Method for monitoring the quality of ultrasonic welded joints of fiber-reinforced thermoplastic composites

Abstract

The application discloses a kind of fiber reinforced thermoplastic composite ultrasonic welding joint quality monitoring methods, belong to composite ultrasonic welding joint quality monitoring technical field, comprising: two to be welded workpieces are fixed on anvil;Reflective film is pasted on the surface of to be welded workpiece;Ultrasonic welding head is selected and image calibration point is set on its surface, while setting ultrasonic welding parameter and amplitude threshold range;The angle of high-speed camera, first laser displacement sensor and second laser displacement sensor is adjusted, and ultrasonic welding is started;During welding process, the amplitude of ultrasonic welding head, the amplitude of upper layer to be welded workpiece, the amplitude of lower layer to be welded workpiece are monitored in real time, are transmitted to amplitude acquisition comparison system and compared with preset amplitude, judge whether welding process is normal, and stop welding when abnormal.The application reflects the transmission of ultrasonic energy to to-be-welded workpiece by real-time monitoring the vibration of ultrasonic welding head and to-be-welded workpiece during welding process, and realizes the monitoring of joint forming.

Description

Technical Field

[0001] This invention relates to the field of quality monitoring technology for ultrasonic welded joints of composite materials, and in particular to a method for monitoring the quality of ultrasonic welded joints of fiber-reinforced thermoplastic composite materials. Background Technology

[0002] Fiber-reinforced thermoplastic composites have become the most attractive lightweight engineering materials due to their numerous advantages, such as short molding cycle, high specific strength, reusability and recyclability, and ease of connection and repair. They have been widely used in fields such as rail transportation, aerospace and defense.

[0003] As equipment becomes increasingly complex, joining processes inevitably arise to reduce forming costs. Currently, commonly used joining techniques include mechanical joining, bonding, and welding. Mechanical joining and bonding, however, increase processing costs due to the need for additional joining materials. Given the remelting and molding capabilities of thermoplastic materials, welding technology has become a promising forming process for joining continuous fiber-reinforced thermoplastic composites. Ultrasonic welding, in particular, has become the most widely applicable joining technology due to its high efficiency, minimal surface damage, and ease of automation.

[0004] However, due to the short duration of ultrasonic welding and the lack of effective methods for real-time monitoring of the composite welding process, current monitoring of composite welding mainly involves real-time monitoring of welding parameters such as ultrasonic amplitude, frequency, and welding energy, as well as the joint temperature. These monitoring methods cannot directly reflect the changes in the joint during ultrasonic welding.

[0005] Therefore, in order to ensure the reliability of joint formation, there is an urgent need for a real-time monitoring technology to directly monitor the ultrasonic welding process of composite materials, so as to ensure the formation of high-quality welded joints. Summary of the Invention

[0006] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a method for monitoring the quality of ultrasonic welded joints of fiber-reinforced thermoplastic composites. By real-time monitoring of the vibration of the ultrasonic welding head and the workpiece to be welded during the welding process to reflect the transmission of ultrasonic energy to the workpiece, the joint formation can be monitored, thereby solving the problem of difficulty in real-time monitoring during ultrasonic welding of fiber-reinforced thermoplastic composites.

[0007] According to one aspect of the present invention, a method for monitoring the quality of ultrasonically welded joints of fiber-reinforced thermoplastic composites is provided. The monitoring method uses a monitoring system comprising an ultrasonic welding machine controller, an amplitude acquisition and comparison system, an ultrasonic welding machine, an anvil, a fixture, an ultrasonic welding head, a high-speed camera, a first laser displacement sensor, and a second laser displacement sensor. The high-speed camera is located to the side of the ultrasonic welding head, and the first and second laser displacement sensors are located above the upper and lower workpieces to be welded, respectively. The monitoring method includes: assembling the upper and lower workpieces to be welded in a single-lap joint configuration and fixing them to the anvil using the fixture; attaching reflective films to the surfaces of the upper and lower workpieces to be welded, the reflective films being used to reflect laser light to measure the vibration amplitude of the workpieces; selecting a corresponding ultrasonic welding head according to the material and thickness of the workpieces to be welded and setting image calibration points on the side surface of the ultrasonic welding head, while simultaneously setting ultrasonic welding parameters and an amplitude threshold range; and adjusting the high-speed camera and the first laser displacement sensor. The positions and angles of the high-speed camera and the ultrasonic welding head are adjusted so that the first laser displacement sensor is perpendicular to the surface of the upper workpiece to be welded, and the second laser displacement sensor is perpendicular to the surface of the lower workpiece to be welded. Ultrasonic welding is initiated. During the welding process, the high-speed camera monitors the first amplitude of the ultrasonic welding head in real time and transmits it to the amplitude acquisition and comparison system for comparison with a preset amplitude. The first laser displacement sensor monitors the second amplitude of the upper workpiece to be welded in real time and transmits it to the amplitude acquisition and comparison system for comparison with a preset amplitude. The second laser displacement sensor monitors the third amplitude of the lower workpiece to be welded in real time and transmits it to the amplitude acquisition and comparison system for comparison with a preset amplitude. Based on the comparison results, the amplitude acquisition and comparison system determines whether the welding process is normal and triggers a stop welding signal to the ultrasonic welding machine controller to stop welding in case of an abnormality, thereby completing the ultrasonic welding and achieving real-time monitoring of the composite material ultrasonic welding process.

[0008] Optionally, the material of the workpiece to be welded is a thermoplastic composite material reinforced with short fibers or continuous fibers, wherein the type of fiber is carbon fiber, glass fiber, basalt fiber or natural fiber, and the matrix is ​​polyphenylene sulfide, polyether ether ketone or polyetherimide.

[0009] Optionally, the tooling fixture includes clamping plates that conform to the shape of the workpiece to be welded. One clamping plate is used to fix the upper workpiece to be welded, and the other clamping plate is used to fix the lower workpiece to be welded. A process pad of the same thickness as the lower workpiece to be welded is placed below the upper workpiece to be welded. The two clamping plates press the opposite ends of the two workpieces to be welded respectively and are connected to the anvil by bolts.

[0010] Optionally, the reflective film is pasted on the upper surface of the workpiece to be welded, at a position close to but not in contact with the outer edge of the ultrasonic welding head, and the light beams emitted by the first laser displacement sensor and the second laser displacement sensor are respectively perpendicularly irradiated onto the upper workpiece reflective film and the lower workpiece reflective film; the reflective film is made of aluminum film, gold film or silver film.

[0011] Optionally, the ultrasonic welding parameters include: ultrasonic frequency of 20kHz, ultrasonic amplitude of 20~40μm, welding pressure of 0.1~0.3MPa, welding time of 0.2~9.9s, and welding speed of 5~15mm / s.

[0012] Optionally, the ultrasonic welding head is made of 7-series high-strength aluminum alloy or TC4 titanium alloy, and the side surface of the ultrasonic welding head is kept smooth.

[0013] Optionally, the image calibration points are black and white alternating dots, stripes, or grid patterns, located in the center of the high-speed camera's field of view.

[0014] Optionally, if the first amplitude exceeds ±10% of the preset ultrasonic amplitude, it is determined to be abnormal and welding is stopped; otherwise, it is determined to be normal and welding continues.

[0015] Optionally, if the second amplitude is higher than 90% of the preset ultrasonic amplitude, it is determined to be normal and welding continues; if the second amplitude is lower than 90% of the preset ultrasonic amplitude but higher than 70% of the preset ultrasonic amplitude, the weld point is marked as a suspicious weld point and welding continues; if the second amplitude is lower than 70% of the preset ultrasonic amplitude, it is determined to be abnormal and welding stops.

[0016] Optionally, if the third amplitude is less than 10% of the preset ultrasonic amplitude, it is determined to be normal and welding continues; if the third amplitude is more than 10% of the preset ultrasonic amplitude, it is determined to be abnormal and welding stops.

[0017] The beneficial effects of this invention are:

[0018] This invention monitors the joint welding quality by real-time monitoring of the vibration amplitude of the ultrasonic welding head, the upper workpiece, and the lower workpiece during ultrasonic welding. Ultrasonic welding of fiber-reinforced thermoplastic composites relies on the ultrasonic welding head transmitting ultrasonic vibrations to the workpiece, causing it to viscoelastically generate heat, melting the resin, and forming a good joint. Compared to existing technologies that monitor welding parameters and interface temperatures, these methods do not directly reflect changes in the workpiece and joint formation during the welding process. This invention, by real-time monitoring of the vibration amplitude of the ultrasonic welding head, the upper workpiece, and the lower workpiece, directly reflects the changes in these components and the joint formation process. Furthermore, it allows for the identification of problems based on changes observed at different monitoring locations, leading to the development of corresponding improvement measures and processes. This invention more directly reflects the joint formation process during welding, is simpler to implement, highly practical, and easily applicable in industrial production. Attached Figure Description

[0019] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings:

[0020] Figure 1 This is a schematic diagram of the ultrasonic welding joint of fiber-reinforced thermoplastic composite in this invention;

[0021] Figure 2 This is a flowchart of the method for monitoring the quality of ultrasonic welded joints of fiber-reinforced thermoplastic composites in this invention;

[0022] The components are: 1-Ultrasonic welding machine controller; 2-Amplitude acquisition and comparison system; 3-Ultrasonic welding machine; 4-Anvil; 5-Tooling fixture; 6-Ultrasonic welding head; 7-Workpiece to be welded; 8-Upper workpiece reflective film; 9-Lower workpiece reflective film; 10-High-speed camera; 11-First laser displacement sensor; 12-Second laser displacement sensor; 13-Process pad; 14-Image calibration point. Detailed Implementation

[0023] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, and not all of them. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present application. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present application can be combined with each other.

[0024] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0025] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0026] Example 1: This example provides a method for monitoring the quality of ultrasonically welded joints of fiber-reinforced thermoplastic composites. It uses a monitoring system, referring to... Figure 1 The monitoring system includes an ultrasonic welding machine controller 1, an amplitude acquisition and comparison system 2, an ultrasonic welding machine 3, an anvil 4, tooling fixtures 5, an ultrasonic welding head 6, a high-speed camera 10, a first laser displacement sensor 11, and a second laser displacement sensor 12.

[0027] The high-speed camera 10, the first laser displacement sensor 11, and the second laser displacement sensor 12 are fixed on the ultrasonic welding machine 3 and are respectively located on the side of the ultrasonic welding head 6, above the upper workpiece to be welded (also referred to as the upper workpiece) and the lower workpiece to be welded (also referred to as the lower workpiece), and are used to detect the vibration amplitude of the ultrasonic welding head, the upper workpiece to be welded, and the lower workpiece to be welded during the welding process.

[0028] like Figure 2 The monitoring method includes the following steps:

[0029] S1, fix the two workpieces 7 to be welded on the anvil 4 respectively by the tooling fixture 5 to ensure the forming accuracy of the joint. The assembly method of the workpieces 7 to be welded is single lap joint.

[0030] The workpiece 7 to be welded is made of short fiber or continuous fiber reinforced thermoplastic composite material; the fiber type is carbon fiber, glass fiber, basalt fiber or natural fiber, and the matrix is ​​polyphenylene sulfide, polyether ether ketone or polyetherimide.

[0031] The two tooling fixtures 5 each include a clamping plate that conforms to the shape of the workpiece 7 to be welded. One clamping plate is used to fix the upper workpiece to be welded, and the other clamping plate is used to fix the lower workpiece to be welded. A process pad 13 of the same thickness as the lower workpiece to be welded is placed under the upper workpiece to be welded. The two clamping plates press the opposite ends of the two workpieces to be welded respectively, and are connected to the anvil 4 by bolts, so that the clamping plates can press the workpiece 7 to be welded and prevent the workpiece 7 to be welded from shifting during the welding process. The shape of the anvil 4 is adapted to the workpiece 7 to ensure the welding accuracy.

[0032] S2, reflective films are pasted on the surfaces of the upper and lower workpieces to be welded respectively. The reflective films are pasted on the upper surfaces of the two workpieces to be welded 7, and are positioned near the outer edge of the ultrasonic welding head 6, but not in contact with the ultrasonic welding head 6.

[0033] The reflective film is used to reflect the laser beam emitted by the laser displacement sensor. The material is aluminum film, gold film or silver film. In practical applications, it can be selected according to the specific laser wavelength used to increase the sensitivity of the test.

[0034] S3. Select an ultrasonic welding head 6 of appropriate material according to the type (i.e. the material type and thickness of the workpiece 7 to be welded) and the image calibration point 14 is set on the side of the ultrasonic welding head 6, and set appropriate ultrasonic welding parameters and amplitude threshold range through the ultrasonic welding machine controller 1.

[0035] The ultrasonic welding head 6 is made of 7-series high-strength aluminum alloy or TC4 titanium alloy, and the side surface of the ultrasonic welding head 6 is kept smooth to improve the testing accuracy.

[0036] The ultrasonic welding parameters are as follows: ultrasonic frequency of 20kHz, ultrasonic amplitude of 20~40μm, welding pressure of 0.1~0.3MPa, welding time of 0.2~9.9s, and welding speed of 5~15mm / s.

[0037] Image calibration points 14 are black and white alternating dots, stripes or grid patterns, drawn on the side surface of ultrasonic welding head 6 to increase the sensitivity of the test (the high-speed camera judges the amplitude of the ultrasonic welding head based on the moving distance of the image calibration points).

[0038] S4, adjust the angles of the high-speed camera 10, the first laser displacement sensor 11, and the second laser displacement sensor 12 so that the high-speed camera 10, the first laser displacement sensor 11, and the second laser displacement sensor 12 are perpendicular to the corresponding test piece;

[0039] Specifically, the angle between the high-speed camera 10 and the side of the ultrasonic welding head 6 is 90°, the angle between the first laser displacement sensor 11 and the upper surface of the upper workpiece 7 to be welded is 90°, the angle between the second laser displacement sensor 12 and the upper surface of the lower workpiece 7 to be welded is 90°, the image calibration point 14 is located in the central area of ​​the field of view of the high-speed camera 10, and the light beams emitted by the first laser displacement sensor 11 and the second laser displacement sensor 12 are perpendicularly irradiated onto the corresponding reflective film (the laser displacement sensor measures the distance to the object to be measured by emitting a laser beam and receiving the reflected light, thereby determining the moving distance of the object to be measured, i.e., the vibration amplitude of the workpiece to be welded).

[0040] S5, start ultrasonic welding, monitor the first amplitude of the ultrasonic welding head in real time through the high-speed camera 10, and compare it with the ultrasonic amplitude preset in step S3 (i.e., the ultrasonic amplitude in the ultrasonic welding parameters) in the amplitude acquisition and comparison system 2: the amplitude deviation of the ultrasonic welding head should be within ±10% of the preset ultrasonic amplitude. If the first amplitude exceeds ±10% of the preset ultrasonic amplitude (i.e., the amplitude of the ultrasonic welding head has not reached the specified value), it means that there is a problem with the amplitude of the ultrasonic welding head 6, and welding should be stopped immediately.

[0041] S6, the second amplitude of the upper workpiece to be welded is monitored in real time by the first laser displacement sensor 11, and compared with the preset ultrasonic amplitude in the amplitude acquisition and comparison system 2: if the second amplitude is higher than 90% of the preset ultrasonic amplitude, it is judged to be normal and welding continues; if the second amplitude is lower than 90% of the preset ultrasonic amplitude but higher than 70% of the preset ultrasonic amplitude, welding can continue and this weld point is marked as a suspicious weld point; if the second amplitude is lower than 70% of the preset ultrasonic amplitude, it means that there is a problem with the amplitude transmission from the ultrasonic welding head 6 to the workpiece, and welding is stopped immediately.

[0042] S7, the third amplitude of the lower workpiece to be welded is monitored in real time by the second laser displacement sensor 12, and compared with the preset ultrasonic amplitude in the amplitude acquisition and comparison system 2: if the third amplitude is lower than 10% of the preset ultrasonic amplitude, it is judged to be normal and welding continues; if the third amplitude is higher than 10% of the preset ultrasonic amplitude, it means that there is a problem with the assembly of the workpiece, which may cause the workpiece to be welded 7 to overheat, and welding is stopped immediately.

[0043] S8 completes ultrasonic welding and enables real-time monitoring of the ultrasonic welding process of composite materials.

[0044] Example 2: This example provides a method for monitoring the quality of a single-point fiber-reinforced thermoplastic composite ultrasonic welded joint, including:

[0045] S1, a 2mm thick continuous glass fiber reinforced polyphenylene sulfide composite material plate is selected as the welding material. The single-lapped workpiece 7 to be welded is fixed on the anvil 4 by the tooling fixture 5 to ensure the forming accuracy of the joint.

[0046] S2, attach the workpiece reflective film 8 and the lower workpiece reflective film 9 to the upper surface of the two workpieces 7 to be welded, respectively, to increase the sensitivity of the test. The reflective film is made of aluminum film. Specifically, the upper workpiece reflective film 8 is attached to the upper surface of the workpiece 7 to be welded below the first laser displacement sensor 11, and the lower workpiece reflective film 9 is attached to the upper surface of the workpiece 7 to be welded below the second laser displacement sensor 12.

[0047] S3. Select an ultrasonic welding head 6 of appropriate material according to the type and thickness of the workpiece 7 to be welded, and set image calibration points 14 on the side surface of the ultrasonic welding head 6. At the same time, set appropriate ultrasonic welding parameters: select an ultrasonic welding head of 7075 aluminum alloy, select a black and white grid pattern as image calibration points, ultrasonic frequency of 20kHz, ultrasonic amplitude of 30μm, welding pressure of 0.15MPa, and welding time of 2s.

[0048] S4, adjust the angles of the high-speed camera 10, the first laser displacement sensor 11 and the second laser displacement sensor 12 so that the three are at 90° to the ultrasonic welding head 6, the upper workpiece reflective film 8 and the lower workpiece reflective film 9 respectively. At the same time, the image calibration point 14 is located at the center of the field of view of the high-speed camera 10, and the laser beams emitted by the first laser displacement sensor 11 and the second laser displacement sensor 12 can respectively illuminate the corresponding reflective films.

[0049] S5, monitor the first amplitude test result and compare it with the preset ultrasonic amplitude in step S3: monitor the ultrasonic amplitude output of the ultrasonic welding head 6 in real time through the high-speed camera 10, and compare the test result with the preset value through the amplitude acquisition and comparison system 2. The real-time amplitude is 30±1.5μm, which meets the requirements.

[0050] S6, monitor the second amplitude test result and compare it with the preset amplitude; monitor the amplitude of the upper workpiece 7 to be welded in real time through the first laser displacement sensor 11, and compare the test result with the preset value through the amplitude acquisition and comparison system 2. The real-time amplitude is 24.3±0.8μm, which meets the requirements;

[0051] S7, monitor the third amplitude test result and compare it with the preset amplitude; monitor the amplitude of the lower workpiece 7 to be welded in real time through the second laser displacement sensor 12, and compare the test result with the preset value through the amplitude acquisition and comparison system 2. The real-time amplitude is 2.5±0.27μm, which meets the requirements;

[0052] S8 completes ultrasonic welding and achieves real-time monitoring of the ultrasonic welding process of composite materials. If the monitored amplitude meets the set threshold, the ultrasonic welding process is judged to be good, and a high-quality welded joint is formed.

[0053] Example 3: This example provides a method for monitoring the quality of ultrasonically welded joints of continuous fiber-reinforced thermoplastic composites, including:

[0054] S1, a 1.5mm thick short-cut carbon fiber reinforced polyether ether ketone composite material plate is selected as the welding material. The single-lapped welding workpiece 7 is fixed on the anvil 4 by the tooling fixture 5 to ensure the forming accuracy of the joint.

[0055] S2, attach the workpiece reflective film 8 and the lower workpiece reflective film 9 to the upper surface of the two workpieces 7 to be welded, respectively, to increase the sensitivity of the test. The reflective film is made of aluminum film. Specifically, the upper workpiece reflective film 8 is attached to the upper surface of the workpiece 7 to be welded below the first laser displacement sensor 11, and the lower workpiece reflective film 9 is attached to the upper surface of the workpiece 7 to be welded below the second laser displacement sensor 12.

[0056] S3. Select an ultrasonic welding head 6 of appropriate material according to the type and thickness of the workpiece 7 to be welded, and set image calibration points 14 on the side surface of the ultrasonic welding head 6. At the same time, set appropriate ultrasonic welding parameters: select an ultrasonic welding head of TC4 titanium alloy material, select a black and white striped pattern as the image calibration points, ultrasonic frequency of 20kHz, ultrasonic amplitude of 38μm, welding pressure of 0.2MPa, and welding speed of 10mm / s.

[0057] S4, adjust the angles of the high-speed camera 10, the first laser displacement sensor 11 and the second laser displacement sensor 12 so that the three are at 90° to the ultrasonic welding head 6, the upper workpiece reflective film 8 and the lower workpiece reflective film 9 respectively. At the same time, the image calibration point 14 is located at the center of the field of view of the high-speed camera 10, and the laser beams emitted by the first laser displacement sensor 11 and the second laser displacement sensor 12 can respectively illuminate the corresponding reflective films.

[0058] S5, monitor the first amplitude test result and compare it with the preset ultrasonic amplitude in step S3; monitor the ultrasonic amplitude output of the ultrasonic welding head 6 in real time through the high-speed camera 10, and compare the test result with the preset value through the amplitude acquisition and comparison system 2. The real-time amplitude is 35±0.85μm, which meets the requirements.

[0059] S6, monitor the second amplitude test result and compare it with the preset amplitude; monitor the amplitude of the upper workpiece 7 to be welded in real time through the first laser displacement sensor 11, and compare the test result with the preset value through the amplitude acquisition and comparison system 2. The real-time amplitude is 30.9±0.43μm, which meets the requirements;

[0060] S7, monitor the third amplitude test result and compare it with the preset amplitude; monitor the amplitude of the lower workpiece 7 to be welded in real time through the second laser displacement sensor 12, and compare the test result with the preset value through the amplitude acquisition and comparison system 2. The real-time amplitude is 1.7±0.11μm, which meets the requirements;

[0061] S8 completes ultrasonic welding and achieves real-time monitoring of the ultrasonic welding process of composite materials. If the monitored amplitude meets the set threshold, the ultrasonic welding process is judged to be good, and a high-quality welded joint is formed.

[0062] The sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0063] The steps in the method of this invention can be adjusted, combined, or deleted according to actual needs. The technical features can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the embodiments are described. However, as long as the combinations of these technical features do not contradict each other, they should all be considered within the scope of this invention.

[0064] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for monitoring the quality of ultrasonically welded joints of fiber-reinforced thermoplastic composites, characterized in that, The monitoring method uses a monitoring system, which includes an ultrasonic welding machine controller, an amplitude acquisition and comparison system, an ultrasonic welding machine, an anvil, tooling fixtures, an ultrasonic welding head, a high-speed camera, a first laser displacement sensor, and a second laser displacement sensor. The high-speed camera is located to the side of the ultrasonic welding head, and the first and second laser displacement sensors are located above the upper and lower workpieces to be welded, respectively. The monitoring method includes: The upper and lower workpieces to be welded are assembled in a single lap joint and fixed on the anvil by tooling fixtures. Reflective films are attached to the surfaces of the upper and lower workpieces to be welded, respectively. The reflective films are used to reflect laser light to measure the vibration amplitude of the workpieces to be welded. Select the appropriate ultrasonic welding head according to the material and thickness of the workpiece to be welded, set image calibration points on the side surface of the ultrasonic welding head, and set the ultrasonic welding parameters and amplitude threshold range. Adjust the position and angle of the high-speed camera, the first laser displacement sensor, and the second laser displacement sensor so that the high-speed camera is perpendicular to the side of the ultrasonic welding head, the first laser displacement sensor is perpendicular to the surface of the upper workpiece to be welded, and the second laser displacement sensor is perpendicular to the surface of the lower workpiece to be welded. Initiate ultrasonic welding. During the welding process, the first amplitude of the ultrasonic welding head is monitored in real time by the high-speed camera. The first amplitude is then transmitted to the amplitude acquisition and comparison system and compared with the preset amplitude. The second amplitude of the upper workpiece to be welded is monitored in real time by the first laser displacement sensor, and the second amplitude is transmitted to the amplitude acquisition and comparison system for comparison with the preset amplitude. The third amplitude of the lower workpiece to be welded is monitored in real time by the second laser displacement sensor, and the third amplitude is transmitted to the amplitude acquisition and comparison system for comparison with the preset amplitude. The amplitude acquisition and comparison system determines whether the welding process is normal based on the comparison results, and triggers a stop welding signal to be sent to the ultrasonic welding machine controller to stop welding when there is an abnormality, thereby completing ultrasonic welding and realizing real-time monitoring of the ultrasonic welding process of composite materials. If the first amplitude exceeds ±10% of the preset ultrasonic amplitude, it is determined to be abnormal and welding is stopped; otherwise, it is determined to be normal and welding continues. If the second amplitude is higher than 90% of the preset ultrasonic amplitude, it is considered normal and welding continues; If the second amplitude is lower than 90% but greater than 70% of the preset ultrasonic amplitude, then mark this weld point as a suspicious weld point and continue welding. If the second amplitude is less than 70% of the preset ultrasonic amplitude, it is considered abnormal and welding is stopped. If the third amplitude is less than 10% of the preset ultrasonic amplitude, it is considered normal and welding continues. If the third amplitude is higher than 10% of the preset ultrasonic amplitude, it is determined to be abnormal and welding is stopped.

2. The monitoring method according to claim 1, characterized in that, The workpiece to be welded is made of short fiber or continuous fiber reinforced thermoplastic composite material, wherein the fiber is carbon fiber, glass fiber, basalt fiber or natural fiber, and the matrix is ​​polyphenylene sulfide, polyether ether ketone or polyetherimide.

3. The monitoring method according to claim 1, characterized in that, The tooling fixture includes clamps that conform to the shape of the workpiece to be welded. One clamp is used to fix the upper workpiece to be welded, and the other clamp is used to fix the lower workpiece to be welded. A process pad of the same thickness as the lower workpiece to be welded is placed below the upper workpiece to be welded. The two clamps press the opposite ends of the two workpieces to be welded respectively and are connected to the anvil by bolts.

4. The monitoring method according to claim 1, characterized in that, The reflective film is pasted on the upper surface of the workpiece to be welded, near the outer edge of the ultrasonic welding head but not in contact with it. The light beams emitted by the first laser displacement sensor and the second laser displacement sensor respectively irradiate the upper workpiece reflective film and the lower workpiece reflective film perpendicularly. The reflective film is made of aluminum film, gold film or silver film.

5. The monitoring method according to claim 1, characterized in that, The ultrasonic welding parameters include: ultrasonic frequency of 20kHz, ultrasonic amplitude of 20~40μm, welding pressure of 0.1~0.3MPa, welding time of 0.2~9.9s, and welding speed of 5~15mm / s.

6. The monitoring method according to claim 1, characterized in that, The ultrasonic welding head is made of 7-series high-strength aluminum alloy or TC4 titanium alloy, and the side surface of the ultrasonic welding head is kept smooth.

7. The monitoring method according to claim 1, characterized in that, The image calibration points are black and white alternating dots, stripes, or grid patterns, located in the center of the high-speed camera's field of view.

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