Pseudo-beta forging process of high toughness titanium alloy

Abstract

The application discloses a pseudo-beta forging process of high-strength and high-toughness titanium alloy, and the process comprises the following steps: heating a high-strength and high-toughness titanium alloy forging blank to above a phase transition point and keeping warm, slowly cooling the forging blank in a furnace to below the phase transition point and keeping warm, and then performing forging processing to obtain a relatively coarse lamellar alpha phase structure with deformation and staggered arrangement. The heating mode of the application ensures that the high-strength and high-toughness titanium alloy with high molybdenum equivalent obtains regularly arranged lamellar alpha phases with large size, and the deformation and fracture of the lamellar alpha phases are combined with the forging processing to form the relatively coarse lamellar alpha phase structure with deformation and staggered arrangement, so that the conflict between the strength, plasticity and fracture toughness and the microstructure requirements of the titanium alloy can be effectively coordinated after subsequent heat treatment, the comprehensive mechanical properties of the titanium alloy are improved, the high-strength and high-toughness titanium alloy with high molybdenum equivalent cannot form lamellar alpha phases with large size due to high beta stability coefficient and low phase transition point, the plasticity and fracture toughness are poor, and the high-strength titanium alloy cannot meet the high damage tolerance performance requirements under the high-strength state.

Description

Technical Field

[0001] This invention belongs to the field of hot working technology of titanium alloy materials, and specifically relates to a pseudo-β forging process for high-strength and high-toughness titanium alloys. Background Technology

[0002] High-strength and high-toughness titanium alloys are one of the most important branches in the field of titanium alloys. They are mainly used in the main load-bearing structural components of aviation and aerospace, and are an indispensable key material for high-end aircraft.

[0003] Forging is one of the important means to improve the microstructure and properties of high-strength and high-toughness titanium alloys. Through forging, grains can be effectively refined, microstructure morphology can be controlled, and the mechanical properties of the alloy can be significantly improved. The service environment of high-strength and high-toughness titanium alloys requires the alloy to simultaneously meet the comprehensive mechanical property requirements of high strength, high plasticity, and high toughness, but it is difficult to coordinate the requirements of these properties with the microstructure. The mutual constraint between microstructure and properties is the bottleneck of hot working and heat treatment technology for high-strength and high-toughness titanium alloys. In order to coordinate the contradiction between microstructure and various properties, the hot working of high-strength and high-toughness titanium alloys often has certain special characteristics and complexities, and it is desirable to obtain a "compromise" microstructure. To this end, for the processing technology of high-strength and high-toughness titanium alloys, scholars at home and abroad have proposed "β forging", "near β forging", "quasi-β forging" and "post-forging water cooling" technologies, mainly to control the microstructure. "β forging" can obtain lamellar Widmanstätten or basketweave microstructure, "near β forging" can obtain a three-state microstructure with about 15% primary α phase, and "quasi-β forging" can obtain a basketweave microstructure with cross-lamellae. These structures can, to some extent, reconcile the contradictions between strength, plasticity, and toughness. However, for high-strength and high-toughness titanium alloys with a Mo equivalent in the range of 12 to 15, it is often difficult to achieve ideal results. This is mainly because the Mo equivalent of this type of high-strength and high-toughness titanium alloy is relatively high, and traditional processing techniques make it difficult to obtain large-sized α-lamellae, which is detrimental to the fracture toughness and resistance to fatigue propagation of the alloy. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide a pseudo-β forging process for high-strength and high-toughness titanium alloys, addressing the shortcomings of the prior art. This process employs a heating method where a high-strength and high-toughness titanium alloy forging billet is heated above its phase transformation point and held at that temperature, then slowly cooled in the furnace to below the phase transformation point and held at that temperature. Combined with forging, this forms a relatively coarse lamellar α-phase microstructure with deformation and interlacing. Subsequent heat treatment effectively reconciles the conflicting requirements of strength, plasticity, and fracture toughness on the microstructure of the titanium alloy, improving the overall mechanical properties of the titanium alloy and solving the problem that excessively high Mo equivalent prevents the formation of relatively coarse lamellar α-phase.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a pseudo-β forging process for high-strength and high-toughness titanium alloy, characterized in that the specific process is as follows: heating the high-strength and high-toughness titanium alloy forging billet to above the phase transformation point and holding it at that temperature, slowly cooling it in the furnace to below the phase transformation point and holding it at that temperature, and then forging it to obtain a relatively coarse lamellar α phase structure with deformation and interlaced arrangement.

[0006] For high-strength and high-toughness titanium alloys with high Mo equivalent, due to their high β stability coefficient and low phase transformation point, traditional forging processes cannot form thicker and coarser lamellar α phases, especially for high-strength and high-toughness titanium alloys with Mo equivalent of 12-15. The heat treatment process of β annealing with slow cold aging can solve the above problems and obtain larger α phases. However, the α phases obtained by this heat treatment process are regularly arranged, which is not conducive to improving the overall mechanical properties. To address this, the present invention heats a high-strength and high-toughness titanium alloy forging billet above its phase transformation point and holds it at that temperature, then slowly cools it in the furnace to below its phase transformation point and holds it at that temperature. This heating method effectively ensures that high-strength and high-toughness titanium alloys with high molybdenum equivalents, especially those with Mo equivalents of 12-16, obtain large-sized, regularly arranged lamellar α phases. Subsequent forging ensures that these regularly arranged α phases undergo a certain degree of deformation and fracture, thereby forming a relatively coarse lamellar α phase structure with deformation and staggered arrangement. After subsequent heat treatment, the conflict between strength, plasticity, and fracture toughness requirements on the alloy's microstructure can be effectively reconciled, improving the alloy's comprehensive mechanical properties and achieving a good balance of strength, plasticity, and toughness.

[0007] like Figure 1 As shown, in the pseudo-β forging process of the high-strength and high-toughness titanium alloy of this invention, the high-strength and high-toughness titanium alloy undergoes a process ABCDEF from heating to cooling, then to forging and cooling. Point A represents the initial state of the high-strength and high-toughness titanium alloy forging billet, corresponding to the original microstructure of the forging billet. Point B represents the state when heated above the phase transformation point. Point C represents the state of the high-strength and high-toughness titanium alloy forging billet after being heated to the β single-phase region above the phase transformation point and held at that temperature, corresponding to a full β microstructure. Point D represents the state after being slowly cooled in the furnace to below the phase transformation point and held at that temperature, corresponding to a lamellar α phase microstructure. DE represents the forging stage. Point E represents the state after forging, and the corresponding microstructure diagram shows that after forging, the lamellar α phase and grain boundaries are broken, forming a microstructure with large-sized lamellar α phases arranged in an alternating pattern. F represents the state after cooling to room temperature after forging. Figure 1 As shown in the phase diagram, the heating start temperature of the forging process of this invention is in the β phase region, while the actual forging temperature is in the dual-state region where the furnace cools down (the area between the two black dots in the phase diagram is the thickened line segment). Therefore, it is called "pseudo-β forging". The pseudo-β forging process of this invention is significantly different from the existing forging process. For high-strength and high-toughness titanium alloys with high Mo equivalent, it creates a microstructure with large-sized lamellar α-phases arranged in an alternating pattern, giving it a good balance of strength, plasticity, and toughness.

[0008] The aforementioned pseudo-β forging process for a high-strength and tough titanium alloy is characterized in that the high-strength and tough titanium alloy forging billet is a near-β or metastable β titanium alloy with a bimodal structure and fine grains, and the Mo equivalent is 12-16. Since the process of this invention involves the design forging of the final microstructure of the finished part, a bimodal structure forging billet with sufficiently refined grains is selected as the raw material. Simultaneously, the Mo equivalent of the high-strength and tough titanium alloy forging billet is controlled to be 12-16. When the Mo equivalent is too low, large lamellar α phases can be formed without the slow furnace cooling process of this invention. When the Mo equivalent is too high, due to the high stability of the β phase, even slow cooling cannot form large lamellar α phases. Therefore, the process of this invention is suitable for high-strength and tough titanium alloy forging billets with a Mo equivalent of 12-16.

[0009] The aforementioned pseudo-β forging process for a high-strength and high-toughness titanium alloy is characterized in that the high-strength and high-toughness titanium alloy forging billet is heated to 20°C to 40°C above the phase transformation point and held at that temperature for T1 minutes, where T1 = D / 2 + (20~40), and D is the equivalent diameter of the cross-section of the high-strength and high-toughness titanium alloy forging billet in mm. This invention ensures sufficient heating of the high-strength and high-toughness titanium alloy forging billet by controlling the holding time after heating.

[0010] The aforementioned pseudo-β forging process for high-strength and high-toughness titanium alloys is characterized in that the high-strength and high-toughness titanium alloy forging billet is heated above the phase transformation point and held at that temperature before being slowly cooled in the furnace to 20°C to 35°C below the phase transformation point and held at that temperature for T2 minutes, where T2 = D / 4 + (10~20), and D is the equivalent diameter of the cross-section of the high-strength and high-toughness titanium alloy forging billet in mm. This invention ensures sufficient cooling of the high-strength and high-toughness titanium alloy forging billet by controlling the holding time after cooling.

[0011] The aforementioned pseudo-β forging process for high-strength and high-toughness titanium alloys is characterized by a slow cooling rate of 0.2℃ / min to 1.2℃ / min, which decreases as the Mo equivalent of the high-strength and high-toughness titanium alloy forging billet increases. Since the β phase in high-strength and high-toughness titanium alloy forging billets with high Mo equivalent exhibits strong stability, a slow cooling rate is required for the precipitation of larger α phases. This invention ensures that the larger α phases in high-strength and high-toughness titanium alloy forging billets with high Mo equivalent and strong β phase stability are fully precipitated through a slower cooling rate and a longer cooling time by controlling the slow cooling rate to decrease as the Mo equivalent of the high-strength and high-toughness titanium alloy forging billet increases.

[0012] The above-mentioned pseudo-β forging process for a high-strength and high-toughness titanium alloy is characterized in that the deformation amount of the forging process is 100% to 200%, and the forging is followed by air cooling to room temperature.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] 1. This invention employs a heating method that involves heating a high-strength and high-toughness titanium alloy forging billet above the phase transformation point and holding it at that temperature, then slowly cooling it in the furnace to below the phase transformation point and holding it at that temperature. This ensures that the high-strength and high-toughness titanium alloy with high molybdenum equivalent obtains large-sized, regularly arranged lamellar α phases. Combined with forging, the regularly arranged α phases undergo a certain degree of deformation and fracture, thereby forming a relatively coarse lamellar α phase structure with deformation and staggered arrangement. After subsequent heat treatment, this effectively coordinates the conflict between strength, plasticity, and fracture toughness requirements on the microstructure of the titanium alloy, thereby improving the comprehensive mechanical properties of the titanium alloy.

[0015] 2. The process of this invention is simple and easy to operate, enabling high-molybdenum equivalent high-strength and high-toughness titanium alloys to achieve a good balance of strength, plasticity, and toughness.

[0016] 3. The staggered and relatively coarse lamellar α-phase structure obtained by the present invention is a novel forging structure.

[0017] 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

[0018] Figure 1 This is a simulation diagram of the pseudo-β forging process of the high-strength and high-toughness titanium alloy of the present invention, as well as the corresponding microstructure of each process stage.

[0019] Figure 2 This is a microstructure diagram of the Ti-1300 high-strength and high-toughness titanium alloy in Example 1 of the present invention after being slowly cooled in the furnace to below the phase transformation point and held at that temperature.

[0020] Figure 3 This is a microstructure diagram of the Ti-1300 high-strength and high-toughness titanium alloy after forging in Example 1 of the present invention. Detailed Implementation

[0021] Example 1

[0022] The specific process of this embodiment is as follows: a Ti-1300 high-strength and high-toughness titanium alloy forging billet with a Mo equivalent of 13.4 and a diameter of Φ200mm is heated to 30°C above the phase transformation point and held for 130 minutes. It is then slowly cooled in the furnace at a rate of 0.7°C / min to 28°C below the phase transformation point and held for 70 minutes. Then, it is forged with a deformation of 200% and air-cooled to room temperature after forging to obtain a relatively coarse lamellar α-phase structure with deformation and interlacing.

[0023] In this embodiment, the Ti-1300 high-strength and high-toughness titanium alloy, after forging, has a relatively coarse lamellar α-phase structure with deformation and interlacing. After subsequent heat treatment, it can effectively coordinate the strength, plasticity and fracture toughness of the titanium alloy and improve the comprehensive mechanical properties of the titanium alloy.

[0024] Figure 2This is a microstructure image of the Ti-1300 high-strength and high-toughness titanium alloy after it has been slowly cooled in the furnace to below its phase transformation point and held at that temperature. Figure 2 It can be seen that the tissue has large-sized, regularly arranged lamellar α phases.

[0025] Figure 3 This is a microstructure image of the Ti-1300 high-strength and high-toughness titanium alloy after forging in this embodiment. Figure 3 It can be seen that the α lamellae in this structure are relatively coarse and arranged in an interlaced manner, with no obvious grain boundary α phase, which is something that cannot be obtained by traditional forging processes.

[0026] Example 2

[0027] The specific process of this embodiment is as follows: a TB8 high-strength and high-toughness titanium alloy forging billet with a Mo equivalent of 15.1 and a diameter of Φ180mm is heated to 40°C above the phase transformation point and held for 130 minutes. It is then slowly cooled in the furnace at a rate of 0.2°C / min to 35°C below the phase transformation point and held for 70 minutes. Then, it is forged with a deformation of 150%. After forging, it is air-cooled to room temperature to obtain a relatively coarse lamellar α-phase structure with deformation and interlacing.

[0028] In this embodiment, the TB8 high-strength and high-toughness titanium alloy, after forging, has a relatively coarse lamellar α-phase structure with deformation and interlacing. After subsequent heat treatment, it can effectively coordinate the strength, plasticity, and fracture toughness of the titanium alloy and improve the comprehensive mechanical properties of the titanium alloy.

[0029] Example 3

[0030] The specific process of this embodiment is as follows: a Ti5321 high-strength and high-toughness titanium alloy forging billet with a Mo equivalent of 12.1 and a side length of □200mm is heated to 20°C above the phase transformation point and held for 120 minutes. It is then slowly cooled in the furnace at a rate of 1.2°C / min to 20°C below the phase transformation point and held for 60 minutes. Then, it is forged, with a deformation of 100%. After forging, it is air-cooled to room temperature to obtain a relatively coarse lamellar α-phase structure with deformation and interlacing arrangement.

[0031] In this embodiment, the Ti5321 high-strength and high-toughness titanium alloy, after forging, has a relatively coarse lamellar α-phase structure with deformation and interlacing arrangement. After subsequent heat treatment, it can effectively coordinate the strength, plasticity and fracture toughness of the titanium alloy and improve the comprehensive mechanical properties of the titanium alloy.

[0032] Example 4

[0033] The specific process of this embodiment is as follows: a Ti-1300 high-strength and high-toughness titanium alloy forging billet with a Mo equivalent of 13.4 and a diameter of Φ200mm is heated to 40°C above the phase transformation point and held for 130 minutes. It is then slowly cooled in the furnace at a rate of 0.8°C / min to 25°C below the phase transformation point and held for 70 minutes. Then, it is forged, with a deformation of 100%. After forging, it is air-cooled to room temperature to obtain a relatively coarse lamellar α-phase structure with deformation and interlacing arrangement.

[0034] In this embodiment, the Ti-1300 high-strength and high-toughness titanium alloy, after forging, has a relatively coarse lamellar α-phase structure with deformation and interlacing. After subsequent heat treatment, it can effectively coordinate the strength, plasticity and fracture toughness of the titanium alloy and improve the comprehensive mechanical properties of the titanium alloy.

[0035] 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 pseudo-β forging process for high-strength and high-toughness titanium alloys, characterized in that, The specific process of this technology is as follows: the high-strength and high-toughness titanium alloy forging billet is heated to above the phase transformation point and held at that temperature, then slowly cooled to below the phase transformation point in the furnace and held at that temperature, and then forged to obtain a relatively coarse lamellar α phase structure with deformation and interlacing arrangement. The high-strength and high-toughness titanium alloy forging billet is a near-β or metastable β titanium alloy with a fine-grained bimodal structure and a Mo equivalent of 12~16. The high-strength and high-toughness titanium alloy forging billet is heated to 20℃~40℃ above the phase transformation point and held for T1 min, where T1 = D / 2 + (20~40), and D is the equivalent diameter of the cross section of the high-strength and high-toughness titanium alloy forging billet in mm. After being heated to above the phase transformation point and held, the high-strength and high-toughness titanium alloy forging billet is slowly cooled in the furnace to 20℃~35℃ below the phase transformation point and held for T2 min, where T2 = D / 4 + (10~20), and D is the equivalent diameter of the cross section of the high-strength and high-toughness titanium alloy forging billet in mm. The slow cooling rate is 0.2℃ / min~1.2℃ / min, and the slow cooling rate decreases as the Mo equivalent of the high-strength and high-toughness titanium alloy forging billet increases.

2. The pseudo-β forging process for a high-strength and high-toughness titanium alloy according to claim 1, characterized in that, The deformation during the forging process is 100%~200%, and the material is air-cooled to room temperature after forging.

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