Ultrasonic welding is a very fast joining technique well suited for thermoplastic composites, which does not require the use of foreign materials at the welding interface for either carbon or glass fibre-reinforced substrates. Despite very interesting investigations carried out by several researchers on different aspects of the process, ultrasonic welding of thermoplastic composite parts is not well understood yet. This article presents a deep experimental analysis of the transformations and heating mechanisms at the welding interface and their relationship with the dissipated power and the displacement of the sonotrode as provided by a microprocessor-controlled ultrasonic welder. The main aim of this research is to build up the knowledge to enable straightforward monitoring of the process and ultimately of the weld quality through the feedback provided by the ultrasonic welder.

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A model for the mechanics (oscillating deformation), heat transfer including viscoelastic heat generation and friction dissipation, and degree of adhesion (intimate contact and healing) is proposed for the initial transient heating phase. 10Numerical resolution was performed using a multi-physical finite element code. The predicted dissipated power evolution exhibits a good correlation with previous experimental measurement of delivered power, and shows that the apparatus has a global efficiency of 13%. The predicted degree of adhesion also confirms the experimental observation that adhesion starts at the edge of the contact area, and progressively extends to the whole contact area. 15The numerical model was further used to investigate the physical mechanisms occurring during the welding process. As suggested in the literature, the first heating mechanism is confirmed to be due to interfacial friction. Bulk viscoelastic dissipation becomes predominant when the interface reaches higher temperatures. The dissipated power is suddenly increased when the whole interface reaches the glass transition temperature.

The possibility of assembling through welding is one of the major features of thermoplastic composites and it positively contributes to their cost-effectiveness in manufacturing. This article presents a comparative evaluation of ultrasonic, induction and resistance welding of individual carbon fibre-reinforced polyphenylene sulphide (PPS) thermoplastic composite samples that comprises an analysis of the static and dynamic mechanical behaviour of the joints as well as of the main process variables. The induction welding process as used in this research benefitted from the conductive nature of the reinforcing fibres. Hence, no susceptor was placed at the welding interface. Resistance welding used a fine-woven stainless-steel mesh as the heating element and low welding pressures and times were applied to prevent current leakage. Triangular energy directors moulded on a separate tape of PPS resin were used to concentrate ultrasonic heat at the welding interface. The static single-lap shear strength of the joints was found similar for induction and ultrasonic welding. A 15% drop in the static mechanical properties of the resistance welded joints was attributed to incomplete welded overlaps following current leakage prevention. However, the fatigue performance relative to the static one was similar for the three sorts of joints. A comparative analysis of process variables such as welding time, required power and energy was also carried out.

A three-dimensional finite element model of the induction welding of carbon fiber/polyphenylene sulfide thermoplastic composites is developed. The model takes into account a stainless steel mesh heating element located at the interface of the two composite adherends to be welded. This heating element serves to localize the heating where it is needed most, i.e. at the weld interface. The magnetic, electrical, and thermal properties of the carbon fiber/polyphenylene sulfide composite and other materials are identified experimentally or estimated and implemented in the model. The model predicts the temperature–time curves during the heating of the composite and is used to define processing parameters leading to high-quality welded joints. The effect of the heating element size and input current on the thermal behavior is investigated, both experimentally and using the developed model. The welds quality is assessed through microscopic observations of the weld interfaces, mechanical testing, and observations of the fracture surfaces. A comparison with two other welding processes, namely resistance welding and ultrasonic welding is finally conducted.