The induction welding technology for thermoplastic carbon fiber composites is still in the early stages.
The global economic downturn, coupled with potential significant changes in the international situation and the saturation of demand for low-end carbon fiber, collectively determines the contraction of the global carbon fiber market. However, this is not the final outcome. The performance of mid-to-high-end carbon fibers remains essential for industries such as aerospace, medical, and automotive manufacturing. Additionally, from an environmental perspective, the application prospects of thermoplastic carbon fiber composites are quite promising. Thermoplastic carbon fiber can be reshaped multiple times, and its processing can be intelligently controlled. In the future, industrial components for aircraft and spacecraft will likely use this as their base material.
To achieve better performance from thermoplastic carbon fiber components, in addition to custom production, they should also possess post-forming processability features, such as welding. This article will introduce knowledge related to the welding of thermoplastic carbon fiber industrial components, particularly focusing on induction welding.
Introduction to Five Welding Methods for Thermoplastic Carbon Fiber Composites
Unlike thermosetting composites, thermoplastic composites can still melt after molding. The connection of thermoplastic carbon fiber parts can be achieved through secondary melting and applying pressure, which can be regarded as a welding process. Currently, commonly used welding techniques for thermoplastic carbon fiber composites include hot gas, resistance, ultrasonic, induction, and laser welding. Each welding method has its advantages and disadvantages, and the choice of method should be based on different scenarios and requirements.
1.Hot Gas Welding:
Description: Hot gas welding utilizes a stream of hot gas (usually nitrogen) to melt and fuse the thermoplastic materials at the joint.
Process: The surface of the materials is heated with hot gas, and pressure is applied to connect them together.
Advantages: There is precise control over temperature and pressure, making it suitable for various thermoplastic composites.
Considerations: Care must be taken to prevent overheating and damaging the carbon fiber.
2.Resistance Welding:
Description: Resistance welding involves passing an electric current through the materials, generating heat at the joint.
Process: Two components are pressed together, and the current flows through the joint, causing localized heating.
Advantages: The process is quick, suitable for large structures, and can be automated.
Considerations: The materials must possess sufficient conductivity, and there is a risk of localized overheating.
3.Ultrasonic Welding:
Description: Ultrasonic welding uses high-frequency vibrations to generate heat at the joint, thereby melting and fusing the thermoplastic materials.
Process: Ultrasonic vibrations are applied to the interface, causing localized heating and bonding.
Advantages: The processing speed is fast, making it suitable for small and complex parts, with minimal thermal impact on surrounding areas.
Considerations: Proper frequency and amplitude settings are crucial, and this method may not be suitable for all thermoplastic composites.
4.Induction Welding:
Description: Induction welding uses electromagnetic induction to heat the thermoplastic materials at the joint.
Process: An induction coil induces heat within the materials, creating a localized melting zone for welding.
Advantages: There is precise control over heating, making it suitable for large structures with minimal impact on surrounding areas.
Considerations: The materials must have sufficient conductivity, and this method is not universally applicable.
5.Laser Welding:
Description: Laser welding employs a highly focused laser beam to heat and melt the materials at the joint, forming a bond as they cool.
Process: The laser beam is directed to the interface, rapidly heating the thermoplastic material. The components are then pressed together, forming a weld as it solidifies.
Advantages: Laser welding provides high precision and control over thermal input, relatively fast welding speeds, and is suitable for mass production. It creates minimal heat-affected zones, preserves material properties, and poses a lower risk of contamination.
Considerations: Care must be taken during laser welding to protect the carbon fiber from overheating to prevent damage.
Mature Induction Welding Technology for Thermoplastic Carbon Fiber Benefits the Aerospace Industry
Induction welding technology is particularly suitable for joining carbon fiber reinforced thermoplastic composite structures. Since carbon fiber is conductive and can generate eddy currents when subjected to an alternating magnetic field, there is no need to introduce additional induction materials when welding carbon fiber reinforced thermoplastic composites.
As the manufacturing technology for aerospace thermoplastic composites matures and production costs decrease, their application in aerospace manufacturing will significantly increase. Additionally, the complex structure of aerospace components requires simple parts to be assembled into a whole through connection technologies. Therefore, developing welding technologies for aerospace thermoplastic composites, including induction welding, has become an urgent need in advanced aircraft manufacturing research, and it will remain a long-term task in the future.
Currently, the induction welding technology for thermoplastic carbon fiber faces challenges such as low maturity and the fact that it has not yet entered the engineering prototype and practical product application stages. However, research on induction welding of thermoplastic composites for civil aircraft is still in its early stages abroad, with various key technologies pending breakthroughs. The technological gap among countries is not very pronounced. Therefore, China should accelerate the development and application efforts in this area to shorten the gap with foreign advanced materials and manufacturing technologies for aircraft. Only by truly mastering core technologies can we benefit the domestic aerospace industry.
Research Progress on Induction Welding of Thermoplastic CF/PPS Composites in China
Some research teams have studied the effects of welding power and time on the lap shear strength (LSS) using a spot welding approach. They also explored the feasibility of different implanted layers for induction welding of CF/PPS thermoplastic composites. The research found that excessive welding power or prolonged welding time could lead to overheating of the samples, resulting in chemical reactions such as cross-linking, oxidation, and degradation of the resin matrix, which significantly reduce the mechanical properties of the welded joints and even the internal properties of the composites.
1. Maximum Time Data for Induction Welding of CF/PPS Composites
Experimental results indicate that when the relative power is within the range of 400 to 800, the intermediate layer exhibits the highest rate of temperature rise. As the relative power increases, the rate of temperature rise becomes faster, and the smoking time occurs earlier. When the welding time exceeds a certain value, smoking will inevitably appear in the middle of the panels. The occurrence of smoking is primarily due to the degradation of the resin or the volatilization of residual small molecules, both of which can adversely affect the welding quality and the bonding performance between the two panels. Therefore, it is necessary to avoid this situation.
2. Effects of Welding Power and Time on Shear Strength (LSS)
Induction welding was performed on two CF/PPS composite materials using a spot welding method, followed by applying pressure with rollers after heating. The resulting lap shear strength (LSS) was tested. The results indicate that during the induction welding process, due to the relatively short welding time, the outflow of resin is not severe, allowing the weld surface to retain a certain amount of resin. At a relative power of 500, the shear strength (LSS) value reaches its maximum at a heating time of 65 seconds, indicating that the heating time should neither be too short nor too long.
3. Effect of Implant Layer on Shear Strength (LSS)
Using two CF/PPS composite materials, along with a CF/PPS prepreg that has the same specifications (same raw materials, fabric form, fiber volume content, etc.) as the composites, an implant layer was used for spot welding. The results indicate that the addition of the implant layer generally led to a decrease in shear strength (LSS), which may be attributed to the implant layer restricting heat generation and conduction; however, the maximum LSS still reached 24.8 MPa.
