Method for eliminating layered structure of TiAl alloy in electron beam 3D printing

Abstract

The application discloses a method for eliminating layered structure of electron beam 3D printing TiAl alloy, and the method comprises the following steps: heating the electron beam 3D printing TiAl alloy to 1200-1230 DEG C and keeping the temperature for 1-3 hours, immediately placing low-carbon steel gaskets on the upper and lower end surfaces of the electron beam 3D printing TiAl alloy, and then performing unidirectional forging to obtain a TiAl alloy ingot, wherein the TiAl alloy ingot has uniform duplex or near lamellar structure. The method can eliminate the inhomogeneous layered structure in the original ingot, and obtain the uniform duplex or near lamellar structure. The method is simple in process, low in requirement for equipment, and suitable for industrial production.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of titanium-aluminum intermetallic compound processing, and particularly relates to a method for eliminating layered structure of TiAl alloy in electron beam 3D printing. BACKGROUND

[0002] The specific gravity of TiAl alloy is only half of that of nickel-based high-temperature alloy, and meanwhile, the TiAl alloy has excellent high-temperature specific strength and specific stiffness, and outstanding high-temperature oxidation resistance, creep resistance and fatigue resistance, and other outstanding advantages, and is a kind of light high-temperature structural material which can replace nickel-based high-temperature alloy between 600 DEG C and 900 DEG C. Traditional TiAl-based alloy parts are often prepared by precision casting or plastic forming method. In recent years, with the development of additive manufacturing technology, additive manufacturing of TiAl alloy parts has also become a hot spot. At present, the methods for additive manufacturing of TiAl alloy mainly include laser metal deposition (LMD), selective laser melting (SLM) and selective electron beam melting (SEBM). Among them, SEBM technology has the advantages of low residual stress, high deposition efficiency and high vacuum cleanliness, and is considered as one of the most suitable technologies for additive manufacturing of TiAl alloy.

[0003] However, the manufacturing concept based on layer-by-layer accumulation and long-time intrinsic annealing lead to the uneven distribution of Al element in the solidification process, and the TiAl alloy after SEBM forming usually shows the microstructure characteristics of layered heterogeneous, that is, the single-phase gamma coarse crystal + (gamma + alpha2 / gamma) bimodal fine crystal layer alternately distributed along the printing direction. This layered structure will cause the material to exhibit obvious anisotropy of tensile mechanical properties and fatigue properties, which is extremely unfavorable for engineering materials. In order to avoid the influence of uneven organization on mechanical properties, most researchers control the organization of TiAl alloy through heat treatment. However, according to the Ti-Al phase diagram, the fine crystal region with low Al content has a lower temperature, which means that it is difficult to realize synchronous organization transformation of coarse crystal and fine crystal region, so the layered structure also responds unevenly to the heat treatment system. Existing research shows that to realize the homogenization of the organization, it is necessary to first heat treat in the alpha single-phase region, then form the feather-like gamma organization by oil quenching, and finally anneal at 1100 DEG C for 24h to form uniform near-gamma organization. This processing method is not only long and complex, but also may cause the microstructure to be coarse, and more importantly, the near-gamma organization TiAl alloy formed after treatment has poor comprehensive performance and is not suitable for industrial application. α Temperature is also low, which means that it is difficult to realize synchronous organization transformation of coarse crystal and fine crystal region, so the layered structure also responds unevenly to the heat treatment system. Existing research shows that to realize the homogenization of the organization, it is necessary to first heat treat in the alpha single-phase region, then form the feather-like gamma organization by oil quenching, and finally anneal at 1100 DEG C for 24h to form uniform near-gamma organization. This processing method is not only long and complex, but also may cause the microstructure to be coarse, and more importantly, the near-gamma organization TiAl alloy formed after treatment has poor comprehensive performance and is not suitable for industrial application.

[0004] Since TiAl alloy has good organization stability in the two-phase region, grain growth and transformation into near-gamma organization are not easy to occur during the heat treatment process, so the two-phase region heat treatment is an ideal process for layered heterogeneous organization TiAl alloy. However, due to its organization stability characteristics, simple two-phase region heat treatment cannot eliminate this layered heterogeneous uneven organization.

[0005] When the TiAl alloy is deformed at a specific temperature and strain rate in the two-phase region, the alloy phase transition is promoted under the double action of stress and temperature, thereby forming a dual-state or near lamellar structure. The uniform dual-state and near lamellar structure can be directly used due to its good comprehensive performance. Therefore, the realization of the uniformization of the lamellar heterogeneous structure of the TiAl alloy requires the combination of phase transition and plastic forming synergy theory, and the realization of the uniformity of the material after forging also needs to overcome the inherent heterogeneous deformation characteristics in the deformation process. The existing technology often uses multi-directional forging to realize it, however, multi-directional forging is a large plastic deformation technology, and if it is used for 3D printing materials, the technical characteristics of 3D printing are lost.

[0006] Therefore, for the forging deformation of 3D printing materials, it is necessary to meet the small deformation (less than 50%) and uniform plastic deformation at the same time, which is also a technical bottleneck that has not been solved in the field at present. SUMMARY

[0007] In view of the shortcomings of the prior art, the purpose of the present application is to provide a method for eliminating the lamellar structure of electron beam 3D printed TiAl alloy. The present application aims at the problem of large difference in mechanical properties and anisotropy caused by the lamellar heterogeneous structure in the existing electron beam 3D printed TiAl alloy, and combines phase transition + uniform deformation to promote alloy phase transition, so that the original lamellar heterogeneous structure is transformed into a uniform dual-state or near lamellar structure.

[0008] The method for eliminating the lamellar structure of electron beam 3D printed TiAl alloy of the present application heats the electron beam 3D printed TiAl alloy to 1200-1230 DEG C and keeps it for 1-3 hours, then immediately pads low-carbon steel pads on the upper and lower end surfaces of the electron beam 3D printed TiAl alloy, and then performs single-direction forging to obtain a TiAl alloy ingot, wherein the TiAl alloy ingot has a uniform dual-state or near lamellar structure.

[0009] The method provided by the application first heats the electron beam 3D printed TiAl alloy to slightly below the phase transition temperature, and after heat preservation is completed, inserts low carbon steel gaskets on the upper and lower end surfaces of the electron beam 3D printed TiAl alloy blank, and then performs unidirectional forging. On the one hand, the insertion of the low carbon steel gaskets on the upper and lower end surfaces of the electron beam 3D printed TiAl alloy blank before forging can play a lubricating role, ensuring uniform deformation of the end part. At the same time, the low carbon steel gaskets as soft pads can make the TiAl blank as a whole in the easy deformation zone of forging, thereby further avoiding the uneven deformation of the alloy. On the other hand, the deformation under the action of heat and force can promote the electron beam 3D printed TiAl alloy to undergo gamma to alpha phase transition, forming an alpha2 / gamma lamellar structure. In particular, the faster the deformation rate, the higher the proportion of the alpha2 / gamma lamellar structure. At the same time, in the process of thermal deformation, high-density dislocations and deformation twins also promote the dynamic recrystallization of the alloy, and the newly formed fine recrystallized grains gradually replace the coarse grains. These factors are all conducive to eliminating the original layered heterogeneous organization of the alloy "single-phase gamma coarse grain + (gamma + alpha2 / gamma) bimodal fine grain", and ultimately obtaining a uniform bimodal or near lamellar structure after the method of the application.

[0010] In the application, the T ɑ The phase transition temperature is 1235 DEG C, and the application controls the heating temperature to be slightly lower than the phase transition temperature and in a relatively small range. The inventor finds that in this temperature range, recrystallization occurs during the thermal deformation process, fine recrystallized grains are obtained, and if the heating temperature is too high, the temperature of the blank exceeds the phase transition temperature, the grains of the alloy grow rapidly, and the purpose of fine grains after forging cannot be achieved. If the temperature is too low, due to the poor plastic deformation ability of titanium aluminum alloy, low-temperature forging is prone to cracking, and is also not conducive to recrystallization, so the grain refinement cannot be achieved.

[0011] In a preferred embodiment, the electron beam 3D printed TiAl alloy is a gamma-TiAl intermetallic compound.

[0012] In a preferred embodiment, the microstructure of the electron beam 3D printed TiAl alloy is a layered heterogeneous inhomogeneous organization in which single-phase gamma coarse grains and (gamma + alpha2 / gamma) bimodal fine grains are alternately distributed.

[0013] In a preferred embodiment, the surface of the electron beam 3D printed TiAl alloy is first coated with an anti-oxidation coating, then wrapped with asbestos as a whole, and then heated.

[0014] In the application, the anti-oxidation coating used is a high-temperature anti-oxidation coating of model TL1330-3 produced by Beijing Tianlichuang Glass Technology Development Co., Ltd.

[0015] In the present application, by wrapping the electron beam 3D printed TiAl alloy blank with asbestos as a whole, the temperature drop during the entire process of taking out from the heating furnace to forging can be ensured to be small after the electron beam 3D printed TiAl alloy is heated, and if asbestos wrapping is not performed, a larger temperature drop may be caused, resulting in cracking during forging or failing to achieve the effect of uniformizing the structure.

[0016] In actual operation, the loose layer of the electron beam 3D printed TiAl alloy is removed first, and then an anti-oxidation coating is applied to the surface thereof.

[0017] Preferably, the thickness of the low-carbon steel gasket is 25% to 35% of the height of the electron beam 3D printed TiAl alloy.

[0018] In the present application, by controlling the thickness of the low-carbon steel gasket within the range of the present application, it can be ensured that the TiAl blank as a whole is in the easy deformation zone during forging, thereby further avoiding uneven deformation of the alloy. If the thickness of the low-carbon steel gasket is too large, the overall height-to-diameter ratio will increase, and the forging process will be prone to instability (such as tilting), and if the thickness is too small, it cannot ensure that the TiAl blank as a whole is in the easy deformation zone during forging, and if the blank as a whole is not entirely in the easy deformation zone, it will result in uneven structure in different regions.

[0019] Preferably, the temperature of the low-carbon steel gasket is 890 to 910 DEG C. In the present application, by controlling the temperature of the low-carbon steel gasket within the above range, the final effect is optimal, if the temperature is too low, the temperature drop of the electron beam 3D printed TiAl alloy will be too large, and a better effect cannot be achieved, and if the temperature is too high, it will affect the performance of the low-carbon steel gasket, thereby further affecting the deformation effect.

[0020] Preferably, the rate of the single-direction forging is 1 to 5 s -1 , and the total deformation amount with the gasket is 40 to 45%.

[0021] In the present application, by controlling the total deformation amount of the single-direction forging to be 40 to 45%, the deformation amount of the electron beam 3D printed TiAl alloy is about 20% to 27%, which belongs to a small strain range, and at the same time, the structure uniformization can be achieved.

[0022] Preferably, after the single-direction forging is completed, the obtained blank is wrapped with asbestos, and the TiAl alloy ingot blank is obtained by natural cooling.

[0023] Preferably, the difference between the mechanical properties of the TiAl alloy ingot blank perpendicular to the printing direction and parallel to the printing direction is less than 5%.

[0024] The electron beam 3D printed TiAl alloy has a characteristic of layered structure, and the mechanical properties of the alloy with such structure are quite different along the layered direction and perpendicular to the layered direction (i.e. perpendicular to the printing direction and parallel to the printing direction). By the method of the present application, the obtained TiAl alloy ingot has a uniform duplex or near lamellar structure. When the layered structure is eliminated and is changed into a uniform structure, the performance difference in each direction tends to be consistent.

[0025] Principle and advantage

[0026] The method provided by the present application first heats the electron beam 3D printed TiAl alloy blank to slightly below the phase transition temperature, and after heat preservation is completed, low-carbon steel gaskets are placed on the upper and lower end surfaces of the electron beam 3D printed TiAl alloy blank, and then unidirectional forging is performed. On the one hand, the insertion of low-carbon steel gaskets into the upper and lower end surfaces of the electron beam 3D printed TiAl alloy blank before forging can play a lubricating role, ensuring uniform deformation of the end surfaces. At the same time, the low-carbon steel gaskets as soft pads can make the TiAl blank as a whole in the easy deformation zone of forging, thereby further avoiding uneven deformation of the alloy. On the other hand, deformation under the action of heat and force can promote the electron beam 3D printed TiAl alloy to undergo γ→α phase transition and form an α2 / γ lamellar structure. In particular, the faster the deformation rate, the higher the proportion of the α2 / γ lamellar structure. At the same time, in the process of thermal deformation, high-density dislocations and deformation twins also promote the occurrence of dynamic recrystallization in the alloy, and the newly formed fine recrystallized grains gradually replace the coarse grains. These factors are all conducive to eliminating the original "single-phase γ coarse grain + (γ + α2 / γ) duplex fine grain" layered heterogeneous structure of the alloy, and ultimately obtaining a uniform duplex or near lamellar structure after the method of the present application.

[0027] The present application uses a simple process to convert the original layered heterogeneous structure of the electron beam 3D printed TiAl alloy into a uniform duplex or near lamellar structure. The uniformity of the microstructure of the alloy is improved, and at the same time, the mechanical property difference of the material perpendicular to the printing direction and parallel to the printing direction is significantly reduced, and the strength difference is less than 5%.

[0028] The process of the present application is simple, has low requirements on equipment, and is suitable for industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 Layered heterogeneous structure of the electron beam 3D printed TiAl alloy.

[0030] Figure 2 Uniform microstructure of the TiAl alloy ingot obtained in Example 1.

[0031] Figure 3 Uniform microstructure of the TiAl alloy ingot obtained in Example 2.

[0032] Figure 4 The non-uniform microstructure of the TiAl alloy ingot obtained in Comparative Example 1. Detailed Implementation

[0033] Example 1

[0034] An alloy with the composition Ti-48Al-2Cr-2Nb (at.%) was selected as the billet for electron beam 3D printing. The billet's height × width × length dimensions were 100mm × 80mm × 80mm. Its layered heterogeneous microstructure, characterized by "single-phase γ coarse grains + (γ+α2 / γ) dual-state fine grains," was as follows: Figure 1 As shown. First, the printed loose layer (2mm) on the surface of the billet is removed by grinding. Then, a high-temperature resistant and anti-oxidation coating is applied to the surface of the billet, and the entire billet is covered with high-temperature asbestos. The covered billet is placed in a heating furnace at 1200℃ and held for 3 hours. After the holding period, unidirectional forging is performed on a forging press. Before forging, low-carbon steel shims heated to 900℃±10℃ and with a thickness of 25mm are placed on the upper and lower end faces of the billet. The forging speed is 1s. -1 The total deformation including the gasket is 40%; after forging, asbestos is wrapped and naturally cooled to obtain an ingot with a uniform bimorphic structure, the microstructure of which is as follows: Figure 2 As shown, the yield strength of the treated ingot under 800℃ high-temperature tensile testing along the printing direction is 460MPa and the tensile strength is 538MPa. The yield strength perpendicular to the printing direction is 468MPa and the tensile strength is 540MPa. The maximum difference in its mechanical properties is 1.7%.

[0035] Example 2

[0036] An alloy with the composition Ti-48Al-2Cr-2Nb (at.%) was selected as the billet for electron beam 3D printing. The billet's height × width × length dimensions were 100mm × 80mm × 80mm, and its microstructure was a layered heterogeneous inhomogeneous structure consisting of "single-phase γ coarse grains + (γ+α2 / γ) biphase fine grains" (and...). Figure 1 (Similar). First, the printed loose layer (2mm) on the surface of the billet is removed by grinding. Then, a high-temperature resistant and anti-oxidation coating is applied to the surface of the billet, and the entire billet is covered with high-temperature asbestos. The covered billet is placed in a heating furnace at 1220℃ and held for 2 hours. After the holding time, unidirectional forging is performed on a forging press. Before forging, low-carbon steel shims heated to 900℃±10℃ and with a thickness of 30mm are placed on the upper and lower end faces of the billet. The forging speed is 3s. -1, the total deformation amount including the gasket is 42%; after the forging is finished, the asbestos is naturally cooled to obtain the ingot billet with the uniform "double-state" structure. The yield strength of the treated ingot billet is 475 MPa, and the tensile strength is 546 MPa along the printing direction at 800 DEG C high temperature stretching, the yield strength is 487 MPa, and the tensile strength is 552 MPa perpendicular to the printing direction, and the maximum difference of the mechanical properties is 2.5%.

[0037] Example 3

[0038] The alloy with the composition of Ti-48Al-2Cr-2Nb (at.%) selected by electron beam 3D printing is used as the blank, the size of the blank is 100mmx80mmx80mm, and the microstructure thereof is a layered heterogeneous structure of "single-phase gamma coarse crystal + (gamma + alpha2 / gamma) double-state fine crystal" (similar to Figure 1 The printing loose layer (2mm) on the surface of the blank is first polished and removed, then a high-temperature oxidation-resistant coating layer is coated on the surface of the blank, and the whole is coated with high-temperature asbestos, the coated ingot billet is placed in a heating furnace at 1230 DEG C for 1 hour, after the heat preservation is finished, one-way forging is carried out on the forging press, a low-carbon steel gasket heated at 900 DEG C ± 10 DEG C is added on the upper and lower end surfaces of the blank before forging, the thickness of the gasket is 35mm, and the forging rate is 5s -1 , the total deformation amount including the gasket is 45%; after the forging is finished, the asbestos is naturally cooled to obtain the ingot billet with the uniform "near lamellar" structure, and the microstructure thereof is shown in Figure 3 The yield strength of the treated ingot billet is 510 MPa, and the tensile strength is 566 MPa along the printing direction at 800 DEG C high temperature stretching, the yield strength is 527 MPa, and the tensile strength is 575 MPa perpendicular to the printing direction, and the maximum difference of the mechanical properties is 3.3%.

[0039] Comparative Example 1

[0040] The alloy with the composition of Ti-48Al-2Cr-2Nb (at.%) selected by electron beam 3D printing is used as the blank, the size of the blank is 100mmx80mmx80mm, and the microstructure thereof is a layered heterogeneous structure of "single-phase gamma coarse crystal + (gamma + alpha2 / gamma) double-state fine crystal" (similar to Figure 1 The printing loose layer (2mm) on the surface of the blank is first polished and removed, then a high-temperature oxidation-resistant coating layer is coated on the surface of the blank, and the whole is coated with high-temperature asbestos, the coated ingot billet is placed in a heating furnace at 1150 DEG C for 3 hours, after the heat preservation is finished, one-way forging is carried out on the forging press, a low-carbon steel gasket heated at 900 DEG C ± 10 DEG C is added on the upper and lower end surfaces of the blank before forging, the thickness of the gasket is 30mm, and the forging rate is 0.5s -1, the total deformation amount including the gasket is 40%; after the forging is completed, the asbestos is naturally cooled, because the forging temperature is low and the forging strain rate is small, the alloy cannot complete phase transformation and recrystallization, and the obtained structure still presents the characteristics of "single-phase γ coarse crystal + (γ + α2 / γ) bimodal fine crystal", which is only slightly refined compared with the structure before forging, and still has the characteristics of uneven structure, as shown in Figure 4 The yield strength of the ingot along the printing direction at a high temperature of 800 °C is 484 MPa, the tensile strength is 503 MPa, the yield strength perpendicular to the printing direction is 512 MPa, and the tensile strength is 546 MPa, and the maximum difference in mechanical properties is 8.5%.

[0041] Comparative Example 2

[0042] The alloy with a composition of Ti-48Al-2Cr-2Nb (at.%) selected by electron beam 3D printing is used as the blank, and the size of the blank is 100 mm x 80 mm x 80 mm, and the microstructure is a layered heterogeneous structure of "single-phase γ coarse crystal + (γ + α2 / γ) bimodal fine crystal" (similar to Figure 1 First, the printing loose layer (2 mm) on the surface of the blank is polished and removed, then a high-temperature oxidation-resistant coating is coated on the surface of the blank, and the whole blank is coated with high-temperature asbestos, and the blank after coating is placed in a heating furnace at 1280 °C for 3 hours, and after the heat preservation is completed, the blank is subjected to unidirectional forging on a forging press, a low-carbon steel gasket heated to 900 °C ± 10 °C is added on the upper and lower end surfaces of the blank before forging, the thickness of the gasket is 30 mm of the blank, and the forging rate is 10 s -1 , the total deformation amount including the gasket is 40%; after the forging is completed, the asbestos is naturally cooled, because the forging temperature is low and the forging strain rate is small, the alloy cannot complete phase transformation and recrystallization, and the obtained structure still presents the characteristics of "single-phase γ coarse crystal + (γ + α2 / γ) bimodal fine crystal", which is only slightly refined compared with the structure before forging, and still has the characteristics of uneven structure, as shown in

[0043] Comparative Example 3

[0044] The other conditions are the same as in Example 1, and only the upper and lower end surfaces of the blank are not provided with gaskets during the forging process. During unidirectional forging, the upper and lower end surfaces of the blank are in contact with the press head. On the one hand, the temperature of the upper and lower end surfaces of the blank is reduced due to heat transfer, and the deformation ability of the end surfaces is reduced. On the other hand, due to the friction between the end surfaces and the press head, the deformation of the end surfaces is blocked. Finally, the upper and lower end surfaces of the blank after forging have a difficult-to-deform area, and the structure does not change, so that the overall uniformity of the structure of the blank cannot be achieved.

[0045] Comparative Example 4

[0046] Other conditions are same with example 1, only change the thickness of the shim to 10mm, theoretically calculate, the difficult deformation zone thickness of the whole sample (blank+shim) after forging is far more than 10mm, about 15~20mm, it shows that part of the TiAl blank is still in the difficult deformation zone, the organization in the difficult deformation zone does not change, and the uniformity of the whole organization of the blank cannot be realized.

[0047] Comparative example 5

[0048] Other conditions are same with example 1, only control the total deformation amount of the blank with shim to 20%, the deformation amount of the blank is about 10%, because the deformation amount is too small, the coarse grain layer in the lamellar structure is not enough to occur dynamic recrystallization (dynamic recrystallization needs to reach the critical deformation amount to occur), and the organization refinement and the whole uniformity cannot be realized.

[0049] The above only describes the preferred embodiments of the present application, without departing from the principles of the present process innovation, any equivalent process transformation using the content of the present application specification, or direct or indirect use in other related technical fields, is considered as the patent protection range of the present application.

Claims

1. A method for eliminating the lamellar structure of TiAl alloy in electron beam 3D printing, characterized by: The electron beam 3D printed TiAl alloy is heated to 1200-1230 DEG C and kept for 1-3 hours, then low carbon steel gaskets are immediately placed on the upper and lower end surfaces of the electron beam 3D printed TiAl alloy, and then unidirectional forging is carried out to obtain a TiAl alloy ingot, the TiAl alloy ingot has a uniform duplex or near lamellar structure. The thickness of the low carbon steel gasket is 25-35% of the height of the electron beam 3D printed TiAl alloy. The temperature of the low carbon steel gasket is 890-910 DEG C. The rate of the one-way forging is 1~5s -1 The total deformation of the low-carbon steel gasket is 40~45%. The mechanical property difference of the TiAl alloy ingot perpendicular to the printing direction and parallel to the printing direction is less than 5%.

2. The method for eliminating layered structure of TiAl alloy in electron beam 3D printing according to claim 1, characterized in that: The electron beam 3D printed TiAl alloy has a gamma-TiAl intermetallic compound structure.

3. The method for eliminating layered structure of TiAl alloy in electron beam 3D printing according to claim 1, characterized in that: The microstructure of the electron beam 3D printed TiAl alloy is a layered heterogeneous structure of alternating distribution of single-phase gamma coarse crystals and (gamma + alpha2 / gamma) duplex fine crystals.

4. The method for eliminating layered structure of TiAl alloy in electron beam 3D printing according to any one of claims 1-3, characterized in that: The surface of the electron beam 3D printed TiAl alloy is first coated with an anti-oxidation coating, then wrapped with asbestos, and then heated.

5. The method for eliminating layered structure of TiAl alloy in electron beam 3D printing according to any one of claims 1-3, characterized in that: After unidirectional forging, the obtained blank is wrapped with asbestos and naturally cooled to obtain a TiAl alloy ingot.

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