Heat treatment method for improving obdurability matching of Ti55531 alloy

Abstract

The invention discloses a heat treatment method for improving matching of strength, plasticity and toughness of a Ti55531 alloy, which is specifically characterized in that the whole heat treatment is carried out under the protection of argon, so that the titanium alloy is effectively prevented from absorbing hydrogen and even nitriding at a high temperature, serious reduction of local performance is avoided, and the heat treatment method is suitable for heat treatment of titanium alloy bars, forging stocks and complex workpieces; and a two-section annealing process of first rapid cooling and then slow cooling is adopted, a fine lamellar layer is formed through rapid cooling to improve the strength, then slow cooling is performed to weave a part of thicker lamellar structures from a fine needle-shaped secondary alpha phase to improve the toughness of the material, so that strength and toughness matching is realized, a lamellar alpha-phase structure with an ideal thickness is formed more accurately, and the strength of the material is improved. And the challenging matching requirements of strength, plasticity and toughness can be met.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for titanium alloy materials, specifically to a heat treatment method for improving the strength and toughness matching of Ti55531 alloy. Background Technology

[0002] With the continuous development of the aviation industry, higher requirements have been placed on the strength, ductility, and toughness matching of titanium alloys. High-strength and high-toughness titanium alloys play an irreplaceable role in the design of high-performance aviation equipment and are widely used in structural components and connectors such as fuselage landing gear, wings, and nose.

[0003] Currently, the high-strength and high-toughness titanium alloys used in the aerospace industry are mainly β-series and near-β-series, with corresponding grades including TC18, Ti-55531, Ti-5553, TB6, TB17, and TC21. These titanium alloys are generally extremely sensitive to the parameters of thermomechanical processing (such as forging and rolling) and subsequent heat treatment (such as solution temperature, cooling rate, aging temperature, and time). There is an urgent need to develop new, stable, and efficient heat treatment methods to achieve precise control of the microstructure and properties of high-strength and high-toughness titanium alloys.

[0004] Ti55531 alloy was jointly developed in the 1990s by VSMPO and Airbus based on BT22, with a nominal composition of (Ti-5Al-5Mo-5V-3Cr-1Zr). Compared with Ti-1023 and BT22 (TC18) alloys, this alloy has several advantages, including reduced compositional segregation, higher strength, better hardenability, and a wider machining window. Currently, this alloy has been successfully used in the manufacture of key load-bearing components such as aircraft landing gear, engine nacelle pylon joints, and wing-pylon connection devices. However, this alloy also has problems such as a narrow machining window, poor forgeability, and large differences in strength, ductility, and longitudinal and edge-center properties. To address these issues, many researchers have innovated heat treatment methods to control the uniformity of the microstructure, aiming to achieve a better balance of strength, ductility, and toughness.

[0005] Patent CN119082643 B discloses a "vacuum heat treatment process for high-strength and high-toughness titanium alloy workpieces." This method achieves performance control of Ti55531 workpieces by adjusting process parameters such as cooling rate and heating temperature in a vacuum environment to meet the personalized requirements of high toughness for titanium alloys used in aerospace structural components. However, its high-vacuum heat treatment has two major bottlenecks: First, for large-scale equipment, achieving and maintaining this vacuum level is extremely costly (this invention requires a vacuum level < 6.7 × 10⁻⁶). -2Firstly, the high technical difficulty limits the size of workpieces that can be processed. Secondly, heat transfer under extremely low vacuum relies on radiation, resulting in low efficiency and poor controllability, which is not conducive to precise process control. Patent CN 119956144 A discloses an "integral β-annealed aged Ti55531 titanium alloy bar and its preparation method," which optimizes the traditional multi-stage forging process into a forging + rolling process, combined with an integral β-annealing aging heat treatment. This method is mainly a comprehensive solution for Ti55531 titanium alloy bar melting + forging + heat treatment. However, it is not very applicable to complex workpieces. The β-annealing heating temperature is relatively high, and uneven heating can easily cause warping in complex-shaped Ti55531 workpieces.

[0006] To ensure that the mechanical properties of Ti55531 alloy are superior to those of TC18, many novel heat treatment methods have been developed, including vacuum heat treatment, zone laser melting heat treatment, multi-stage solution treatment combined with aging, and forging and rolling combined with controlled cooling rate β-annealing. However, Ti55531 alloy currently exhibits relatively low plasticity and toughness under traditional annealing conditions, while achieving a good balance between strength, plasticity, and toughness under β-annealing conditions is challenging, especially considering the high KIC (≥70 MPa·m) requirements for aerospace titanium alloys. 1 / 2 The requirements severely restrict the application of Ti55531 alloy in more complex scenarios. Summary of the Invention

[0007] To address the shortcomings of the existing technology, the purpose of this invention is to provide a heat treatment method that improves the strength-ductility-toughness matching of Ti55531 alloy. This method enables Ti55531 to form a lamellar α-phase structure of ideal thickness more precisely, thereby achieving a good balance between strength, ductility, and toughness.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A heat treatment method for improving the strength-ductility-toughness balance of Ti55531 alloy, characterized by comprising the following steps: Step 1: Preheat the heat treatment furnace to the temperature of the alloy T. β - (10-50℃), then the Ti55531 titanium alloy material is fed into the warm charging furnace, the T β This is the β transition temperature; Step 2: Evacuate the heat treatment furnace until the pressure inside the furnace is ≤10Pa. Then, introduce argon gas into the heat treatment furnace until the pressure inside the furnace reaches 10000-50000Pa. After the pressure inside the furnace stabilizes after the heat treatment furnace is removed, stop the argon filling. Use this constant pressure for the first heat treatment, and the heat treatment time is 30min-90min. Step 3: Raise the furnace temperature to T β+ (5-50℃), Ti55531 titanium alloy material is subjected to a second heat preservation under argon atmosphere protection for 60min-180min; Step 4: Control the heat treatment furnace to rapidly cool to T at a first constant cooling rate under argon atmosphere protection. β - (50-200℃), where the first constant cooling rate is 3℃ / min-4℃ / min, and then under argon atmosphere protection, it is slowly cooled to 450℃-600℃ at the second constant cooling rate, where the second constant cooling rate is 1℃ / min-2℃ / min. Then, the Ti55531 titanium alloy material is held at 450℃-600℃ under argon atmosphere protection for a third time for 300min-600min, and then cooled to below 300℃ in the furnace under argon atmosphere protection. Step 5: Depressurize, open the heat treatment furnace, and externally air cool the Ti55531 titanium alloy material obtained from the heat treatment.

[0009] Preferably, in step 1 above, the preheating temperature of the heat treatment furnace is T. β - (20-40℃).

[0010] Preferably, in step 2 above, the furnace pressure is evacuated to ≤5Pa, argon gas is introduced to the furnace pressure to 10000-30000Pa, and the holding time is 20min-40min.

[0011] Preferably, in step 3 above, the second heat preservation temperature is T. β + (20-40℃), heat preservation time is 60min-120min.

[0012] Preferably, in step 4 above, the first constant cooling rate is 3℃ / min-3.5℃ / min, and the temperature is rapidly cooled to T at the first constant cooling rate. β - (50-150℃).

[0013] Preferably, in step 4 above, the second constant cooling rate is 1.5℃ / min-2℃ / min, and the temperature is rapidly cooled to 520℃-580℃ at the second constant cooling rate.

[0014] Preferably, in step 4 above, the third heat preservation temperature is 520℃-580℃ and the heat preservation time is 360min-480min.

[0015] Compared with the prior art, the present invention has the following innovative features and beneficial effects: 1. The entire heat treatment process of this invention is carried out under argon protection, which not only effectively prevents titanium alloys from absorbing hydrogen or even nitriding at high temperatures, avoiding serious local performance degradation, but also makes it suitable for heat treatment of titanium alloy bars, forgings and complex workpieces. Furthermore, the argon cooling of this invention stabilizes the cooling rate to a certain extent, promotes uniform heating of titanium alloy heat-treated materials, and effectively reduces warping of heat-treated parts. 2. This invention designs a two-stage annealing process of "rapid cooling first and slow cooling later". First, rapid cooling is used to form fine lamellar layers to improve strength. Then, slow cooling is used to weave a part of the coarser lamellar structure from the fine needle-like secondary α phase to improve the toughness of the material. This achieves a match between strength and toughness and more accurately forms a lamellar α phase structure of ideal thickness to meet the more challenging requirements of strength-ductility-toughness matching. 3. This invention employs a three-stage heat treatment process. The first heat treatment allows the heat-treated part to be fully preheated to the set temperature in an argon atmosphere, ensuring thorough quenching of cross-sections of varying thicknesses and avoiding uneven phase transformation caused by temperature differences between the material's edges and center, and between its transverse and longitudinal directions, during direct heating to the single-phase region. The second heat treatment allows all α-phase above the phase transformation point to dissolve back, transforming into a β-matrix. This process aims to transform the equiaxed structure into a lamellar structure. The third heat treatment involves isothermally holding the supercooled metastable β-phase or α-phase for an extended period. The purpose is to fully precipitate fine secondary α-phases, allowing these fine α-phase clusters to disperse and strengthen the material, thereby increasing its strength. In this design, the first heat treatment is fundamental to the subsequent phase transformation and strengthening processes. Its core function is to provide a uniform and stable temperature field and microstructure preparation conditions for the heat-treated part, ensuring the effective implementation of the subsequent two heat treatment processes and reducing microstructure anisotropy. Attached Figure Description

[0016] Figure 1 The heat treatment curve provided for this invention; Figure 2 These are comparison images of typical microstructures of Example 1 and Comparative Examples 1 to 6 of the present invention. Detailed Implementation

[0017] The present invention will now be further described with reference to specific embodiments. Parts not detailed below are based on existing technology in the field. Any equivalent or similar substitutions that do not depart from the concept of the present invention should fall within the protection scope of the present invention.

[0018] Example 1 Step 1: Preheat the heat treatment furnace to the temperature of the alloy T. β At -35℃, place Ti55531 titanium alloy bars, forgings, or complex workpieces into a warm-loading furnace. Step 2: Evacuate the heat treatment furnace until the pressure inside the furnace is ≤10Pa, then introduce argon gas into the heat treatment furnace until the pressure inside the furnace reaches 30000 Pa. After the pressure inside the heat treatment furnace stabilizes, stop the argon introduction and hold the furnace at this temperature and pressure for 30 minutes for the first time. Step 3: Raise the furnace temperature to T β +20℃, heating time: 20min, then hold at this temperature for a second time for 120min; Step 4: Control the cooling rate of the heat treatment furnace to 3.0℃ / min, and cool to T. β -150℃, then control the cooling rate of the heat treatment furnace to 1.5℃ / min to cool down to 520℃, and then perform a third heat preservation for 480min. The heat-treated material is cooled down to below 300℃ in the furnace under the protection of argon atmosphere. Step 5: Depressurize, open the heat treatment furnace, and place the heat-treated material in an external air cooler at room temperature.

[0019] Note: This embodiment represents an ideal heat treatment regime with a balance of strength, ductility, and toughness, as explored in previous practical studies. The mechanical properties of the product are shown in Table 1. Under this heat treatment regime, the requirements for high-strength and high-toughness titanium alloys used in aerospace structural components are met with sufficient margin (tensile strength ≥ 1150 MPa, fracture toughness ≥ 75 MPa·m). 0.5 ).

[0020] Table 1: Mechanical properties of Embodiment 1 of the present invention

[0021] Comparative Example 1 Step 1: Preheat the heat treatment furnace to the temperature of the alloy T. β At -35℃, place Ti55531 titanium alloy bars, forgings, or complex workpieces into a warm-loading furnace. Step 2: Evacuate the heat treatment furnace to a pressure ≤10Pa, and hold at this temperature and pressure for 30 minutes for the first time. Step 3: Raise the furnace temperature to T β +20℃, heating time: 20min, then hold at this temperature for a second time for 120min; Step 4: Control the cooling rate of the heat treatment furnace to 3.0℃ / min, and cool to T. β -150℃, then control the cooling rate of the heat treatment furnace to 1.5℃ / min to cool down to 520℃, and then perform a third heat preservation for 480min. The heat-treated material is cooled down to below 300℃ with the furnace. Step 5: Depressurize, open the heat treatment furnace, and place the heat-treated material in an external air cooler at room temperature.

[0022] The mechanical properties of the comparative example are shown in Table 2.

[0023] Comparative Example 2 Step 1: Preheat the heat treatment furnace to the temperature of the alloy T. β At -35℃, place Ti55531 titanium alloy bars, forgings, or complex workpieces into a warm-loading furnace. Step 2: Hold the heat-treated part at this temperature and atmospheric pressure for the first time for 30 minutes; Step 3: Raise the furnace temperature to T β +20℃, heating time: 20min, then hold at this temperature for a second time for 120min; Step 4: Control the cooling rate of the heat treatment furnace to 3.0℃ / min, and cool to T. β -150℃, then control the cooling rate of the heat treatment furnace to 1.5℃ / min to cool down to 520℃, and then perform a third heat preservation for 480min. The heat-treated material is cooled down to below 300℃ with the furnace. Step 5: Open the heat treatment furnace and place the heat-treated material in an external air cooler at room temperature.

[0024] The mechanical properties of the comparative example are shown in Table 2.

[0025] Comparative Example 3 Step 1: Preheat the heat treatment furnace to the temperature of the alloy T. β At -35℃, place Ti55531 titanium alloy bars, forgings, or complex workpieces into a warm-loading furnace. Step 2: Evacuate the heat treatment furnace until the pressure inside the furnace is ≤10Pa, then introduce argon gas into the heat treatment furnace until the pressure inside the furnace reaches 30000 Pa. Stop purging argon gas after the pressure inside the heat treatment furnace stabilizes. Step 3: Raise the furnace temperature to T β +20℃, heating time: 20min, then hold at this temperature for the first time for 120min; Step 4: Control the cooling rate of the heat treatment furnace to 3.0℃ / min, and cool to T. β -150℃, then control the cooling rate of the heat treatment furnace to 1.5℃ / min to cool down to 520℃, and then perform a second heat preservation for 480min. The heat-treated material is cooled down to below 300℃ in the furnace under the protection of argon atmosphere. Step 5: Depressurize, open the heat treatment furnace, and place the heat-treated material in an external air cooler at room temperature.

[0026] The mechanical properties of the comparative example are shown in Table 2.

[0027] Comparative Example 4 Step 1: Preheat the heat treatment furnace to the temperature of the alloy T.β At -35℃, place Ti55531 titanium alloy bars, forgings, or complex workpieces into a warm-loading furnace. Step 2: Evacuate the heat treatment furnace until the pressure inside the furnace is ≤10Pa, then introduce argon gas into the heat treatment furnace until the pressure inside the furnace reaches 30000 Pa. After the pressure inside the heat treatment furnace stabilizes, stop the argon introduction and hold the furnace at this temperature and pressure for 30 minutes for the first time. Step 3: Raise the furnace temperature to T β +20℃, heating time: 20min, then hold at this temperature for a second time for 120min; Step 4: Control the cooling rate of the heat treatment furnace to 1.5℃ / min, and cool to T. β -150℃, then control the cooling rate of the heat treatment furnace to 3.0℃ / min to cool down to 520℃, and then perform a third heat preservation for 480min. The heat-treated material is cooled down to below 300℃ in the furnace under the protection of argon atmosphere. Step 5: Depressurize, open the heat treatment furnace, and place the heat-treated material in an external air cooler at room temperature.

[0028] The mechanical properties of the comparative example are shown in Table 2.

[0029] Comparative Example 5 Step 1: Preheat the heat treatment furnace to the temperature of the alloy T. β At -35℃, place Ti55531 titanium alloy bars, forgings, or complex workpieces into a warm-loading furnace. Step 2: Evacuate the heat treatment furnace until the pressure inside the furnace is ≤10Pa, then introduce argon gas into the heat treatment furnace until the pressure inside the furnace reaches 30000 Pa. After the pressure inside the heat treatment furnace stabilizes, stop the argon introduction and hold the furnace at this temperature and pressure for 30 minutes for the first time. Step 3: Raise the furnace temperature to T β +20℃, heating time: 20min, then hold at this temperature for a second time for 120min; Step 4: Control the cooling rate of the heat treatment furnace to 2.0℃ / min to cool down to 520℃, and then perform a third heat preservation for 480min. The heat-treated material is cooled down to below 300℃ in the furnace under the protection of argon atmosphere. Step 5: Depressurize, open the heat treatment furnace, and place the heat-treated material in an external air cooler at room temperature.

[0030] The mechanical properties of the comparative example are shown in Table 2.

[0031] Comparative Example 6 Step 1: Place the heat-treated part in the heat treatment furnace, heat the furnace to 795℃ and hold for 2 hours, then remove it from the furnace and air cool. Step 2: Place the heat-treated part back into the heat treatment environment and hold it at 600℃ for 8 hours, then air cool it.

[0032] The mechanical properties of the comparative example are shown in Table 2.

[0033] Table 2: Summary of Mechanical Properties of Comparative Examples 1-6 of the Invention .

[0034] The following conclusions can be drawn from the performance data analysis in Tables 1 and 2 above: Example 1 and Comparative Examples 1 and 2 were set up with different furnace atmospheres (argon protection, vacuum and atmospheric pressure). The results showed that the material mechanical properties were least anisotropic under argon protection, followed by atmospheric pressure. This is because the surface of the heat-treated part does not form an oxygen-rich "α-embrittlement layer" under argon protection, which changes the heat treatment and heat transfer of the workpiece to a certain extent. In contrast, the vacuum environment mainly relies on radiation heat transfer, resulting in greater differences in the transverse and longitudinal directions. 2. Example 1 and Comparative Example 3 were compared with three insulation cycles and two insulation cycles. The stability and differences in mechanical properties showed that the uniformity of mechanical properties was significantly improved after three insulation cycles, and the fluctuation range of tensile properties in the transverse, longitudinal, and single-direction tests was small.

[0035] 3. Comparison of Example 1 with Comparative Examples 4 and 5: “rapid cooling + slow cooling”, “slow cooling + rapid cooling”, and “constant cooling rate” were set up for comparison. Example 1 used a “rapid cooling + slow cooling” cooling regime. From the perspective of phase precipitation, rapid cooling first accelerated the precipitation of fine needle-like secondary α phase to improve the material strength. Then, slow cooling promoted the epitaxial growth of some needle-like secondary α phase structures, the connection and extension of adjacent lamellars, and the merging and coarsening of unidirectional lamellars, ultimately forming the “triangular multi-scale structure” of Example 1 in Figure 1. Comparative Example 4 used a “slow cooling + rapid cooling” cooling regime. In the slow cooling stage, the thicker secondary α phase had already been fully precipitated. Then, through slow cooling and subsequent aging stages, the precipitation of metastable secondary phases was limited (as shown in Comparative Example 4). Therefore, the overall strength was lower, but the fracture toughness was relatively better. Comparative Example 5 used a “constant cooling rate” cooling. The overall strength was comparable to that of Example 1, but the degree of consistency of secondary phase orientation in the microstructure was higher. There was insufficient stacking fault between the coarse and fine lamellars. Therefore, the fracture toughness and plasticity were not as good as those of Example 1.

[0036] 4. Example 1 and Comparative Example 6 compare the mechanical properties of the present invention with those of conventional solution-treated and aged annealed materials. The comparative data show that the strength of the solution-treated and aged Ti55531 alloy is about 50 MPa higher than that of the present example. The microstructure is a dual state, and the overall fracture toughness is significantly different. The strength, plasticity and toughness are poorly matched, making it difficult to meet the requirements of aerospace materials for plasticity and toughness. The overall performance is significantly different from that of the example.

[0037] This invention improves the uniformity of the alloy's microstructure and enhances the anisotropy of its mechanical properties by using "argon atmosphere protection" and "triple heat preservation system"; furthermore, it uses a time-sequential cooling system of "rapid cooling + slow cooling" to directionally regulate the microstructure and construct a "triangular multi-scale structure" to achieve simultaneous improvement in strength, plasticity and toughness.

Claims

1. A heat treatment method for improving the strength-ductility balance of Ti55531 alloy, characterized by, It comprises the following steps: Step 1, preheat the furnace temperature of the heat treatment furnace to the alloy T β - (10-50°C) and then load the Ti55531 titanium alloy material into the furnace at temperature, said T β is the beta transus temperature; Step 2, vacuumize the heat treatment furnace to a furnace pressure of ≤10 Pa, then introduce argon into the heat treatment furnace to a furnace pressure of 10000-50000 Pa, stop the argon introduction after the furnace pressure stabilizes, and perform the first heat preservation at the constant pressure, with a heat preservation time of 30 min-90 min; Step 3, the temperature of the heat treatment furnace is raised to T β + (5-50℃), the Ti55531 titanium alloy material is subjected to the second heat preservation under the protection of the argon atmosphere, and the heat preservation time is 60 min-180 min. Step 4, control the heat treatment furnace to cool down to T β - (50-200℃), wherein the first constant cooling rate is 3℃ / min-4℃ / min, then slowly cool to 450℃-600℃ under the protection of argon atmosphere at a second constant cooling rate of 1℃ / min-2℃ / min, then the Ti55531 titanium alloy material is subjected to a third holding at 450℃-600℃ under the protection of argon atmosphere, the holding time is 300min-600min, and then the furnace is cooled to below 300℃ under the protection of argon atmosphere; Step 5, release the pressure, open the heat treatment furnace, and air cool the Ti55531 titanium alloy material obtained by the heat treatment outside.

2. The heat treatment method for improving the strength-ductility matching of Ti55531 alloy according to claim 1, characterized in that, In the above step 1, the preheating temperature of the heat treatment furnace is T β - (20-40°C).

3. The heat treatment method for improving the strength-ductility matching of Ti55531 alloy according to claim 1 or 2, characterized in that, In the above step 2, the vacuumization is to a furnace pressure of ≤5 Pa, the argon introduction is to a furnace pressure of 10000-30000 Pa, and the heat preservation time is 20 min-40 min.

4. The heat treatment method for improving the strength-ductility matching of Ti55531 alloy according to claim 3, characterized in that, In the above step 3, the second holding temperature is T β + (20-40℃) for 60-120 min.

5. The heat treatment method for improving the strength-ductility matching of Ti55531 alloy according to claim 4, characterized in that, In the above step 4, the first constant cooling rate is 3°C / min to 3.5°C / min, and the temperature is rapidly cooled to T β (50 to 150°C).

6. The heat treatment method for improving the strength-ductility matching of Ti55531 alloy according to claim 5, characterized in that, In the above step 4, the second constant cooling rate is 1.5℃ / min-2℃ / min, and the fast cooling is performed to 520℃-580℃ at the second constant cooling rate.

7. The heat treatment method for improving the strength-ductility matching of Ti55531 alloy according to claim 6, characterized in that, In the above step 4, the third heat preservation temperature is 520℃-580℃, and the heat preservation time is 360 min-480 min.

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