Gradient copper plating method and er50-6 solid wire based on the method

Abstract

The application relates to a gradient copper plating method and an ER50-6 solid welding wire based on the method, and at least comprises the following steps: first low-temperature bottom layer electroplating: first electroplating is carried out in a first bottom layer electroplating solution at 26-31 DEG C to form an ultra-fine grain combined bottom layer; second middle-temperature surface layer electroplating: second electroplating is carried out in a second bottom layer electroplating solution at 48-53 DEG C to form a wear-resistant surface layer on the surface of the ultra-fine grain combined bottom layer. The low-temperature condition is used to inhibit the rapid crystallization of copper ions, copper atoms are uniformly nucleated along the micro concave-convex sites of the matrix, penetrate a trace amount of residual oxidized layer at the interface, form a metallurgical bonding interface, and eliminate interlayer defects.

Description

Technical Field

[0001] This invention belongs to the field of welding wire electroplating technology, specifically relating to a gradient copper plating method with high interfacial bonding strength and low welding spatter, and an ER50-6 solid core welding wire based on this method. Background Technology

[0002] ER50-6 (corresponding to GB / T 8110 ER50-6 and AWS ER70S-6) is currently the most widely used general-purpose solid welding wire for gas shielded welding in China, and is widely used in steel structure manufacturing, engineering machinery welding, shipbuilding, pressure vessel and pipeline installation, and other fields. The finished product of ER50-6 solid welding wire is a composite structure with a copper-plated layer on the surface of a steel substrate. The copper plating layer in ER50-6 solid welding wire performs three core functions: conductivity, lubrication, and protection.

[0003] To achieve the aforementioned copper plating structure, the industry has developed a technical approach based on acidic copper sulfate as the mother liquor and single-tank electroplating as the main process through long-term production practice. The industry has long held the following technical understanding: lowering the electroplating temperature significantly slows down the copper deposition rate, and the adhesion of the plating layer obtained under conventional pretreatment conditions at low temperatures is insufficient to meet the requirements of high-speed wire feeding. Therefore, existing mass production lines generally adopt a constant-temperature electroplating process of 40-45℃ to balance deposition efficiency and plating adhesion performance. This process deposits a single-layer copper plating on the surface of the continuously feeding wire substrate in a single pass using direct current electrodeposition. It has advantages such as low equipment investment, simple operation, and good compatibility with integrated production lines for continuous wire drawing, copper plating, and winding.

[0004] However, the above-mentioned technical routes have long been limited to the adjustment of plating solution formula and control of total plating thickness in terms of process optimization, resulting in several defects that cannot be solved simultaneously in actual production and use: (1) Oxide interlayer exists at the plating interface: Conventional pickling can only remove the thick oxide scale on the surface, and cannot effectively remove the grease remaining in the drawing process and the Si-Mn composite oxide particles embedded in the micro-pits of the substrate surface. The above residues form a discontinuous covering film on the surface of the welding wire, which blocks the direct contact between Cu2+ and Fe during electroplating, so that the Cu deposition layer cannot form a metal bond with the substrate at the interface. The copper layer is actually attached to the residue covering layer, and its bonding method is only mechanical interlocking. Under high-speed wire feeding friction and high-temperature thermal cycling of electric arc, the brittle residue layer cracks first, and the coating peels off in flakes. The copper shavings that fall off accumulate on the inner wall of the conductive nozzle, causing nozzle blockage and shutdown; (2) Single-temperature electroplating causes coating quality defects: constant medium-temperature electroplating copper crystallizes quickly, the coating grains are coarse, the internal tensile stress is high, and the number of micropores is large; under the high temperature of the welding arc, the gas inside the pores expands, which intensifies the splashing of molten droplets. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a gradient copper plating method.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: a gradient copper plating method, comprising at least the following steps: First-stage low-temperature underlayer electroplating: First-stage electroplating is performed in a first-stage underlayer electroplating solution at 26~31℃ to form an ultra-fine crystal bonding underlayer. The first-stage bottom layer electroplating solution comprises the following components: copper sulfate pentahydrate 125~140g / L, sulfuric acid 55~62g / L, chloride ions 32~38mg / L, interface wetting additive 0.6~1.0g / L, and the balance being deionized water; Second-stage medium-temperature surface plating: Second-stage electroplating is performed in a second-stage electroplating solution at 48~53℃ to form a wear-resistant surface layer on the surface of the ultrafine grain bonded substrate. The second-stage electroplating solution comprises the following components: copper sulfate pentahydrate 175~190g / L, sulfuric acid 42~48g / L, chloride ions 22~26mg / L, grain leveling agent 0.25~0.4g / L, internal stress retardant 0.15~0.22g / L, and the balance being deionized water.

[0007] Optimally, the interface wetting additive is a compound of polyethylene glycol and fatty alcohol polyoxyethylene ether in a mass ratio of 2.5~3.5:1, the grain leveling agent is sodium polydisulfide dipropane sulfonate, the internal stress retardant is a compound of PEG and modified polyethyleneimine alkyl derivative in a mass ratio of 1.5~2.5:1, and the chloride ions are provided by sodium chloride.

[0008] Ideally, before the first-stage low-temperature bottom plating step, a pre-plating substrate interface activation treatment is also included: ultrasonic treatment with an organic acid composite activation solution is used to control the substrate interface roughness Ra to 0.32~0.38μm, and to remove interface grease, Si-Mn oxides and free carbon.

[0009] Furthermore, after the second-stage medium-temperature surface plating, the following steps are also included: Pure water closed-loop rinsing: Pure water rinsing is performed using a two-way counter-current method, with a rinsing water temperature of 36~40℃; Low-temperature shaping and drying: hot air drying at 62~68℃.

[0010] Furthermore, the components in the organic acid composite activation solution, by mass percentage, are: citric acid 10%~14%, hydrogen peroxide 2.5%~4.5%, sodium dodecyl sulfonate 0.15%~0.35%, and the remainder is deionized water; The activation temperature is 42~46℃, and the ultrasonic activation time is 2.5~4min; after activation, it is rinsed with 0.38~0.42MPa high-pressure pure water for 70~90s.

[0011] Ideally, the current density of the first-order electroplating is 12~15 A / dm². 2 The second-order electroplating current density is 18~22 A / dm². 2 .

[0012] Ideally, the thickness of the ultrafine crystalline bonding substrate is 0.12~0.20μm, and the thickness of the wear-resistant surface layer is 0.75~0.95μm; The total thickness of the finished coating is 0.87~1.15μm, and the residual internal stress of the coating is ≤16MPa.

[0013] Another object of the present invention is to provide an ER50-6 solid welding wire, comprising an ER50-6 substrate and a double-layer gradient copper plating layer formed on the surface of the ER50-6 substrate, wherein the double-layer gradient copper plating layer is prepared by any of the above methods.

[0014] Ideally, the chemical composition of the ER50-6 matrix, by mass fraction, is as follows: C 0.065%~0.075%, Si 0.85%~0.93%, Mn 1.68%~1.74%, P≤0.015%, S≤0.009%, Cr≤0.025%, Ni≤0.025%, Cu≤0.010%, Mo≤0.004%, Al≤0.004%, Ti 0.006%~0.009%, V≤0.002%, with the balance being iron and unavoidable impurities.

[0015] Ideally, the porosity of the double-layer gradient copper plating is ≤0.08%, and the interfacial bonding strength is ≥5.8N / mm.

[0016] Due to the application of the above technical solutions, the present invention has the following advantages compared with the prior art: The gradient copper plating method of the present invention performs a first-stage low-temperature bottom layer electroplating (26~31℃) and a second-stage medium-temperature surface layer electroplating (48~53℃) in sequence. The low temperature condition inhibits the rapid crystallization of copper ions, and copper atoms uniformly nucleate along the micro-concave and convex sites of the substrate, penetrate the trace amount of residual oxide layer at the interface, form a metallurgical bonding interface, eliminate interlayer defects, and improve the anti-peeling ability of the plating layer from the root. The moderate temperature increase improves the deposition efficiency, and the combination of leveling and stress relief additives refines the surface grains, seals the micro-pores of the bottom layer, reduces the surface friction coefficient of the welding wire, and stabilizes the conductivity uniformity during the high-speed wire feeding process.

[0017] Furthermore, ultrasonic organic acid directional activation is preferred to precisely remove free carbon, metal oxides, and drawing lubricant residues from the interface, controlling the interface roughness within a narrow range, improving the wettability of the plating solution, and avoiding substrate corrosion caused by over-activation; the ultra-thin metallurgical underlayer ensures adhesion, the dense surface layer is responsible for lubrication and rust prevention, and the total thickness is controlled within a narrow range to balance wire feeding resistance, conductivity, and corrosion resistance, avoiding the two defects of traditional plating layers being too thick or too thin. Attached Figure Description

[0018] The following sections will describe some specific embodiments of the invention in a detailed manner, by way of example and not limitation, with reference to the accompanying drawings. In the drawings: Figure 1 Photographs of the plating layers for different embodiments: (a) typical copper layer of the solder wire in Example 1, (b) typical copper layer of the solder wire in Comparative Example 1. Detailed Implementation

[0019] The technical solutions of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0020] This invention provides a gradient copper plating method and an ER50-6 solid core welding wire based on this method. The overall process of the gradient copper plating method includes five core steps: pre-plating substrate interface activation treatment, first-stage low-temperature bottom layer electroplating, second-stage medium-temperature surface layer electroplating, pure water closed-loop rinsing, and low-temperature shaping and drying. The specific process is as follows: (1) Pre-plating substrate interface activation treatment: Before the first-stage low-temperature bottom layer electroplating step, the ER50-6 welding wire substrate is ultrasonically activated using an organic acid composite activation solution to precisely remove residual grease, Si-Mn oxides, and free carbon impurities from the substrate surface. At the same time, the substrate interface roughness Ra is precisely controlled within the range of 0.32~0.38μm to ensure good wettability of the plating solution and effectively avoid over-activation that could cause substrate corrosion defects. The organic acid composite activation solution is prepared by mass percentage as follows: citric acid 10%~14%, hydrogen peroxide 2.5%~4.5%, sodium dodecyl sulfonate 0.15%~0.35%, with the remainder being deionized water. The activation process parameters are controlled as follows: activation temperature 42~46℃, ultrasonic activation time 2.5~4min. After activation, the substrate is rinsed with 0.38~0.42MPa high-pressure pure water for 70~90s to thoroughly remove residual activation solution from the surface.

[0021] (2) First-stage low-temperature undercoat electroplating: The pretreated welding wire substrate is placed in a first-stage undercoat electroplating solution and electroplated at 26~31℃ and a current density of 12~15A / dm² to form a uniform and dense ultrafine crystalline bonding undercoat on the substrate surface. The first-stage undercoat electroplating solution includes the following components: copper sulfate pentahydrate 125~140g / L, sulfuric acid 55~62g / L, chloride ions 32~38mg / L, wherein the chloride ions are provided by sodium chloride (i.e., the corresponding concentration of chloride ions is obtained by adding chemically pure sodium chloride to the first-stage undercoat electroplating solution, the same below); interface wetting additive 0.6~1.0g / L, wherein the interface wetting additive is preferably a mixture of polyethylene glycol 4000 and C12~C14 fatty alcohol polyoxyethylene ether AEO-9 in a mass ratio of 2.5~3.5:1 (more preferably 3:1), and the balance is deionized water.

[0022] (3) Second-stage medium-temperature surface electroplating: After the bottom layer electroplating is completed, the welding wire is sent into the second-stage electroplating solution and the second-stage electroplating is carried out at 48~53℃ and current density of 18~22A / dm² to deposit a wear-resistant protective surface layer on the surface of the ultra-fine crystal bonded bottom layer. The second-stage electroplating solution comprises the following components: copper sulfate pentahydrate 175~190 g / L, sulfuric acid 42~48 g / L, chloride ions 22~26 mg / L, wherein the chloride ions are provided by sodium chloride; grain leveling agent 0.25~0.4 g / L, wherein the grain leveling agent is sodium polydithiopropane sulfonate (SPS); internal stress stabilizing agent 0.15~0.22 g / L, wherein the internal stress stabilizing agent is a mixture of polyethylene glycol 2000 and modified polyethyleneimine alkyl derivative (the modified polyethyleneimine alkyl derivative is C8~C14 long-chain alkyl hydrophobic grafted modified polyethyleneimine, preferably dodecyl hydrophobic modified polyethyleneimine (abbreviated as PEI-C12)) at a mass ratio of 1.5~2.5:1 (more preferably 2:1), and the balance is deionized water.

[0023] (4) Pure water closed-loop rinsing: After the second-stage electroplating is completed, the surface of the welding wire is cleaned by a two-way countercurrent pure water rinsing process. The rinsing water temperature is controlled at 36~40℃ to thoroughly remove residual plating solution and trace organic additives from the surface of the welding wire, so as to avoid defects in the subsequent plating layer.

[0024] (5) Low temperature setting and drying: After rinsing, the welding wire is placed in a hot air environment of 62~68℃ for setting and drying, so as to achieve rapid curing and setting of the coating and ensure that the coating structure is stable and uniform.

[0025] This invention employs a gradient temperature difference and gradient formulation differential electroplating core mechanism. The first-stage low-temperature process inhibits the rapid and disordered crystallization of copper ions, promoting uniform nucleation of copper atoms along the microscopic unevenness of the welding wire substrate. This allows copper atoms to penetrate the trace residual oxide layer on the substrate surface, constructing a high-strength metallurgical bonding interface. This completely eliminates defects such as interlayering, incomplete plating, and pinholes present in traditional single-layer copper plating, fundamentally improving the coating's anti-peeling and anti-copper-removal performance. The second-stage medium-temperature process effectively improves copper layer deposition efficiency. Combined with a dedicated grain leveling agent and internal stress relief agent, it precisely refines the surface copper grains, seals the micropores in the bottom layer, and relieves the internal stress of the thick coating lattice. Through narrow-range parameter control, this invention constructs a double-layer gradient structure of "ultra-thin metallurgical bonding bottom layer + dense wear-resistant surface layer," precisely balancing the welding wire feeding resistance, conductivity, and corrosion resistance. This significantly reduces welding arc fluctuations and welding spatter, avoiding the two core defects of traditional coatings: excessively thick coatings result in high resistance, excessively thick coatings are prone to wear, and excessively thin coatings provide poor protection.

[0026] The ER50-6 solid welding wire prepared based on the above gradient copper plating method includes an ER50-6 substrate and a double-layer gradient copper plating layer formed on the surface of the substrate. The thickness of the ultrafine grain bonding underlayer is 0.12~0.20μm, the thickness of the wear-resistant surface layer is 0.75~0.95μm, the total thickness of the finished double-layer gradient copper plating layer is controlled at 0.87~1.15μm, the residual internal stress of the plating layer is ≤16MPa, the porosity of the plating layer is ≤0.08%, and the interfacial bonding strength between the plating layer and the substrate is ≥5.8N / mm. The overall plating performance is excellent.

[0027] The chemical composition of the ER50-6 welding wire matrix, by mass fraction, is as follows: C 0.065%~0.075%, Si 0.85%~0.93%, Mn 1.68%~1.74%, P≤0.015%, S≤0.009%, Cr≤0.025%, Ni≤0.025%, Cu≤0.010%, Mo≤0.004%, Al≤0.004%, Ti 0.006%~0.009%, V≤0.002%, with the balance being iron and unavoidable impurities.

[0028] To further illustrate the technical solution and beneficial effects of the present invention in detail, the present invention will be described in detail below with reference to specific embodiments and comparative examples. The performance tests of all embodiments and comparative examples adopt a unified testing standard, as detailed below: Performance testing standards: The interfacial bonding strength is evaluated quantitatively using the cross-cut adhesion test combined with the peeling method, in accordance with GB / T 5270 "Test Method for Adhesion Strength of Electrodeposited and Chemically Deposited Coatings on Metallic Substrates". Coating porosity is assessed using the metallographic cross-section observation method, counting the number of pores per unit area of ​​the coating cross-section under a 500x optical microscope and converting it to an area percentage. The neutral salt spray test is conducted according to GB / T 10125 "Artificial Atmosphere Corrosion Test - Salt Spray Test", using continuous spraying at a temperature of (35±2)℃, a NaCl solution concentration of (50±5) g / L, and a pH value of 6.5~7.2. Corrosion is observed after continuous spraying. Weld spatter rate is assessed using the weighing method, weighing the test plate and welding torch protective cover before and after welding, and calculating the percentage of spatter mass relative to the total welding wire consumption. Contact resistance fluctuation is assessed using a DC resistance meter, continuously detecting the contact resistance between the welding wire and the contact tip under a steady-state welding current of 250A, and recording the percentage fluctuation. Example 1

[0029] This embodiment provides a gradient copper plating method and an ER50-6 solid core welding wire based on this method. The specific process steps are as follows: (1) Pre-plating activation: Prepare an organic acid composite activation solution by mass percentage: citric acid 12%, hydrogen peroxide 3.5%, sodium dodecyl sulfate 0.25%, with the remainder being deionized water. Select ER50-6 welding wire substrate, with the following chemical composition by mass fraction: C 0.070%, Si 0.90%, Mn 1.72%, P 0.012%, S 0.007%, Cr 0.018%, Ni 0.020%, Cu 0.008%, Mo 0.003%, Al 0.003%, Ti 0.008%, V 0.001%, with the remainder being iron. Place the welding wire substrate in the above organic acid composite activation solution, set the ultrasonic power intensity to 350W / m², the activation temperature to 44℃, and the ultrasonic activation time to 3min. After activation, the surface roughness Ra of the substrate interface was 0.35 μm. After activation, the substrate was rinsed with 0.40 MPa high-pressure pure water for 80 s to completely remove residual activation solution and reaction impurities.

[0030] (2) First-stage low-temperature bottom layer electroplating: The activated and rinsed welding wire substrate is placed in the first-stage bottom layer electroplating solution for electroplating to form a uniform and dense ultrafine crystal bonding bottom layer; the formula of the first-stage bottom layer electroplating solution is: copper sulfate pentahydrate 135g / L, sulfuric acid 60g / L, chloride ion 35mg / L (provided by chemically pure sodium chloride), interface wetting additive 0.8g / L (comprised of polyethylene glycol 4000 and C12~C14 fatty alcohol polyoxyethylene ether AEO-9 in a mass ratio of 3:1), and the balance is deionized water; the specific electroplating parameters are: liquid temperature 28℃, current density 13.5A / dm², and the thickness of the ultrafine crystal bonding bottom layer is 0.16μm.

[0031] (3) Second-stage medium-temperature surface electroplating: After the first-stage bottom layer electroplating is completed, the second-stage surface electroplating solution is immediately transferred to form a dense wear-resistant protective surface layer on the surface of the ultra-fine crystal bonded bottom layer. The formula of the second-stage surface electroplating solution is: 185g / L copper sulfate pentahydrate, 45g / L sulfuric acid, 24mg / L chloride ions (provided by chemically pure sodium chloride), 0.3g / L grain leveling agent (sodium polydisulfide dipropane sulfonate SPS), 0.18g / L internal stress retardant (composed of polyethylene glycol 2000 and modified polyethyleneimine alkyl derivative (PEI-C12) in a mass ratio of 2:1), with the remainder being deionized water. The specific electroplating parameters are: liquid temperature 50℃, current density 20A / dm², and the wear-resistant surface layer thickness is 0.84μm, with a total coating thickness of 1.00μm.

[0032] (4) Rinsing and drying: The bidirectional countercurrent pure water rinsing process is adopted, and the rinsing water temperature is 38℃. After rinsing, the 65℃ hot air is used for shaping and drying. The above pre-plating activation, double-stage electroplating, rinsing and drying processes are all carried out online continuously, and the wire feeding speed of the production line is 1.38m / s.

[0033] Finished product performance: interfacial bonding strength 6.1 N / mm, porosity 0.06%, resistance fluctuation 2.2%, splashing rate 0.95%, no rust or blackening in 72h neutral salt spray test. Example 2

[0034] This embodiment provides a gradient copper plating method and an ER50-6 solid core welding wire based on the method. The process steps, raw material formula, matrix composition and treatment method are completely consistent with those of Embodiment 1. The only difference is that the two-stage electroplating adopts the lower limit setting of the parameters of this invention: the first-stage electroplating bath temperature is 26℃, and the thickness of the ultrafine crystal bonding underlayer is 0.12μm; the second-stage electroplating bath temperature is 48℃, and the thickness of the wear-resistant surface layer is 0.75μm, with a total coating thickness of 0.87μm.

[0035] Finished product performance: interfacial bonding strength 5.8 N / mm, porosity 0.07%, resistance fluctuation 2.6%, splashing rate 1.10%, no rust in 72h neutral salt spray test. Example 3

[0036] This embodiment provides a gradient copper plating method and an ER50-6 solid core welding wire based on the method. The process steps, raw material formula, matrix composition and treatment method are completely consistent with those of Embodiment 1. The only difference is that the two-stage electroplating adopts the upper limit setting of the parameters of this invention: the first-stage electroplating liquid temperature is 31°C, and the thickness of the ultrafine crystal bonding underlayer is 0.20μm; the second-stage electroplating liquid temperature is 53°C, and the thickness of the wear-resistant surface layer is 0.95μm, with a total coating thickness of 1.15μm.

[0037] Finished product performance: interfacial bonding strength 5.9 N / mm, porosity 0.08%, resistance fluctuation 2.8%, splashing rate 1.15%, no rust in 72h neutral salt spray test.

[0038] Comparative Example 1 This comparative example uses traditional single-tank single-layer electroplating to provide a copper plating method and an ER50-6 solid core welding wire based on this method. The substrate composition, pre-plating activation, rinsing and drying process, and total plating thickness are all the same as in Example 1. The only difference is that the gradient two-stage electroplating process of this invention is cancelled, the low-temperature bottom layer electroplating and medium-temperature surface layer electroplating are not performed in stages, a single tank and a single temperature electroplating are used, the electroplating solution is the second-stage surface electroplating solution of Example 1, the electroplating temperature is 42℃, the current density is 20A / dm², the electroplating is completed in one go, and the total plating thickness is controlled to be 1.0μm.

[0039] Finished product performance: interfacial bonding strength 2.2 N / mm, porosity 1.12%, resistance fluctuation 7.6%, splashing rate 2.10%, localized pitting corrosion appeared in the 48h neutral salt spray test.

[0040] Comparative Example 2 This comparative example uses a gradient-free, isothermal two-stage electroplating method to provide a gradient copper plating method and an ER50-6 solid core welding wire based on this method. The substrate composition, pre-plating activation, two-stage electroplating solution formulation, current density, plating thickness, and rinsing and drying process are all the same as in Example 1. The only difference is that the gradient temperature difference core process is cancelled, and the first-stage and second-stage electroplating temperatures are uniformly set to 42℃, without temperature differentiation control between the low-temperature bottom layer and the medium-temperature surface layer.

[0041] Finished product performance: interfacial bonding strength 3.5 N / mm, porosity 0.45%, resistance fluctuation 5.2%, splashing rate 1.70%, slight surface blackening and no obvious corrosion were observed in the 60h neutral salt spray test.

[0042] Comparative Example 3 This comparative example uses conventional hydrochloric acid pickling without ultrasonic precise activation to provide a gradient copper plating method and an ER50-6 solid core welding wire based on this method. The two-stage electroplating formula, process parameters, plating thickness, and rinsing and drying process are all the same as in Example 1. The only difference is that the organic acid composite ultrasonic activation process of this invention is cancelled. Before plating, conventional industrial dilute hydrochloric acid (hydrochloric acid concentration of 130g / L) is used for pickling, without ultrasonic-assisted activation, and only static immersion pickling for 3 minutes is performed.

[0043] Finished product performance: interfacial bonding strength 3.1 N / mm, porosity 0.58%, resistance fluctuation 5.8%, splashing rate 1.85%, and localized fine rust spots appeared in the 54-hour neutral salt spray test.

[0044] The above embodiments are only for illustrating the technical concept and features of the present invention. Their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be used to limit the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A gradient copper plating method, characterized in that, At least the following steps are included: First-stage low-temperature underlayer electroplating: First-stage electroplating is performed in a first-stage underlayer electroplating solution at 26~31℃ to form an ultra-fine crystal bonding underlayer. The first-stage bottom layer electroplating solution comprises the following components: copper sulfate pentahydrate 125~140g / L, sulfuric acid 55~62g / L, chloride ions 32~38mg / L, interface wetting additive 0.6~1.0g / L, and the balance being deionized water; Second-stage medium-temperature surface plating: Second-stage electroplating is performed in a second-stage electroplating solution at 48~53℃ to form a wear-resistant surface layer on the surface of the ultra-fine grain bonded substrate. The second-stage electroplating solution comprises the following components: copper sulfate pentahydrate 175~190g / L, sulfuric acid 42~48g / L, chloride ions 22~26mg / L, grain leveling agent 0.25~0.4g / L, internal stress retardant 0.15~0.22g / L, and the balance being deionized water.

2. The gradient copper plating method according to claim 1, characterized in that: The interface wetting additive is a compound of polyethylene glycol and fatty alcohol polyoxyethylene ether in a mass ratio of 2.5~3.5:1; the grain leveling agent is sodium polydisulfide dipropane sulfonate; the internal stress retardant is a compound of PEG and modified polyethyleneimine alkyl derivative in a mass ratio of 1.5~2.5:1; and the chloride ions are provided by sodium chloride.

3. The gradient copper plating method according to claim 1, characterized in that: Before the first-stage low-temperature bottom plating step, a pre-plating substrate interface activation treatment is also included: ultrasonic treatment is performed using an organic acid composite activation solution to control the substrate interface roughness Ra to 0.32~0.38μm, and to remove interface grease, Si-Mn oxides and free carbon.

4. The gradient copper plating method according to claim 1 or 2, characterized in that: Following the second-stage medium-temperature surface plating, the following steps are also included: Pure water closed-loop rinsing: Pure water rinsing is performed using a two-way counter-current method, with a rinsing water temperature of 36~40℃; Low-temperature shaping and drying: hot air drying at 62~68℃.

5. The gradient copper plating method according to claim 2, characterized in that, The components of the organic acid composite activation solution, by mass percentage, are: citric acid 10%~14%, hydrogen peroxide 2.5%~4.5%, sodium dodecyl sulfonate 0.15%~0.35%, with the remainder being deionized water; The activation temperature is 42~46℃, and the ultrasonic activation time is 2.5~4min; after activation, it is rinsed with 0.38~0.42MPa high-pressure pure water for 70~90s.

6. The gradient copper plating method according to claim 1, characterized in that: The current density of the first-stage electroplating is 12~15A / dm², and the current density of the second-stage electroplating is 18~22A / dm².

7. The gradient copper plating method according to claim 1, characterized in that: The thickness of the ultrafine crystalline bonding substrate is 0.12~0.20μm, and the thickness of the wear-resistant surface layer is 0.75~0.95μm; The total thickness of the finished coating is 0.87~1.15μm, and the residual internal stress of the coating is ≤16MPa.

8. An ER50-6 solid core welding wire, comprising an ER50-6 substrate and a double-layer gradient copper plating formed on the surface of the ER50-6 substrate, characterized in that: The double-layer gradient copper plating is prepared by the method described in any one of claims 1 to 6.

9. The ER50-6 solid welding wire according to claim 8, characterized in that, The chemical composition of the ER50-6 matrix, by mass fraction, is as follows: C 0.065%~0.075%, Si 0.85%~0.93%, Mn 1.68%~1.74%, P≤0.015%, S≤0.009%, Cr≤0.025%, Ni≤0.025%, Cu≤0.010%, Mo≤0.004%, Al≤0.004%, Ti 0.006%~0.009%, V≤0.002%, with the balance being iron and unavoidable impurities.

10. The ER50-6 solid welding wire according to claim 8, characterized in that: The double-layer gradient copper plating has a plating porosity of ≤0.08% and an interfacial bonding strength of ≥5.8N / mm.

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