Fine copper wire-oriented high-performance laser spot welding process and equipment

Abstract

The application discloses a kind of high-performance laser spot welding processes and equipment for fine copper wire, the process includes the following steps: before welding, by coating a layer of coating on the surface of copper wire to be welded, to improve the laser absorption rate of copper wire;Two copper wires to be welded contain the side of coating and are fixed on the welding tool, according to the diameter of copper wire to be welded, select corresponding welding parameters;Adjust the laser focal point to be located at the butt joint of copper wire to be welded, emit laser to complete spot welding;The application solves the problem of high laser reflectivity of copper wire welding and the positioning of copper wire to be welded;The application can also greatly reduce the reflectivity of fine copper wire to laser, while effectively improving the positioning efficiency of spot welding, with the advantages of simple process, high precision, good welding quality, high efficiency and the like.

Description

Technical Field

[0001] This invention belongs to the field of copper wire laser welding, and specifically relates to a high-performance laser spot welding process and equipment for fine copper wires. Background Technology

[0002] Copper and its alloys possess excellent ductility, thermal and electrical conductivity, corrosion resistance, and processing performance, making them widely used in electrical, chemical, and mechanical fields. Copper wire is commonly used in products such as conductors, coils, and springs. With industrial development, the demand for copper wire connections is increasing. However, due to the small size of copper wires, especially those with a diameter of 0-3mm, positioning the welding area is difficult, making connection challenging.

[0003] Traditional arc welding methods, such as gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), and manual arc welding, suffer from large heat source volume, high heat input, and low energy density, often leading to problems like melting and deformation when welding copper wire. Laser welding offers advantages such as high positioning accuracy, high energy density, and low heat input, making it more suitable for joining smaller copper wire products. However, copper at room temperature has a reflectivity of approximately 95% for commonly used near-infrared lasers, resulting in poor laser welding stability and a tendency for poor weld formation and spatter. Furthermore, reflected laser light can easily damage operators and equipment. In contrast, copper has a higher absorption rate for blue lasers at room temperature, but it still does not exceed 50%. Moreover, the development of commercial blue lasers is currently immature, with low power and high cost, making their industrial application still a long way off.

[0004] To address the issue of copper's high reflectivity to near-infrared lasers, patent CN109014570A discloses a laser welding method and apparatus for copper, employing a combination of green and near-infrared lasers to weld copper workpieces. Similarly, CN104907695A discloses a laser welding apparatus and method for copper, which preheats the copper plate using heating and heat preservation mechanisms to improve the copper plate's laser absorption rate. However, these existing technologies still have the following shortcomings: First, while they primarily improve copper's laser absorption rate through preheating, copper's absorption rate to near-infrared lasers in its molten state is still less than 15%, resulting in extremely limited effectiveness. Second, they do not consider the welding point positioning issues for small, high-precision workpieces like copper wires. In conclusion, existing technologies cannot yet achieve effective connection of copper wires. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned defects in the prior art and provide a high-performance laser spot welding process and equipment for fine copper wires. This process can greatly reduce the reflectivity of fine copper wires to lasers and effectively improve spot welding positioning efficiency. It has the advantages of simple process, high precision, good welding quality and high efficiency.

[0006] To achieve the above objectives, the present invention provides a high-performance laser spot welding process for fine copper wires, comprising the following steps: Step S1: Before welding, a coating is applied to the surface of the copper wire to be welded to improve the absorption rate of the copper wire to the laser. Step S2: Attach the coated sides of the two copper wires to be welded to the welding fixture, and select the appropriate welding parameters according to the diameter of the copper wires to be welded; Step S3: Adjust the laser focus to the joint of the copper wires to be welded, and emit the laser to complete the spot welding.

[0007] Preferably, the width of the coating should be greater than the radius of the laser spot used for spot welding.

[0008] Preferably, the coating extends to the circumferential surface of the copper wire and also to the vertical cross-section of the copper wire to be welded.

[0009] Preferably, the welding parameters include laser power, laser defocusing amount, and spot welding time; the laser power is determined according to the diameter of the copper wire to be welded, and the laser power range is 100~1000W when the diameter of the copper wire to be welded is less than 3mm; the laser defocusing amount ranges from -2 to +2mm; and the spot welding time ranges from 0.05 to 0.6s.

[0010] Preferably, the coating is prepared by coating, spraying, wetting or electroplating processes.

[0011] Preferably, the laser spot welding process can employ either pulsed laser mode or continuous laser mode.

[0012] This invention also provides a high-performance laser spot welding equipment for fine copper wires, used to perform the high-performance laser spot welding process for fine copper wires described above; it also includes a three-axis motion mechanism, a weld seam identification device, a laser welding head, a C-frame, and welding fixtures; the three-axis motion mechanism is mounted on the top of the C-frame; the laser welding head is mounted on the three-axis motion mechanism and is driven by the three-axis motion mechanism to move freely in three-dimensional space, thereby adjusting the laser spot position and focal length; the laser welding head emits a laser beam, which acts on the joint of the copper wires to be welded, thereby melting the copper wires to complete the welding.

[0013] Preferably, the laser beam focusing diameter of the laser welding head is 0.1~0.4mm; the weld identification device is coaxially mounted with the laser welding head and is used to identify and locate the joint of the copper wires to be welded.

[0014] Preferably, the welding fixture is set at the bottom of the C-shaped frame to fix the copper wire to be welded, ensuring that the copper wire does not deform after welding.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention provides a high-performance laser spot welding process for fine copper wires. By applying a coating to the surface of the copper wire to be welded through a surface pretreatment process, the absorption rate of the copper wire to the laser is greatly improved, which can stabilize the welding process and improve the spot welding quality.

[0016] 2. This invention provides a high-performance laser spot welding equipment for fine copper wires. By adopting a weld seam recognition device, it can achieve rapid positioning of the weld points of the copper wire to be welded, which greatly improves the spot welding efficiency. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flow chart of a high-performance laser spot welding process for fine copper wires provided by the present invention; Figure 2 This is a schematic diagram of a coating provided by the present invention that can improve the laser absorption rate of copper wire; Figure 3 This is a scanning electron microscope image of a spring-type copper wire completed according to a high-performance laser spot welding process for fine copper wire provided by the present invention. Figure 4 This is a schematic diagram of a high-performance laser spot welding equipment for fine copper wires provided by the present invention.

[0019] The diagram includes: 1. Copper wire C1 to be welded; 2. Copper wire C2 to be welded; 3. Coating; 4. Three-axis motion mechanism; 5. Weld seam identification device; 6. Laser welding head; 7. C-frame; 8. Tooling bracket; 9. Welding fixture; 10. Joint of copper wires to be welded; d. Width of coating. 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 one embodiment 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] Example 1: Please refer to Figures 1 to 3 This invention provides a high-performance laser spot welding process for fine copper wires.

[0022] like Figure 1 As shown, a high-performance laser spot welding process for fine copper wire constructed according to Embodiment 1 includes the following steps: Step 1: Before welding, a coating is applied to the surface of the copper wire to be welded to significantly improve the absorption rate of the copper wire to the laser.

[0023] Furthermore, in this first embodiment, the coating is a multi-component composite material with a higher laser absorption rate than copper; this improves laser energy absorption efficiency and reduces reflection loss, thus enabling better welding of copper wires, better protection of the laser welding head 6, and extension of the lifespan of the laser welding head 6; it also allows for high-quality welding with lower laser power, reducing equipment energy consumption and the range of the heat-affected zone.

[0024] Furthermore, the above surface treatment process can greatly improve the absorption rate of the copper wire to the laser, thereby stabilizing the welding process and improving the efficiency and quality of copper wire spot welding.

[0025] To avoid introducing impurities into the coating that could affect the welding quality, the surface of the copper wire to be welded must be wiped with organic solvents such as acetone before the surface treatment process to remove oil and impurities, and then the copper wire to be welded must be dried.

[0026] like Figure 2 As shown, coating 3 exists on the surfaces of copper wires C1 and C2 to be welded. The width d of the coating should not be less than the radius of the laser spot used for spot welding, so as to ensure that the laser can act uniformly on the surface of the copper wires to be welded and stabilize the welding process.

[0027] In this first embodiment, the position and range of the coating 3 are not limited or fixed. Generally, the coating 3 can cover the circumferential side of the copper wire to be welded, or it can be set on the cross-section of the joint of the copper wire to be welded, which is circular.

[0028] Step 2: Attach the two copper wires to be welded together on the welding fixture with the coated side facing each other, and select the appropriate welding parameters according to the diameter of the copper wires to be welded.

[0029] Furthermore, the welding parameters include laser power, laser defocusing amount, and spot welding time.

[0030] In this first embodiment, the laser power is determined based on the diameter of the copper wire to be welded. When the diameter of the copper wire to be welded is less than 3mm, the laser power ranges from 100 to 1000W; the laser defocusing amount ranges from -2 to +2mm; and the spot welding time ranges from 0.05 to 0.6s. This first embodiment does not limit or fix the welding parameters.

[0031] Step 3: Adjust the laser focus to the center of the copper wire joint to be welded, and emit the laser to complete the spot welding.

[0032] Furthermore, the laser spot welding process can employ near-infrared lasers, green lasers, and blue lasers, and the laser mode can be either pulsed laser mode or continuous laser mode. Of course, this embodiment does not limit or fix the laser wavelength and mode.

[0033] Since the absorption rate of copper wire for green and blue lasers is significantly higher than that for near-infrared lasers, green and blue lasers can convert energy into heat more efficiently at the same power. Near-infrared lasers have a low absorption rate, which can easily lead to energy waste and unstable welding. Therefore, the above method has significant process optimization value for near-infrared lasers (λ≈1064nm) – by improving the surface absorption rate, it can effectively make up for its inherent defects, resulting in more uniform weld penetration, less spatter, and denser weld formation.

[0034] like Figure 3 As shown, the high-performance laser spot welding process for fine copper wires provided by this invention achieves high-performance laser spot welding of 1.2mm diameter fine copper wires by coating the surface to be welded. The welds are aesthetically pleasing and free from defects such as spatter, porosity, and cracks. Actual testing verified that this process achieves a 99.8% first-pass yield in batch welding, with the weld tensile strength consistently at 185±5MPa, far exceeding the 150MPa benchmark value of traditional processes. Furthermore, the heat-affected zone width is reduced to less than 45μm, significantly improving the conductivity and service reliability of the joint.

[0035] Example 2: The present invention also provides a laser spot welding equipment for performing the above-mentioned high-performance laser spot welding process for fine copper wires. The equipment mainly includes a three-axis motion mechanism 4, a weld seam recognition device 5, a laser welding head 6, a C-frame 7, and a welding fixture 9.

[0036] In the actual assembly process, such as Figure 4 As shown, the C-shaped frame 7 has a certain space inside for installing equipment and welding copper wires; the three-axis motion mechanism 4 is installed on the top of the C-shaped frame 7 and has enough space to move freely in the three-dimensional space inside the C-shaped frame 7.

[0037] The laser welding head 6 is mounted on the three-axis motion mechanism 4. The laser spot position and focal length are adjusted by the three-axis motion mechanism 4, and then the laser is emitted to act on the joint 10 of the copper wires to be welded, melting the copper wires and completing the welding.

[0038] Furthermore, the laser beam focusing diameter of the laser welding head 6 is 0.1~0.4mm. The weld seam identification device 5 is coaxially mounted with the laser welding head 6 and is controlled by an external computer, enabling it to accurately identify and locate the joint 10 of the copper wires to be welded.

[0039] like Figure 4 As shown, the welding fixture 9 is fixed to the bottom of the C-shaped frame 7 via the fixture bracket 8, and is used to fix the copper wires to be welded, ensuring that the copper wires do not deform after welding. During welding, the welding fixture 9 fixes the copper wires C1 and C2 to be welded respectively; after being precisely clamped, the copper wires C1 and C2 can move laterally, approaching each other until their end faces are tightly fitted, and can also move longitudinally to adjust the gap between adjacent wires. The change in the gap between the end faces of the copper wires is fed back in real time by a high-precision displacement sensor, ensuring that the docking accuracy is better than ±2μm, creating ideal initial conditions for precise laser melting; at the same time, during the welding process, the copper wires C1 and C2 can also automatically approach each other to compensate for the micro-displacement caused by melt shrinkage, maintaining continuous tight contact between the end faces.

[0040] In this embodiment, the type, quantity, and position of the welding fixture 9 can be flexibly adjusted according to actual needs to adapt to copper wires of different diameters. The welding fixture 9 has a spring inside, which uses elastic force to flexibly press the copper wire to avoid micro-deformation or surface scratches caused by rigid clamping; it can also adapt to copper wires of different diameters to achieve adaptive clamping.

[0041] Example 3: Copper has a reflectivity of over 95% for near-infrared lasers (such as fiber lasers, λ≈1064nm), making direct welding extremely difficult. The core task of the light-absorbing coating is to efficiently convert light energy into heat energy and transfer it to the copper substrate within a very short time (milliseconds) of laser irradiation.

[0042] The coating must have both strong light absorption and controllable thermal conductivity—too much light absorption but too little thermal conductivity will cause heat to accumulate on the surface and not be able to be transferred to the welding interface, which is meaningless; it must have good adhesion and be easy to remove—the coating must adhere firmly before welding and the residue must be wiped off easily after welding, so as not to become a contaminant; it must be stable at high temperatures and not ablate or splatter—the coating must not disintegrate and splatter under the instantaneous high temperature of the laser, thus not contaminating the optical lens, nor should it vaporize and generate back pressure that blows away the molten pool; it must be thin and evenly coated—the diameter of the fine copper wire is only a few tenths of a millimeter; if the coating is too thick, it will change the size of the weld joint; if it is too thin, the light absorption will be insufficient; and if it is uneven, the welding quality will be inconsistent.

[0043] The coating components (based on a total mass of 100g) include: graphite: 30.0~35g, carbon black: 10.0~16g, resin: 25.0~35.0g, bentonite: 4.0~7.0g, borax: 2.0~5.0g, water: 15.0~20.0g, and a dry film thickness of 20.0~25.0μm. Preferably, the composition is: graphite 32g, carbon black 13g, resin 30g, bentonite 5g, borax 3g, water 17g, and a dry film thickness of 22μm.

[0044] Among them, 32g of flake graphite powder is used as the main absorber; 13g of pigment carbon black is used as the auxiliary absorber / broad-spectrum enhancer; 30g of waterborne acrylic resin (40% solid content) is used as the binder and film-forming agent; 5g of organic modified bentonite is used as the thickening thixotropic agent; 3g of sodium tetraborate (borax) is used as the fluxing and dispersing agent; and 17g of deionized water is used as the diluent.

[0045] The mechanism of action of each component in the formulation: The flake graphite powder, as the main light-absorbing framework, has a unique layered crystal structure. Within the layers, carbon atoms form a conjugated π-bond system with sp² hybridization, allowing electrons to move freely along the layer plane. When irradiated by near-infrared laser, the π electrons undergo collective oscillation to absorb photon energy. This absorption mechanism is essentially similar to the plasma absorption of metals, resulting in extremely high absorption efficiency. The reasons for choosing flake graphite instead of spherical graphite are as follows: During the coating drying process, flake graphite naturally tends to align parallel to the substrate surface, forming a stacked "scale structure." This structure causes the laser to undergo multiple reflections and absorptions within the coating (similar to the blackbody cavity effect), significantly increasing the effective optical path. Flake graphite exhibits anisotropic thermal conductivity—fast in-plane thermal conductivity and slow inter-plane thermal conductivity. This is precisely what facilitates the uniform diffusion of heat along the coating plane before slowly transferring it to the copper substrate, avoiding localized overheating and ablation. The dosage of 32g is based on the following balance: if it exceeds 40g, the coating becomes too "loose," increasing the contact resistance between the flake graphite pieces and reducing the heat transfer efficiency; if it is below 25g, the absorbance density is insufficient.

[0046] The pigment-grade carbon black exhibits broad-spectrum absorption supplementation and enhanced absorption density. The absorption mechanism of carbon black differs from that of graphite—carbon black is amorphous carbon with numerous defect sites and dangling bonds. These defect energy levels can absorb photons across a wider wavelength range. The reason for choosing pigment-grade carbon black (particle size approximately 20-40 nm, specific surface area >200 m² / g) instead of ordinary conductive carbon black or furnace-processed carbon black is its extremely small particle size and enormous specific surface area, forming numerous nanoscale "light traps" in the coating. The absorbance of pigment-grade carbon black is high in the near-infrared range. With a light absorption rate exceeding 98%, it is one of the carbon materials with the highest known light absorption efficiency. Its extremely small primary particle size (nanoscale) gives it better suspension stability in slurry. The dosage of 13g is based on the following deduction: when the mass ratio of carbon black to graphite is about 1:2.5, the carbon black nanoparticles can just fill the gaps formed by the stacking of sheet-like graphite microflakes (particle size about 5-15μm), realizing a densely packed structure of "micron flakes + nanoparticles", so that the coating can achieve high-efficiency absorption even at a very thin thickness (<50μm).

[0047] The water-based acrylic resin is chosen for bonding and film formation. The reasons for choosing water-based acrylic resin over epoxy or polyurethane are as follows: the molecular chain of acrylic resin contains carboxyl groups (-COOH), which can form coordination bonds or hydrogen bonds with the copper surface, providing adhesion without damaging the copper wire surface; the water-based system decomposes into CO2 and H2O during laser welding, releasing no corrosive gases, thus not contaminating optical lenses or corroding the welding area; the decomposition temperature range of acrylic resin (250-400℃) is just below the melting point of copper (1083℃) but above the boiling point of water, ensuring that the coating dries and cures first during the initial laser heating stage, followed by rapid vaporization and decomposition. The gas pressure generated during decomposition helps to "push" the molten carbonaceous layer towards the copper surface, promoting thermal contact; the dosage of 30g (approximately 12g of solid resin after solid content conversion) is based on the ratio between the total powder mass of 50g (graphite + carbon black) and the solid resin mass of 12g—the powder is just coated by the resin, but there are still exposed light-absorbing particles on the surface, and the resin does not overflow excessively from the surface.

[0048] The organically modified bentonite exhibits rheological regulation and suspension stability. Bentonite is a layered silicate mineral; after organic modification, the interlayer becomes hydrophobic, forming a three-dimensional "cardboard house" network structure in an aqueous system. Its triple function in this formulation is: anti-settling: the three-dimensional network structure encapsulates graphite and carbon black particles, preventing the rapid sedimentation of heavier graphite (2.2 g / cm³) in the slurry and extending the coating's shelf life; thixotropy: high apparent viscosity at rest, but a sudden drop in viscosity under shear force during application (brushing or spraying) facilitates leveling; viscosity recovers after shear loss, preventing coating sagging—especially important for curved substrates like fine copper wire; water retention: slows down the coating's drying speed, preventing excessively rapid drying and cracking after application. The dosage of 5 g is based on the minimum critical concentration of bentonite forming an effective network structure in an aqueous system, approximately 3-5%. Below this value, suspension stability is insufficient; above this value, the slurry viscosity is too high, which is detrimental to coating.

[0049] Sodium tetraborate (borax); flux and surfactant; Borax plays a unique and easily overlooked role in this formulation: pH buffering and dispersion: Borax aqueous solution is weakly alkaline (pH≈9.2), which can ionize the carboxyl groups in acrylic resin, increase the electrostatic repulsion between resin molecular chains, and promote the uniform dispersion of graphite and carbon black particles in the slurry; Fluxing effect: Borax melts and decomposes at 741℃ to generate B2O3. B2O3 forms a liquid glassy film upon laser heating, which encapsulates carbon particles and wets the copper surface, improving the thermal contact between the carbon layer and the copper substrate. This is equivalent to introducing a "thermal bridge" between the carbon light-absorbing layer and the copper substrate; Copper surface activation: Molten borax can dissolve the CuO and Cu2O oxide layers on the copper surface, exposing a fresh copper surface, allowing heat to be effectively transferred rather than being blocked by the oxide layer; The dosage of 3g is based on the following considerations: when the borax content is less than 2g, the fluxing effect is not obvious; when it is more than 5g, the residual borosilicate phase increases, which may contaminate the solder joints and increase the difficulty of post-soldering cleaning. 3G is a compromised middle ground.

[0050] Preparation and coating process parameters Step 1: Premix graphite and carbon black, grind and mix in a mortar for 15 minutes; Step 2: Dissolve bentonite and borax in water, stir in 60℃ warm water until completely dissolved and dispersed, then cool and set aside. Step 3: Add acrylic resin and slowly stir to mix; Step 4: Add the powder mixture to the resin solution and disperse at high speed (2000 rpm) for 30 minutes; Step 5: Three-roll mill or ball mill, grind to a fineness of <15μm; Step 6: Pre-treatment of copper wire surface: degrease with acetone, immerse in 5% dilute sulfuric acid for 10 seconds, rinse with deionized water, and dry with nitrogen (apply immediately to prevent re-oxidation). Step 7: Apply, spray or dip coat, with the wet film thickness controlled at approximately 50 μm; Step 8: Drying, dry in an 80℃ oven for 15 minutes; Step 9: Laser welding, the dry film thickness of the coating is about 20-25μm.

[0051] Technical effect Enhanced absorption: Bare copper has a reflectivity of about 95% for 1064nm laser (with an absorption rate of only 5%). After coating with this coating, the expected surface absorption rate can reach 85-92%, achieving an absorption enhancement of at least 17-18 times.

[0052] Improved welding quality: Under the same laser power, the weld penetration depth can be increased by 2-3 times, enabling low-power lasers (such as below 200W) to weld copper wires that originally required high power (>1000W).

[0053] Welding spatter is significantly reduced—due to improved energy coupling efficiency, there is no longer a need to drastically increase laser power to compensate for reflection losses.

[0054] Coating easy to remove: The combination of acrylic resin, bentonite and borax leaves a loose, grayish-white, brittle thin layer after laser heating. It can be removed by gently wiping with a lint-free cloth dampened with alcohol without damaging the surface finish of the copper wire. The resistivity of the copper wire surface does not change significantly after welding.

[0055] Process tolerance: Effective for coating thicknesses ranging from 15-40μm, with no stringent requirements for coating precision, suitable for manual or automated spraying production.

[0056] The core innovation of this formula lies in the synergistic ratio of the five components rather than the selection of a single component: A densely packed light-absorbing layer structure of "sheet-like micron particles + spherical nanoparticles" is achieved by using graphite and carbon black in a mass ratio of 2.46:1. This allows the coating to achieve effective light absorption with a thickness of 20μm, which is crucial for maintaining the flexibility and solderability of fine copper wires (diameter <1mm).

[0057] The introduction of bentonite not only solves the problem of slurry storage stability, but also utilizes its thixotropy to achieve uniform coating on the curved surface of fine filaments during the coating process.

[0058] Borax achieves its four functions (pH adjustment, dispersant, flux, and oxide layer remover) in a trace amount (3%), embodying the efficient formulation design concept of "one substance, multiple uses".

[0059] The all-water-based system avoids the potential corrosion of copper wires and environmental problems caused by organic solvents.

[0060] Experimental conditions laser Fiber laser, wavelength 1064nm laser power 200W (Continuous Mode) Spot diameter 0.3mm Welding time 200ms copper wire material Copper (T2), 0.8mm in diameter Coating thickness Dry film approximately 20-25 μm Absorption rate test Integral sphere method, 1064nm At room temperature, the absorption rate of polished copper surfaces to 1064nm laser is only about 5% (the reflectivity is about 95%).

[0061] Absorption rate test data (comparison of absorption rates of coatings for different formulations) A0 Bare copper (polished surface) 5.2 0.8 A1 The formula of this invention (graphite 32g + carbon black 13g + resin 30g + bentonite 5g + borax 3g + water 17g) 89.6 1.2 A2 Pure graphite coating (50g graphite + 30g resin + 20g water) 68.4 2.5 A3 Pure carbon black coating (50g carbon black + 30g resin + 20g water) 72.1 2.8 A4 Graphite + Carbon Black (without bentonite) 81.3 3.6 A5 Graphite + Carbon Black + Bentonite (Borax-free) 84.7 2.0 Interpretation of Absorption Rate Data The absorption rate of A1 (the formulation of this invention) reaches 89.6%, which is about 17 times higher than that of bare copper (5.2%). The absorption rate of graphite for 1064nm infrared laser is generally 60%~80%. This invention improves the absorption rate to nearly 90% through the synergistic effect of carbon black compounding, bentonite dispersion, and borax fluxing.

[0062] Comparison between A2 and A3: The pure carbon black coating (72.1%) is slightly better than the pure graphite coating (68.4%), but both are lower than the compound solution (A1's 89.6%), proving that the compound effect of graphite and carbon black is significant. Carbon black can achieve an absorbance of over 77% in the near-infrared region, and the sheet-like structure of graphite provides multiple reflection absorption channels. The combined effect of the two is far greater than that of a single component.

[0063] Comparison between A4 and A1: Without bentonite, the absorption rate dropped to 81.3% and the standard deviation increased (3.6%), indicating that bentonite improved the suspension dispersibility, making the light-absorbing particles more evenly distributed in the coating, thus improving the consistency and repeatability of the absorption rate.

[0064] Comparison between A5 and A1: The absorption rate of A5 without borax is 84.7%, which is lower than that of A1 (89.6%). This proves that the fluxing effect of borax improves the thermal contact between the carbonaceous layer and the copper surface, further enhancing the effective absorption rate.

[0065] Experimental data of laser spot welding process (welding penetration and width (laser power 200W, welding time 200ms)) B0 Bare copper Approximately 20 (not fully melted) Approximately 180 Surface oxidation and discoloration, no obvious molten pool observed. B1 The formula of this invention 285 420 The molten pool is full and round, with no splashing. B2 Pure graphite coating 165 340 There is a molten pool with slightly irregular edges. B3 pure carbon black coating 178 355 There is a molten pool with slight carbon residue on the surface. B1 (the formula of this invention) achieves a penetration depth of 285μm, approximately 14 times that of bare copper (about 20μm), and a penetration width of 420μm, resulting in round, spatter-free solder joints. The significant improvement in energy coupling efficiency allows copper wire welding, which previously required over 1000W, to be accomplished with a low-power laser of 200W.

[0066] The melting depths of B2 and B3 are 165 μm and 178 μm, respectively, which are only 58% and 62% of those of B1. The absorption rate of a single carbon-based coating is insufficient (approximately 68%~72%), and it lacks the fluxing effect of borax, thus limiting the heat transfer efficiency.

[0067] Coating adhesion and removability test, coating adhesion (cross-cut test, rating scale: 0 is the best, 5 is the worst) C1 The formula of this invention Level 1 Extremely easy to remove (remove with alcohol after one wipe). C2 Pure graphite coating Level 2 Relatively easy (wipe with alcohol 3 times) C3 pure carbon black coating Level 3 Medium (wipe with alcohol 5 times) Adhesion and Removability: C1 (this invention) exhibits Grade 1 adhesion, and post-soldering residue is extremely easy to remove. The water-based acrylic resin contains carboxyl groups (-COOH), which can form coordination bonds with the copper surface to provide adhesion; after laser heating, the resin completely decomposes into CO2 and H2O, leaving a loose and brittle carbonaceous layer that can be easily removed by wiping with alcohol.

[0068] Process stability test: batch-to-batch absorbance repeatability (5 batches were prepared for each formulation, and 5 samples were tested for each batch). D1 The formula of this invention 89.6 2.1 1.2 D2 Graphite + Carbon Black (without bentonite) 81.3 7.8 3.6 D3 Graphite + Carbon Black + Bentonite (Borax-free) 84.7 3.5 2.0 Stability analysis: D1 (the present invention) exhibits a batch-to-batch range of only 2.1% and an intra-batch standard deviation of only 1.2%, making it the best among the three groups. Bentonite prevents the sedimentation of light-absorbing particles, while borax promotes uniform dispersion; together, they ensure a high degree of consistency between batches.

[0069] The batch-to-batch variation of D2 (without bentonite) is as high as 7.8%. Due to the high specific gravity of graphite (2.2 g / cm³), it is easy to settle when there is no bentonite, resulting in uneven coating composition among different batches.

[0070] D3 (borax-free) has better stability than D2 but not as good as D1, indicating that bentonite contributes more to stability than borax, but the combination of the two has the best effect.

[0071] Summary of Comparative Examples Absorption rate (%) 5.2 68.4 72.1 81.3 84.7 89.6 Melt depth (μm) ~20 165 178 — — 285 Adhesion (Grade) — 2 3 — — 1 Clearance Difficulty — relatively easy medium — — Extremely easy Batch stability (range %) — — — 7.8 3.5 2.1 Data shows that the formulation of this invention, through the synergistic effect of five components—graphite + carbon black (high absorption), bentonite (uniform dispersion + thixotropic coating), and borax (fluxing + interface activation)—is superior to all comparative solutions in five dimensions: absorption rate, welding quality, adhesion, removability, and process stability.

[0072] Experimental data on variations of each component in the formulation of this invention This section focuses on the core formula (32g graphite + 13g carbon black + 30g resin + 5g bentonite + 3g borax + 17g water, dry film thickness 22μm). The dosage of each individual component was changed while other components remained constant. The changes in various performance indicators were systematically recorded to verify the scientific basis for the optimal dosage of each component.

[0073] I. Experiment on variation of graphite dosage Fixation conditions: 13g carbon black, 30g resin, 5g bentonite, 3g borax, 17g water 15 38.2 69.8 156 318 The coating is too thin, and the substrate is clearly visible. Level 2 20 40.8 76.3 183 356 The coating basically covers the entire area, with some areas slightly thinner. Level 2 25 43.6 83.1 228 388 Even coverage, smooth surface Level 1 30 46.2 88.4 276 414 Dense coverage, smooth surface Level 1 32 (Recommended) 47.0 89.6 285 420 Dense coverage and excellent flatness Level 1 35 47.8 89.8 281 416 Slightly grainy surface Level 1-2 40 49.2 89.3 268 398 The surface is rough and the particles are clearly raised. Level 2 Laser conditions: 200W, 200ms, 0.8mm copper wire When the graphite content is less than 25g, the density of light-absorbing particles is insufficient, resulting in unsatisfactory absorption rate and penetration depth. When the content is greater than 35g, although the absorption rate remains high (around 89%), the increased roughness and porosity of the coating lead to decreased adhesion. Simultaneously, the excessively thick carbon layer during welding hinders heat transfer to the copper substrate, causing the penetration depth to decrease (from 285μm to 268μm). 32g represents the optimal balance between light absorption density and coating density.

[0074] II. Experiment on the variation of carbon black dosage Fixation conditions: 32g graphite, 30g resin, 5g bentonite, 3g borax, 17g water 0 68.4 165 none uniform 8.2 5 76.8 198 none uniform 7.6 10 85.2 252 none uniform 6.8 13 (Recommended) 89.6 285 Very slight uniform 6.2 16 91.0 279 slight uniform 5.5 20 91.8 258 medium Mild reunion 4.8 25 92.2 226 serious Obvious agglomeration, rough coating 3.9 Laser conditions: 200W, 200ms, 0.8mm copper wire As the carbon black addition increased from 0 to 13g, the absorption rate linearly increased from 68.4% to 89.6%, with a simultaneous increase in melting depth. However, when the carbon black addition exceeded 16g, the agglomeration effect of nano-sized carbon black intensified. Although the absorption rate still increased slightly (due to the extremely high light absorption efficiency of carbon black itself), the agglomerates led to microscopic inhomogeneity in the coating, resulting in localized hot spots during laser heating, increased spatter, and a decrease in melting depth instead of an increase. Simultaneously, the relatively low specific gravity of carbon black (approximately 1.8g / cm³) increased the density difference with graphite (2.2g / cm³) after adding large amounts, exacerbating sedimentation and stratification. 13g represents the optimal boundary between light absorption efficiency and dispersion uniformity.

[0075] III. Experiment on varying graphite:carbon black ratio (with a fixed total carbon content of 45g) Fixed conditions: Total absorber (graphite + carbon black) = 45g, resin 30g, bentonite 5g, borax 3g, water 17g 45:0 (pure graphite) 68.4 165 340 The molten pool is relatively shallow and the surface is slightly rough. 40:5 74.2 188 362 Improved molten pool forming 35:10 85.6 258 408 Full molten pool 32:13 (Recommended) 89.6 285 420 The molten pool is round and full, without defects. 28:17 90.8 274 405 Surface micro-splash 25:20 91.5 252 386 Increased splashing, uneven edges 20:25 92.1 218 358 There was obvious splashing, and black spots remained on the surface. Laser conditions: 200W, 200ms, 0.8mm copper wire While pure graphite's flake structure provides multiple reflections and absorptions, its absorptivity is only 68.4%. As the proportion of carbon black increases, nanoparticles fill the gaps between graphite flakes, forming a "close-packed" structure, achieving an absorptivity of 89.6% when the graphite:carbon black ratio is approximately 2.46:1 (i.e., 32:13). However, when the carbon black ratio exceeds 1:1, excess nanoparticles form agglomerates. Although the absorptivity continues to rise (nano-carbon black itself has extremely strong light absorption), these agglomerates cause localized overheating and spattering during laser heating, deteriorating the weld quality—the weld penetration actually decreases from 285 μm to 218 μm. This reveals a key principle: a higher absorptivity is not necessarily better; the goal is effective energy coupling in welding.

[0076] IV. Experiment on variations in bentonite dosage Fixation conditions: 32g graphite, 13g carbon black, 30g resin, 3g borax, 17g water 0 420 1.8 81.3 ±8.5 18.6 (Completely compacted) Level 2 2 780 2.5 85.1 ±5.2 12.4 Level 2 3 1120 3.1 87.6 ±3.8 9.1 Level 1-2 5 (Recommended) 1850 4.2 89.6 ±1.8 6.2 Level 1 7 2650 5.6 89.0 ±2.1 3.8 Level 1 10 4200 8.3 87.2 ±4.5 1.5 Level 1-2 15 6800 11.2 83.8 ±7.2 0.8 Level 2-3 Thixotropic index = viscosity at 6 rpm / viscosity at 60 rpm. The higher the value, the stronger the thixotropy (the thicker it is at rest and the thinner it is when sheared). Without bentonite (0g), the slurry viscosity is only 420mPa·s, similar to diluted ink, making it extremely prone to sagging during application. The coating thickness on the fine copper wire surface is uneven (±8.5μm), with an absorption rate of only 81.3%. Adding bentonite forms a three-dimensional network structure, significantly increasing the viscosity. 5g is the inflection point for rheological properties—at this point, the thixotropic index reaches 4.2, giving the slurry the ideal thixotropic characteristics of "thick at rest, thinner under shear": the viscosity drops sharply under brushing / spraying shear force during application, facilitating leveling, and the viscosity recovers after application, preventing sagging. Above 7g, the excessively high viscosity leads to difficulties in application and an overly thick coating (dry film >30μm), which in turn reduces absorption rate and adhesion.

[0077] V. Experiment on variation of borax dosage Fixation conditions: 32g graphite, 13g carbon black, 30g resin, 5g bentonite, 17g water 0 6.8 84.7 238 176 2 times Slight oxidation color 1 7.6 86.5 256 194 2 times Very slight 2 8.4 88.2 272 208 1-2 times No visible oxidation 3 (Recommended) 9.2 89.6 285 215 1 time No visible oxidation 4 9.6 90.1 279 211 1 time No visible oxidation 5 9.8 89.8 268 202 1-2 times Very slight white residue 7 10.1 88.6 244 185 2-3 times Small amount of white borate Laser conditions: 200W, 200ms, 0.8mm copper wire Borax plays four roles in this formulation: ① pH adjustment – ​​adjusting the slurry from acidic to weakly alkaline (pH≈9.2), promoting the ionization of resin carboxyl groups, and enhancing dispersion stability; ② flux – molten B2O3 wets the copper surface and dissolves the oxide layer; ③ surfactant – improving the thermal contact between the carbonaceous layer and the copper substrate; ④ post-weld oxidation prevention – molten borax covers the molten pool, providing temporary protection.

[0078] 3g is the optimal value: below 2g, the flux is insufficient, the oxide layer is not thoroughly removed, and the melt depth and strength are low; above 5g, the borate residue increases, the number of post-weld cleanings increases, and the excessive B2O3 forms a glassy isolation layer on the surface of the molten pool, which hinders heat transfer (the melt depth decreases from 285μm to 244μm, and the strength decreases from 215MPa to 185MPa).

[0079] VI. Experiment on the variation of resin (water-based acrylic resin) dosage Fixed conditions: 32g graphite, 13g carbon black, 5g bentonite, 3g borax, 17g water Laser conditions: 200W, 200ms, 0.8mm copper wire 15 43.6 Level 4 92.3 198 2% >20mm (brittle, prone to cracking) 20 44.8 Level 3 91.8 226 4% 15mm 25 46.4 Level 2 90.5 258 8% 8mm 30 (Recommended) 48.2 Level 1 89.6 285 12% 5mm 35 49.6 Level 1 87.8 276 22% 3mm 40 50.8 Level 1-2 85.1 248 35% <2mm (flexible) 50 52.6 Level 2 81.6 206 52% <2mm (too soft, not wear-resistant) The resin is an aqueous acrylic resin emulsion (solid content 40wt%). The "resin usage" in the table refers to the mass of the emulsion, and the solid content has been converted to the effective solid content.

[0080] The core contradiction of the resin lies in the conflict between its adhesiveness and its removability. When there is too little resin (15~20g), the coating adhesion is extremely poor (3~4 level), and it will peel off with the slightest touch, rendering it useless. When there is too much resin (40~50g), although the adhesion is restored and the coating becomes flexible, the residue increases significantly (35%~52%) after the resin decomposes at high temperatures, requiring multiple wiping or even sanding to remove it, thus losing the core advantage of this formula: "easy to wipe off".

[0081] 30g is optimal: at this point, the solid content is 48.2%, the adhesion reaches level 1 (peeling area by cross-cut method <5%), the post-soldering residue is only 12% and is in loose powder form (removable by wiping with alcohol once), and the flexibility (bending a 5mm diameter wire without cracking) is sufficient to adapt to the bending operation of fine copper wire.

[0082] VII. Water Consumption Variation Experiment (Solid Content Adjustment) Fixation conditions: 32g graphite, 13g carbon black, 30g resin, 5g bentonite, 3g borax 8 55.3 3850 It has a paste-like consistency and cannot be sprayed; it can only be applied by scraping. 48 88.2 12 51.6 2680 Viscous, difficult to spray, can be brushed. 35 89.0 17 (Recommended) 48.2 1850 Suitable for spraying / dipping / brushing 22 89.6 22 45.4 1260 Too thin, obvious paint dripping. 16 85.8 27 43.0 880 If the coating is too thin, it won't adhere to the surface of the copper wire. 10 76.2 35 40.2 580 The paint was severely dripping and could not form a continuous film. 6 60.4 The coating method is uniformly spraying, with a spray gun nozzle diameter of 1.0mm, a spraying distance of 150mm, and an air pressure of 0.3MPa; the dry film thickness is uniformly controlled to 22μm by adjusting the number of spray passes (if the thickness cannot be achieved due to viscosity limitations when the water volume is extreme, the actual film thickness shall prevail). The amount of water determines the suitability of the slurry for coating processes. With 17g of water, the solid content is 48.2%, the viscosity is 1850 mPa·s, resulting in good atomization and no sagging during spraying, allowing for a uniform dry film of approximately 22μm in a single coat. Too much water (>22g) results in a thin slurry, making it difficult for the coating to adhere to the surface of fine copper wires, leading to insufficient film thickness and a sharp decrease in absorption rate. Too little water (<12g) results in a paste-like slurry, making spraying impossible and resulting in extremely low coating efficiency. 17g is the optimal solution balancing coating efficiency and film thickness accuracy.

[0083] VIII. Comprehensive Comparison: Optimal Formulation vs. Worst-Case Values ​​of Each Component Absorption rate (%) 89.6 69.8 / 88.1 68.4 / 92.2 81.3 84.7 92.3 / 81.6 88.2 / 60.4 Melt depth (μm) 285 156 / 242 165 / 226 238 238 198 / 206 288 / 108 Adhesion (Grade) 1 2 / 2~3 2 / 2~3 2 1 4 / 2 2 / 4 Number of clears 1 time 3 times / 1 time 3 times / 3~4 times 2 times 2 times 1 time / 5 times 1 time / 3 times Slurry stability (settlement in mm after 15 days) 6.2 8.5 / 4.8 8.2 / 3.9 18.6 6.8 6.5 / 5.8 8.1 / 3.2 Optimal formula: 32g graphite + 13g carbon black + 30g resin + 5g bentonite + 3g borax + 17g water IX. Data Summary and Design Insights graphite 30~35g 32g The coating is rough and porous, resulting in a reduced penetration depth. Insufficient light absorption density and insufficient melting depth carbon black 10~16g 13g Agglomeration → Increased splashing, decreased melt depth Low absorption rate and insufficient melting depth Graphite: Carbon Black 2.0~3.0:1 2.46:1 Severe splashing / residue Insufficient absorption rate Bentonite 4~7g 5g The viscosity is too high, making it difficult to apply. Slurry settling and crusting, uneven coating Borax 2~5g 3g Increased residue and decreased melting depth Incomplete removal of the oxide layer results in poor adhesion. resin 25~35g 30g The residue has increased significantly, making it difficult to remove. Poor adhesion, coating easily peels off water 15~20g 17g Insufficient membrane thickness leads to a sharp drop in absorption rate. The viscosity is too high to be applied. There is a precise synergistic balance among the components of this formulation. The total amount and proportion of carbon-based light absorbers (graphite + carbon black) determine the upper limit of light absorption efficiency; bentonite ensures the stability of the slurry and the uniformity of the coating; borax acts as a "catalyst" for the effective transfer of heat to the copper substrate; the resin represents a delicate balance between adhesion and removability; and the amount of water is a prerequisite for all process performance. If any component deviates from its optimal range, it will cause a significant degradation at some point in the performance chain. All five components are indispensable, and adding or subtracting any one will also be detrimental.

[0084] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A high-performance laser spot welding process for fine copper wires, characterized in that: Includes the following steps: Step S1: Before welding, a coating is applied to the surface of the copper wire to be welded to improve the absorption rate of the copper wire to the laser. Step S2: Attach the coated sides of the two copper wires to be welded to the welding fixture, and select the appropriate welding parameters according to the diameter of the copper wires to be welded; Step S3: Adjust the laser focus to the joint of the copper wires to be welded, and emit the laser to complete the spot welding.

2. The high-performance laser spot welding process for fine copper wires according to claim 1, characterized in that: The width of the coating should be greater than the radius of the laser spot used for spot welding.

3. The high-performance laser spot welding process for fine copper wires according to claim 1, characterized in that: The coating extends to the circumferential surface of the copper wire and also to the vertical cross-section of the copper wire to be welded.

4. The high-performance laser spot welding process for fine copper wires according to claim 1, characterized in that: The welding parameters include laser power, laser defocusing amount, and spot welding time; the laser power is determined according to the diameter of the copper wire to be welded, and the laser power range is 100~1000W when the diameter of the copper wire to be welded is less than 3mm; the laser defocusing amount ranges from -2 to +2mm; and the spot welding time ranges from 0.05 to 0.6s.

5. The high-performance laser spot welding process for fine copper wires according to claim 1, characterized in that: The coating is made by coating, spraying, wetting or electroplating processes.

6. The high-performance laser spot welding process for fine copper wires according to claim 1, characterized in that: The laser spot welding process can use either pulsed laser mode or continuous laser mode.

7. A high-performance laser spot welding equipment for fine copper wires, characterized in that: The process is used to perform a high-performance laser spot welding process for fine copper wires as described in any one of claims 1 to 6; it also includes a three-axis motion mechanism (4), a weld seam identification device (5), a laser welding head (6), a C-frame (7), and a welding fixture (9); the three-axis motion mechanism (4) is mounted on the top of the C-frame (7); the laser welding head (6) is mounted on the three-axis motion mechanism (4) and is driven by the three-axis motion mechanism (4) to move freely in three-dimensional space, thereby adjusting the position and focal length of the laser spot; the laser welding head (6) emits a laser, which acts on the joint of the copper wires to be welded, thereby melting the copper wires to complete the welding.

8. A high-performance laser spot welding equipment for fine copper wires according to claim 7, characterized in that: The laser beam focusing diameter of the laser welding head (6) is 0.1~0.4mm.

9. A high-performance laser spot welding equipment for fine copper wires according to claim 8, characterized in that: The weld seam identification device (5) is coaxially mounted with the laser welding head (6) and is used to identify and locate the joint of the copper wires to be welded.

10. A high-performance laser spot welding equipment for fine copper wires according to claim 9, characterized in that: The welding fixture (9) is set at the bottom of the C-frame (7) to fix the copper wire to be welded, ensuring that the copper wire does not deform after welding.

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