Welding process of TA18 titanium alloy thin-wall pipe

Abstract

The invention discloses a welding process of a TA18 titanium alloy thin-wall pipe, and relates to the technical field of metal material welding, in particular to the welding process of the TA18 titanium alloy thin-wall pipe, which is characterized in that the welding current is 40 amperes, and the welding voltage is 25 volts so as to control penetration and maintain electric arc stability; high-purity argon with the purity not lower than 99.5% is adopted as protective gas and is synchronously conveyed on the front side and the rear side of a welding seam; and the welding advancing speed is controlled to be 12-15 mm / min. Oxidation film removal, oil removal and drying treatment are carried out on a groove and an adjacent area before welding; argon needs to be continuously introduced after welding to carry out hysteresis protection on a welding seam and a heat affected zone until the temperature is reduced to 250 DEG C or below. According to the process, no filling material is added, the process is suitable for welding connection of the circumferential weld or the longitudinal weld of the TA18 titanium alloy pipe with the wall thickness not larger than 2.0 mm, and the welding quality can be effectively guaranteed.

Description

Technical Field

[0001] This invention relates to the field of metal material welding technology, specifically a welding process for TA18 titanium alloy thin-walled pipes. Background Technology

[0002] TA18 titanium alloy (nominal composition Ti-3Al-2.5V) is a near-alpha type titanium alloy widely used in aerospace, petrochemical, and shipbuilding industries due to its excellent cold and hot working properties, moderate room temperature and high temperature strength, outstanding corrosion resistance, and good plasticity-toughness balance. Particularly in the hydraulic, fuel, and environmental control systems of aircraft and engines, TA18 titanium alloy thin-walled tubing is a key structural component for achieving lightweight, high reliability, and long-life connections. These piping systems typically need to withstand medium pressure, mechanical vibration, and thermal cycling loads under complex operating conditions, thus placing extremely stringent requirements on the quality of welded joints between pipes. These joints must possess high strength, good sealing performance, and excellent fatigue and stress corrosion resistance.

[0003] However, the physical and chemical properties of titanium alloys, especially their extremely high chemical reactivity at high temperatures, pose a severe challenge to the welding process. When the temperature exceeds 400°C, titanium alloys begin to absorb hydrogen significantly; at temperatures above 600°C, they react violently with oxygen and nitrogen. During welding, the temperature of the arc zone and the molten pool is far above this critical point. If not effectively isolated from air, the titanium alloy will absorb these interstitial elements, leading to a sharp decrease in the plasticity and toughness of the weld joint area, a significant increase in brittleness, and potentially inducing cracks, severely damaging the mechanical properties and service safety of the joint. Therefore, one of the core technical challenges in titanium alloy welding lies in achieving thorough and continuous inert gas protection for the weld pool, arc, and heat-affected zone exceeding 250°C, preventing any air intrusion.

[0004] Currently, welding methods for titanium alloys mainly include gas tungsten inert gas welding (GTAW / TIG), plasma arc welding (PAW), electron beam welding (EBW), and laser welding (LBW). For welding thin-walled pipes (typically referring to pipes with a wall thickness of no more than 2.0 mm), GTAW has become one of the most commonly used and reliable joining technologies due to its ease of precise heat input control, good process visibility, simple equipment, and flexible operation. However, directly applying the conventional GTAW process to TA18 thin-walled pipes still faces several challenges: 1. Insufficient protection: Relying solely on the welding torch nozzle to provide protective gas is insufficient. The back of the weld, the front of the weld that has solidified but is still at a high temperature, and the heat-affected zone are highly susceptible to oxidation and discoloration (from slight blue or purple to severe gray or white powder) due to insufficient shielding or inadequate delayed protection time. This means that the joint performance has been compromised.

[0005] Improper heat input control: Thin-walled pipes are extremely sensitive to heat input. Insufficient current may lead to incomplete penetration, while excessive current or slow welding speed can easily cause burn-through, collapse, and coarse grains, thereby reducing joint strength. The stability of the voltage directly affects the arc concentration and energy density, influencing the penetration depth and weld formation.

[0006] Insufficient cleanliness: Impurities such as oxide film, oil, and moisture on the surface of titanium alloys will decompose and generate gas at high welding temperatures. This may not only cause porosity defects, but the hydrogen and oxygen elements in the gas will also dissolve directly into the molten pool, causing pollution.

[0007] Filler material matching: For thin-walled pipes, adding filler wire is sometimes not necessary, but if the process control is not proper, self-fusion welding may result in poor weld formation or uneven performance due to fluctuations in the composition of the base material or the influence of assembly gaps.

[0008] While existing titanium alloy welding technologies have solved some problems to a certain extent, for thin-walled tubular components of specific alloys like TA18, achieving effective protection and precise heat input control throughout the entire process—from pre-weld preparation and welding to post-weld cooling—through a refined and highly repeatable manual welding process to consistently obtain silver-white or light yellow (indicating good protection), aesthetically pleasing, internally defect-free welded joints with excellent mechanical properties remains a goal that those skilled in the art continue to explore and optimize. Especially in manual operation scenarios, ensuring high-quality welds for every operator through clearly defined process parameters and operating procedures, under conditions lacking the high precision and stability of automated equipment, is crucial for improving product quality consistency. Therefore, developing a targeted, parameter-defined, and comprehensive manual argon arc welding process for TA18 titanium alloy thin-walled tubular components has significant engineering application value. Summary of the Invention

[0009] The purpose of this invention is to provide a welding process for TA18 titanium alloy thin-walled pipes. By precisely controlling key parameters such as welding current, voltage, and travel speed, and combining strict beveling before welding with high-purity argon gas protection measures throughout the welding process and after welding, the high-temperature oxidation of titanium alloys can be effectively prevented, thereby obtaining well-formed and reliable welds. This process is particularly suitable for high-quality connections of circumferential or longitudinal seams in pipes with a wall thickness of no more than 2.0 mm.

[0010] To achieve the above objectives, this invention provides the following technical solution: a welding process for TA18 titanium alloy thin-walled tubing. This welding process is designed specifically for TA18 titanium alloy thin-walled tubing and employs manual argon arc welding as the welding method. During the welding process, inert gas protection is implemented throughout the entire welding process, from start to finish, to ensure that the titanium alloy does not react with oxygen, nitrogen, or other gases in the air at high temperatures, thereby preventing weld oxidation, nitriding, and the resulting degradation of mechanical properties. The operational flexibility of manual argon arc welding is suitable for the precision welding of thin-walled tubing and allows for localized heat input control.

[0011] Furthermore, the welding current parameter was set to 40 amperes, a value determined based on the physical properties of the TA18 titanium alloy thin-walled tubing. A current of 40 amperes provides suitable heat input, ensuring full penetration of the tubing while avoiding burn-through or deformation due to excessive heat input. Stable application of this current parameter helps control weld formation, ensuring a uniform and smooth weld that meets the quality requirements of thin-walled structures.

[0012] Furthermore, the welding voltage parameter was set to 25 volts, in conjunction with a current parameter of 40 amps. The 25-volt voltage helps maintain the length and stability of the arc, preventing arc drift or interruption during welding. A stable arc is crucial for ensuring weld continuity and consistency, especially important for welding thin-walled pipes, as it reduces weld defects.

[0013] Furthermore, high-purity argon is used as the shielding gas, supplied simultaneously from both the front and back sides of the weld area. This double-sided protection mechanism covers the weld and its heat-affected zone from two directions, effectively eliminating air and reducing blind spots. The argon gas forms a uniform gas barrier in the welding area, improving the protection effect on the high-temperature areas of the titanium alloy.

[0014] Furthermore, the volume purity of high-purity argon gas must be no less than 99.5%. High purity is fundamental to ensuring the effectiveness of inert gas protection; insufficient purity may introduce impurities, weakening the protective capability. Argon gas with a purity of 99.5% or higher can more effectively isolate the high-temperature areas of titanium alloys, preventing contamination from elements such as oxygen and nitrogen, and ensuring the chemical composition and metallurgical quality of the weld metal.

[0015] Furthermore, the welding travel speed is controlled within a continuous range of 12 to 15 millimeters per minute. This speed range is set based on the heat input sensitivity of TA18 titanium alloy thin-walled tubing. Too high a speed may result in insufficient penetration or incomplete weld penetration; too low a speed can easily cause overheating, burn-through, or deformation. Maintaining a speed of 12-15 millimeters per minute balances heat input and cooling rate, ensuring the stability of the welding process.

[0016] Furthermore, before welding begins, the bevel and adjacent areas of the TA18 titanium alloy base material must undergo pretreatment, including oxide film removal, degreasing, and drying. Removing the oxide film and oil eliminates sources of contamination and prevents defects such as weld porosity and inclusions; drying prevents moisture introduction and reduces the risk of hydrogen-induced cracking. Pretreatment is a fundamental step in ensuring the cleanliness of the weld joint and the quality of the weld.

[0017] Furthermore, argon gas must be continuously introduced after welding to provide delayed protection for the weld and heat-affected zone until the temperature in this area drops below 250 degrees Celsius. Titanium alloys retain high chemical reactivity during the high-temperature cooling phase and readily react with air. Delayed protection extends the gas protection time, preventing weld contamination during cooling. Once the temperature drops below 250 degrees Celsius, the reactivity of the titanium alloy significantly decreases, and protection can be discontinued.

[0018] Furthermore, this manual argon arc welding process does not add external filler material, directly utilizing the self-fusion of the base metal to form the weld. Self-fusion welding relies on the melting and fusion of the workpiece edges and is suitable for butt welding of thin-walled pipes. This method can reduce welding heat input, avoid compositional deviations or operational complexity caused by the introduction of filler material, and at the same time help maintain the consistency of weld composition with the base metal.

[0019] Furthermore, this process is applicable to the welding connections of TA18 titanium alloy pipes with a wall thickness of no more than 2.0 mm, including circumferential and longitudinal welds. Welding thin-walled pipes requires controlled heat input to avoid deformation or burn-through; this process meets this requirement through parameter matching and protective measures. Circumferential welds are used for circumferential connections of the pipes, and longitudinal welds are used for longitudinal joints. This process effectively addresses the welding challenges of different joint types.

[0020] This invention provides a welding process for TA18 titanium alloy thin-walled tubing, which has the following beneficial effects: 1. This process sets the welding current to 40 amps and the voltage to 25 volts, strictly limiting the welding speed to a narrow range of 12 to 15 millimeters per minute. The core advantage of this parameter combination lies in achieving precise control over the welding heat input. For thin-walled titanium alloy tubes like TA18, they are extremely sensitive to heat input: excessive heat input leads to rapid grain coarsening and an excessively wide heat-affected zone, thus reducing the plasticity and toughness of the joint; while excessive heat input may cause quality problems such as incomplete fusion. This process, by fixing the current and voltage and proceeding at a constant low speed, ensures that the energy absorbed per unit length of weld is stable and moderate. This precise control is sufficient to ensure the tube penetrates and forms a good weld shape, while also minimizing the tendency of titanium alloy grain growth at high temperatures, avoiding overheated structures in the weld joint. This lays a solid foundation for obtaining weld joints with excellent mechanical properties (such as high strength and high fatigue life), and is particularly suitable for thin-walled structural components with high reliability requirements.

[0021] The most significant feature of this process is its systematic gas protection strategy. TA18 titanium alloy begins to absorb oxygen, nitrogen, and hydrogen from the air at temperatures above 300 degrees Celsius, leading to weld embrittlement and severe performance degradation. This process not only uses high-purity argon gas (99.5% purity) as the protective medium, but more importantly, it employs a dual mechanism of "synchronous protection before and after welding" and "delayed protection after welding." During welding, argon gas is simultaneously supplied before and after the weld, forming a complete protective gas curtain that completely isolates the molten pool and high-temperature area (including the area to be welded in front of the welding torch) from the air. After welding, argon gas continues to be supplied until the temperature drops below 250 degrees Celsius. This measure is crucial because it protects the still-high-temperature weld metal and heat-affected zone from oxidation during cooling. This protective scheme, which extends throughout the welding process, ensures that the titanium alloy remains in an inert atmosphere throughout the entire process of melting, solidification, and cooling. This results in a beautiful weld with a silvery-white or metallic color, and its intrinsic quality (such as plasticity and toughness) is fundamentally guaranteed, avoiding welding defects caused by gas contamination.

[0022] This process explicitly employs a welding method without adding external filler materials, directly utilizing the self-fusion of the base metal to form the weld. For thin-walled pipes with a wall thickness of no more than 2.0 mm, this choice offers multiple benefits. First, it simplifies pre-welding preparation, eliminating the need for rigorous cleaning, drying, and matching of the filler wire, reducing operational steps and potential sources of contamination. Second, during welding, the welder does not need to simultaneously control the welding torch angle, travel speed, and wire feed speed, reducing operational difficulty and making it easier to achieve uniform weld formation, reducing defects such as uneven welds and undercut caused by unstable wire feeding. Most importantly, self-fusion welding ensures that the composition of the weld metal is completely consistent with the base metal, avoiding metallurgical incompatibility issues that may arise from differences in the composition of the filler material and the base metal, such as the formation of low-melting-point eutectic or brittle phases. This results in a high degree of matching between the weld and the base metal in terms of composition, microstructure, and properties, making it particularly suitable for the connection of thin-walled precision pipe fittings where strict requirements for compositional uniformity are necessary.

[0023] This process not only focuses on the welding process itself but also emphasizes quality control in the two critical pre- and post-weld stages. Pre-weld treatment involves rigorous removal of oxide film, oil, and drying of the bevel and adjacent areas, a prerequisite for successful welding. Any trace of oil, moisture, or scale will decompose under the high temperature of the electric arc, and its products (such as hydrogen and oxygen) will dissolve into the molten pool, leading to porosity, inclusions, or hydrogen-induced cracks, severely compromising the weld's density and mechanical properties. This rigorous pre-treatment eliminates these sources of contamination at the source. Post-weld hysteresis protection is a crucial supplement to the welding thermal cycle endpoint, ensuring the joint's safety at its most vulnerable red-hot state. These two measures complement each other, forming a closed-loop quality control system from "clean start" to "safe finish," significantly improving process reliability and weld quality consistency. For TA18 titanium alloy pipe welding under harsh conditions, this comprehensive quality control approach is key to ensuring long-term stable product operation.

[0024] The various parameters of this welding process (40A current, 25V voltage, speed 12-15mm / min, high-purity argon gas protection, etc.) are not simply listed, but constitute a highly synergistic and mutually matched organic whole. For example, the 25V voltage and 40A current are matched to maintain a stable and concentrated arc, which is a prerequisite for achieving good penetration and weld formation. The set welding speed range is based on the optimal window for achieving ideal penetration and preventing overheating under the above electrical parameters. This inherent logical relationship between parameters makes the entire process highly systematic and operable. Explicitly defining it as manual argon arc welding and providing specific values ​​or ranges for all key parameters greatly reduces over-reliance on individual welder experience. Welders of different skill levels can more easily obtain repeatable welding results with stable quality and consistent performance when following this process specification. This is of great significance for quality control and standardized operations in mass production, effectively reducing product quality variability. Attached Figure Description

[0025] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0026] Figure 1 This is a flowchart illustrating the core selection process for the welding method of this invention. Figure 2 This is a flowchart of the core welding parameter setting process for this invention. Figure 3 This is a flowchart of the gas protection system of the present invention; Figure 4 This is a flowchart of the pre-welding pretreatment process of the present invention; Figure 5 The flowchart summarizes the applicability of the process of this invention. Detailed Implementation

[0027] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this disclosure. Rather, they are merely examples of apparatuses consistent with some aspects of this disclosure as detailed in the appended claims.

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0029] How to use: I. Preparations before welding Thorough preparation is a prerequisite for successful welding; any negligence may lead to welding defects such as porosity, inclusions, oxidation, or cracks.

[0030] Base material inspection and pretreatment: Confirming the base material: First, it must be confirmed that the pipe to be welded is TA18 titanium alloy, and its specifications (outer diameter, wall thickness) must be checked to ensure they meet the process requirements (wall thickness ≤ 2.0 mm).

[0031] Beveling preparation: For pipe butt welding, a suitable beveling must be designed and processed according to the wall thickness. For thin-walled pipes (e.g., ≤2.0mm), an I-bevel (i.e., no beveling) is usually used for butt welding. The gap must be strictly controlled, generally recommended to be between 0-0.5mm, to ensure full penetration and prevent collapse.

[0032] Cleaning (crucial): Strictly follow the requirements of claim 7. Thoroughly remove the oxide film from the bevel and at least 20 mm on both sides using a stainless steel wire brush or specialized sandpaper (avoid tools previously used on other metals to prevent contamination). Then, use high-purity acetone or a specialized metal cleaner, along with a non-woven cloth, to thoroughly remove oil, moisture, fingerprints, and other contaminants. Welding should be performed immediately after cleaning to avoid secondary contamination. If immediate welding is not possible, cover with a clean plastic film for protection.

[0033] Inspection of welding equipment and tooling: Welding machine: Select a stable DC TIG welding machine and ensure that its current and voltage regulation is accurate, and that the high-frequency arc ignition and attenuation functions are normal.

[0034] Welding torch: Select a welding torch of appropriate size to ensure that its ceramic nozzle diameter can provide a sufficient gas protection range.

[0035] Gas protection system: This is the key to titanium alloy welding.

[0036] Main gas line: Connect to a high-purity argon cylinder (purity ≥99.5%, claim 5), and install a pressure reducing gauge, flow meter, and solenoid valve. Check the gas line for leaks.

[0037] Back cover (rear protection): A back cover (or rear protection cover) that matches the curvature of the pipe must be provided to continuously protect the front of the weld and the heat-affected zone, which has solidified but is still at a high temperature, during the welding process.

[0038] Backside protection: For circumferential welds on pipes, internal argon purging protection must be implemented. This can be achieved by using a specially made retractable plug depending on the pipe diameter, or by using sealed plastic bags filled with argon, ensuring the backside of the weld is in an inert gas environment during welding.

[0039] Tooling and fixtures: Use appropriate fixtures to accurately align and secure the pipes, ensuring uniform assembly gaps and effectively controlling welding deformation. The fixtures should be reasonably designed to avoid excessive interference with the protective gas flow.

[0040] Preset parameters: According to claims 2, 3, and 6, the welding machine parameters are pre-set to: welding current 40 amperes and welding voltage 25 volts. This parameter combination is designed to provide a stable arc and achieve controlled melting of thin-walled TA18 alloy.

[0041] Set the argon gas flow rate properly: the main gas flow rate for the welding torch is typically 8-15 L / min, the flow rate for the shielding tube is 15-25 L / min, and the flow rate for the back protection needs to be adjusted according to the pipe diameter and cavity volume to ensure complete air replacement. All flow rates must be precisely controlled by a flow meter; too low a flow rate will result in insufficient protection, while too high a flow rate will easily generate turbulence and draw in air.

[0042] The operator needs to familiarize themselves with and fine-tune the welding speed through trial welding (on a test plate of the same material), controlling it within the range of 12-15 mm / min (claim 6) to ensure that the weld formation is uniform, the width is consistent, and the penetration depth is appropriate.

[0043] III. Welding Operation Procedures 1. Final inspection before welding: Double-check the cleanliness of the workpieces and the quality of the assembly.

[0044] Turn on the argon gas and pre-purge the welding torch, shielding, and inside the pipes for a period of time (usually no less than 30 seconds) to completely purge air from the pipes and welding area. You can use a lighter or smoke to test the gas flow at the shielding gas outlet to feel if the flow is stable.

[0045] Arc striking and welding: High-frequency arc ignition or pulse arc ignition is used to ignite the electric arc at the start of the weld.

[0046] Maintain a 75°-85° angle between the welding torch and the workpiece surface, and keep the arc length (distance from the tip of the tungsten electrode to the workpiece) stable at 1.5-3mm.

[0047] Welding Process: As described in claim 9, this process employs autogenous welding without filler wire. The operator must concentrate on observing the formation and changes of the molten pool. A qualified molten pool should exhibit a bright, clear mirror-like appearance and flow smoothly forward with the movement of the arc. The welding speed should be strictly controlled within the predetermined range by moving the welding torch evenly and smoothly. The weld should be completed continuously in one go, avoiding interruptions.

[0048] Synergy of gas protection: During the welding process, the argon gas from the welding torch provides primary protection for the molten pool area.

[0049] The shield must follow closely behind the arc, completely covering the red-hot weld and heat-affected zone, ensuring that the area remains in an argon atmosphere until it cools to the critical temperature.

[0050] Argon protection inside the pipeline must remain effective until the entire weld is completed.

[0051] IV. Post-weld treatment and quality control 1. Delayed protection: After welding is completed, the welding torch and shroud should not be removed immediately. The current reduction button on the welding torch should be pressed to extinguish the arc, but the argon supply must continue. The welding torch should remain above the weld termination point while argon continues to flow, and the shroud should continue to cover the weld. This delayed protection must continue until the temperature of the weld and heat-affected zone metal drops below 250 degrees Celsius (claim 8). The temperature can be determined using an infrared thermometer or by observing the weld color empirically—when the surface turns silvery-white or light straw-colored, the temperature is sufficiently low. This is the final critical barrier to prevent high-temperature oxidation of the titanium alloy.

[0052] Weld cleaning and inspection: After the hysteresis protection ends, allow the weldment to cool naturally to room temperature in still air.

[0053] After cooling, use a stainless steel wire brush to gently brush away any very thin layer of oxide color that may be present on the weld surface (if well protected, it should be silvery-white).

[0054] Visual inspection: Check the weld formation; it should be uniform and aesthetically pleasing, free from surface defects such as undercut, weld beads, depressions, and cracks. The weld reinforcement and width should be uniform.

[0055] Non-destructive testing (NDT): Depending on product requirements, 100% dye penetrant testing (PT) or radiographic testing (RT) is performed to check for defects such as porosity, incomplete penetration, and lack of fusion inside the weld.

[0056] Performance verification: For critical components, welding procedure qualification test specimens must be prepared according to standards, and mechanical property tests (such as tensile and bending tests) and metallographic analysis must be conducted to verify whether the performance of the welded joint meets the design requirements.

[0057] Example: Example 1: Standardized Operation of Parallel Butt Circumferential Welds This embodiment describes a parallel (1G position) butt weld of a φ25mm×1.5mm TA18 titanium alloy pipe used in an aviation hydraulic system. The operation was performed in a clean, dedicated welding workshop. First, a precision pipe fitting machine was used to cut the pipe, ensuring that the perpendicularity error of the pipe end face was less than 0.1mm to achieve a gapless butt weld. The cleaning stage was particularly critical. The operator wore brand-new white cotton gloves and used a 304 stainless steel wire brush specifically designed for titanium alloys to repeatedly polish a 20mm area on both the inner and outer surfaces along the circumference of the pipe end until a uniform metallic luster was achieved. Subsequently, medical-grade absorbent cotton soaked in sufficient analytical-grade acetone was used to wipe the pipe in the same direction. The absorbent cotton was replaced after each wipe until no stains remained on the final absorbent cotton. Immediately after cleaning, the pipe fitting was placed into the butt welding fixture. The V-block of the fixture was lined with a copper pad to prevent scratches and aid in heat dissipation.

[0058] Welding parameters were set strictly according to the claims: 40A current, 25V voltage. The gas protection system was specially configured: the welding torch gas flow rate was set to 10L / min, using a φ12mm diameter ceramic nozzle. To ensure foolproof back protection, a "removable sealed gas chamber" method was adopted: two temporary plugs were made from thin aluminum plates, tightly fitting the inner diameter of the pipe, with an inlet pipe and a micro-pressure gauge inserted in the center of each plug. Before welding, argon was purged at a flow rate of 5L / min for approximately 2 minutes. After confirming positive pressure and stability within the chamber using the micro-pressure gauge, the flow rate was adjusted to the required 3L / min for welding. The shroud was handcrafted from 0.1mm thick stainless steel sheet, forming an arc shape approximately 80mm long to match the outer diameter of the pipe, and filled with multiple layers of stainless steel wire mesh to ensure uniform airflow. Its gas supply flow rate was set to 18L / min.

[0059] After arc ignition, the operator observed a rapid formation of the molten pool, exhibiting a bright silver, mirror-like finish—a direct indication of good protection. The welding torch advanced at a constant speed, precisely controlled at approximately 13 mm / min using a ruler and stopwatch beside the weld bead. During welding, the welding shield was pulled by an assistant using an insulated rod, closely following approximately 10 mm behind the molten pool. The entire weld circle was completed in one pass, with the current decay function used during arc termination. The gas supply to the welding torch and shield was maintained for at least 5 minutes, until the temperature of the weld area, measured with an infrared thermometer, dropped below 80°C. The final weld exhibited a uniform silver-white color with a fine, continuous fish-scale pattern on the surface. 100% dye penetrant testing revealed no surface defects. This example demonstrates the standardized, high-quality results achievable under ideal tooling and preparation conditions, with strict adherence to process parameters.

[0060] Example 2: Application of transition welding for pipes with different wall thicknesses In actual maintenance, we encountered situations where a 32mm × 1.8mm TA18 pipe needed to be butt-welded to another pipe of the same diameter but with a wall thickness of 1.5mm. This embodiment illustrates the adaptive application of the core process under this asymmetrical wall thickness condition. During pretreatment, a small chamfer of approximately 15° was applied to the outer side of the thick-walled pipe (1.8mm) end to form a shallow V-shape, smoothly transitioning to the end face of the thin-walled pipe (1.5mm). This step aims to balance the fusion amount on both sides and prevent the weld from shifting towards the thin-walled side. During assembly, the inner walls of the pipes were ensured to be flush, while the outer walls naturally formed a misalignment of approximately 0.3mm.

[0061] The core welding parameters remained based on a 40A current and 25V voltage, but were fine-tuned. Considering the differences in wall thickness and the heat capacity changes caused by the chamfering, the welding speed was slightly increased to 14.5mm / min, close to the upper limit of the range, and the arc dwell time on the thicker side was slightly increased. The gas protection scheme was crucial for success. Because the different wall thicknesses on both sides could lead to uneven heat dissipation, the length of the shield was intentionally extended to 100mm, ensuring its arc perfectly conformed to the outer surfaces of both pipe diameters. The argon flow rate was increased to 22L / min to cover a wider heat-affected zone. High-density sponge plugs were used as temporary stoppers for the back protection; their flexibility allowed them to adapt well to slight differences in pipe inner diameter, ensuring a tight seal.

[0062] Welding begins with an arc on the thin-walled side of the tube, pointing slightly towards the chamfered area of ​​the thick-walled tube. The operator dynamically adjusts the torch angle by observing the width and sag of the molten pool: when the molten pool width slightly widens on the thin-walled side, the arc is slightly directed towards the thick-walled side; when the molten pool shows a good "penetration" (indicating near penetration) and the back formation is visible (observed through an internal tube light) and uniform, a constant speed is maintained. Post-weld, the hysteresis shielding time is extended to 6 minutes because the thick-walled section cools slightly slower. The final weld has a uniform silvery-white appearance, a smooth transition area, and radiographic testing shows good weld fusion, uniform root penetration, and no incomplete fusion or concave defects due to wall thickness differences. This example demonstrates the ability to handle non-ideal joint forms within a given process framework by fine-tuning speed, gas shielding, and operating techniques.

[0063] Example 3: Construction of circumferential welds on fixed pipe sections in confined spaces This embodiment simulates a scenario where two fixed pipe sections are welded within a confined space during the installation of a spacecraft propulsion system piping. The pipes are φ18mm×1.2mm TA18 alloy, and the welding position is all-position (5G), meaning the pipes are horizontally fixed, and the welder needs to weld around the pipes. The biggest challenge is the limited space, making the implementation of argon filling and drag shield protection extremely difficult. We designed a miniature local protection scheme. For back protection, we abandoned overall argon filling and adopted a "flexible gasbag local isolation method": a slender gasbag is made of high-temperature resistant polyimide film, which is inserted through a process hole on one side of the pipe and aligned with the weld position through a thin copper tube. After argon filling, it expands and adheres tightly to the inner wall of the pipe, forming a local gas chamber.

[0064] Due to space constraints, a standard welding torch could not be used. We created a custom "finger-shaped welding torch": several thin-walled copper tubes with an outer diameter of φ4mm were arranged side-by-side, bent into an arc shape matching the outer diameter of the pipes. One side of each copper tube had a row of densely packed fine holes, and the other end was connected to the argon gas main pipe. This device was fixed to an openable ring clamp, allowing it to be held behind the weld seam. A special model with an extra-long neck and a small nozzle was selected for the welding torch to adapt to the confined space. The welding parameters were set to the lower limits: current 40A, voltage 25V, but the welding speed was dynamically adjusted according to the position: approximately 12mm / min in flat and overhead welding positions, and slightly faster to approximately 14mm / min in vertical welding positions, to counteract the effect of gravity on the molten pool.

[0065] During operation, the welder needs to constantly change body posture and welding torch angle. The arc initiation point is selected at the top (12 o'clock position). During welding, the assistant monitors the position and fullness of the back gasbag through an observation lens to ensure it always covers the molten pool area. Argon gas is delivered at a high flow rate (20L / min) through the finger-shaped drag shield, forming a dense gas curtain. Despite the harsh operating conditions, the molten pool remains bright silver due to the targeted protective measures. After the entire circle is welded, gas protection continues until the weld no longer emits any visible red light in the dark (below approximately 500°C), then the gasbag is slowly withdrawn. The weld passed inspection, proving that this process can still be reliably implemented in extremely confined environments through flexible design of protective fixtures.

[0066] Example 4: Welding and Deformation Control of Longitudinal Seams in Thin-Walled Tubes This embodiment involves rolling and welding a 1.0mm thick TA18 titanium alloy sheet into a longitudinal straight seam for a φ60mm pipe. The weld length is approximately 190mm. The core challenge lies in controlling the thermal deformation and burn-through of the thin sheet during the welding process. We used a precision plate rolling machine to ensure a tight fit between the butt edges, with a gap of less than 0.1mm. The cleaning area of ​​the sheet metal was extended to 50mm wide on both sides of the weld. To control deformation, a rigid welding fixture with a copper backing was fabricated. The copper backing has a forming groove 0.5mm deep and 3mm wide, and the back is cooled by water.

[0067] During welding, the clamped workpiece is placed in a flat position. For parameter selection, considering the ultra-thin 1.0mm wall thickness, the current is slightly reduced to 38A, while the voltage is maintained at 25V to improve control precision. The welding speed is set at a relatively high 15mm / min to reduce heat input. Special protection measures are required: in addition to the standard welding torch protection (flow rate 10L / min), a protective shield is used on both sides of the weld. The front shield is a long, flat strip covering the weld and both sides; the back shield, located within the copper gasket groove, uses tiny pores inside the gasket to deliver argon gas (flow rate 5L / min), achieving double-sided three-dimensional protection.

[0068] A segmented back-welding method was employed to disperse thermal stress: the 190mm weld was divided into three segments. The middle segment (approximately 70mm) was welded first, then the last segment was welded from the end towards the middle, and finally the first segment was welded from the arc initiation point towards the middle, with each segment overlapping by 10mm at the joint. Gas was strictly supplied in advance for each arc initiation, and the current was attenuated and the gas was cut off after a delay upon arc termination. During welding, through the welding torch filter, the molten pool could be clearly seen forming smoothly against the copper pad, slightly "sinking" into the pad groove without burning through, forming a uniform back weld bead. After welding, the clamps were released, and the pipe body showed only minimal longitudinal shrinkage deformation, with roundness measurements meeting requirements. This example demonstrates that this process, combined with specialized tooling and a segmented welding strategy, can effectively solve the challenges of burn-through and deformation control in thin-walled longitudinal seams.

[0069] Example 5: Practice of Multi-pass Cycle Welding of Small Diameter Thin-walled Tubes This embodiment addresses a specific component: it is formed by bending a 10mm × 1.0mm TA18 ultra-thin-walled tube, and requires welding three closely spaced circumferential welds (minimum interval of only 15mm). Heat accumulation and heat-affected zone overlap are the main risks. We developed a multi-pass welding scheme of "low heat input, intermittent cooling." First, a miniature lathe is used to precision machine all the pipe ends to be welded, ensuring that the gap after assembly is almost zero. Cleaning is performed under a 10x magnifying glass to ensure microscopic cleanliness.

[0070] For each weld, parameters were strictly controlled at the lower limit of the process parameters: current 36A (slightly lower than the claimed value due to the small pipe diameter and rapid heat conduction), voltage 24V, and welding speed at the upper limit of 15mm / min. Protective measures were specialized due to the small pipe diameter: the welding torch used a φ6mm micro-nozzle; the drag shield was a custom-made small cylindrical copper shield lined with fine steel wire mesh; and the back protection used extremely flexible latex tubing as a temporary air plug. The welding sequence was carefully planned: the middle weld was welded first, followed by the two ends, to distribute the heat input as much as possible.

[0071] After each weld is completed, the weld is immediately cooled by concentrated delayed purging using an independent, flow-controlled argon gas nozzle. Simultaneously, the entire component is moved to the vicinity of a water-cooled copper base (non-contact) for radiation-assisted cooling. The next weld is only performed after the temperature of the weld area has dropped below 50°C (measured with a handheld thermocouple) and the area has been left to stand for at least 15 minutes. When welding adjacent welds, the already welded areas are covered with damp asbestos cloth to prevent secondary heating of their heat-affected zones. This extremely cautious "weld one, cool one through" approach, while extending the total work time, successfully prevented grain growth and deformation of the thin tubes due to heat accumulation. All three welds ultimately turned silvery-white, with stable dimensions. Microscopic metallographic examination showed that the heat-affected zones of each weld were narrow and non-overlapping, and the base material properties were not significantly affected. This embodiment demonstrates the level of precision control achievable when dealing with high-density joints and easily deformable components by combining core processes with strict process management and cooling regimes.

[0072] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A welding process for TA18 titanium alloy thin-walled tubing, characterized in that: The welding method used is manual argon arc welding, and the welding process requires inert gas protection throughout.

2. The welding process for a TA18 titanium alloy thin-walled tube according to claim 1, characterized in that: The welding current parameter used during welding is 40 amperes, which is suitable for the penetration and forming control of TA18 titanium alloy thin-walled tubes.

3. The welding process for a TA18 titanium alloy thin-walled tube according to claim 1, characterized in that: The welding voltage parameter used during welding is 25 volts, which is matched with the current parameter to maintain arc stability.

4. The welding process for a TA18 titanium alloy thin-walled tube according to claim 1, characterized in that: The protective gas is high-purity argon, which is supplied and protected simultaneously on both the front and back sides of the weld area.

5. The welding process for a TA18 titanium alloy thin-walled tube according to claim 4, characterized in that: The volume purity of the high-purity argon gas is not less than 99.5% to ensure effective isolation and protection of the high-temperature areas of the titanium alloy.

6. The welding process for a TA18 titanium alloy thin-walled tube according to claim 1, characterized in that: The travel speed during the welding process should be controlled within a continuous range of 12 to 15 millimeters per minute.

7. The welding process for a TA18 titanium alloy thin-walled tube according to any one of claims 1 to 6, characterized in that: Before welding begins, the bevel and adjacent areas of the TA18 titanium alloy base material must be pretreated by removing the oxide film, oil, and drying.

8. The welding process for a TA18 titanium alloy thin-walled tube according to claim 4 or 5, characterized in that: Argon gas should be continuously introduced after welding to provide delayed protection for the weld and heat-affected zone until the temperature in the area drops below 250 degrees Celsius.

9. The welding process for a TA18 titanium alloy thin-walled tube according to claim 1, characterized in that: The manual argon arc welding process does not add external filler material and directly utilizes the self-fusion of the base material to form the weld.

10. The welding process for a TA18 titanium alloy thin-walled tube according to claim 1, characterized in that: The process is applicable to the welding connection of circumferential or longitudinal welds of TA18 titanium alloy pipes with a wall thickness of no more than 2.0 mm.

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