10Nb, characterized in that: The steps include: Step 1: Place aluminum block, titanium sponge and niobium aluminum alloy into a water-cooled copper crucible in sequence according to alloy composition; Step 2: Evacuate the ingot and, under protective gas, raise the heating power to the highest power at a certain temperature. After all the raw materials are melted, keep the temperature for 5 to 7 minutes. Then, reduce the power to 0 at a certain temperature-lowering power. After the ingot cools down, take it out, turn it over, and place it in a water-cooled copper crucible. Step 3, repeat the smelting process of step 2 twice. In the cooling stage of the third smelting process, the temperature of the alloy is reduced to 1500°C-1560°C at a cooling power of 20 kW / min, and the temperature is kept at this temperature for 5-30 minutes. Then, the temperature is further reduced to 1420°C-1490°C, and the temperature reaches the (α+β) two-phase region, and the temperature is kept at this temperature for 10-60 minutes. After the temperature is kept at this temperature, the power is reduced to 0 at a power of 20 kW / min, and the ingot is taken out after it cools down. Step 4: Cut the ingot into required size and perform heat treatment.

2. The method according to claim 1, wherein Vacuum to 9×10 -3 Pa.

3. The method according to claim 1, wherein The protective gas used is argon.

4. The method according to claim 1, wherein In step 2, the heating power and the cooling power are both 50 kW / min.

5. The method according to claim 1, wherein The heat treatment process is as follows: keep the sample at 1330℃-1380℃ for 3-9h, take it out and air cool it, and then keep the sample at 900℃ for 30min after cooling to room temperature.

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