A two-pass friction stir welding method for copper or copper alloys

Abstract

The present application belongs to the technical field of metal welding, and particularly relates to a double-pass friction stir welding method for copper or copper alloy. The double-pass friction stir welding method comprises the following steps: fixing a to-be-welded piece by pressing a backing, performing main welding on the to-be-welded piece to obtain a plasticized penetration-welded piece, performing cover welding on the obtained plasticized penetration-welded piece to obtain a surface-finished welded piece, and the main welding and the cover welding are both performed by using friction stir welding; the direction of the cover welding is opposite to the direction of the main welding. Compared with the prior art, the double-pass friction stir welding method can take into account deep fusion and surface quality of the large-thickness copper or copper alloy, and realizes efficient welding of the large-thickness copper or copper alloy.

Description

Technical Field

[0001] This invention relates to a two-pass friction stir welding method for copper or copper alloys, belonging to the field of metal welding technology. Background Technology

[0002] In fields such as power electrical engineering, new energy vehicles, and rail transportation, copper alloys are indispensable core structural materials due to their excellent electrical and thermal conductivity and superior mechanical strength. However, their high thermal conductivity, susceptibility to high-temperature oxidation, and large coefficient of thermal expansion also make them widely recognized as difficult-to-weld materials. Especially for copper alloy structural components with a thickness ≥20mm, heat dissipates rapidly during welding, and high-temperature oxidation and thermal stress severely threaten the integrity and precision of the joint. Therefore, achieving high-quality, non-destructive welding penetration of copper alloys has been a long-standing technical challenge in this field.

[0003] Currently, the welding process for thick copper alloys still mainly relies on fusion welding technologies such as Tungsten Inert Gas (TIG) welding and Melt Inert Gas (MIG) welding. The inherent melting-solidification process of these techniques easily leads to defects such as weld cracks, structural deformation, and decreased electrical conductivity. While other solid-state joining technologies such as diffusion welding and brazing can avoid the melting defects associated with fusion welding, they are limited by high equipment costs, complex processes, and low production efficiency, making it difficult to meet the large-scale demands of modern manufacturing for efficient and high-quality joining.

[0004] For the friction stir welding (FSW) solution for thick copper alloys, there are problems such as the lack of systematic quality assurance methods in the process implementation. This leads to unstable welding process, joint defects such as incomplete welding and poor surface formation, poor performance consistency, and difficulty in meeting the stringent requirements for high-reliability joints in fields such as power equipment.

[0005] Therefore, developing a friction stir welding method for thick copper or copper alloys that balances deep fusion and surface quality is of great practical significance. Summary of the Invention

[0006] This invention addresses the problems of high-cost welding methods required for deep penetration welding of thick copper or copper alloys in existing technologies, and the inefficiency of existing friction stir welding for thick copper or copper alloys. It provides a friction stir welding method for copper or copper alloys.

[0007] In this invention, the aforementioned thick copper or copper alloy specifically refers to copper or copper alloy plates with a thickness of 20-30 mm.

[0008] The technical solution of this invention to solve the above-mentioned technical problems is as follows: A method for two-pass friction stir welding of copper or copper alloys, comprising the following steps: (1) Press and fix the parts to be welded onto the backing; (2) Perform main welding on the parts to be welded to obtain plasticized and fully penetrated welded parts; (3) Perform cover welding on the plasticized welded part obtained in step (2) to obtain a surface finished welded part; Both the main welding and the cap welding are performed using friction stir welding. The direction of the cover welding is reversed along the main welding direction.

[0009] Preferably, the thickness of the copper or copper alloy is 20-30 mm.

[0010] Preferably, the friction stir welding method further includes a pretreatment step, the pretreatment step comprising: The flatness of the surface to be welded should be ≤0.1mm and the surface roughness should be ≤6.3μm.

[0011] More preferably, the pretreated workpiece is welded within 8-12 hours after pretreatment.

[0012] More preferably, the preprocessing step further includes: More preferably, the pretreatment step includes alkaline degreasing with a NaOH solution of 5-8% by mass, acid washing with H2SO4 of 10-15% by mass, rinsing with deionized water, and drying.

[0013] The above pretreatment steps are intended to provide the necessary surface quality foundation for achieving a tight fit between the surfaces to be welded during subsequent assembly.

[0014] Preferably, the butt joint gap of the parts to be welded is ≤0.2mm.

[0015] Preferably, the stirring pin of the friction stir welding includes a shoulder and a stirring pin coaxially fixed to the end face of the shoulder; The selection of the stirring needle is based on the following rules: 10-20mm thick plates: stirring needle diameter range 6-8mm, effective working length range 9-19mm; For 20-30mm thick plates: the diameter of the stirring needle is suitable for 8-10mm, and the effective working length is suitable for 18-28.5mm. The effective working length of the stirring pin specifically refers to the vertical distance from the lower end face of the shoulder to the tip of the stirring pin in the initial state before pressing down. The difference between its length and the preset welding depth is <0.5mm. The preset welding depth is 90-95% of the plate thickness.

[0016] Preferably, the backing in step (1) has a bending strength ≥250MPa, an elastic modulus ≥50GPa, and a thermal conductivity ≥5W / (m·K) at room temperature (23±5℃).

[0017] Preferably, the clamping in step (1) is achieved using a multi-point lateral clamping mechanism.

[0018] More preferably, the clamping force in step (1) is 8-12 kN.

[0019] Preferably, the alignment accuracy between the edge of the workpiece to be welded and the preset welding trajectory in step (1) is -0.1 to 0.1 mm.

[0020] Preferably, the purpose of fixing in step (1) includes limiting the maximum displacement of the workpiece to be welded during the welding process to ≤0.02mm.

[0021] Preferably, the process parameters for the main welding in step (2) are: Stirring needle rotation speed: 800-1200 rpm; welding speed: 50-80 mm / min; axial pressure: 15-25 kN.

[0022] More preferably, the process parameters for the main welding in step (2) are: Stirring needle rotation speed: 900-1150 rpm; welding speed: 50-80 mm / min; axial pressure: 15-25 kN.

[0023] More preferably, the main welding step in step (2) is as follows: After the stirring needle is pre-rotated and stabilized at a preset stirring needle speed, it is pressed down axially and moves along the welding trajectory.

[0024] More preferably, the axial downward pressing speed is 300 mm / min.

[0025] More preferably, the target of the axial downward pressure is a downward pressure of 0.12-0.18 mm on the shoulder.

[0026] It should be noted that the pressure applied is relative to the surface of the workpiece to be welded.

[0027] It should be noted that the aforementioned steps are intended to ensure that the shoulder fits tightly against the surface of the workpiece to be welded.

[0028] More preferably, the timing of traveling along the welding trajectory is when the welding torque fluctuation ranges from -5% to 5%.

[0029] Preferably, the process parameters for the cover welding in step (3) are: Stirring needle rotation speed: 900-1350 rpm; welding speed: 47-72 mm / min; axial pressure: 15-25 kN.

[0030] More preferably, the process parameters for the cover welding in step (3) are: Stirring needle rotation speed: 1030-1290 rpm; welding speed: 56-60 mm / min; axial pressure: 16-21 kN.

[0031] It should be noted that cover welding involves re-stirring the surface material of the weld to optimize its surface morphology and microstructure.

[0032] Preferably, the parameter design of the main welding in step (2) and the cover welding in step (3) follows the dynamic balance of heat input; The aforementioned dynamic balance of heat input refers to controlling the welding heat input (Q) per unit length by means of the stirring pin rotation speed (N), axial pressure (F), and welding speed (v), i.e., Q∝(N×F) / v, so that Q can meet the requirements of friction stir welding.

[0033] Preferably, the main welding in step (2) and the cover welding in step (3) are both performed under inert gas protection.

[0034] More preferably, the inert gas is argon with a purity of ≥99.99%.

[0035] More preferably, the inert gas flow rate for the main welding in the protection step (2) is 25-30 L / min, and the air replacement time is 30-60 s.

[0036] More preferably, the inert gas flow rate for the cover welding in the protection step (3) is 30-35 L / min, and the air replacement time is 30-60 s.

[0037] Preferably, the friction stir welding method further includes a post-weld treatment step.

[0038] More preferably, the post-weld treatment method includes the following steps: Cool the surface finishing welded part described in step (3) and remove the keyhole and defective areas.

[0039] More preferably, the post-weld treatment method further includes annealing.

[0040] It should be noted that the annealing step is applicable to products with requirements for dimensional stability or resistance to stress corrosion. The annealing is carried out in a protective atmosphere (such as argon), and the temperature and time are strictly set according to the alloy grade.

[0041] The present invention also provides welded parts obtained by the aforementioned two-pass friction stir welding method.

[0042] The beneficial effects of this invention are as follows: This invention uses quantitative clamping control and dual-pass welding heat input management as its core methods to specifically solve the problem that more expensive welding methods are required for deep penetration welding of thick copper or copper alloys. It provides a friction stir welding method and overcomes the problem of poor welding effect of conventional friction stir welding methods for thick copper or copper alloys. It achieves high-quality and high-reliability welding and promotes the application of friction stir welding technology in the field of high-performance copper or copper alloy thick plate joining.

[0043] The core of this invention lies in the synergistic process of deep plasticizing and penetration welding combined with surface finishing and re-stirring. The first pass of the secondary main weld ensures full-thickness plasticizing and penetration welding at the joint root, establishing a core load-bearing zone; the capping weld performs re-stirring and thermal control on the surface layer, simultaneously achieving defect elimination, grain refinement, and stress field optimization. This constructs a high-quality joint interface with homogeneous microstructure, uniform properties, and stable performance across the entire weld cross-section, promoting the engineering application of friction stir welding technology in the field of high-performance copper or copper alloy thick plates. Attached Figure Description

[0044] Figure 1 The flowchart provided by this invention illustrates the execution order and logical relationship of each step, in order to help understand the technical solution of this invention. Figure 2 This is a schematic diagram showing the shape and forces acting on the stirring needle component; Figure 3 This is a schematic diagram of the main welding process according to an embodiment of the present invention, which specifically shows the movement posture and travel path of the stirring pin; The attached diagram is labeled as follows: 1. Shoulder; 2. Stirring needle; 3. Left-hand thread; 4. Copper plate to be welded. Detailed Implementation

[0045] The principles and features of the present invention are described below. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0046] Figure 2 This is a schematic diagram of the shape and forces of the stirring pin component, where v is the welding speed, ω is the rotational speed of the stirring pin, P is the downward pressure exerted by the shoulder on the material below it, and F is the frictional force between the shoulder and the material below it.

[0047] Figure 3 This is a schematic diagram of the main welding process according to an embodiment of the present invention; combined with Figure 2 and 3The welding structure employed in this invention involves a shoulder 1 rigidly connected coaxially to the main shaft of a friction stir welding machine at its upper end, and a lower center rigidly fastened coaxially to the upper end of a stirring pin 2. A left-hand thread 3 is integrally formed on the outer wall of the stirring pin 2. During the welding process, after the stirring pin pre-rotates to a stable state, it is pressed axially downwards into the butt joint of the copper plates to be welded; subsequently, it moves at a uniform speed along the welding trajectory to complete the welding work.

[0048] In the embodiments of the present invention, the "high rigidity thermally conductive backing" mentioned refers to a backing with a bending strength ≥250MPa, an elastic modulus ≥50GPa, and a thermal conductivity ≥5W / (m·K) at 20℃; the butt joint gap of the parts to be welded is ≤0.2mm.

[0049] The stirring pin used in this embodiment of the invention is cylindrical with a left-handed spiral pattern on its outer surface. Its diameter and effective working length are selected to match the thickness of the copper alloy plate to be welded, and are based on the following rules: 10-20mm thick sheet metal, stirring needle diameter range 6-8mm, effective working length range 9-19mm; 20-30mm thick sheet metal, stirring needle diameter range 8-10mm, effective working length range 18-28.5mm; The effective working length of the stirring pin specifically refers to the vertical distance from the lower end face of the shoulder to the tip of the stirring pin in the initial state before pressing down. Its length matches the preset welding depth, which is 90-95% of the plate thickness. During actual welding, the value can be finely adjusted within the above matching range according to the specific process conditions to ensure that the tip of the stirring pin approaches but does not penetrate the back of the plate, achieving full penetration and no root forming defects.

[0050] Example 1 S1. Workpiece pretreatment: Select a 25mm thick pure copper (T2) plate as the workpiece to be welded. First, use a carbide end mill to rough mill and remove the surface oxide scale, and then use a diamond end mill for finish milling. Use cutting fluid to control the machining temperature throughout the process to ensure that the flatness of the surface to be welded after machining is ≤0.1mm and the surface roughness Ra is ≤6.3μm.

[0051] The surface is then cleaned: alkaline degreasing is performed using a 5% NaOH solution, acid pickling is performed using a 10% H2SO4 solution, and rinsing and drying are carried out with deionized water to make the surface to be welded clean and present a uniform metallic luster.

[0052] After pretreatment, subsequent assembly and welding should be carried out within 8 hours to avoid secondary oxidation of the surfaces to be welded.

[0053] S2. Fixing and Positioning: The pre-treated sheet metal is placed in the welding fixture and rigidly fixed using a high-rigidity thermally conductive backing and a multi-point lateral clamping mechanism. The alignment accuracy between the workpiece edge to be welded and the preset welding trajectory should be controlled within ≤±0.1mm, and a clamping force of 8kN should be applied to ensure that the maximum displacement of the workpiece during welding does not exceed 0.02mm.

[0054] S3. Welding Process Design: Select a stirring pin with a nominal diameter of 9mm and an effective working length of 22.2mm, matched with a preset welding depth of 22.5mm; based on the pure copper material, develop a two-pass process scheme with main welding parameters and capping welding parameters as follows: Table 1. Process Parameters for Example 1

[0055] Note: Protective gas parameters: Argon purity ≥ 99.99%. The flow rate corresponding to the main welding pass is 27 L / min; the flow rate corresponding to the capping welding pass is 32 L / min.

[0056] S4. Layered Collaborative Welding Operation: The entire welding process is carried out under argon protection. Before welding, argon gas is introduced to replace the air in the area; this process lasts for 50 seconds. Welding operations are then performed after the atmosphere has stabilized. S4.1 Main Welding: The stirring needle is first pre-rotated at a speed of 1100 rpm. After the speed stabilizes, it is axially pressed down at a speed of 300 mm / min to make the shoulder reach the preset reference pressing amount of 0.12 mm, so that the shoulder is tightly attached to the workpiece surface. The axial pressure of 17 kN is controlled to move at a uniform welding speed of 65 mm / min along the positive direction of the butt joint to complete the deep welding.

[0057] S4.2 Cover weld: After the main weld is completed, weld in the reverse direction along the same weld line. Follow the cover weld procedure specified in S3.

[0058] S5. Post-weld treatment: After the welded joint has cooled, the keyhole areas at both ends of the weld are first mechanically removed (machining allowance has been reserved) and then ground and polished; subsequently, the weld is cleaned and treated with anti-oxidation measures.

[0059] In some embodiments, a low-temperature stress-relief annealing process is further performed at 450°C for 4 hours under an argon protective atmosphere, followed by furnace cooling to room temperature.

[0060] Example 2 S1. Workpiece pretreatment: Select a 25mm thick chromium zirconium copper plate (chemical composition by mass percentage: Cr 0.95%, Zr 0.1%, balance Cu) as the workpiece to be welded. First, use a carbide end mill to rough mill and remove the surface oxide scale, and then use a diamond end mill for finish milling. Use cutting fluid to control the machining temperature throughout the process to ensure that the flatness of the surface to be welded after machining is ≤0.1mm and the surface roughness Ra is ≤6.3μm.

[0061] The surface is then cleaned: it undergoes alkaline degreasing with 8% NaOH solution, pickling with 15% H2SO4, rinsing (with deionized water), and drying to make the surface to be welded clean and give it a uniform metallic luster.

[0062] Subsequent assembly and welding should be carried out within 12 hours after pretreatment to avoid secondary oxidation of the surfaces to be welded.

[0063] S2. Fixing and Positioning: The pre-treated sheet metal is placed in the welding fixture and rigidly fixed using a high-rigidity thermally conductive backing and a multi-point lateral clamping mechanism. The alignment accuracy between the workpiece edge to be welded and the preset welding trajectory should be controlled within the range of ≤±0.1mm, and a clamping force of 12kN should be applied to ensure that the maximum displacement of the workpiece during welding does not exceed 0.02mm.

[0064] S3. Welding Process Design: A stirring pin with a nominal diameter of 9mm and an effective working length of 22.2mm is selected, matched with a preset welding depth of 22.5mm; a two-pass process scheme with main welding parameters and capping welding parameters is formulated based on the chromium-zirconium copper material, as follows: Table 2 Process Parameters for Example 2

[0065] Note: Protective gas parameters: Argon purity ≥ 99.99%. The flow rate corresponding to the main welding pass is 27 L / min; the flow rate corresponding to the capping welding pass is 32 L / min.

[0066] S4. Layered Collaborative Welding Operation: The entire welding process is carried out under argon protection. Before welding, argon gas is introduced to replace the air in the area; this process lasts for 50 seconds. Welding operations are then performed after the atmosphere has stabilized. S4.1 Main Welding: The stirring needle is first pre-rotated at a speed of 900 rpm. After the speed stabilizes, it is axially pressed down at 300 mm / min to make the shoulder reach the preset reference pressing amount of 0.13 mm, so that the shoulder is tightly attached to the workpiece surface. The axial pressure of 21 kN is controlled to move at a uniform welding speed of 65 mm / min along the positive direction of the butt joint to complete the deep welding. S4.2 Cover weld: After the main weld is completed, weld in the reverse direction along the same weld line. Follow the cover weld procedure specified in S3. S5. Post-weld treatment: After the welded joint has cooled, the keyhole areas at both ends of the weld are first mechanically removed (machining allowance has been reserved) and then ground and polished; subsequently, the weld is cleaned and treated with anti-oxidation measures. In some embodiments, a low-temperature stress-relief annealing process is further performed at 500°C for 4 hours under an argon protective atmosphere, followed by furnace cooling to room temperature.

[0067] Example 3 The only difference between this embodiment and Embodiment 1 is that the workpiece thickness is adjusted to 20mm, and the welding process parameters are adjusted accordingly. Specifically, only steps S1, S3, and S4 are adjusted as follows: S1. Workpiece pretreatment: Select a pure copper (T2) plate with a thickness of 20mm. The pretreatment steps (milling, cleaning, etc.) are the same as in Example 1.

[0068] S3. Welding Process Formulation: A stirring pin with a nominal diameter of 8mm and an effective working length of 18.5mm is selected, matched with a preset welding depth of 18.5mm; based on the heat input requirements of a 20mm thick plate, the parameters for the main welding and cover welding are adjusted as follows: Table 3 Process Parameters for Example 3

[0069] S4. Layered collaborative welding operation: The steps are the same as in Example 1, and the main welding and cover welding are performed according to the parameters in Table 3.

[0070] Example 4 The only difference between this embodiment and Embodiment 1 is that the thickness of the workpiece to be welded is adjusted to 30mm, and the welding process parameters are adjusted accordingly. Specifically, only steps S1, S3, and S4 are adjusted as follows: S1. Workpiece pretreatment: The workpiece to be welded is replaced with a 30mm thick pure copper (T2) plate, and the rest is the same as in Example 1.

[0071] S3. Welding Process Formulation: A stirring pin with a nominal diameter of 10mm and an effective working length of 27.0mm is selected, matched with a preset welding depth of 27.0mm; based on the heat input requirements of a 25mm plate thickness, the main welding parameters are adjusted as follows, and the cover welding parameters are determined according to the core relationship of this invention, as follows: Table 4 Process Parameters for Example 4

[0072] S4. Layered collaborative welding operation: The steps are the same as in Example 1, and the main welding and cover welding are performed according to the parameters in Table 4.

[0073] Example 5 The difference between this embodiment and Embodiment 1 lies only in the adjustment of welding process parameters. Specifically, the process parameter table for step S3 is adjusted as follows: Table 5 Process Parameters for Example 5

[0074] Example 6 The difference between this embodiment and Embodiment 1 lies only in the adjustment of welding process parameters. Specifically, the process parameter table for step S3 is adjusted as follows: Table 6 Process Parameters for Example 6

[0075] Example 7 The difference between this embodiment and Embodiment 1 lies only in the adjustment of welding process parameters. Specifically, the process parameter table for step S3 is adjusted as follows: Table 7 Process Parameters for Example 7

[0076] Comparative Example 1 The difference between this comparative example and Example 1 is that the welding process is limited to a single pass. Specifically, only steps S3 and S4 are adjusted as follows: S3. Welding Process Development: Based on the pure copper material, a single-pass welding parameter process plan is developed, with the following parameters: Table 8 Comparative Example 1 Process Parameter Table

[0077] S4. Single-pass welding: The entire welding process is carried out under argon protection; the stirring needle is first pre-rotated and stabilized at a speed of 1100 rpm, and then axially pressed down at 300 mm / min to make the shoulder reach the preset reference pressing amount of 0.12 mm, so that the shoulder is in close contact with the workpiece surface; the axial pressure is controlled at 17 kN and the welding speed is uniformly advanced along the positive direction of the butt joint at 65 mm / min to complete the welding.

[0078] Comparative Example 2 The difference between this comparative example and Example 2 is that the welding process is limited to a single pass. Specifically, only steps S3 and S4 are adjusted as follows: S3. Welding Process Development: Select a stirring pin with a nominal diameter of 9mm and an effective working length of 22.2mm, matched with a preset welding depth of 22.5mm; develop a single-pass welding parameter process plan based on the chromium-zirconium-copper material, as follows: Table 9 Comparative Example 2 Process Parameter Table

[0079] S4. Single-pass welding: The entire welding process is carried out under argon protection; the stirring needle is first pre-rotated and stabilized at a speed of 900 rpm, and then axially pressed down at 300 mm / min to make the shoulder reach the preset reference pressing amount of 0.13 mm, so that the shoulder is in close contact with the workpiece surface; the axial pressure is controlled at 21 kN, and the welding is carried out at a uniform speed of 65 mm / min along the positive direction of the butt joint to complete the welding.

[0080] Comparative Example 3 The core difference between this comparative example and Example 1 is that it employs a two-pass welding scheme with identical parameters for both the main welding and the capping welding; all other conditions remain the same as in Example 1. Specifically, only steps S3 and S4 are adjusted as follows: S3. Welding Process Development: Based on the pure copper material, a double-pass welding parameter process plan is developed, with the following parameters: Table 10 Comparative Example 3 Process Parameter Table

[0081] S4. Layered collaborative welding operation: The steps are the same as in Example 1, and the main welding and cover welding are performed according to the parameters in Table 10.

[0082] Comparative Example 4 The only difference between this comparative example and Example 1 is the double-pass welding process parameters. The specific steps S3 and S4 are adjusted as follows: S3. Welding Process Development: Based on the pure copper material, a double-pass welding parameter process plan is developed, with the following parameters: Table 11 Comparative Example 4 Process Parameter Table

[0083] S4. Layered collaborative welding operation: The steps are the same as in Example 1, and the main welding and cover welding are performed according to the parameters in Table 11.

[0084] Comparative Example 5 The only difference between this comparative example and Example 1 is the double-pass welding process parameters. Specifically, only steps S3 and S4 are adjusted as follows: S3. Welding Process Development: Based on the pure copper material, a double-pass welding parameter process plan is developed, with the following parameters: Table 12 Comparative Example 5 Process Parameter Table

[0085] S4. Layered collaborative welding operation: The steps are the same as in Example 1, and the main welding and cover welding are performed according to the parameters in Table 12.

[0086] Comparative Example 6 The only difference between this comparative example and Comparative Example 1 is that the thickness of the workpiece is adjusted to 15mm, a stirring needle with a nominal diameter of 7mm and an effective working length of 14.2mm is selected, and a preset welding depth of 14.5mm is matched; all other conditions are the same as in Example 1.

[0087] Comparative Example 7 The only difference between this comparative example and Comparative Example 1 is that the workpiece to be welded is changed to a 25mm Q&P980 high-strength steel plate. All other conditions are the same as in Example 1.

[0088] Test case The mechanical properties of the welded joints were tested strictly according to GB / T2651-2008 standard. The test used circular proportional specimens with a nominal diameter d=10mm, a short proportional gauge length L0=50mm, and a parallel section length L... c The sample diameter was ≥60mm, the sample processing accuracy met the standard requirements, the surface roughness Ra≤3.2μm, and there were no stress concentration defects. Mechanical property tests were performed on the examples and comparative examples, and the weld appearance was evaluated according to the weld morphology (flatness and whether there were surface overmelting wrinkles, double-sided flash, and / or uneven weld width defects). The results are shown in Table 13 below.

[0089] Table 13 Data from Examples and Comparative Examples

[0090] The obtained tensile strength, yield strength and elongation after fracture data are objective quantitative criteria for evaluating welding quality factors such as the internal density of the joint, the interfacial bonding performance and the uniformity of the microstructure.

[0091] As can be seen from the comparison between Comparative Example 1 and Example 1, Comparative Example 2 and Example 2, and Table 14, under the same basic process conditions, the weld quality of the double-pass process of the present invention is better, while the copper plate welded by the conventional single-pass process has surface defects and lower mechanical properties. This fully demonstrates that the conventional single-pass process is difficult to balance the deep fusion and surface forming quality of the thick plate weld, and is prone to forming defects and performance degradation, thus highlighting its process limitations in thick plate welding.

[0092] The welds obtained by Embodiments 1 and 2 of the present invention have beautiful and smooth welds and high mechanical strength, which fully demonstrates that the present invention successfully provides a high-quality friction stir welding method that can be applied to thick copper or copper alloys.

[0093] Table 14 Data from Examples and Comparative Examples

[0094] A comparison of Example 1 and Comparative Example 3, along with Table 15, reveals that under the same double-pass welding conditions, the weld quality obtained using the tailored matching parameters is excellent. In contrast, the copper plate obtained by reciprocating welding without parameter matching exhibits defects such as surface over-melting wrinkles, excessive flash on both sides, and uneven weld width, and also has lower mechanical properties. This indicates that reciprocating welding without matching parameters cannot meet the capping requirements of thick pure copper plates. The reason lies in the lack of heat input optimization for surface forming, which easily leads to surface forming defects and joint performance degradation.

[0095] Table 15 Data from Examples and Comparative Examples

[0096] As shown in Example 1, Comparative Examples 4 and 5, and Table 16, under the same double-pass welding conditions, the weld quality achieved using parameters within the limits of this invention is excellent. However, copper plates welded using parameters outside this range exhibit surface cracks and large-area wrinkles, resulting in a significant reduction in mechanical properties. This indicates that parameters exceeding the scope of this invention make reliable welding of thick pure copper plates difficult. This is because the heat input deviates significantly from the optimal welding window, easily leading to fatal forming defects and a rapid deterioration in joint performance.

[0097] Table 16 Data from Examples and Comparative Examples

[0098] A comparison of Comparative Examples 1 and 6, along with Table 17, reveals that under the same process conditions, the weld quality of thin pure copper plates is better, while thicker copper plates exhibit surface roughness, poor uniformity in forming, and lower mechanical properties. This indicates that as the plate thickness increases, the material's heat capacity increases significantly, and fixed heat input parameters are insufficient to meet the fusion and forming requirements of thick plates, leading to a significant increase in weld defect sensitivity and further highlighting the difficulty of welding thick copper plates.

[0099] Table 17 Comparative Experiment Data Table

[0100] As shown in Comparative Example 1 and Comparative Example 7, and Table 17, the weld formation quality of high-strength steel plates is good under the same process conditions. This indicates that welding pure copper plates is significantly more difficult. This suggests that copper's extremely high thermal conductivity leads to rapid heat dissipation, making it difficult to establish a stable high-temperature plasticization zone. This is the fundamental reason why copper has more forming defects, lower joint performance, and is far more difficult to weld than high-strength steel.

[0101] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for two-pass friction stir welding of copper or copper alloys, characterized in that, Includes the following steps: (1) Press and fix the parts to be welded onto the backing; (2) Perform main welding on the parts to be welded to obtain plasticized and fully penetrated welded parts; (3) Perform cover welding on the plasticized welded part obtained in step (2) to obtain a surface finished welded part; Both the main welding and the cap welding are performed using friction stir welding. The direction of the cover welding is reversed along the main welding direction.

2. The double-pass friction stir welding method according to claim 1, characterized in that, It also includes a preprocessing step, which includes: The flatness of the surface to be welded should be ≤0.1mm and the surface roughness should be ≤6.3μm.

3. The double-pass friction stir welding method according to claim 1, characterized in that, The backing described in step (1) has a bending strength ≥250MPa, an elastic modulus ≥50GPa, and a thermal conductivity ≥5W / (m·K) at 23±5℃.

4. The double-pass friction stir welding method according to claim 1, characterized in that, The clamping force described in step (1) is 8-12 kN.

5. The double-pass friction stir welding method according to claim 1, characterized in that, The process parameters for the main welding in step (2) are as follows: Stirring needle rotation speed: 800-1200 rpm; welding speed: 50-80 mm / min; axial pressure: 15-25 kN.

6. The double-pass friction stir welding method according to claim 5, characterized in that, The main welding steps described in step (2) are as follows: After the stirring needle is pre-rotated and stabilized at a preset stirring needle speed, it is pressed down axially and moves along the welding trajectory.

7. The double-pass friction stir welding method according to claim 6, characterized in that, The axial downward pressing speed is 300 mm / min.

8. The double-pass friction stir welding method according to claim 1, characterized in that, The process parameters for the cover welding in step (3) are as follows: Stirring needle rotation speed: 880-1380 rpm; welding speed: 47-72 mm / min; axial pressure: 15-25 kN.

9. The double-pass friction stir welding method according to claim 1, characterized in that, The main welding in step (2) and the cover welding in step (3) are both carried out under inert gas protection.

10. The double-pass friction stir welding method according to claim 9, characterized in that, The inert gas is argon.

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