A free forging method for preparing AAA non-destructive testing grade large-size TC4 alloy bars

Abstract

This invention relates to a free forging method for preparing large-size TC4 alloy bars for AAA non-destructive testing, belonging to the field of titanium alloy hot working technology. The method utilizes high-temperature and large-deformation forging during the billet preparation stage to fully break down and refine the as-cast grains; it promotes high-temperature dynamic recrystallization through "high-low-high" heating forging below and above the phase transformation point to homogenize the grains; it significantly reduces dead zones in edge deformation through multi-stage diagonal elongation forging; and it uses a U-shaped anvil during the forming stage to reduce metal flow in all directions during the rounding stage. This free forging method for large-size TC4 alloy bars improves the uniformity of the TC4 alloy bar structure, meeting the requirements for AAA-level non-destructive testing.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy hot working technology, specifically relating to a free forging method for preparing AAA non-destructive testing grade large-size TC4 alloy bars. Background Technology

[0002] TC4 alloy is a medium-strength α+β type two-phase titanium alloy developed by the Watertown Arsenal in the United States in 1954. It contains 6% α-stabilizing element Al and 4% β-stabilizing element V. This alloy has excellent comprehensive properties and good processability, and can operate at temperatures up to 400°C for extended periods. In the aerospace industry, it is mainly used to manufacture engine fans and compressor discs and blades, as well as important load-bearing components such as beams, joints, and bulkheads in aircraft structures.

[0003] Due to the poor thermal conductivity, high deformation resistance, and sensitivity of microstructure to temperature and deformation process parameters, titanium alloys exhibit reduced deformation uniformity during forging, easily leading to defects such as overheating, localized coarse grains, and bright bands. This severely affects the uniformity of their microstructure and mechanical properties. Furthermore, to ensure the reliability of TC4 titanium alloy parts during use, ultrasonic testing is essential for bars, forgings, free forgings, and die forgings used in the manufacture of compressor discs, blades, and aircraft structural components. Existing research indicates that the main factors affecting ultrasonic testing results and noise generation in metallic materials are concentrated in the microstructure. However, existing large-diameter TC4 bars (Φ160–Φ260 mm) generally exhibit large grain size, low α-phase spheroidization, and inhomogeneous microstructure. These characteristics increase noise reflection during ultrasonic testing, severely impacting the ultrasonic testing level of large-diameter TC4 bars and failing to guarantee compliance with specified performance requirements. Summary of the Invention

[0004] This invention, based on current industry forging techniques, provides a free forging method for preparing large-diameter TC4 alloy bars for AAA non-destructive testing, addressing the technical problem that Φ160~Φ260mm TC4 alloy bars cannot meet AAA-level ultrasonic testing requirements. The Φ160~Φ260mm TC4 bars prepared by this invention meet AAA-level ultrasonic testing requirements, exhibit uniform microstructure, and achieve excellent mechanical properties after heat treatment, satisfying the existing requirements for TC4 bars.

[0005] To achieve the above objectives, the present invention provides the following technical solution.

[0006] On one hand, this invention provides a free forging method for preparing large-size TC4 alloy bars of AAA non-destructive testing grade, comprising:

[0007] Step 1: Forging the TC4 alloy ingot:

[0008] The TC4 alloy ingot is subjected to cross-heating forging at the phase transformation point, and the furnace temperature is gradually reduced with each heat. The ingot is forged into a square billet by a combination of upsetting and reversing elongation. After forging, it is cooled to fully break up the coarse structure in the as-cast state and to allow the forged billet to recrystallize fully at high temperature to refine the grains.

[0009] Step 2: Perform forging below the phase transformation point and cross-fire forging above the phase transformation point on the square billet ingot obtained in Step 1:

[0010] The square billet ingot obtained in step one is heated and forged below the phase transformation point. The deformation process is upsetting and diagonal drawing. Then it is heated back to the phase transformation point and deformed into an intermediate square billet through upsetting and diagonal drawing. After forging, it is cooled.

[0011] Step 3: Perform multi-fire forging in the two-phase region on the intermediate square billet obtained in Step 2:

[0012] The intermediate square billet obtained in step two is subjected to multiple upsetting and diagonal drawing to obtain a square billet, which is then cooled after forging.

[0013] Step 4: U-shaped anvil rolling and forging:

[0014] The square billet obtained in step three is formed into a bar using U-shaped anvil rolling and precision forging, so that the edges and faces of the square billet are deformed into circumferential surfaces.

[0015] Furthermore, step one involves two forging processes, specifically as follows:

[0016] First heating: Heating temperature 1100~1200℃, heating and holding coefficient 0.6~0.8, place the TC4 alloy ingot cylinder diagonally perpendicular to the ground on the anvil for upsetting, and then draw it out to form a square billet; return to the furnace and hold at 1050~1100℃, heating and holding coefficient 0.1~0.3, deformation 60%~80%;

[0017] Second heating: heating temperature 1000~1100℃, heating and heat preservation coefficient 0.6~0.8; return to furnace heat preservation temperature 1000~1050℃, heating and heat preservation coefficient 0.1~0.3, deformation amount 70%~80%, for upsetting and reversing drawing forging.

[0018] Furthermore, in step two, the heating temperature for forging below the phase transformation point is 15–40°C below the phase transformation point, with a heating and holding coefficient of 0.6–0.8; the heating temperature for forging above the phase transformation point is 30–50°C above the phase transformation point, with a holding coefficient of 0.3–0.4, and a total of two upsetting and two drawing forging deformations are performed, with a deformation amount of 70%–80%.

[0019] Furthermore, in step two, chamfering is performed after drawing to prevent cracking and folding during subsequent forging deformation.

[0020] Furthermore, in step three, the height-to-diameter ratio of the billet after upsetting is 0.9–1.0, and the height-to-diameter ratio of the billet after diagonal drawing is 1.9–2.0. A total of four heating cycles are performed. The specific process is as follows: the heating temperature for each heating cycle is 15–40°C below the phase transformation point, the heating and heat preservation coefficient is 0.6–0.8, the heat preservation coefficient for reheating is 0.1–0.3, and the deformation is 70%–80%.

[0021] Furthermore, in step four, the square billet is drawn into an octagon and then rolled into a round shape. The specific process is as follows:

[0022] Lengthening and sculpting into an octagon: Forging is carried out using upper and lower flat anvils, with a heating temperature of 15-40℃ below the phase transformation point, a heating and heat preservation coefficient of 0.6-0.8, and a deformation of 30%-40%. The cross section is forged into an octagon and then rolled in the furnace.

[0023] Rounding: U-shaped anvil forging is used, the heating temperature is 15-40℃ below the phase transformation point, the heating and heat preservation coefficient is 0.1-0.3, the deformation amount of each heat treatment is 30%-40%, and then precision forging is performed.

[0024] Furthermore, in step four, the precision forging process involves setting the number of forging passes according to the finished product size, with each pass resulting in a deformation of 20% to 30%. The forging process is carried out at a uniform drawing speed, and the product is air-cooled to room temperature after forging.

[0025] Furthermore, in steps one and four, the cooling method is air cooling, and in steps two and three, the cooling method is water cooling. The specific process is as follows:

[0026] In steps one and four, after forging is completed, the forging is air-cooled to room temperature and surface cracks and other defects are polished.

[0027] In steps two and three, after forging is completed, the forging is placed in a water-cooling zone for 15 to 60 minutes.

[0028] On the other hand, the present invention also provides a large-size TC4 alloy bar of AAA non-destructive testing grade, which is prepared by the free forging method described above for preparing large-size TC4 alloy bars of AAA non-destructive testing grade.

[0029] Furthermore, the dimensions of the prepared AAA non-destructive testing grade large-size TC4 alloy bars are Φ160~Φ260mm.

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

[0031] The billet preparation stage employs high-temperature heating and prolonged holding heating. The billet undergoes sufficient recrystallization at high temperatures to refine the microstructure, and the cast grains are thoroughly broken up during large deformation and reversible elongation forging. After billet preparation, heating forging is performed both below and above the phase transformation point. Heating forging below the phase transformation point provides sufficient distortion energy for secondary high-temperature forging, ensuring sufficient recrystallization and microstructure refinement during the secondary high-temperature heating and forging processes. In the two-phase region forging, a diagonal elongation process is used, with alternating deformation of edges and faces during elongation, mitigating the dead zones caused by single-face deformation. Post-forging water cooling is employed, rapidly preserving the high-temperature microstructure in a low-temperature environment, providing sufficient crystal for the subsequent dynamic recrystallization process. The defect forming stage utilizes U-shaped anvil rolling forging to restrict axial metal flow and improve the overall microstructure uniformity of the bar.

[0032] The 160-260mm TC4 alloy bars obtained by this method have a typical equiaxed microstructure, with the primary α phase accounting for more than 40%. Ultrasonic testing achieves the AAA level in AMS STD-2154E. After heat treatment, the tensile strength of bars of different specifications is similar, with the range of average tensile strength less than 10MPa, and all mechanical properties meet the requirements of existing aero-engines and other products. Attached Figure Description

[0033] Figure 1 This is a low-magnification corrosion image of a 180mm TC4 alloy bar prepared according to Example 1 of the present invention;

[0034] Figure 2 This is a transverse 100x corrosion image of a Φ180mm TC4 alloy bar prepared in Example 1 of this invention;

[0035] Figure 3 This is a longitudinal 100x corrosion image of a Φ180mm TC4 alloy bar prepared in Example 1 of this invention;

[0036] Figure 4 These are low-magnification corrosion images of TC4 alloy bars with a specification of Φ230mm prepared in Example 2 of this invention;

[0037] Figure 5 This is a transverse 100x corrosion image of a Φ230mm TC4 alloy bar prepared in Example 2 of this invention;

[0038] Figure 6 This is a longitudinal 100x corrosion image of a 230mm TC4 alloy bar prepared according to Example 2 of the present invention;

[0039] Figure 7 These are low-magnification corrosion images of 250mm TC4 alloy bars prepared in Example 3 of this invention;

[0040] Figure 8 This is a transverse 100x corrosion image of a Φ250mm TC4 alloy bar prepared in Example 3 of this invention;

[0041] Figure 9 This is a longitudinal corrosion image (100x magnification) of a 250mm TC4 alloy bar prepared according to Example 3 of the present invention. Detailed Implementation

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention.

[0043] In this invention, numerical intervals (i.e., numerical ranges) are involved. Unless otherwise specified, the selected numerical distributions within the aforementioned numerical intervals are considered continuous and include the two endpoints (i.e., the minimum and maximum values) of the numerical range, as well as every value between these two endpoints. Unless otherwise specified, when a numerical interval refers only to integers within that interval, it includes the two endpoint integers of the numerical range, as well as every integer between the two endpoints. In this document, this is equivalent to directly listing every integer. For example, if t is an integer selected from 1 to 10, it means that t is any integer selected from the group of integers consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. Furthermore, when multiple ranges are provided to describe features or characteristics, these ranges can be merged. In other words, unless otherwise specified, the ranges disclosed herein should be understood to include any and all subranges to which they are included.

[0044] Unless otherwise specified, the temperature parameters in this invention can be either constant temperature treatment or variations within a certain temperature range. It should be understood that the constant temperature treatment allows temperature fluctuations within the precision range controlled by the instrument. Fluctuations are permitted within ranges such as ±5℃, ±4℃, ±3℃, ±2℃, and ±1℃.

[0045] This invention provides a free forging method for preparing large-size TC4 alloy bars of AAA non-destructive testing grade, comprising:

[0046] Step 1: Forging the TC4 alloy ingot:

[0047] The TC4 alloy ingot is subjected to cross-heating forging at the phase transformation point, and the furnace temperature is gradually reduced with each heat. The ingot is forged into a square billet by a combination of upsetting and reversing elongation. After forging, it is cooled to fully break up the coarse structure in the as-cast state and to allow the forged billet to recrystallize fully at high temperature to refine the grains.

[0048] Step 2: Perform forging below the phase transformation point and cross-fire forging above the phase transformation point on the square billet ingot obtained in Step 1:

[0049] The square billet ingot obtained in step one is heated and forged below the phase transformation point. The deformation process is upsetting and diagonal drawing. Then it is heated back to the phase transformation point and deformed into an intermediate square billet through upsetting and diagonal drawing. After forging, it is cooled.

[0050] Step 3: Perform multi-fire forging in the two-phase region on the intermediate square billet obtained in Step 2:

[0051] The intermediate square billet obtained in step two is subjected to multiple upsetting and diagonal drawing to obtain a square billet, which is then cooled after forging.

[0052] Step 4: U-shaped anvil rolling and forging:

[0053] The square billet obtained in step three is formed into a bar using U-shaped anvil rolling and precision forging, so that the edges and faces of the square billet are deformed into circumferential surfaces.

[0054] In some embodiments, the present invention provides a free forging method for preparing large-size TC4 alloy bars of AAA non-destructive testing grade, comprising:

[0055] Step 1: Forging the TC4 alloy ingot: The forging stage involves forging the TC4 alloy ingot at the phase transformation point using a series of reverse heating processes, with the reflow temperature decreasing with each subsequent heating. A combination of upsetting with a flat hammer and reverse drawing is used to forge the square ingot. After forging, the ingot is cooled to fully break down the coarse as-cast structure and allow the forged ingot to recrystallize fully at high temperatures to refine the grains. The specific process is as follows: First heating: heating temperature 1100~1200℃, heating and heat preservation coefficient 0.6~0.8, the TC4 alloy ingot cylinder is placed diagonally perpendicular to the ground on the anvil for upsetting, and then drawn and squared into a square billet with a deformation of 60%~80%; the furnace holding temperature is 1050~1100℃, heating and heat preservation coefficient 0.1~0.3, and then air-cooled and ground; Second heating: heating temperature 1000~1100℃, heating and heat preservation coefficient 0.6~0.8, upsetting and reverse drawing forging are performed, and the reverse drawing is drawn into a square billet along the radial direction of the original square billet after upsetting; the furnace holding temperature is 1000~1050℃, heating and heat preservation coefficient 0.1~0.3, deformation of 70%~80%, and then air-cooled and ground.

[0056] Step 2: Perform forging below the phase transformation point and cross-forging above the phase transformation point on the square billet ingot obtained in Step 1: The square billet ingot obtained in Step 1 is heated and forged below the phase transformation point, with the deformation process involving upsetting and diagonal drawing. It is then reheated in the furnace to the phase transformation point, and deformed into an intermediate square billet through upsetting and diagonal drawing, followed by cooling. Specific process: The heating temperature for forging below the phase transformation point is 15–40℃ below the phase transformation point, with a heating and holding coefficient of 0.6–0.8; the heating temperature for cross-forging above the phase transformation point is 30–50℃ above the phase transformation point, with a holding coefficient of 0.3–0.4. A total of two upsetting and two drawing processes are performed, with a deformation amount of 70%–80%. After forging, it is water-cooled for 15–60 minutes. Preferably, chamfering is performed after drawing to prevent cracking, folding, and other adverse effects during subsequent forging deformation.

[0057] Step 3: The intermediate square billet obtained in Step 2 undergoes multi-stage forging in the two-phase region, namely, multi-stage upsetting and diagonal drawing, to obtain the square billet material, which is then cooled after forging. The specific process is as follows: the heating temperature for each stage is 15–40℃ below the phase transformation point, with a heating holding coefficient of 0.6–0.8. Upsetting and diagonal drawing are performed, with a deformation of 70%–80%. After upsetting, the height-to-diameter ratio of the square billet is 0.9–1.0, and after diagonal drawing, the height-to-diameter ratio is 1.9–2.0. The furnace holding coefficient is 0.1–0.3, and the billet is water-cooled after forging for 15–60 minutes. The multi-stage diagonal drawing fully deforms the square billet, improving the homogeneity of the microstructure.

[0058] Step 4: U-shaped anvil rolling and forging. The square billet obtained in Step 3 is formed into a bar using U-shaped anvil rolling and precision forging to deform the edges and faces of the billet into circumferential surfaces. First, elongation and octagonal deformation are performed using upper and lower flat anvils, forging the cross-section to an octagonal shape, followed by remelting. After remelting, U-shaped anvil rolling is used, followed by final precision forging. Specific process: Elongation and octagonal deformation: Forging is performed using upper and lower flat anvils at a heating temperature of 15–40°C below the phase transformation point, with a heating and holding coefficient of 0.6–0.8, and a deformation amount of 30%–40%, forging the cross-section to an octagonal shape. After remelting and holding at a high temperature, rounding is performed with a holding coefficient of 0.1–0.3. Rounding: Forging is performed using a U-shaped anvil at a heating temperature of 15–40°C below the phase transformation point, with a heating and holding coefficient of 0.1–0.3, and a deformation amount of 30%–40%, followed by precision forging. The precision forging process involves setting the number of forging passes according to the finished product dimensions. Each pass is heated to 15–40°C below the phase transformation point, with a heating and heat preservation coefficient of 0.7–0.8 and a deformation of 20%–30%. The forging process involves uniform elongation, and the resulting bar stock is then air-cooled to room temperature.

[0059] To clarify the purpose, technical solution, and advantages of this invention, the invention will be described in detail below with reference to specific embodiments and accompanying drawings. The specific embodiments described herein are only used to explain the invention, and the invention is not limited thereto.

[0060] Example 1: Preparation of Φ180mm TC4 alloy bar

[0061] Step 1: Forging the TC4 alloy ingot: The forging stage involves forging the TC4 alloy ingot at its phase transformation point using a series of alternating heating processes. The furnace temperature is gradually reduced with each heating process. A combination of flat hammer head riveting and upsetting, along with alternating drawing, is used to forge the ingot into a square billet, which is then cooled after forging. This stage effectively breaks down the coarse as-cast structure and allows the forged billet to recrystallize sufficiently at high temperatures to refine the grains. The specific process is as follows: First heating: heating temperature 1150℃, heating and heat preservation coefficient 0.6, the TC4 alloy ingot cylinder is placed diagonally perpendicular to the ground on the anvil for upsetting, and then drawn and squared into a square billet; the furnace holding temperature is 1100℃, heating and heat preservation coefficient 0.2, deformation amount 65%, and air-cooled grinding after forging; Second heating: heating temperature 1080℃, heating and heat preservation coefficient 0.7, upsetting and reverse drawing forging, deformation amount 70%; the furnace holding temperature is 1050℃, heating and heat preservation coefficient 0.2, and air-cooled grinding after forging.

[0062] Step 2: Perform forging below the phase transformation point and forging above the phase transformation point on the square billet ingot obtained in Step 1: Specific process: Heat the square billet ingot obtained in Step 1 at 960℃ with a heating and holding coefficient of 0.7; the heating temperature for forging above the phase transformation point is 1020℃ with a holding coefficient of 0.4. A total of two upsetting and two drawing forging deformations are performed, with a deformation amount of 70%. After forging, water cooling is performed for 30 minutes.

[0063] Step 3: Perform two-phase multi-fire forging on the intermediate square billet obtained in Step 2: Specific process: The heating temperature for each fire is 960℃, the heating and heat preservation coefficient is 0.7, and upsetting and diagonal drawing forging are performed. After upsetting, the height-to-diameter ratio of the square billet is 1.0, and after diagonal drawing, the height-to-diameter ratio of the square billet is 2.0, with a deformation of 70%; the heat preservation coefficient for returning to the furnace is 0.2, and the billet is water-cooled after forging for 30 minutes.

[0064] Step 4, U-shaped anvil rounding forging: Specific process: Drawing and octagonal shaping: Forging is performed using upper and lower flat anvils, heating temperature 960℃, heating and holding coefficient 0.7, deformation amount 40%, forging the cross-section into an octagon, then reheating and holding at the furnace before rounding, holding coefficient 0.2; Rounding: Forging is performed using a U-shaped anvil, heating temperature 960℃, heating and holding coefficient 0.2, deformation amount 30%. The finishing forging is a single forging, heating temperature 960℃, heating and holding coefficient 0.7, deformation amount 20%, uniform drawing during finishing forging, obtaining the shaped bar, and air cooling to room temperature. Figures 1-3As shown, the Φ180mm rod prepared in this embodiment exhibits uniform microstructure at both high and low magnification, with a high degree of spheroidization. The high-magnification microstructure is a processed microstructure in the α+β two-phase region, lacking intact original β grain boundaries. There is no significant difference in the high-magnification microstructure between the transverse and longitudinal directions, with the primary α phase content exceeding 50% and an average grain size of only 21.9 μm. The mechanical properties of the TC4 alloy rod prepared in this embodiment (heat treatment regime: 780℃ / 170min, air cooling) are shown in Table 1. In Table 1, L represents the gauge length of the tensile specimen, and D represents the specimen diameter.

[0065] Table 1 Mechanical properties of Φ180mm TC4 alloy bars

[0066]

[0067] As can be seen from the test results in Table 1, the mechanical properties of the head and tail of the bar are similar, with an average tensile strength and elongation after fracture of 956 MPa and 19.1%, respectively. The prepared bar has excellent strength-plasticity matching and meets the AAA grade standard in AMSSTD-2154E in ultrasonic flaw detection.

[0068] Example 2: Preparation of Φ230mm TC4 alloy rod

[0069] Step 1: Forging the TC4 alloy ingot: First forging: Heating temperature 1150℃, heating and holding coefficient 0.6. The cylindrical TC4 alloy ingot is placed diagonally perpendicular to the ground on the anvil for upsetting. After upsetting, it is drawn and squared into a square billet. The furnace holding temperature is 1100℃, heating and holding coefficient 0.2, deformation amount 70%. After forging, it is air-cooled and ground. Second forging: Heating temperature 1100℃, heating and holding coefficient 0.6. Upsetting and reversing drawing and forging are performed, deformation amount 70%. The furnace holding temperature is 1050℃, heating and holding coefficient 0.2. After forging, it is air-cooled and ground.

[0070] Step 2: Perform forging below the phase transformation point and forging above the phase transformation point on the square billet ingot obtained in Step 1. Specific process: The square billet ingot obtained in Step 1 is heated at 960℃ with a heating and holding coefficient of 0.7; the heating temperature for forging above the phase transformation point is 1020℃ with a holding coefficient of 0.4. A total of two upsetting and two drawing forging deformations are performed, with a deformation amount of 75%. After forging, it is water-cooled for 30 minutes.

[0071] Step 3: Perform two-phase multi-fire forging on the intermediate square billet obtained in Step 2: Specific process: The heating temperature for each fire is 960℃, the heating and heat preservation coefficient is 0.7, and upsetting and diagonal drawing forging are performed. After upsetting, the height-to-diameter ratio of the square billet is 1.0, and after diagonal drawing, the height-to-diameter ratio of the square billet is 2.0, with a deformation of 75%; the heat preservation coefficient for returning to the furnace is 0.2, and the billet is water-cooled after forging for 30 minutes.

[0072] Step 4: U-shaped anvil rounding and forging: Drawing and octagonal shaping: Forging is performed using upper and lower flat anvils at a heating temperature of 960℃, a heating and holding coefficient of 0.7, and a deformation of 40%. The cross-section is forged into an octagon, then reheated and held at the furnace before rounding, with a holding coefficient of 0.2. Rounding: Forging is performed using a U-shaped anvil at a heating temperature of 960℃, a heating and holding coefficient of 0.2, and a deformation of 30%. The final forging is a single pass at a heating temperature of 960℃, a heating and holding coefficient of 0.7, and a deformation of 20%. During final forging, the section is drawn at a uniform speed, resulting in a shaped bar, which is then air-cooled to room temperature. Figures 4-6 As shown, the Φ230mm rod prepared in this embodiment exhibits uniform microstructure at both high and low magnification. The high-magnification microstructure is a processed microstructure in the α+β two-phase region, without complete original β grain boundaries. There is no significant difference in the high-magnification microstructure between the transverse and longitudinal directions. The primary α phase content is greater than 40%, and the average grain size is 26.1μm. The mechanical properties of the rod in this embodiment are shown in Table 2 (heat treatment regime: 780℃ / 240min, air cooling).

[0073] Table 2 Mechanical properties of Φ230mm TC4 alloy bars

[0074]

[0075] In Table 2, L represents the gauge length of the tensile specimen, and D represents the specimen diameter. The test results in Table 2 show that the mechanical properties of the bar are similar at both ends, with an average tensile strength of 949 MPa and an elongation after fracture of 18.5%. The prepared bar exhibits excellent strength-plasticity matching and meets the AAA grade standard in AMS STD-2154E during ultrasonic testing.

[0076] Example 3: Preparation of Φ250mm TC4 alloy rod

[0077] Step 1: Forging the TC4 alloy ingot: First forging: Heating temperature 1150℃, heating and holding coefficient 0.6. The cylindrical TC4 alloy ingot is placed diagonally perpendicular to the ground on the anvil for upsetting. After upsetting, it is drawn and squared into a square billet. The furnace holding temperature is 1100℃, heating and holding coefficient 0.2, deformation amount 70%. After forging, it is air-cooled and ground. Second forging: Heating temperature 1100℃, heating and holding coefficient 0.6. Upsetting and reversing drawing and forging are performed, deformation amount 70%. The furnace holding temperature is 1050℃, heating and holding coefficient 0.2. After forging, it is air-cooled and ground.

[0078] Step 2: Perform forging below the phase transformation point and forging above the phase transformation point on the square billet ingot obtained in Step 1. Specific process: The square billet ingot obtained in Step 1 is heated at 960℃ with a heating and holding coefficient of 0.7; the heating temperature for forging above the phase transformation point is 1020℃ with a holding coefficient of 0.4. A total of two upsetting and two drawing forging deformations are performed, with a deformation amount of 75%. After forging, it is water-cooled for 30 minutes.

[0079] Step 3: Perform two-phase multi-fire forging on the intermediate square billet obtained in Step 2: Specific process: The heating temperature for each fire is 960℃, the heating and heat preservation coefficient is 0.7, and upsetting and diagonal drawing forging are performed. After upsetting, the height-to-diameter ratio of the square billet is 1.0, and after diagonal drawing, the height-to-diameter ratio of the square billet is 2.0, with a deformation of 75%; the heat preservation coefficient for returning to the furnace is 0.2, and the billet is water-cooled after forging for 30 minutes.

[0080] Step 4: U-shaped anvil rounding and forging: Drawing and octagonal shaping: Forging is performed using upper and lower flat anvils at a heating temperature of 960℃, a heating and holding coefficient of 0.7, and a deformation of 40%. The cross-section is forged into an octagon, then reheated and held at the furnace before rounding, with a holding coefficient of 0.2. Rounding: Forging is performed using a U-shaped anvil at a heating temperature of 960℃, a heating and holding coefficient of 0.2, and a deformation of 30%. The final forging is a single pass at a heating temperature of 960℃, a heating and holding coefficient of 0.7, and a deformation of 20%. During final forging, the section is drawn at a uniform speed, resulting in a shaped bar, which is then air-cooled to room temperature. Figures 7-9 As shown, the Φ250mm rod prepared in this embodiment exhibits uniform microstructure at both high and low magnification. The high-magnification microstructure is a processed microstructure in the α+β two-phase region, without complete original β grain boundaries. There is no significant difference in the high-magnification microstructure between the transverse and longitudinal directions. The primary α phase content is greater than 40%, and the average grain size is 26.2μm. The mechanical properties of the rod in Example 3 are shown in Table 3 (heat treatment regime: 780℃ / 260min, air cooling).

[0081] Table 3 Mechanical properties of Φ250mm TC4 alloy bars

[0082]

[0083] In Table 3, L represents the gauge length of the tensile specimen, and D represents the specimen diameter. The test results in Table 3 show that the mechanical properties of the bar are similar at both ends, with an average tensile strength of 937 MPa and an elongation after fracture of 18.0%. The prepared bar exhibits excellent strength-plasticity matching and meets the AAA grade standard in AMS STD-2154E during ultrasonic testing.

[0084] The three specifications of TC4 bars prepared in Examples 1-3 have uniform microstructures with an average grain size of 21.9 μm to 26.2 μm. The test results of tensile mechanical properties have a certain margin of error compared with the industry standard, and the ultrasonic flaw detection all meet the AAA grade standard in AMSSTD-2154E.

[0085] It should be noted that the embodiments described above are merely preferred embodiments of the present invention. For those skilled in the art, various modifications, improvements, and equivalent substitutions can be made to the present invention without departing from its principles, and such modifications, improvements, and equivalent substitutions are also considered to fall within the protection scope of the claims of the present invention.

Claims

1. A free forging method for preparing AAA non-destructive testing grade large-size TC4 alloy bars, characterized in that, include: Step 1: Forging the TC4 alloy ingot: The TC4 alloy ingot is subjected to cross-heating forging at the phase transformation point, and the furnace temperature is gradually reduced with each heat. The ingot is forged into a square billet by a combination of upsetting and reversing elongation. After forging, it is cooled to fully break up the coarse structure in the as-cast state and to allow the forged billet to recrystallize fully at high temperature to refine the grains. Step 2: Perform forging below the phase transformation point and cross-fire forging above the phase transformation point on the square billet ingot obtained in Step 1: The square billet ingot obtained in step one is heated and forged below the phase transformation point. The deformation process is upsetting and diagonal drawing. Then it is heated back to the phase transformation point and deformed into an intermediate square billet through upsetting and diagonal drawing. After forging, it is cooled. Step 3: Perform multi-fire forging in the two-phase region on the intermediate square billet obtained in Step 2: The intermediate square billet obtained in step two is subjected to multiple upsetting and diagonal drawing to obtain a square billet, which is then cooled after forging. In step three, the height-to-diameter ratio of the billet after upsetting is 0.9–1.0, and the height-to-diameter ratio of the billet after diagonal drawing is 1.9–2.

0. A total of four heating cycles are performed. The specific process is as follows: the heating temperature for each heating cycle is 15–40°C below the phase transformation point, the heating and heat preservation coefficient is 0.6–0.8, the heat preservation coefficient for reheating is 0.1–0.3, and the deformation is 70%–80%. Step 4: U-shaped anvil rolling and forging: The square billet obtained in step three is formed into a bar using U-shaped anvil rolling and precision forging, so that the edges and faces of the square billet are deformed into circumferential surfaces; In step four, the billet is drawn into an octagon and then rolled into a round shape. The specific process is as follows: drawing into an octagon: forging is performed using upper and lower flat anvils, the heating temperature is 15-40℃ below the phase transformation point, the heating and heat preservation coefficient is 0.6-0.8, the deformation is 30%-40%, the cross section is forged into an octagon, and then it is returned to the furnace for rolling. Rounding: U-shaped anvil forging is used, the heating temperature is 15-40℃ below the phase transformation point, the heating and heat preservation coefficient is 0.1-0.3, the deformation amount of each heat treatment is 30%-40%, and then precision forging is performed; In step four, the precision forging process involves setting the number of forging passes according to the finished product size, with each pass resulting in a deformation of 20% to 30%. The forging process is carried out at a uniform drawing speed, and the product is air-cooled to room temperature after forging.

2. The free forging method for preparing AAA non-destructive testing grade large-size TC4 alloy bars as described in claim 1, characterized in that, In step two, the heating temperature for forging below the phase transformation point is 15-40°C below the phase transformation point, with a heating and holding coefficient of 0.6-0.8; the heating temperature for forging above the phase transformation point is 30-50°C above the phase transformation point, with a holding coefficient of 0.3-0.4, and a total of two upsetting and two drawing forging deformations are carried out, with a deformation amount of 70%-80%.

3. The free forging method for preparing AAA non-destructive testing grade large-size TC4 alloy bars as described in claim 1, characterized in that, In step two, after the diagonal elongation, chamfering is performed to prevent cracking and folding during subsequent forging deformation.

4. The free forging method for preparing AAA non-destructive testing grade large-size TC4 alloy bars as described in claim 1, characterized in that, In steps one and four, the cooling method is air cooling; in steps two and three, the cooling method is water cooling. The specific process is as follows: In steps one and four, after forging is completed, the forging is air-cooled to room temperature and surface cracks and defects are polished. In steps two and three, after forging is completed, the forging is placed in a water-cooling zone for 15 to 60 minutes.

5. A large-size TC4 alloy bar of AAA non-destructive testing grade, characterized in that, The free forging method for preparing AAA non-destructive testing grade large-size TC4 alloy bars according to any one of claims 1 to 4.

6. The AAA non-destructive testing grade large-size TC4 alloy bar according to claim 5, characterized in that, The dimensions of the AAA non-destructive testing grade large-size TC4 alloy rod are Φ160~Φ260mm.

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