Oblique piercing method for TC11 titanium alloy thick-wall pipe blank under phase transformation point

Abstract

The invention relates to the technical field of titanium alloy pipe machining, in particular to a cross piercing method for a TC11 titanium alloy thick-wall pipe blank under a phase transformation point. A small-caliber through hole is prefabricated in the bar blank, then heating is conducted, and the heating temperature is lower than the phase transformation point of the material; a lubricant is smeared, a perforating needle of a variable-diameter structure is used for conducting segmented transition type perforating, and in the perforating process, the perforating temperature is controlled to be below the phase transformation point; and then air-cooling to room temperature. Through cooperative use of the prefabricated small-caliber through hole and the variable-diameter perforating needle, the perforating difficulty is greatly reduced, the deformation resistance in the perforating process is reduced, and the heat dissipation efficiency in the severe deformation process of a material is improved; the inner wall of the through hole is lubricated, so that the friction condition in the perforating process is improved; and the performance of the material in subsequent use and processing is ensured by controlling the perforation temperature to be below the phase transformation point. The method can effectively improve the perforating efficiency and the pipe quality, and is suitable for producing and manufacturing the high-strength titanium alloy thick-wall pipe blank.

Description

Technical Field

[0001] The invention relates to the technical field of titanium alloy pipe processing, and in particular to a method for oblique rolling and piercing of a TC11 titanium alloy thick-walled pipe blank at a phase transition point. Background Art

[0002] Titanium alloys are widely used in aerospace, marine engineering, and other fields due to their excellent specific strength, corrosion resistance, and high-temperature performance. The preparation process for thick-walled tubes of TC11 titanium alloy, a high-strength two-phase titanium alloy, has a significant impact on subsequent processing and the performance of the final product. Currently, the main methods for preparing thick-walled titanium alloy tubes include machining piercing and cross-rolling piercing.

[0003] Machining through holes is a traditional method for preparing tube blanks. This involves drilling holes through solid bar stock. While this method is mature, it suffers from high processing costs and low material utilization. This is particularly true for high-strength titanium alloys, where machining efficiency is lower and costs are higher.

[0004] Cross-rolling piercing is an efficient method for preparing tube billets, which has the advantages of high production efficiency and high material utilization, and has become a research hotspot. For example, CN110340148A discloses a cross-rolling piercing method for two-phase titanium alloy seamless tube billets for marine use. Through the combined design of head and tail centering holes, the use of anti-oxidation coatings, and a heating process combining induction heating with resistance furnace insulation, it is possible to use existing equipment to efficiently and stably produce two-phase titanium alloy seamless tube billets. CN110252814B discloses a two-roll cross-rolling piercing method for titanium alloy solid bar billets. By controlling parameters such as the head extension, feed angle, rolling angle, reduction rate, and roll speed, and heating the titanium alloy cylindrical bar billet to 930℃~990℃, the temperature rise of the tube during the cross-rolling piercing process is effectively controlled, thereby improving the quality of the cross-rolling piercing of the titanium alloy solid bar billet. For cross-rolling and piercing of difficult-to-deform alloys, CN110252813B provides a two-roll cross-rolling and piercing method for nickel-based high-temperature alloy solid bar stock. By rationally combining heating temperature, roll speed, reduction ratio, and plug extension, this method effectively avoids the sticking phenomenon caused by the alloy's high deformation resistance. CN110743916A discloses a cross-rolling and piercing method for bearing bar stock, which increases the yield rate of the traditional production process from 26.7% to over 95%, greatly improving material utilization and labor efficiency.

[0005] However, although oblique rolling piercing has the advantages of high production efficiency and high material utilization, due to the severe deformation characteristics and poor plasticity of TC11 titanium alloy, the use of conventional oblique rolling piercing methods is prone to problems such as unstable piercing and poor inner surface quality; and the existing oblique rolling piercing process is usually carried out at a temperature higher than the alloy phase transition point, which results in coarse structure of thick-walled tube blanks and poor performance, making it difficult to meet subsequent processing and use requirements. Summary of the Invention

[0006] In order to solve the above technical problems, the present invention provides a method for oblique rolling and piercing of TC11 titanium alloy thick-walled tube billets at the phase transformation point. The method of the present invention can ensure the yield rate and reduce processing costs while ensuring that the tube billets have good inner surface quality and organizational properties, which can greatly improve the subsequent processing and use performance of the tube billets.

[0007] The present invention is specifically implemented through the following technical solutions.

[0008] The present invention provides a method for oblique rolling and piercing of a thick-walled tube blank of TC11 titanium alloy at a phase transition point, comprising the following steps: Step 1: Prefabricate a small-diameter through hole along the central axis in a TC11 titanium alloy rod to be pierced to obtain a pretreated rod.

[0009] It should be noted that by prefabricating small-diameter through holes, a good guidance and heat dissipation channel is provided for the subsequent perforation process, which significantly reduces the difficulty of perforation.

[0010] Step 2: Apply anti-oxidation coating to the pre-treated rod blank, and then heat it. During the heating process, the heating temperature is lower than the phase transition point of the material. After the heating treatment, apply lubricant to the inner wall of the small-diameter through hole; It should be noted that applying lubricant to the inner wall of the through-hole significantly reduces friction during the piercing process, preventing adhesion between the piercing needle and the inner wall of the rod blank, thereby improving piercing quality and efficiency. Furthermore, the even distribution of lubricant ensures stability during the piercing process and reduces wear on the piercing needle.

[0011] Step 3: Use a perforating needle with a variable diameter structure to perform segmented transition perforation at the location of the small-diameter through-hole. During the perforation process, control the perforation temperature below the phase transition point. It should be noted that the use of a variable-diameter piercing needle for segmented transitional piercing allows for a gradually increasing piercing process, significantly reducing the difficulty of a single piercing, lowering the piercing force, and improving piercing quality and efficiency. Furthermore, the segmented piercing process allows for more uniform material flow, reducing the occurrence of inner wall defects.

[0012] Furthermore, by controlling the piercing temperature below the phase transition point, grain coarsening and structural inhomogeneity at high temperatures are avoided, maintaining the material's favorable microstructure and mechanical properties. Furthermore, low-temperature piercing reduces oxidation and improves surface quality. Perforated tubes produced under these process conditions exhibit excellent subsequent processing properties, making them suitable for the production of precision tubing.

[0013] Step 4: After the perforation is completed, the perforated tube billet is air-cooled to room temperature, and then the tube billet is subjected to a solid solution aging heat treatment to obtain a finished tube billet.

[0014] Preferably, the diameter of the small-diameter through hole is 5 mm to 50 mm.

[0015] Preferably, during the heating process, the temperature is T β -(10~100)℃, holding time is 0.5 δ +50min, of which T β is the phase transition temperature of TC11 titanium alloy, in °C; δ is the rod diameter, in mm.

[0016] Preferably, the piercing needle with a variable diameter structure includes, in sequence from head to tail, multiple variable diameter sections and a forming section. Adjacent variable diameter sections, as well as a forming section and an adjacent variable diameter section, are connected by a transition section. Furthermore, the diameters of the multiple variable diameter sections and the forming section increase sequentially from head to tail, with the diameter of the forming section being equal to the inner diameter of the finished tube. It should be noted that for a specific variable diameter section, its diameter remains constant, i.e., it is of constant diameter. The "variable diameter" in the variable diameter section refers to the diameter of the entire piercing needle, and the diameters of different "sections" vary. Furthermore, the diameters of the multiple variable diameter sections increase sequentially until they reach the same diameter as the forming section, i.e., the desired inner diameter of the finished tube.

[0017] More preferably, the ratio of the diameter of the head of the piercing needle to the diameter of the small-diameter through hole is 1.2-2:1.

[0018] The number of the diameter-reducing sections is 3 to 5, and the total length of each diameter-reducing section and the transition section close to the piercing needle head is 50 mm to 500 mm.

[0019] The angle between each transition section and the center axis of the rod blank is 8°~15°.

[0020] During segmented transition piercing, the piercing speed is 20 mm / min to 60 mm / min. The piercing speed decreases as the diameter increases from the head to the tail of the piercing needle.

[0021] Preferably, the rod blank rotates at 30 rpm to 50 rpm during the segmented transition piercing process, and the piercing temperature is stabilized at T β -(10~100)℃ range, where T β is the phase transition temperature of TC11 titanium alloy, in °C.

[0022] Preferably, during the segmented transition perforation process, the change in perforation force is monitored in real time by a pressure sensor, and the position of the perforation needle is monitored by a displacement sensor.

[0023] Preferably, when prefabricating a small-diameter through hole, drilling is performed in stages, and an intermittent feeding method is adopted in each drilling stage. As the drilling depth increases, the spindle speed and the feed speed are reduced.

[0024] Preferably, the lubricant is a graphite-based high-temperature lubricant, which forms a lubricating layer with a thickness of 0.2 mm to 0.5 mm on the inner wall after being applied.

[0025] The outer diameter of the finished tube blank obtained by the method of the present invention is 100 mm to 300 mm, and the wall thickness is 10 mm to 100 mm.

[0026] Compared with the prior art, the present invention has the following beneficial effects: The present invention prefabricates a small-diameter through hole on the end face of a TC11 titanium alloy bar billet, applies an anti-oxidation coating to the bar billet with the through hole, and then performs high-temperature heating and heat preservation. During the heating treatment, the temperature is controlled to be lower than the phase transition point of the material. After the heat preservation is completed, a lubricant is applied to the inner wall of the through hole of the heated bar billet; then, a perforating needle with a variable diameter structure is used to perform segmented transition-type oblique rolling perforation on the bar billet; the perforation temperature is controlled below the phase transition point, the microstructure and performance are good, and the subsequent processing performance of the material is improved.

[0027] The small-diameter through-hole prefabricated on the end face of the titanium alloy billet is simple to machine and can be lubricated. This through-hole provides guidance, lubrication, and heat dissipation during the piercing process, ensuring precise positioning of the subsequent variable-diameter piercing needle, improving the uniformity of the billet's wall thickness and significantly reducing the difficulty of piercing. Furthermore, compared to existing technologies, the variable-diameter piercing needle employed in this invention enables segmented, transitional piercing of the billet with gradually increasing diameters, reducing severe material deformation and improving the quality of the billet's inner surface.

[0028] Furthermore, the present invention performs perforation at the phase transition point, ensuring high-quality perforation even at this point. This significantly improves the yield rate of difficult-to-process, high-strength TC11 titanium alloy thick-walled tube billets and significantly reduces processing costs. Compared to conventional cross-rolling perforation methods that use temperatures above the alloy's phase transition point, the thick-walled tube billets produced by this method offer superior performance and meet subsequent processing and application requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the process flow chart for the oblique rolling and piercing of TC11 high-strength titanium alloy thick-walled tube billets.

[0030] Figure 2 This is a schematic structural diagram of the perforating needle with a variable diameter structure adopted in the present invention.

[0031] Figure 3 Schematic diagram of the perforating needle with variable diameter structure used in Example 1.

[0032] Figure 4The microstructure diagram of the thick-walled tube in Example 1, wherein (a) is 500X in the transverse direction and (b) is 500X in the longitudinal direction.

[0033] Figure 5 The microstructure diagram of the thick-walled tube in Example 2, where (a) is 500X in the horizontal direction and (b) is 500X in the vertical direction.

[0034] Figure 6 These are the actual pictures of the bar billet after the oblique rolling piercing failed in Comparative Example 1, where (a) is the actual picture of the starting end of the bar billet piercing, and (b) is the actual picture of the entire bar billet. DETAILED DESCRIPTION

[0035] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention is further described below with reference to specific examples and accompanying drawings. However, the examples are not intended to limit the present invention. The experimental methods and detection methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0036] The present invention preforms a small-diameter through hole along the central axis in a TC11 titanium alloy rod to be punched to obtain a pretreated rod; applies an anti-oxidation coating to the pretreated rod, and then performs a heat treatment. During the heating process, the temperature is controlled to be lower than the phase transition point of the material; after the heating treatment, a lubricant is applied to the inner wall of the small-diameter through hole; a perforating needle with a variable diameter structure is used to perform segmented transition perforation at the position of the small-diameter through hole. During the perforation process, the perforation temperature is controlled to be below the phase transition point; after the perforation is completed, the perforated tube is air-cooled to room temperature, and then the tube is subjected to a solution aging heat treatment to obtain a finished tube. The main steps are as follows: Figure 1 shown.

[0037] Among them, Figure 2 As shown, the perforating needle of the variable diameter structure of the present invention includes a plurality of variable diameter sections 1 (such as Figure 2 The first reducing section 11, the second reducing section 12 and the third reducing section 13) and a forming section 3, between two adjacent reducing sections, the forming section 3 and the adjacent reducing section are connected by a transition section 2, such as Figure 2 In the embodiment, the first diameter-reducing section 11 and the second diameter-reducing section 12 are connected by a first transition section 21, the second diameter-reducing section 12 and the third diameter-reducing section 13 are connected by a second transition section 22, and the third diameter-reducing section 13 and the forming section 3 are connected by a third transition section 23; and along the direction from the head to the tail, the diameters of the multiple diameter-reducing sections and the forming section increase in sequence (for example, Figure 2As shown, the diameters of the first, second, and third reducing sections 11, 12, and 13 increase sequentially. However, it should be noted that the diameters of the reducing sections within a given section remain constant. For example, the diameter of the first reducing section 11 remains constant. The diameter of the forming section 3 is equal to the inner diameter of the finished tube. During segmented transition piercing, the piercing speed ranges from 20 mm / min to 60 mm / min. The piercing speed decreases as the diameter increases from the tip of the piercing needle to the tail.

[0038] The present invention will be described in detail below through the following examples and comparative examples.

[0039] Example 1 The production of high-strength TC11 thick-walled tube with an outer diameter of 150 mm and an inner diameter of 80 mm includes the following steps: Step 1: Drill small-diameter deep holes on the bar blank for oblique rolling and piercing to prefabricate small-diameter through holes; Specifically, TC11 titanium alloy bar blanks with a diameter of 150mm and a length of 1200mm were selected as the raw materials for oblique rolling and perforation. A deep-hole drill made of high-speed steel was used with a diameter of 12mm and a length of 1300mm to ensure that it could penetrate the entire bar blank. The bar blank was fixed on a special fixture that could provide a 360° rotation function to ensure that the bar blank remained stable during the drilling process. The spindle speed of the drilling machine was set to 800rpm and the feed rate was 0.05mm / rev. To prevent the heat generated during the drilling process from damaging the drill bit and the bar blank, a high-pressure cutting fluid cooling system was used with a cutting fluid pressure of 5MPa and a flow rate of 30L / min.

[0040] The drilling process is divided into three stages: in the first stage, the drilling depth is 0-400mm, and intermittent feeding is adopted. The tool is retracted every 50mm of feed to clear chips; in the second stage, the drilling depth is 400-800mm, the spindle speed is reduced to 600rpm, the feed rate is reduced to 0.03mm / rev, and the tool is retracted every 30mm of feed; in the third stage, the drilling depth is 800-1200mm, the spindle speed is further reduced to 500rpm, the feed rate is reduced to 0.02mm / rev, and the tool is retracted every 20mm of feed to ensure drilling accuracy and surface quality.

[0041] After drilling is complete, the inner wall of the through-hole is inspected using an endoscope to ensure the absence of visible cracks, burrs, and surface defects. A pneumatic pressure test is performed to verify the through-hole's integrity, with the pressure set at 0.6 MPa for 60 seconds to confirm the absence of leaks. Prefabricating small-diameter through-holes provides a good guide and heat dissipation path for subsequent drilling operations, significantly reducing the difficulty of drilling.

[0042] Step 2: After heating, apply lubricant to the inner wall of the through hole to optimize the lubrication conditions; Specifically, the rod blanks with prefabricated through holes are first coated with anti-oxidation coating (anti-oxidation coating source: CMS-19B3, Beijing Tianlichuang Glass Technology Development Co., Ltd.), and then placed in a resistance furnace for heating. The heating temperature is controlled at 930°C, which is lower than the phase change point of the material (about 1000°C). The holding time is 125 minutes to ensure uniform temperature of the rod blanks.

[0043] After the billet is heated, a lubricant coating device is used to lubricate the inner wall of the through hole. The lubricant is a high-temperature graphite-based lubricant with an operating temperature range of 800-1000°C. It has good thermal stability and is not easy to decompose.

[0044] The lubricant applicator consists of a 10mm diameter stainless steel rod and a custom applicator head. The head is made of high-temperature resistant ceramic and features spiral grooves on its surface, which evenly apply the lubricant to the inner wall of the through-hole. During the application process, the applicator rotates at 60rpm and feeds at 100mm / min, ensuring even distribution of the lubricant along the inner wall of the through-hole. This application process is repeated three times, with each application applying approximately 0.1mm of lubricant to a thickness of approximately 0.3mm.

[0045] By applying lubricant to the inner wall of the through-hole, friction during the piercing process is significantly reduced, preventing the piercing needle from sticking to the inner wall of the rod, and improving piercing quality and efficiency. At the same time, the even distribution of lubricant ensures stability during the piercing process and reduces wear on the piercing needle.

[0046] Step 3: Use a perforating needle with a variable diameter structure to perform segmented transition perforation; Specifically, the piercing needle adopts a special variable diameter structure design, is made of high-speed heat-resistant steel, and has undergone special heat treatment to achieve a hardness of HRC62-65, with good wear resistance and high temperature strength. Figure 3 As shown, the perforating needle is divided into four different diameter sections: the first section length ( Figure 3 The length of the middle section (A) is 50mm and the diameter is 15mm; the length of the second section ( Figure 3 The length of the middle section B) is 100mm, the diameter of the first section is 30mm; the length of the third section ( Figure 3 The length of the middle section C is 200mm, the diameter of the same section is 60mm; the length of the fourth section ( Figure 3 The diameter of the middle D section is 1800, and the diameter of the unchanged section is 80mm. A 10° tapered transition is used between each section, and the transition section is at the section with the larger diameter to ensure a smooth transition during the perforation process.

[0047] The piercing equipment is driven by a servo motor with a maximum thrust of 200kN. The piercing speed is divided into four stages: the first stage (corresponding to the first stage of the piercing needle) is 50mm / min; the second stage (corresponding to the second stage of the piercing needle) drops to 40mm / min; the third stage (corresponding to the third stage of the piercing needle) drops further to 30mm / min; and the fourth stage (corresponding to the fourth stage of the piercing needle) reaches 20mm / min. During the piercing process, the billet rotates at 30rpm to ensure uniform perforation.

[0048] During the piercing process, a pressure sensor monitors the changes in piercing force in real time. When the piercing force exceeds the set value (180kN), the system automatically reduces the piercing speed to ensure safety. At the same time, a displacement sensor monitors the position of the piercing needle to ensure precise control of the piercing process.

[0049] By using a variable-diameter piercing needle for segmented transitional piercing, a gradually increasing diameter piercing process is achieved, significantly reducing the difficulty of one-time piercing, reducing piercing force, and improving piercing quality and efficiency. Furthermore, during the segmented piercing process, the material flows more evenly, reducing the occurrence of inner wall defects.

[0050] It should be noted that the perforation temperature should be controlled below the phase change point to improve the subsequent processing performance of the material.

[0051] Specifically, during the piercing process, the surface temperature of the billet is monitored in real time using an infrared thermometer with a measurement range of 600-1200°C and an accuracy of ±5°C. The piercing temperature is controlled between 900-960°C. When the billet temperature falls below 900°C, an auxiliary heating system (electric induction heating) with a heating power of 50kW is used to provide heat. If the billet temperature exceeds 960°C, the piercing process is suspended until the temperature drops.

[0052] Step 4: After the perforation is completed, the perforated tube is air-cooled to room temperature, and then the tube is subjected to solid solution aging heat treatment, wherein the solid solution temperature is 930℃, the holding time is 1h, the cooling method is air cooling, the aging temperature is 550℃, the holding time is 6h, and the cooling method is also air cooling. After the heat treatment is completed, the deformation sections at both ends of the tube are cut off, and then the tube is subjected to room temperature tensile performance test. The microstructure of the tube is shown in FIG. Figure 4 , where (a) is 500X horizontally and (b) is 500X vertically. The performance test results are shown in Table 1. Figure 4 The microstructure of the medium tube blank presents a typical uniform equiaxed structure. At the same time, it can be seen from the data in Table 1 that the tensile strength of the tube blank is ≥1112 MPa and the elongation is ≥12%, indicating that the forming method provided by the present invention can obtain a high-strength TC11 titanium alloy thick-walled tube blank with excellent strength and plasticity matching.

[0053] Table 1 Test results of room temperature tensile properties of tube blanks By controlling the piercing temperature below the phase transition point, grain coarsening and structural inhomogeneity at high temperatures are avoided, maintaining the material's favorable microstructure and mechanical properties. Furthermore, low-temperature piercing reduces oxidation and improves surface quality. Perforated tubes produced under these conditions exhibit excellent subsequent processing properties and are suitable for the production of precision tubing.

[0054] Example 2 The production of high-strength TC11 thick-walled tube with an outer diameter of 250 mm and an inner diameter of 150 mm includes the following steps: Step 1: Drill small-diameter deep holes on the bar blank for oblique rolling and piercing to prefabricate small-diameter through holes; Specifically, TC11 titanium alloy bar blanks with a diameter of 250mm and a length of 1500mm were selected as the raw materials for oblique rolling and perforation. A deep-hole drill made of carbide was used with a diameter of 30mm and a length of 1600mm to ensure that it could penetrate the entire bar blank. The bar blank was fixed on a special fixture that could provide a 360° rotation function to ensure that the bar blank remained stable during the drilling process. The spindle speed of the drilling machine was set to 600rpm and the feed rate was 0.04mm / rev. To prevent the heat generated during the drilling process from damaging the drill bit and the bar blank, a high-pressure cutting fluid cooling system was used with a cutting fluid pressure of 6MPa and a flow rate of 40L / min.

[0055] The drilling process is divided into four stages: in the first stage, the drilling depth is 0~400mm, and intermittent feeding is adopted. The tool is retracted every 40mm of feed to clear chips; in the second stage, the drilling depth is 400~800mm, the spindle speed is reduced to 500rpm, the feed speed is reduced to 0.03mm / rev, and the tool is retracted every 30mm of feed; in the third stage, the drilling depth is 800~1200mm, the spindle speed is further reduced to 400rpm, the feed speed is reduced to 0.02mm / rev, and the tool is retracted every 20mm of feed; in the fourth stage, the drilling depth is 1200~1500mm, the spindle speed is reduced to 300rpm, the feed speed is reduced to 0.015mm / rev, and the tool is retracted every 15mm of feed to ensure drilling accuracy and surface quality.

[0056] After drilling is complete, the inner wall of the through-hole is inspected using an endoscope to ensure the absence of visible cracks, burrs, and surface defects. A pneumatic pressure test is performed to verify the through-hole's integrity, with the pressure set at 0.8 MPa for 90 seconds to confirm the absence of leaks. Prefabricating small-diameter through-holes provides a good guide path for subsequent perforation processes, significantly reducing the difficulty of perforation.

[0057] Step 2: After heating, apply lubricant to the inner wall of the through hole to optimize the lubrication conditions; Specifically, the rod blank with prefabricated through holes is first coated with anti-oxidation coating (anti-oxidation coating source: CMS-19B3, Beijing Tianlichuang Glass Technology Development Co., Ltd.), and then the rod blank is placed in a resistance furnace for heating. The heating temperature is controlled at 960°C, which is lower than the phase transition point of TC11 titanium alloy (about 1000°C). The heating time is 175 minutes to ensure uniform temperature of the rod blank.

[0058] After the billet is heated, a lubricant coating device is used to lubricate the inner wall of the through hole. The lubricant is a high-temperature graphite-based lubricant with an operating temperature range of 800-1000°C. It has good thermal stability and is not easy to decompose.

[0059] The lubricant applicator consists of a 20mm diameter heat-resistant alloy rod and a custom applicator head. The head, made of a high-temperature alloy material, features a double helical groove on its surface, ensuring a more even distribution of lubricant across the inner wall of the through-hole. During the application process, the applicator rotates at 80rpm and feeds at 120mm / min, ensuring even distribution of the lubricant across the inner wall of the through-hole. This process is repeated four times, with each application applying approximately 0.08mm of lubricant to a thickness of approximately 0.32mm.

[0060] By applying lubricant to the inner wall of the through-hole, friction during the piercing process is significantly reduced, preventing the piercing needle from sticking to the inner wall of the rod, and improving piercing quality and efficiency. At the same time, the even distribution of lubricant ensures stability during the piercing process and reduces wear on the piercing needle.

[0061] Step 3: Use a perforating needle with a variable diameter structure to perform segmented transition perforation; Specifically, the piercing needle features a unique variable-diameter design. Made from high-speed, heat-resistant steel, it undergoes special heat treatment and surface nitriding, achieving a hardness of HRC65-68, offering excellent wear resistance and high-temperature strength. The piercing needle is divided into five diameter segments: the first segment is 50mm long and 50mm in diameter; the second segment is 100mm long and 75mm in diameter; the third segment is 150mm long and 100mm in diameter; the fourth segment is 200mm long and 125mm in diameter; and the fifth segment is 2500mm long and 150mm in diameter. Each segment features an 8° tapered transition, with the transition occurring at the larger diameter segment, to ensure a smooth transition during the piercing process.

[0062] The piercing equipment uses a hydraulic drive system with a maximum thrust of 600kN. The piercing speed is divided into five stages: the first stage (corresponding to the first stage of the piercing needle) is 60mm / min; the second stage (corresponding to the second stage of the piercing needle) reduces the piercing speed to 50mm / min; the third stage (corresponding to the third stage of the piercing needle) reduces the piercing speed to 40mm / min; the fourth stage (corresponding to the fourth stage of the piercing needle) reduces the piercing speed to 30mm / min; and the fifth stage (corresponding to the fifth stage of the piercing needle) reaches a piercing speed of 20mm / min. During the piercing process, the billet rotates at 40rpm to ensure uniform perforation.

[0063] During the piercing process, the changes in piercing force and torque are monitored in real time through torque sensors and pressure sensors. When the piercing force exceeds the set value (570kN), the system automatically reduces the piercing speed to ensure the safety of the piercing process.

[0064] By using a variable-diameter piercing needle for segmented transitional piercing, a gradually increasing diameter piercing process is achieved, significantly reducing the difficulty of one-time piercing, reducing piercing force, and improving piercing quality and efficiency. Furthermore, during the segmented piercing process, the material flows more evenly, reducing the occurrence of inner wall defects.

[0065] It should be noted that the perforation temperature should be controlled below the phase change point to improve the subsequent processing performance of the material.

[0066] Specifically, during the piercing process, a multi-point infrared temperature measurement system monitors the surface temperature of the billet in real time. Its measurement range is 600-1000°C, with an accuracy of ±3°C. The piercing temperature is controlled between 900-990°C. When the billet temperature falls below 900°C, an auxiliary heating system (electric induction heating) provides heat at 80kW. When the billet temperature exceeds 990°C, a forced air cooling system is activated at a cooling velocity of 15m / s to ensure the temperature does not exceed the phase transition point.

[0067] Step 4: After the perforation is completed, the perforated tube is air-cooled to room temperature, and then the tube is subjected to solid solution aging heat treatment, wherein the solid solution temperature is 930℃, the holding time is 1h, the cooling method is air cooling, the aging temperature is 550℃, the holding time is 6h, and the cooling method is also air cooling. After the heat treatment is completed, the deformation sections at both ends of the tube are cut off, and then the tube is subjected to room temperature tensile performance test. The microstructure of the tube is shown in FIG. Figure 5 , where (a) is 500X horizontally and (b) is 500X vertically. The performance test results are shown in Table 2. Figure 5The microstructure of the medium tube blank presents a typical uniform equiaxed structure. At the same time, it can be seen from the data in Table 2 that the tensile strength of the tube blank is ≥1128 MPa and the elongation is ≥12.5%, indicating that the forming method provided by the present invention can obtain a high-strength TC11 titanium alloy thick-walled tube blank with excellent strength and plasticity matching.

[0068] Table 2 Test results of room temperature tensile properties of tube blanks By controlling the piercing temperature below the phase transition point, grain coarsening and structural inhomogeneity at high temperatures are avoided, maintaining the material's favorable microstructure and mechanical properties. Furthermore, low-temperature piercing reduces oxidation and improves surface quality. Perforated tubes produced under these conditions exhibit excellent subsequent processing properties and are suitable for the production of precision tubing.

[0069] It should be noted that both Example 1 and Example 2 are methods for prefabricating through-holes in rod blanks for oblique rolling and piercing.

[0070] In order to highlight the key factors in the method of the present invention, the present invention also provides the following comparative examples.

[0071] Comparative Example 1 Using conventional non-variable diameter piercing needles and TC11 titanium alloy bar billets with non-through shallow centering holes at both ends as raw materials, high-strength TC11 thick-walled tubes with an outer diameter of 220 mm and an inner diameter of 160 mm were prepared by oblique rolling and piercing.

[0072] Specifically: TC11 titanium alloy bar billets with a diameter of 220 mm and a length of 1500 mm were selected as the raw materials for oblique rolling and piercing. Non-through centering holes with a diameter of 30 mm and a depth of 20 mm were added to both ends of the billet through a drilling machine. The main function is to guide the piercing needle and to center the billet during the piercing process.

[0073] The billet is then heated in a resistance furnace at 1050°C, above the phase transition point of TC11 titanium alloy (approximately 1000°C), for 160 minutes to ensure uniform temperature. After heating, the billet is lubricated using a lubricant applicator. A high-temperature graphite-based lubricant is used, operating in a temperature range of 900-1200°C and exhibiting excellent thermal stability and resistance to decomposition.

[0074] The rods were pierced using a conventional, non-variable extrusion needle and a hydraulically driven piercing machine with a maximum thrust of 6000kN. The piercing speed was controlled at 30mm / min throughout the process. At a piercing depth of approximately 150mm, the piercing force exceeded the machine's maximum thrust limit, causing the machine to stall and the piercing process to cease. This indicates that the cross-rolling piercing of the rod failed.

[0075] After the billet is cooled, the sample obtained is as follows Figure 6 As shown, (a) is the actual picture of the beginning of the rod billet perforation, and (b) is the actual picture of the entire rod billet. Figure 6 It is not difficult to find at the beginning of the perforation of the medium billet that even if the perforation is performed at a temperature higher than the alloy phase transition point, the billet still undergoes a high degree of uneven deformation, and tearing occurs locally in the billet. This result shows that compared with the new oblique rolling perforation process provided by the present invention, for high-strength titanium alloys such as TC11 that are more difficult to deform, the conventional non-variable diameter perforation needle and the TC11 titanium alloy billet with non-through shallow centering hole structures at both ends are used as raw materials for oblique rolling perforation. Even if the deformation is performed at the alloy phase transition point, the equipment tonnage is still required to be higher, and there is a risk of equipment jamming and perforation failure due to excessive thrust. At the same time, the violent deformation characteristics during the perforation process can easily lead to defects such as material tearing, which seriously affects the perforation quality of the tube billet.

[0076] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications of the present invention fall within the scope of the claims and their equivalents, such changes and modifications are intended to be included.

Claims

1. A method for oblique rolling and piercing of thick-walled TC11 titanium alloy tube billets at a phase transition point, characterized in that: The following steps are involved: Prefabricating a small-diameter through hole along the central axis in a TC11 titanium alloy bar to be pierced to obtain a pretreated bar; Apply anti-oxidation coating to the pre-treated rod blank, and then heat it. During the heating process, the temperature is controlled to be lower than the phase transition point of the material; After the heat treatment, apply lubricant to the inner wall of the small-diameter through hole; Use a perforating needle with a variable diameter structure to perform segmented transition perforation at the location of the small-diameter through-hole. During the perforation process, control the perforation temperature below the phase transition point. After the piercing is completed, the pierced tube blank is air-cooled to room temperature, and then the tube blank is subjected to solid solution aging heat treatment to obtain the finished tube blank.

2. The oblique rolling and piercing method of TC11 titanium alloy thick-walled tube billet at the phase transition point according to claim 1, characterized in that: The piercing needle with a variable diameter structure includes multiple variable diameter sections and a forming section in the direction from the head to the tail. Two adjacent variable diameter sections, as well as a forming section and an adjacent variable diameter section are connected by a transition section respectively; and along the direction from the head to the tail, the diameters of the multiple variable diameter sections and the forming section increase in sequence, and the diameter of the forming section is equal to the inner diameter of the finished tube blank.

3. The oblique rolling and piercing method of TC11 titanium alloy thick-walled tube billet at the phase transition point according to claim 2, characterized in that: The ratio of the diameter of the piercing needle head to the diameter of the small-diameter through hole is 1.2~2:1; The number of the reducing sections is 3 to 5, and the total length of each reducing section and the transition section close to the piercing needle head is 50 mm to 500 mm; The angle between each transition section and the center axis of the billet is 8°~15°; During segmented transition piercing, the piercing speed is 20 mm / min to 60 mm / min. The piercing speed decreases as the diameter increases from the head to the tail of the piercing needle.

4. The oblique rolling and piercing method of TC11 titanium alloy thick-walled tube billet below the phase transition point according to claim 1, characterized in that: During the segmented transition piercing process, the billet rotates at a speed of 30rpm~50rpm, and the piercing temperature is stabilized by real-time temperature monitoring. T β -(10~100)℃ range, where T β is the phase transition temperature of TC11 titanium alloy, in °C.

5. The oblique rolling and piercing method of TC11 titanium alloy thick-walled tube billet below the phase transition point according to claim 1, characterized in that: During the segmented transition perforation process, the changes in perforation force are monitored in real time by a pressure sensor, and the position of the perforation needle is monitored by a displacement sensor.

6. The oblique rolling and piercing method of TC11 titanium alloy thick-walled tube billet below the phase transition point according to claim 1, characterized in that: The diameter of the small-diameter through hole is 5mm~50mm.

7. The oblique rolling and piercing method of TC11 titanium alloy thick-walled tube billet below the phase transition point according to claim 1, characterized in that: During the heating process, the temperature is T β -(10~100)℃, holding time is 0.5 δ +50min, of which T β is the phase transition temperature of TC11 titanium alloy, in °C; δ is the rod diameter, in mm.

8. The oblique rolling and piercing method of TC11 titanium alloy thick-walled tube billet below the phase transition point according to claim 1, characterized in that: When prefabricating small-diameter through holes, drilling is carried out in stages, and intermittent feeding is adopted in each drilling stage. As the drilling depth increases, the spindle speed and feed speed are reduced.

9. The oblique rolling and piercing method of TC11 titanium alloy thick-walled tube billet below the phase transition point according to claim 1, characterized in that: The lubricant is a graphite-based high-temperature lubricant, which forms a 0.2mm~0.5mm thick lubricating layer on the inner wall after application.

10. The oblique rolling and piercing method of TC11 titanium alloy thick-walled tube billet below the phase transition point according to claim 1, characterized in that: The outer diameter of the finished tube is 100mm~300mm, and the wall thickness is 10mm~100mm.

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