A heat treatment method for improving the impact toughness of near-alpha titanium alloy welds

Abstract

The application discloses a heat treatment method for improving the impact toughness of a near-alpha titanium alloy weld, and belongs to the technical field of titanium alloys. The titanium alloy weld is kept at a first temperature until the martensite alpha' phase in the weld structure is fully transformed into alpha+beta phase; and the titanium alloy weld is kept at a second temperature to release residual stress. The application achieves the purpose of regulating the mechanical properties of the weld by regulating the microstructure of the weld. The application transforms the martensite alpha' into alpha+beta lath, which has the advantage of reducing the residual stress, lattice distortion, dislocation density and local stress concentration of the weld. Meanwhile, the beta phase has better plasticity and can be used as a buffer layer to absorb part of the energy and coordinate the deformation between the alpha phase laths. During the crack propagation process, the alpha laths and beta lamellas are alternately distributed, the crack needs to deflect the direction for many times, and the energy absorption is higher.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy technology, and specifically relates to a heat treatment method for improving the impact toughness of near-α titanium alloy welds. Background Technology

[0002] Titanium alloys, as low-density, high-specific-strength, and highly corrosion-resistant materials, are widely used in marine equipment, aerospace, and oil pipelines. Ti75, with its nominal composition of Ti-3Al-2Mo-2Zr, is a marine titanium alloy specifically developed for deep-sea equipment. Electron beam welding, with its advantages of high efficiency, low heat input, and good protection, is a commonly used welding method for titanium alloys. The high-temperature environment during welding causes coarsening of the β-grains in titanium alloys, while rapid cooling leads to the formation of a large amount of martensite α' in the weld, severely deteriorating the weld's toughness. This results in titanium alloy electron beam welded joints having the same strength as the base metal in the as-welded state, but with an impact toughness only about 30% of the base metal. Therefore, post-weld heat treatment to control and optimize the morphology of the weld microstructure and the stress distribution in the weld, thereby improving the mechanical properties of the welded joint, is a relatively ideal method.

[0003] Harbin Welding Research Institute [Harbin Welding Research Institute Co., Ltd. of China Academy of Machinery Science and Technology. A heat treatment method to improve the plasticity and toughness of titanium alloy welded joints: 202211633826.2 [P]. 2023-08-08.] proposed a heat treatment method to improve the plasticity and toughness of TC4 titanium alloy welded joints. The method involves heating the laser-welded joint in a vacuum environment at 800~1200℃ for a period of time, and then cooling it to room temperature, which improves the elongation after fracture and impact toughness. Zhu He et al. [Zhu He, Liu Yanmei, Zhao Dong, et al. Effect of post-weld heat treatment on bending properties of TA15 titanium alloy medium-thick plate welded joints [J]. Heat Treatment of Metals, 2024, 49(02):179-182. DOI:10.13251 / j.issn.0254-6051.2024.02.027.] used a furnace cooling annealing process of 850℃×120min to heat treat the TA15 tungsten inert gas welded joints. The columnar crystals were refined, the β phase was more uniformly distributed, and the average bending properties were improved. Ma Quan et al. [Ma Quan, Xin Shewei, Song Kai, et al. Effect of heat treatment on microstructures and mechanical properties of vacuum laser welding Ti-1300 high-strength titanium alloy [J]. Rare Metals Materials and Engineering, 2019, 48(08):2723-2728.] subjected the welded joints to three heat treatments: 700℃ / 2h, 540℃ / 4h, and 760℃ / 1h AC + 540℃ / 4h AC. Their study found that heat treatment altered the distribution trend of precipitated α phase at grain boundaries, thus affecting weld performance. When more α phase precipitated at grain boundaries, the strengthening effect was significant, and the weld strength was greater than that of the matrix. Wang et al. [Cong Wang, Nuo Xu, Guoyu Zhang, Guojian Xu, Fei Xing, Effect of heat treatment on microstructures and properties of vacuum laser welding Ti–6Al–4V titanium alloy [J], Journal of Materials Research and Technology, 2024, 30: [6309-6320] The Ti-6Al-4V vacuum laser welded head was subjected to two post-weld heat treatments: 850℃ / 2h and 980℃ / 10min+720℃ / 2h. The weld microstructure consisted of columnar crystals and grain boundary α´. After the 850℃ / 2h heat treatment, the microstructure transformed into acicular α and α+β basketweave microstructures. The tensile strength decreased to 988MPa (97% of the base material) and the impact toughness increased to 17J (130% of the base material). After the 980℃ / 10min+720℃ / 2h heat treatment, the tensile strength decreased to 586MPa (58% of the base material) and the impact toughness decreased to 1.4J (10% of the base material). Summary of the Invention

[0004] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the scope of the invention; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0005] In view of the problems existing in the above and / or prior art, the present invention is proposed.

[0006] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a heat treatment method for improving the impact toughness of near-α titanium alloy welds.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution, including maintaining the titanium alloy weld at a first temperature until the martensite α´ phase in the weld microstructure is fully transformed into the α+β phase; and maintaining the titanium alloy weld at a second temperature to release residual stress.

[0008] The first temperature is 30-50°C below the phase transformation temperature of the titanium alloy; the second temperature is 660°C.

[0009] Preferably, the first temperature is 30°C below the phase transformation temperature of the titanium alloy.

[0010] As a preferred embodiment of the heat treatment method for improving the impact toughness of near-α titanium alloy welds according to the present invention, the titanium alloy weld is heated to a first temperature at a heating rate of 10±1℃ / min.

[0011] As a preferred embodiment of the heat treatment method for improving the impact toughness of near-α titanium alloy welds according to the present invention, the titanium alloy weld is cooled to a second temperature at a cooling rate of 10±1℃ / min at a first temperature.

[0012] As a preferred embodiment of the heat treatment method for improving the impact toughness of near-α titanium alloy welds according to the present invention, the method further includes: cooling the titanium alloy weld to 300℃±50℃ at a cooling rate of 10±1℃ / min at a second temperature, followed by furnace cooling.

[0013] As a preferred embodiment of the heat treatment method for improving the impact toughness of near-α titanium alloy welds according to the present invention, the near-α titanium alloy includes TA24 (Ti-3Al-2Mo-2Zr), TA18 (Ti-3Al-2.5V), TA11 (Ti-8Al-1Mo-1V), TA15 (Ti-6.5Al-2Zr-1Mo-1V), TA19 (Ti-6Al-2Sn-4Zr-2Mo-0.08Si), TA29 (Ti-5.8Al-4Sn-4Zr-0.7Nb-1.5Ta-0.4Si-0.06C), and TA33 (Ti-5.8Al-4Sn-3.5Zr-0.7Mo-0.5Nb-1.1Ta-0.4Si-0.06C).

[0014] As a preferred embodiment of the heat treatment method for improving the impact toughness of near-α titanium alloy welds according to the present invention, the welding method of the weld includes electron beam welding.

[0015] As a preferred embodiment of the heat treatment method for improving the impact toughness of near-α titanium alloy welds according to the present invention, the titanium alloy weld is held at the first temperature for 1.5 to 2 hours.

[0016] As a preferred embodiment of the heat treatment method for improving the impact toughness of near-α titanium alloy welds according to the present invention, the titanium alloy weld is held at the second temperature for 1.5 to 2 hours.

[0017] Preferably, the titanium alloy weld is maintained at the first temperature and the second temperature for 2 hours.

[0018] As a preferred embodiment of the heat treatment method for improving the impact toughness of near-α titanium alloy welds according to the present invention, the heat treatment method reduces the martensite α´ content in the titanium alloy weld, transforms acicular martensite α´ into an α+β dual-phase structure, and improves the impact toughness of the titanium alloy weld.

[0019] As a preferred embodiment of the heat treatment method for improving the impact toughness of near-α titanium alloy welds according to the present invention, wherein: the impact toughness of the titanium alloy weld is improved by 163% to 177% in the upper part and by 213% to 212% in the lower part.

[0020] Beneficial effects of this invention: (1) The heat treatment method provided by the present invention reduces the α´ content of martensite in titanium alloy and greatly improves the impact toughness of titanium alloy weld.

[0021] (2) This invention achieves the goal of controlling the mechanical properties of the weld by regulating the microstructure of the weld. Through a specific heat treatment process, all martensite α´ is transformed into α+β laths. This reduces the residual stress, lattice distortion, dislocation density, and local stress concentration in the weld. At the same time, the β phase has better plasticity and can act as a buffer layer to absorb some of the energy, while coordinating the deformation between the α phase laths. During crack propagation, the α laths and β lamellae are distributed alternately, and the crack needs to deflect multiple times, resulting in higher energy absorption.

[0022] (3) This invention greatly improves impact toughness by sacrificing less than 13% of tensile strength, achieving an impact toughness improvement of more than 200%, reaching 78.5J (92.4% of the base material), far exceeding the engineering requirement of 47J. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein: Figure 1 This is a diagram showing the steps of the heat treatment process.

[0024] Figure 2 This is a comparison diagram of the microstructure in the welded state and the heat-treated state.

[0025] Figure 3 These are photographs of the microstructure of welds in the as-welded and heat-treated states.

[0026] Figure 4 XRD patterns for the welded and heat-treated states.

[0027] Figure 5 IPF diagrams for welds in the as-welded and heat-treated states.

[0028] Figure 6 KAM diagrams of welds in the as-welded and heat-treated states. Detailed Implementation

[0029] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0030] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0031] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0032] Unless otherwise specified, all raw materials used in this invention are commercially available.

[0033] Example 1

[0034] This embodiment provides a heat treatment method for improving the impact toughness of titanium alloy welds, the steps of which are as follows: Figure 1 And Table 1, specifically: The phase transformation temperature of Ti75 titanium alloy (grade TA24) was measured to be 965℃. Electron beam welding (welding parameters: high voltage 85kV, electron beam current 70mA, welding speed 200mm / min, working distance 30mm, focusing current 390mA) was used to produce welds that were held at 35℃ (i.e. 930℃) and 660℃ below the phase transformation temperature for 2 hours respectively. The temperature was increased and decreased at 10℃ / min, and then cooled in the furnace to 300℃.

[0035] Table 1 Comparison of Heat Treatment Process Steps

[0036] Figure 2 This is a comparison image of the microstructure in the welded and heat-treated states. Figure 3 The images show the microstructure of the weld in the as-welded and heat-treated states. It can be seen that in the as-welded state, the upper part of the weld is dominated by martensite α´, while the lower part is martensite α´+αs phase. After heat treatment, the heat-affected zone of the weld disappears, the weld grains grow significantly, and both the upper and lower parts of the weld become α+β lamellar structures (Widmanstätten lamellar structure).

[0037] Figure 4 The XRD patterns of the welded and heat-treated states are shown in Table 2, and the statistical results of the full width at half maximum (FWHM) are also shown.

[0038] Table 2. Statistical results of half-width at half-maximum (WHM) for as-welded and heat-treated states.

[0039] Combine Table 2 and Figure 4 It can be seen that the peak shape is sharper after heat treatment, indicating a reduction in lattice distortion and residual stress, and a transformation from non-equilibrium martensite to stable lamellar structure. Combined with microstructure analysis, it can be seen that the weld microstructure after heat treatment decomposes from acicular martensite into a lamellar two-phase structure dominated by α phase and containing a small amount of β phase.

[0040] Figure 5The images show the IPF diagrams of the weld in the as-welded and heat-treated states. The as-welded state exhibits a disordered microstructure with a large number of acicular martensite α´ phases. After heat treatment, the acicular martensite α´ phases disappear, and the microstructure becomes more homogeneous.

[0041] Figure 6 The KAM diagrams of the weld in the as-welded and heat-treated states are shown. The as-welded weld region exhibits a high local orientation difference, indicating the presence of high-density dislocations and residual stress. After heat treatment, the KAM value decreases significantly, with most areas transitioning to a low-strain state. Only at the grain boundaries does a high orientation difference remain, indicating that the material has undergone a significant recovery and stress release process.

[0042] The mechanical properties of the samples were tested, and the results are shown in Table 3 below.

[0043] Table 3 Mechanical Properties of Samples

[0044] It can be seen that there is no significant difference in tensile strength between the upper and lower parts of the weld. After heat treatment, the impact toughness of the upper part of the weld increased by 170%, and the impact toughness of the lower part increased by 213%; the tensile strength decreased by 12.6%.

[0045] Comparative Example 1

[0046] Wang et al. [doi:10.1016 / j.jmrt.2024.04.265] used vacuum laser welding to weld Ti-6Al-4V and compared the microstructure and properties of the as-welded state and the state after furnace cooling at 980℃ for 10 min followed by furnace cooling at 720℃ for 2 h. The tensile strength decreased from 1009 MPa in the as-welded state to 586 MPa, a decrease of 42%, and the impact toughness decreased from 13 J to 1.4 J, a decrease of 89%. Both tensile strength and impact toughness showed a significant decline.

[0047] Comparative Example 2

[0048] Reda et al. [doi:10.17654 / IJMETJan2015_001_021] heat-treated the EBW weld joint of a Ti-6Al-4V casting. The heat treatment process was as follows: holding at 900℃ for 10 min, furnace cooling to 700℃, holding at 700℃ for 30 min, followed by water quenching. The tensile strength increased from 903 MPa in the welded state to 930 MPa in the heat-treated state, and the impact absorption energy increased from 6 J to 10 J, which is much lower than the 72.9~78.5 J in Example 1.

[0049] Comparative Example 3

[0050] He Yifan et al. [doi:10.7513 / j.issn.1004-7638.2021.06.024] studied the post-weld heat treatment of TC4 titanium alloy MIG welded joints at 550~650℃. The results showed that the room temperature impact absorption energy of the welded joint was 37.83J, while the room temperature impact absorption energy of the joint softened after heat treatment and was lower than that of the welded joint.

[0051] Comparative Example 4

[0052] Li et al. [doi:10.3390 / aerospace10050436] conducted different heat treatment processes on TA15 titanium alloy and found that the mechanical properties were the highest at 810℃. After heat treatment at 940℃, the bending strength decreased from 5572MPa to 1773MPa, the impact toughness decreased from 31J to 13.13J, and the tensile strength decreased from 987MPa to about 610MPa.

[0053] Comparative Example 5

[0054] Wang et al. [doi:10.3390 / mi13020331] subjected Ti-6Al-4V titanium alloy (near-α titanium alloy) to a heat treatment process at 1050℃. The results showed that the grains were severely coarsened after heat treatment, and the tensile strength decreased from 1204MPa to 877MPa.

[0055] Comparative Example 6

[0056] Boccardo et al. [doi:10.1016 / j.matdes.2024.112949] conducted a heat treatment process of 650℃ on Ti-6Al-4V titanium alloy (near-α titanium alloy). Their study found that α′ martensite could undergo phase decomposition at 650℃ for a sufficient time, but significant morphological evolution was only observed at higher temperatures. This demonstrates that excessively low heat treatment cannot effectively control the microstructure.

[0057] The comparative examples show that excessively high or long heat treatments can lead to coarsening of the α / β microstructure, excessive growth of lamellar α or secondary α, and deterioration of local hardness matching, thereby causing a decrease in impact toughness, strength, and overall mechanical properties. On the other hand, excessively low heat treatment temperatures or excessively short holding times are insufficient to fully change the metastable microstructure in the weld and cannot effectively control the mechanical properties.

[0058] In summary, this invention achieves the control of weld mechanical properties by regulating the weld microstructure. Through a specific heat treatment process, all martensite α´ is transformed into α+β laths. This reduces residual stress, lattice distortion, dislocation density, and localized stress concentration in the weld. Simultaneously, the β phase exhibits better plasticity, acting as a buffer layer to absorb some energy and coordinate deformation between α phase laths. During crack propagation, the alternating distribution of α laths and β lamellae necessitates multiple crack deflections, resulting in higher energy absorption. At the cost of less than 13% tensile strength, impact toughness is significantly improved, achieving an increase of over 200% to 78.5 J (92.4% of the base metal), far exceeding the engineering requirement of 47 J.

[0059] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A heat treatment method for improving the impact toughness of near-α titanium alloy welds, characterized in that: include, The titanium alloy weld was held at a first temperature until the martensite α´ phase in the weld microstructure fully transformed into the α+β phase; and... The titanium alloy weld was kept at a second temperature to release residual stress; Wherein, the first temperature is 30~50°C below the phase transformation temperature of the titanium alloy; the second temperature is 660°C.

2. The heat treatment method for improving the impact toughness of near-α titanium alloy welds as described in claim 1, characterized in that: The titanium alloy weld is heated to the first temperature at a heating rate of 10±1℃ / min.

3. The heat treatment method for improving the impact toughness of near-α titanium alloy welds as described in claim 2, characterized in that: The titanium alloy weld was cooled to the second temperature at a cooling rate of 10±1℃ / min from the first temperature.

4. The heat treatment method for improving the impact toughness of near-α titanium alloy welds as described in any one of claims 1 to 3, characterized in that: It also includes cooling the titanium alloy weld to 300℃±50℃ at a cooling rate of 10±1℃ / min at a second temperature, followed by furnace cooling.

5. The heat treatment method for improving the impact toughness of near-α titanium alloy welds as described in claim 1, characterized in that: The titanium alloy weld is kept at the first temperature for 1.5 to 2 hours.

6. The heat treatment method for improving the impact toughness of near-α titanium alloy welds as described in claim 1, characterized in that: The titanium alloy weld is kept at the second temperature for 1.5 to 2 hours.

7. The heat treatment method for improving the impact toughness of near-α titanium alloy welds as described in any one of claims 1 to 3, 5, and 6, characterized in that: The near-α titanium alloys include TA24, TA18, TA11, TA15, TA19, TA29, and TA33.

8. The heat treatment method for improving the impact toughness of near-α titanium alloy welds as described in claim 1, characterized in that: The welding method for the weld seam includes electron beam welding.

9. The heat treatment method for improving the impact toughness of near-α titanium alloy welds as described in any one of claims 1 to 3, 5, 6, and 8, characterized in that: The improvement of impact toughness of titanium alloy welds is achieved by increasing the impact toughness of the upper part of the weld by 163% to 177% and the impact toughness of the lower part by 213% to 212%.

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