Preparation method for improving surface quality of titanium alloy seamless tube

Abstract

The invention relates to a preparation method for improving the surface quality of a titanium alloy seamless tube, and belongs to the technical field of metal material machining. The method comprises the following steps that S1, grain refining pretreatment is conducted on the surface layer of a blank, specifically, a titanium alloy bar is subjected to surface shot peening grain refining treatment and then subjected to low-temperature annealing treatment; s2, a gradient composite lubricating layer and a multi-layer composite sheath are prepared, specifically, the surface of the bar is sprayed with a bottom-layer glass lubricant, after solidification, the surface of the bar is sequentially sleeved with an inner-layer titanium foil sheath and an outer-layer metal sheath, the bar is filled with a solid lubricant powder layer and then is welded and sealed, and then the outer surface of a metal soft sheath is sprayed with an outer-layer glass lubricant; s3, variable-diameter lubricating groove double-cone die extrusion is carried out; s4, carrying out online acid pickling treatment; s5, neutral electrolytic polishing; s6, cleaning and air drying. According to the method, the surface quality can be remarkably improved, meanwhile, the interface purity is guaranteed, and the cost is reduced. The technical problems that an existing full-titanium sheath is high in cost, and a single-layer glass lubricant is prone to damage and difficult to stably lubricate are solved.

Description

Technical Field

[0001] This invention relates to a method for preparing seamless titanium alloy tubes with improved surface quality, belonging to the field of metal material processing technology. Background Technology

[0002] Titanium alloys are characterized by low density, high specific strength, and excellent corrosion resistance, and are widely used in aerospace, petrochemical, and marine engineering fields. As an important structural component, the surface quality of seamless titanium alloy tubes directly affects the product's service life and reliability.

[0003] However, due to its low thermal conductivity (approximately 6.7 W / (m·K)), the temperature difference between the surface and core of the titanium alloy seamless tube can reach 200-250℃ during hot extrusion. This leads to increased surface metal strength and decreased plasticity, forming a "hard shell" that easily causes surface cracks. Simultaneously, at high temperatures of 980-1030℃, titanium undergoes a eutectic reaction with the die material, generating brittle intermetallic compounds such as TiFe and TiNi, causing surface contamination and die wear.

[0004] To improve forming quality, existing technologies often employ a combination of encapsulation extrusion and glass lubricant. For example, Chinese patent CN 119347343 A discloses a method for extruding seamless titanium alloy tubes and its products. This method uses a steel + copper bimetallic encapsulation to avoid copper contamination; it uses a double-cone die extrusion but without lubrication. Chinese patent CN 118513635 A discloses a coating method for preparing a surface coating layer on a billet used in large-diameter titanium alloy tubes, and proposes an encapsulation for thin-walled welded titanium alloy tubes, supplemented by a glass lubricant to reduce the coefficient of friction from 0.8 to 0.1-0.2.

[0005] However, the existing technologies still have significant shortcomings: single steel / copper cladding is prone to producing brittle interfacial phases, while full titanium cladding is costly; glass lubricants are mostly single-layer coatings, which are easily damaged at high temperatures, making it difficult to achieve stable lubrication throughout the entire process. Furthermore, traditional acid pickling and strong acid electrolytic polishing are inefficient, have high waste liquid treatment costs, and cause serious environmental pollution.

[0006] Therefore, there is an urgent need to develop a synergistic control method that integrates surface microstructure modification, composite encapsulation structure, active mold lubrication, and environmentally friendly post-treatment to systematically solve the problem of surface quality control in the hot extrusion process of titanium alloys. Summary of the Invention

[0007] The technical problem to be solved by the present invention is that the existing full titanium sheath is costly and the single-layer glass lubricant is easily damaged and difficult to lubricate stably.

[0008] The technical solution adopted by this invention to solve its technical problem is: a preparation method for improving the surface quality of seamless titanium alloy tubes, comprising the following steps: S1. Pre-treatment of the surface of the billet by fine graining: The titanium alloy bar is subjected to surface shot peening and fine graining treatment, followed by low-temperature annealing. S2. Preparation of gradient composite lubricating layer and multi-layer composite sheath: A bottom layer of glass lubricant is sprayed on the surface of the pretreated titanium alloy rod. After the bottom layer of lubricant has cured, an inner layer of titanium foil sheath and an outer layer of metal sheath are sequentially sheathed on the surface of the rod. A solid lubricant powder layer is filled between the inner and outer layers of sheath. The double-layer composite sheath is welded and sealed. Then, an outer layer of glass lubricant is sprayed on the outer surface of the metal soft sheath. S3. Variable diameter lubrication groove double cone die extrusion; S4. Online pickling treatment; S5. Neutral electrolytic polishing; S6. Wash and air dry.

[0009] Furthermore, in step S1, a centrifugal shot peening machine is used to perform shot peening and fine grain treatment on the surface of the bar, and high-hardness ceramic or glass shot is used as the shot medium to bombard the outer surface of the bar at high speed. The shot diameter is 0.2-0.8 mm, the shot peening intensity is 0.2-0.6 mmA, and the surface coverage is 100%-200%, forming an ultra-fine crystalline structure layer with a thickness of 50-150 μm on the surface of the bar; the annealing temperature is 450-550℃, the holding time is 30-60 min, and air cooling is performed.

[0010] Furthermore, the bottom layer glass lubricant described in step S2 has a temperature tolerance of 800-900℃ and a spraying thickness of 50-100μm; the outer layer glass lubricant has a temperature tolerance of 900-1050℃ and a spraying thickness of 80-150μm.

[0011] Furthermore, the inner titanium foil sheath is made of TA1 or TA2 pure titanium foil with a thickness of 0.1-0.3 mm, and is tightly attached to the surface of the rod by spiral winding; the outer metal sheath is a thin-walled sleeve of low carbon steel or stainless steel with a thickness of 0.5-1.2 mm; the solid lubricant powder layer filling the space between the inner and outer sheaths has a thickness of 0.2-0.5 mm; and the inner titanium foil sheath is sealed by argon arc welding, while the outer metal sheath is sealed by laser welding or plasma welding.

[0012] Furthermore, in step S2, the raw material weight percentage composition of the bottom layer glass lubricant is: SiO2 40-50%, CaO 5-10%, MgO 2-5%, Al2O3 3-6%, Na2O 15-25%, and binder 15-25%; the raw material weight percentage composition of the outer layer glass lubricant is: SiO2 45-55%, CaO 8-12%, MgO 3-6%, Al2O3 4-8%, K2O 5-10%, B2O3 5-10%, and binder 10-20%.

[0013] Furthermore, in step S3, the titanium alloy bar obtained in step S2 is heated to 950-1050℃ and held for 60-120 minutes, and then extruded using a double-cone die with a variable-diameter lubrication groove; the double-cone die includes a first conical surface and a second conical surface, the first conical surface has a cone angle of 30-45°, the second conical surface has a cone angle of 15-25°, a spiral micro-oil reservoir is opened on the second conical surface, the cross-sectional shape of the oil reservoir is semi-circular or trapezoidal, the groove depth is 0.1-0.3mm, the groove width is 0.2-0.5mm, the distance between adjacent grooves is 0.5-1.2mm, the spiral angle is 30-60°, and the oil reservoir is filled with solid lubricant; the extrusion ratio is 8-15, and the extrusion speed is 50-100mm / s.

[0014] Furthermore, the solid lubricant in step S3 is a mixture of molybdenum disulfide and graphite in a mass ratio of 1:1 to 3:1, and 5-10% cerium oxide is added to the solid lubricant as a high-temperature stabilizer.

[0015] Furthermore, in step S4, after the seamless titanium alloy tube extruded in step S3 is demolded, it is immediately subjected to online spray pickling treatment at 200-300°C using the residual heat of the tube itself. The pickling solution consists of the following components by weight percentage: 5-10% citric acid, 3-8% oxalic acid, 0.5-1.5% corrosion inhibitor, 0.1-0.5% surfactant, and the remainder is deionized water. The spraying pressure is 0.3-0.6 MPa, and the spraying time is 10-30 seconds.

[0016] Furthermore, in step S5, the seamless titanium alloy tube processed in step S4 is subjected to neutral electrolytic polishing. The electrolyte is a neutral salt solution composed of the following components by weight percentage: 10-15% sodium chloride, 5-10% sodium sulfate, 3-6% sodium citrate, and the remainder is deionized water, with a pH value of 6.5-7.5; the electrolysis voltage is 10-20V, the current density is 20-50A / dm², and the polishing time is 2-5min.

[0017] Furthermore, in step S6, the seamless titanium alloy tube obtained in step S5 is rinsed with clean water on both the inner and outer surfaces, then immersed in a 2-5% sodium carbonate neutralization solution for 1-2 minutes, rinsed with clean water again, and finally rapidly cooled under nitrogen protection.

[0018] The beneficial effects of this invention are: (1) Significantly improved surface quality: Through the pretreatment of fine grain formation on the surface of the billet, a fine grain structure is formed on the surface of the bar, which exhibits superplasticity during high-temperature deformation and can better replicate the mold surface, fundamentally suppressing the generation of "orange peel" defects. Testing showed that the inner surface roughness Ra of the seamless titanium alloy tube prepared by the method of this invention was <0.6μm, and the outer surface roughness Ra was <0.8μm, with no surface defects such as cracks, folds, or scratches.

[0019] (2) The inner titanium foil (0.1-0.3mm) is made of the same material as the billet, and its deformation behavior is consistent at high temperatures, completely eliminating the risk of the formation of brittle intermetallic compounds such as TiFe and TiFe2, and avoiding iron contamination; the outer steel sleeve (0.5-1.2mm) provides structural strength and protects the inner titanium foil from damage during heating and extrusion; the intermediate lubricating powder layer (0.2-0.5mm) melts during heating to form an initial lubricating film, achieving continuous lubrication throughout the entire process from heating to extrusion. This structure ensures interface purity while reducing costs by more than 60% compared to a full titanium cladding, demonstrating significant economic advantages.

[0020] (3) High oxide layer removal efficiency: Online reduction spray treatment utilizes the waste heat of the pipe, and organic acids selectively dissolve the oxide layer, avoiding excessive corrosion of the substrate; neutral electrolytic polishing further removes the micron-level surface defect layer, obtaining a bright surface. Compared with the traditional pickling process, titanium metal loss is reduced by more than 50%, there is no risk of hydrogen absorption, and production efficiency is increased by 100%.

[0021] (4) High integration of process flow: The surface pretreatment, forming lubrication and online post-treatment are integrated into a single process. The process is closely connected, energy saving and consumption reduction are achieved, which is suitable for continuous industrial production. The investment in technological transformation is small and the investment recovery period is about 2-3 years, resulting in significant economic benefits.

[0022] (5) Wide range of applications: The method of this invention is applicable to the production of seamless titanium alloy tubes of various grades, especially for high-quality seamless titanium alloy tubes used in aerospace and marine engineering. In the domestic industry, it can promote the localization of high-end titanium alloy tubes, with annual comprehensive benefits reaching hundreds of millions of yuan; in the international market, it has obvious environmental protection advantages, conforms to the trend of green manufacturing, and has the dual potential for product export and technology output. The invention is technologically mature, has a broad market, and has good prospects for promotion and application. Detailed Implementation

[0023] The present invention will be further described below with reference to embodiments and comparative examples.

[0024] A method for improving the surface quality of seamless titanium alloy tubes includes the following steps: S1. Pre-treatment of the surface of the billet by fine graining: The titanium alloy bar is subjected to surface shot peening and fine graining treatment, followed by low-temperature annealing. S2. Preparation of gradient composite lubricating layer and multi-layer composite sheath: A bottom layer of glass lubricant is sprayed onto the surface of the pretreated titanium alloy rod. After the bottom layer of lubricant has cured, an inner layer of titanium foil sheath and an outer layer of metal sheath are sequentially sheathed onto the surface of the rod to form a double-layer composite sheath structure. A solid lubricant powder layer is filled between the inner and outer layers of sheath. The double-layer composite sheath is then welded and sealed. Finally, an outer layer of glass lubricant is sprayed onto the outer surface of the metal soft sheath. S3. Variable diameter lubrication groove double cone die extrusion; S4. Online pickling treatment; S5. Neutral electrolytic polishing; S6. Cleaning and air drying. This method solves common technical problems in the hot extrusion process of titanium alloys, such as easy oxidation, easy adhesion, easy cracking, and difficult post-processing, through the systematic integration of processes such as shot peening fine grain pretreatment, three-layer composite cladding, self-lubricating double cone die extrusion, online residual heat pickling, and neutral electrolytic polishing.

[0025] Preferably, in step S1, a centrifugal shot peening machine is used to perform shot peening and fine grain treatment on the surface of the bar, using high-hardness ceramic or glass pellets as the shot medium to bombard the outer surface of the bar at high speed. The pellet diameter is 0.2-0.8 mm, the shot peening intensity is 0.2-0.6 mmA, and the surface coverage is 100%-200%, meaning that after complete coverage, an over-peening process of the same duration is performed. This forms an ultrafine crystalline layer with a thickness of 50-150 μm on the surface of the bar; the annealing temperature is 450-550℃, the holding time is 30-60 min, and air cooling is performed.

[0026] Preferably, in step S2, the bottom layer glass lubricant has a temperature tolerance of 800-900℃ and a spraying thickness of 50-100μm; the outer layer glass lubricant has a temperature tolerance of 900-1050℃ and a spraying thickness of 80-150μm.

[0027] Preferably, the inner titanium foil sheath is a TA1 or TA2 pure titanium foil with a thickness of 0.1-0.3 mm, which is tightly attached to the surface of the rod by spiral winding; the outer metal sheath is a low carbon steel or stainless steel thin-walled sleeve with a thickness of 0.5-1.2 mm; the solid lubricant powder layer filling the space between the inner and outer sheaths has a thickness of 0.2-0.5 mm; and the inner titanium foil sheath is sealed by argon arc welding, while the outer metal sheath is sealed by laser welding or plasma welding.

[0028] Preferably, in step S2, the raw material weight percentage composition of the bottom layer glass lubricant is: SiO2 40-50%, CaO 5-10%, MgO 2-5%, Al2O3 3-6%, Na2O 15-25%, and binder 15-25%; the raw material weight percentage composition of the outer layer glass lubricant is: SiO2 45-55%, CaO 8-12%, MgO 3-6%, Al2O3 4-8%, K2O 5-10%, B2O3 5-10%, and binder 10-20%.

[0029] Preferably, in step S3, the titanium alloy bar obtained in step S2 is heated to 950-1050℃ and held for 60-120 minutes, and then extruded using a double-cone die with a variable-diameter lubrication groove; the double-cone die includes a first conical surface and a second conical surface, the first conical surface has a cone angle of 30-45°, the second conical surface has a cone angle of 15-25°, a spiral micro-oil storage groove is opened on the second conical surface, the cross-sectional shape of the oil storage groove is semi-circular or trapezoidal, the groove depth is 0.1-0.3mm, the groove width is 0.2-0.5mm, the distance between adjacent grooves is 0.5-1.2mm, the spiral angle is 30-60°, and the oil storage groove is filled with solid lubricant; the extrusion ratio is 8-15, and the extrusion speed is 50-100mm / s.

[0030] Preferably, the solid lubricant in step S3 is a mixture of molybdenum disulfide and graphite in a mass ratio of 1:1 to 3:1, and 5-10% cerium oxide is added to the solid lubricant as a high-temperature stabilizer.

[0031] Preferably, in step S4, after the seamless titanium alloy tube extruded in step S3 is demolded, it is immediately subjected to online spray pickling treatment at 200-300°C using the residual heat of the tube itself. The pickling solution consists of the following components by weight percentage: 5-10% citric acid, 3-8% oxalic acid, 0.5-1.5% corrosion inhibitor, 0.1-0.5% surfactant, and the remainder is deionized water. The spraying pressure is 0.3-0.6 MPa, and the spraying time is 10-30 seconds.

[0032] Preferably, in step S5, the seamless titanium alloy tube treated in step S4 is subjected to neutral electrolytic polishing. The electrolyte is a neutral salt solution composed of the following components by weight percentage: 10-15% sodium chloride, 5-10% sodium sulfate, 3-6% sodium citrate, and the remainder is deionized water, with a pH value of 6.5-7.5; the electrolysis voltage is 10-20V, the current density is 20-50A / dm², and the polishing time is 2-5min.

[0033] Preferably, in step S6, the seamless titanium alloy tube obtained in step S5 is rinsed with clean water on both the inner and outer surfaces, then immersed in a 2-5% sodium carbonate neutralization solution for 1-2 minutes, rinsed with clean water again, and finally rapidly cooled under nitrogen protection.

[0034] Example 1 S1. Pretreatment for refining the surface of the billet TC4 titanium alloy bars (120mm in diameter, 500mm in length) were selected and subjected to surface shot peening for fine grain refinement using a centrifugal shot peening machine. High-hardness ceramic shot with a diameter of 0.5mm was used, with a shot peening intensity of 0.4 mmA and a surface coverage of 150% (followed by a 50% transition shot peening time after complete coverage), forming an ultrafine crystalline layer approximately 100μm thick on the surface of the bars. After shot peening, the bars underwent low-temperature annealing at 500℃ for 45 minutes, followed by air cooling.

[0035] S2. Preparation of Gradient Composite Lubricating Layer and Multilayer Composite Encasing A base layer of glass lubricant (formula: 45% SiO2, 8% CaO, 3% MgO, 35% Al2O3, 20% Na2O, 19% binder) was sprayed onto the surface of the pretreated titanium alloy rod to a thickness of 80 μm, and then cured for later use. An inner titanium foil sheath and an outer metal sheath were then sequentially fitted onto the surface of the rod: the inner titanium foil sheath was made of 0.2 mm thick TA2 pure titanium foil, tightly bonded using a spiral winding method; the outer metal sheath was a 0.8 mm thick low-carbon steel thin-walled sleeve; a solid lubricant powder layer (molybdenum disulfide:graphite = 2:1, with 8% cerium oxide added) was filled between the inner and outer sheaths to a thickness of 0.3 mm. The ends of the composite sheath were vacuum-sealed, with the inner titanium foil layer welded using argon arc welding and the outer steel sleeve welded using laser welding. Then, an outer layer of glass lubricant (formula: 50% SiO2, 10% CaO, 4% MgO, 6% Al2O3, 8% K2O, 8% B2O3, and 14% binder) is sprayed onto the outer surface of the metal sheath, with a coating thickness of 120 μm.

[0036] S3. Variable Diameter Lubricating Groove Double Cone Die Extrusion The titanium alloy rod prepared in step two was heated to 1000℃ and held for 90 minutes. It was then extruded using a double-cone die with variable-diameter lubrication grooves. The first cone of the double-cone die had a cone angle of 38°, and the second cone had a cone angle of 20°. Spiral micro-oil reservoirs were formed on the second cone, with a semi-circular cross-section, a depth of 0.2 mm, a width of 0.3 mm, a spacing of 0.8 mm between adjacent reservoirs, and a spiral angle of 45°. The reservoirs were filled with a solid lubricant (molybdenum disulfide:graphite = 2:1, with 8% cerium oxide added). The extrusion ratio was 12, and the extrusion speed was 80 mm / s.

[0037] S4. Online pickling treatment After extrusion, the seamless titanium alloy tubes are immediately subjected to online pickling at 250°C using the residual heat of the tubes themselves. The pickling solution formula (by weight percentage) is: 8% citric acid, 5% oxalic acid, 1.0% hexamethylenetetramine (corrosion inhibitor), 0.3% sodium dodecylbenzenesulfonate (surfactant), and the remainder is deionized water. The spraying pressure is 0.5 MPa, and the spraying time is 20 seconds.

[0038] S5. Neutral Electropolishing The seamless titanium alloy tubes, after acid pickling, underwent neutral electrolytic polishing. The electrolyte formula (by weight percentage) was: sodium chloride 12%, sodium sulfate 8%, sodium citrate 5%, and the remainder was deionized water, with a pH of 7.0. The electrolysis voltage was 15V, the current density was 35A / dm², and the polishing time was 3 minutes. Ultrasonic vibration was applied during the polishing process at a power of 350W and a frequency of 30kHz.

[0039] S6. Wash and air dry After electropolishing, the inner and outer surfaces of the pipe are rinsed with water, then immersed in a neutralizing solution (3% sodium carbonate solution) for 1.5 minutes, rinsed with water again, and finally rapidly cooled under nitrogen protection to obtain a seamless titanium alloy pipe with improved surface quality.

[0040] Testing revealed that the obtained seamless titanium alloy tube had a surface roughness Ra of 0.35 μm on both the inner and outer surfaces, with no microcracks, scratches, or adhesion marks. The yield was 18% higher than that of conventional processes. Metallographic observation showed that the fine-grained structure on the surface retained its fine-grained genetic effect after extrusion, with a grain size of approximately 5-8 μm. Corrosion resistance tests indicated that the passivation film formed by neutral electrolytic polishing was complete and dense, reducing the corrosion rate by 32% compared to traditional pickling treatment.

[0041] Example 2 S1. Pretreatment for refining the surface of the billet TA15 titanium alloy bars (150mm in diameter, 600mm in length) were selected and subjected to surface shot peening for fine grain refinement using a centrifugal shot peening machine. High-hardness glass pellets with a diameter of 0.6mm were used, with a shot peening intensity of 0.5 mmA and a surface coverage of 200% (followed by a 100% transition shot peening time after complete coverage), forming an ultrafine crystalline layer approximately 120μm thick on the surface of the bars. After shot peening, the bars underwent low-temperature annealing at 480℃ for 60 minutes, followed by air cooling.

[0042] S2. Preparation of Gradient Composite Lubricating Layer and Multilayer Composite Encasing A base layer of glass lubricant (formula: SiO2 42%, CaO 9%, MgO 4%, Al2O3 4%, Na2O 22%, binder 19%) was sprayed onto the surface of the pretreated titanium alloy rod to a thickness of 60 μm and cured for later use. An inner titanium foil sheath and an outer metal sheath were then sequentially fitted onto the rod surface: the inner titanium foil sheath was made of 0.15 mm thick TA1 pure titanium foil, tightly bonded using a spiral winding method; the outer metal sheath was a 1.0 mm thick stainless steel thin-walled sleeve (304 stainless steel); a solid lubricant powder layer (molybdenum disulfide:graphite = 1.5:1, with 6% cerium oxide added) was filled between the inner and outer sheaths to a thickness of 0.4 mm. The ends of the composite sheath were vacuum-sealed, with the inner titanium foil sheath using argon arc welding and the outer stainless steel sheath using plasma welding. Then, an outer layer of glass lubricant (formula: 48% SiO2, 11% CaO, 5% MgO, 35% Al2O3, 7% K2O, 39% B2O, and 15% binder) is sprayed onto the outer surface of the metal sheath, with a coating thickness of 100 μm.

[0043] S3. Variable Diameter Lubricating Groove Double Cone Die Extrusion The titanium alloy rod prepared in step two was heated to 980℃ and held for 100 min, then extruded using a double-cone die with variable-diameter lubrication grooves. The first cone of the double-cone die had a cone angle of 35°, and the second cone had a cone angle of 18°. Spiral micro-oil reservoirs were formed on the second cone, with a trapezoidal cross-section (wider at the top and narrower at the bottom, 0.3 mm wide at the top, 0.2 mm wide at the bottom, and 0.25 mm deep). The spacing between adjacent reservoirs was 1.0 mm, and the spiral angle was 50°. The oil reservoirs were filled with a solid lubricant (molybdenum disulfide:graphite = 2.5:1, with 10% cerium oxide added). The extrusion ratio was 10, and the extrusion speed was 60 mm / s.

[0044] S4. Online pickling treatment After extrusion, the seamless titanium alloy tubes are immediately subjected to online pickling at 220°C using the residual heat of the tubes themselves. The pickling solution formula (by weight percentage) is: 6% citric acid, 6% oxalic acid, 1.2% thiourea (corrosion inhibitor), 0.4% OP-10 (surfactant), and the remainder is deionized water. The spraying pressure is 0.4 MPa, and the spraying time is 25 seconds.

[0045] S5. Neutral Electropolishing The pickled seamless titanium alloy tubes were subjected to neutral electrolytic polishing. The electrolyte formula (by weight percentage) was: sodium chloride 14%, sodium sulfate 6%, sodium citrate 4%, and the remainder was deionized water, with a pH of 6.8. The electrolysis voltage was 12V, the current density was 30A / dm², and the polishing time was 4 minutes. The polishing process was supplemented with ultrasonic vibration at a power of 400W and a frequency of 25kHz.

[0046] S6. Wash and air dry After electropolishing, the inner and outer surfaces of the pipe are rinsed with water, then immersed in a neutralizing solution (4% sodium carbonate solution) for 1 minute, rinsed with water again, and finally rapidly cooled under nitrogen protection to obtain a seamless titanium alloy pipe with improved surface quality.

[0047] Effect evaluation: Testing revealed that the obtained seamless titanium alloy tubes had an inner and outer surface roughness Ra of 0.32 μm, with no microcracks, scratches, or adhesion marks. The yield was 20% higher than that of conventional processes. Metallographic observation showed that the fine-grained structure on the surface retained its fine-grained genetic effect after extrusion, with a grain size of approximately 4-7 μm. Corrosion resistance tests indicated that the corrosion rate was reduced by 35% compared to traditional pickling treatment.

[0048] Comparative Example 1 S1. Billet Preparation The same batch of TC4 titanium alloy rods (120 mm in diameter and 500 mm in length) as in Example 1 were selected. No shot peening pretreatment was performed; only routine cleaning and degreasing were carried out.

[0049] S2. Encapsulation Preparation A single-layer low-carbon steel sheath (2.0 mm wall thickness) is directly fitted onto the surface of a titanium alloy rod. No base layer of glass lubricant is sprayed, and no intermediate lubricating powder layer is provided. The two ends of the sheath are sealed by conventional welding. A single layer of glass lubricant (formula same as the outer layer glass lubricant in Example 1) is sprayed onto the outer surface of the sheath, with a coating thickness of 150 μm.

[0050] S3. Extrusion molding The encapsulated titanium alloy bar was heated to 1000℃ and held for 90 minutes. It was then extruded using a standard single-cone die (30° cone angle), without an oil reservoir. The extrusion ratio was 12, and the extrusion speed was 80 mm / s. No additional solid lubricant was added during the extrusion process.

[0051] S4. Post-processing After the extruded pipes cool, they are pickled using a conventional pickling process (HF + HNO3 mixed acid) to remove oxide scale for 5 minutes. Electropolishing is not performed. After pickling, the pipes are rinsed with water and allowed to air dry.

[0052] Testing revealed that the inner and outer surface roughness Ra of the obtained seamless titanium alloy tube was 1.8 μm, with obvious scratches and localized adhesion marks, and microcracks (0.5-2 mm in length) visible in some areas. The yield rate was only 72%. Metallographic observation showed a brittle TiFe reaction layer of approximately 15 μm thickness at the interface between the surface and the steel cladding. After pickling, the surface color was uneven, and localized over-corrosion was observed.

[0053] Comparative Example 2 S1. Billet Preparation The same batch of TC4 titanium alloy rods (120 mm in diameter and 500 mm in length) as in Example 1 were selected. No shot peening pretreatment was performed; only routine cleaning and degreasing were carried out.

[0054] S2. Encapsulation Preparation A single-layer TA2 titanium cladding (1.5 mm wall thickness) is fitted onto the surface of a titanium alloy rod. The cladding is a prefabricated thin-walled welded pipe with a gap of ≤0.5 mm between it and the billet, and both ends are vacuum-sealed. A single layer of glass lubricant (formula same as the outer glass lubricant in Example 1) is sprayed onto the outer surface of the cladding, with a coating thickness of 150 μm. No intermediate lubricating powder layer is provided.

[0055] S3. Extrusion molding The encapsulated titanium alloy bar was heated to 1000°C and held for 90 minutes. A double-cone die (first cone angle 38°, second cone angle 20°) was used, identical to that in Example 1, but the second cone surface of the die did not have a spiral oil reservoir and was not filled with solid lubricant. The extrusion ratio was 12, and the extrusion speed was 80 mm / s.

[0056] S4. Post-processing After the extruded pipes are cooled, they are subjected to the same online pickling process as in Example 1 (citric acid + oxalic acid formula, 250°C residual heat spray for 20s), and then the same neutral electrolytic polishing treatment as in Example 1.

[0057] Testing revealed that the obtained seamless titanium alloy tube had an inner and outer surface roughness Ra of 0.92 μm, with no microcracks, but some minor scratches (approximately 1-3 mm in length and 0.05 mm in depth) and uneven surface finish. The yield was 84%. Compared to Example 1, the lack of an intermediate lubricating powder layer and the continuous lubrication provided by the mold oil reservoir resulted in localized damage to the lubricating film during the later stages of extrusion, leading to surface scratches. Metallographic observation showed no brittle reaction layer at the interface between the titanium sheath and the billet, demonstrating the advantage of the titanium sheath's interface purity; however, insufficient lubrication resulted in a surface quality still lower than that of Example 1.

[0058] Comparative Example 3 (no pretreatment, otherwise the same as Example 1) Difference: Step S1 (pre-treatment of fine graining of billet surface) is omitted, and raw TC4 rods that have not undergone laser nano-processing are used directly.

[0059] Other steps: Steps S2 to S6 are exactly the same as in Example 1.

[0060] Comparative Example 4 (no gradient composite lubrication layer, using single lubrication, otherwise the same as Example 1) Differences: The preparation of the gradient composite lubricating layer in step S2 is omitted, and the traditional single lubrication method is adopted: only a single glass lubricant with a thickness of 200μm (the composition is the same as the outer lubricant in Example 1) is sprayed on the surface of the titanium alloy rod, and no bottom lubricant and metal soft sleeve are set.

[0061] Other steps: Steps S1, S3 to S6 are exactly the same as in Example 1.

[0062] The following table summarizes the comparison of the effects of Examples 1-2 and Comparative Examples 1-4: In summary, the surface roughness Ra of the products obtained in Examples 1-2 is ≤0.4μm, which is much lower than that of Comparative Examples 1-4; and the products obtained in Examples 1-2 have high surface quality, high yield, and low corrosion rate.

Claims

1. A method for preparing seamless titanium alloy tubes with improved surface quality, characterized in that... Includes the following steps: S1. Pre-treatment of the surface of the billet by fine graining: The titanium alloy bar is subjected to surface shot peening and fine graining treatment, followed by low-temperature annealing. S2. Preparation of gradient composite lubricating layer and multi-layer composite sheath: A bottom layer of glass lubricant is sprayed on the surface of the pretreated titanium alloy rod. After the bottom layer of lubricant has cured, an inner layer of titanium foil sheath and an outer layer of metal sheath are sequentially sheathed on the surface of the rod. A solid lubricant powder layer is filled between the inner and outer layers of sheath. The double-layer composite sheath is welded and sealed. Then, an outer layer of glass lubricant is sprayed on the outer surface of the metal soft sheath. S3. Variable diameter lubrication groove double cone die extrusion; S4. Online pickling treatment; S5. Neutral electrolytic polishing; S6. Wash and air dry.

2. The method for preparing a seamless titanium alloy tube with improved surface quality according to claim 1, characterized in that: In step S1, a centrifugal shot peening machine is used to perform shot peening and fine grain treatment on the surface of the bar. High-hardness ceramic or glass shot is used as the shot medium to bombard the outer surface of the bar at high speed. The shot diameter is 0.2-0.8 mm, the shot peening intensity is 0.2-0.6 mmA, and the surface coverage is 100%-200%, forming an ultra-fine crystalline structure layer with a thickness of 50-150 μm on the surface of the bar. The annealing temperature is 450-550℃, the holding time is 30-60 min, and the bar is air-cooled.

3. The method for preparing a seamless titanium alloy tube with improved surface quality according to claim 1, characterized in that: The bottom layer glass lubricant described in step S2 has a temperature tolerance of 800-900℃ and a spraying thickness of 50-100μm; the outer layer glass lubricant has a temperature tolerance of 900-1050℃ and a spraying thickness of 80-150μm.

4. The method for preparing a seamless titanium alloy tube with improved surface quality according to claim 3, characterized in that: The inner titanium foil sheath is made of TA1 or TA2 pure titanium foil with a thickness of 0.1-0.3 mm, and is tightly attached to the surface of the rod by spiral winding; the outer metal sheath is a thin-walled sleeve of low carbon steel or stainless steel with a thickness of 0.5-1.2 mm; the solid lubricant powder layer filling the space between the inner and outer sheaths has a thickness of 0.2-0.5 mm; and the inner titanium foil sheath is sealed by argon arc welding, while the outer metal sheath is sealed by laser welding or plasma welding.

5. The method for preparing a seamless titanium alloy tube with improved surface quality according to claim 1, characterized in that: In step S2, the raw material weight percentage composition of the bottom layer glass lubricant is: SiO2 40-50%, CaO 5-10%, MgO 2-5%, Al2O3 3-6%, Na2O 15-25%, and binder 15-25%; the raw material weight percentage composition of the outer layer glass lubricant is: SiO2 45-55%, CaO 8-12%, MgO 3-6%, Al2O3 4-8%, K2O 5-10%, B2O3 5-10%, and binder 10-20%.

6. The method for preparing a seamless titanium alloy tube with improved surface quality according to claim 1, characterized in that: In step S3, the titanium alloy bar obtained in step S2 is heated to 950-1050℃ and held for 60-120 minutes. Then, it is extruded using a double-cone die with a variable-diameter lubrication groove. The double-cone die includes a first conical surface and a second conical surface. The cone angle of the first conical surface is 30-45°, and the cone angle of the second conical surface is 15-25°. A spiral micro-oil storage groove is opened on the second conical surface. The cross-sectional shape of the oil storage groove is semi-circular or trapezoidal, with a groove depth of 0.1-0.3 mm, a groove width of 0.2-0.5 mm, a groove spacing of 0.5-1.2 mm, and a spiral angle of 30-60°. The oil storage groove is filled with solid lubricant. The extrusion ratio is 8-15, and the extrusion speed is 50-100 mm / s.

7. The method for preparing a seamless titanium alloy tube with improved surface quality according to claim 6, characterized in that: The solid lubricant in step S3 is a mixture of molybdenum disulfide and graphite in a mass ratio of 1:1 to 3:1, and 5-10% cerium oxide is added to the solid lubricant as a high-temperature stabilizer.

8. The method for preparing a seamless titanium alloy tube with improved surface quality according to claim 1, characterized in that: Step S4: After the seamless titanium alloy tube extruded in step S3 is demolded, it is immediately subjected to online spray pickling treatment at 200-300℃ using the residual heat of the tube itself. The pickling solution consists of the following components by weight percentage: 5-10% citric acid, 3-8% oxalic acid, 0.5-1.5% corrosion inhibitor, 0.1-0.5% surfactant, and the remainder is deionized water; the spraying pressure is 0.3-0.6 MPa, and the spraying time is 10-30 s.

9. The method for preparing a seamless titanium alloy tube with improved surface quality according to claim 1, characterized in that: In step S5, the seamless titanium alloy tube processed in step S4 is subjected to neutral electrolytic polishing. The electrolyte is a neutral salt solution composed of the following components by weight percentage: sodium chloride 10-15%, sodium sulfate 5-10%, sodium citrate 3-6%, and the remainder is deionized water, with a pH value of 6.5-7.

5. The electrolysis voltage is 10-20V, the current density is 20-50A / dm², and the polishing time is 2-5min.

10. The method for preparing a seamless titanium alloy tube with improved surface quality according to claim 1, characterized in that: In step S6, the seamless titanium alloy tube obtained in step S5 is rinsed with clean water on both the inner and outer surfaces, then immersed in a 2-5% sodium carbonate neutralization solution for 1-2 minutes, rinsed with clean water again, and finally rapidly cooled under nitrogen protection.

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