Tc18 titanium alloy bar and method of making

Abstract

The application discloses a kind of TC18 titanium alloy bar and preparation method thereof, belong to titanium alloy technical field.The method comprises the following steps: S1, according to TC18 titanium alloy target component allocation, is made into ingot by multiple VAR melting;S2, the ingot is heated to 1140~1210 ℃, after heat preservation, multiple fire times upset forging is carried out, and first grade forging blank is obtained;S3, the first grade forging blank is in the range of 50~200 ℃ above beta phase point and is carried out multiple fire times upset forging, and secondary forging blank is obtained;S4, the secondary forging blank is in the range of 40~60 ℃ below beta phase point and is carried out multiple fire times upset forging, and tertiary forging blank is obtained;S5, the tertiary forging blank is in the range of 50~70 ℃ below beta phase point and is carried out one fire time lengthening forging, and bar is obtained;S6, the bar is heat treated, and finished product TC18 titanium alloy bar is obtained.The scheme of the application realizes the synergy optimization of the strength and toughness of TC18 titanium alloy bar by the synergistic control of component optimization design, multi-stage forging process and specific heat treatment system, and significantly improves the organization uniformity of TC18 titanium alloy bar.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy technology, specifically relating to a TC18 titanium alloy bar and its preparation method. Background Technology

[0002] Titanium alloys, as a lightweight metal material, possess excellent comprehensive properties such as high specific strength, corrosion resistance, non-magnetic properties, and weldability, making them a key structural component material in the current aerospace field. High-strength titanium alloys generally refer to titanium alloys with a tensile strength of over 1000 MPa, and are widely used in critical components such as aircraft main landing gear struts, engine plugs, nozzles, and fuselage load-bearing frames. Although significant progress has been made in the research and application of high-strength titanium alloys both domestically and internationally, while strength has been continuously improved, the plasticity, toughness, and damage tolerance of titanium alloys have decreased to varying degrees. Achieving a better balance between strength and toughness is key to promoting the further application of high-strength titanium alloys in aircraft.

[0003] TC18 titanium alloy (Russian designation BT22, nominal composition Ti-5Al-5Mo-5V-1Cr-1Fe) is a transitional α+β titanium alloy, possessing the performance characteristics of both α+β and β titanium alloys. It has comprehensive advantages such as excellent hardenability and forgeability, with a hardenability depth of up to 250mm. Its tensile strength in the annealed state can reach over 1080MPa, and its strength in the strengthened heat-treated state can reach 1230MPa. It is mainly used in aircraft landing gear components, high-load-bearing devices and components, and engine fixed support structures.

[0004] The existing technology for preparing large-diameter TC18 titanium alloy bars has the following shortcomings: First, the β-forging process has difficulty guaranteeing the axial microstructure uniformity of large-diameter bars. Some studies have attempted to solve the microstructure uniformity problem by using conventional forging followed by β-heat treatment, but β-heat treatment is prone to causing grain coarsening and the formation of continuous grain boundaries, making the microstructure difficult to control, especially for bars with large cross-sectional dimensions, where the microstructure differences between the core surface and the edges are more significant.

[0005] Secondly, existing forging processes struggle to achieve a synergistic optimization of strength and impact toughness. Studies show that TC18 titanium alloy can achieve equiaxed or biphasic microstructures when forged in the two-phase region, but with lower fracture toughness. When forged above the β-phase transformation point, the primary α-phase morphology changes from equiaxed to lamellar, significantly improving fracture toughness, but potentially decreasing plasticity and impact toughness. How to maintain high strength while improving impact toughness is a key technical challenge in the fabrication of large-diameter TC18 titanium alloy bars.

[0006] Third, the strength-toughness matching process for TC18 bars with a diameter of Ф350mm and above is not yet mature. In the existing technology, the preparation of TC18 titanium alloy bars is mostly concentrated on medium-sized bars (Ф50~90mm), and there is little research on the control of microstructure uniformity and the synergistic optimization of strength and toughness for large-sized bars with a diameter of Ф350mm and above. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a method for preparing TC18 titanium alloy bars. Through the synergistic control of composition optimization design, multi-stage forging process, and specific heat treatment regime, the strength and toughness of large-size TC18 titanium alloy bars are synergistically optimized, and the microstructure uniformity of large-size TC18 titanium alloy bars is significantly improved.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: According to a first aspect of the present invention, a method for preparing TC18 titanium alloy bars is provided, comprising the following steps: S1. According to the target composition of TC18 titanium alloy, the ingot is produced by multiple VAR melting processes. The target composition of TC18 titanium alloy, by mass percentage, includes: Al: 4.8~5.0%, V: 5.8~6.0%, Mo: 5.6~5.8%, Cr: 1.2~1.5%, Fe: 0.8~1.2%, Zr: 0.5~1.0%, O: 0.15~0.18%, with the balance being Ti and unavoidable impurities. S2. The ingot is heated to 1140~1210℃, held at that temperature, and then subjected to multiple upsetting and drawing forging to obtain a first-grade forging billet. The upsetting pressure is 35~40%, the drawing is done in a square manner, and the height-to-diameter ratio is controlled at 1.9~2.1 after drawing. S3. The first-stage forging billet is subjected to multiple upsetting and drawing forging processes within a temperature range of 50~200℃ above the β phase transformation point to obtain a second-stage forging billet. The upsetting pressure is 45~50%, the drawing is done using square drawing, and the height-to-diameter ratio after drawing is controlled at 1.9~2.1. S4. The secondary forging billet is subjected to multiple upsetting and drawing forging in the range of 40~60℃ below the β phase transformation point to obtain the tertiary forging billet. The upsetting pressure is 33~35%, the drawing is diagonal square drawing, and the height-to-diameter ratio after drawing is controlled at 1.9~2.1. S5. The three-stage forging billet is subjected to one-time drawing forging in the range of 50~70℃ below the β phase transformation point to obtain a bar. S6. The bar is heat-treated to obtain the finished TC18 titanium alloy bar.

[0009] As a further implementation scheme, in step S2, the ingot is subjected to two-stage upsetting and drawing forging: the first-stage upsetting and drawing forging heats the ingot to 1200±10℃ and holds it at that temperature, and then performs two upsetting and drawing operations; the second-stage upsetting and drawing forging heats the billet to 1150±10℃ and holds it at that temperature, and then performs two upsetting and drawing operations.

[0010] As a further implementation scheme, in step S3, the primary forging billet is subjected to three-stage upsetting and drawing forging, with each stage involving XYZ three-axis reversing forging.

[0011] As a further implementation scheme, in step S3, the heating temperature of multi-stage upsetting and drawing forging gradually decreases as the number of stages increases.

[0012] As a further implementation scheme, in step S4, the secondary forging billet is subjected to four upsetting and drawing forging cycles. Each upsetting and drawing cycle adopts a two-upsetting and two-drawing process. After completing one upsetting and one drawing, the forging billet is kept in the furnace at the original forging temperature for 1 to 2 hours before being taken out of the furnace to carry out the remaining one upsetting and one drawing.

[0013] As a further implementation scheme, in step S5, the elongation deformation route is sequentially square billet, octagonal billet, and rounding.

[0014] As a further implementation scheme, in step S5, the single drawing reduction is 0.5 to 0.6 times the side length of the square billet, the reduction speed is 10 to 12 mm / s, and when the surface temperature of the forging billet is below 700℃, it is returned to the furnace for heat preservation for 1 to 1.5 hours before continuing forging.

[0015] As a further implementation, in step S5, a heat-insulating coating is applied before the drawing and forging process.

[0016] As a further implementation scheme, in step S6, the heat treatment process is as follows: the bar is heated to 10~20℃ below the β phase transformation point, then placed in a furnace and held for 2~3 hours, and then air-cooled after removal from the furnace; the bar is heated again to 30~40℃ below the β phase transformation point, then placed in a furnace and held for 2~3 hours; then furnace-cooled to 620~650℃ at a rate of 4~6℃ / min, held for 8~10 hours, and then air-cooled to obtain the finished TC18 titanium alloy bar.

[0017] According to a second aspect of the present invention, a TC18 titanium alloy bar is provided, which is prepared by the above method.

[0018] By adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art: The method of this invention achieves synergistic optimization of the strength and toughness of TC18 titanium alloy bars through composition optimization design, multi-stage forging process and specific heat treatment system. The prepared TC18 titanium alloy bars have a tensile strength ≥1200MPa and an impact energy KU2 ≥23J, which simultaneously meets the dual requirements of high strength and high toughness for key load-bearing components of aircraft. The method of the present invention effectively eliminates the deformation inhomogeneity during the forging process by combining a specific forging method with precise control of the height-to-diameter ratio at each stage, and significantly improves the microstructure uniformity of TC18 titanium alloy bars. The method and process of this invention are highly feasible, suitable for industrial production, and meet the development requirements of the aerospace industry for high-quality TC18 titanium alloy large-size bars. Attached Figure Description

[0019] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description of the present invention will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0020] Figure 1 A flowchart illustrating the preparation method of TC18 titanium alloy rods provided by this invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0022] Specific embodiments of the invention are disclosed herein as needed; however, it should be understood that the embodiments disclosed herein are merely examples of the invention that may be implemented in various alternative forms. In the following description, various operating parameters and components are described in several contemplated embodiments. These specific parameters and components are provided as examples only and are not intended to be limiting.

[0023] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0024] The first aspect of this invention provides a method for preparing TC18 titanium alloy bars. Figure 1 This is a flowchart illustrating the preparation method of the TC18 titanium alloy bar provided by the present invention. Figure 1 As shown, the method includes the following steps: S1. According to the target composition of TC18 titanium alloy, the ingot is produced by multiple VAR melting processes. The target composition of TC18 titanium alloy by mass percentage includes: Al: 4.8~5.0%, V: 5.8~6.0%, Mo: 5.6~5.8%, Cr: 1.2~1.5%, Fe: 0.8~1.2%, Zr: 0.5~1.0%, O: 0.15~0.18%, with the balance being Ti and unavoidable impurities. S2. Heat the ingot to 1140~1210℃, hold it at that temperature, and then perform multi-fire upsetting and drawing forging to obtain a first-grade forging billet. The upsetting pressure is 35~40%, and the drawing is done using square drawing. After drawing, the height-to-diameter ratio is controlled at 1.9~2.1. S3. The first-stage forging billet is subjected to multiple upsetting and drawing forging processes within a temperature range of 50~200℃ above the β phase transformation point to obtain the second-stage forging billet. The upsetting reduction is 45~50%, and the drawing is done using a square drawing method. After drawing, the height-to-diameter ratio is controlled at 1.9~2.1. S4. The secondary forging billet is subjected to multiple upsetting and drawing forging in the range of 40~60℃ below the β phase transformation point to obtain the tertiary forging billet. The upsetting reduction is 33~35%, and the drawing is done diagonally. After drawing, the height-to-diameter ratio is controlled at 1.9~2.1. S5. The third-level forging billet is subjected to one-time drawing forging in the range of 50~70℃ below the β phase transformation point to obtain bar stock; S6. Heat treat the bar to obtain the finished TC18 titanium alloy bar.

[0025] The method of this invention achieves synergistic optimization of the strength and toughness of TC18 titanium alloy bars through composition optimization design, multi-stage forging process and specific heat treatment regime. The prepared TC18 titanium alloy bars have a tensile strength ≥1200MPa and an impact energy KU2 ≥23J, which simultaneously meets the dual requirements of high strength and high toughness for key load-bearing components of aircraft. The method of this invention effectively eliminates the deformation inhomogeneity during the forging process by combining specific forging methods with precise control of the height-to-diameter ratio at each stage, and significantly improves the microstructure uniformity of TC18 titanium alloy bars.

[0026] The optimization design of the conventional TC18 titanium alloy composition in this invention specifically includes: adding Zr to the TC18 titanium alloy and limiting the Zr content to the range of 0.5% to 1.0%. Zr is used to refine the grains and improve plasticity and toughness. In contrast, existing conventional TC18 titanium alloys only use the nominal composition Ti-5Al-5Mo-5V-1Cr-1Fe without intentionally adding Zr. Existing conventional TC18 titanium alloys only provide broad ranges for Al, V, Mo, Cr, and Fe, while this invention strictly limits these elements to narrow ranges: Al 4.8–5.0%, V 5.8–6.0%, Mo 5.6–5.8%, Cr 1.2–1.5%, and Fe 0.8–1.2%, precisely controlling the matrix phase ratio. Through the optimized design of the conventional TC18 titanium alloy composition, combined with subsequent forging and heat treatment processes, this invention achieves synergistic optimization of strength and impact toughness.

[0027] Step S1 involves a smelting process. Preferably, the multiple VAR smelting specifically involves three VAR smelting processes. Preferably, after the ingot is formed, the ingot body is peeled off before proceeding with subsequent steps.

[0028] In some implementations, in step S1, a 3t ingot with a shape of Φ(700~750)mm×L is prepared by three VAR melting processes, and the ingot body is peeled off.

[0029] In this invention, the multi-stage forging process includes step S2 (blanking forging), step S3 (single-phase forging), step S4 (two-phase forging), and step S5 (drawing forging). The ingot has a coarse as-cast structure with compositional segregation. Step S2 (blanking forging) employs a large deformation (35-40% upsetting reduction) and high-temperature upsetting and drawing forging to thoroughly break down the coarse as-cast structure of the ingot, eliminating compositional segregation and providing a relatively uniform β-grain structure in terms of composition and grain size for all subsequent processes. Step S3 (single-phase forging) involves forging above the β-phase transformation point (T... β Forging at +50~200℃, at which point the material is entirely β phase. Through multiple forging processes, the β grains undergo dynamic recrystallization, regrowing into fine, uniform equiaxed grains, constructing an isotropic β grain "skeleton," providing an ideal basic microstructure for the subsequent uniform precipitation of the α phase. Step S4, the two-phase forging process, occurs below the β phase transformation point (T...). βForging in the α+β two-phase region (-40~60℃) involves precise control of deformation (upsetting pressure 33~35%), combined with a diagonal square drawing method, to ensure stress fully penetrates to the core of the forged billet, achieving uniform plastic deformation. The content, size, and distribution of the primary α phase directly affect the strength and toughness of the material. In step S5, the drawing forging process controls the forging temperature at 50~70℃ below the β phase transformation point to obtain an equiaxed microstructure with an equiaxed α phase size of less than 5μm. Simultaneously, lowering the drawing temperature avoids excessively rapid deformation during drawing, preventing localized overheating and ensuring the excellent microstructure established in previous passes is properly transferred to the final finished bar. Finally, by controlling the deformation path and process parameters, the billet is precisely forged into a round bar of the target specifications, ensuring the dimensional accuracy, surface quality, and cross-sectional microstructure uniformity of the finished product. These four processes are interconnected and indispensable. Through this collaborative process, the TC18 titanium alloy bar achieves a synergistic match of tensile strength ≥1200MPa, impact energy KU2 ≥23J, and excellent uniformity of the cross-sectional structure of the bar.

[0030] Step S2 involves the billet forging process. The billet forging temperature can typically, but is not limited to, be set to 1140°C, 1150°C, 1160°C, 1170°C, 1180°C, 1190°C, 1200°C, or 1210°C; the upsetting pressure during billet forging can typically, but is not limited to, be set to 35%, 36%, 37%, 38%, 39%, or 40%; and the height-to-diameter ratio after elongation during billet forging can typically, but is not limited to, be set to 1.9, 2.0, or 2.1. The "shaft upsetting and elongation" mentioned in this invention refers to the forging operation of upsetting and elongating along the axial direction of the billet, the purpose of which is to fully break down the coarse as-cast structure of the ingot and improve the uniformity of the structure; the "square elongation" refers to the operation of forging the billet into a square cross-section and elongating it along the axial direction.

[0031] Preferably, during the billet forging process, the ingot is subjected to two-stage upsetting and drawing forging: the first stage of upsetting and drawing forging heats the ingot to 1200±10℃ and holds it at that temperature, and then performs two upsetting and drawing operations; the second stage of upsetting and drawing forging heats the billet to 1150±10℃ and holds it at that temperature, and then performs two upsetting and drawing operations.

[0032] Step S3 involves a single-phase forging process. The single-phase forging temperature can typically, but is not limited to, be set to 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, or 200°C above the β-phase transformation point; the upsetting pressure during single-phase forging can typically, but is not limited to, be set to 45%, 46%, 47%, 48%, 49%, or 50%; and the height-to-diameter ratio after drawing during single-phase forging can typically, but is not limited to, be set to 1.9, 2.0, or 2.1.

[0033] Preferably, during single-phase forging, the primary forging billet undergoes three-stage upsetting and drawing forging, with each stage involving XYZ three-axis reversal forging. The heating temperature for multi-stage upsetting and drawing forging gradually decreases with each stage. The "XYZ three-axis reversal forging" refers to sequentially upsetting and drawing forging along the three orthogonal directions (X, Y, and Z) of the forging billet. After forging in one direction, the forging billet is rotated 90° to proceed to the next direction. This operation helps eliminate anisotropy during forging and promotes microstructure homogenization.

[0034] Step S4 involves a two-phase forging process. The two-phase forging temperature is typically, but not limited to, 40°C, 50°C, or 60°C below the β-phase transformation point; the upsetting pressure during two-phase forging is typically, but not limited to, 33%, 34%, or 35%; and the height-to-diameter ratio after drawing during two-phase forging is typically, but not limited to, 1.9, 2.0, or 2.1. The term "diagonal square drawing" refers to the process where the diagonal direction of the forging blank corresponds to the forging direction. Compared to conventional square drawing, this optimizes the stress state of the forging and helps improve the uniformity of the microstructure.

[0035] Preferably, during the two-phase forging process, the secondary forging billet is subjected to four upsetting and drawing forging cycles. Each upsetting and drawing cycle employs a two-upsetting and two-drawing process. After completing one upsetting and one drawing cycle, the forging billet is kept in the furnace at the original forging temperature for 1 to 2 hours before being taken out of the furnace for the remaining one upsetting and one drawing cycle.

[0036] Step S5 involves the drawing and forging process. The drawing and forging temperature can typically, but not limitedly, be set to 50°C, 60°C, or 70°C below the β phase transformation point.

[0037] Preferably, during the drawing and forging process, the drawing deformation path is sequentially square billet, octagonal billet, and rounding. The single drawing reduction is 0.5 to 0.6 times the side length of the square billet, the reduction speed is 10 to 12 mm / s, and when the surface temperature of the forged billet is below 700℃, it is reheated in the furnace for 1 to 1.5 hours before continuing forging. The "octagonal billet" refers to a forged billet with an octagonal cross-section, which is an intermediate shape for the transition from a square billet to a round bar, and is beneficial for ensuring the roundness and dimensional accuracy of the final bar.

[0038] Preferably, a heat-insulating coating is applied before drawing and forging to reduce heat loss during the forging process and ensure the final forging temperature.

[0039] Step S6 involves a heat treatment process. Preferably, the heat treatment process is as follows: the bar is heated to 10-20°C below the β phase transformation point, then placed in a furnace and held for 2-3 hours, followed by air cooling; the temperature is then raised again to 30-40°C below the β phase transformation point, and held in a furnace for 2-3 hours; finally, it is furnace cooled to 620-650°C at a rate of 4-6°C / min, held for 8-10 hours, and then air cooled to obtain the finished TC18 titanium alloy bar. This invention employs a two-stage solution treatment + stepped aging heat treatment regime: the first solution treatment is carried out at 10-20°C below the β phase transformation point to ensure sufficient solution and homogenization of the alloying elements; the second solution treatment is carried out at 30-40°C below the β phase transformation point, combined with aging treatment of furnace cooling to 620-650°C at a rate of 4-6°C / min and holding for 8-10 hours, which can further control the morphology, size, and distribution of the α phase to achieve the best match between strength and toughness.

[0040] A second aspect of this invention provides a TC18 titanium alloy bar, which is prepared using the method described above. The bar has a tensile strength ≥1200 MPa and an impact energy KU2 ≥23 J.

[0041] Preferably, the diameter of the TC18 titanium alloy bar is Ф350mm.

[0042] The present invention will be further described below with reference to embodiments.

[0043] Example 1 The target alloy composition of TC18 titanium alloy is: Al 5.0wt.%, V 6.0wt.%, Mo 5.8wt.%, Cr 1.2wt.%, Fe 1.0wt.%, Zr 0.8wt.%, O 0.15wt.%, with the balance being Ti and unavoidable impurities.

[0044] The β-phase transformation point T of the TC18 titanium alloy β The actual temperature was 870℃.

[0045] TC18 titanium alloy bars are prepared by the following steps: Step 1 (S1: Smelting): Prepare an ingot with a single weight of 3t and an ingot shape of Φ(700~750)mm×L by three VAR smelting processes, and peel off the skin of the ingot body.

[0046] Step Two (S2: Billet Forging): The ingot is heated in an electric furnace, initially at 850℃ and held for 1.5~2.5 hours, then increased to 1200℃ at a rate of 80℃ / hour and held for 7 hours. After exiting the furnace, the ingot undergoes two rounds of upsetting and drawing, with an upsetting reduction of 40% and a four-way drawing process, maintaining a height-to-diameter ratio of 2.0. It is then returned to the furnace at 1150℃ and held for 3.0~4.0 hours. After exiting the furnace, the ingot undergoes two more rounds of upsetting and drawing, with an upsetting reduction of 40% and a four-way drawing process, maintaining a height-to-diameter ratio of 2.0.

[0047] Step 3 (S3: Single-phase forging, first heating pass): The forged billet obtained in Step 2 is heated in an electric furnace, charged to 850℃ and held for 1.5~2.5h, then heated to 1060℃ (T) at a rate of 60℃ / h. β +190℃), hold for 7 hours. After exiting the furnace, perform three XYZ three-way reversing upsetting and drawing forging, with an upsetting pressure of 48%, and perform four-way drawing. After drawing, the height-to-diameter ratio is controlled at 2.0.

[0048] Step 4 (S3: Single-phase forging, second heating): The forged billet obtained in Step 3 is heated in an electric furnace, charged to 850℃ and held for 1.5~2.5h, then heated to 980℃ (T) at a rate of 60℃ / h. β +110℃), hold for 7 hours. After exiting the furnace, perform three XYZ three-way reversing upsetting and drawing forging, with an upsetting pressure of 48%, and perform four-way drawing. After drawing, the height-to-diameter ratio is controlled at 2.0.

[0049] Step 5 (S3: Single-phase forging, third heating): The forged billet obtained in Step 4 is heated in an electric furnace, charged to 800℃ and held for 1.5~2.5h, then heated to 920℃ (T) at a rate of 50℃ / h. β +50℃), hold for 7 hours. After exiting the furnace, perform three XYZ three-way reversing upsetting and drawing forging, with an upsetting pressure of 48%, and perform four-way drawing. After drawing, the height-to-diameter ratio is controlled at 2.0.

[0050] Step Six (S4: Two-phase zone forging, first heating): The forged billet obtained in Step Five is heated in an electric heating furnace, loaded into the furnace at below 700℃, and then heated to 810℃ (T) at a rate of 50℃ / h. β -60℃), hold for 8 hours. After exiting the furnace, perform two screwing and pulling operations (two screwings and two pullings). The screwing is done with a 35% reduction in thickness, and the pulling is done diagonally in a square manner. After pulling, the height-to-diameter ratio is controlled at 2.0. After each screwing and pulling, the furnace needs to be returned to the original temperature for 2 hours of holding.

[0051] Step Seven (S4: Two-Phase Forging, Second Heat): The forged billet obtained in Step Six is ​​heated in an electric furnace, loaded into the furnace at below 700℃, and then heated to 810℃ (T) at a rate of 50℃ / h. β -60℃), hold for 8 hours. After exiting the furnace, perform two screwing and pulling operations (two screwings and two pullings). The screwing is done with a 35% reduction in thickness, and the pulling is done diagonally in a square manner. After pulling, the height-to-diameter ratio is controlled at 2.0. After each screwing and pulling, the furnace needs to be returned to the original temperature for 2 hours of holding.

[0052] Step 8 (S4: Two-phase zone forging, third heating): The forged billet obtained in Step 7 is heated in an electric heating furnace, loaded into the furnace at below 700℃, and then heated to 810℃ (T) at a rate of 50℃ / h. β-60℃), hold for 8 hours. After exiting the furnace, perform two screwing and pulling operations (two screwings and two pullings). The screwing is done with a 35% reduction in thickness, and the pulling is done diagonally in a square manner. After pulling, the height-to-diameter ratio is controlled at 2.0. After each screwing and pulling, the furnace needs to be returned to the original temperature for 2 hours of holding.

[0053] Step Nine (S4: Two-Phase Forging, Fourth Heat): The forged billet obtained in Step Eight is heated in an electric furnace, loaded into the furnace at below 700℃, and then heated to 810℃ (T) at a rate of 50℃ / h. β -60℃), hold for 8 hours. After exiting the furnace, perform two screwing and pulling operations (two screwings and two pullings). The screwing is done with a 35% reduction in thickness, and the pulling is done diagonally in a square manner. After pulling, the height-to-diameter ratio is controlled at 2.0. After each screwing and pulling, the furnace needs to be returned to the original temperature for 2 hours of holding.

[0054] Step 10 (S5: Drawing and Forging): The forged billet obtained in Step 9 is coated with heat-insulating paint, loaded into the furnace at a temperature below 700℃, and then heated to 800℃ (T) at a rate of 50℃ / h. β -70℃), hold at that temperature for 8 hours. After exiting the furnace, draw the billet to □360mm×L with a single reduction of 0.5d (d is the side length of the square billet) and a reduction speed of 10mm / s. Use a temperature gun to check the surface temperature of the forging billet. If it is below 700℃, return it to the furnace at the original temperature for 1~1.5 hours. After exiting the furnace, continue drawing until it reaches □360mm×L. Then return it to the furnace at the original temperature for 1~1.5 hours. After exiting the furnace, draw it to 380mm×L (octagonal). Then return it to the furnace at the original temperature for 1~1.5 hours. After exiting the furnace, roll it to Ф360mm×L.

[0055] Step 11 (S6: Heat Treatment): The forged bar is heated to 855℃ in the furnace. β (T) β Furnace assembly at -15℃ and hold for 2 hours → air cooling → Furnace assembly and hold for 3 hours when furnace temperature reaches 835℃ (Tβ-35℃) → furnace cooling to 630℃ at a rate of 4℃ / min, hold for 8 hours → air cooling.

[0056] Performance test results: The tensile strength of the bar was 1251 / 1248 / 1237 MPa, and the impact energy KU2 was 25 / 30 / 28 J.

[0057] Example 2 The target alloy composition of TC18 titanium alloy is: Al 4.8wt.%, V 6.0wt.%, Mo 5.8wt.%, Cr 1.2wt.%, Fe 1.0wt.%, Zr 1.0wt.%, O 0.18wt.%, with the balance being Ti and unavoidable impurities.

[0058] Steps one through eleven are the same as in Example 1.

[0059] Performance test results: The tensile strength of the bar was 1288 / 1310 / 1283 MPa, and the impact energy KU2 was 24 / 23 / 25 J.

[0060] Example 3 The target alloy composition of TC18 titanium alloy is consistent with that of Example 1.

[0061] Steps one through ten are the same as in Example 1.

[0062] Step 11 (S6: Heat Treatment): The forged bar is heated to 858℃ (T) in the furnace. β The furnace is installed at -12℃ and kept at that temperature for 2 hours → air-cooled → the furnace temperature is raised to 840℃ (T β The furnace is set at -30℃ and held for 3 hours → furnace cooled to 650℃ at a rate of 4℃ / min, held for 8 hours → air cooled.

[0063] Performance test results: The tensile strength of the bar was 1202 / 1211 / 1204 MPa, and the impact energy KU2 was 28 / 31 / 28 J.

[0064] As can be seen from the above embodiments, by using the preparation method provided by the present invention, the tensile strength of TC18 titanium alloy bars is ≥1200MPa and the impact energy KU2 is ≥23J, achieving a synergistic match between high strength and high impact toughness, and the microstructure of the Ф350mm specification bars is excellent.

[0065] Finally, it should be noted that the embodiments described above are only some, not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing TC18 titanium alloy bars, characterized in that, Includes the following steps: S1. According to the target composition of TC18 titanium alloy, the ingot is produced by multiple VAR melting processes. The target composition of TC18 titanium alloy, by mass percentage, includes: Al: 4.8~5.0%, V: 5.8~6.0%, Mo: 5.6~5.8%, Cr: 1.2~1.5%, Fe: 0.8~1.2%, Zr: 0.5~1.0%, O: 0.15~0.18%, with the balance being Ti and unavoidable impurities. S2. The ingot is heated to 1140~1210℃, held at that temperature, and then subjected to multiple upsetting and drawing forging to obtain a first-grade forging billet. The upsetting pressure is 35~40%, the drawing is done in a square manner, and the height-to-diameter ratio is controlled at 1.9~2.1 after drawing. S3. The first-stage forging billet is subjected to multiple upsetting and drawing forging processes within a temperature range of 50~200℃ above the β phase transformation point to obtain a second-stage forging billet. The upsetting pressure is 45~50%, the drawing is done using square drawing, and the height-to-diameter ratio after drawing is controlled at 1.9~2.

1. S4. The secondary forging billet is subjected to multiple upsetting and drawing forging in the range of 40~60℃ below the β phase transformation point to obtain the tertiary forging billet. The upsetting pressure is 33~35%, the drawing is diagonal square drawing, and the height-to-diameter ratio after drawing is controlled at 1.9~2.

1. S5. The three-stage forging billet is subjected to one-time drawing forging in the range of 50~70℃ below the β phase transformation point to obtain a bar. S6. The bar is heat-treated to obtain the finished TC18 titanium alloy bar.

2. The method for preparing TC18 titanium alloy rods according to claim 1, characterized in that, In step S2, the ingot is subjected to two-stage upsetting and drawing forging: the first stage of upsetting and drawing forging heats the ingot to 1200±10℃ and holds it at that temperature, and then performs two upsetting and drawing operations; the second stage of upsetting and drawing forging heats the billet to 1150±10℃ and holds it at that temperature, and then performs two upsetting and drawing operations.

3. The method for preparing TC18 titanium alloy rods according to claim 1, characterized in that, In step S3, the primary forging billet is subjected to three-stage upsetting and drawing forging, with each stage involving XYZ three-axis reversal forging.

4. The method for preparing TC18 titanium alloy rods according to claim 3, characterized in that, In step S3, the heating temperature of multi-stage upsetting and drawing forging gradually decreases as the number of stages increases.

5. The method for preparing TC18 titanium alloy rods according to claim 1, characterized in that, In step S4, the secondary forging billet is subjected to four upsetting and drawing forging processes. Each upsetting and drawing process is adopted. After one upsetting and one drawing is completed, the forging billet is kept in the furnace at the original forging temperature for 1 to 2 hours before being taken out of the furnace for the remaining one upsetting and one drawing.

6. The method for preparing TC18 titanium alloy rods according to claim 1, characterized in that, In step S5, the elongation deformation path is sequentially square billet, octagonal billet, and rounding.

7. The method for preparing TC18 titanium alloy rods according to claim 6, characterized in that, In step S5, the single drawing reduction is 0.5 to 0.6 times the side length of the square billet, the reduction speed is 10 to 12 mm / s, and when the surface temperature of the forging billet is below 700℃, it is returned to the furnace for heat preservation for 1 to 1.5 hours before continuing forging.

8. The method for preparing TC18 titanium alloy rods according to claim 1, characterized in that, In step S5, a heat-insulating coating is applied before the drawing and forging process.

9. The method for preparing TC18 titanium alloy rods according to claim 1, characterized in that, In step S6, the heat treatment process is as follows: the bar is heated to 10-20°C below the β phase transformation point and held in a furnace for 2-3 hours, then removed from the furnace and air-cooled; the temperature is then raised again to 30-40°C below the β phase transformation point and held in a furnace for 2-3 hours; the bar is then furnace-cooled to 620-650°C at a rate of 4-6°C / min, held for 8-10 hours, and then air-cooled to obtain the finished TC18 titanium alloy bar.

10. A TC18 titanium alloy bar, characterized in that, It is prepared by any one of claims 1-9.

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