A preparation method of TC4 titanium alloy forgings

Abstract

The present invention discloses a method for preparing TC4 titanium alloy forgings. The raw material bar is blanked to produce bar segments. The bar segments are then sequentially forged through (α+β) zone pre-forging, pretreatment, (α+β) zone final forging, solution treatment, and aging treatment to produce TC4 alloy forgings with high impact toughness. This method achieves the transition between the α and β phases by adding a combined high-temperature and low-temperature pretreatment process. A pretreatment step is added before the final forging to adjust the content and microstructure of the primary α phase. This process, combined with final forging and solution aging heat treatment in the upper portion of the two-phase zone, produces TC4 titanium alloy forgings with high impact toughness while maintaining strength compared to conventional processes, meeting engine service requirements.

Description

Technical Field

[0001] The invention belongs to the field of metallurgy and relates to a titanium alloy hot processing technology, specifically a preparation method of a TC4 titanium alloy forging. Background Art

[0002] TC4 is a typical representative of the (α+β) two-phase alloy among titanium alloy materials. It is widely used in the aerospace field, primarily for the manufacture of disks, blades, structural components, and fasteners for aircraft engines. To achieve good overall performance, TC4 titanium alloy forgings are generally forged in the (α+β) two-phase region and then subjected to a solution-aging heat treatment to obtain a typical dual-state structure with an excellent balance of strength, plasticity, and toughness. However, as the strength and fatigue performance requirements of aircraft engine designs increase, conventional forging preparation methods increase the cooling rate to improve strength and fatigue performance. The secondary α phase in the structure becomes fine, needle-like, resulting in a decrease in impact toughness and plasticity.

[0003] Su Huabing et al. analyzed the effects of different heat treatment processes (annealing, solution treatment + aging) on the impact toughness of TC4 alloy blade forgings in the article "Effects of Forging and Heat Treatment Processes on the Microstructure and Impact Toughness of TC4 Titanium Alloy" in the 4th issue of Volume 38, 2023, pages 36-39 of "Heat Treatment" magazine. The impact toughness of the forgings under the cold and hot treatment process after annealing is the best, but the strength of the forgings under this process is obviously insufficient.

[0004] In the article "Microstructure and Impact Toughness of TC4 Titanium Alloy," published in the June 2013 issue of Hot Working Technology, Liu Jianqiang conducted various heat treatments on TC4 titanium, achieving a variety of microstructures. The equiaxed + massive dual-morph microstructure, obtained through a heat treatment process combining β-solution (air cooling) + solution (furnace cooling) + aging (air cooling), exhibited the highest impact energy. However, due to its coarse Widmanstätten microstructure, it was clear that high impact toughness was achieved at the expense of strength, making it unsuitable for forging applications.

[0005] Therefore, it is urgent to develop a preparation method for TC4 alloy forgings to further improve the impact toughness and plasticity without sacrificing strength performance, so as to meet the service requirements of aircraft engines. Summary of the Invention

[0006] In view of the above technical problems in the prior art, the present invention provides a method for preparing a TC4 titanium alloy forging. The method for preparing the TC4 titanium alloy forging is intended to solve the technical problem of insufficient strength of the TC4 titanium alloy in the prior art.

[0007] The present invention provides a method for preparing a TC4 alloy forging, comprising the following steps:

[0008] 1) Blanking the raw material bar to obtain bar segments;

[0009] 2) a pre-forging step in the (α+β) region: the rod segment described in step 1) is forged at a heating temperature of Tβ-(30-70)°C, a heating coefficient of (0.5-1) min / mm, and a total deformation amount controlled between 40% and 80%, to obtain an intermediate billet A;

[0010] 3) a pretreatment step: pretreating the intermediate billet A described in step 2), and then air-cooling the intermediate billet to room temperature to obtain an intermediate billet B;

[0011] 4) a step of performing final forging in the (α+β) region: performing one heat final forging on the intermediate billet B described in step 3), wherein the heating temperature of the final forging is Tβ-(30-60)°C, the heating coefficient is (0.5-1) min / mm, the total deformation is controlled between 40% and 80%, and the forging is cooled to room temperature to obtain a forging C;

[0012] 5) a solution treatment step: performing a solution treatment on the forging C in step 4), wherein the solution heating temperature is Tβ-(30-50)°C, the holding time is (2-4) hours, and the forging is removed from the furnace and water-cooled to room temperature to obtain the forging D;

[0013] 6) An aging treatment step: performing aging treatment on the forging D described in step 5) at an aging temperature of 520°C to 580°C for a holding time of (2 to 4) hours, and then air-cooling to room temperature to obtain a TC4 alloy forging with high impact toughness.

[0014] Furthermore, the microstructure of the raw material bar in step 1) is an equiaxed structure, wherein the content of the β-transformed structure does not exceed 30%, and the size of the equiaxed α phase is ≤30 μm.

[0015] Furthermore, the forging speed in step 2) is 5-10 mm / s, which can reduce the temperature rise while fully ensuring the deformation amount and deformation efficiency.

[0016] Furthermore, the pretreatment in step 3) adopts a gradient heating method, which consists of a high-temperature heating process + a low-temperature heating process. The temperature of the high-temperature heating process is: Tβ-(40~60)℃ / (2~4)h, and the temperature of the low-temperature heating process is: (650℃~700℃) / (4~8)h.

[0017] Preferably, the forging speed in step 4) is 0.05-5 mm / s, which can fully ensure complete metal filling, avoid defects, and fully achieve dynamic recovery and dynamic recrystallization during the forming process;

[0018] Further preferably, in step 4), water cooling is immediately performed after forging is completed, and the transfer time after forging is ≤25s, so as to reduce the exposure time of the blank to the air, increase the final forging temperature of the forging, and ensure the water quenching effect;

[0019] Preferably, the water cooling transfer time after the solution treatment in step 5) is ≤30s, which can control the morphology and content of the α phase.

[0020] Specifically, Tβ refers to the β phase transition point of titanium alloy, indicating that above the temperature T, it is all β phase, and below the temperature T, the β phase gradually begins to transform into the α phase. If Tβ is 1000℃, Tβ-50℃ refers to 50℃ below the β phase transition point of titanium alloy, that is, 950℃.

[0021] Specifically, the element content (mass fraction) of the raw material of the TC4 titanium alloy bar conforms to the standard range of GB / T 3620.1-2016.

[0022] The present invention sequentially subjects the raw material bar to (α+β) zone pre-forging, pretreatment, (α+β) zone final forging, solution treatment and aging treatment, realizes the transformation between α phase and β phase by adding a high-temperature + low-temperature combined pretreatment process, adds a pretreatment process before the final forming of the forging to adjust the content of the primary α phase (ratio 15%-40%) and the microstructure (primary α phase size: 10-15 μm, secondary α phase lamella thickness: 2-4 μm, primary α phase equiaxility: aspect ratio ≤1.2), and then combines the final forging and solution aging heat treatment in the upper part of the two-phase zone. Compared with the traditional process, the TC4 titanium alloy forging with high impact toughness is obtained while ensuring the strength.

[0023] In the preparation method of the present invention:

[0024] Titanium alloy forgings have very high requirements for strength, plasticity, and impact toughness. Under existing forging and heat treatment requirements, their strength has essentially reached the material's upper limit through post-forging water cooling and solution heat treatment. It is currently known that microstructural morphology significantly influences impact toughness and plasticity, especially the thickness of the secondary α phase, which has a certain relationship with impact toughness. Under multiple water cooling conditions, needle-shaped martensite α' precipitates within the intragranular structure and decomposes into a stable secondary needle-shaped α phase after aging. During deformation, stress concentration is easily formed at the tips of the needle-shaped α phase. When subjected to stress, cracks form at the needle-shaped tips until they break, reducing impact toughness.

[0025] On the other hand, when the equiaxed morphology of the primary α phase is good (ideal aspect ratio 1:1), the content increases, and the content of secondary needle-shaped α decreases, the orientations of the equiaxed α phases and between the equiaxed α phases and the matrix are arbitrary, the chances of deformation coordination increase, the coordination is good, and it contributes to providing impact toughness to a certain extent.

[0026] The present invention achieves the transformation between the α phase and the β phase by adding pretreatment before final forging and heat preservation at high and low temperatures. Since the titanium alloy will produce nucleation points of new grains, such as grain boundaries, dislocations, vacancies, stacking faults, etc., during the heating and cooling process, these defects will serve as the locations of preferential nucleation or fracture of the original long strip α phase when the high temperature section of the pretreatment is first heat-insulated. In this process, the spherical α phase uses distortion energy as the driving force for nucleation and growth, and static recrystallization behavior occurs. As new grains are generated, the original long strip α grains are truncated. When the low temperature section of the pretreatment is heat-insulated, new defects are formed, which serve as nucleation points for the next round of primary α phase spheroidization.

[0027] At the same time, the present invention utilizes the different specific volume differences between the α phase and the β phase. During the heat preservation process of the pretreatment high-temperature section, due to the tip effect, the α phase will first begin to dissolve and transform into the β phase from a position with a relatively small contact area, further promoting the equiaxialization of the primary α phase.

[0028] Because a large number of defects accumulate within the microstructure after pretreatment, the distortion energy within it becomes the driving force for dynamic recrystallization nucleation during the final forging process. Forging deformation based on this, on the one hand, promotes element diffusion, phase dissolution, and growth of undissolved phases in the α and β phases; on the other hand, it increases the degree of kinking, bending, and fracture of the lamellar α phase, and with it, the number of defects. These defects and deformation energy storage become the nucleation points and driving force for dynamic recrystallization, thereby significantly improving the equiaxed effect of the α phase.

[0029] Finally, the volume fraction of the primary α phase is controlled by high-temperature solution treatment to obtain the metastable β phase, and then the metastable β phase is fully decomposed and the internal stress is completely eliminated through low-temperature aging heat treatment. The structure is close to the equilibrium state and a certain degree of aging strengthening is produced to ensure the structural stability of the alloy during long-term service.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The α and β phases in titanium alloys exhibit a strict crystallographic orientation relationship, resulting in a strong "heritability" within the structure. Conventional forging deformation cycles and heat treatments are difficult to alter. The forging preparation method provided by the present invention uses pretreatment to adjust and control the microstructure to produce an equiaxed primary α phase with excellent spheroidization and a secondary α phase with a defined lamellar thickness. This achieves high impact toughness while maintaining strength. Specifically, the forging exhibits room temperature strength ≥950 MPa, elongation ≥15%, reduction of area ≥40%, and impact toughness ≥38 J across all parts. This superior overall performance meets even higher service performance requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1This is a microstructure photograph of the TC4 titanium alloy forging in Example 1 of the present invention.

[0033] Figure 2 This is a microstructure photograph of the TC4 titanium alloy forging in Example 2 of the present invention.

[0034] Figure 3 This is a microstructure photograph of the TC4 titanium alloy forging in the comparative example of the present invention. DETAILED DESCRIPTION

[0035] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0036] Example 1

[0037] The present invention provides a method for preparing a TC4 alloy forging, which comprises the following steps:

[0038] 1) The Φ210 mm forged bar was sawn and blanked to obtain rod segments with a size of Φ210×580 mm and a weight of 90 kg. The phase transition point Tβ was measured by metallographic method to be 1000°C. The microstructure of the raw material bar was equiaxed, with the β transformation structure content not exceeding 30%, and the size of the equiaxed α phase was 20 μm.

[0039] 2) (α+β) zone pre-forging: The rod segment described in step 1) is forged at a heating temperature of 930°C, a heating coefficient of 0.5 min / mm, a heating time of 105 min, and one forging pass with a deformation of 40% at a forging speed of 5 mm / s to obtain an intermediate billet A;

[0040] 3) Pretreatment: Pretreatment of the intermediate billet A described in step 2) at 940°C for 2 hours and 650°C for 4 hours, followed by air cooling to room temperature to obtain intermediate billet B;

[0041] 4) (α+β) zone final forging: The intermediate billet B described in step 3) is subjected to final forging, wherein the heating temperature for the final forging is 940°C, the heating coefficient is 0.5 min / mm, the heating time is 105 min, the deformation is controlled to be 40%, and the forging is transferred to water within 15 seconds after forging and water-cooled to room temperature to obtain a forging C;

[0042] 5) Solution treatment: The forging C in step 4) is subjected to solution treatment at a solution heating temperature of 950° C. for a holding time of 2 h. Within 20 seconds after exiting the furnace, the forging is transferred into water and water-cooled to room temperature to obtain a forging D.

[0043] 6) Aging treatment: The forging D in step 5) is subjected to aging treatment at an aging temperature of 520° C. for 2 h, and then air-cooled to room temperature after being taken out of the furnace.

[0044] Example 2

[0045] The present invention provides a method for preparing a TC4 alloy forging, which comprises the following steps:

[0046] 1) The Φ310 mm forged bar was sawn and blanked to obtain rod segments with a size of Φ310×830 mm and a weight of 290 kg. The phase transition point Tβ was measured by metallographic method to be 1000°C. The microstructure of the raw material bar was equiaxed, with the β transformation structure content not exceeding 30%, and the size of the equiaxed α phase ≤30 μm.

[0047] 2) (α+β) zone pre-forging: The rod segment described in step 1) is forged at a heating temperature of 960°C, a heating coefficient of 1 min / mm, a heating time of 310 min, and a forging speed of 10 mm / s with an 80% deformation in one fire to obtain an intermediate billet A;

[0048] 3) Pretreatment: Pretreatment of the intermediate billet A described in step 2) at 960°C for 4 hours and 700°C for 8 hours, followed by air cooling to room temperature to obtain intermediate billet B;

[0049] 4) (α+β) zone final forging: The intermediate billet B described in step 3) is subjected to final forging, wherein the heating temperature for the final forging is 970°C, the heating coefficient is 1 min / mm, the heating time is 310 min, the deformation is controlled to be 70%, and the forging is transferred to water within 25 seconds after forging and water-cooled to room temperature to obtain a forging C;

[0050] 5) Solution treatment: The forging C in step 4) was subjected to solution treatment at a solution heating temperature of 970° C. for a holding time of 4 h. The forging was transferred into water within 30 s after being taken out of the furnace and water-cooled to room temperature to obtain the forging D.

[0051] 6) Aging treatment: The forging D in step 5) is subjected to aging treatment at an aging temperature of 580° C. for 4 hours, and then air-cooled to room temperature after being taken out of the furnace.

[0052] The TC4 alloy forgings obtained in the examples of the present invention were sampled and inspected, and the mechanical properties of the forgings were all qualified, meeting the design standard requirements. The mechanical properties data of Examples 1 and 2 are shown in Table 1.

[0053] Comparative Example

[0054] This example is a comparative example of Example 2, which includes the following steps:

[0055] 1) The raw material bar is blanked to obtain rod segments with a size of Φ310×830 mm and a weight of 290 kg. The phase transition point Tβ is measured by metallographic method to be 1000°C. The microstructure of the raw material bar is equiaxed, with the β transformation structure content not exceeding 30%, and the size of the equiaxed α phase ≤30 μm.

[0056] 2) (α+β) zone pre-forging: The rod segment described in step 1) is forged at a heating temperature of 960°C, a heating coefficient of 1 min / mm, a heating time of 310 min, and a forging speed of 20 mm / s with a deformation of 80% in one fire to obtain an intermediate billet A;

[0057] 3) (α+β) zone final forging: The intermediate billet B described in step 3) is subjected to final forging, wherein the heating temperature for the final forging is 970°C, the heating coefficient is 1 min / mm, the heating time is 310 min, the deformation is controlled to be 70%, and the forging is transferred to water within 25 seconds after forging and water-cooled to room temperature to obtain a forging C;

[0058] 4) Solution treatment: The forging C in step 4) was subjected to solution treatment at a solution heating temperature of 970° C. for a holding time of 4 h. The forging was transferred into water within 30 s after being taken out of the furnace and water-cooled to room temperature to obtain a forging D.

[0059] 5) Aging treatment: The forging D in step 5) is subjected to aging treatment at an aging temperature of 580° C. for 4 hours, and then air-cooled to room temperature after being taken out of the furnace.

[0060] The TC4 alloy forgings obtained in the embodiment of the present invention were sampled and inspected, and the mechanical property test results thereof were all qualified, meeting the design standard requirements.

[0061] The mechanical properties data of Examples 1-2 and Comparative Example are shown in Table 1.

[0062] Table 1

[0063]

[0064] As can be seen from the above table, the present invention improves the elongation, cross-sectional shrinkage and room temperature impact toughness of various parts of the forging by adding pretreatment before final forging.

Claims

1. A method for preparing TC4 titanium alloy forgings, characterized in that The following steps are involved: 1) Blanking the raw material bar to obtain bar segments; 2) a pre-forging step in the (α+β) region: the rod segment described in step 1) is forged at a heating temperature of Tβ-(30-70)°C, a heating coefficient of 0.5-1 min / mm, and a total deformation amount controlled between 40% and 80%, to obtain an intermediate billet A; 3) a pretreatment step: pretreating the intermediate billet A described in step 2), and then air-cooling it to room temperature to obtain an intermediate billet B; the pretreatment comprises a high-temperature heating process and a low-temperature heating process, wherein the high-temperature heating process is performed at a temperature of Tβ-(40-60)°C / (2-4) hours, and the low-temperature heating process is performed at a temperature of (650-700°C) / (4-8) hours; 4) a step of performing final forging in the (α+β) region: performing one heat final forging on the intermediate billet B described in step 3), wherein the heating temperature for the final forging is Tβ-(30-60)°C, the heating coefficient is 0.5-1 min / mm, the total deformation is controlled between 40% and 80%, and the forging is cooled to room temperature to obtain a forging C; 5) a solution treatment step: performing a solution treatment on the forging C in step 4), wherein the solution heating temperature is Tβ-(30-50)°C, the holding time is 2-4 hours, and the forging is removed from the furnace and water-cooled to room temperature to obtain the forging D; 6) An aging treatment step: performing aging treatment on the forging D described in step 5) at an aging temperature of 520° C. to 580° C. for 2 to 4 hours, and then air-cooling to room temperature to obtain a TC4 titanium alloy forging with high impact toughness.

2. The method for preparing a TC4 titanium alloy forging according to claim 1, wherein: The microstructure of the raw material bar in step 1) is an equiaxed structure, wherein the content of the β-transformed structure does not exceed 30%, and the size of the equiaxed α phase is ≤30 μm.

3. The method for preparing a TC4 titanium alloy forging according to claim 1, wherein: The forging speed in step 2) is 5~10mm / s.

4. The method for preparing a TC4 titanium alloy forging according to claim 1, wherein: The forging speed in step 4) is 0.05~5mm / s.

5. The method for preparing a TC4 titanium alloy forging according to claim 1, wherein: Step 4) After forging is completed, water cooling is carried out immediately, and the transfer time after forging is ≤25s.

6. The method for preparing a TC4 titanium alloy forging according to claim 1, wherein: In step 5), the water cooling transfer time after solution treatment is ≤30s.

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