Ultrasonic sound can be used to weld thermoplastic materials. During this process high frequency vibrations are locally applied under pressure to the thermoplastic materials by means of a sonotrode and this creates a solid weld. This method can also be used to weld thermoplastic composites. To achieve a good welding, energy directors are often used. Energy directors are manmade resin protrusions, do not contain any fibers, and they are placed at the interface during welding.
For the Helacs project, a HandyStar welding device from Telsonic Ultrasonics was used to show the proof of concept of welding (see figure 2) and debonding. The welding frequency was 35 kHz and the power was 1 kW. To test the concept of welding, two polyamide (PA) test dog bones with a thickness of 4 mm were welded together by overlapping the two pieces and pressing the horn during 10s.
As a result, the two pieces are welded. However, some part of the surface at the welding spot was also melted, but the two pieces are nicely joined together. To debond the samples the same welding device was used, but now we had to pull from the side to remove the top PA dog bone. As a result, the 2 pieces are separated, but the surface of the 2 pieces is damaged. The same process was repeated to weld a PA dog bone with a 20% carbon fiber filled PA dog bone. The surface was again damaged, but to show that the two samples are nicely welded, the PA dog bone was bended, and a white crack could be observed before the weld broke.
To further investigate the possibilities of ultrasonic debonding, a second series of trials was set up. Now a PA carbon composite plate (thickness 2 mm, 50% carbon fiber) and a carbon fiber PA dog bone (thickness 4mm) were used as testing materials.
The same welding device was used and welding of the two composites could be easily done.
Afterwards, it was tried to separate the 2 components via the ultrasonic welding device. By pulling perpendicular to the welding device, it was possible to debond the 2 composites in less than 30s. Figures show the composites after separation. The surface is just a little bit damaged at the welding spot. So, to reuse the part some repairment of the surface would be advised. Other recycling options are reforming the composite by thermomoulding or removing the damaged part by cutting and use the downsized composite plate.
To proof the concept with recycled carbon fibers, we also tested ultrasonic bonding/debonding with PA recycled carbon nonwoven composites that were developed in the Helacs project.
Two PA/carbon composite plates with a thickness of 1mm and 20% of carbon fibers were placed partially on top of each other. Because the surface of the composite was already rich in resin, it could act as a flat ED at the welding interface. Welding was achieved by holding the welding horn for 10 seconds at the top of the composite along the welding zone. As a result, the composites were welded together.
To debond the sample again, we used the same procedure as before and it was possible to separate the plates.
The same procedure was tried to weld PAEK composites received from Aitiip with a thickness of 2 mm. Unfortunately, with our welding device it was not possible to join the composites by ultrasonic welding. Even when a lower melting PA sheet was placed at the welding interface to act as a kind of flat energy director, no welding occurred. It seemed that the ultrasonic vibrations could not travel through the composite as no temperature increase could be measured. Only a scratch at the surface could be observed indicating that the energy was concentrated at the horn composite interface instead of at the welding interface.
