Multi-process coupling laser welding strengthening method for copper and aluminum dissimilar metal

Abstract

The invention discloses a copper-aluminum dissimilar metal multi-process coupling laser welding strengthening method which is characterized in that in-situ coordination of welding, stress inversion, targeted strengthening and closed-loop repair is realized by integrating a three-layer coaxial nozzle end effector, a composite laser source and a digital twin system; synchronously stabilizing the temperature of the welding pool at 2200-2500K through PID (Proportion Integration Differentiation) closed-loop control; after welding, residual stress and IMC distribution are inversed based on a digital twin heat-force coupling model, and a targeted impact path is generated; switching short pulse laser to combine with a constraint water film to realize impact strengthening, introducing + 800-+ 1200 MPa residual compressive stress, and inducing a nanocrystalline structure; and finally, through hot nitrogen transient drying and ultrasonic closed-loop repairing, joint manufacturing is completed. According to the method, the process period is shortened, the joint tensile strength is improved, and the problems of excessive brittle phases, stress degradation and low efficiency of traditional copper-aluminum welding are effectively solved.

Description

Technical Field

[0001] This invention relates to a multi-process coupled laser welding strengthening method for dissimilar copper and aluminum metals. Background Technology

[0002] Copper-aluminum dissimilar metal joints are urgently needed in fields such as deep-sea cables, fast-charging connectors for new energy vehicles, and power supply systems for rail transit. Due to the significant differences in the physicochemical properties of copper and aluminum (melting point difference of approximately 423K, thermal conductivity difference of approximately 200W / m·K, and significant differences in the coefficient of thermal expansion), traditional fusion welding methods are prone to generating brittle intermetallic compounds (IMCs) at the weld interface, leading to a deterioration in the mechanical properties and corrosion resistance of the joint.

[0003] The existing Gaussian beam laser welding technology mainly faces the following technical bottlenecks:

[0004] Firstly, under the action of welding thermal cycling, copper and aluminum atoms undergo intense inter-diffusion, resulting in the formation of a brittle intermetallic compound layer (mainly Al2Cu, Al4Cu9, etc.) with a thickness of 8-15μm on both sides of the weld. This brittle layer becomes a preferred channel for crack initiation and propagation, severely weakening the joint's toughness and fatigue life, making the welded joint a weak link in the overall structure.

[0005] Secondly, while laser shock peening (LSP) technology can introduce residual compressive stress into the material surface to improve fatigue performance, existing processes mostly employ a separate production model of "welding-transfer-strengthening." After welding, the workpiece needs to undergo cooling, disassembly, transfer, and reclamping, a process that takes 2-4 hours. During this process, the residual welding stress has been largely released, and the secondary clamping introduces positioning errors, making it difficult to accurately align the strengthened area with the heat-affected zone of the weld. This results in a significant reduction in the strengthening effect, failing to meet the requirements of zero-defect manufacturing.

[0006] Third, in multi-point parallel welding scenarios, the thermal interference problem between adjacent welding units is prominent. Due to the high thermal diffusivity of copper and aluminum, the heat accumulation of adjacent weld points leads to local temperature field runaway, causing fluctuations in weld quality, and the finished product qualification rate is only maintained at 85-90%.

[0007] To address the aforementioned issues, the academic community has proposed improvements such as asymmetric bevel design, filler intermediate layer alloys, and pulsed laser modulation. However, asymmetric bevels cannot simultaneously accommodate the differences in laser absorption rates and thermal expansion coefficients between dissimilar materials like copper and aluminum; a single filler material cannot simultaneously optimize multiple objectives such as IMC thickness control, residual stress regulation, and joint corrosion resistance; and existing research is mostly limited to single-process parameter optimization, failing to establish a systematic synergistic coupling mechanism between welding and strengthening processes, making it difficult to overcome the inherent contradiction between efficiency and precision. Summary of the Invention

[0008] The present invention provides a method for strengthening copper-aluminum dissimilar metals through multi-process coupled laser welding in order to solve the problems existing in the prior art.

[0009] The technical solutions adopted in this invention are as follows:

[0010] A multi-process coupled laser welding strengthening method for dissimilar copper and aluminum metals, employing a composite laser source and a three-layer coaxial nozzle end effector, characterized in that: the composite laser source includes a long-pulse welding laser with a pulse width in the microsecond range and a short-pulse strengthening laser with a pulse width in the nanosecond range, and the three-layer coaxial nozzle includes a central channel, a middle annular channel, and an outer air knife channel;

[0011] The method includes the following steps:

[0012] S1: Fix the dissimilar copper and aluminum workpieces to the tooling fixture and establish a thermo-mechanical coupling model;

[0013] S2: Start welding mode. The central channel sprays protective gas, the long pulse welding laser outputs laser to scan and weld, and the welding process data is collected in real time.

[0014] S3: After welding is completed, the welding process data is loaded into the thermo-mechanical coupling model to calculate the residual stress distribution and generate a targeted impact strengthening path plan;

[0015] S4: Close the protective gas in the central channel and the long-pulse welding laser, open the middle annular channel to deliver constrained water to form a water film, and start the short-pulse strengthening laser to perform scanning impact strengthening according to the targeted impact strengthening path plan;

[0016] S5: After reinforcement is completed, the outer air knife channel is opened to spray gas for surface drying. Defect assessment is carried out by ultrasonic testing, and a decision is made on whether to implement targeted reinforcement in the defect area based on the assessment results.

[0017] Furthermore, the composite laser source combines a long-pulse welding laser and a short-pulse enhancement laser into a single optical fiber using a dual-wavelength optical combiner. The combining efficiency of the dual-wavelength optical combiner is greater than 95%, and the diameter of the optical fiber is 200 μm.

[0018] Furthermore, the inner diameter of the central channel is 3-5mm, the middle diameter of the middle annular channel is 8-12mm, the outer diameter of the outer air knife channel is 15-20mm and adopts a Laval nozzle structure with a jet angle of 30-45°.

[0019] Furthermore, in step S1, the thermo-mechanical coupling model is established by the digital twin system as a thermo-mechanical coupling finite element model with an element size of 0.1-0.2 mm.

[0020] Furthermore, in step S2, the protective gas is argon, and the flow rate is 15-25 L / min;

[0021] In step S5, the injected gas is hot nitrogen with a temperature greater than or equal to 80°C, a flow rate of 20-30 m / s, and a flow rate of 50-80 L / min.

[0022] Furthermore, in step S3, generating the targeted impact enhancement path plan includes determining the impact point spacing to be 2-3 mm and the energy density to be 0.5-3.0 GW / cm³. 2 .

[0023] Furthermore, in step S4, the water film is a laminar water film with a thickness of 1-2 mm and a water flow velocity of 3-5 m / s.

[0024] Furthermore, the short-pulse enhanced laser has a power of 400-500W, a frequency of 2-3kHz, a pulse width of 8-12ns, and a scanning speed of 30-50mm / min.

[0025] The long-pulse welding laser has a power of 250-350W, a pulse width of 200-500μs, and a scanning speed of 100-120mm / min.

[0026] Furthermore, in step S5, the criterion for defect assessment is that the area of ​​the pore defect is greater than 0.5 mm. 2 If the depth is greater than 0.2 mm, and a defect is determined to exist, an additional 3-5 high-energy impact reinforcements will be applied within a 2-3 mm radius around the defect area (increasing the impact energy density to 3.0 GW / cm²). 2 )

[0027] Furthermore, when the method is used for parallel processing of multiple welding points, a time-domain staggered peak control strategy is adopted to divide the total cycle into two time windows on average, and the laser energy of adjacent welding units is staggered and started in different time windows.

[0028] The present invention has the following beneficial effects:

[0029] (1) The welding shielding gas, constrained water film and drying air knife are switched in place quickly by the three-layer coaxial nozzle end effector. Combined with the long pulse welding and short pulse strengthening mode conversion of the composite laser source, the welding and laser shock strengthening processes are integrated into the same station, eliminating the time loss and positioning error caused by workpiece transfer and secondary clamping, significantly shortening the process cycle and reducing equipment investment costs.

[0030] (2) The thermo-mechanical coupling model established based on the digital twin system is used to invert the distribution of welding residual stress in real time and generate a targeted impact strengthening path plan. This allows the short-pulse strengthening laser to accurately impact the area of ​​concentrated residual tensile stress immediately after welding, induce the formation of nanostructures on the surface of the material, introduce residual compressive stress in the deep layer, effectively offset the welding residual tensile stress, significantly improve the tensile strength and toughness of the joint, and greatly extend the fatigue life.

[0031] (3) By controlling the temperature of the weld pool precisely within a reasonable range through closed-loop temperature control, the excessive diffusion of copper and aluminum atoms is suppressed, the thickness of the intermetallic compound layer is effectively reduced, the corrosion resistance of the joint is improved, and the mechanical properties are deteriorated due to excessive growth of brittle phase.

[0032] (4) A closed-loop iterative mechanism of transient drying and ultrasonic defect assessment is adopted. After the strengthening is completed, surface drying and defect detection are performed immediately. When pore-type defects are detected, secondary reinforcement is automatically triggered to achieve quality self-repair in the manufacturing process, significantly improve the finished product qualification rate and reduce the unit production cost.

[0033] (5) In the scenario of parallel processing of multiple welding points, the total cycle is divided into two time windows by adopting the time-domain staggered peak control strategy, so that the laser energy of adjacent welding units is staggered to start, effectively controlling the thermal interference of adjacent welding points, eliminating the quality fluctuation caused by heat accumulation, and ensuring the batch consistency of multiple welding points. Attached Figure Description

[0034] Figure 1 This is a top view of the welding system corresponding to the present invention.

[0035] Figure 2 This is a flowchart of a PID closed-loop control based on real-time temperature feedback.

[0036] Figure 3 This is a schematic diagram of water cooling. Detailed Implementation

[0037] The invention will now be further described with reference to the accompanying drawings.

[0038] This embodiment uses high-voltage connection of dissimilar metals such as copper and aluminum for deep-sea cables as the application scenario. The workpieces to be welded are T2 copper and 6061 aluminum alloy cables or terminals. The thermal conductivity of copper is 400W / m·K and the melting point is 1083℃. The thermal conductivity of aluminum is 200W / m·K and the melting point is 660℃. The specific parameters of the workpieces include diameter d, contact area S and surface roughness Ra.

[0039] The core of the welding strengthening system used in this invention includes a composite laser source, a three-layer coaxial nozzle end effector, a tooling platform, and a multimodal sensing system. The composite laser source integrates a long-pulse welding laser with a pulse width in the microsecond range and a short-pulse strengthening laser with a pulse width in the nanosecond range. The two laser beams are combined into a single 200μm diameter optical fiber through a dual-wavelength optical combiner (combining efficiency greater than 95%) to ensure optical path coaxiality.

[0040] The power of long-pulse welding lasers is 250-350W, the pulse width is 200-500μs, the pulse frequency is 50-100kHz, and the scanning speed is 100-120mm / min; the power of short-pulse enhanced lasers is 400-500W, the frequency is 2-3kHz, the pulse width is 8-12ns, and the scanning speed is 30-50mm / min.

[0041] The three-layer coaxial nozzle end effector integrates a central channel, a middle annular channel, and an outer air knife channel. The central channel has an inner diameter of 3-5mm and is used to carry the core beam and protective airflow. The middle annular channel has a middle diameter of 8-12mm and is used to deliver constrained water flow. The outer air knife channel has an outer diameter of 15-20mm, adopts a Laval nozzle structure, and has an air jet angle of 30-45° for transient drying and surface resetting.

[0042] The tooling platform adopts a variable stiffness acoustic impedance matching design and is equipped with a hydraulically driven dynamic backplate at the bottom. By adjusting the filling pressure of the hydraulic bladder, it can quickly switch between welding mode (backplate descends, Z-axis distance from workpiece ≥ 50mm) and reinforcement mode (backplate rises, close to the bottom surface of workpiece), with a switching time of less than 200ms. A 1-2mm thick acoustic impedance matching medium layer (polymer or hydrogel) is laid between the backplate and the workpiece to effectively absorb the transmitted shock wave energy and prevent secondary damage to the workpiece.

[0043] The multimodal sensing system consists of an infrared thermal imager 1, an ultrasonic probe 2, a high-speed camera 3, and a laser power detector. The infrared thermal imager has a sampling frequency of 100Hz and an accuracy of ±2%, and is used to acquire the temperature field of the weld area in real time. The ultrasonic probe has a center frequency of 5-10MHz and is used for defect assessment after welding. The high-speed camera has a frame rate of more than 500fps and is used to capture the dynamic morphology of the molten pool and the spatter situation. The laser power detector is used to monitor the deviation between the actual output power and the set value.

[0044] Combination Figures 1 to 3 The specific implementation steps of the copper-aluminum dissimilar metal multi-process coupled laser welding strengthening method of the present invention are as follows:

[0045] First, workpiece clamping and initial modeling are performed. The dissimilar copper-aluminum workpieces to be welded are fixed on the tooling fixture. The clamping preload data is obtained through strain gauge sensors built into the fixture. The preload range is 500-1000N to ensure contact rigidity. The digital twin system reads the material properties and geometric model of the workpiece and establishes an initial thermo-mechanical coupled finite element mesh model. The element size of this model is 0.1-0.2mm, and the total number of meshes is greater than 50,000, providing a basic model support for subsequent stress field inversion.

[0046] The digital twin system has a built-in real-time model calibration module that dynamically corrects the boundary conditions and material parameters of the thermo-mechanical coupled finite element model based on the actual defect data of the welded joint collected by the ultrasonic probe, ensuring that the inversion accuracy of residual stress and IMC distribution is less than 10%.

[0047] The welding mode is then activated. Shielding gas, argon, is injected through the central channel of the three-layer coaxial nozzle end effector at a flow rate of 15-25 L / min to prevent oxidation of the welding area. The composite laser source is switched to long-pulse welding laser output mode, and the laser head scans the weld seam at a constant speed for 24-30 seconds. During this welding process, a multimodal sensing system works synchronously. An infrared thermal imager and a high-speed camera collect welding process data in real time, including temperature field, molten pool morphology, and spatter conditions. All collected data is transmitted in real time to the cloud computing module of the digital twin system, providing basic thermal history data for subsequent stress field inversion. At the same time, the weld pool temperature is precisely maintained within a reasonable range of 2200-2500K through PID closed-loop control. Specifically, the sampling period of this PID control is 10ms, the update frequency is 100Hz, and its control algorithm formula is: P_pid=Kp⋅ΔT+Ki⋅∫ΔT⋅dt+Kd⋅d(ΔT) / dt, where the proportional coefficient Kp is 0.5-1.0, the integral coefficient Ki is 0.01-0.05, and the derivative coefficient Kd is 0.1-0.3. The calculated control quantity needs to be processed by power limiting to constrain the core power of the laser within a safe range of 200W-350W. Finally, a laser power command is generated and sent to the laser power supply to achieve precise and stable control of the welding pool temperature. Its response time is less than 500ms, and the stability error is less than 50K, effectively suppressing the excessive diffusion of copper and aluminum atoms.

[0048] After welding, within a time window of no more than 30 seconds, the thermal history data collected during the welding process is loaded into the thermo-mechanical coupled finite element model. The digital twin system quickly calculates the residual stress distribution (-400~-600MPa tensile stress) in the weld area, the IMC thickened area (predicted thickness 2-5μm), and the stress concentration factor distribution in the heat-affected zone. It also automatically generates a targeted impact strengthening path plan, determining the impact point spacing to be 2-3mm and the energy density to be 0.5-3.0GW / cm². 2 With a coverage rate of ≥90%, the impact sequence adopts a strategy of "radiating outward from the stress peak" to maximize the influence depth of compressive stress.

[0049] Following this, a rapid environmental switch and targeted impact enhancement are performed. The end effector completes the rapid switching of the three-layer channels within less than 50ms, shutting off the welding laser and shielding gas channels, and opening the middle annular channel to deliver constrained water flow, constructing a stable laminar water film with a thickness of 1-2mm on the weld surface. The water flow velocity is 3-5m / s, and the water temperature is ambient temperature +15℃±5℃. The composite laser source switches to short-pulse laser output mode and performs scanning impact enhancement according to the generated targeted impact enhancement path plan, with 30-40 pulses and a total enhancement time of 15-20 seconds. During the impact process, a high-speed camera monitors the splash and water film stability in real time. Combined with data from the water flow pressure sensor, if an anomaly is detected (excessive splashing, water film rupture), the system adaptively adjusts the water flow velocity (within the range of 3-5m / s) and impact energy density (0.5-3.0GW / cm²) through a PID algorithm. 2 Within the specified range, the system ensures a continuous and stable water film. For areas where the IMC predicted thickness is >5 μm, the system automatically increases the impact energy density to a maximum of 3.0 GW / cm². 2 To achieve targeted reinforcement based on individual needs, the high strain rate (>10) generated by the shock wave. 3 The surface layer (0-50μm) induces a gradient nanocrystalline structure (grain size 10-500nm), while the deep layer (50-100μm) introduces residual compressive stress of +800~+1200MPa, effectively offsetting the residual tensile stress of welding.

[0050] After the strengthening process is complete, the laser head immediately shuts off, and the end effector opens the outer air knife channel, spraying high-speed hot nitrogen gas. The hot nitrogen gas temperature is ≥80℃, the flow rate is 20-30m / s, and the flow rate is 50-80L / min, instantly blowing away residual moisture on the surface within 3 seconds to prevent water immersion and corrosion. Simultaneously, an ultrasonic probe is activated to assess defects. The criterion for defect assessment is a pore defect area greater than 0.5mm. 2If the depth is greater than 0.2mm, and a defect that meets this standard is detected, the system will automatically trigger a secondary reinforcement process. Within a 2-3mm radius around the defect area, an additional 3-5 high-energy impacts (pulse energy increased to the maximum value) will be performed for targeted repair.

[0051] If it is multi-layer additive manufacturing (such as stacking multi-layer reinforced joints), the above welding, stress field inversion and strengthening strategy generation, targeted impact strengthening and transient drying and closed-loop iteration steps are repeated, layer by layer to build and optimize until the entire joint is manufactured. The control logic of closed-loop iteration ensures that each joint is adaptively designed based on the actual welding thermal history of the previous layer.

[0052] When this method is used for parallel processing of multiple weld points, a time-domain staggered peak control strategy is adopted, which divides the total cycle into two time windows, each accounting for 1 / 2. The laser energy of adjacent welding units is staggered to ensure that the peak temperature difference between adjacent weld points is <±15%, effectively controlling the thermal interference between adjacent weld points, eliminating quality fluctuations caused by heat accumulation, and ensuring batch consistency of multiple weld points.

[0053] Through the above process, the final copper-aluminum dissimilar metal welded joint can achieve a tensile strength of 75-85 MPa, an elongation of 3-5%, and a fatigue life (R=-1, 10). 7 (times) > 2 × 10 7 After one cycle, the corrosion current density in the corrosion resistance test (3.5% NaCl, 360h) decreased to 10-20 μA / cm². 2 The total process cycle (welding + cooling + strengthening + drying) is 40-60 seconds, and the pass rate of a single batch (100 pieces) is consistently 98-99%.

[0054] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements without departing from the principle of the present invention, and these improvements should also be considered within the scope of protection of the present invention.

Claims

1. A multi-process coupled laser welding strengthening method for dissimilar copper and aluminum metals, employing a composite laser source and a three-layer coaxial nozzle end effector, characterized in that: The composite laser source includes a long-pulse welding laser with a pulse width in the microsecond range and a short-pulse enhancement laser with a pulse width in the nanosecond range. The three-layer coaxial nozzle includes a central channel, a middle annular channel, and an outer air knife channel. The method includes the following steps: S1: Fix the dissimilar copper and aluminum workpieces to the tooling fixture and establish a thermo-mechanical coupling model; S2: Start welding mode. The central channel sprays protective gas, the long pulse welding laser outputs laser to scan and weld, and the welding process data is collected in real time. S3: After welding is completed, the welding process data is loaded into the thermo-mechanical coupling model to calculate the residual stress distribution and generate a targeted impact strengthening path plan; S4: Close the protective gas in the central channel and the long-pulse welding laser, open the middle annular channel to deliver constrained water to form a water film, and start the short-pulse strengthening laser to perform scanning impact strengthening according to the targeted impact strengthening path plan; S5: After the strengthening is completed, open the outer air knife channel to spray gas for surface drying and perform defect assessment. Based on the assessment results, decide whether to repeat steps S2 to S5.

2. The copper-aluminum dissimilar metal multi-process coupled laser welding strengthening method as described in claim 1, characterized in that: The composite laser source combines a long-pulse welding laser and a short-pulse enhancement laser into a single optical fiber through a dual-wavelength optical combiner. The combining efficiency of the dual-wavelength optical combiner is greater than 95%, and the diameter of the optical fiber is 200 μm.

3. The copper-aluminum dissimilar metal multi-process coupled laser welding strengthening method as described in claim 1, characterized in that: The inner diameter of the central channel is 3-5mm, the middle diameter of the middle annular channel is 8-12mm, the outer diameter of the outer air knife channel is 15-20mm and adopts a Laval nozzle structure with a jet angle of 30-45°.

4. The copper-aluminum dissimilar metal multi-process coupled laser welding strengthening method as described in claim 1, characterized in that: In step S1, the thermo-mechanical coupling model is established by the digital twin system as a thermo-mechanical coupling finite element model with an element size of 0.1-0.2 mm.

5. The copper-aluminum dissimilar metal multi-process coupled laser welding strengthening method as described in claim 1, characterized in that: In step S2, the protective gas is argon, and the flow rate is 15-25 L / min; In step S5, the injected gas is hot nitrogen with a temperature of ≥80°C and a flow rate of 20-30 m / s.

6. The copper-aluminum dissimilar metal multi-process coupled laser welding strengthening method as described in claim 1, characterized in that: In step S3, generating the targeted impact enhancement path plan includes determining the impact point spacing to be 2-3 mm and the energy density to be 0.5-3.0 GW / cm³. 2 .

7. The copper-aluminum dissimilar metal multi-process coupled laser welding strengthening method as described in claim 1, characterized in that: In step S4, the water film is a laminar water film with a thickness of 1-2 mm and a water flow velocity of 3-5 m / s.

8. The copper-aluminum dissimilar metal multi-process coupled laser welding strengthening method as described in claim 1, characterized in that: The short-pulse enhanced laser has a power of 400-500W, a frequency of 2-3kHz, a pulse width of 8-12ns, and a scanning speed of 30-50mm / min. The long-pulse welding laser has a power of 250-350W, a pulse width of 200-500μs, and a scanning speed of 100-120mm / min.

9. The method for strengthening copper-aluminum dissimilar metals through multi-process coupled laser welding as described in claim 1, characterized in that: In step S5, the criterion for defect assessment is that the area of ​​the pore defect is greater than 0.5 mm. 2 If the depth is greater than 0.2 mm, and a defect is determined to exist, the repeated execution of steps S2 to S5 involves performing 3 to 5 additional high-energy impact reinforcements within a 2-3 mm radius around the defect area.

10. The copper-aluminum dissimilar metal multi-process coupled laser welding strengthening method as described in claim 1, characterized in that: When the method is used for parallel processing of multiple welding points, a time-domain staggered peak control strategy is adopted, which divides the total cycle into two time windows on average, and the laser energy of adjacent welding units is staggered and started in different time windows.

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