A preparation method for small-sized thick-walled titanium alloy tubes

Abstract

The present invention discloses a preparation method for small-sized thick-walled titanium alloy tubes, comprising the following steps: preparing a titanium alloy ingot, forging a billet for extrusion, drilling a hole along the central axis of the billet with a hole diameter of 42-50 mm, and then using a steel plate and a copper plate with a thickness of 0.8-1.2 mm to perform composite sheathing on the inside and outside of the extruded billet; feeding the extruded blank into an industrial frequency induction furnace or a resistance furnace for heating, and performing forward extrusion after heat preservation for 0.5-4 h; when extruding, ensure that after the extruded billet is placed in the extrusion cylinder, the central axis of the extrusion cylinder coincides with the axial central axis of the billet, and at the same time, insert a stepped extrusion needle into the central hole of the extruded billet and protrude 20-50 mm. During extrusion, the extrusion needle remains stationary, and the extrusion shaft applies pressure to the extruded billet and controls the extrusion speed to be 175-185 mm / s to obtain a thick-walled titanium alloy tube blank; and then obtaining a high-strength thick-walled titanium alloy tube through straightening, heat treatment, and surface treatment. The preparation method for small-sized thick-walled titanium alloy tubes provided by the present invention has a tensile strength of the prepared thick-walled tube greater than 1150 Mpa, high elongation, and good plasticity.

Description

Technical Field

[0001] The present invention relates to the technical field of non-ferrous metal processing, and particularly relates to a preparation method for small-sized thick-walled titanium alloy tubes. Background Art

[0002] TC4 titanium alloy is a typical α+β type two-phase titanium alloy, which has excellent comprehensive properties and has been most widely used in the aviation and aerospace industries. There are two types of titanium alloy tubes: welded tubes and seamless tubes. Among them, welded tubes have low pressure resistance and are difficult to be applied in the aviation and aerospace fields.

[0003] At home and abroad, methods such as cross-rolling piercing rolling, extrusion, drawing, and welding are mainly used to produce seamless titanium alloy tubes. The production process technology for ordinary titanium alloy tubes with an outer diameter / wall thickness (D / S) of 10-40 is relatively mature. However, for thick-walled titanium alloy tubes with D / S < 10, if produced by cold rolling / drawing methods, the tensile strength after heat treatment is generally 960 MPa, and the working pressure is also low; for tubes with a tensile strength greater than 1050 MPa and a working pressure higher than 21 MPa, due to low plasticity, they cannot be rolled and produced by cold rolling and other methods. Thick-walled titanium alloy tubes are difficult to process due to their large thickness and small inner hole, high requirements for tissue uniformity and mechanical properties, and the current production technology is not yet mature. Since there is no effective method for producing long-length seamless titanium alloy tubes, such medium and high-strength seamless titanium alloy tubes have not been widely used.

[0004] In view of this, it is necessary to provide a new process to solve the above technical problems. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a preparation method for small-sized thick-walled titanium alloy tubes, and the prepared thick-walled tubes have a tensile strength greater than 1150 Mpa, high elongation, and good plasticity.

[0006] To solve the above problems, the technical solution of the present invention is as follows:

[0007] A preparation method for small-sized thick-walled titanium alloy tubes includes the following steps:

[0008] Step S1, melting titanium alloy raw materials through a vacuum consumable arc furnace for no less than two times to prepare a titanium alloy ingot;

[0009] Step S2, heating the titanium alloy ingot and then forging it into a billet for extrusion through upsetting and drawing free forging;

[0010] Step S3, drilling a hole along the central axis of the billet prepared in Step S2, with a hole diameter of 40-50 mm, and then using a steel plate and a copper plate with a thickness of 0.8-1.2 mm to perform composite sheathing on the inside and outside of the extrusion billet, where the steel sleeve is closely attached to the extrusion billet, and the copper sleeve is closely attached to the outside of the steel sleeve;

[0011] Among them, the drilling hole diameter can be 40 mm, 42 mm, 45 mm, 48 mm, 50 mm, or other hole diameter values within this range;

[0012] Step S4: Provide a variable-diameter extrusion needle, which includes a detachable front section and a rear section of the extrusion needle. In the extrusion process, the front section of the extrusion needle protrudes 20 - 50 mm from the extrusion billet. The front section of the extrusion needle includes a variable-diameter area and an equal-diameter area. The variable-diameter area is connected to the rear section of the extrusion needle, and the diameter of the equal-diameter area is equivalent to the inner diameter of the thick-walled pipe.

[0013] Feed the extrusion billet obtained in step S3 into an industrial frequency induction furnace or a resistance furnace for heating. After heat preservation for 0.5 - 4 h, perform forward extrusion. When extruding, ensure that after the extrusion billet is placed in the extrusion cylinder, the center line of the extrusion cylinder coincides with the axial center line of the billet. At the same time, insert the variable-diameter extrusion needle into the central hole of the extrusion billet and make it protrude 20 - 50 mm. Keep the extrusion needle stationary during extrusion. The extrusion shaft applies pressure to the extrusion billet and controls the extrusion speed to be 175 - 185 mm / s to achieve the plastic deformation of the extrusion billet and obtain a thick-walled titanium alloy pipe billet;

[0014] Among them, the extrusion speed can be 175 mm / s, 178 mm / s, 180 mm / s, 182 mm / s, 185 mm / s, or other values within this range;

[0015] Step S5: Roller straighten the pipe billet obtained in step S4 within the temperature range of 300 - 500 °C. The straightness of the straightened pipe is ≤ 3 mm / m;

[0016] Step S6: Perform double heat treatment on the pipe billet obtained in step S5 in a resistance furnace. The solution temperature is controlled within the range of 20 - 60 °C below the phase transition point, and the aging temperature is controlled between 460 - 600 °C;

[0017] Step S7: Remove the surface oxides and extrusion grooves of the titanium alloy pipe through precision machining to make the surface roughness ≤ 1.6 μm and obtain a high-strength thick-walled titanium alloy pipe.

[0018] Furthermore, the variable-diameter extrusion needle includes a detachable front section and a rear section of the extrusion needle. In the extrusion process, the front section of the extrusion needle protrudes 20 - 50 mm from the extrusion billet. The front section of the extrusion needle includes a variable-diameter area and an equal-diameter area. The variable-diameter area is connected to the rear section of the extrusion needle, and the diameter of the equal-diameter area is equivalent to the inner diameter of the thick-walled pipe.

[0019] Furthermore, the size of the variable-diameter area of the front section of the extrusion needle gradually decreases from the end close to the rear section of the extrusion needle to the end close to the equal-diameter area and smoothly transitions to the equal-diameter area.

[0020] Furthermore, the slope of the diameter-changing area at the front section of the extrusion needle is greater than 0.02.

[0021] Furthermore, in step S3, the diameter of the hole drilled in the billet is 5 - 12 mm larger than the diameter of the rear section of the extrusion needle.

[0022] Furthermore, the variable-diameter extrusion needle further includes:

[0023] a connecting piece with an external thread, and the connecting piece is fixedly connected to the right end of the front section of the extrusion needle;

[0024] an embedding hole with an internal thread, and the embedding hole is opened on the left side of the rear section of the extrusion needle; the connecting piece is screwed into the embedding hole to realize the connection between the front section and the rear section of the extrusion needle.

[0025] Furthermore, a plugging groove and a plugging block that is in fit connection with the plugging groove are opened at the bottom of the rear section of the extrusion needle, and sliding grooves are opened on both sides of the bottom of the rear section of the extrusion needle, and sliders are slidably connected inside the sliding grooves.

[0026] Furthermore, a spring is arranged on the outer side of the slider and inside the sliding groove, and an L-shaped plugging rod is fixedly installed at the bottom of the slider.

[0027] Furthermore, a fixed limiting block is fixedly connected to the bottom of the plugging block, and a through jack is opened inside the fixed limiting block.

[0028] Furthermore, a connecting rope is connected to the jack.

[0029] Compared with the prior art, the preparation method of small-specification thick-walled titanium alloy tubes provided by the present invention has the following beneficial effects:

[0030] 1. In the preparation method of small-specification thick-walled titanium alloy tubes provided by the present invention, a variable-diameter extrusion needle is used to extrude the billet. The front section of the extrusion needle belongs to the diameter-changing section and serves as the main working section, bearing the compressive stress and tensile stress of the titanium alloy deformation under high temperature and high pressure. The rear section of the extrusion needle serves as a structurally stable section, bearing the maximum tensile force during the extrusion process, ensuring the smooth realization of the extrusion process and preventing the needle from breaking. The variable-diameter extrusion needle ensures the stability of the needle during the extrusion process, is beneficial to the concentricity of the outer diameter and inner diameter of the extruded tube, and reduces the wall thickness deviation of the thick-walled extruded tube; by improving the slope of the working section of the extrusion needle, it is beneficial to the metal flow during the extrusion process, thereby reducing the metal deformation resistance and facilitating the smooth extrusion of the tube through the die hole. The extruded titanium alloy tube blank is prepared into thick-walled high-strength titanium alloy tubes through subsequent solution aging treatment, thus solving the production problem of small-diameter tubes.

[0031] II. The preparation method of small-sized thick-walled titanium alloy tubes provided by the present invention can produce seamless titanium alloy tubes with a wall thickness greater than 20 mm and a length exceeding 4 m. Moreover, the prepared tubes have good tissue uniformity, strong performance consistency, a tensile strength exceeding 1150 Mpa, and an elongation greater than 11.5%, which can well meet the requirements of the aerospace field for high-strength seamless tubes.

[0032] III. In the preparation method of small-sized thick-walled titanium alloy tubes provided by the present invention, the front section and the rear section of the variable-diameter extrusion needle are detachably connected, and the front section / and the rear section of the extrusion needle can be changed according to processing requirements, so that the processing of tubes with any inner diameter size can be quickly realized, the production is convenient, the efficiency is high, and the tooling materials are saved.

[0033] IV. In the preparation method of small-sized thick-walled titanium alloy tubes provided by the present invention, the finished product rate of tube preparation is high, and the finished product rate of ultrasonic testing is 100%. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0035] Figure 1 It is a schematic structural diagram of Embodiment 1 of the variable-diameter extrusion needle in the present invention; Figure 2 It is Figure 1 a schematic structural diagram of the front section of the extrusion needle in the shown variable-diameter extrusion needle;

[0036] Figure 3 It is Figure 1 a schematic structural diagram of the rear section of the extrusion needle in the shown variable-diameter extrusion needle;

[0037] Figure 4 It is a schematic structural diagram of Embodiment 2 of the variable-diameter extrusion needle in the present invention;

[0038] Figure 5 It is Figure 5 a schematic structural diagram of another angle of the insertion slot in;

[0039] Figure 6 It is an assembly schematic diagram of the variable-diameter extrusion needle and the extrusion billet in the extrusion process. DETAILED DESCRIPTION OF THE INVENTION

[0040] In order to enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention and make the above-mentioned objects, features, and advantages of the present invention more obvious and understandable, the following further explains the specific embodiments of the present invention in conjunction with the drawings.

[0041] It should be noted here that the description of these embodiments is used to help understand the present invention, but does not limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0042] Embodiment 1

[0043] Please refer to Figures 1 - 3 , in the variable-diameter extrusion needle 100 used in the extrusion process of the present invention, it includes an extrusion needle front section 1 and an extrusion needle rear section 2, and the extrusion needle front section 1 and the extrusion needle rear section 2 are detachably connected. The extrusion needle front section / and the extrusion needle rear section are replaced according to the actual processing dimension requirements, so as to meet the processing of pipes with different inner diameters and wall thicknesses. Among them, the extrusion needle front section 1 is a variable-diameter section.

[0044] Specifically, the extrusion needle front section 1 includes a variable-diameter area 111 and an equal-diameter area 112. The variable-diameter area 111 is connected to the extrusion needle rear section 2, and the diameter of the equal-diameter area 112 is equivalent to the inner diameter of the thick-walled pipe. The size of the variable-diameter area of the extrusion needle front section 1 gradually decreases from the end close to the extrusion needle rear section to the end close to the equal-diameter area, and is smoothly transitioned with the equal-diameter area. The slope of the variable-diameter area is greater than 0.02, and can be specifically determined according to the inner diameter of the thick-walled pipe and the diameter of the extrusion needle rear section.

[0045] The diameter of the extrusion needle rear section 2 is related to the drilling aperture of the bar blank pipe in the pipe processing technology. Specifically, the bar blank drilling aperture is 5-12 mm larger than the diameter of the extrusion needle rear section.

[0046] In this embodiment, the extrusion needle rear section 2 and the extrusion needle front section 1 are connected by threads. Specifically, one end of the extrusion needle rear section 2 is provided with an insertion hole 221, and the insertion hole has an internal thread, while one end of the extrusion needle front section 1 connected thereto is provided with a connecting member 113, and the connecting member is provided with an external thread matching the internal thread. The connecting member 113 is screwed into the insertion hole to realize the detachable connection between the two.

[0047] The working principle of the variable-diameter extrusion needle provided by the present invention is as follows:

[0048] When in use, the connecting member and the insertion hole are manually aligned and the connecting member is inserted into the interior of the insertion hole. The extrusion needle rear section is rotated, and the rotation of the extrusion needle rear section drives the insertion hole to rotate. At this time, the extrusion needle front section and the connecting member are in a stationary state, so that the connecting member is inserted into the interior of the insertion hole, and the two are connected together by threads, so that the extrusion needle front section and the extrusion needle rear section are installed and fixed, and at this time they form a whole. The extrusion needle front section is inserted into the extrusion bar blank and protrudes 20-50 mm for extrusion.

[0049] Embodiment 2

[0050] Based on the variable-diameter extrusion needle provided in Embodiment 1, the variable-diameter extrusion needle of Embodiment 2 is a preferred mode of Embodiment 1 and will not affect the individual implementation of Embodiment 1.

[0051] Please refer to Figure 4 and Figure 5 For this embodiment, the difference between the variable-diameter extrusion needle of this embodiment and that of Embodiment 1 is that: a socket groove 8 is provided at the bottom of the rear section of the extrusion needle, and sliding grooves 9 are provided on both sides of the bottom of the rear section of the extrusion needle, and a slider 10 is slidably connected inside the sliding groove 9.

[0052] The socket groove 8 and the socket block 13 are adapted to each other. When the socket block 13 is completely inserted into the inside of the socket groove 8, the top of the fixed limit block 15 contacts the bottom of the rear section of the extrusion needle.

[0053] A spring 11 is provided on the outside of the slider 10 and inside the sliding groove 9, and an L-shaped insertion rod 12 is fixedly installed at the bottom of the slider 10.

[0054] Through the elastic deformation of the spring 11, the slider 10 can be forced inward, so that the L-shaped insertion rod 12 can be forced to be stably inserted into the inside of the through insertion hole 14.

[0055] A socket block 13 is provided at the bottom of the rear section of the extrusion needle and at the bottom of the socket groove 8.

[0056] The bottom of the socket block 13 is fixedly connected to a fixed limit block 15, and a through insertion hole 14 is provided inside the fixed limit block 15.

[0057] The use diameter of the through insertion hole 14 is adapted to the use diameter of the L-shaped insertion rod 12, so that the L-shaped insertion rod 12 can be inserted into the inside of the through insertion hole 14 to fix the socket block 13 inside the socket groove 8.

[0058] The bottom of the fixed limit block 15 is fixedly connected to a connecting rope 16.

[0059] The working principle of the variable-diameter extrusion needle of this embodiment is as follows:

[0060] When it is not in use as a whole, manually push the slider 10 outward simultaneously, causing the slider 10 to be forced to move from the inside to the outside within the chute 9. By moving the slider 10, the spring 11 is compressed, causing the spring 11 to undergo elastic deformation. Then, by moving the slider 10, the L-shaped insertion rod 12 is driven to move until the slider 10 moves to the outermost position within the chute 9. At this time, the spring 11 is compressed to the smallest state, and the L-shaped insertion rod 12 disengages from the outside of the insertion slot 8, so that the bottom position of the insertion slot 8 is in a state without any object blocking. At this time, manually insert the insertion block 13 into the insertion slot 8 from the outside opening position of the insertion slot 8 until the insertion block 13 is completely inserted into the insertion slot 8.

[0061] At this time, the two open positions of the through-hole 14 are aligned with the L-shaped insertion rod 12. Then, by releasing the outward push on the slider 10, the slider 10 is pushed inward by the elastic deformation of the spring 11, causing the slider 10 to move from the outside to the inside within the chute 9. Then, by moving the slider 10, the L-shaped insertion rod 12 is driven to move, so that the L-shaped insertion rod 12 is inserted into the through-hole 14, thereby fixing the insertion block 13 within the insertion slot 8. At this time, the fixed limit block 15 is fixed at the bottom of the rear section of the extrusion needle, so that the connecting rope 16 and the rear section of the extrusion needle are connected together. At this time, the rear section of the extrusion needle can be hung at a suitable position through the connecting rope 16.

[0062] The deformable extrusion needle of this embodiment can be hung by using the connecting rope 16 during its non-use period through the mutual cooperation among structures such as the insertion slot 8, the chute 9, the slider 10, the spring 11, the L-shaped insertion rod 12, the insertion block 13, the through-hole 14, the fixed limit block 15, and the connecting rope 16, avoiding loss during non-use. And the hanging position is near the working position, which is convenient for later use. The structure is simple, the operation is convenient, and the connection and separation with the connecting rope 16 can be carried out quickly.

[0063] Embodiment 3

[0064] A preparation method for small-sized titanium alloy thick-walled tubes includes the following steps:

[0065] Step S1, melting the titanium alloy raw material by a vacuum consumable arc furnace for no less than two times to prepare a φ700mm titanium alloy ingot;

[0066] Step S2, heating the titanium alloy ingot and then performing upsetting and drawing free forging to forge it into a Φ210mm extrusion billet;

[0067] Step S3: Drill a hole along the central axis of the billet prepared in Step S2 with a hole diameter of 42 mm. Then, use a steel plate and a copper plate with a thickness of 0.8 - 1.2 mm to perform composite sheathing on the inside and outside of the extruded billet. The steel sheath is in close contact with the extruded billet, and the copper sheath is in close contact with the outside of the steel sheath.

[0068] Step S4: Feed the extruded blank obtained in Step S3 into a power frequency induction furnace or a resistance furnace for heating. After heat preservation for 0.5 - 4 h, perform forward extrusion. When extruding, ensure that after the extruded billet is placed in the extrusion cylinder, the central axis of the extrusion cylinder coincides with the axial central axis of the billet. At the same time, insert a stepped extrusion needle (the structure is as in Example 1) into the central hole of the extruded billet, and the front section of the extrusion needle protrudes 20 - 50 mm. During extrusion, the extrusion needle remains stationary, and the extrusion shaft applies pressure to the extruded billet and controls the extrusion speed to 180 mm / s to achieve the plastic deformation of the extruded billet and obtain a thick-walled titanium alloy tube blank with a diameter of φ84×23.5 mm. Among them, the assembly schematic diagram of the stepped extrusion needle 100 and the extruded billet 200 is as Figure 6 shown;

[0069] Step S5: Perform roller straightening on the tube blank obtained in Step S4 within the temperature range of 300 - 500 °C. The straightness of the straightened tube is ≤ 3 mm / m.

[0070] Step S6: Perform double heat treatment on the tube blank obtained in Step S5 in a resistance furnace. The solution temperature is controlled within the range of 20 - 60 °C below the phase transformation point, and the aging temperature is controlled between 460 - 600 °C.

[0071] Step S7: Remove the surface oxides and extrusion grooves of the titanium alloy tube through precision machining to make the surface roughness ≤ 1.6 μm and obtain a high-strength titanium alloy thick-walled tube.

[0072] Perform performance testing on the high-strength titanium alloy thick-walled tube of Example 2. The test results are shown in Table 1:

[0073] Table 1: Performance test results of the titanium alloy thick-walled tube prepared in Example 2

[0074] Tensile strength (MPa) Elongation rate (%) Working pressure (MPa) Example 2 1197 13.5 42

[0075] Compared with the prior art, the preparation method of the small-sized titanium alloy thick-walled tube provided by the present invention has the beneficial effects that:

[0076] 1. The preparation method of small-sized thick-walled titanium alloy tubes provided by the present invention uses a variable-diameter extrusion needle to extrude a billet. The front section of the extrusion needle belongs to the variable-diameter section and serves as the main working section, which bears the compressive stress and tensile stress of the titanium alloy deformation under high temperature and high pressure. The rear section of the extrusion needle serves as a structurally stable section, which bears the maximum tensile force during the extrusion process to ensure the smooth realization of the extrusion process without needle breakage. The variable-diameter extrusion needle ensures the stability of the needle during the extrusion process, is conducive to the concentricity of the outer diameter and inner diameter of the extruded tube, and reduces the wall thickness deviation of the thick-walled extruded tube. By improving the slope of the working section of the extrusion needle, it is conducive to the metal flow during the extrusion process, thereby reducing the metal deformation resistance and facilitating the smooth extrusion of the tube through the die hole. The extruded titanium alloy tube blank is prepared into thick-walled high-strength titanium alloy tubes through subsequent solution aging treatment, thus solving the production problem of small-diameter tubes.

[0077] 2. The preparation method of small-sized thick-walled titanium alloy tubes provided by the present invention can prepare seamless titanium alloy tubes with a wall thickness greater than 20 mm and a length exceeding 4 m. Moreover, the prepared tubes have good tissue uniformity, strong performance consistency, a tensile strength exceeding 1150 Mpa, and an elongation greater than 11.5%, which can well meet the requirements of the aerospace field for high-strength seamless tubes.

[0078] 3. The preparation method of small-sized thick-walled titanium alloy tubes provided by the present invention has a detachable connection between the front section and the rear section of the variable-diameter extrusion needle. The front section and / or the rear section of the extrusion needle can be changed according to the processing requirements, so that the processing of tubes with any inner diameter size can be quickly realized, the production is convenient, the efficiency is high, and the tooling materials are saved.

[0079] 4. The preparation method of small-sized thick-walled titanium alloy tubes provided by the present invention has a high finished product rate for tube preparation, and the finished product rate of ultrasonic testing is 100%.

[0080] The above has made a detailed description of the embodiments of the present invention in conjunction with the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations made to these embodiments without departing from the principle and spirit of the present invention still fall within the protection scope of the present invention.

Claims

1. A method for preparing small-sized thick-walled titanium alloy tubes, characterized in that, It includes the following steps: Step S1: Melting the titanium alloy raw material in a vacuum consumable arc furnace for no less than two times to prepare a titanium alloy ingot; Step S2: Heating the titanium alloy ingot and then performing upsetting and drawing free forging to forge it into a billet for extrusion; Step S3: Drilling the billet prepared in Step S2 along the central axis with a hole diameter of 40 - 50 mm, and then using a steel plate and a copper plate with a thickness of 0.8 - 1.2 mm to perform composite sheathing on the inside and outside of the extrusion billet, where the steel sheath is closely attached to the extrusion billet and the copper sheath is closely attached to the outside of the steel sheath; Step S4: Providing a variable-diameter extrusion needle, the variable-diameter extrusion needle includes a detachable extrusion needle front section and an extrusion needle rear section. In the extrusion process, the extrusion needle front section protrudes 20 - 50 mm from the extrusion billet. The extrusion needle front section includes a variable-diameter area and an equal-diameter area. The variable-diameter area is connected to the extrusion needle rear section, and the diameter of the equal-diameter area is equivalent to the inner diameter of the thick-walled pipe; Feeding the extrusion blank prepared in Step S3 into an industrial frequency induction furnace or a resistance furnace for heating, and performing forward extrusion after heat preservation for 0.5 - 4 h; when extruding, ensure that after the extrusion billet is placed in the extrusion cylinder, the central axis of the extrusion cylinder coincides with the axial central axis of the billet. At the same time, insert the variable-diameter extrusion needle into the central hole of the extrusion billet and protrude 20 - 50 mm. Keep the extrusion needle stationary during extrusion. The extrusion shaft applies pressure to the extrusion billet and controls the extrusion speed at 175 - 185 mm / s to achieve the plastic deformation of the extrusion billet and obtain a thick-walled titanium alloy pipe blank; Step S5: Straightening the pipe blank prepared in Step S4 by roll straightening within the temperature range of 300 - 500 °C, and the straightness of the straightened pipe ≤ 3 mm / m; Step S6: Performing double heat treatment on the pipe blank prepared in Step S5 in a resistance furnace, controlling the solution temperature within the range of 20 - 60 °C below the phase transformation point, and controlling the aging temperature between 460 - 600 °C; Step S7: Removing the surface oxides and extrusion grooves of the titanium alloy pipe through precision machining to make the surface roughness ≤ 1.6 um and obtain a high-strength titanium alloy thick-walled pipe; the size of the variable-diameter area of the extrusion needle front section gradually decreases from the end close to the extrusion needle rear section to the end close to the equal-diameter area and smoothly transitions to the equal-diameter area; the slope of the variable-diameter area of the extrusion needle front section is greater than 0.02; in Step S3, the hole diameter of the billet drilling is 5 - 12 mm larger than the diameter of the extrusion needle rear section.

2. The method for preparing small-sized thick-walled titanium alloy tubes according to claim 1, characterized in that, The variable-diameter extrusion needle further includes: A connector with an external thread, the connector is fixedly connected to the right end of the extrusion needle front section; An embedding hole with an internal thread, the embedding hole is opened on the left side of the extrusion needle rear section; the connector is screwed into the embedding hole to realize the connection between the extrusion needle front section and the extrusion needle rear section.

3. The method for preparing small-sized thick-walled titanium alloy tubes according to claim 1, characterized in that, A plugging groove and a plugging block that cooperates with the plugging groove are opened at the bottom of the extrusion needle rear section. Sliding grooves are opened on both sides of the bottom of the extrusion needle rear section, and sliders are slidably connected inside the sliding grooves.

4. The method for preparing small-sized thick-walled titanium alloy tubes according to claim 3, characterized in that, A spring is arranged on the outside of the slider and inside the sliding groove, and an L-shaped insertion rod is fixedly installed at the bottom of the slider.

5. The method for preparing small-sized thick-walled titanium alloy tubes according to claim 3, characterized in that, A fixed limit block is fixedly connected to the bottom of the plugging block, and a through insertion hole is opened inside the fixed limit block.

6. The method for preparing small-sized thick-walled titanium alloy tubes according to claim 5, characterized in that, A connecting rope is connected to the insertion hole.

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