A selective ultrasonic head for overcoming the edge effect of resistance welding and a resistance welding method thereof

Abstract

The application relates to a selected area ultrasonic head for overcoming the edge effect of resistance welding and a resistance welding method thereof, and relates to the field of resistance welding methods. In order to solve the problem that the edge effect of a resistance welding head is prone to occurring in the current welding process, the selected area ultrasonic head comprises a titanium shell, a brass static pressure block and a spring, wherein the brass static pressure block is connected through the spring and is embedded into the bottom end face of the titanium shell to form a working face; the brass static pressure block in the working face is a symmetrical structure and has a concave shape with wide ends and a narrow middle part. The method comprises the following steps: cleaning thermoplastic composite laminated plates and heating elements; building a welding joint structure; adjusting the pressure of a welding pressure head and welding current to carry out welding; before the welding is completed, ultrasonic vibration is applied to complete the welding. The application has the advantages of simple operation, low cost and higher joint strength. The application is used for resistance welding of thermoplastic resin-based composites.

Description

Technical Field

[0001] This invention relates to the field of resistance welding methods. Background Technology

[0002] The ongoing demand for lightweight structures in the aerospace field has strongly driven the development and application of high-performance composite materials. Fiber-reinforced thermoplastic composites, with their high specific strength, high specific modulus, excellent fatigue and corrosion resistance, as well as unique recyclability and re-molding capabilities, have become one of the ideal choices for next-generation aircraft materials. Compared to traditional thermoset composites, thermoplastic composites not only have shorter molding cycles but also allow for efficient connections through welding technology. Resistance welding, as an efficient, stable, and easily automated connection process, effectively avoids fiber cutting and stress concentration problems associated with riveting. While ensuring structural integrity and weight reduction, it also improves manufacturing efficiency and joint performance, providing a feasible technical route for the integrated manufacturing of large and complex aerospace components. However, during resistance welding, the different boundary conditions at different edges of the joint lead to uneven temperature distribution within the lap area. When current flows through the edge, the heating element near the edge outside the weld generates heat, creating an additional heating effect on the edge, resulting in a higher temperature at the edge than other areas, forming the so-called "edge effect." Figure 1 As shown, edge effects can severely impact joint strength.

[0003] To mitigate the edge effect at the joint, researchers have undertaken extensive work, such as using liquid nitrogen quenching or air cooling to lower the temperature in the high-temperature zone. However, these methods are difficult to control precisely and cannot guarantee stable welding quality. In addition, the uneven temperature field in resistance welding also suffers from a "size effect," which is amplified as the joint size increases. Therefore, the edge effect urgently needs to be addressed. Summary of the Invention

[0004] In order to solve the problem of edge effect of resistance welding joint that easily occurs in the current welding process, the present invention provides a selective ultrasonic head and resistance welding method to overcome the edge effect of resistance welding.

[0005] The ultrasonic head of this invention employs a copper block and a spring in the middle of its bottom end face. This ultrasonic head can selectively apply ultrasonic waves (the spring serves to weaken the effect of the ultrasonic waves) and also effectively dissipate excess heat from the joint during welding (the copper block plays a role in this process). The ultrasonic waves are applied in the final stage of the resistance welding process. Using this ultrasonic head, a tight bond in thermoplastic composite materials can be achieved under conventional resistance welding parameters, effectively overcoming edge effects in conventional resistance welding. This method is simple to operate, low in cost, and yields joints with higher strength.

[0006] A selective ultrasonic head for overcoming the edge effect of resistance welding, the selective ultrasonic head includes a titanium shell, a brass static pressure block and a spring, wherein the brass static pressure block is connected by the spring and embedded in the bottom end face of the titanium shell to form a working surface, and the brass static pressure block in the working surface has a symmetrical structure and is a concave shape that is wide at both ends and narrow in the middle.

[0007] Furthermore, the width of the brass static pressure block in the working surface is half the width of its two ends.

[0008] The main body of the ultrasonic head is made of titanium alloy, with brass in the middle. The reasons for choosing these two materials are as follows: titanium has a lower thermal conductivity, while brass has a higher thermal conductivity, resulting in greater heat dissipation in the middle compared to the sides during welding. Simultaneously, the brass section in the middle is connected by springs, which dampen and block the transmission of ultrasonic waves, concentrating the energy of the transmitted waves on the top and bottom sides, increasing heat generation on both sides, and improving the distribution of the welding temperature field. Viewed from bottom to top, the brass head is narrow in the middle and wide at the sides, with the middle width approximately half the width of the two edges.

[0009] This invention also proposes a resistance welding method that incorporates selective ultrasonic wave assistance (ultrasonic amplitude greater than 20kHz) during the welding process. This method applies selective ultrasonic waves simultaneously in the final stage of the resistance welding process, achieving a tight bond in thermoplastic composite materials under traditional resistance welding parameters. It is simple to operate, low in cost, and yields joints with higher strength.

[0010] A selective ultrasonic head method for resistance welding, which overcomes the edge effect of resistance welding, is adopted, and is carried out in the following steps:

[0011] 1. Clean the thermoplastic composite laminate and heating element to be welded;

[0012] 2. Apply a layer of thermoplastic resin film to the overlapping area of ​​the two thermoplastic composite laminates to be welded, and then place the heating element at the welding position of the overlapping area to overlap. The overlapping area forms an overlapping structure of thermoplastic composite laminate to be welded - thermoplastic resin film - heating element - thermoplastic resin film - thermoplastic composite laminate to be welded.

[0013] 3. Using a selective ultrasonic head that overcomes the edge effect of resistance welding as a welding head, pressure is applied above the lap structure described in step 2, the heating element is turned on to heat, and the pressure of the welding head and the current of the heating element are adjusted to perform welding.

[0014] Fourth, before the welding in step three is completed, apply ultrasonic vibration to finish the welding.

[0015] Furthermore, the thermoplastic composite laminate described in step one is made of fiber-reinforced thermoplastic resin-based composite material.

[0016] Furthermore, in the fiber-reinforced thermoplastic resin-based composite material, the fiber is carbon fiber, glass fiber, or aramid fiber; the thermoplastic resin is polyphenylene sulfide, nylon, polyetherimide, polyacrylonitrile-butadiene-styrene, polycarbonate, or polyetheretherketone.

[0017] Furthermore, for composite materials based on thermosetting resins such as epoxy resin (EP), phenolic resin (PF), furan resin (FF), and polybutadiene resin (PB), since they themselves lack thermoplasticity, surface modification is required to impart weldability. One effective method is to laminate a layer of thermoplastic resin onto their surface, forming a weldable interface. Suitable thermoplastic resins include polyphenylene sulfide (PPS), nylon (PA), polyetherimide (PEI), polyacrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), and polyetheretherketone (PEEK).

[0018] Furthermore, the heating element described in step one includes a stainless steel mesh, a titanium mesh, a carbon fiber unidirectional belt, or a carbon fiber fabric.

[0019] Furthermore, the thickness of the thermoplastic resin film described in step two is 100 μm.

[0020] Furthermore, in step three, the upper limit of the heating element's heating temperature is 50~1000℃, the current is 5A~220A, the welding pressure is 0.1~2.0MPa, and the welding time is 10~240s.

[0021] Furthermore, in step four, ultrasonic vibration is applied, with the ultrasonic vibration frequency controlled at 20~40kHz, the amplitude at 2~50µm, and the ultrasonic introduction time at 1~20s.

[0022] Furthermore, after completing the welding in step four, simultaneously turn off the resistance welding power supply and ultrasonic assistance, maintain the pressure in the welding area, and allow it to cool until the temperature drops to room temperature.

[0023] Conventional resistance welding suffers from edge effects, a major factor limiting joint quality. To address this, this invention designs an ultrasonic head for selectively applying ultrasonic waves to specific areas and applies it to the resistance welding process. This effectively overcomes the edge effects and improves joint strength. During welding, selective ultrasonic application has the following effects: 1. Applying ultrasound to the edge area raises the welding temperature to the center, creating a uniform temperature field; 2. The use of brass in the center area allows heat to dissipate faster than on the sides, reducing the temperature difference between the center and sides; 3. Regarding resin flow and fusion, edge ultrasound eliminates non-fusion at the weld edge, improving the weld ratio; 4. Simultaneously, it promotes resin flow from the edge to the center, suppressing central void defects caused by resin outflow.

[0024] This invention addresses the edge effect problem present in conventional resistance welding processes by designing an ultrasonic head with titanium sides and brass in the middle, connected to the titanium by a spring, enabling selective ultrasonic application. The core problem solved by this invention is the edge effect in the resistance welding of composite materials. Ultrasonic waves are applied by the edge-selective ultrasonic vibrating head, working in conjunction with a thermoplastic composite laminate, a heating element, and a thermoplastic film to construct the weld joint. The heating element generates heat through resistance to provide the necessary heat for resin melting; the selective ultrasonic action effectively solves the edge effect during welding and improves the wetting effect of the molten resin on the implant, enhancing the temperature uniformity of the weld interface; the thermoplastic film acts as a filler medium, fully filling the mesh of the heating element and reducing the porosity of the joint area. Based on this synergistic mechanism, this process significantly enhances the mechanical properties of the resin-based composite resistance welded joint.

[0025] Beneficial effects of this invention:

[0026] 1) A selective ultrasonic-assisted resistance welding process is adopted, which organically combines ultrasonic vibration heat generation with resistance heating effect, giving full play to the synergistic advantages of the two, and features simple process and high efficiency. During the welding process, the resistance heating element is retained inside the joint as an implant. The joint can be disassembled, repaired and re-welded by secondary power supply, which improves the flexibility and repairability of the process.

[0027] 2) A special ultrasonic head was designed, based on the temperature field distribution characteristics during the welding process. Figure 1 The ultrasonic head end face is made of titanium on both sides and brass in the middle, with the brass in contact with the titanium via springs. Titanium has a lower thermal conductivity, while brass has a higher thermal conductivity; their combination allows the welded joint to have greater heat dissipation in the middle than at the top and bottom. Simultaneously, the spring in the middle acts as a shock absorber, blocking the transmission of ultrasonic waves and concentrating the energy of the waves at the edges, effectively improving the uniformity of the welding temperature field.

[0028] 3) The added selective ultrasonic waves can promote the heating and melting of the unfused areas of the welding interface, improve the temperature uniformity of the welding interface, increase the area of ​​the interface fusion zone, and effectively prevent the occurrence of unwelded or thermally degraded phenomena.

[0029] 4) The applied ultrasonic vibration can use ultrasonic energy to cause the molten thermoplastic resin to wet the heating element to a greater extent, thereby achieving a better interface bond between the heating element surface and the thermoplastic resin, reducing defects such as porosity. This process is efficient, low-cost, highly adaptable, and easy to promote industrially.

[0030] 5) This invention uses ultrasonic vibration-assisted resistance welding of composite materials, utilizing the ultrasonic capability to improve the flow and filling ability of thermoplastic resin.

[0031] 6) The welding strength can be significantly improved compared to traditional resistance welding methods.

[0032] This invention is used for resistance welding of thermoplastic resin-based composite materials. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the edge effect in resistance welding.

[0034] Figure 2 The image shows a physical and schematic diagram of a selective ultrasonic head for overcoming the edge effect of resistance welding as described in Specific Embodiment 1; 1 is a titanium shell, 2 is a spring, and 3 is a brass static pressure block;

[0035] Figure 3 The image shows a partial physical and schematic diagram of a selective ultrasonic head for overcoming the edge effect of resistance welding as described in Specific Embodiment 1; 1 is a titanium shell, 2 is a spring, and 3 is a brass static pressure block;

[0036] Figure 4 The image shows a physical view and a schematic diagram of the bottom end face of a selective ultrasonic head for overcoming the edge effect of resistance welding as described in Specific Embodiment 1; 1 is a titanium shell, 2 is a spring, and 3 is a brass static pressure block;

[0037] Figure 5 Comparison of cross-sectional morphology of welded joint in Example 1;

[0038] Figure 6 This is a comparison chart of the mechanical properties of the welded joint in Example 1. Detailed Implementation

[0039] Specific Implementation Method 1: This implementation method provides a selective ultrasonic head that overcomes the edge effect of resistance welding. The selective ultrasonic head includes a titanium shell, a brass static pressure block, and a spring. The brass static pressure block is connected to and embedded in the bottom end face of the titanium shell through the spring to form a working surface. The brass static pressure block in the working surface has a symmetrical structure and is concave in shape, which is wide at both ends and narrow in the middle.

[0040] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the width of the middle section of the brass static pressure block in the working surface is half the width of both ends. Everything else is the same as in Specific Implementation Method One.

[0041] Features and effects of this implementation method:

[0042] 1. The selective ultrasonic-assisted composite ultrasonic head can be used without modifying the resistance welding process, without the need for additional mechanical structures, and is easy to operate, reducing the cost of use;

[0043] 2. Selective ultrasound-assisted composite ultrasonic head is simple to manufacture, low in cost, and has advantages for widespread application;

[0044] 3. The addition of a selective ultrasonic-assisted composite structure ultrasonic head layer strengthens the weld, optimizes the temperature field of the welding interface, increases the actual connection area, and improves the joint connection strength.

[0045] Specific Implementation Method 3: This implementation method employs a selective ultrasonic head to overcome the edge effect of resistance welding, specifically following these steps:

[0046] 1. Clean the thermoplastic composite laminate and heating element to be welded;

[0047] 2. Apply a layer of thermoplastic resin film to the overlapping area of ​​the two thermoplastic composite laminates to be welded, and then place the heating element at the welding position of the overlapping area to overlap. The overlapping area forms an overlapping structure of thermoplastic composite laminate to be welded - thermoplastic resin film - heating element - thermoplastic resin film - thermoplastic composite laminate to be welded.

[0048] 3. Using a selective ultrasonic head that overcomes the edge effect of resistance welding as a welding head, pressure is applied above the lap structure described in step 2, the heating element is turned on to heat, and the pressure of the welding head and the current of the heating element are adjusted to perform welding.

[0049] Fourth, before the welding in step three is completed, apply ultrasonic vibration to finish the welding.

[0050] Specific Implementation Method Four: This implementation method differs from Specific Implementation Method Three in that the thermoplastic composite laminate described in step one is made of fiber-reinforced thermoplastic resin-based composite material. Everything else is the same as in Specific Implementation Method Three.

[0051] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods Three or Four in that: in the fiber-reinforced thermoplastic resin-based composite material, the fiber is carbon fiber, glass fiber, or aramid fiber; the thermoplastic resin is polyphenylene sulfide, nylon, polyetherimide, polyacrylonitrile-butadiene-styrene, polycarbonate, or polyetheretherketone. Everything else is the same as in Specific Implementation Methods Three or Four.

[0052] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods Three to Five in that the heating element described in step one includes a stainless steel mesh, a titanium mesh, a carbon fiber unidirectional belt, or a carbon fiber fabric. Everything else is the same as in Specific Implementation Methods Three to Five.

[0053] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods Three to Six in that the thickness of the thermoplastic resin film described in step two is 100 μm. Everything else is the same as in Specific Implementation Methods Three to Four to Six.

[0054] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods Three to Seven in that: in step three, the upper limit of the heating element's heating temperature is 50~1000℃, the current is 5A~220A, the welding pressure is 0.1~2.0MPa, and the welding time is 10~240s. Everything else is the same as in Specific Implementation Methods Three to Seven.

[0055] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods Three to Eight in that: in step four, ultrasonic vibration is applied, with the ultrasonic vibration frequency controlled at 20~40kHz, the amplitude at 2~50µm, and the ultrasonic introduction time at 1~20s. Everything else is the same as in Specific Implementation Methods Three to Eight.

[0056] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods Three to Nine in that: after welding is completed in step four, the resistance welding power supply and ultrasonic assistance are simultaneously turned off, the pressure in the welding area is maintained, and the area is cooled until the temperature drops to room temperature. Everything else is the same as in Specific Implementation Methods Three to Nine.

[0057] The scope of this invention is not limited to the above-described embodiments; a combination of one or more specific embodiments can also achieve the purpose of the invention.

[0058] Example 1:

[0059] The glass fiber reinforced polyphenylene sulfide (GF / PPS) composite laminate and PPS film were wiped three times with ethanol to remove surface stains; at the same time, the stainless steel mesh heating element was ultrasonically cleaned to ensure its surface was clean.

[0060] Fabrication of selective composite ultrasonic heads, such as Figure 2 , 3 4. The ultrasonic head includes a titanium shell, a brass static pressure block, and a spring. The brass static pressure block is connected to and embedded in the bottom end face of the titanium shell via the spring, forming a working surface. The brass static pressure block in the working surface has a symmetrical structure and is concave in shape, wider at both ends and narrower in the middle. The width of the middle part of the brass static pressure block in the working surface is half the width at both ends. The spring dampens and blocks the transmission of ultrasonic waves, concentrating the energy of the transmitted ultrasonic waves on the upper and lower sides.

[0061] In sequence, the GF / PPS composite laminate, PPS film (100 μm thick), stainless steel mesh (200 mesh, wire diameter 0.05 mm, aperture 0.08 mm), another layer of PPS film, and another GF / PPS composite laminate are stacked together.

[0062] The specific operation is as follows: First, the PPS film is attached to the upper surface of the lower GF / PPS laminate. The heating element is placed at the predetermined welding position of the film. Then, the upper GF / PPS laminate with the PPS film attached to its lower surface is pressed together. Subsequently, pressure is applied to the surface of the upper GF / PPS laminate. The welding current is set to 16 A, the welding pressure to 440 N, the welding time to 120 s, and the ultrasonic assistance time to 6 s. Welding is compared using a conventional ultrasonic head and a selective area composite structure ultrasonic head.

[0063] During resistance welding, when the current is turned on and the welding time is 114 s, ultrasonic assistance is applied at the same time. When the welding time is 120 s, the power supply and ultrasonic waves are turned off simultaneously. After welding, the resin-based composite material joint is allowed to cool naturally, and a well-connected joint is finally obtained.

[0064] like Figure 5 As shown, comparing the cross-sections of joints obtained using conventional ultrasonic assistance reveals that, under conventional ultrasonic assistance conditions (without selective ultrasonic assistance), the central region exhibits good fusion due to concentrated heat. While the edge regions are effectively heated by ultrasonic assistance, the central region still suffers from uneven temperature distribution due to the ultrasonic application, failing to achieve full fusion across the entire interface and resulting in uneven weld bonding. However, with selective ultrasonic assistance, the width of the fusion zone along the weld increases, and the thickness of the fusion zone at the weld edge increases, becoming more consistent with the thickness of the central fusion zone. This effectively suppresses the unevenness of the temperature field, resulting in a joint with more uniform heat input.

[0065] The two types of joints were subjected to single-lap tensile and shear mechanical property tests, and the results are as follows: Figure 6 As shown, the weld joint strength under conventional ultrasonic assistance conditions is 7.07 kN, while the joint strength increased to 8.09 kN, an increase of 14.43%, after ultrasonic assistance with a selectively applied composite ultrasonic head. This indicates that selective ultrasonic assistance significantly enhances the mechanical properties of the joint.

Claims

1. A selective ultrasonic head for overcoming the edge effect of resistance welding, characterized in that... The selective ultrasonic head includes a titanium shell, a brass static pressure block and a spring. The brass static pressure block is connected to and embedded in the bottom end face of the titanium shell through the spring to form a working surface. The brass static pressure block in the working surface has a symmetrical structure and is a concave shape that is wide at both ends and narrow in the middle. The width of the brass static pressure block in the working surface is half the width of its two ends.

2. A method for resistance welding using a selective ultrasonic head as described in claim 1 to overcome the edge effect of resistance welding, characterized in that... This method is specifically carried out in the following steps:

1. Clean the thermoplastic composite laminate and heating element to be welded; 2. Apply a layer of thermoplastic resin film to the overlapping area of ​​the two thermoplastic composite laminates to be welded, and then place the heating element at the welding position of the overlapping area to overlap. The overlapping area forms an overlapping structure of thermoplastic composite laminate to be welded - thermoplastic resin film - heating element - thermoplastic resin film - thermoplastic composite laminate to be welded.

3. Using a selective ultrasonic head that overcomes the edge effect of resistance welding as a welding head, pressure is applied above the lap structure described in step 2, the heating element is turned on to heat, and the pressure of the welding head and the current of the heating element are adjusted to perform welding. Fourth, before the welding in step three is completed, apply ultrasonic vibration to finish the welding.

3. The method for resistance welding using a selective ultrasonic head to overcome the edge effect of resistance welding according to claim 2, characterized in that... The thermoplastic composite laminate described in step one is made of fiber-reinforced thermoplastic resin-based composite material.

4. The method for resistance welding using a selective ultrasonic head to overcome the edge effect of resistance welding according to claim 3, characterized in that... In the fiber-reinforced thermoplastic resin-based composite material, the fiber is carbon fiber, glass fiber, or aramid fiber; the thermoplastic resin is polyphenylene sulfide, nylon, polyetherimide, polyacrylonitrile-butadiene-styrene, polycarbonate, or polyetheretherketone.

5. A method for resistance welding using a selective ultrasonic head to overcome the edge effect of resistance welding according to claim 2, characterized in that... The heating element described in step one includes a stainless steel mesh, a titanium mesh, a carbon fiber unidirectional belt, or a carbon fiber fabric.

6. A method for resistance welding using a selective ultrasonic head to overcome the edge effect of resistance welding according to claim 2, characterized in that... The thickness of the thermoplastic resin film described in step two is 100 μm.

7. A method for resistance welding using a selective ultrasonic head to overcome the edge effect of resistance welding according to claim 2, characterized in that... Step 3: The upper limit of the heating element's heating temperature is 50~1000℃, the current is 5A~220A, the welding pressure is 0.1~2.0MPa, and the welding time is 10~240s.

8. A method for resistance welding using a selective ultrasonic head to overcome the edge effect of resistance welding according to claim 2, characterized in that... Step 4: Apply ultrasonic vibration, controlling the ultrasonic vibration frequency to be 20~40kHz, the amplitude to be 2~50um, and the ultrasonic introduction time to be 1~20s.

9. A method for resistance welding using a selective ultrasonic head to overcome the edge effect of resistance welding according to claim 2, characterized in that... After completing the welding in step four, simultaneously turn off the resistance welding power supply and ultrasonic assistance, maintain the pressure in the welding area, and allow it to cool until the temperature drops to room temperature.

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