A filler wire for aluminum alloy CMT welding, a welding method

Abstract

The present application belongs to the technical field of welding, and specifically relates to a filler tape for CMT welding of aluminum alloy and a welding method. The filler tape comprises: an outer skin made of aluminum alloy foil, which has a melting point not higher than that of the base material to be welded; a core powder wrapped inside the outer skin, the core powder being an alloy powder containing Sc and Zr micro-alloying elements; and a gas-permeable exhaust structure for providing an escape channel for the interstitial gas and volatile matter of the core powder during the welding process. The present application realizes high-quality, crack-free and low-porosity welding of medium-thick plate 7-series aluminum alloy by designing a composite filler tape with a gas-permeable exhaust function and cooperating with a specific sidewall presetting and wetting melting mode.

Description

Technical Field

[0001] This invention mainly relates to the technical field of aluminum alloy welding, specifically a filler strip and welding method for aluminum alloy CMT welding. Background Technology

[0002] 7-series aluminum alloys (such as 7075 and 7050) are widely used in aerospace, rail transportation, and weaponry due to their high specific strength. However, 7-series aluminum alloys are typical hot-cracking sensitive materials and are prone to porosity during welding, and have long been considered "unweldable." Existing methods to improve the weldability of 7-series aluminum alloys mainly include using high-toughness welding wires (such as 5-series welding wires) or adding microalloying elements (such as Sc and Zr) for modification treatment. Among these, adding Sc and Zr can form Al3(Sc,Zr) dispersed phases, effectively refining grains and suppressing hot cracking.

[0003] Currently, the main technical problems associated with adding Sc / Zr elements to welds are as follows:

[0004] 1. Powder feeding method interferes with the arc: If coaxial or off-axis airflow is used for powder feeding, the powder-carrying airflow will seriously disrupt the laminar flow state of the protective airflow, thereby seriously interfering with the stability of the CMT arc and causing defects such as edge biting and lack of fusion.

[0005] 2. High cost and fixed composition of finished welding wire: The preparation process of special welding wire containing Sc / Zr is complicated. Not only is there a problem of difficult drawing, resulting in high price, but the addition ratio of elements cannot be flexibly adjusted according to different base material thicknesses and heat inputs.

[0006] 3. Pre-placed powder is easily blown away and porosity is generated: If simple pre-placed dry powder or ordinary flux-cored tape is used, the plasma flow force of the CMT arc can easily blow the lightweight alloy powder away from the molten pool area; at the same time, the gas and binder volatiles remaining between the powder particles are difficult to escape in the rapidly solidifying molten pool, which can easily form dense metallurgical pores inside the weld.

[0007] Therefore, there is an urgent need for a welding process and supporting materials that can ensure the stability of the CMT welding arc of aluminum alloys, effectively prevent powder scattering, and solve the porosity problem. Summary of the Invention

[0008] To address the shortcomings of current technologies, this invention combines existing technologies and, based on practical applications, provides a filler strip and welding method for CMT welding of aluminum alloys. By designing a composite filler strip with ventilation and exhaust functions, and in conjunction with specific sidewall pre-positioning and wetting melting methods, high-quality, crack-free, and low-porosity welding of medium-thick 7-series aluminum alloys can be achieved.

[0009] The technical solution of the present invention is as follows:

[0010] According to one aspect of the present invention, a filler strip for CMT welding of aluminum alloys is provided, comprising:

[0011] The outer skin is made of aluminum alloy foil, the melting point of which is not higher than the melting point of the base material to be welded;

[0012] The core powder is encapsulated inside the outer skin, and the core powder is an alloy powder containing Sc and Zr microalloying elements;

[0013] A permeable venting structure and / or venting agent are used to allow gases and volatiles from the gaps in the core powder to escape during the welding process.

[0014] Furthermore, the outer skin has a thickness of 0.05mm to 0.1mm and is made of Al-Zn-Mg-Cu alloy foil.

[0015] Furthermore, the core powder is an Al-Mg-Sc-Zr intermediate alloy powder with a particle size of 15μm~53μm.

[0016] Furthermore, the mass percentages of each component in the core powder are as follows: 4.71% Mg, 0.86% Sc, 0.27% Zr, and the remainder is Al.

[0017] Furthermore, the breathable and exhaust structure includes micropores, which are spaced in one or more rows along the length of the outer skin surface.

[0018] Furthermore, the diameter of the micropores is 0.1mm to 0.3mm, and the spacing between adjacent micropores is 5mm to 10mm.

[0019] Furthermore, the exhaust agent is mixed into the core powder, and the volume percentage of the exhaust agent in the core powder is 1% to 3%.

[0020] Furthermore, the exhaust agent is a foaming agent or a slagging agent.

[0021] According to another aspect of the present invention, a method for CMT welding of aluminum alloys using the above-mentioned filler strip is provided, comprising the following steps:

[0022] Step 1, Beveling and Pre-positioning: Beveling the aluminum alloy base material to be welded, and pre-positioning the filler strip at the step of the beveling sidewall or the edge of the interlayer weld bead, avoiding the center of the beveling.

[0023] Step 2, CMT welding: Using a CMT welding heat source, control the welding torch to align with the center of the bevel or the center of the weld bead to initiate the arc; use the arc heat to melt the sidewall base material to form liquid metal, and use the surface tension and capillary action of the liquid metal to wet and draw in the filler strip pre-placed on the sidewall, so that the core powder in the filler strip can smoothly transition into the weld pool.

[0024] Furthermore, during welding, an oscillating welding mode is adopted, with the oscillation trajectory being a transverse sine wave or a triangle, and the oscillation amplitude covering the preset position of the filler strip.

[0025] The beneficial effects of this invention are:

[0026] 1. Completely eliminate airflow interference: This invention uses a solid pre-filled tape to replace the traditional airflow powder feeding, which suppresses the influence of powder-carrying airflow on the stability of the CMT arc flow field and temperature field, and ensures the aesthetic appearance of the weld.

[0027] 2. Solving the problem of powder scattering: The filling strip of this invention physically constrains the powder through the aluminum foil outer skin, preventing the powder from being blown away by the plasma arc flow at the moment of arc ignition; in conjunction with the pre-placed sidewall, it avoids the direct bombardment of the powder by the arc, and instead utilizes the "wetting and absorption" effect of liquid metal to allow the powder to smoothly transition into the depth of the molten pool.

[0028] 3. Significantly reduced porosity: The laser micropores on the outer skin or the slag-forming design of the core of the filling tape of this invention provide a low-resistance escape channel for gas and binder volatiles in the powder gaps, avoiding the formation of pores by gas being trapped in the rapidly solidifying molten pool.

[0029] 4. Refined grain size and crack resistance: The Sc / Zr elements in this invention are uniformly transferred into the weld, forming a large number of Al3(Sc,Zr) heterogeneous nucleation cores, which significantly refines the micro-grain size of the 7-series aluminum alloy weld and greatly improves the resistance to hot cracking from a metallurgical mechanism perspective. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the cross-sectional structure of the filling strip of the present invention.

[0031] Figure 2 This is a top view of the vent hole of the filling strip of the present invention.

[0032] Figure 3 This is a schematic diagram illustrating the principle of the filler strip pre-positioned on the bevel sidewall and the CMT welding fluid dynamics process of the present invention.

[0033] Figure 4 The image shows a metallographic image of the weld area after welding using the process of the present invention in Example 1.

[0034] The labels shown in the attached diagram are: 1. Outer skin; 2. Slag-forming / foaming agent; 3. Core powder; 4. Micropores; 5. Welding torch oscillation direction; 6. CMT welding torch; 7. Electric arc; 8. Gas exhaust path; 9. Bevel sidewall; 10. Molten pool. Detailed Implementation

[0035] The present invention will be further described in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined in this application.

[0036] This embodiment provides a breathable and venting Sc / Zr filler tape for CMT welding of aluminum alloys. The filler tape mainly includes an outer skin 1, a core powder 3, a breathable and venting structure and / or a venting agent.

[0037] The outer skin 1 is made of Al-5.93Zn-2.58Mg-1.55Cu(7075) aluminum alloy foil with a thickness of 0.05mm~0.1mm, and its melting point is lower than or equal to that of the base material.

[0038] The core powder 3 is wrapped inside the outer skin 1 and is composed of Al-4.71Mg-0.86Sc-0.27Zr intermediate alloy powder with a particle size of 15μm~53μm.

[0039] This embodiment provides two methods for the escape of gas and binder volatiles in the powder gaps: a breathable venting structure or a venting agent. The two methods can be used simultaneously or individually.

[0040] In one of the ventilation and exhaust methods provided in this embodiment, micropores 4 are used for exhaust. Specifically, a nanosecond or picosecond-level ultrafast laser is used to create one or more rows of micropores 4 at intervals along the length of the outer skin 1 of the filling strip. The diameter of the micropores 4 is 0.1mm to 0.3mm, and the spacing is 5mm to 10mm. In this embodiment, the design of the laser micropores cleverly utilizes the differences in physical properties of gas-solid multiphase flow.

[0041] Firstly, the unidirectional exhaust mechanism: Under the intense thermal cycling of the CMT arc, a significant temperature gradient is generated inside the filler strip near the front end of the molten pool 10. Residual air and trace amounts of adsorbed moisture in the powder gaps rapidly expand and vaporize upon heating, creating localized high pressure in the core of the filler strip. Due to the extremely low kinematic viscosity of the gas phase, driven by the pressure gradient, the gas can preferentially escape through the micropores 4 with minimal fluid resistance.

[0042] Secondly, the mechanical arching leak-proof mechanism: During static pre-setting and dynamic feeding, the multi-faceted Sc / Zr alloy powder spontaneously forms a "mechanical arching" effect at the micropore outlet due to the friction and van der Waals forces between particles. This microscopic powder bridging structure effectively blocks the free flow of solid particles, ensuring that the powder will not leak macroscopically from the micropores before melting.

[0043] Thirdly, liquid-phase encapsulation and multiphase flow entrainment: When the electric arc heat source reaches the micropore region, the 0.05mm~0.1mm thick low-melting-point aluminum foil outer layer 1 rapidly melts to form a liquid film. At this time, the surface tension of the high-temperature liquid aluminum instantly destroys the aforementioned "powder arch" and rapidly wets the solid powder particles. Under the combined traction of strong Marangoni convection and capillary action inside the molten pool 10, the pure solid powder that has already completed "pre-venting" is steadily and continuously entrained into the depths of the liquid molten pool, thereby completely cutting off the nucleation source of pores before metallurgical crystallization.

[0044] In another ventilation and venting method provided in this embodiment, a mixed venting agent is used. That is, a foaming agent and / or slagging agent with a volume ratio of 1% to 3% is mixed into the core powder 3. By utilizing the in-situ thermal decomposition reaction under the high temperature of the electric arc, microbubbles are generated inside the molten pool, and venting is achieved through the rising and escaping of the bubbles.

[0045] The foaming agent (such as sodium bicarbonate, calcium carbonate, etc.) undergoes a thermal decomposition reaction at the core of the filler tape under the high temperature of an electric arc, producing... Gases. These gases are generated in situ in the pores between the three core powder particles, forming tiny bubble nuclei. When the filler tape is wetted and drawn in by the liquid metal of the sidewall substrate, the tiny bubbles generated by the decomposition of the core foaming agent... Bubbles expand rapidly in the high-temperature molten pool and rise from the inside of the pool. This rising process not only... The foaming agent not only removes gas from the molten pool, but more importantly, it generates physical stirring and a "pneumatic traction" effect, carrying away and expelling the harmful gases (hydrogen) originally dissolved in the molten pool, thus achieving the effect of venting gas with gas and actively removing gas. CMT welding itself is characterized by low heat input and rapid solidification, and the molten pool exists for a very short time. If air bubbles are not actively introduced, the air and decomposition volatiles remaining in the powder gaps are easily "frozen" in the solidified weld, forming dense pores. The air bubbles artificially introduced by the foaming agent are equivalent to opening "venting channels" inside the molten pool, providing a preferred escape path for the gas in the molten pool. The air bubbles of the foaming agent are a process tool, not a residual defect. These air bubbles disappear after rising to the surface of the molten pool and will not form residual pores in the final weld. The stirring effect of the oscillating arc can help the uniform distribution and orderly escape of air bubbles, while extending the existence time window of the molten pool, ensuring that air bubbles escape fully before the molten pool solidifies. The location of bubbles generated by the foaming agent cannot be precisely controlled. The decomposition of the foaming agent produces bubbles through a random nucleation process that occurs throughout the core powder. The upward path of the bubbles through the molten pool is also random. This "disordered venting" can be promoted by the stirring action of the molten pool and the oscillating welding mode (lateral sine wave or triangular oscillation with amplitude covering the pre-set position of the filler strip) in CMT welding to promote the uniform distribution and orderly escape of bubbles.

[0046] The selection of foaming agents mainly focuses on CO2-type foaming agents, including , Since these two foaming agents have different decomposition temperatures, both can be used simultaneously: the low-temperature decomposition type... High-temperature decomposition type The compound can produce a "tiered exhaust" effect. Gas generation begins at the periphery of the molten pool, helping the existing gas in the powder gaps to escape first; Secondary gas generation occurs in the high-temperature core zone of the molten pool, deeply expelling dissolved hydrogen.

[0047] Slag-forming agents can physically adsorb or chemically react with impurities such as oxides and sulfides in the molten pool to form liquid slag with low density, low melting point, and easy floating. After floating to the surface of the molten pool, this slag is pushed to the edge of the weld bead by the arc blowing force or subsequent weld beads, and is finally removed by post-treatment such as grinding, thus purifying the weld metal.

[0048] In welding metallurgy, the physical properties of slag directly affect its adsorption and separation efficiency for inclusions. Slag-forming agents (such as fluorite CaF2) can lower the overall melting point of the slag, improve its fluidity at high temperatures, and make it easier for it to migrate from the interior of the molten pool to the surface, facilitating subsequent removal.

[0049] While physically removing slag, the bubbles generated by the decomposition or reaction of slag-forming agents (such as SiF4, CO2, etc.) also carry harmful gases from the molten pool out during their flotation process, playing an auxiliary role in "gas-driven gas removal". Some slag-forming agents themselves or their reaction products have a certain deoxidizing ability, which can reduce the dissolved oxygen content in the molten pool, thereby indirectly reducing the formation of oxidized inclusions.

[0050] The slag-forming agents selected are mainly KAlF4 and K2TiF6. The former focuses on breaking down the oxide film and forming slag, while the latter also has the function of refining grains. In this embodiment, a combination of these two slag-forming agents is used. KAlF4 slag-forming agent can actively remove the oxide film, clearing the physical obstacles for subsequent reactions. The resulting fluid slag can encapsulate and adsorb oxide inclusions in the molten pool. The slag floating process also plays a "gas stirring" role, physically driving bubbles to float and escape. At the same time, the gas microbubbles generated by the reaction of the slag-forming agent with the oxide film can also play a similar expulsion effect as a foaming agent. K2TiF6 reacts with the molten pool to form the TiAl3 phase in situ, becoming a heterogeneous nucleation core during the solidification of aluminum melt, which can significantly refine the grains.

[0051] Based on the filler tape provided in the above embodiments, this embodiment also provides a CMT welding method for Sc / Zr filler tape based on the sidewall wetting mechanism, including the following steps:

[0052] Step 1, Preparation of breathable and venting Sc / Zr filled tape: Low melting point aluminum foil is used as the outer skin to wrap Sc / Zr modified powder, and micro venting holes are set on the outer skin or venting agent is mixed in;

[0053] Step 2, beveling and pre-positioning of filler band: beveling the 7-series aluminum alloy medium-thick plate to be welded, and pre-positioning the prepared filler band at the step of the bevel sidewall or the edge of the interlayer weld, rather than at the center of the bevel; preferably, the distance between the pre-positioned position of the filler band and the center line of the bevel is 1 / 3 to 1 / 2 of the weld width.

[0054] Step 3, CMT welding: Using a CMT welding heat source, the welding torch is aimed at the center of the bevel or the center of the weld bead to initiate arc welding; the arc heat melts the base material of the sidewall to form liquid metal, and the surface tension and capillary action of the liquid metal wets and "draws in" the filler band at the sidewall, allowing the Sc / Zr powder to enter the molten pool. Preferably, CMT welding adopts an oscillating welding mode, with the oscillation trajectory being a transverse sine wave or triangle, and the oscillation amplitude covering the preset position of the filler band, using the arc digging force to assist the molten pool in wetting and drawing in the filler band.

[0055] Example 1:

[0056] This embodiment describes butt welding of 10mm thick 7075-T6 aluminum alloy sheet. The main steps are as follows.

[0057] Preparation of the filler strip: 0.08 mm thick 7075 aluminum foil was selected as the outer skin. The core powder was an Al-4.71Mg-0.86Sc-0.27Zr master alloy powder with a particle size of 15 μm to 53 μm. The powder was wrapped in the aluminum foil to form a strip structure with a width of 4 mm. Holes (50 W laser power, 2000 mm / s scanning speed) were drilled on one side of the outer skin of the filler strip, with a hole diameter of 0.2 mm and a hole spacing of 8 mm, to serve as venting channels.

[0058] Welding preparation: Cut an X-shaped bevel in the plate with a 2mm blunt edge. After the root pass is completed, perform the filler pass welding. Place the filler strip on the step of the bevel sidewall (approximately 3mm from the weld center).

[0059] CMT welding process: ER5356 welding wire (1.2mm diameter) was used as filler wire. A CMT power supply was used with a welding current of 95A, a voltage of 13.5V, and a welding speed of 0.5m / min. The welding torch was used in a transverse sinusoidal oscillation with an amplitude of 6mm. The arc primarily acted on the center of the bevel; when the arc oscillated to the sidewall, the molten aluminum base material rapidly wetted the filler band. Due to the matching melting point of the 7075 material, the filler band melted rapidly. Under the combined action of Marangoni convection and surface tension within the molten pool, the Sc / Zr powder was stably "drawn" into the depths of the molten pool. During welding, the gas within the filler band was smoothly discharged through the pre-drilled laser micro-holes, preventing the formation of porosity.

[0060] Results analysis: such as Figure 4 As shown, microstructure analysis revealed that the grain size at the weld center changed from coarse columnar grains without Sc / Zr filler bands to fine equiaxed grains, with the average grain size refined to about 50 μm, and no hot cracks were generated.

Claims

1. A filler strip for CMT welding of aluminum alloys, characterized in that, include: The outer skin is made of aluminum alloy foil, the melting point of which is not higher than the melting point of the base material to be welded; The core powder is encapsulated inside the outer skin, and the core powder is an alloy powder containing Sc and Zr microalloying elements; A permeable venting structure and / or venting agent are used to allow gases and volatiles from the gaps in the core powder to escape during the welding process.

2. The filler strip for aluminum alloy CMT welding according to claim 1, characterized in that, The outer skin has a thickness of 0.05mm to 0.1mm and is made of Al-Zn-Mg-Cu alloy foil.

3. The filler strip for aluminum alloy CMT welding according to claim 1, characterized in that, The core powder is an Al-Mg-Sc-Zr intermediate alloy powder with a particle size of 15μm~53μm.

4. The filler strip for aluminum alloy CMT welding according to claim 3, characterized in that, The mass percentages of each component in the core powder are as follows: 4.71% Mg, 0.86% Sc, 0.27% Zr, and the remainder is Al.

5. The filler strip for aluminum alloy CMT welding according to claim 1, characterized in that, The breathable and exhaust structure includes micropores, which are spaced in one or more rows along the length of the outer skin surface.

6. The filler strip for aluminum alloy CMT welding according to claim 5, characterized in that, The diameter of the micropores is 0.1mm to 0.3mm, and the spacing between adjacent micropores is 5mm to 10mm.

7. The filler strip for aluminum alloy CMT welding according to claim 1, characterized in that, The exhaust agent is mixed into the core powder, and the volume percentage of the exhaust agent in the core powder is 1% to 3%.

8. The filler strip for aluminum alloy CMT welding according to claim 7, characterized in that, The exhaust agent is a foaming agent or a slag-forming agent.

9. A method for CMT welding of aluminum alloys using the filler strip according to any one of claims 1-8, characterized in that, The steps include the following: Step 1, Beveling and Pre-positioning: Beveling the aluminum alloy base material to be welded, and pre-positioning the filler strip at the step of the beveling sidewall or the edge of the interlayer weld bead, avoiding the center of the beveling. Step 2, CMT welding: Using a CMT welding heat source, control the welding torch to ignite the arc at the center of the bevel or the center of the weld bead; use the arc heat to melt the sidewall base material to form liquid metal, and use the surface tension and capillary action of the liquid metal to wet and draw in the filler strip pre-placed on the sidewall, so that the core powder in the filler strip can smoothly transition into the weld pool.

10. The welding method according to claim 9, characterized in that, During welding, an oscillating welding mode is used, with the oscillation trajectory being a transverse sine wave or triangle, and the oscillation amplitude covering the preset position of the filler strip.

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