Electron beam additive manufacturing method of fine-grained full lamellar microstructure ti-al alloy
By combining micro-boron doping and in-situ annealing, the problems of bulk phase transformation and coarse grains in electron beam additive manufacturing of TiAl alloys were solved, and a fine-grained, fully lamellar microstructure with high strength and high plasticity was prepared, which significantly improved the mechanical properties of TiAl alloys.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- NORTHEASTERN UNIV CHINA
- Filing Date
- 2023-10-25
- Publication Date
- 2026-06-12
AI Technical Summary
Existing electron beam additive manufacturing of TiAl alloys suffers from problems such as easy powder bed collapse, powder blowing, bulk phase transformation, and coarse γ-equiaxed grain structure, resulting in low density, poor room temperature plasticity, and insufficient high temperature strength.
A fine-grained, fully lamellar TiAl alloy was prepared by using trace boron-doped alloy powder, in-situ annealing and additional ex-situ annealing pretreatment during electron beam additive manufacturing, combined with full lamellarization heat treatment.
It significantly improves the room temperature plasticity and high temperature strength of TiAl alloy, reduces the grain size to 1/5 of that of conventional heat treatment, increases the room temperature tensile strength by more than 50%, and increases the tensile strength at 800℃ by more than 25%.
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Figure CN117548687B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of additive manufacturing technology for lightweight, high-temperature resistant metal materials, and particularly to an electron beam additive manufacturing method for a fine-grained, fully lamellar TiAl alloy. Background Technology
[0002] TiAl alloys (γ-TiAl-based intermetallic compounds) have a low density (4.0 g / cm³). 3 TiAl alloys possess outstanding advantages such as high high-temperature specific strength and specific modulus, excellent oxidation resistance and creep resistance, making them a novel metallic structural material capable of replacing denser nickel-based superalloys in the 600–850℃ range. They hold significant application potential in aerospace engines and thermal protection structures. Precision casting and die forging of TiAl alloy turbine blades and valves have already seen widespread application. Compared to powder bed laser additive manufacturing, electron beam additive manufacturing technology offers significant advantages such as high vacuum, high substrate preheating temperature, and high forming efficiency, making it an ideal method for rapidly and precisely manufacturing brittle and difficult-to-machine TiAl alloy parts. Currently, companies such as GE (USA), AVIO (Italy), Xi'an Sailon (China), and the Beijing Aeronautical Manufacturing Engineering Research Institute have successfully manufactured TiAl alloy turbine blades and turbochargers using electron beam additive manufacturing technology.
[0003] However, current electron beam additive manufacturing of TiAl alloys still faces drawbacks such as easy powder bed collapse, powder splattering, bulk phase transformation, and the formation of coarse γ-equiaxed or alternating banded structures, which severely affect the density, room temperature plasticity, and high-temperature strength of TiAl alloys. The coarse γ-equiaxed structure formed by bulk phase transformation, or the alternating γ-coarse grains and γ+α2 banded structures, is commonly found in typical Ti-48Al-2Cr-2Nb and Ti-45Al-8Nb alloys. The bulk phase transformation of additively manufactured TiAl alloys is mainly affected by Al content and cooling rate. In particular, the deposited layer is subjected to rapid heating and cooling during the periodic thermal cycling of the electron beam, which easily induces bulk phase transformation. TiAl alloys with bulk phase transformation and banded structures inherently have low strength, especially poor high-temperature strength and creep resistance, requiring heat treatment to achieve a fully lamellar structure. Fine-grained fully lamellar microstructure or near-fully lamellar microstructure dominated by lamellar clusters exhibits the best combined mechanical properties at both room temperature and high temperature, making it the ideal service microstructure type for TiAl alloys.
[0004] Due to hereditary issues with microstructure, it is difficult to produce fine-grained, fully lamellar TiAl alloys with coarse bulk phase transformation and alternating banded microstructures through α-single-phase annealing heat treatment. This inevitably worsens room-temperature plasticity and significantly reduces room-temperature and high-temperature strength. Therefore, it is urgent to address the issues in the in-situ processes of electron beam additive manufacturing and post-annealing treatments to suppress bulk phase transformation and avoid the formation of coarse equiaxed grains, thereby producing fine-grained, fully lamellar TiAl alloys. Summary of the Invention
[0005] This invention addresses the prominent problems commonly found in electron beam additive manufacturing of TiAl alloys, such as coarse blocky phase transformation structures and alternating banded structures, resulting in poor room temperature plasticity and low high temperature strength. It adopts innovative measures such as micro-boron doping of alloy powder, enhanced in-situ annealing during electron beam additive manufacturing, and additional ex-situ annealing pretreatment to achieve high-strength, high-plasticity fine-grained, fully lamellar structure TiAl alloys in electron beam additive manufacturing.
[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: an electron beam additive manufacturing method for a fine-grained, fully lamellar TiAl alloy, comprising the following steps:
[0007] S1, boron-doped alloy powder is selectively melted by electron beam;
[0008] S2. In-situ annealing of the selected area melted and solidified layer;
[0009] S3. Annealing pretreatment based on the product obtained in step S2;
[0010] S4. Full-layer heat treatment based on the product obtained in step S3.
[0011] In step S1, the boron-doped alloy powder is a spherical powder with a particle size of 40–130 micrometers and an atomic percentage content of boron of 0.1%–0.3%. Extensive experiments have demonstrated that using TiAl alloy spherical powder with a particle size of 40–130 micrometers and a boron doping content of 0.1%–0.3% helps improve the stability of the powder bed. While achieving high-density sample fabrication, it directly controls the grain size of the γ phase in the initial microstructure to within 6 micrometers and the size of the α2 / γ lamellar clusters to within 20 micrometers.
[0012] In step S2, the in-situ annealing of the solidified layer involves repeatedly scanning the surface of the solidified layer with an electron beam, raising the temperature of the top 200–900 micrometer-thick deposited layer to 1250–1340°C, which is located in the high-temperature region of the α+γ two-phase interval. After periodic selective electron beam melting and in-situ annealing, an electron beam additive manufacturing TiAl alloy is obtained.
[0013] The electron beam current is 35–44 mA, the scanning speed is 15–25 m / s, and the continuous scanning time of the solidified layer surface is 25–50 seconds. Experimental studies have shown that after the electron beam selective melting and solidification of each layer of boron-doped alloy powder, a periodic in-situ strengthening annealing treatment is applied to bring the temperature of the top solidified deposited layer into the high-temperature region of the α+γ dual-phase range of the TiAl alloy. Further powder spreading and selective melting operations then help to eliminate the bulk phase transformation products of the high-temperature α phase in situ, allowing the high-temperature α phase to transform into a mixed structure of fine γ grains and fine α2 / γ lamellar clusters through solid-state phase transformation. Specifically, the γ grain size is less than 8 micrometers, and the α2 / γ cluster size is less than 15 micrometers.
[0014] In step S3, the annealing pretreatment specifically involves placing the TiAl alloy manufactured by electron beam additive manufacturing in a vacuum annealing furnace or an atmosphere-protected annealing furnace and holding it at a temperature of 1000–1260°C for 2–6 hours to obtain the annealed TiAl alloy.
[0015] In step S3, the annealing pretreatment specifically involves placing the TiAl alloy manufactured by electron beam additive manufacturing in a hot isostatic pressing furnace and holding it at a temperature of 1000–1260°C and a pressure of 120–200 MPa for 2–6 hours to obtain the annealed TiAl alloy.
[0016] Based on numerous experimental results, it has been demonstrated that annealing pretreatment at temperatures of 1000–1260 °C within the α+γ dual-phase region of electron beam additive manufacturing TiAl alloy samples can effectively decompose α2 / γ lamellae, promote γ grain spheroidization, generate an alloy microstructure dominated by γ phase fine grains, and improve the uniformity of the microstructure in the height direction.
[0017] In step S4, the full lamellar heat treatment involves annealing the pre-annealed TiAl alloy at a temperature within the α single-phase region to prepare a fine-grained full lamellar structure.
[0018] In step S4, the annealing temperature for the full-layer heat treatment is between 1265 and 1370°C, the annealing time is between 5 and 60 minutes, and the cooling method is air cooling or furnace cooling. Specifically, the annealing temperature in the α single-phase region can be selected within the range of 1265 to 1370°C depending on the alloy composition, the annealing holding time is between 5 and 60 minutes, and the cooling method can be air cooling or furnace cooling.
[0019] The heat treatment equipment used can be one of the following: a conventional high-temperature muffle furnace, a high-temperature atmosphere protection furnace, or a vacuum annealing furnace.
[0020] Extensive experimental results have demonstrated that strictly adhering to steps S1-S4 can achieve the preparation of a fine-grained, fully lamellar structure in TiAl alloys using electron beam additive manufacturing, effectively preventing cracking and deformation of the samples during the additive manufacturing process. The lamellar cluster size of the electron beam additively manufactured biphase TiAl alloy can reach as small as 25 micrometers. Furthermore, the holding time in step S4 can be flexibly adjusted according to the required mechanical properties to prepare a fine-grained, near-fully lamellar structure, i.e., a microstructure containing less than 20% γ-grains and more than 80% lamellar clusters.
[0021] The beneficial effects of this invention are as follows: This invention provides an electron beam additive manufacturing method for a fine-grained, fully lamellar TiAl alloy, achieved through measures such as micro-boron-doped spherical alloy powder, in-situ strengthening annealing, annealing pretreatment, and fully lamellar heat treatment. Compared with previous electron beam additive manufacturing methods for TiAl alloys, the material has higher density and a lower tendency to crack and deform, making it suitable for electron beam additive manufacturing of complex parts made of high-plasticity, fine-grained TiAl alloys.
[0022] Specifically, this method directly prevents the bulk phase transformation of TiAl alloys during electron beam additive manufacturing, avoiding the formation of coarse bulk transformation structures or coarse banded structures, and directly generating an initial structure dominated by fine equiaxed γ-phase grains. Annealing pretreatment in the α+γ dual-phase region further decomposes the lamellar structure, promotes spheroidization of γ grains, and significantly increases the content of fine γ grains. The fine-grained fully lamellar structure prepared by the final fully lamellar heat treatment has a grain size that is only 1 / 5 of the grain size of the fully lamellar structure obtained by conventional heat treatment methods. In terms of mechanical properties, the room temperature plasticity of the fine-grained fully lamellar structure is increased by more than 50%, the room temperature tensile strength is increased by about 30%, and the tensile strength at 800℃ is increased by more than 25%.
[0023] The Ti-45Al-6Nb-0.2B (atomic percentage) alloy turbine blades additively manufactured using this method have an average lamellar cluster size of 25μm, a room temperature tensile strength ≥850MPa, an elongation after fracture ≥1.0%, and a tensile strength at 800℃ ≥650MPa. The Ti-48Al-2Cr-2Nb-0.15B (atomic percentage) alloy additively manufactured using this method has an average lamellar cluster size of less than 60μm, a room temperature tensile strength as high as 800MPa, an elongation after fracture ≥1.5%, and a tensile strength at 800℃ ≥550MPa. Attached Figure Description
[0024] Figure 1 Morphology of trace boron-doped spherical TiAl alloy powder as observed by scanning electron microscopy;
[0025] Figure 2 Microstructure characteristics generated by in-situ annealing of the solidified layer for observation by scanning electron microscopy;
[0026] Figure 3 The fine-grained, full-lamellar microstructure features observed by scanning electron microscopy;
[0027] Figure 4 The characteristics of the massive transformation coarse-grained structure observed under an optical microscope;
[0028] Figure 5 Flowchart of electron beam additive manufacturing method for fine-grained, fully lamellar TiAl alloys. Detailed Implementation
[0029] This invention provides an electron beam additive manufacturing method for a fine-grained, fully lamellar TiAl alloy, such as... Figure 5 As shown, the process includes the following steps: S1, selective melting of boron-doped alloy powder; S2, in-situ annealing of the solidified layer; S3, annealing pretreatment; S4, full lamellar heat treatment. The process flow of this invention is simple, highly controllable, and suitable for TiAl alloys of any alloy composition. The innovation of this invention lies in the fact that by combining trace amounts of boron-doped spherical powder with enhanced in-situ annealing, the bulk phase transformation of TiAl alloys manufactured by electron beam additive manufacturing is completely suppressed, grain coarsening is avoided, and lamellar structure decomposition is promoted, directly generating an initial microstructure dominated by γ-phase equiaxed fine grains. This effectively avoids the microstructure characteristics of coarse bulk γ-grains or columnar lamellae clusters commonly found in current electron beam additive manufacturing of TiAl alloys. Furthermore, the trace amount of boron-doped γ+α2 biphase equiaxed fine-grained microstructure produced by this additive manufacturing effectively suppresses high-temperature α-grain coarsening during α single-phase annealing, ultimately generating a fine-grained full lamellar structure with a cluster size of only 25 micrometers. In addition, the fine-grained, fully lamellar additive manufacturing technology for TiAl alloys proposed in this invention can significantly improve room temperature plasticity and high temperature strength; wherein, the room temperature tensile strength is higher than 850 MPa, the elongation at break is increased to more than 1.0%, and the tensile strength at 800℃ is not less than 550 MPa.
[0030] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Specific Implementation Example 1
[0032] This embodiment provides an electron beam additive manufacturing method for a fine-grained, fully lamellar Ti-45Al-6Nb-0.2B (atomic percentage) titanium-aluminum alloy. First, step S1 involves selective electron beam melting of micro-boron-doped TiAl alloy spherical powder. Spherical Ti-45Al-6Nb-0.2B alloy powder with a particle size of 40–130 micrometers is selected and electron beam selective melting is performed according to a conventional powder bed additive manufacturing process. The forming chamber is first evacuated to 1.0 × 10⁻⁶. -4The atmosphere pressure is increased to 0.1 Pa by filling with high-purity helium gas, and powder deposition and selective electron beam melting begin. The following settings can be selected: substrate preheating temperature 1100℃, layer thickness 60 μm, pass spacing 110 μm, powder bed preheating current 40 mA, powder bed preheating scan speed 18 m / s, repeated scans for 15 seconds, selective melting current 12 mA, and scan speed during melting 7 m / s. Figure 1 The image shows the morphological characteristics of titanium-aluminum alloy spherical powder with a particle size of 40–130 micrometers as observed by scanning electron microscopy.
[0033] Next, the in-situ annealing of the solidified layer in process step S2 begins. After the alloy deposit layer completed in step S1 with selective melting and solidification, a high-energy electron beam is used to repeatedly scan the surface of the solidified layer to raise the temperature of the top approximately 600 micrometer-thick deposit layer to between 1260 and 1300 °C. The high-energy electron beam current is controlled at 42 mA, the scanning speed at 20 m / s, and the scanning is repeated continuously for 40 seconds. Then, cyclical operations such as powder spreading, powder bed preheating, selective melting, and in-situ strengthening annealing are continued. After cooling, the TiAl alloy is produced. Figure 2 Scanning electron micrographs of the equiaxed fine-grained structure produced in step S2 are shown. It can be seen that this fine-grained structure is almost entirely composed of equiaxed grains of α2 (white) and γ phases, with grain sizes all within 6 micrometers and only a small number of lamellar structures.
[0034] Then, the annealing pretreatment in step S3 is performed. The annealing pretreatment involves placing the Ti-45Al-6Nb-0.2B alloy sample manufactured by electron beam additive manufacturing in step S2 in an argon atmosphere tube furnace, holding it at 1240℃ for 2 hours in the α+γ two-phase region, and then air-cooling it. The purpose of this annealing pretreatment is to further decompose the residual lamellar structure within the alloy sample manufactured in step S2, generating more fine γ and α2 grains.
[0035] Finally, step S4, full-layer heat treatment, is performed. The alloy sample pretreated by annealing in step 3 is placed in a high-temperature tube furnace under a protective atmosphere and held at 1330°C for 10 minutes in the single α-phase region of the alloy, followed by furnace cooling. By completing steps S1-S4 sequentially, the fabrication of a fine-grained, fully lamellar TiAl alloy sample is complete. Figure 3 The morphological characteristics of a fine-grained, fully lamellar structure fabricated by electron beam additive manufacturing, as observed under a scanning electron microscope, are presented. It is evident that the fine-grained, fully lamellar structure contains only α2 / γ lamellar clusters, all with a size less than 25 micrometers. Trace amounts of submicron-sized TiB exist within the lamellar clusters in a needle-like form. Mechanical property testing shows that the fine-grained, fully lamellar TiAl alloy fabricated using this embodiment exhibits a room-temperature tensile strength of up to 900 MPa, a fracture elongation of 1.5%, and a tensile strength of 750 MPa at 800°C. Specific Implementation Example 2
[0037] This embodiment provides an electron beam additive manufacturing method for a fine-grained, fully lamellar Ti-48Al-2Cr-2Nb-0.15B (atomic percentage) titanium-aluminum alloy. First, step S1 involves selective electron beam melting of micro-boron-doped TiAl alloy spherical powder. Spherical Ti-48Al-2Cr-2Nb-0.15B alloy powder with a particle size of 40–130 micrometers is selected and electron beam selective melting is performed according to a conventional powder bed additive manufacturing process. The forming chamber is first evacuated to 0.8 × 10⁻⁶. -4 Pa, then high-purity helium is introduced. When the helium pressure reaches 0.1 Pa, powder spreading and selective electron beam melting begin. The following parameters can be selected: substrate preheating temperature 1100℃, layer thickness 80 μm, pass spacing 110 μm, powder bed preheating current 38 mA, powder bed preheating scan speed 20 m / s, preheating scan 20 seconds, selective melting current 10 mA, and scanning speed during melting 6 m / s.
[0038] Next, the in-situ annealing treatment of the solidified layer in process step S2 was initiated. After the selected melting and solidification in step S1, the solidified deposit layer surface was repeatedly scanned using a high-energy electron beam to raise the temperature of the top 650-micrometer-thick deposit layer to between 1280 and 1340°C. The high-energy electron beam current was controlled at 42 mA and the scanning speed at 22 m / s, with continuous reciprocating scanning for 40 seconds. Then, operations such as powder spreading, powder bed preheating, electron beam selected melting, and in-situ strengthening annealing were performed periodically. After cooling, a TiAl alloy was produced. The TiAl alloy produced by in-situ strengthening annealing additive manufacturing had γ-grain sizes all within 8 μm.
[0039] Then, the annealing pretreatment in step S3 is performed. The annealing pretreatment involves placing the Ti-48Al-2Cr-2Nb-0.15B alloy sample manufactured by electron beam additive manufacturing in step S2 into a hot isostatic pressing furnace, holding it at 1260℃ and 165MPa argon pressure for 3 hours in the α+γ dual-phase region of the alloy, and then furnace cooling. This annealing pretreatment further decomposes the residual lamellar structure within the alloy sample manufactured in step S2, spheroidizes the γ grains, and generates more fine γ and α2 grains; simultaneously, the sample's density is increased to over 99.9%.
[0040] Finally, step S4, the full-lamellar heat treatment, is performed. The Ti-48Al-2Cr-2Nb-0.15B alloy sample, which underwent annealing pretreatment in step 3, is placed in a high-temperature tube furnace under a protective atmosphere and held at 1360℃ for 10 minutes in the single α-phase region of the alloy, followed by furnace cooling. By completing steps S1-S4 sequentially, a TiAl alloy sample with a fine-grained, fully lamellar microstructure is produced. The fine-grained, fully lamellar Ti-48Al-2Cr-2Nb-0.15B alloy fabricated by electron beam additive manufacturing consists only of α2 / γ lamellar clusters, with the size of each cluster around 50 micrometers. Trace amounts of submicron-sized TiB exist in needle-like forms within the lamellar clusters. Mechanical property testing revealed that the fine-grained, fully lamellar TiAl alloy fabricated using this embodiment exhibits a room temperature tensile strength of up to 780 MPa, a fracture elongation of 1.8%, and a tensile strength of 620 MPa at 800℃.
[0041] Comparative Example 1
[0042] This embodiment provides an electron beam additive manufacturing method for a trace amount of boron-doped TiAl alloy, Ti-48Al-2Cr-2Nb-0.15B (atomic percentage), which mainly includes the following steps: S1, selective melting of alloy powder; S2, in-situ annealing of the solidified layer; S3, full-layer heat treatment. First, electron beam selective melting of the TiAl alloy spherical powder is performed in step S1. Spherical Ti-48Al-2Cr-2Nb-0.15B alloy powder with a particle size of 40–130 micrometers is selected, and electron beam selective melting is performed according to the conventional powder bed additive manufacturing process. The forming chamber is first evacuated to 1×10⁻⁶. -4 The pressure is increased to Pa, then high-purity helium is introduced. When the helium pressure reaches 0.12 Pa, powder spreading and electron beam selective melting operations begin. The following parameters can be selected: substrate preheating temperature 1100℃, layer thickness 60 μm, pass spacing 110 μm, powder bed preheating current 35 mA, powder bed preheating scan speed 20 m / s, preheating scan 20 seconds, selective melting current 11 mA, and scanning speed during melting 6.5 m / s.
[0043] Next, the in-situ annealing treatment of the solidified layer in process step S2 was carried out. The parameters of the in-situ annealing treatment were the same as those in Example 2. Then, operations such as powder spreading, powder preheating, electron beam selective melting, and in-situ annealing were performed periodically, and after cooling, a high-density TiAl alloy was produced.
[0044] Then, the full-lamellar heat treatment in step S3 is directly performed. The Ti-48Al-2Cr-2Nb alloy sample manufactured by electron beam additive manufacturing in step S2 is placed in a high-temperature tube furnace under atmosphere protection and held at 1360°C for 10 minutes in the single α-phase region of the alloy, and then cooled with the furnace. In this way, after completing steps S1-S3 in sequence, a TiAl alloy sample with full-lamellar microstructure is produced. This conventional electron beam additive manufactured full-lamellar Ti-48Al-2Cr-2Nb alloy is also composed only of α2 / γ lamellar clusters, but some of them are abnormally large lamellar clusters, with the largest cluster size reaching 150 micrometers. Through mechanical property testing, it was found that the full-lamellar TiAl alloy manufactured by this embodiment has a relative density of 99.8%, a room temperature tensile strength of 700 MPa, a fracture elongation of only 0.8%, and a tensile strength of 560 MPa at 800°C.
[0045] Comparative Example 2
[0046] This embodiment provides an electron beam additive manufacturing method for conventional titanium-aluminum alloy Ti-48Al-2Cr-2Nb (atomic percentage), which mainly includes the following steps: S1, selective melting of alloy powder; S2, in-situ annealing of the solidified layer; S3, annealing pretreatment; S4, full-layer heat treatment. First, selective electron beam melting of the TiAl alloy spherical powder in step S1 is performed. Ti-48Al-2Cr-2Nb alloy spherical powder with a particle size of 40–130 micrometers is selected, and selective electron beam melting is performed according to the conventional powder bed additive manufacturing process. First, the forming chamber is evacuated to 0.75 × 10⁻⁶. -4 Pa, then high-purity helium is introduced. When the helium pressure reaches 0.09 Pa, powder spreading and selective electron beam melting begin. The following parameters can be selected: substrate preheating temperature 1150℃, layer thickness 75 μm, pass spacing 120 μm, powder bed preheating current 35 mA, powder bed preheating scan speed 18 m / s, preheating scan 15 seconds, selective melting current 11 mA, and scanning speed during melting 7 m / s.
[0047] Next, the in-situ annealing of the solidified layer in process step S2 was initiated. After the selected melting and solidification in step S1, the solidified deposit layer was repeatedly scanned using a high-energy electron beam to raise the temperature of the top approximately 650-micrometer-thick deposit layer to between 1280 and 1340°C. The high-energy electron beam current was controlled at 40 mA, the scanning speed at 20 m / s, and the scanning was repeated continuously for 40 seconds. Then, operations such as powder spreading, powder bed preheating, electron beam selected melting, and in-situ annealing were performed periodically, followed by cooling to produce the TiAl alloy. The TiAl alloy produced by in-situ annealing additive manufacturing exhibited γ-grain sizes all within 10 μm, with occasional observation of a small number of coarse lamellar structures.
[0048] Then, the annealing pretreatment in process step S3 is performed. The annealing pretreatment involves placing the Ti-48Al-2Cr-2Nb alloy sample manufactured by electron beam additive manufacturing in step S2 in a hot isostatic pressing furnace, holding it at 1260℃ and 165MPa argon pressure for 3 hours in the α+γ dual-phase region of the alloy, and then furnace cooling. This annealing pretreatment further decomposes the residual lamellar structure within the alloy sample manufactured in step S2, spheroidizes the γ grains, and generates more γ and α2 fine grains; simultaneously, the sample's density is increased to over 99.9%.
[0049] Finally, step S4, the full-lamellar heat treatment, is performed. The Ti-48Al-2Cr-2Nb alloy sample pretreated in step 3 is placed in a high-temperature tube furnace under a protective atmosphere and held at 1360℃ for 10 minutes in the single α-phase region of the alloy, followed by furnace cooling. By completing steps S1-S4 sequentially, a TiAl alloy sample with a fine-grained, fully lamellar microstructure is produced. This fully lamellar Ti-48Al-2Cr-2Nb alloy manufactured by electron beam additive manufacturing consists only of α2 / γ lamellar clusters, with most lamellar clusters around 80 micrometers in size, but a small number of coarse lamellar clusters of 150 micrometers in size still appear. Mechanical property testing revealed that the fine-grained, fully lamellar TiAl alloy manufactured using this embodiment has a room temperature tensile strength as high as 740 MPa, a fracture elongation of 1.0%, and a tensile strength of 585 MPa at 800℃.
[0050] Comparative Example 3
[0051] This embodiment provides an electron beam additive manufacturing method for Ti-48Al-2Cr-2Nb-0.15B (atomic percentage) titanium-aluminum alloy with a fully lamellar microstructure. The main steps include: S1, selective melting of alloy powder; S2, annealing pretreatment; S3, full lamellarization heat treatment. First, step S1, selective electron beam melting of trace boron-doped TiAl alloy spherical powder, is performed. Spherical Ti-48Al-2Cr-2Nb-0.15B alloy powder with a particle size of 40–130 micrometers is selected, and selective electron beam melting is performed according to the conventional powder bed additive manufacturing process. The forming chamber is first evacuated to 0.8 × 10⁻⁶. -4 Pa, then high-purity helium is introduced, and when the helium pressure reaches 0.1 Pa, powder spreading and selective electron beam melting begin. The following parameters can be selected: substrate preheating temperature 1100℃, layer thickness 80 μm, pass spacing 110 μm, powder bed preheating current 38 mA, powder bed preheating scanning speed 20 m / s, preheating scan 20 seconds, selective melting current 10 mA, and scanning speed during melting 6 m / s. The TiAl alloy manufactured by this selective electron beam melting exhibits a typical bulk phase transformation in step S2 of the comparative example, generating coarse bulk transformation microstructure characteristics, such as... Figure 4 As shown in the optical microscope images, the coarse γ grains are between 10 and 40 micrometers in size, with almost no lamellar structure.
[0052] Next, the annealing pretreatment in process step S2 is performed directly. The annealing pretreatment involves placing the Ti-48Al-2Cr-2Nb-0.15B alloy sample manufactured by electron beam additive manufacturing in step S1 into a hot isostatic pressing furnace, holding it at 1260℃ and 165MPa argon pressure for 3 hours in the α+γ two-phase region of the alloy, and then furnace cooling. This annealing pretreatment increases the density of the alloy sample manufactured in step S1 to over 99.9% and eliminates internal stress, but further coarsens the γ and α2 grains.
[0053] Finally, step S3, full-lamellar heat treatment, is performed. The Ti-48Al-2Cr-2Nb-0.15B alloy sample, which underwent annealing pretreatment in step 2, is placed in a high-temperature tube furnace under a protective atmosphere and held at 1360°C for 6 minutes in the single α-phase region of the alloy, followed by furnace cooling. By completing steps S1-S3 sequentially, a TiAl alloy sample with full-lamellar microstructure characteristics is produced. Although this electron beam additive manufacturing of TiAl alloy without in-situ annealing consists only of α2 / γ lamellar clusters, the lamellar clusters are exceptionally large, generally exceeding 120 micrometers in size, with some clusters reaching up to 200 micrometers. Mechanical property testing revealed that the coarse-grained full-lamellar TiAl alloy produced using this embodiment exhibits a room temperature tensile strength of 650 MPa, a fracture elongation of 0.6%, and a tensile strength of 520 MPa at 800°C.
[0054] In summary, the electron beam additive manufacturing method for fine-grained, fully lamellar TiAl alloys proposed in this invention is highly feasible, simple in process, and economical. It completely solves the problems of blocky transformation structure and large grain size after full lamellar heat treatment in current electron beam additive manufacturing of TiAl alloys, as well as the resulting poor room temperature plasticity and insufficient high temperature strength.
[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art can make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.
Claims
1. A method for electron beam additive manufacturing of a fine-grained, fully lamellar TiAl alloy, characterized in that, Includes the following steps: S1, boron-doped alloy powder is subjected to selective electron beam melting; after selective melting, the grain size of the γ phase is controlled to be within 6 micrometers, and the size of the α2 / γ lamellar clusters is controlled to be within 20 micrometers. S2. In-situ annealing of the selected area melting solidified layer; the in-situ annealing of the solidified layer is as follows: the surface of the solidified layer is repeatedly scanned by an electron beam to raise the temperature of the top 200-900 micrometer thick deposited layer to 1250-1340℃, which is located in the high temperature region of the α + γ two-phase interval, to obtain the electron beam additive manufacturing TiAl alloy; the electron beam current is 35-44 mA, the scanning speed is 15-25 m / s, and the continuous scanning time of the solidified layer surface is 25-50 seconds. S3. Annealing pretreatment based on the product obtained in step S2; S4. Based on the product obtained in step S3, perform full lamellar heat treatment to prepare a fine-grained full lamellar structure containing less than 20% γ grains and more than 80% lamellar clusters; the annealing temperature of the full lamellar heat treatment is 1265~1370℃, the annealing time is 5~60 minutes, and the cooling method is air cooling or furnace cooling.
2. The electron beam additive manufacturing method for a fine-grained, fully lamellar TiAl alloy according to claim 1, characterized in that, In step S1, the boron-doped alloy powder is a spherical powder with a particle size of 40 to 130 micrometers and an atomic percentage content of boron of 0.1% to 0.3%.
3. The electron beam additive manufacturing method for a fine-grained, fully lamellar TiAl alloy according to claim 1, characterized in that, In step S3, the annealing pretreatment specifically involves placing the electron beam additive manufacturing TiAl alloy in a vacuum annealing furnace or an atmosphere-protected annealing furnace and holding it at a temperature of 1000~1260℃ for 2~6 hours to obtain the annealed TiAl alloy.
4. The electron beam additive manufacturing method for a fine-grained, fully lamellar TiAl alloy according to claim 1, characterized in that, In step S3, the annealing pretreatment specifically involves placing the electron beam additive manufacturing TiAl alloy in a hot isostatic pressing furnace and holding it at a temperature of 1000~1260℃ and a pressure of 120~200 MPa for 2~6 hours to obtain the annealed TiAl alloy.