Through the thickness heating in the ultrasonic welding of unidirectional thermoplastic composites

Abstract

Research and development of thermoplastic composites has been ever increasing owing to the advantages brought by it over conventional materials such as metals and thermoset composites. Higher processing rates, automation, the possibility to recycle, weldability and better mechanical and chemical properties are some of them. Fusion bonding of TPCs that involves application of heat and pressure at the localised bonding area promises faster assembly times and weight reduction and is thus attractive to the aerospace industry. When comparing different welding techniques, ultrasonic welding has the lowest welding times and energy consumption and also overcomes certain challenges that come with the other techniques. The ultrasonic welding process has been a part of the plastic industry for decades, and with time, the research has also shown promising results in welding advanced thermoplastic composites. Heat generation in this mechanical/frictional welding technique occurs due to surface friction and viscoelastic heating caused due to high frequency and low amplitude vibrations. The process creates quality welds in seconds and can be controlled in-situ, ensuring robustness and repeatability. Scaling up the static process to a continuous process enables continuous sealed welds having a more uniform distribution of load and higher load-carrying capability. However, the continuous process brings challenges. The state of the art continuous ultrasonic welding (CUW) equipment at TU Delft has demonstrated quality welds for CF/PPS fabric laminates. However, problems like excessive through the thickness heating (TTH) and deconsolidation of welds need to be understood and overcome.

As the industry moves toward welding larger and more complex composites structures such as stringers, frames and skin joints, the CUW process becomes more attractive. The primary thermoplastic structures would involve the use of unidirectional materials. The material that has been proposed for the future structures is the Carbon fiber/ Low Melt - PolyArylEtherKetone composite. LM-PAEK polymer exhibits comparable properties to the other advanced thermoplastic materials like PEEK and PEKK but has a lower processing temperature enabling faster processing. To the author's knowledge, no literature is currently available related to CUW of this material. It was believed that when welding UD / LM-PAEK, the problem of excessive through the thickness heating might be present, leading to fibre and polymer squeeze-out and porosity in the adherends and the need for longer consolidation time. Therefore, this research was focused on investigating the extent of TTH in the ultrasonic welding of this UD material and the solutions to mitigate the effects of TTH on the welds. Static and continuous welding experiments were conducted, and temperature readings were obtained from the adherends and the interface. Along with this, different characterisation techniques such as lap shear tests, fractography and cross-sectional microscopy were used to observe the effects of TTH.

CUW showed overall higher TTH. It was observed that changing the fibre architecture from fabric to UD exacerbates the effects of excessive TTH in the adherends. Also, the change in the boundary conditions from that of a static weld to that of a spot or a continuous weld brings in the differences in the viscoelastic bulk heat generation in the top adherend. The cause of this difference is yet to be understood. Changing the process parameters and shape of the sonotrode helped in mitigating the overall TTH in the CUW. The use of a round sonotrode helped in reducing the preheating during welding and the lower welding speeds used, brought the advantage of consolidating the welds for longer times. In conclusion, it was found that changes in the welding and consolidation parameters, and the equipment, can help mitigate the effects of excessive TTH on the adherends. Overcoming the problem of excessive TTH is one of the first steps toward enabling CUW of UD/LM-PAEK composites for future applications.