A heat treatment method for improving the strength and plasticity of additive manufacturing Ti2AlNb alloy

Abstract

The application discloses a heat treatment method for improving the strength and plasticity of additive manufacturing Ti2AlNb alloy, and the specific process is as follows: firstly, a 980 DEG C to 1020 DEG C homogenization solid solution treatment is carried out on an additive manufacturing Ti2AlNb alloy blank for 1.5 h to 3 h, then stress pre-aging treatment is carried out under an external tensile stress field of 180 MPa to 420 MPa and a heat field of 820 DEG C to 860 DEG C for 6 h to 12 h, and then 720 DEG C to 780 DEG C medium-temperature annealing treatment is carried out for 3 h to 5 h, so that the heat-treated Ti2AlNb alloy is obtained. The stress pre-aging treatment is used to induce the ordered precipitation of fine strip-shaped O strengthening phase, the B2 / O phase ratio and the phase interface state are improved, the medium-temperature annealing treatment is combined to balance the stress release and the stability of the precipitated phase, and the strength and plasticity of the alloy are simultaneously improved, so that the application can be widely applied to the preparation of high-temperature load-bearing parts of an aero-engine.

Description

Technical Field

[0001] This invention belongs to the field of metal additive manufacturing technology, specifically relating to a heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloys. Background Technology

[0002] Ti2AlNb alloy is a key material for high-temperature components such as turbine blades, guide vanes, casings, and high-temperature load-bearing frames in aircraft engines. Electron beam powder bed additive manufacturing technology offers vacuum high-temperature forming and uniform heat input, enabling the one-piece forming of brittle and difficult-to-machine Ti2AlNb high-temperature alloy components with complex internal cavities and irregular cross-sections. This significantly reduces machining allowances, shortens the preparation cycle, and is suitable for customized production of irregular aerospace components. However, the layer-by-layer melting and solidification forming characteristics of electron beam powder bed additive manufacturing can lead to defects such as residual thermal stress in the molten pool, uneven grain orientation, disordered second-phase precipitation, and significant anisotropy in the microstructure within the alloy billet. Directly formed Ti2AlNb alloys generally suffer from poor strength-plasticity matching, low room temperature plasticity, insufficient tensile elongation at 750℃, and imbalance in high and low temperature mechanical properties, failing to meet the usage standards for long-term service and alternating load bearing of aerospace high-temperature components.

[0003] Conventional heat treatment processes, employing only a single solution treatment followed by stress-free aging, can only slightly improve the alloy's strength. During aging, the disordered precipitation of O and α2 phases easily disrupts the matrix continuity, leading to a significant decrease in alloy plasticity. It is difficult to simultaneously maintain room-temperature and high-temperature strength and plasticity. Furthermore, existing processes do not specifically address the residual stress inherent in additive printing, making components prone to microcracks during service and resulting in poor service safety. Currently, the industry lacks a dedicated heat treatment process suitable for electron beam powder bed additive manufacturing of Ti2AlNb alloys, capable of simultaneously optimizing high and low temperature strength and plasticity, and suitable for industrial mass production. This severely limits the large-scale engineering application of complex Ti2AlNb additive manufacturing components in the field of high-end hot-end components for aero-engines.

[0004] Therefore, there is a need to provide a heat treatment method to improve the strength and plasticity of additively manufactured Ti2AlNb alloys. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a heat treatment method to improve the strength and plasticity of additively manufactured Ti2AlNb alloys, addressing the shortcomings of the prior art. This method employs a composite process of high-temperature homogenization solution treatment + external tensile stress coupled with a thermal field pre-aging + medium-temperature annealing to directionally control the morphology, size, and distribution of the B2 and O phases in the alloy matrix. This eliminates the original residual stress from additive manufacturing, weakens the anisotropy of the microstructure, and simultaneously improves the room temperature and high-temperature tensile strength and elongation of the alloy. This achieves a synergistic improvement in strength and plasticity, solving the technical problems of residual stress accumulation within the alloy, easy cracking of finished products, coarse grains, and the inability to simultaneously achieve both material strength and plasticity.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloy, characterized in that the specific process of the method is as follows: firstly, the additively manufactured Ti2AlNb alloy billet is subjected to homogenization solution treatment at 980℃~1020℃ for 1.5h~3h, then subjected to stress pre-aging treatment at an external tensile stress field of 180MPa~420MPa and a thermal field of 820℃~860℃ for 6h~12h, and then subjected to medium-temperature annealing treatment at 720℃~780℃ for 3h~5h to obtain the heat-treated Ti2AlNb alloy.

[0007] In this invention, the additively manufactured Ti2AlNb alloy billet is first subjected to homogenization solution treatment. The selected temperature range of 980℃~1020℃ is the stable B2 single-phase region of the alloy. This effectively avoids the problems of insufficient solution temperature, which fails to dissolve the brittle α2 phase and agglomerated O phase generated during additive manufacturing, leading to unavoidable compositional segregation, and excessive solution temperature, which causes grain coarsening and distortion cracking of the billet. At the same time, by controlling the solution treatment time to 1.5h~3h, the invention avoids insufficient homogeneity in the core of the billet due to too short a treatment time, and excessive grain growth due to too long a treatment time. This results in a homogeneous B2 matrix, eliminates the anisotropy of the additive structure, and provides a microstructure basis for subsequent directional precipitation.

[0008] Next, the additively manufactured Ti2AlNb alloy billet after homogenization and solution treatment is subjected to stress pre-aging treatment for 6 to 12 hours under an external tensile stress field of 180 MPa to 420 MPa and a thermal field of 820°C to 860°C. This is to directionally control the morphology, arrangement, and size of the strengthening phase, induce the orderly precipitation of fine strip-shaped O strengthening phase, and improve the B2 / O phase ratio and phase interface state. The selected temperature range of 820°C to 860°C is the optimal precipitation temperature range for fine strip-shaped plastic O phase, avoiding insufficient O phase nucleation and low product strength due to excessively low temperatures. Excessive temperature can generate large, brittle O phases that fracture the matrix. Simultaneously, the applied tensile stress field is controlled to be 80MPa~420MPa to prevent disordered O phase precipitation and lack of aging synergy caused by tensile stress below the stress-induced critical threshold of 180MPa, and to prevent microcracks from initiating in the matrix due to excessive tensile stress. The processing time is controlled to be 6h~12h to avoid insufficient precipitation of the strengthening phase due to too short a time, and coarsening and embrittlement of the precipitated phase due to too long a time. This invention uses a pre-aging coupled stress field to directionally regulate the O phase arrangement, balancing strengthening and plasticity, and eliminating residual stress from additive manufacturing.

[0009] Finally, the present invention performs a medium-temperature annealing treatment at 720℃~780℃ for 3h~5h on the additively manufactured Ti2AlNb alloy billet after stress pre-aging treatment to balance stress release and precipitate stability, and to repair and stabilize the O / B2 two-phase interface. This process avoids the inability to repair dislocations and interface defects and the accumulation of residual stress due to excessively low temperature, and the failure of oriented O phase spheroidization and high-temperature strength decay of the alloy due to excessively high temperature by controlling the treatment temperature. At the same time, by controlling the treatment temperature, it avoids incomplete stress release due to insufficient medium-temperature annealing time and precipitation of secondary brittle α2 phase due to excessively long medium-temperature annealing time, thereby balancing stress release and precipitate stability and synergistically improving the high and low temperature plasticity of the alloy.

[0010] The aforementioned heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloys is characterized in that the additively manufactured Ti2AlNb alloy blank is an electron beam powder bed additively manufactured Ti2AlNb alloy profile. This invention preferably uses an electron beam powder bed additively manufactured Ti2AlNb alloy profile, which has low oxygen content, few pores, and uniform axial residual stress, allowing for matching with an axial tensile stress lattice control mechanism. It also exhibits low heat treatment distortion and high finished product purity, making it suitable for use as components in high-end equipment such as aerospace equipment.

[0011] The aforementioned heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloys is characterized in that the chemical composition of the additively manufactured Ti2AlNb alloy blank, in atomic percentage, is: Al 18%~24%, Nb 23%~26%, with the balance being Ti and unavoidable impurities. This invention controls the O phase content by limiting the Al atomic percentage in the alloy blank, avoiding insufficient high-temperature oxidation of the blank due to excessively low Al content, and excessive matrix brittleness due to excessively high Al content; by limiting the Nb atomic percentage in the alloy blank, it stabilizes the B2 matrix, avoiding insufficient matrix decomposition and insufficient strength due to excessively low Nb content, and excessive Nb content leading to the formation of Nb-based brittle compounds and increased costs. This chemical composition ratio is adapted to the entire heat treatment phase transformation law of this invention, achieving optimal strength and plasticity compatibility.

[0012] The aforementioned heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloys is characterized by subjecting the additively manufactured Ti2AlNb alloy blanks to sandblasting and low-temperature pre-stress relief treatment before the homogenization solution treatment. The sandblasting pressure is 0.2 MPa to 0.4 MPa, and the low-temperature pre-stress relief treatment temperature is 540℃ to 560℃ with a holding time of 1.5 h to 2.5 h. This invention, by controlling the sandblasting pressure, avoids incomplete impurity removal due to insufficient pressure and damage to the surface matrix due to excessive pressure. By controlling the low-temperature pre-stress relief treatment temperature, it avoids stress release failure due to excessively low temperature and premature precipitation of irreversible brittle phases due to excessively high temperature. By controlling the holding time to adapt to blanks of different wall thicknesses, it can mitigate more than 80% of the original additive manufacturing stress, eliminating deformation and inclusion defects in subsequent heat treatment.

[0013] The aforementioned heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloys is characterized in that the heat-treated Ti2AlNb alloy exhibits a room temperature tensile strength ≥1200 MPa, a yield strength ≥860 MPa, and an elongation ≥8%; and a 750℃ tensile strength ≥690 MPa, a yield strength ≥560 MPa, and an elongation ≥13%. The heat-treated Ti2AlNb alloy prepared by this invention possesses mechanical properties that meet the service entry standards for 750℃-grade engine load-bearing components in aerospace applications, satisfying requirements for brittle fracture resistance during assembly and high-temperature alternating fatigue resistance. Compared to conventional heat treatment, its plasticity is significantly improved, making it valuable for industrial application.

[0014] Compared with the prior art, the present invention has the following advantages: 1. This invention employs a composite process of high-temperature homogenization solution treatment, external tensile stress coupled thermal field pre-aging, and medium-temperature annealing to effectively weaken the anisotropy of the microstructure of Ti2AlNb components manufactured by electron beam powder bed additive manufacturing, and simultaneously achieve a synergistic improvement in strength and plasticity at room temperature and high temperature, breaking through the technical bottleneck of the mutual exclusion of strength and plasticity in conventional heat treatment.

[0015] 2. This invention adds an external tensile stress field to assist in pre-aging and directionally controls the arrangement of strengthening phase morphology. It eliminates the need for plastic processing processes such as forging and rolling, and only requires pure heat treatment to optimize the alloy microstructure. The process is suitable for processing irregular, thin-walled, and complex additive components, and is suitable for the mass production of irregular aerospace parts.

[0016] 3. The present invention has a wide process window and strong parameter controllability. There are no flammable or explosive media or special cooling media throughout the process. The heat treatment energy consumption is low and the process is continuous. It can be used in conjunction with existing industrial heat treatment furnaces and stress aging integrated equipment for mass production.

[0017] 4. The heat-treated Ti2AlNb alloy obtained by this invention has excellent and stable mechanical properties: room temperature tensile strength ≥1200MPa, yield strength ≥860MPa, elongation ≥8%; 750℃ tensile strength ≥690MPa, yield strength ≥560MPa, elongation ≥13%, which meets the service indicators of high temperature alternating load components of aero-engines at 750℃.

[0018] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] Figure 1 This is a heat treatment process diagram for additive manufacturing of Ti2AlNb alloy according to the present invention.

[0020] Figure 2 This is an optical microstructure of the Ti2AlNb alloy after heat treatment in Example 1 of the present invention.

[0021] Figure 3 This is an optical microstructure of the Ti2AlNb alloy after heat treatment in Comparative Example 1 of this invention. Detailed Implementation

[0022] Example 1 like Figure 1 As shown, this embodiment includes the following steps: Step 1: Sandblasting and Low-Temperature Pre-Stress Relief Treatment: The electron beam powder bed additive manufacturing Ti2AlNb alloy billet is sandblasted at a pressure of 0.2 MPa, and then sent to a heat treatment furnace for low-temperature pre-stress relief treatment at 540℃ for 2.5 hours, followed by air cooling to room temperature. The chemical composition of the additive manufacturing Ti2AlNb alloy billet, in atomic percentage, is: Al 18%, Nb 26%, with the balance being Ti and unavoidable impurities. Step 2, homogenization solution treatment: The electron beam powder bed additive manufacturing Ti2AlNb alloy billet after sandblasting and low temperature pre-stress relief treatment in Step 1 is subjected to solution treatment at 980℃ for 3 hours in a heat treatment furnace and then cooled to room temperature with argon gas. Step 3, stress pre-aging treatment: Apply a constant axial tensile stress of 180MPa to the electron beam powder bed additive manufacturing Ti2AlNb alloy billet that was air-cooled to room temperature in Step 2, and simultaneously heat it to 820℃ and hold it for 12h, and then cool it down with the furnace while maintaining the stress. Step 4, medium-temperature annealing: The external tensile stress of the electron beam powder bed additive manufacturing Ti2AlNb alloy billet after furnace cooling in Step 3 is removed, and then the temperature is raised to 720℃ and held for 5 hours. After furnace cooling to room temperature, the heat-treated Ti2AlNb alloy is obtained.

[0023] Figure 2 The image shows the optical microstructure of the heat-treated Ti2AlNb alloy in this embodiment. Figure 2 As shown, the heat-treated Ti2AlNb alloy matrix is ​​a continuous and dense B2 phase, with lath-shaped strengthening O phases that are evenly distributed, small and regular in size, and clean phase interfaces without pores or microcracks. The matrix has good continuity and weak anisotropy, which is conducive to simultaneously improving the alloy's room temperature and high temperature strength and plasticity. The test results show that the room temperature tensile strength of the heat-treated Ti2AlNb alloy is 1240 MPa, the yield strength is 875 MPa, and the elongation is 8%. The 750℃ tensile strength is 695 MPa, the yield strength is 582 MPa, and the elongation is 15%.

[0024] Comparative Example 1 The difference between this comparative example and Example 1 is that the constant tensile stress applied in step three is cancelled, that is, only the sandblasting and low-temperature pre-stress relief treatment in step one, the homogenization and solution treatment in step two, the heating and holding time treatment in step three, and the medium-temperature annealing treatment in step four are used.

[0025] Figure 3 The image shows the optical microstructure of the heat-treated Ti2AlNb alloy in this comparative example. Figure 3 As shown, the B2 phase of the heat-treated Ti2AlNb alloy matrix has poor integrity, the O phase precipitates irregularly and randomly, and mostly aggregates into coarse, brittle, blocky phases. The grain boundaries are enriched with large brittle phases, and there are micropores and stress concentration sites at the grain boundaries. The second phase disrupts the connectivity of the matrix, making it prone to rapid initiation and propagation of cracks under external forces. The room temperature elongation of the heat-treated Ti2AlNb alloy is only 4.5%, and its plasticity is extremely poor, which cannot meet the service requirements.

[0026] Comparing Example 1 with Comparative Example 1, it can be seen that the present invention adds external tensile stress through stress pre-aging treatment, which can fundamentally change the morphology and distribution of O strengthening phase precipitation inside the alloy, avoid the problem of disordered agglomeration and precipitation of brittle second phase and splitting of matrix under stress-free aging, and can significantly improve the room temperature and high temperature strength and plasticity of the alloy while ensuring that the high and low temperature tensile strength of the alloy meets the standards, thus breaking the bottleneck of mutual exclusion between strength and plasticity and low plasticity of Ti2AlNb alloy.

[0027] Comparative Example 2 The difference between this comparative example and Example 1 is that the sandblasting and low-temperature pre-stress relief treatment in step one are omitted, that is, only the homogenization solution treatment in step two, the stress pre-aging treatment in step three, and the medium-temperature annealing treatment in step four are used.

[0028] Testing revealed that the heat-treated Ti2AlNb alloy in this comparative example exhibited deformation, warping, and inclusion defects. The heat-treated Ti2AlNb alloy had a room temperature tensile strength of 860 MPa, a yield strength of 672 MPa, and an elongation of 5.2%. At 750℃, the tensile strength was 520 MPa, the yield strength was 380 MPa, and the elongation was 7%.

[0029] Comparing Example 1 with Comparative Example 2, it can be seen that the present invention can effectively remove inclusions, release stress, and prevent deformation and inclusion defects in subsequent heat treatment by sandblasting and low-temperature pre-stress relief treatment of additively manufactured Ti2AlNb alloy blanks, which is beneficial to improving the plasticity and strength of the components.

[0030] Example 2 like Figure 1 As shown, this embodiment includes the following steps: Step 1: Sandblasting and Low-Temperature Pre-Stress Relief Treatment: The electron beam powder bed additive manufacturing Ti2AlNb alloy billet is sandblasted at a pressure of 0.3 MPa, and then sent to a heat treatment furnace for low-temperature pre-stress relief treatment at 550℃ for 2 hours, followed by air cooling to room temperature. The chemical composition of the additive manufacturing Ti2AlNb alloy billet, in atomic percentage, is: Al 22%, Nb 25%, with the balance being Ti and unavoidable impurities. Step 2, Homogenization and Solution Treatment: The electron beam powder bed additive manufacturing Ti2AlNb alloy billet after sandblasting and low-temperature pre-stress relief treatment in Step 1 is subjected to solution treatment at 1000℃ for 2.5h in a heat treatment furnace and then cooled to room temperature with argon gas. Step 3, Stress pre-aging treatment: Apply a constant axial tensile stress of 280MPa to the electron beam powder bed additive manufacturing Ti2AlNb alloy billet that was air-cooled to room temperature in Step 2, and simultaneously heat it to 840℃ and hold it for 9 hours, and then cool it down with the furnace while maintaining the stress. Step 4, medium-temperature annealing: The external tensile stress of the electron beam powder bed additive manufacturing Ti2AlNb alloy billet after furnace cooling in Step 3 is removed, and then the temperature is raised to 750℃ and held for 4 hours. After furnace cooling to room temperature, the heat-treated Ti2AlNb alloy is obtained.

[0031] Testing revealed that the Ti2AlNb alloy matrix after heat treatment in this embodiment consisted of a continuous and dense B2 phase, with lath-shaped reinforcing O phases that were uniformly dispersed, small, and regular in size. The phase interfaces were clean, free of pores and microcracks, and the matrix exhibited good continuity and weak anisotropy, which is beneficial for simultaneously improving the alloy's room temperature and high temperature strength and plasticity. The room temperature tensile strength of the heat-treated Ti2AlNb alloy was 1238 MPa, the yield strength was 892 MPa, and the elongation was 13%. At 750°C, the tensile strength was 725 MPa, the yield strength was 560 MPa, and the elongation was 13%.

[0032] Example 3 like Figure 1 As shown, this embodiment includes the following steps: Step 1: Sandblasting and Low-Temperature Pre-Stress Relief Treatment: The electron beam powder bed additive manufacturing Ti2AlNb alloy billet is sandblasted at a pressure of 0.4 MPa, and then sent to a heat treatment furnace for low-temperature pre-stress relief treatment at 560℃ for 1.5 hours, followed by air cooling to room temperature. The chemical composition of the additive manufacturing Ti2AlNb alloy billet, in atomic percentage, is: Al 24%, Nb 23%, with the balance being Ti and unavoidable impurities. Step 2, Homogenization and Solution Treatment: The electron beam powder bed additive manufacturing Ti2AlNb alloy billet after sandblasting and low-temperature pre-stress relief treatment in Step 1 is subjected to solution treatment at 1020℃ for 1.5h in a heat treatment furnace and then cooled to room temperature with argon gas. Step 3, Stress pre-aging treatment: Apply a constant axial tensile stress of 420MPa to the electron beam powder bed additive manufacturing Ti2AlNb alloy billet that was air-cooled to room temperature in Step 2, and simultaneously heat it to 860℃ and hold it for 6 hours, then cool it down with the furnace while maintaining the stress. Step 4, medium-temperature annealing: The external tensile stress of the electron beam powder bed additive manufacturing Ti2AlNb alloy billet after furnace cooling in Step 3 is removed, and then the temperature is raised to 780℃ and held for 3 hours. After furnace cooling to room temperature, the heat-treated Ti2AlNb alloy is obtained.

[0033] Testing revealed that the Ti2AlNb alloy matrix after heat treatment in this embodiment consisted of a continuous and dense B2 phase, with lath-shaped reinforcing O phases that were uniformly dispersed, small, and regular in size. The phase interfaces were clean, free of pores and microcracks, and the matrix exhibited good continuity and weak anisotropy, which is beneficial for simultaneously improving the alloy's room temperature and high temperature strength and plasticity. The room temperature tensile strength of the heat-treated Ti2AlNb alloy was 1208 MPa, the yield strength was 865 MPa, and the elongation was 12%. At 750°C, the tensile strength was 692 MPa, the yield strength was 620 MPa, and the elongation was 16%.

[0034] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloys, characterized in that, The specific process of this method is as follows: First, the additively manufactured Ti2AlNb alloy billet is subjected to homogenization solution treatment at 980℃~1020℃ for 1.5h~3h, then subjected to stress pre-aging treatment at an external tensile stress field of 180MPa~420MPa and a thermal field of 820℃~860℃ for 6h~12h, and then subjected to medium-temperature annealing treatment at 720℃~780℃ for 3h~5h to obtain the heat-treated Ti2AlNb alloy.

2. The heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloy according to claim 1, characterized in that, The additively manufactured Ti2AlNb alloy billet is an electron beam powder bed additively manufactured Ti2AlNb alloy profile.

3. The heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloy according to claim 1, characterized in that, The chemical composition of the additively manufactured Ti2AlNb alloy billet, in atomic percentage, is: Al 18%~24%, Nb 23%~26%, with the balance being Ti and unavoidable impurities.

4. The heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloy according to claim 1, characterized in that, Prior to the homogenization solution treatment, the additively manufactured Ti2AlNb alloy billet is subjected to sandblasting and low-temperature pre-stress relief treatment. The sandblasting pressure is 0.2MPa~0.4MPa, and the low-temperature pre-stress relief treatment temperature is 540℃~560℃, with a holding time of 1.5h~2.5h.

5. The heat treatment method for improving the strength and plasticity of additively manufactured Ti2AlNb alloy according to claim 1, characterized in that, The heat-treated Ti2AlNb alloy has a room temperature tensile strength ≥1200MPa, a yield strength ≥860MPa, and an elongation ≥8%; a 750℃ tensile strength ≥690MPa, a yield strength ≥560MPa, and an elongation ≥13%.

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