An active metal powder re-compression plasma arc welding process and apparatus for stainless steel

Abstract

This invention provides a process and apparatus for recompressing plasma arc welding of stainless steel using active metal powder. The apparatus includes a welding torch, a powder delivery system, and a gas path control unit. The welding torch contains an ion gas channel and an active metal powder channel, with the outlet of the active metal powder channel facing the arc column region of the plasma arc. The process includes simultaneously feeding the active metal powder into the arc column region of the plasma arc at a preset feed rate, wherein the feed rate is determined based on the welding speed. In this invention, the powder particles melt and evaporate in the arc column region, generating thermal and electromagnetic contraction effects, thus recompressing the arc and effectively increasing the weld penetration without increasing heat input or compressing corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of plasma arc welding technology for stainless steel, and particularly to a process and apparatus for recompressed plasma arc welding of stainless steel using active metal powder. Background Technology

[0002] Stainless steel, especially 316L austenitic stainless steel, is widely used in fields with stringent corrosion resistance requirements, such as chemical engineering, marine engineering, nuclear power, food and pharmaceuticals, due to its excellent corrosion resistance and good comprehensive mechanical properties. Plasma arc welding, as a high-energy-density fusion welding method, has significant advantages such as good arc stiffness, strong penetration ability, and high welding efficiency, and is an important process for welding medium and heavy plate stainless steel components.

[0003] However, in conventional plasma arc welding, the energy density and penetration capability of the arc still have certain physical limits after mechanical compression by the nozzle and thermal compression by the gas flow. When single-pass penetration of thick plates needs to be achieved with a relatively small welding current or a relatively high welding speed, conventional plasma arc welding often exhibits insufficient penetration depth. While increasing the welding current can improve penetration depth, it also brings a series of negative effects such as increased heat input, intensified welding deformation, and coarsening of the weld microstructure. How to effectively increase the penetration depth of plasma arc welds without excessively increasing welding heat input or sacrificing weld quality has always been a research hotspot and engineering challenge in this field.

[0004] To address this issue, activated plasma welding technology offers a feasible solution. The core concept of activated plasma welding is to introduce trace amounts of active substances into the welding zone. By utilizing the physicochemical interaction between these active substances and the electric arc or molten pool, the behavior of the arc and the flow state of the molten pool are altered, thereby increasing the penetration depth. In the field of plasma arc welding, activated plasma arc welding technology has been developed. The traditional method involves pre-applying a paste or suspension containing active components to the surface of the workpiece to be welded, allowing it to dry before welding. Relying on the differences in ionization energy of certain elements or compounds in the activator under the action of the arc, the cooling and contraction effect on the arc, and the change in the surface tension of the molten pool, activated plasma arc welding does indeed achieve an increase in penetration depth to a certain extent.

[0005] However, this pre-coating process has revealed several significant shortcomings in engineering practice. First, there is the challenge of controlling coating uniformity. Manual application makes it difficult to ensure that the thickness and density of the activator coating are completely consistent throughout the weld. Inconsistent coating directly leads to fluctuations in weld penetration along its length, affecting the uniformity and reliability of weld quality. Second, since the activator is applied to the workpiece surface in paste or suspension form, the direct blowing of the electric arc easily disperses or peels off any unreacted active material. Therefore, in engineering practice, organic binders must be introduced into the activator formulation to enhance the adhesion between the coating and the workpiece surface. This approach, however, introduces new and often more serious problems: the organic binder decomposes at the high temperature of the electric arc, inevitably introducing carbon into the weld metal. For ultra-low carbon austenitic stainless steels like 316L, the introduction of carbon directly leads to a significant deterioration in its resistance to intergranular corrosion, which contradicts the fundamental requirements of stainless steel welding. Furthermore, the pre-coating method requires coating the entire weld seam and a wide area on both sides before welding, which is not only cumbersome and inefficient, but also wastes a considerable portion of the activator without actually playing an effective role during the welding process.

[0006] On the other hand, in plasma arc spraying and plasma arc additive manufacturing technologies, techniques have emerged that allow metal powder to be directly fed into the plasma arc region via powder channels. However, in these technologies, the metal powder serves as a filler material for cladding or additive manufacturing, aiming for the highest possible powder deposition rate and cladding efficiency. Therefore, the powder feed rate is typically large, generally above 20 g / min, with the powder being fed to the edge or surface of the molten pool. The process objective and technical effect are fundamentally different from the core concept of micro-control in active welding.

[0007] In summary, there is an urgent need in the existing technology for a new active plasma arc welding technology that can effectively increase the penetration depth of stainless steel plasma arc welds, fundamentally avoid the introduction of harmful impurities such as carbon to ensure or even improve the corrosion resistance of the welds, and at the same time have good process controllability and be suitable for industrial mass production. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a novel active metal powder recompression plasma arc welding process and apparatus for stainless steel. This innovative active plasma arc welding technology for stainless steel not only increases weld penetration but also ensures or even enhances the corrosion resistance of the weld. It is particularly suitable for high-end welding manufacturing scenarios of austenitic stainless steels such as 316L, where high standards are required for both penetration and corrosion resistance. It has outstanding technical advantages and engineering application value.

[0009] The present invention achieves the above-mentioned technical objectives through the following technical means.

[0010] A recompression plasma arc welding process using active metal powder for stainless steel includes the following steps:

[0011] Premixing of active metal powders, wherein the active metal powders include stainless steel powder, SiO2 powder and Al2O3 powder;

[0012] During the welding process, the active metal powder is simultaneously fed into the arc column area of ​​the plasma arc at a preset powder feeding amount, wherein the powder feeding amount is determined according to the welding speed, and the higher the welding speed, the greater the powder feeding amount.

[0013] By interacting with the electric arc plasma in the arc column region, the electric arc is recompressed to increase the weld penetration depth.

[0014] Furthermore, the metal powder of the stainless steel material is the same as the stainless steel material to be welded; the volume ratio of the Al2O3 powder, SiO2 powder and the metal powder of the stainless steel material is 1:1:2.

[0015] Furthermore, the particle size of the stainless steel metal powder is 100-200 μm, the particle size of the SiO2 powder is 60-100 μm, and the particle size of the Al2O3 powder is 70-110 μm.

[0016] Furthermore, the powder feeding amount is determined according to the following relationship: Q = A × v;

[0017] Where Q is the powder feeding rate in g / min; v is the welding speed in m / min; and A is a coefficient ranging from 0.5 to 0.8.

[0018] Furthermore, the active metal powder is directly fed into the plasma arc column region via a carrier gas flow through at least one powder channel provided in the welding torch; the carrier gas is pure argon gas, and the flow rate of the carrier gas is 1.0 to 2.0 L / min.

[0019] Furthermore, the premixing of the active metal powder includes the following steps:

[0020] The stainless steel metal powder, SiO2 powder, and Al2O3 powder were dried respectively.

[0021] Then, the powder is mixed evenly using a mechanical rolling mixing method according to a predetermined ratio.

[0022] After mixing, seal and store.

[0023] Furthermore, the welding process parameters are as follows: ion gas flow rate 3.0 L / min, shielding gas flow rate 20 L / min, welding current 200A, welding speed 100 mm / min, distance from welding torch nozzle to workpiece surface 5mm, and tungsten electrode indentation 5mm.

[0024] A welding apparatus for implementing the aforementioned active metal powder recompression plasma arc welding process for stainless steel, comprising:

[0025] The welding torch has an ion gas channel and at least one powder feeding channel inside. The outlet end of the powder feeding channel faces the arc column area of ​​the plasma arc and is used to directly feed active metal powder into the arc column area of ​​the plasma arc.

[0026] A powder conveying system for storing active metal powder and outputting the active metal powder at a controllable rate;

[0027] The gas path control unit is connected to the gas path of the powder conveying system and the active metal powder channel of the welding torch, and is used to regulate the carrier gas flow to send the active metal powder into the arc column area through the active metal powder channel;

[0028] The powder feeding amount of the powder conveying system is configured to be determined based on a preset welding speed, and the higher the welding speed, the greater the powder feeding amount.

[0029] The beneficial effects of this invention are as follows:

[0030] 1. The active metal powder recompression plasma arc welding process for stainless steel described in this invention involves directly feeding a mixture of 316L metal powder, SiO2 powder, and Al2O3 powder into the plasma arc column region in a micro-scale, synchronous manner. This induces a recompression effect on the arc, where the powder particles melt and evaporate instantaneously in the high-temperature zone of the arc column, strongly absorbing heat from the outer periphery and producing a significant thermal contraction effect. Simultaneously, SiO2 and Al2O3 powders, as high-ionization-energy materials, alter the ionization balance and conductive cross-section of the arc plasma, generating a further electromagnetic contraction effect. The synergistic effect of these two effects significantly increases the arc energy density and enhances penetration capability. Furthermore, the micro-scale introduction of active components into the molten pool alters the surface tension temperature coefficient, causing the molten pool flow pattern to shift from outward to inward, concentrating arc heat at the bottom of the molten pool. The combined effect of these three factors effectively increases weld penetration without increasing welding heat input.

[0031] 2. The active metal powder recompression plasma arc welding process for stainless steel described in this invention, by employing a powder formulation system completely free of organic binders or polymers, fundamentally eliminates the pathway for carbon elements to be introduced into the weld metal, thus avoiding the problem of weld corrosion resistance degradation caused by the decomposition of organic binders, which is common in traditional pre-coated active welding processes. Simultaneously, the 316L metal powder in the active metal powder serves as a matrix powder homogeneous with the base metal, ensuring the compatibility and stability of the main weld components.

[0032] 3. The active metal powder recompression plasma arc welding process for stainless steel described in this invention achieves a balance between increased penetration depth and improved corrosion resistance by using Al2O3 powder, SiO2 powder, and 316L metal powder in a volume ratio of 1:1:2 and selecting them according to a specific particle size range. A very small amount of Al2O3 transitions into the weld pool and disperses under the action of the arc, playing a beneficial role in grain refinement and microstructure stabilization in the weld metal. Electrochemical test results show that the weld obtained using this invention exhibits a 12.02% reduction in self-corrosion current density compared to the condition using pure 316L powder, while its corrosion resistance is actually improved, achieving an unexpected effect that traditional active welding techniques could not achieve.

[0033] 4. The active metal powder recompression plasma arc welding process for stainless steel described in this invention establishes a linear quantitative matching relationship of Q = A × v between the powder feed rate (g / min) and the welding speed (m / min), and strictly controls the coefficient A within the range of 0.5 to 0.8. This ensures that the supply of active metal powder is proportionally linked to the welding travel speed, guaranteeing that the quality of active powder participating in the effective action per unit weld length is constant and controllable. This fundamentally solves the technical problem of uneven distribution and uncontrollable quality of activator in traditional pre-coating processes, providing a reliable guarantee for the engineering and mass application of this process.

[0034] 5. The active metal powder recompression plasma arc welding process for stainless steel described in this invention completely eliminates cumbersome pre-coating and drying processes by directly feeding the active metal powder into the arc column area instead of pre-coating it onto the workpiece surface. This simplifies the operation process and significantly improves production efficiency. Furthermore, the powder is only fed in and consumed simultaneously during the welding arc's movement, eliminating the problem of waste due to unused powder. This greatly improves the utilization rate of active materials and results in excellent process economy. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings described below are some embodiments of the present invention. For those skilled in the art, it is obvious that other drawings can be obtained from these drawings without creative effort.

[0036] Figure 1 This is a schematic diagram of the active metal powder recompression plasma arc welding device for stainless steel described in this invention.

[0037] Figure 2 This is a schematic diagram of the welding torch structure described in this invention.

[0038] Figure 3 Macroscopic morphology of the weld seam obtained by welding using pure 316L powder.

[0039] Figure 4 Macroscopic morphology of a weld seam produced using active metal powder.

[0040] Figure 5 Comparison of weld penetration depth between weld samples made of pure 316L powder and weld samples made of active metal powder.

[0041] Figure 6 Electrochemical impedance spectroscopy of the weld surface of a weld sample made of pure 316L powder and a weld sample made of active metal powder.

[0042] Figure 7 Potential dynamic polarization curves of the weld surface of a weld sample made of pure 316L powder and a weld sample made of active metal powder.

[0043] In the picture:

[0044] 1-Plasma arc welding power source; 2-Welding torch; 3-Mechanical motion worktable; 4-Powder conveying system; 5-Gas circuit control unit; 6-Workpiece clamping device; 7-Cooling and auxiliary protection system; 2-1-Ion gas channel; 2-2-Powder feeding channel; 2-3-Shielding gas channel. Detailed Implementation

[0045] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0046] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "axial," "radial," "vertical," "horizontal," "inner," and "outer," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0047] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0048] like Figure 1 As shown, the active metal powder recompression plasma arc welding apparatus for stainless steel of the present invention includes a plasma arc welding power source 1, a welding torch 2 containing an active metal powder channel, a mechanical motion worktable 3, a powder conveying system 4, a gas path control unit 5, a workpiece clamping device 6, and a cooling and auxiliary protection system 7.

[0049] The output of the plasma arc welding power source 1 is electrically connected to the electrode of the welding torch 2, used to generate and maintain a stable pinhole plasma arc between the tungsten electrode and the workpiece, providing the necessary energy for the welding process. The mechanical motion table 3 carries the workpiece and moves it smoothly along the welding direction at a preset welding speed, achieving relative movement between the welding torch and the workpiece. The workpiece clamping device 6 is mounted on the table surface of the mechanical motion table 3, used to reliably fix and clamp the workpiece to be welded, preventing displacement due to thermal stress or mechanical vibration during welding. The cooling and auxiliary protection system 7 is connected to the cooling channel of the welding torch 2, continuously dissipating heat from the welding torch through circulating coolant, while also providing auxiliary gas protection on the back of the weld, ensuring long-term stable operation of the equipment and weld formation quality. All these components can adopt conventional configurations in this field, and their internal structures will not be described in detail here.

[0050] like Figure 2As shown, in addition to the conventional ion gas channel 2-1 and shielding gas channel 2-3, the welding torch 2 is specially equipped with four independent active metal powder channels 2-2. These four powder channels are evenly arranged around the circumference of the welding torch, and their outlet ends all face the plasma arc column area below the central axis of the welding torch. The angle between the axis of each active metal powder channel 2-2 and the central axis of the welding torch is set between 30 and 45 degrees. This multi-channel, obliquely converging channel configuration allows the active metal powder to flow evenly from multiple directions into the central high-temperature area of ​​the arc column in a dispersed particle flow, ensuring sufficient and uniform interaction between the powder particles and the arc plasma, rather than simply accumulating on the surface of the molten pool. The feed end of each active metal powder channel 2-2 is connected to the powder outlet of the powder conveying system 4 through a pipeline.

[0051] The powder conveying system 4 is used to control the supply of active metal powder. The powder conveying system 4 includes a powder storage tank and a powder feeder. The powder storage tank stores pre-prepared active metal powder, while the powder feeder stably outputs powder at a preset rate with an adjustable rotation speed. The powder outlet of the powder feeder is divided into four paths by a powder separator, each connected to one of the four active metal powder channels 2-2 of the welding torch. The correspondence between the powder feeder rotation speed and the powder output needs to be determined in advance through calibration so that the targeted powder supply can be accurately matched at different welding speeds.

[0052] The gas path control unit 5 is used to independently and precisely regulate the flow rate of each gas path during the welding process. It controls at least three gases: ionizing gas, shielding gas, and powder delivery gas. The powder delivery gas acts as a carrier gas, carrying the active metal powder output from the powder delivery system 4 through the active metal powder channel 2-2 into the plasma arc column region. The stability of the powder delivery gas flow rate directly affects the continuity and uniformity of powder delivery; therefore, the gas path control unit 5 can adjust the powder delivery gas flow rate within a low flow range (e.g., 1.0 to 2.0 L / min). The welding torch 2, powder delivery system 4, and gas path control unit 5 of this invention work together to achieve precise, minute, and controllable direct delivery of active metal powder into the plasma arc column region.

[0053] The active metal powder of this invention consists of three powder components with different functions: stainless steel metal powder, SiO2 powder, and Al2O3 powder. The selection and proportioning of these three components underwent systematic theoretical analysis and experimental screening. The stainless steel metal powder is a homogeneous powder whose chemical composition is completely matched with that of the base material to be welded. Its function is to melt and transition into the weld pool during the welding process, ensuring the compatibility and stability of the main components of the weld and preventing damage to the corrosion resistance of the weld due to the incorporation of dissimilar materials. SiO2 powder and Al2O3 powder are the core components for achieving the active function. The two work synergistically to recompress the electric arc through their respective physicochemical properties.

[0054] In the preferred formulation, the volume ratio of Al2O3 powder, SiO2 powder, and stainless steel powder is 1:1:2. The particle size selection was also verified through repeated experiments: the particle size of the stainless steel powder is preferably controlled between 100 and 200 μm, the SiO2 powder between 60 and 100 μm, and the Al2O3 powder between 70 and 110 μm. If the particle size is too large, melting and evaporation will be insufficient, weakening the arc compression effect; if the particle size is too small, the flowability will be poor, and the powder feeding stability will be difficult to guarantee. The above particle size range represents a reasonable compromise that balances both requirements.

[0055] This embodiment takes two rolled 316L stainless steel plates with welding specifications of 100mm×75mm×10mm and a butt joint as an example. The active metal powder recompression plasma arc welding process for stainless steel described in this invention includes the following steps:

[0056] S01: Preparation of active metal powder, specifically:

[0057] Al2O3 powder, SiO2 powder, and 316L metal powder were placed separately in an oven and thoroughly dried at an appropriate temperature to completely remove the moisture adsorbed on the particle surface. This step is crucial for preventing powder agglomeration during storage and transportation, and for reducing the tendency for hydrogen porosity to form in the weld.

[0058] After drying, the powders are placed in a clean, dry, sealed container in the aforementioned 1:1:2 volume ratio and mixed evenly over a long period of time using a mechanical rolling mixing method.

[0059] Rolling mixing achieves homogenization through repeated tumbling and mutual diffusion between particles, without damaging the original morphology of the powder particles. After mixing, the resulting active metal powder should be immediately transferred to a sealed container for storage, and used only as needed to avoid prolonged exposure to air and moisture absorption.

[0060] S02: Pre-welding preparation

[0061] Cut the 316L stainless steel sheet to be welded to the required dimensions. Carefully grind both sides of the sheet and the mating surfaces until a bright metallic luster is fully revealed, thoroughly removing oxide scale, oil, and other surface deposits formed during rolling and storage. After grinding, repeatedly wipe the ground area with a lint-free cloth or cotton wool soaked in anhydrous ethanol, then place the sheet in a clean, dry place to air dry naturally until ready for use.

[0062] Take the pre-prepared and sealed active metal powder and transfer it from the sealed container to the powder storage tank of powder conveying system 4 under a dry environment. The powder pouring operation should be as rapid as possible to minimize the time the powder is exposed to air. At the same time, as a preparation for the control experiment, take another 316L of unmixed metal powder, dry it in the same way, and pour it into another powder feeder for later use.

[0063] S03: Clamping and Parameter Setting

[0064] Place the two processed plates onto the mechanical motion worktable 3 and secure them firmly and reliably using the workpiece clamping device 6, ensuring that the mating surfaces of the two plates are tightly fitted and in a horizontal position. During clamping, care should be taken to avoid generating additional assembly stress or warping deformation in the plates.

[0065] The water cooling machine, plasma arc welding power source 1, mechanical motion worktable 3 control system, gas circuit control unit 5, and powder conveying system 4 controller are started in sequence to put the entire welding system into standby mode.

[0066] Adjust the welding torch 2 to a tungsten electrode retraction of 5mm, and input the welding process parameters through the control platform. The main welding process parameters used in this embodiment, after optimization, are determined as follows: welding current 200A, welding speed 0.1m / min (i.e., 100mm / min), and distance from the welding torch nozzle end face to the workpiece upper surface 5mm. Regarding the gas path, the gas path control unit 5 sets the ionizing gas flow rate to 3.0L / min, the shielding gas flow rate to 20L / min, and the powder feeding gas flow rate to 1.0L / min. High-purity argon is used for all three gas paths.

[0067] S04: Determining the amount of powder delivered;

[0068] Unlike existing technologies that rely on experience or trial and error to determine the powder feeding rate, the powder feeding rate in this invention must be strictly determined based on a preset welding speed, with a positively correlated linear matching relationship between the two. Specifically, the powder feeding rate (in g / min) is determined according to the following formula: Q = A × v; where Q is the powder feeding rate in g / min; v is the welding speed in m / min, which is taken as 0.1 in this embodiment; and A is an adjustment coefficient, which, based on data fitting from numerous process experiments, has a reasonable range of 0.5 to 0.8. In this embodiment, A is selected as 0.5, so the required targeted powder feeding rate Q = 0.5 × 0.1 = 0.05 g / min. According to the pre-calibrated powder feeder speed-powder feeding rate curve, the powder feeder speed is adjusted to the gear corresponding to the targeted powder feeding rate, typically corresponding to a specific calibrated value in the range of 3 to 5 r / min.

[0069] The quantitative matching relationship between powder feed rate and welding speed ensures that the mass of active metal powder fed into the arc column zone per unit weld length remains constant regardless of welding speed variations. As welding speed increases, the powder feed rate increases proportionally; conversely, as welding speed decreases, the powder feed rate decreases accordingly. This dynamic matching ensures consistent weld penetration and process robustness, providing a crucial guarantee for achieving mass production and engineering applications.

[0070] S05: Perform welding;

[0071] Before the actual arc ignition welding, conduct a trial run of the welding torch. Let the welding torch travel along the preset welding path once to confirm that the welding start and end points are accurately positioned and that there is no risk of collision or interference throughout the movement.

[0072] After successful trial operation, the gas control unit 5 sequentially opens the three gas channels in the order of ionizing gas, powder delivery gas, and protective gas. Once the flow rates of each gas channel reach their set values ​​and stabilize, the powder feeder of the powder delivery system 4 is activated. The active metal powder, carried by the powder delivery gas flow, is then transported stably as multiple fine particle streams through the four active metal powder channels 2-2 inside the welding torch 2 towards the predetermined area below the welding torch nozzle. Subsequently, the control system triggers high-frequency arc ignition, and the pinhole plasma arc is excited between the tungsten electrode and the workpiece, quickly reaching a stable combustion state, and the welding process officially begins.

[0073] Throughout the stable combustion of the welding arc, the powder delivery system 4 continuously and stably delivers the active metal powder into the high-temperature central region of the plasma arc column via four active metal powder channels 2-2 at an extremely low powder delivery rate of 0.05 g / min. The recompression effect referred to in this invention is dynamically realized during this stage through the complex interaction between the powder particles and the arc plasma. The specific physical process is as follows: micron-sized powder particles are instantaneously heated, melted, and partially evaporated in the extreme high-temperature environment of tens of thousands of degrees Celsius in the arc column region. This phase transition process absorbs a large amount of heat from the outer region of the arc column, causing a sharp drop in the temperature and ionization of the gas outside the arc column. The conductive channel of the arc is forced to narrow towards the center, i.e., the so-called thermal contraction effect, achieving a first recompression of the arc. Simultaneously, SiO2 and Al2O3, as substances with high ionization energy, alter the ionization balance of the plasma in the arc column. The distribution of charge carriers tends to concentrate in the central region of the arc column, thus significantly increasing the current density, i.e., the electromagnetic contraction effect further constricts the arc, which is the second recompression mechanism. Furthermore, a small amount of incompletely vaporized powder particles enter the weld pool below with the arc gas flow, altering the temperature coefficient of the weld pool's surface tension. This causes the flow direction of the molten metal to reverse from the traditional center-to-outer periphery to the periphery-to-center. This inward-converging flow pattern more effectively transfers the heat input from the arc to the bottom of the weld pool, essentially completing a third recompression at the fluid dynamics level. The synergistic effect of these three mechanisms enables this invention to achieve a significant leap in plasma arc penetration and a significant increase in weld penetration depth without any additional heat input.

[0074] S06: The welding procedure terminates when the welding torch reaches the preset welding termination point. First, the plasma arc welding power supply 1 should be cut off to extinguish the arc. Then, immediately shut off the powder feeder of the powder delivery system 4 to stop further powder supply. One operational detail deserves emphasis here: after stopping powder feeding, all gases should not be shut off immediately. The ionizing gas, powder feeding gas, and shielding gas should continue to flow for at least several seconds until no residual powder is observed being ejected from the welding torch nozzle. Then, shut off each gas in the order of powder feeding gas, ionizing gas, and shielding gas. This is to prevent residual powder from sintering and clogging the internal channels of the high-temperature welding torch, and to ensure that the high-temperature weld and its proximal zone remain under sufficient protection by inert gas until cooled to a safe temperature, preventing oxidation and discoloration.

[0075] After the workpiece has completely cooled to room temperature, release the workpiece clamping device 6 and remove the welded plate from the worktable.

[0076] Effect verification and comparative analysis

[0077] A comparative experiment was conducted under identical base materials, clamping conditions, welding current, welding speed, gas flow rate, and nozzle distance: one group used the active metal powder of the present invention (316L + SiO2 + Al2O3 ternary mixed powder), and the other group used pure 316L metal powder without the addition of active components.

[0078] After welding, the samples were obtained through wire cutting, cold mounting, progressive grinding, mechanical polishing, and chemical etching. The macroscopic morphology of the weld cross-section was then observed and measured under a stereomicroscope. (See attached image.) Figure 3 , Figure 4 and Figure 5 As shown, the weld penetration depth obtained using active metal powder is approximately 0.6 mm greater than that obtained using pure 316L metal powder. This increase in penetration depth demonstrates the existence of the recompression effect.

[0079] Regarding changes in corrosion resistance, the weld surface area was used as the working electrode, and electrochemical tests were conducted using a standard three-electrode system in a 3.5 wt.% NaCl aqueous solution. (See attached image.) Figure 6 and attached Figure 7 As shown, the Nyquist plot of the electrochemical impedance spectroscopy test reveals that the capacitive arc radius of the active metal powder weld is significantly larger than that of the pure 316L powder weld. This indicates that the former has a greater charge transfer resistance of the surface passivation film and superior corrosion resistance. Quantitative fitting results of the potentiodynamic polarization curve further confirm that the self-corrosion current density of the active metal powder weld is 8.2243 × 10⁻⁶. -8 A / cm 2 The self-corrosion current density under pure 316L powder conditions is 9.3469 × 10⁻⁶. -8 A / cm 2 The rate of corrosion decreased by 12.02%. This decrease signifies a substantial slowdown in the corrosion rate. In contrast, the process of this invention not only did not impair the corrosion resistance of stainless steel, but also brought quantifiable performance gains.

[0080] The active metal powder recompression plasma arc welding process for stainless steel described in this invention controls the powder feeding rate to an extremely low level of less than 0.1 g / min, and precisely feeds a specific ratio of active metal powder into the arc column area. Combined with the quantitative matching and control of the powder feeding rate and welding speed, this process increases the weld penetration depth and ensures or even improves the corrosion resistance of the weld.

[0081] It should be understood that although this specification is described according to various embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0082] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A recompression plasma arc welding process for stainless steel using active metal powder, characterized in that, Includes the following steps: Premixing of active metal powders, wherein the active metal powders include stainless steel powder, SiO2 powder and Al2O3 powder; During the welding process, the active metal powder is simultaneously fed into the arc column area of ​​the plasma arc at a preset powder feeding amount, wherein the powder feeding amount is determined according to the welding speed, and the higher the welding speed, the greater the powder feeding amount. By interacting with the electric arc plasma in the arc column region, the electric arc is recompressed to increase the weld penetration depth.

2. The active metal powder recompression plasma arc welding process for stainless steel according to claim 1, characterized in that, The stainless steel metal powder is a matrix powder of the same composition as the base material to be welded; the volume ratio of the Al2O3 powder, SiO2 powder and stainless steel metal powder is 1:1:

2.

3. The active metal powder recompression plasma arc welding process for stainless steel according to claim 2, characterized in that, The stainless steel powder has a particle size of 100–200 μm, the SiO2 powder has a particle size of 60–100 μm, and the Al2O3 powder has a particle size of 70–110 μm.

4. The active metal powder recompression plasma arc welding process for stainless steel according to claim 1, characterized in that, The powder feeding amount is determined according to the following relationship: Q=A×v; Where Q is the powder feeding rate in g / min; v is the welding speed in m / min; and A is a coefficient ranging from 0.5 to 0.

8.

5. The active metal powder recompression plasma arc welding process for stainless steel according to claim 1, characterized in that, The active metal powder is fed directly into the plasma arc column region via a carrier gas flow through at least one powder channel in the welding torch; the carrier gas is pure argon, and the flow rate of the carrier gas is 1.0 to 2.0 L / min.

6. The active metal powder recompression plasma arc welding process for stainless steel according to claim 1, characterized in that, The premixing of active metal powders includes the following steps: The stainless steel metal powder, SiO2 powder, and Al2O3 powder were dried respectively. Then, the powder is mixed evenly using a mechanical rolling mixing method according to a predetermined ratio.

7. The active metal powder recompression plasma arc welding process for stainless steel according to claim 1, characterized in that, The welding process parameters are as follows: ion gas flow rate 3.0 L / min, shielding gas flow rate 20 L / min, welding current 200A, welding speed 100 mm / min, distance from welding torch nozzle to workpiece surface 5mm, and tungsten electrode indentation 5mm.

8. A welding apparatus for implementing the active metal powder recompression plasma arc welding process for stainless steel according to any one of claims 1-7, characterized in that, include: The welding torch (2) is provided with an ion gas channel (2-1) and at least one powder feeding channel (2-2) inside. The outlet end of the powder feeding channel (2-2) faces the arc column area of ​​the plasma arc and is used to directly feed active metal powder into the arc column area of ​​the plasma arc. Powder conveying system (4) is used to store active metal powder and output the active metal powder at a controllable rate; The gas path control unit (5) is connected to the gas path of the powder conveying system (4) and the active metal powder channel (2-2) of the welding torch (2), and is used to regulate the carrier gas flow to send the active metal powder into the arc column area through the active metal powder channel (2-2); The powder feeding amount of the powder conveying system (4) is configured to be determined based on a preset welding speed, and the greater the welding speed, the greater the powder feeding amount.

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