Preparation method of TC4 titanium alloy forge piece

Abstract

The invention discloses a preparation method of a TC4 titanium alloy forge piece. The preparation method comprises the following steps: blanking a raw material bar to obtain a bar section; and then the bar forging is sequentially subjected to (alpha + beta) area pre-forging, pretreatment, (alpha + beta) area finish forging, solution treatment and aging treatment, and the TC4 alloy forge piece with the high impact toughness is obtained. According to the method, conversion between the alpha phase and the beta phase is achieved by adding the high-temperature and low-temperature combined pretreatment process, the pretreatment procedure is added before final forming of the forge piece to adjust the content and the structure form of the primary alpha phase, and then finish forging and solid solution aging heat treatment on the upper portion of a two-phase region are combined. And the TC4 titanium alloy forge piece with high impact toughness is obtained on the premise that the strength is guaranteed, and the service requirement of an engine is met.

Description

Technical Field

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

[0002] TC4 is a typical representative of (α + β) two-phase alloys in titanium alloy materials. It is widely used in the aerospace field, mainly for manufacturing components such as disks, blades, structural parts, and fasteners of aeroengines. To obtain good comprehensive properties, TC4 titanium alloy forgings are generally heat-treated by solution + aging after forging in the (α + β) two-phase region to obtain a typical bimodal structure, which has good strength-plasticity and toughness matching. However, with the increasing requirements for strength and fatigue performance in aeroengine design, the conventional forging preparation method improves strength and fatigue performance by further increasing the cooling rate, and the secondary α-phase in the structure is fine needle-shaped, resulting in a decrease in impact toughness and plasticity.

[0003] Su Huabing et al. analyzed the influence of different heat treatment processes (annealing, solution + aging) on the impact toughness of TC4 alloy blade forgings in the literature "Influence of Forging and Heat Treatment Processes on the Microstructure and Impact Toughness of TC4 Titanium Alloy" on pages 36 - 39, Volume 38, Issue 4 of the journal Heat Treatment in 2023. The impact toughness is the best under the heat treatment process of furnace cooling after annealing, but the strength of the forgings under this process is significantly insufficient.

[0004] Liu Jianqiang carried out heat treatment of TC4 titanium alloy with different processes in the literature "Microstructure and Impact Resistance of TC4 Titanium Alloy" on pages 63 - 66, Volume 42, Issue 12 of the journal Hot Working Technology in June 2013, and obtained various tissue morphologies. The impact energy of the equiaxed + blocky bimodal structure obtained by the heat treatment process of β solution (air cooling) + solution (furnace cooling) + aging (air cooling) is the highest. However, because its original structure is a coarse Widmanstätten structure, it is obvious that it sacrifices strength performance in order to obtain high impact toughness, which does not meet the use requirements of forgings.

[0005] Therefore, there is an urgent need to develop a method for preparing a TC4 alloy forging to further improve impact toughness and plasticity without sacrificing strength performance, so as to meet the service requirements for aeroengines. 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, and the method for preparing a TC4 titanium alloy forging of the present invention aims 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: 1) Blanking the raw material bar to obtain bar segments; 2) A pre-forging step in the (α+β) region: Forging the bar segments described in step 1), the forging heating temperature is Tβ-(30~70)°C, the heating coefficient is (0.5~1) min / mm, and the total deformation is controlled between 40% and 80% to obtain intermediate blank A; 3) A pre-treatment step: Pre-treating the intermediate blank A described in step 2), and then air-cooling to room temperature to obtain intermediate blank B; 4) A final forging step in the (α+β) region: Performing one-pass final forging on the intermediate blank B described in step 3), the final forging heating temperature is Tβ-(30~60)°C, the heating coefficient is (0.5~1) min / mm, and the total deformation is controlled between 40% and 80%. After forging, it is cooled to room temperature to obtain forging C; 5) A solution treatment step: Performing solution treatment on the forging C described in step 4), the solution heating temperature is Tβ-(30~50)°C, the holding time is (2~4) h, and it is water-cooled to room temperature after being taken out of the furnace to obtain forging D; 6) An aging treatment step: Performing aging treatment on the forging D described in step 5), the aging temperature is 520°C~580°C, the holding time is (2~4) h, and it is air-cooled to room temperature after being taken out of the furnace to obtain a TC4 alloy forging with high impact toughness.

[0008] Furthermore, the microstructure of the raw material bar described in step 1) is equiaxed structure, the content of β transformation structure therein does not exceed 30%, and the size of its equiaxed α phase is ≤30μm.

[0009] Furthermore, the forging reduction rate in step 2) is 5~10mm / s, which can reduce the temperature rise while fully ensuring the deformation and deformation efficiency.

[0010] Furthermore, the pre-treatment 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)°C / (2~4) h, and the temperature of the low-temperature heating process is: (650°C~700°C) / (4~8) h.

[0011] Preferably, the forging reduction rate in step 4) is 0.05~5mm / s, which can fully ensure the complete filling of the metal and avoid defects, and fully realize dynamic recovery and dynamic recrystallization during the forming process; More preferably, in step 4), it is immediately water-cooled after forging, and the transfer time after forging is ≤25s, which reduces the exposure time of the blank in the air, increases the final forging temperature of the forging, and ensures the water quenching effect; Preferably, the water-cooling transfer time after solution treatment in step 5) ≤ 30 s, which can control the morphology and content of the α phase.

[0012] Specifically, Tβ refers to the β phase transformation point of the titanium alloy, indicating that above this temperature T is the full β phase, and below it, the transformation from the β phase to the α phase gradually begins. If Tβ is 1000 °C, Tβ - 50 °C refers to 50 °C below the β phase transformation point of the titanium alloy, that is, 950 °C.

[0013] Specifically, the elemental content (mass fraction) of the raw material of the TC4 titanium alloy bar meets the standard range of GB / T 3620.1 - 2016.

[0014] In the present invention, the raw material bar is successively subjected to (α + β) zone pre-forging, pre-treatment, (α + β) zone final forging, solution treatment, and aging treatment. By adding a pre-treatment process of high temperature + low temperature combination, the transformation between the α phase and the β phase is realized. Before the final forming of the forging, a pre-treatment process is added to adjust the content (proportion 15% - 40%) and microstructure morphology of the primary α phase (primary α phase size: 10 - 15 μm, secondary α phase lamellar thickness: 2 - 4 μm, primary α phase equiaxed degree: aspect ratio ≤ 1.2). Then, combined with the final forging and solution aging heat treatment in the upper part of the two-phase zone, compared with the traditional process, a TC4 titanium alloy forging with high impact toughness is obtained on the premise of ensuring strength.

[0015] In the preparation method described in the present invention: Titanium alloy forgings have very high requirements for strength, plasticity, and impact toughness. Under the existing forging and heat treatment requirements, through post-forging water cooling and solution heat treatment strengthening, its strength has basically reached the material limit. At present, it is known that the microstructure morphology has a great influence on its impact toughness and plasticity, especially the thickness of the secondary α phase has a certain relationship with the impact toughness. Under multiple water-cooling conditions, needle-shaped martensite α' precipitates in the intragranular structure and decomposes into stable secondary needle-shaped α phases after aging treatment. During the deformation process, stress concentration is easily formed at the two tips of the needle-shaped α, and cracks are generated at the tips of the needles during loading until fracture, resulting in a decrease in impact toughness.

[0016] On the other hand, when the equiaxed morphology of the primary α phase is good (ideal aspect ratio 1:1) and its content increases, and the content of the secondary needle-shaped α decreases, the orientations between the equiaxed αs and between the equiaxed α and the matrix are arbitrary, and the opportunity for deformation coordination increases, with good coordination, which makes a certain contribution to providing impact toughness.

[0017] In the present invention, by adding a pretreatment before final forging, the transformation between the α-phase and the β-phase is achieved through high- and low-temperature heat preservation. Since new grain nucleation sites, such as grain boundaries, dislocations, vacancies, stacking faults, etc., will be generated during the heating and cooling of titanium alloys, during the high-temperature section heat preservation of the pretreatment, these defect sites will serve as the positions for preferential nucleation or the fracture of the original long-strip α-phase. During this process, the spherical α-phase takes the distortion energy as the driving force for nucleation and growth, and static recrystallization occurs. With the generation of new grains, the original long-strip α-grains are truncated. When the low-temperature section of the pretreatment is heat-preserved, new defects are formed again, serving as the nucleation sites for the next round of spheroidization of the primary α-phase.

[0018] Meanwhile, the present invention utilizes the different specific volume differences between the α-phase and the β-phase. During the high-temperature section heat preservation of the pretreatment, due to the tip effect, the α-phase will first start to dissolve and transform into the β-phase from the position with a relatively small contact area, further promoting the equiaxedization of the primary α-phase.

[0019] Since a large number of defects are accumulated inside the structure after the pretreatment, the distortion energy therein becomes the driving force for dynamic recrystallization nucleation during the final forging process. On this basis, forging deformation is carried out. On the one hand, it can promote element diffusion, phase dissolution, and the growth of undissolved phases in the α-phase and the β-phase; on the other hand, it increases the kinking, bending, and fracture degrees of the lamellar α-phase, and the defects also increase. These defects and deformation energy storage become the nucleation sites and driving force for dynamic recrystallization, thus significantly improving the equiaxedization effect of the α-phase.

[0020] Finally, by controlling the volume fraction of the primary α-phase through high-temperature solution treatment, a metastable β-phase is obtained, and then through low-temperature aging heat treatment, the metastable β-phase is fully decomposed and the internal stress is completely eliminated. The structure is close to the equilibrium state, and a certain degree of aging strengthening is generated, ensuring the structural stability of the alloy during long-term service.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: There is a strict crystallographic orientation relationship between the α-phase and the β-phase in the titanium alloy structure. Therefore, it has a strong "heredity" in its structure, and it is very difficult to change its structure with a limited number of forging deformation times and conventional heat treatment processes under conventional processes. The forging preparation method provided by the present invention adjusts and controls the microstructure through pretreatment to equiaxed primary α-phase with very good spheroidization effect and secondary α-phase with a certain lamellar thickness, achieving high impact toughness on the premise of ensuring strength. Its specific manifestations are: the room temperature strength of each part of the forging ≥ 950 MPa, elongation ≥ 15%, reduction of area ≥ 40%, impact toughness ≥ 38 J, with excellent comprehensive performance, and can meet higher service performance requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a microstructural photograph of the TC4 titanium alloy forging in Example 1 of the present invention.

[0023] Figure 2 This is the microstructural photograph of the TC4 titanium alloy forging in Example 2 of the present invention.

[0024] Figure 3 This is the microstructural photograph of the TC4 titanium alloy forging in the comparative example of the present invention. Detailed implementation manners

[0025] The present invention will be further described below in conjunction with the embodiments and the accompanying drawings.

[0026] Example 1

[0027] The present invention provides a method for preparing a TC4 alloy forging, which includes the following steps: 1) Saw and cut a forged bar with a diameter of Φ210mm to obtain a bar segment. The bar segment has a specification of Φ210×580mm and a weight of 90kg. The phase transformation point Tβ = 1000°C is measured by the metallographic method. The microstructure of the raw material bar is equiaxed structure, and the content of β transformation structure therein does not exceed 30%. The size of the equiaxed α phase is 20μm.

[0028] 2) Pre-forging in the (α+β) region: Forge the bar segment in step 1). The forging heating temperature is 930°C, the heating coefficient is 0.5min / mm, the heating time is 105min, and it is forged with a deformation amount of 40% in 1 heat treatment at a forging speed of 5mm / s to obtain an intermediate blank A; 3) Pretreatment: Perform pretreatment on the intermediate blank A in step 2). The pretreatment process is 940°C / 2h + 650°C / 4h, and it is air-cooled to room temperature to obtain an intermediate blank B; 4) Final forging in the (α+β) region: Perform final forging on the intermediate blank B in step 3). The final forging heating temperature is 940°C, the heating coefficient is 0.5min / mm, the heating time is 105min, the deformation amount is controlled at 40%, and it is transferred into water within 15s after forging and water-cooled to room temperature to obtain a forging C; 5) Solution treatment: Perform solution treatment on the forging C in step 4). The solution heating temperature is 950°C, the holding time is 2h, and it is transferred into water within 20s after being taken out of the furnace and water-cooled to room temperature to obtain a forging D; 6) Aging treatment: Perform aging treatment on the forging D in step 5). The aging temperature is 520°C, the holding time is 2h, and it is air-cooled to room temperature after being taken out of the furnace.

[0029] Example 2

[0030] The present invention provides a method for preparing a TC4 alloy forging, which includes the following steps: 1) The forged bar with a diameter of Φ310mm is sawed and blanked to obtain a bar segment. The specifications of the bar segment are Φ310×830mm, with a weight of 290kg. The phase transformation point Tβ = 1000°C is measured by the metallographic method. The microstructure of the raw material bar is equiaxed structure, and the content of β transformation structure is not more than 30%, and the size of its equiaxed α phase is ≤30μm.

[0031] 2) Pre-forging in the (α + β) region: The bar segment described in step 1) is forged. The forging heating temperature is 960°C, the heating coefficient is 1min / mm, the heating time is 310min, and it is forged with a deformation amount of 80% in one heat, and the forging speed is 10mm / s to obtain intermediate blank A; 3) Pretreatment: The intermediate blank A described in step 2) is pretreated. The pretreatment process is 960°C / 4h + 700°C / 8h, and it is air-cooled to room temperature to obtain intermediate blank B; 4) Final forging in the (α + β) region: The intermediate blank B described in step 3) is finally forged. The final forging heating temperature is 970°C, the heating coefficient is 1min / mm, the heating time is 310min, the deformation amount is controlled at 70%, and it is transferred into water within 25s after forging and water-cooled to room temperature to obtain forging C; 5) Solution treatment: The forging C described in step 4) is solution-treated. The solution heating temperature is 970°C, the holding time is 4h, it is transferred into water within 30s after leaving the furnace, and water-cooled to room temperature to obtain forging D; 6) Aging treatment: The forging D described in step 5) is aged. The aging temperature is 580°C, the holding time is 4h, and it is air-cooled to room temperature after leaving the furnace.

[0032] Samples of the TC4 alloy forgings obtained in the embodiments of the present invention are taken for inspection, and the mechanical property test results are all qualified, meeting the requirements of the design standards. The mechanical property data of Examples 1-2 are shown in Table 1.

[0033] Comparative example This example is a comparative example of Example 2 and includes the following steps: 1) The raw material bar is blanked to obtain a bar segment. The specifications of the bar segment are Φ310×830mm, with a weight of 290kg. The phase transformation point Tβ = 1000°C is measured by the metallographic method. The microstructure of the raw material bar is equiaxed structure, and the content of β transformation structure is not more than 30%, and the size of its equiaxed α phase is ≤30μm.

[0034] 2) Pre-forging in the (α + β) region: The bar segment described in step 1) is forged. The forging heating temperature is 960°C, the heating coefficient is 1min / mm, the heating time is 310min, and it is forged with a deformation amount of 80% in one heat, and the forging speed is 20mm / s to obtain intermediate blank A; 3) Finish forging in the (α + β) region: The intermediate billet B described in step 3) is finish forged. The heating temperature for the finish forging is 970 °C, the heating coefficient is 1 min / mm, the heating time is 310 min, the deformation amount is controlled at 70%, and it is transferred into water within 25 s after forging and water-cooled to room temperature to obtain the forging C; 4) Solution treatment: The forging C described in step 4) is solution treated. The solution heating temperature is 970 °C, the holding time is 4 h, it is transferred into water within 30 s after being taken out of the furnace, and water-cooled to room temperature to obtain the forging D; 5) Aging treatment: The forging D described in step 5) is aged. The aging temperature is 580 °C, the holding time is 4 h, and it is air-cooled to room temperature after being taken out of the furnace.

[0035] Samples of the TC4 alloy forgings obtained in the embodiments of the present invention are taken for inspection, and the mechanical property test results are all qualified, meeting the requirements of the design standards.

[0036] The mechanical property data of Examples 1-2 and the comparative examples are shown in Table 1.

[0037] Table 1

[0038] As can be seen from the above table, by adding a pretreatment before finish forging, the present invention improves the elongation, cross-sectional shrinkage rate, and room temperature impact toughness of each part of the forging.

Claims

1. A preparation method of a TC4 titanium alloy forging, characterized in that It includes the following steps: 1) Blanking the raw material bar to obtain bar segments; 2) A pre-forging step in the (α+β) region: Forging the bar segments described in step 1), the heating temperature for forging is Tβ-(30~70)°C, the heating coefficient is 0.5~1 min / mm, and the total deformation is controlled between 40% and 80% to obtain intermediate blank A; 3) A pre-treatment step: Pre-treating the intermediate blank A described in step 2), and then air-cooling to room temperature to obtain intermediate blank B; 4) A final forging step in the (α+β) region: Performing one-pass final forging on the intermediate blank B described in step 3), the heating temperature for the final forging is Tβ-(30~60)°C, the heating coefficient is 0.5~1 min / mm, and the total deformation is controlled between 40% and 80%. After forging, cooling to room temperature to obtain forging C; 5) A solution treatment step: Performing solution treatment on the forging C described in step 4), the solution heating temperature is Tβ-(30~50)°C, the holding time is 2~4 h, and after furnace cooling and water cooling to room temperature to obtain forging D; 6) An aging treatment step: Performing aging treatment on the forging D described in step 5), the aging temperature is 520°C~580°C, the holding time is 2~4 h, and after furnace cooling and air cooling to room temperature to obtain a TC4 titanium alloy forging with high impact toughness.

2. The preparation method of a TC4 titanium alloy forging according to claim 1, characterized in that, The microstructure of the raw material bar described in step 1) is equiaxed structure, the content of β transformation structure therein does not exceed 30%, and the size of its equiaxed α phase is ≤30 μm.

3. The preparation method of a TC4 titanium alloy forging as described in claim 1, characterized in that The forging reduction rate in step 2) is 5~10 mm / s.

4. The preparation method of a TC4 titanium alloy forging according to claim 1, wherein The pre-treatment described in step 3) consists of a high-temperature heating process + a low-temperature heating process. The temperature of the high-temperature heating process is: Tβ-(40~60)°C / (2~4) h, and the temperature of the low-temperature heating process is: (650°C~700°C) / (4~8) h.

5. The preparation method of a TC4 titanium alloy forging according to claim 1, characterized in that, The forging reduction rate in step 4) is 0.05~5 mm / s.

6. The preparation method of a TC4 titanium alloy forging as described in claim 1, characterized in that, In step 4), water cooling is immediately carried out after forging is completed, and the transfer time after forging is ≤25 s.

7. The preparation method of a TC4 titanium alloy forging as described in claim 1, characterized in that, In step 5), the water cooling transfer time after solution treatment is ≤30 s.

Images