Ultrasonic welding of thermoplastic composite materials is a promising joining technique that is now moving towards up-scaling, i.e. the assembling of large industrial parts. Despite its growing technological maturation, the assumed physical mechanisms underlying ultrasonic heating (viscoelastic heating, friction) are still insufficiently understood and modelled. In particular, the hammering phenomenon, resulting from the periodic loss of contact between the sonotrode and adherends due to the high frequency vibration caused to the former, directly impacts the heating efficiency. We propose in this work an original experimental and modelling approach towards a better understanding of the hammering effect. This approach makes combined use of: (i) an experimental static welding setup provided with a high-frequency laser sensor to analyse the vibration amplitude transmitted to the adherends and (ii) an improvement of the multiphysical finite element model already presented in previous works. Results show it is possible to obtain a good estimation of the vibration transmitted to the upper adherend from laser measurements close to the sonotrode. The hammering effect is shown to decrease during the welding process, due to the heating of the interface which directly affects further heat generation. Quantitative introduction of this hammering effect in the existing numerical model results in improved predictions in terms of dissipated power in time.

Continuous ultrasonic welding (CUW) is an innovative high-speed joining method for thermoplastic composites. Currently, thin flat energy directors (EDs) are used to focus the heat generation at the weld line. The resulting fracture surfaces exhibit large areas of intact ED, resulting in a non-uniform weld, and significantly lowering the strength. The goal of this study is to improve the weld uniformity of continuous ultrasonically welded joints. In the first part of this paper we found that a 0.20 mm-thick woven mesh ED significantly improved the weld uniformity and strength in comparison to a 0.08 mm thick flat ED. The second part the paper focuses on understanding why the mesh gives this improved weld uniformity by analyzing the feedback data from the welder and by performing a microscopy analysis of the weld line at different moments during the static welding process. It was found that at the beginning of the welding process the mesh filaments expand within the open areas of the mesh while flattening; the mesh is being pre-formed in between the adherends. This pre-forming most likely created a good uniform intimate contact between the ED and adherends, which most likely resulted in a uniform heat generation and therefore created a uniform weld line. Because energy directing meshes make it possible to create uniform weld lines, they are expected to play an important role in the future for the continuous ultrasonic welding of thermoplastic composites.