Effect of consolidation parameters on (de)consolidation in ultrasonic welding of thermoplastic composites

Abstract

Thermoplastic composites are gaining prominence in the aerospace industry owing to their higher damage tolerance, cost-efficient means of manufacturing, and the possibility to be recycled. One of the major benefits of thermoplastics compared to thermosets is their ability to be welded, and one of the most promising welding techniques for thermoplastic composites is ultrasonic welding. Ultrasonic welding is the fastest welding technique currently known, with typical weld times of a few hundred milliseconds.

Studies have been conducted in plenty regarding static ultrasonic welding of thermoplastic composites. But the industrialisation of the process involves the development of a robust continuous ultrasonic welding process which can weld the entire span of the joints, thus enabling higher load transfer and reduced stress concentrations. However, the state-of-the-art continuous ultrasonic welded joints contain voids at various locations within the weld which are assumed to appear due to a lack of consolidation during the welding process. Unlike the static ultrasonic welding, where the sonotrode can both transfer the vibrational energy to the adherends being welded and provide consolidation force, the continuous ultrasonic welding requires a separate consolidation device to provide consolidation pressure application. This makes it necessary to expand the understanding of the consolidation process to improve the weld quality and increase the Technology Readiness Level (TRL) of the ultrasonic welding process before it can be industrially used. While a lot of research to date focused on the vibration phase of the process, not much information is available regarding the consolidation phase. This research project thus explores the effect of consolidation pressure and time on (de)consolidation in ultrasonic welding of thermoplastic composites.

An experimental study was carried out on the consolidation in static welding of CF/PPS test coupons, and the knowledge obtained was extended to the continuous ultrasonic welding process. The consolidation in the continuous ultrasonic welding process was provided by a separate consolidation device or ”consolidator” placed behind the sonotrode. Various characterisation techniques including lap shear strength, void content assessment and fracture surface analysis were used to analyse the results obtained. The experiments revealed that for semi-crystalline polymer PPS, consolidation should start when the polymer is in its melt state and extend until the interface temperature of the weld drops below the crystallisation temperature of the polymer. The results obtained indicated that the voids in ultrasonic welding were formed due to a combination of shrinkage due to crystallisation, fibre decompaction, the choice of the clamps used and excessive squeeze out of the resin. In continuous ultrasonic welding, the location of the consolidator behind the sonotrode and the consolidation pressure was found to influence the weld quality. The research conclusions serve as a first step towards developing a robust consolidation process in continuous ultrasonic welding of thermoplastic composites.