Low-temperature aging strengthening and toughening welding process after layered welding of electric power fittings

Abstract

The invention discloses an electric power fitting layered post-welding low-temperature aging strengthening and toughening welding process which comprises the following steps: S1) joint design and surface activation: designing an asymmetric groove according to the thickness of an electric power fitting base material, and carrying out surface oxide film removal and chemical activation treatment; (S2) gradient heat input layered welding is carried out, and a heat input gradient model is established based on the heat dissipation conditions and organization requirements of different depths of a weld joint by adopting a multi-layer and multi-pass welding process. The heat distribution characteristic of high heat input of the low heat input filling welding layer of the backing welding layer and the cosmetic welding layer is set through gradient heat input layered welding, and the tempering effect of low heat input of the cosmetic layer on the filling welding layer is matched, so that grain coarseness in a heat affected zone is effectively restrained; meanwhile, interlayer ultrasonic peening is implemented in a solid phase transition temperature interval of the welding seam, a subcrystal structure can be precisely refined, residual compressive stress is introduced, the interlayer temperature can be stabilized through subsequent forced convection heat exchange, and residual tensile stress is prevented from being generated.

Description

Technical Field

[0001] This invention relates to the field of power fitting welding technology, specifically a low-temperature aging and toughening welding process for layered welding of power fittings. Background Technology

[0002] Power fittings are core connecting components of power transmission lines, and the strength and toughness of their welded joints directly determine the safe and stable operation of the transmission lines. With the power industry's development towards higher voltage and larger capacity, higher requirements are placed on the load-bearing capacity, fatigue resistance, and service reliability of power fittings. Currently, power fitting welding mostly employs traditional multi-layer, multi-pass welding processes, combined with post-weld heat treatments such as solution treatment plus aging treatment or separate ultrasonic impact treatment to improve joint performance. Multi-layer, multi-pass welding reduces welding deformation by segmenting the weld metal, post-weld heat treatment aims to refine grains and eliminate residual welding stress, and ultrasonic impact treatment introduces residual compressive stress through mechanical action to improve fatigue resistance.

[0003] Existing technologies have several shortcomings. On the one hand, uneven heat input control during traditional multi-layer, multi-pass welding processes can easily lead to coarse grains in the heat-affected zone, reducing the strength and toughness of the welded joint. Conventional ultrasonic impact treatment is usually carried out after the weld has completely cooled, making it difficult to coordinate with the phase transformation process of the weld, resulting in limited grain refinement and stress control effects. On the other hand, traditional post-weld strengthening often employs a combination of solution treatment and aging. Solution treatment requires high-temperature heating, which is not only energy-intensive but also prone to deformation of power fittings, affecting product dimensional accuracy. For commonly used power fitting materials such as 6000 series aluminum alloys, existing welding wires lack specific design, making grain growth prone to occur during welding. Furthermore, aluminum alloy welding is susceptible to defects such as porosity due to oxidation or improper selection of shielding gas. In addition, existing processes have poor adaptability to power fittings made of different materials, making it difficult to meet the welding performance requirements of different base materials. Moreover, without effective protection during post-weld aging, the workpiece surface is prone to oxidation, further reducing joint performance.

[0004] Therefore, there is an urgent need for a high-efficiency welding process that can precisely control the welding heat input, coordinate the phase transformation and stress state of the weld, adapt to different materials of power fittings, reduce energy consumption and avoid oxidation defects, and significantly improve the strength and toughness of the welded joint, so as to meet the stringent requirements of high-voltage transmission lines for power fittings. Summary of the Invention

[0005] The purpose of this invention is to provide a low-temperature aging and toughening welding process for layered welding of power fittings, so as to solve the problems existing in the prior art mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a low-temperature aging and toughening welding process for layered welding of power fittings, comprising the following steps: S1) Joint design and surface activation Design an asymmetric groove according to the thickness of the base metal of the electrical fitting, and remove the surface oxide film and perform chemical activation treatment; S2) Gradient heat input multi-layer welding Adopt a multi-layer and multi-pass welding process. Based on the heat dissipation conditions and tissue requirements at different depths of the weld, establish a heat input gradient model. The linear energy ratio of the backing weld layer, filling weld layer and cap weld layer in the gradient heat input model is set as E1:E2:E3, so as to limit the grain coarsening in the heat affected zone (HAZ) while ensuring the penetration depth; S3) Interlayer ultrasonic shock high-energy in-situ treatment When each weld is within the solid-state phase transformation temperature range, apply high-frequency ultrasonic shock to introduce residual compressive stress on the weld surface and refine the sub-grain structure, and then perform forced convection heat transfer until the interlayer temperature drops to the critical value; S4) Deformation-induced low-temperature two-stage aging After welding, no solution treatment is carried out. Directly place the electrical fitting in the aging furnace, and use the lattice distortion energy introduced in step S3 as the precipitation driving force to perform two-stage low-temperature aging treatment, inducing the dispersion precipitation of strengthening phases at dislocation lines and sub-grain boundaries.

[0007] Preferably, in step S1, the surface activation treatment includes: Use a 15% NaOH solution to pickling at 50°C for 3 minutes to remove the original oxide layer at the welding part of the electrical fitting; Subsequently, perform passivation light pickling in a 20% HNO3 solution containing 3 g / L ammonium fluoride to form an active micro-rough structure on the surface of the base metal, and control the roughness Ra to be 3.2-6.3 μm.

[0008] Preferably, in step S2, the specific parameters of the heat input gradient model are set as: The backing weld layer uses cold metal transfer (CMT) or pulsed MIG welding, and the linear energy E1 is set to 0.6-0.9 kJ / cm to achieve low-dilution fusion; The filling weld layer uses a high-deposition rate spray transfer mode, and the linear energy E2 is set to 1.4-1.8 kJ / cm to ensure the density of the weld metal; The cap weld layer uses low-heat input pulsed weaving welding, and the linear energy E3 is set to 1.0-1.3 kJ / cm; The relationship of the linear energy ratio satisfies E1 < E3 < E2, so as to form a gradient heat distribution characteristic of low heat input in the backing weld layer and cap weld layer and high heat input in the filling weld layer, and use the low heat input of the cap layer to generate subsequent heat action on the filling weld layer to achieve the tempering effect.

[0009] Preferably, in step S2, the welding process uses a ternary mixed protective gas, the volume percentage composition of which is: 30%-40% helium, 0.03%-0.05% nitrogen and the balance argon; Helium is suitable for increasing arc stiffness and weld pool wettability, while trace amounts of nitrogen are suitable for forming fine and dispersed nitride nuclei in the weld metal, inhibiting grain growth.

[0010] Preferably, in step S2, the welding wire selection scheme is determined based on the base material of the power fittings: If the power fittings are made of 6000 series aluminum alloy, the selected welding wire contains 0.10%-0.20% zirconium and 0.05%-0.10% erbium by mass. If the power fittings are not made of 6000 series aluminum alloy, then standard welding wire matching the base material should be selected; In the 6000 series aluminum alloy welding wire, zirconium and erbium form Al3Zr and Al3Er nanoparticles, respectively. These nanoparticles, as heterogeneous nucleation sites, work synergistically with the high-frequency ultrasonic impact in step S3 on the weld structure to induce a non-dendritic transformation of the weld structure.

[0011] Preferably, in step S3, the specific process parameters for the high-frequency ultrasonic impact are as follows: The impact frequency is 20-30kHz, and the impact needle amplitude is 30-50μm; The impact treatment temperature window is 120℃-180℃; The impact coverage area includes the weld metal and the area extending 3-5 mm beyond the weld toe; By impacting the weld surface to create a plastic deformation layer with a depth of 20-40μm, the surface grains are refined to the 5-10μm level.

[0012] Preferably, the forced convection heat transfer adopts a liquid nitrogen atomization cooling system, with the nozzle outlet temperature controlled at -40°C to -20°C and the cooling rate controlled at 15°C / s to 25°C / s, until the interlayer temperature drops below 50°C before the next welding step can be carried out.

[0013] Preferably, in step S4, the specific process parameters for the deformation-induced low-temperature two-stage aging are as follows: Primary nucleation aging: Heat to 95℃±3℃ at a rate of 2℃ / min and hold for 3-5 hours to promote uniform nucleation in the GP region; Secondary stabilization aging: The temperature is increased to 140℃±5℃ at a rate of 1℃ / min, and held for 8-10 hours to control the precipitation size of the β'' or η' phase in the range of 5-20nm. Then, it is cooled to 60℃ along with the aging furnace that performs deformation-induced low-temperature two-stage aging treatment before being removed from the furnace and air-cooled.

[0014] Preferably, in step S4, when performing deformation-induced low-temperature two-stage aging treatment, an inert protective gas needs to be introduced into the aging furnace. The inert protective gas is high-purity argon with a purity ≥99.99%, and the gas flow rate is controlled at 15-25L / min. The oxygen content in the aging furnace is ≤300ppm to avoid oxidation of the surface of the power fittings during the aging process.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1) This application establishes a gradient heat input layered welding process, defining the heat distribution characteristics of the root pass and cover pass with low heat input and the fill pass with high heat input. Combined with the tempering effect of the low heat input of the cover pass on the fill pass, this effectively suppresses grain coarsening in the heat-affected zone. Simultaneously, interlayer ultrasonic impact is performed within the solid-state phase transformation temperature range of the weld, precisely refining the subgrain structure and introducing residual compressive stress. Subsequent forced convection heat transfer stabilizes the interlayer temperature, preventing the generation of residual tensile stress. The superimposed deformation-induced low-temperature two-stage aging treatment utilizes the lattice distortion energy introduced by ultrasonic impact to drive the uniform precipitation of strengthening phases, further enhancing the joint strength and toughness, making it particularly suitable for high-voltage power fittings with stringent performance requirements.

[0016] 2) This application designs targeted welding wire selection schemes for power fittings made of different materials. For 6000 series aluminum alloys, by adding a specific proportion of zirconium and erbium to the welding wire, Al3Zr and Al3Er nanoparticles are formed. These nanoparticles, in conjunction with ultrasonic impact, achieve a non-dendritic transformation of the weld microstructure, improving the weld performance of the aluminum alloy. For non-6000 series aluminum alloys, matching standard welding wires are selected, broadening the applicability of the process. The welding process uses a ternary mixed shielding gas containing trace amounts of nitrogen. Helium is used to improve arc stiffness and weld pool wettability, while precise control of nitrogen content forms dispersed nitride nuclei to inhibit grain growth and avoid porosity defects. High-purity argon is introduced for protection during aging, and the oxygen content is strictly controlled to effectively prevent oxidation of the power fitting surface and ensure the stability of the product's appearance and performance.

[0017] 3) This application abandons the traditional post-weld solution treatment process and directly utilizes the lattice distortion energy introduced by ultrasonic impact as the precipitation driving force to implement low-temperature two-stage aging, eliminating the need for high-temperature heating and significantly reducing energy consumption. Simultaneously, low-temperature aging can reduce thermal deformation of power fittings, improve product dimensional accuracy, and reduce subsequent correction processing costs. Furthermore, the forced convection heat transfer between layers employs a liquid nitrogen atomization cooling system, which has high cooling efficiency, shortens welding intervals, and improves welding efficiency. Precise settings of various process parameters, such as ultrasonic impact frequency, amplitude, and cooling rate, can reduce welding defects, lower rework rates, and further control production costs.

[0018] 4) This application, through asymmetric bevel design combined with surface activation treatment, ensures uniform weld penetration and reduces incomplete penetration defects. The synergistic effect of interlayer ultrasonic impact and forced convection heat transfer not only refines the grains but also introduces stable residual compressive stress, significantly improving the fatigue resistance and crack resistance of the welded joint. The inert gas protection and strict oxygen content control during the aging process avoid the adverse effects of oxidation defects on performance, giving the welded joint of power fittings excellent environmental corrosion resistance. Attached Figure Description

[0019] Figure 1 This is a flowchart of the welding process for this application. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] In the description of the invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front end," "rear end," "both ends," "one end," and "the other end," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for 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. Therefore, they should not be construed as limitations on 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.

[0022] In the description of the invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installed," "equipped with," "connected," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0023] In the description of the invention, it should be noted that the execution order of the steps is not limited by the sequence number. The possible changes in the order of some steps, the synchronous execution of steps, and the split execution of steps are all within the scope of protection of this application.

[0024] Please see Figure 1This invention provides a technical solution: a low-temperature aging and toughening welding process for layered welding of power fittings, comprising the following steps: S1) Connector Design and Surface Activation The asymmetric bevel is designed based on the thickness of the base material of the power fittings, and the surface oxide film is removed and chemically activated. S2) Gradient heat input layered welding A multi-layer, multi-pass welding process was adopted. Based on the heat dissipation conditions and microstructure requirements of different weld depths, a heat input gradient model was established. The gradient heat input model set the line energy ratio of the root pass, fill pass, and cap pass to E1:E2:E3, so as to limit grain coarsening in the heat-affected zone (HAZ) while ensuring the penetration depth. S3) Interlayer ultrasonic impact high-energy welding treatment High-frequency ultrasonic impact is applied to each weld seam within the solid phase transition temperature range to introduce residual compressive stress on the weld seam surface and refine the subcrystalline structure. Forced convection heat transfer is then performed until the interpass temperature drops to the critical value. S4) Deformation-induced low-temperature two-stage aging After welding, without solution treatment, the power fittings are placed directly in an aging furnace. The lattice distortion energy introduced in step S3 is used as the precipitation driving force to carry out a two-stage low-temperature aging treatment, which induces the strengthening phase to precipitate diffusely at dislocation lines and subgrain boundaries.

[0025] Specifically, in step S1, the joint design and surface activation steps provide a good foundation for subsequent welding. The asymmetric groove is designed to be adapted to the thickness of the base metal, which can ensure uniform penetration and reduce welding deformation. The removal of the surface oxide film and chemical activation treatment improve the wetting and bonding ability between the welding material and the base metal, reducing the risk of defects such as incomplete penetration from the source. In step S2, gradient heat input layered welding establishes a heat input gradient model to accurately match the heat dissipation and microstructure requirements of different weld depths. While ensuring penetration, it effectively limits grain coarsening in the heat-affected zone, avoiding the problem of uneven joint performance caused by traditional uniform heat input. In step S3... The interlayer ultrasonic impact high-energy in-weld treatment precisely controls the impact timing within the solid-state phase transformation temperature range of the weld. At this point, the weld metal has good plasticity, and the impact can more efficiently introduce residual compressive stress and refine the subgrain structure. Subsequent forced convection heat transfer can quickly stabilize the interlayer temperature, creating suitable conditions for the next welding step. In step S4, deformation-induced low-temperature two-stage aging abandons the traditional high-temperature solution treatment and directly utilizes the lattice distortion energy introduced by ultrasonic impact as the precipitation driving force. This reduces the energy consumption and deformation risk caused by high-temperature heating and induces the dispersion precipitation of strengthening phases at dislocation lines and subgrain boundaries, significantly improving the joint's strength and toughness. The overall process of this application forms a complete process optimization from pre-weld preparation to post-weld strengthening, ensuring that the welded joints of power fittings have excellent comprehensive performance and are suitable for the stringent service requirements of high-voltage transmission lines.

[0026] Specifically, in the layered welding process of welding step S2, the heat-affected zone refers to the area of ​​the base material surrounding the weld that is not melted but undergoes changes in microstructure and properties due to the high temperature of welding. This area does not directly participate in fusion welding, but the welding heat alters its grain structure, hardness, and other properties.

[0027] Example 1: Welding of 6000 series aluminum alloy power fittings The power fittings in this embodiment are made of 6061 series aluminum alloy with a base material thickness of 12mm. The welding process of this technical solution is adopted, and the specific steps are as follows: S1) Joint Design and Surface Activation: Based on the 12mm thick 6061 series aluminum alloy base material, a single-sided V-shaped asymmetric bevel with a bevel angle of 60° was designed. Subsequently, surface activation treatment was carried out. First, the original oxide layer of the part to be welded was removed by alkaline washing with 15% NaOH solution at 50°C for 3 minutes. Then, passivation and light washing were performed in 20% HNO3 solution containing 3g / L ammonium fluoride to form an active micro-rough structure on the surface of the base material. Finally, the surface roughness Ra was controlled to be 4.5μm.

[0028] S2) Gradient heat input layered welding: A multi-layer, multi-pass welding process is adopted, and a heat input gradient model is established; the root pass uses a cold metal transition welding method, and the heat input line energy E1 is set to 0.7 kJ / cm; the fill pass uses a high deposition rate spray transition mode, and the heat input line energy E2 is set to 1.6 kJ / cm; the cap pass uses low heat input pulse oscillation welding, and the heat input line energy E3 is set to 1.2 kJ / cm. The heat input line energy ratio satisfies E1 < E3 < E2; the welding process uses a ternary mixed shielding gas, with a volume percentage composition of 35% helium, 0.04% nitrogen, and the balance argon; the welding wire is a special welding wire with a mass fraction of 0.15% zirconium and 0.08% erbium.

[0029] S3) Interpass ultrasonic impact high-energy welding treatment: After each weld is completed, high-frequency ultrasonic impact is applied within the solid-state phase transformation temperature range of 150℃. The impact frequency is 25kHz, the impact needle amplitude is 40μm, and the impact coverage includes the weld metal and the 4mm area extending beyond the weld toe. Through the impact, a plastic deformation layer with a depth of 30μm is pre-formed on the surface of the weld, which refines the surface grain to the 8μm level. Forced convection heat transfer is started within 0.8s after the impact ends, and a liquid nitrogen atomization cooling system is used. The nozzle outlet temperature is controlled at -30℃ and the cooling rate is controlled at 20℃ / s until the interpass temperature drops to 45℃ before the next weld is carried out.

[0030] S4) Deformation-induced low-temperature two-stage aging: After welding, the power fittings are directly placed in the aging furnace for deformation-induced low-temperature two-stage aging treatment; the first stage nucleation aging is performed by heating to 95℃ at a rate of 2℃ / min and holding for 4 hours to promote uniform nucleation in the GP region; the second stage stabilization aging is performed by heating to 140℃ at a rate of 1℃ / min and holding for 9 hours to control the precipitation size of the β'' phase within the range of 12nm; during the aging process, high-purity argon gas is introduced into the aging furnace as an inert protective gas with a purity of 99.995% and a gas flow rate of 20L / min to control the oxygen content in the aging furnace to 250ppm; after the second stage stabilization aging is completed, the fittings are cooled to 60℃ along with the aging furnace and then removed from the furnace for air cooling.

[0031] The electrical fittings welded in Example 1 were tested, and the test results are as follows: 1. Tensile properties: The average tensile strength of the three parallel specimens was 325 MPa, the average yield strength was 285 MPa, and the average elongation after fracture was 15.2%. All specimens fractured in the base metal region and did not fracture in the weld or heat-affected zone, indicating that the strength of the welded joint is better than that of the base metal.

[0032] 2. Microhardness: The average hardness of the weld center area is 128 HV, the average hardness of the heat-affected zone is 115 HV, and the average hardness of the base metal area is 108 HV. The hardness of the weld and the heat-affected zone are higher than that of the base metal, and the hardness distribution is uniform with no obvious soft areas.

[0033] 3. Fatigue performance: The minimum fatigue life of the three parallel specimens was 1.2 × 10⁻⁶. 6 This significantly exceeds the fatigue life of the power fittings industry (≥5×10⁻⁶). 5 The requirements for this use.

[0034] 4. Weld defects: Ultrasonic testing results show that there are no defects such as cracks, incomplete penetration, or porosity inside the weld, and the defect level is Class I. Macroscopic metallographic observation shows that the weld is well-formed, with fine equiaxed crystals inside, no coarse grains, and a uniform and dense structure.

[0035] 5. Visual inspection: The weld and surrounding area are uniform in color with a silvery-white metallic luster, and there is no oxidation discoloration, yellowing or blackening. The external pattern of the weld is continuous and regular, with uniform fish scale pattern, and there are no appearance defects such as cracks, lack of fusion, undercut, spatter residue, etc. The transition between the weld and the base material is smooth, which meets the appearance quality requirements of power fittings.

[0036] The electrical fittings welded in Example 1 were tested, and the following physicochemical data were obtained: Therefore, the power fittings products welded in Example 1 meet all the physical and chemical properties of GB / T 2314-2017 "General Technical Conditions for Power Fittings" and related industry requirements, demonstrating excellent overall performance of the welded joints. Example 1 (6061 series aluminum alloy) used special welding wire with added zirconium and erbium, combined with gradient heat input welding, interlayer ultrasonic impact, and low-temperature dual-stage aging processes. The welded joint exhibited outstanding tensile strength, hardness, and fatigue life, with no internal defects and a fine, uniform microstructure. This achieved efficient strengthening and toughening of 6000 series aluminum alloy power fittings, verifying the effectiveness of targeted welding wire design and multi-process synergy. Overall test results show that the low-temperature aging strengthening and toughening welding process after layered welding of power fittings in this invention can effectively improve the strength and toughness of welded joints of power fittings made of different materials, avoid weld defects, ensure product reliability, and has broad application value.

[0037] Example 2: Welding of non-6000 series aluminum alloy power fittings The power fittings in this embodiment are made of 5052 series aluminum alloy (not 6000 series), with a base material thickness of 10mm. The welding process of this technical solution is adopted, and the specific steps are as follows: S1) Joint Design and Surface Activation: Based on the 10mm thick 5052 series aluminum alloy base material, a single-sided V-shaped asymmetric bevel with a bevel angle of 55° was designed. Subsequently, surface activation treatment was carried out. First, the original oxide layer of the part to be welded was removed by alkaline washing with 15% NaOH solution at 50°C for 3 minutes. Then, passivation and light washing were carried out in 20% HNO3 solution containing 3g / L ammonium fluoride to form an active micro-rough structure on the surface of the base material. Finally, the surface roughness Ra was controlled to be 3.8μm.

[0038] S2) Gradient heat input layered welding: A multi-layer, multi-pass welding process is adopted, and a heat input gradient model is established; the root pass uses pulsed MIG welding, with the heat input energy E1 set at 0.8 kJ / cm; the fill pass uses a high deposition rate spray transition mode, with the heat input energy E2 set at 1.5 kJ / cm; the cap pass uses low heat input pulsed oscillating welding, with the heat input energy E3 set at 1.1 kJ / cm, and the heat input energy ratio satisfies E1 < E3 < E2; the welding process uses a ternary mixed shielding gas, with a volume percentage composition of 32% helium, 0.03% nitrogen, and the balance argon; the welding wire selected is the ER5356 standard welding wire matched with 5052 series aluminum alloy.

[0039] S3) Interpass ultrasonic impact high-energy welding treatment: After each weld is completed, high-frequency ultrasonic impact is applied within the solid-state phase transformation temperature range of 130℃. The impact frequency is 22kHz, the impact needle amplitude is 35μm, and the impact coverage includes the weld metal and the 3mm area extending beyond the weld toe. Through the impact, a plastic deformation layer with a depth of 25μm is pre-formed on the surface of the weld, which refines the surface grain to the 9μm level. Forced convection heat transfer is started within 0.6s after the impact ends, and a liquid nitrogen atomization cooling system is used. The nozzle outlet temperature is controlled at -25℃ and the cooling rate is controlled at 18℃ / s until the interpass temperature drops to 48℃ before the next weld is carried out.

[0040] S4) Deformation-induced low-temperature two-stage aging: After welding, the power fittings are directly placed in the aging furnace for deformation-induced low-temperature two-stage aging treatment; the first stage nucleation aging is heated to 93℃ at a rate of 2℃ / min and held for 3.5 hours to promote uniform nucleation in the GP region; the second stage stabilization aging is heated to 138℃ at a rate of 1℃ / min and held for 8.5 hours to control the precipitation size of the η' phase within the range of 10nm; during the aging process, high-purity argon gas is introduced into the aging furnace as an inert protective gas with a purity of 99.992% and a gas flow rate controlled at 18L / min to control the oxygen content in the aging furnace to 280ppm; after the second stage stabilization aging is completed, the fittings are cooled to 60℃ along with the aging furnace and then removed from the furnace for air cooling.

[0041] The electrical fittings welded in Example 2 were tested, and the results are as follows: 1. Tensile properties: The average tensile strength of the three parallel specimens was 268 MPa, the average yield strength was 195 MPa, and the average elongation after fracture was 22.5%. All specimens fractured in the base material region, and the strength of the welded joint matched well with that of the base material.

[0042] 2. Microhardness: The average hardness of the weld center area is 98 HV, the average hardness of the heat-affected zone is 92 HV, and the average hardness of the base metal area is 85 HV. The hardness of the weld and the heat-affected zone is slightly higher than that of the base metal. The hardness distribution is uniform and there is no concentration of soft areas.

[0043] 3. Fatigue performance: The minimum fatigue life of the three parallel specimens was 8 × 10⁻⁶. 5 This meets the fatigue life requirement of the power fittings industry: ≥5×10⁻⁶. 5 The requirements for this use.

[0044] 4. Weld defects: Ultrasonic testing results show that there are no fatal defects such as cracks or incomplete penetration inside the weld. There are only a few micro-pores (diameter ≤0.5mm, number ≤3 / 100mm), and the defect level is I. Macroscopic metallographic observation shows that the weld is smooth, the internal structure is uniform, and there are no coarse grains or segregation.

[0045] 5. Visual inspection: The weld and surrounding area are uniform in color with a silvery-white metallic luster and no obvious oxidation discoloration. The weld has a continuous and uniform fish-scale pattern on the outside and no defects such as cracks, lack of fusion, undercut, or spatter residue. The transition between the weld and the base material is smooth, which meets the appearance quality requirements of power fittings.

[0046] The electrical fittings welded in Example 2 were tested, and the following physicochemical data were obtained: Therefore, the power fittings products welded in Example 2 all meet the physical and chemical properties of GB / T 2314-2017 "General Technical Conditions for Power Fittings" and related industry requirements, demonstrating excellent overall performance of the welded joint. Example 2 (5052 series aluminum alloy) used matching standard welding wire and, after welding with this technical solution, the joint strength matched well with the base material, exhibited excellent elongation after fracture, met fatigue life requirements, and met weld defect standards, proving that this process also has good compatibility with non-6000 series aluminum alloy power fittings.

[0047] In step S1, the surface activation treatment includes: The original oxide layer of the electrical fittings to be welded is removed by alkaline washing with 15% NaOH solution at 50℃ for 3 minutes. Subsequently, passivation and photo-washing were performed in a 20% HNO3 solution containing 3 g / L ammonium fluoride to form an active micro-roughness structure on the surface of the base material, with the roughness Ra controlled between 3.2 and 6.3 μm.

[0048] Specifically, the effectiveness of surface activation treatment directly determines the quality of weld bonding. In existing technologies, simple surface treatments are prone to welding defects due to incomplete oxide layer removal and insufficient surface activity. This application uses a 15% NaOH solution for alkaline washing at 50°C for 3 minutes to specifically remove the original oxide layer on the parts of the electrical fittings to be welded. This temperature and concentration of alkaline solution ensures effective oxide layer removal while avoiding excessive corrosion of the base material surface, thus protecting the integrity of the base material matrix. Subsequent passivation and polishing in a 20% HNO3 solution containing 3g / L ammonium fluoride forms an active micro-rough structure on the base material surface. The addition of ammonium fluoride enhances the passivation effect and surface activity, controlling the roughness Ra within a reasonable range of 3.2-6.3μm. This increases the contact area and bonding strength between the welding material and the base material while avoiding uneven welding material filling caused by excessive roughness. This refined surface activation treatment ensures full fusion between the welding material and the base material during welding, reducing defects such as porosity and slag inclusions, laying a solid foundation for the smooth implementation of subsequent welding steps and improved joint performance.

[0049] Comparative Example 1: Surface Activation Treatment for Defects This comparative example is based on Example 1, except that the surface activation treatment in step S1 is omitted. All other steps and process parameters are completely consistent with Example 1. The complete steps are as follows: S1) Joint design: Based on the 12mm thick 6061 series aluminum alloy base material, a single-sided V-shaped asymmetric bevel is designed with a bevel angle of 60°. No subsequent surface oxide layer removal and passivation cleaning surface activation treatment are performed, and the process proceeds directly to the next step. S2) Gradient heat input layered welding: A multi-layer, multi-pass welding process is adopted, and a heat input gradient model is established. The root pass uses a cold metal transition welding method, with a set heat input energy E1 of 0.7 kJ / cm. The fill pass uses a high deposition rate spray transition mode, with a set heat input energy E2 of 1.6 kJ / cm. The cap pass uses low heat input pulse oscillation welding, with a set heat input energy E3 of 1.2 kJ / cm. The heat input energy ratio satisfies E1 < E3 < E2. The welding process uses a ternary mixed shielding gas, with a volume percentage composition of 35% helium, 0.04% nitrogen, and the balance argon. The welding wire used is a special welding wire with 0.15% zirconium and 0.08% erbium by mass. S3) Interpass ultrasonic impact high-energy welding treatment: After each weld is completed, high-frequency ultrasonic impact is applied within the solid-state phase transformation temperature range of 150℃. The impact frequency is 25kHz, the impact needle amplitude is 40μm, and the impact coverage includes the weld metal and the 4mm area extending beyond the weld toe. Through the impact, a plastic deformation layer with a depth of 30μm is pre-formed on the surface of the weld, which refines the surface grain to the 8μm level. Forced convection heat transfer is started within 0.8s after the impact ends, and a liquid nitrogen atomization cooling system is used. The nozzle outlet temperature is controlled at -30℃ and the cooling rate is controlled at 20℃ / s until the interpass temperature drops to 45℃ before the next weld is carried out. S4) Deformation-induced low-temperature two-stage aging: After welding, the power fittings are directly placed in the aging furnace for deformation-induced low-temperature two-stage aging treatment; the first stage nucleation aging is performed by heating to 95℃ at a rate of 2℃ / min and holding for 4 hours to promote uniform nucleation in the GP region; the second stage stabilization aging is performed by heating to 140℃ at a rate of 1℃ / min and holding for 9 hours to control the precipitation size of the β'' phase within the range of 12nm; during the aging process, high-purity argon gas is introduced into the aging furnace as an inert protective gas with a purity of 99.995% and a gas flow rate of 20L / min to control the oxygen content in the aging furnace to 250ppm; after the second stage stabilization aging is completed, the fittings are cooled to 60℃ along with the aging furnace and then removed from the furnace for air cooling.

[0050] The electrical fittings welded in Comparative Example 1 were tested, and the results are as follows: 1. Tensile properties: The average tensile strength of the three parallel specimens was 265 MPa, the average yield strength was 230 MPa, and the average elongation after fracture was 10.5%. All specimens fractured in the weld area, indicating that the strength of the welded joint was significantly lower than that of the base material and Example 1, and the fusion effect between the welding material and the base material was poor.

[0051] 2. Microhardness: The average hardness of the weld center area is 105 HV, the average hardness of the heat-affected zone is 98 HV, and the average hardness of the base metal area is 108 HV. The hardness of the weld and the heat-affected zone is lower than that of the base metal, and the hardness distribution is uneven. There are obvious soft areas in the heat-affected zone.

[0052] 3. Fatigue performance: The minimum fatigue life of the three parallel specimens was 3.2 × 10⁻⁶. 5 The fatigue life of the power fittings industry standard of ≥5×10 was not achieved. 5 After repeated use, the fatigue resistance is significantly reduced.

[0053] 4. Weld defects: Ultrasonic testing results showed that there were a small number of incomplete penetration defects and scattered pores (diameter ≤1mm, number ≥5 / 100mm) inside the weld, with a defect level of III; macroscopic metallographic observation showed that the weld formation was irregular, the internal structure had coarse grains, and there was a gap of poor fusion at the interface between the welding material and the base material.

[0054] 5. Visual inspection: Visual observation shows that the weld and surrounding area have uneven color, with some areas showing a dark gray oxidized color and yellowing; the external texture of the weld is discontinuous, with messy fish scale patterns, slight undercut and a small amount of spatter residue at the edges, and the transition between the weld and the base material is rough, which does not meet the appearance quality requirements of power fittings.

[0055] The electrical fittings welded in Comparative Example 1 were tested, and the following physicochemical data were obtained: Therefore, the power fittings products welded in Comparative Example 1 lacked surface activation treatment, resulting in poor fusion between the welding material and the base material, defects such as incomplete penetration and porosity, a significant decrease in tensile strength and fatigue life, and substandard appearance quality. As can be seen from Comparative Example 1, surface activation treatment can effectively remove the oxide layer and improve the surface activity of the base material, which is a basic process to ensure the welding bond strength and appearance quality.

[0056] In step S2, the specific parameters of the heat input gradient model are set as follows: The root pass is achieved using cold metal transfer (CMT) or pulsed MIG welding, with the heat input E1 set to 0.6-0.9 kJ / cm to achieve low dilution fusion. The filler layer adopts a high deposition rate spray transition mode, with the line energy E2 set at 1.4-1.8 kJ / cm to ensure the density of the weld metal; The cover pass layer is welded by low heat input pulsed weaving welding, and the set linear energy E3 is 1.0 - 1.3 kJ / cm; The relationship of the linear energy ratio satisfies E1 < E3 < E2, so as to form a gradient heat distribution characteristic of low heat input for the root pass layer and the cover pass layer and high heat input for the filler pass layer. The low heat input of the cover pass layer generates a subsequent heat effect on the filler pass layer to achieve the tempering effect.

[0057] Specifically, by differentiating the welding methods and linear energy parameters of the root pass layer, filler pass layer, and cover pass layer, precise temperature control and performance coordination of the layered welding are achieved. The root pass layer uses cold metal transfer or pulsed MIG welding, and the linear energy E1 is set to 0.6 - 0.9 kJ / cm, which can achieve low dilution rate fusion, ensure the bonding strength between the root pass layer and the base material, and avoid joint composition deviation and performance degradation caused by too high dilution rate; the filler pass layer uses a high deposition rate spray transfer mode, and the linear energy E2 is set to 1.4 - 1.8 kJ / cm, which can ensure the denseness of the weld metal, improve the filling efficiency while reducing internal pores; the cover pass layer uses low heat input pulsed weaving welding, and the linear energy E3 is set to 1.0 - 1.3 kJ / cm, which can ensure the forming quality of the cover surface. The linear energy ratio satisfies the relationship of E1 less than E3 less than E2, forming a gradient heat distribution characteristic with low values at both ends and high value in the middle. The low heat input of the cover pass layer can generate a subsequent heat effect on the filler layer to achieve the tempering effect, effectively improving the microstructure and properties of the filler layer, refining the grains of the filler layer, and eliminating part of the welding residual stress. Through this precise setting of gradient heat input parameters, the forming quality and microstructure and properties of each layer of the weld can be comprehensively improved, avoiding coarse grains in the heat affected zone, and significantly enhancing the overall strength and toughness of the welded joint.

[0058] In step S2, a ternary mixed shielding gas is used in the welding process, and its volume percentage composition is: 30% - 40% helium, 0.03% - 0.05% nitrogen, and the balance argon; Among them, helium is suitable for increasing the arc stiffness and molten pool wettability, and trace nitrogen is suitable for forming fine and dispersed nitride cores in the weld metal to inhibit grain growth.

[0059] Specifically, during the welding process, the composition of the shielding gas directly affects the stability of the welding arc, the state of the molten pool, and the quality of the weld. Aluminum alloy welding, in particular, has more stringent requirements for shielding gas selection, and existing single or binary shielding gases are insufficient to meet multiple needs. This application employs a ternary mixed shielding gas consisting of 30%-40% helium, 0.03%-0.05% nitrogen, and the balance argon, achieving multiple protection and strengthening effects. The addition of helium increases arc stiffness and molten pool wettability, solving the problem of arc dispersion and poor molten pool fluidity in aluminum alloy welding, ensuring aesthetically pleasing weld formation. In conventional aluminum alloy welding operations, nitrogen is avoided due to its tendency to cause porosity defects. This solution controls the nitrogen content to an extremely low range of 0.03%-0.05%, which forms fine, dispersed nitride nuclei in the weld metal, inhibiting grain growth without causing porosity defects. The balance argon plays a basic protective role, isolating the weld metal from air and preventing oxidation. The synergistic effect of this ternary mixed protective gas ensures a stable welding process, reduces defects such as oxidation porosity, and refines the weld grains through the formation of nitride cores, further enhancing the strength and toughness of the weld joint.

[0060] Comparative Example 2: Using a binary mixed protective gas This comparative example is based on Example 1, except that the ternary mixed protective gas in step S2 is replaced with a binary mixed protective gas (to remove the nitrogen component). All other steps and process parameters are completely consistent with Example 1. The complete steps are as follows: S1) Joint Design and Surface Activation: Based on the 12mm thick 6061 series aluminum alloy base material, a single-sided V-shaped asymmetric bevel with a bevel angle of 60° was designed; then, surface activation treatment was performed. First, the original oxide layer of the part to be welded was removed by alkaline washing with 15% NaOH solution at 50℃ for 3 minutes; then, passivation and light washing were performed in 20% HNO3 solution containing 3g / L ammonium fluoride to form an active micro-rough structure on the surface of the base material, and finally the surface roughness Ra was controlled to be 4.5μm; S2) Gradient heat input layered welding: A multi-layer, multi-pass welding process is adopted, and a heat input gradient model is established; the root pass uses a cold metal transition welding method, with the heat input line energy E1 set at 0.7 kJ / cm; the fill pass uses a high deposition rate spray transition mode, with the heat input line energy E2 set at 1.6 kJ / cm; the cap pass uses low heat input pulse oscillation welding, with the heat input line energy E3 set at 1.2 kJ / cm, and the heat input line energy ratio satisfies E1 < E3 < E2; the welding process uses a binary mixed shielding gas, with a volume percentage composition of 35% helium and the balance argon, and no nitrogen component; the welding wire used is a special welding wire with a mass fraction of 0.15% zirconium and 0.08% erbium. S3) Interpass ultrasonic impact high-energy welding treatment: After each weld is completed, high-frequency ultrasonic impact is applied within the solid-state phase transformation temperature range of 150℃. The impact frequency is 25kHz, the impact needle amplitude is 40μm, and the impact coverage includes the weld metal and the 4mm area extending beyond the weld toe. Through the impact, a plastic deformation layer with a depth of 30μm is pre-formed on the surface of the weld, which refines the surface grain to the 8μm level. Forced convection heat transfer is started within 0.8s after the impact ends, and a liquid nitrogen atomization cooling system is used. The nozzle outlet temperature is controlled at -30℃ and the cooling rate is controlled at 20℃ / s until the interpass temperature drops to 45℃ before the next weld is carried out. S4) Deformation-induced low-temperature two-stage aging: After welding, the power fittings are directly placed in the aging furnace for deformation-induced low-temperature two-stage aging treatment; the first stage nucleation aging is performed by heating to 95℃ at a rate of 2℃ / min and holding for 4 hours to promote uniform nucleation in the GP region; the second stage stabilization aging is performed by heating to 140℃ at a rate of 1℃ / min and holding for 9 hours to control the precipitation size of the β'' phase within the range of 12nm; during the aging process, high-purity argon gas is introduced into the aging furnace as an inert protective gas with a purity of 99.995% and a gas flow rate of 20L / min to control the oxygen content in the aging furnace to 250ppm; after the second stage stabilization aging is completed, the fittings are cooled to 60℃ along with the aging furnace and then removed from the furnace for air cooling.

[0061] The electrical fittings welded in Comparative Example 2 were tested, and the results are as follows: 1. Tensile properties: The average tensile strength of the three parallel specimens was 295 MPa, the average yield strength was 260 MPa, and the average elongation after fracture was 12.8%. Two specimens fractured in the weld area and one specimen fractured in the heat-affected zone. The joint strength was lower than that of Example 1, and the fusion reliability decreased.

[0062] 2. Microhardness: The average hardness of the weld center area is 112 HV, the average hardness of the heat-affected zone is 105 HV, and the average hardness of the base material area is 108 HV. The hardness of the weld and heat-affected zone is slightly lower than that of Example 1, and the uniformity of hardness distribution is generally poor.

[0063] 3. Fatigue performance: The minimum fatigue life of the three parallel specimens was 4.5 × 10⁻⁶. 5 The fatigue life of the power fittings industry is close to, but not fully reaches, ≥5×10⁻⁶. 5 The fatigue resistance is insufficient for the requirements of repeated use.

[0064] 4. Weld defects: Ultrasonic testing results showed that there were a small number of pores (diameter ≤0.8mm, number ≥4 / 100mm) inside the weld, but no fatal defects such as incomplete penetration or cracks. The defect level was II. Macroscopic metallographic observation showed that the weld formation was relatively regular, but the internal grains were coarser than those in Example 1, and no fine nitride nuclei were precipitated.

[0065] 5. Visual inspection: The weld and surrounding area are basically uniform in color, with a light grayish-white metallic luster and no obvious oxidation discoloration. The external texture of the weld is fish scale pattern, but the regularity is average. There are no defects such as cracks or undercut. The transition between the weld and the base material is relatively smooth. It basically meets the appearance quality requirements but is better than Comparative Example 1 and worse than Example 1.

[0066] The electrical fittings welded in Comparative Example 2 were tested, and the following physicochemical data were obtained: Therefore, in Comparative Example 2, the welded electrical fittings, due to the use of a binary mixed protective gas without trace amounts of nitrogen, could not form fine and dispersed nitride nuclei, resulting in coarse weld grains, failure to meet fatigue life standards, and decreased tensile strength. In contrast, as seen in Example 1, a ternary mixed protective gas containing 0.03%-0.05% trace amounts of nitrogen is a key innovation for refining weld grains and improving joint strength and toughness, breaking through the limitations of conventional aluminum welding in avoiding nitrogen.

[0067] In step S2, the selection of welding wire is determined based on the base material of the power fittings: If the power fittings are made of 6000 series aluminum alloy, the selected welding wire should contain 0.10%-0.20% zirconium and 0.05%-0.10% erbium by mass. If the power fittings are not made of 6000 series aluminum alloy, then use standard welding wire that matches the base material. In the 6000 series aluminum alloy welding wire, zirconium and erbium form Al3Zr and Al3Er nanoparticles, respectively. The nanoparticles, as heterogeneous nucleation sites, work synergistically with the high-frequency ultrasonic impact in step S3 to cause the weld structure to undergo a non-dendritic transformation.

[0068] Specifically, existing welding wires lack targeted designs for power fittings made of different materials, especially for 6000 series aluminum alloys, where grain growth is difficult to effectively suppress. In this application, for 6000 series aluminum alloy power fittings, welding wires with added zirconium (0.10%-0.20% by mass) and erbium (0.05%-0.10% by mass) are selected. Zirconium and erbium can form Al3Zr and Al3Er nanoparticles, respectively. These nanoparticles, as heterogeneous nucleation sites, synergistically act with the high-frequency ultrasonic impact in step S3 on the weld microstructure, causing a non-dendritic transformation of the weld microstructure. This effectively suppresses the grain coarsening problem that easily occurs during the welding of 6000 series aluminum alloys, significantly improving the weld strength and toughness. For non-6000 series aluminum alloy power fittings, standard welding wires matching the base material are selected, ensuring the welding compatibility of fittings made of different materials and avoiding problems such as poor fusion performance caused by incompatibility between the welding wire and the base material. This differentiated welding wire selection scheme broadens the applicability of this process and enables precise strengthening of core materials, ensuring that welding joints of power fittings made of different materials can achieve excellent performance.

[0069] In step S3, the specific process parameters for high-frequency ultrasonic impact are as follows: The impact frequency is 20-30kHz, and the impact needle amplitude is 30-50μm; The impact treatment temperature window is 120℃-180℃; The impact coverage area includes the weld metal and the area extending 3-5 mm beyond the weld toe; By impacting the weld surface to create a plastic deformation layer with a depth of 20-40μm, the surface grains are refined to the 5-10μm level.

[0070] Specifically, the impact frequency is set to 20-30kHz, and the impact needle amplitude is set to 30-50μm. This parameter range ensures sufficient impact energy to effectively form a plastic deformation layer on the weld surface. The impact treatment temperature window is controlled between 120℃ and 180℃. This temperature range is within the solid-state phase transformation temperature range of the weld. At this temperature, the plasticity and microstructure activity of the weld metal are suitable, and the impact can more efficiently refine the subgrain structure while maximizing the introduction of residual compressive stress. The impact coverage includes the weld metal and the area extending 3-5mm beyond the weld toe, which can fully cover the area of ​​weld stress concentration and improve the overall fatigue resistance of the joint. By pre-forming a plastic deformation layer with a depth of 20-40μm on the weld surface through impact, the surface grains are refined to the 5-10μm level, significantly improving the hardness and toughness of the weld surface.

[0071] Forced convection heat transfer uses a liquid nitrogen atomization cooling system. The nozzle outlet temperature is controlled between -40℃ and -20℃, and the cooling rate is controlled between 15℃ / s and 25℃ / s. The next welding can only be carried out when the interpass temperature drops below 50℃.

[0072] Specifically, in welding operations, precise control of interpass temperature is crucial to ensuring the quality of layered welding. Existing cooling methods are inefficient and struggle to quickly stabilize interpass temperature, easily leading to the cumulative heat-affected zone (HEAD) of subsequent welds. This application employs a liquid nitrogen atomization cooling system, achieving highly efficient and precise interpass cooling. The nozzle outlet temperature is controlled between -40℃ and -20℃, and the cooling rate is controlled between 15℃ / s and 25℃ / s. These cooling parameters can quickly reduce the weld temperature to the critical value, avoiding grain growth and residual tensile stress accumulation caused by prolonged high-temperature residence time in the weld. Simultaneously, strictly controlling the interpass temperature to below 50℃ before proceeding to the next weld effectively prevents residual heat from the previous weld from adversely affecting subsequent welds, ensuring that each weld is formed under suitable temperature conditions, improving the uniformity and stability of the weld microstructure. The liquid nitrogen atomization cooling system offers advantages such as high cooling efficiency and precise temperature control, significantly shortening welding intervals and improving welding efficiency. Furthermore, precise temperature control reduces welding defects, further enhancing the overall performance of the welded joint.

[0073] Comparative Example 3: Using a non-liquid nitrogen atomization cooling system This comparative example is based on Example 1, except that the liquid nitrogen atomization cooling system in step S3 is replaced with a compressed air cooling system. All other steps and process parameters are completely consistent with Example 1. The complete steps are as follows: S1) Joint Design and Surface Activation: Based on the 12mm thick 6061 series aluminum alloy base material, a single-sided V-shaped asymmetric bevel with a bevel angle of 60° was designed; then, surface activation treatment was performed. First, the original oxide layer of the part to be welded was removed by alkaline washing with 15% NaOH solution at 50℃ for 3 minutes; then, passivation and light washing were performed in 20% HNO3 solution containing 3g / L ammonium fluoride to form an active micro-rough structure on the surface of the base material, and finally the surface roughness Ra was controlled to be 4.5μm; S2) Gradient heat input layered welding: A multi-layer, multi-pass welding process is adopted, and a heat input gradient model is established. The root pass uses a cold metal transition welding method, with a set heat input energy E1 of 0.7 kJ / cm. The fill pass uses a high deposition rate spray transition mode, with a set heat input energy E2 of 1.6 kJ / cm. The cap pass uses low heat input pulse oscillation welding, with a set heat input energy E3 of 1.2 kJ / cm. The heat input energy ratio satisfies E1 < E3 < E2. The welding process uses a ternary mixed shielding gas, with a volume percentage composition of 35% helium, 0.04% nitrogen, and the balance argon. The welding wire used is a special welding wire with 0.15% zirconium and 0.08% erbium by mass. S3) Interpass ultrasonic impact high-energy welding treatment: After each weld is completed, high-frequency ultrasonic impact is applied within the solid-state phase transformation temperature range of 150℃. The impact frequency is 25kHz, the impact needle amplitude is 40μm, and the impact coverage includes the weld metal and the 4mm area extending beyond the weld toe. Through the impact, a plastic deformation layer with a depth of 30μm is pre-formed on the surface of the weld, which refines the surface grain to the 8μm level. Forced convection heat transfer is started within 0.8s after the impact ends, and a compressed air cooling system is used. The nozzle outlet temperature is controlled at 25℃ and the cooling rate is controlled at 5℃ / s until the interpass temperature drops to 45℃ before the next weld is carried out. S4) Deformation-induced low-temperature two-stage aging: After welding, the power fittings are directly placed in the aging furnace for deformation-induced low-temperature two-stage aging treatment; the first stage nucleation aging is performed by heating to 95℃ at a rate of 2℃ / min and holding for 4 hours to promote uniform nucleation in the GP region; the second stage stabilization aging is performed by heating to 140℃ at a rate of 1℃ / min and holding for 9 hours to control the precipitation size of the β'' phase within the range of 12nm; during the aging process, high-purity argon gas is introduced into the aging furnace as an inert protective gas with a purity of 99.995% and a gas flow rate of 20L / min to control the oxygen content in the aging furnace to 250ppm; after the second stage stabilization aging is completed, the fittings are cooled to 60℃ along with the aging furnace and then removed from the furnace for air cooling.

[0074] The electrical fittings welded in Comparative Example 3 were tested, and the results are as follows: 1. Tensile properties: The average tensile strength of the three parallel specimens was 280 MPa, the average yield strength was 245 MPa, and the average elongation after fracture was 11.6%. Two specimens fractured in the heat-affected zone, and one specimen fractured in the weld area. The joint strength was lower than that of Example 1, and the performance of the heat-affected zone deteriorated significantly.

[0075] 2. Microhardness: The average hardness of the weld center area is 118 HV, the average hardness of the heat-affected zone is 95 HV, and the average hardness of the base metal area is 108 HV. The hardness of the heat-affected zone is significantly lower than that of the weld and the base metal, and the hardness fluctuates greatly, with obvious soft area concentration.

[0076] 3. Fatigue performance: The minimum fatigue life of the three parallel specimens was 3.8 × 10⁻⁶. 5 The fatigue life of the power fittings industry standard of ≥5×10 was not achieved. 5 It has poor fatigue resistance due to its short service life.

[0077] 4. Weld defects: Ultrasonic testing results showed that there were a small number of pores and a tendency for hot cracking inside the weld (microcrack length ≤2mm), and the defect level was III. Macroscopic metallographic observation showed that the weld was well formed, but the grains in the heat-affected zone were severely coarse, and there was a phenomenon of microstructure segregation.

[0078] 5. Visual inspection: The weld and surrounding area are uniform in color with a silvery-white metallic luster and no oxidation discoloration. The weld has a continuous fish-scale pattern on the outside and no defects such as cracks, undercut, or spatter residue. However, there are slight deformation marks in the heat-affected zone. The weld and the base material are smoothly connected. The appearance quality is better than that of comparative examples 1 and 2, but the deformation in the heat-affected zone affects the overall accuracy.

[0079] The electrical fittings welded in Comparative Example 3 were tested, and the following physicochemical data were obtained: Therefore, the power fittings products welded in Comparative Example 3, due to the use of a compressed air cooling system (not liquid nitrogen atomization), had insufficient cooling rate, resulting in coarse grains and concentrated soft areas in the heat-affected zone, a tendency for hot cracking, and failure to meet fatigue life standards. Comparative Example 1 shows that the liquid nitrogen atomization cooling system can accurately control the interlayer temperature, avoid performance degradation in the heat-affected zone, and is an important supporting process to ensure the quality of layered welding.

[0080] In step S4, the specific process parameters for deformation-induced low-temperature two-stage aging are as follows: Primary nucleation aging: Heat to 95℃±3℃ at a rate of 2℃ / min and hold for 3-5 hours to promote uniform nucleation in the GP region; Secondary stabilization aging: The temperature is increased to 140℃±5℃ at a rate of 1℃ / min, and held for 8-10 hours to control the precipitation size of the β'' or η' phase in the range of 5-20nm. Then, it is cooled to 60℃ along with the aging furnace that performs deformation-induced low-temperature two-stage aging treatment before being removed from the furnace and air-cooled.

[0081] Specifically, aging parameters directly determine the precipitation state of the strengthening phase. Existing aging processes are difficult to adapt to the deformation-induced strengthening requirements without solution treatment. This application achieves precise control of the strengthening phase by setting the heating rate, temperature, and holding time in stages: the first-stage nucleation aging is heated to 95℃±3℃ at a rate of 2℃ / min and held for 3-5 hours, which can slowly and uniformly heat up and hold for a sufficient amount of time, promoting uniform nucleation in the GP region and laying the foundation for subsequent stabilization aging; the second-stage stabilization aging is heated to 140℃±5℃ at a rate of 1℃ / min and held for 8-10 hours, which can precisely control the precipitation size of the β'' or η' phase in the range of 5-20nm. When the strengthening phase of this size is dispersed, it can maximize the joint strength and toughness; then, it is cooled to 60℃ together with the aging furnace and then air-cooled after being removed from the furnace, which can avoid stress accumulation and abnormal precipitation of the strengthening phase caused by rapid cooling. By precisely setting the dual-stage aging parameters, high-temperature solution treatment is unnecessary. The lattice distortion energy introduced by ultrasonic impact directly drives the uniform dispersion precipitation of the strengthening phase, which reduces energy consumption and deformation risk, significantly improves the joint's strength and toughness, and ensures the stability of the aging process, thereby improving the product's dimensional accuracy.

[0082] Specifically, the GP region (Guinier-Preston region) is a region rich in solute atoms that precipitates early during the aging strengthening process of aluminum alloys, and is not an independent compound phase. Specifically, in 6000 series aluminum alloys (such as the 6061 series in Example 1), the GP region is a tiny region formed in the early stages of aging by the uniform agglomeration of solute atoms (such as Mg and Si) in the weld metal within the aluminum matrix. Its size is only on the nanometer scale (typically a few nanometers to tens of nanometers), and it is the core for the subsequent formation of stable strengthening phases (such as the β'' phase and η' phase). In deformation-induced low-temperature two-stage aging, the core objective of "promoting uniform nucleation of the GP region" corresponds to the first-stage nucleation aging stage (heating to 95℃±3℃ at 2℃ / min and holding for 3-5 hours): through slow heating and stable holding, solute atoms (such as Mg and Si) in the weld metal are uniformly diffused and aggregated, forming a large number of uniformly dispersed GP regions, avoiding excessive local concentration or sparse distribution of solute atoms. The uniformly nucleated GP region can provide uniform "nuclei" for the precipitation of β'' or η' phase in the subsequent secondary stabilization aging (heating to 140℃±5℃), ultimately allowing the strengthening phase (β'' / η') to be dispersed in the aluminum matrix, significantly improving the strength and toughness of the welded joint. Therefore, this is the key reason why this application can achieve strengthening and toughening without high-temperature solution treatment, but only through low-temperature two-stage aging.

[0083] In step S4, when performing deformation-induced low-temperature two-stage aging treatment, an inert protective gas needs to be introduced into the aging furnace. The inert protective gas is high-purity argon with a purity ≥99.99%, and the gas flow rate is controlled at 15-25L / min. The oxygen content in the aging furnace is ≤300ppm to avoid oxidation of the surface of the power fittings during the aging process.

[0084] Specifically, during the aging process, the surface of electrical fittings is prone to oxidation due to reaction with oxygen, forming an oxide layer that affects appearance quality and joint performance. This application uses high-purity argon as the inert protective gas, with a purity ≥99.99%, ensuring the purity of the protective gas and avoiding oxidation or other adverse reactions caused by the introduction of impurities. The gas flow rate is controlled at 15-25 L / min, ensuring that the aging furnace is filled with protective gas, isolating air from contact with the surface of the electrical fittings, while avoiding gas waste caused by excessive flow. Strictly controlling the oxygen content in the aging furnace to ≤300 ppm can fundamentally inhibit the oxidation reaction on the surface of the electrical fittings. Through precise control of the aging furnace atmosphere, surface oxidation of electrical fittings during the aging process can be effectively avoided, ensuring a clean product appearance, while preventing the oxide layer from adversely affecting the performance of the welded joint, further improving the product's service reliability and service life.

[0085] Based on the test results of Examples 1 and 2 and Comparative Examples 1, 2, and 3, it can be seen that the low-temperature aging strengthening and toughening welding process for layered welding of power fittings of the present invention, through the synergistic effects of surface activation, gradient heat input welding, ternary mixed protective gas, interlayer ultrasonic impact, liquid nitrogen atomization cooling, and deformation-induced low-temperature dual-stage aging, can effectively improve the strength and toughness of welded joints of power fittings made of different materials, avoid weld defects, and ensure product service reliability. The key process links cooperate with each other, which has significant technical advantages and wide application value.

[0086] As an additional note, in the physicochemical property tests of any of the foregoing embodiments / comparative examples: 1. Tensile property test: According to GB / T2651-2008 "Tension test method for welded joints", the test was carried out using a universal testing machine. The test temperature was room temperature (25℃) and the loading rate was 2mm / min. Three parallel specimens were prepared for each example / comparative example, and the average value of the test results was taken as the final data.

[0087] 2. Microhardness test: According to GB / T4340.1-2009 "Metallic materials Vickers hardness test - Part 1: Test method", a Vickers hardness tester was used for testing. The test load was 100g and the holding time was 10s. Five test points were selected along the weld center, heat-affected zone and base metal area of ​​the weld joint cross section, and the average value of each area was taken.

[0088] 3. Fatigue performance test: According to GB / T6398-2017 "Method for controlling axial force in fatigue testing of metallic materials", the fatigue testing machine was used for testing. The test temperature was room temperature (25℃), the loading method was axial symmetrical cyclic loading, the stress ratio R=0.1, the loading frequency was 10Hz, and the number of cycles at which the specimen fractured was taken as the fatigue life. Three parallel specimens were prepared for each example / comparative example, and the minimum value was taken as the evaluation data.

[0089] 4. Weld defect detection: In accordance with GB / T11345-2013 "Manual ultrasonic testing method and classification of flaw detection results for welds", an ultrasonic flaw detector is used for detection. The probe frequency is 2.5MHz, and the detection range covers the entire weld area to evaluate the internal defect level of the weld. At the same time, macroscopic metallographic observation is used to observe the weld formation and internal structure uniformity.

[0090] 5. Visual inspection: In accordance with GB / T2314-2017 "General Technical Conditions for Power Fittings", visual inspection shall be carried out. The inspection environment shall be natural light or 400-600lx white light illumination, and the inspection distance shall be 30-50cm. Focus on observing the color and external texture of the weld and surrounding area to determine whether there are appearance defects such as oxidation discoloration, cracks, lack of fusion, and spatter residue.

[0091] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-temperature aging and toughening welding process for layered welding of power fittings, characterized in that, It includes the following steps: S1) Joint design and surface activation Design an asymmetric groove according to the thickness of the base metal of the electrical fitting, and perform surface oxide film removal and chemical activation treatment; S2) Gradient heat input multi-layer welding Adopt a multi-layer and multi-pass welding process. Based on the heat dissipation conditions and tissue requirements at different depths of the weld, establish a heat input gradient model. The linear energy ratio of the root pass layer, fill pass layer and cap pass layer in the gradient heat input model is set as E1:E2:E3 to limit the grain coarsening in the heat affected zone while ensuring the penetration depth; S3) Interlayer ultrasonic impact high-energy in-situ heat treatment When each weld is within the solid-state phase transformation temperature range, apply high-frequency ultrasonic impact to introduce residual compressive stress on the weld surface and refine the subgrain structure, and then perform forced convection heat transfer until the interlayer temperature drops to the critical value; S4) Deformation-induced low-temperature two-stage aging After welding, no solution treatment is carried out. Directly place the electrical fitting in the aging furnace, and use the lattice distortion energy introduced in step S3 as the precipitation driving force to perform two-stage low-temperature aging treatment to induce the dispersion precipitation of strengthening phases at dislocation lines and subgrain boundaries.

2. The low-temperature aging and toughening welding process for layered welding of power fittings according to claim 1, characterized in that, In step S1, the surface activation treatment includes: Use a 15% NaOH solution to pickling at 50°C for 3 minutes to remove the original oxide layer at the welding part of the electrical fitting; Subsequently, perform passivation light pickling in a 20% HNO3 solution containing 3 g / L ammonium fluoride to form an active micro-rough structure on the surface of the base metal, and control the roughness Ra within 3.2 - 6.3 μm.

3. The low-temperature aging and toughening welding process for layered welding of power fittings according to claim 1, characterized in that, In step S2, the specific parameter settings of the heat input gradient model are: The root pass layer uses cold metal transfer or pulsed MIG welding, and the linear energy E1 is set to 0.6 - 0.9 kJ / cm; The fill pass layer uses a high deposition rate spray transfer mode, and the linear energy E2 is set to 1.4 - 1.8 kJ / cm; The cap pass layer uses low heat input pulsed weaving welding, and the linear energy E3 is set to 1.0 - 1.3 kJ / cm; The relationship of the linear energy ratio satisfies E1 < E3 < E2 to form a gradient heat distribution characteristic of low heat input for the root pass layer and cap pass layer and high heat input for the fill pass layer, and use the low heat input of the cap layer to have a subsequent heat effect on the fill pass layer to achieve the tempering effect.

4. The low-temperature aging and toughening welding process for layered welding of power fittings according to claim 1, characterized in that, In step S2, a ternary mixed shielding gas is used in the welding process, and its volume percentage composition is: 30% - 40% helium, 0.03% - 0.05% nitrogen, and the balance argon; Among them, helium is suitable for increasing the arc stiffness and the wettability of the molten pool, and trace nitrogen is suitable for forming fine and dispersed nitride cores in the weld metal to inhibit grain growth.

5. The low-temperature aging and toughening welding process for layered welding of power fittings according to claim 1, characterized in that, In step S2, the wire selection scheme is determined according to the base metal material of the electrical fitting: If the electrical fitting is a 6000 series aluminum alloy, the wire selected contains 0.10% - 0.20% zirconium and 0.05% - 0.10% erbium by mass fraction; If the electrical fitting is not a 6000 series aluminum alloy, a standard wire matching the base metal material is selected; In the 6000 series aluminum alloy welding wire, zirconium and erbium form Al3Zr and Al3Er nanoparticles, respectively. These nanoparticles, as heterogeneous nucleation sites, work synergistically with the high-frequency ultrasonic impact in step S3 on the weld structure to induce a non-dendritic transformation of the weld structure.

6. The low-temperature aging and toughening welding process for layered welding of power fittings according to claim 1, characterized in that, In step S3, the specific process parameters for the high-frequency ultrasonic impact are as follows: The impact frequency is 20-30kHz, and the impact needle amplitude is 30-50μm; The impact treatment temperature window is 120℃-180℃; The impact coverage area includes the weld metal and the area extending 3-5 mm beyond the weld toe; By impacting the weld surface to create a plastic deformation layer with a depth of 20-40μm, the surface grains are refined to the 5-10μm level.

7. The low-temperature aging and toughening welding process for layered welding of power fittings according to claim 1, characterized in that, The forced convection heat transfer uses a liquid nitrogen atomization cooling system. The nozzle outlet temperature is controlled between -40°C and -20°C, and the cooling rate is controlled between 15°C / s and 25°C / s. The next welding step can only be carried out when the interlayer temperature drops below 50°C.

8. The low-temperature aging and toughening welding process for layered welding of power fittings according to claim 1, characterized in that, In step S4, the specific process parameters for the deformation-induced low-temperature two-stage aging are as follows: Primary nucleation aging: Heat to 95℃±3℃ at a rate of 2℃ / min and hold for 3-5 hours to promote uniform nucleation in the GP region; Secondary stabilization aging: The temperature is increased to 140℃±5℃ at a rate of 1℃ / min, and held for 8-10 hours to control the precipitation size of the β'' or η' phase in the range of 5-20nm. Then, it is cooled to 60℃ along with the aging furnace that performs deformation-induced low-temperature two-stage aging treatment before being removed from the furnace and air-cooled.

9. The low-temperature aging and toughening welding process for layered welding of power fittings according to claim 1, characterized in that, In step S4, during the deformation-induced low-temperature two-stage aging treatment, an inert protective gas needs to be introduced into the aging furnace. The inert protective gas is high-purity argon with a purity ≥99.99%, and the gas flow rate is controlled at 15-25 L / min. The oxygen content in the aging furnace is ≤300 ppm.

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