A Study on Through-the-Thickness Heating in Continuous Ultrasonic Welding of Thermoplastic Composites

Abstract

Continuous ultrasonic welding is a promising technique for joining thermoplastic composites structures together. The aim of this study was to gain further insight into what causes higher through-the-thickness heating in continuous ultrasonic welding of thermoplastic composites as compared to the static process. Thermocouples were used to measure temperature evolutions at the welding interface and within the adherends. To understand the mechanisms causing the observed temperature behaviours, the results were compared to temperature measurements from an equivalent static welding process and to the predictions from a simplified heat transfer model. Despite the significantly higher temperatures measured at the welding interface for the continuous process, viscoelastic bulk heat generation and not thermal conduction from the interface was identified as the main cause of higher through-the-thickness heating in the top adherend. Interestingly the top adherend seemed to absorb most of the vibrational energy in the continuous process as opposed to a more balanced energy share between the top and bottom adherend in the static process. Finally, the higher temperatures at the welding interface in continuous ultrasonic welding were attributed to pre-heating of the energy director due to the vibrations being transmitted downstream of the sonotrode, to reduced squeeze-flow of energy director due to the larger adherend size, and to heat flux originating downstream as the welding process continues.

On the use of a rounded sonotrode for the welding of thermoplastic composites

“…5 shows only a modest resin squeeze out coming out from the weld line at the edges of the overlap despite the very high welding pressure. Consequently, using a rounded sonotrode provides a solution to the substantial material (fibres and resin) squeeze-out from the top adherend identified in our previous work (Jongbloed et al, 2021(Jongbloed et al, , 2022 as an issue of continuous ultrasonic welding. This is an important issue since it can negatively affect the structural integrity of the adherends through phenomena such as porosity and local fibre misalignment.…”

Effect of Adherend Thickness on Near-Field Ultrasonic Welding of Single-Lap CF/LMPAEK Thermoplastic Composite Joints

“…Temperature measurements (Figure 6) confirm that observation since they show a significant increase in the temperatures within the top adherend as its thickness increases. It should be noted that, due to potential errors in the readings caused by the interaction between the thermocouples and the high-frequency vibrations [19], the temperature data were only considered from a qualitative viewpoint. It is interesting to point out that, even though the images in Figure 7 do not clearly indicate melting of the energy director and hence the creation of a welded joint within the displacement plateau for the thicker adherend, the temperature readings at the welding interface seem to indicate the contrary since they are similar for the thin and the thick adherends (Figure 6).…”

Developments in Laminate Modification of Adhesively Bonded Composite Joints

“…Joining of composites may take place during the manufacture of the original structure or during service, when repairing damage or replacing older components [ 3 ]. Multiple well-established joining methods are available for joining composites, such as riveting, fastening [ 4 ] and fusion bonding/welding [ 5 , 6 , 7 ] (in the case of thermoplastic composites). Different joining methods can also be combined in hybrid processes (using rivets, pins, or bolts) [ 8 , 9 ].…”