Process-material Interaction in Ultrasonic Welding – New Insights Into the Hammering Effect by Ultrahigh- speed Video

Ultrasonic spot welding of unidirectional continuous carbon fiber reinforced high performance thermoplastics (UD-CFR-TP) is a fast and efficient joining technology, utilizing the materials fusion properties. A flat energy director (ED) made from the same thermoplastic resin is placed between the adherends to focus the heating intothe interface by both frictional and viscoelastic effects. Recent studies also showed through-the-thickness-heating in the adherends, which may be caused by the hammering effect. The hammering effect describes the loss of contact between sonotrode and upper adherend and leads to inefficient amplitude transfer between sonotrode and adherends. To this point, the hammering effect and the corresponding hammering coefficient were either discussed theoretically or determined by correlation of experimental power-draw data and multi-physics simulations. To understand the process-material interaction, high speed footage was recorded at 100,000 and 1,000,000 FPS. The footage allowed to identify a loss of contact between sonotrode, adherends and ED which was accompanied by chaotic rigid impact interactions of the sonotrode, adherends and ED and a strong relation of the amplitude and hammering. Lastly, previously found welding stages, characterized by preferential heating of the ED, followed by melting and fiber squeeze out in the adherends were validated. Through this approach, a new understanding of the process was possible, which may also lead to revised modeling approaches in the future, making use of digital image correlation.