Continuous ultrasonic welding is a high-speed joining method for thermoplastic composites. Currently, a thin film energy director is used to focus the heat generation at the interface. However, areas of intact energy director remain in the welded seam, which significantly lowers the weld strength, and result in a non-uniformly welded seam. To improve the weld uniformity of continuous ultrasonically welded joints, we changed to a more compliant energy director. A woven polymer mesh energy director was found to give a significant improvement in weld quality. The mesh was flattened in between the composite adherends during the welding process. This flattening promoted a good contact between the energy director and the adherends, fully wetting the adherend surfaces, resulting in a more uniformly welded seam without areas of intact energy director.

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The research in this paper is an essential part of a bigger effort on developing robust sequential ultrasonic welding of multi-spot welded joints in thermoplastic composites. It mainly focused on assessing the impact of the changes in boundary conditions on the welding process and whether it could be circumvented by using an appropriate process control strategy. A two-step approach was followed by investigating: (1) the effect of boundary conditions on displacement- and energy-controlled single-spot welded joints and (2) displacement- and energy-controlled sequential ultrasonic welding of double-spot welded joints. The results showed that previous spots indeed affect the energy required to obtain an optimum new welded spot, which challenges the use of energy-controlled welding for this application. Contrarily, displacement-controlled welding was shown to provide consistent-quality welds with a constant set of welding parameters and it was hence identified as the most promising welding strategy for sequential ultrasonic welding of thermoplastic composites.

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This paper presents a fully experimental study on melting, flow and weld strength development during ultrasonic welding of CF/PPS composites with integrated triangular energy directors. The main goal of this research was assessing whether the heating time to achieve maximum weld strength could be significantly reduced as compared to ultrasonic welding with flat energy directors. The main conclusion is that, in the specific case under study, the triangular energy directors did heat up, melt and collapse approximately two times faster than the time it took for the flat energy directors to melt and significantly flow. However the heating time needed to achieve maximum weld strength for the integrated triangular energy directors did not differ drastically from that for flat energy directors. This was caused by the fact that a fully welded overlap was not directly achieved right after the collapsing of the triangular energy directors. Instead a solidified resin-rich interface was created which needed to be re-melted as a whole in order to achieve a fully welded overlap and hence maximum weld strength.

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Autoclave manufacturing of fibre metal laminates, such as GLARE, is an expensive process.Therefore, there is an increasing interest to find cost effective out-of-autoclave manufacturing processes without diminishing the laminate quality. The aim of this study is to evaluate the quality of fibre metal laminate panels adhesively bonded and cured using resistance heating. Three manufacturing processes are compared for different layups with an embedded steel mesh at the mid-plane:autoclave curing, resistance bonding of two (autoclave-cured) panels, and complete out-of-autoclave resistance curing of panels. Interlaminar shear strength tests and optical microscopy analysis showed that resistance bonding is a promising technique, leading to results comparable to autoclave curing.Resistance curing led to an interlaminar shear strength decrease of 30-60%. A study of the correlation between degree of cure and distance from the mesh revealed the potential of resistance bonding to be used for flexible embedded mesh geometries and on-site repairs.

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Ultrasonic welding is a promising assembly technique for thermoplastic composites and it is well-suited for spot welding. In this study, two typical welding process control strategies, i.e. displacement-controlled and energy-controlled welding, were selected to manufacture spot-welded joints. The influence of different boundary conditions, provided by different welding jigs, on the welding process and the performance of thus-created welded joints were investigated. The optimum input energy was found dependent on the welding jigs, while the optimum displacement was consistent for achieving the maximum weld strength in both welding jigs. Therefore, displacement-controlled welding showed more potential in producing consistent welds in sequential multi-spot welding.@en

This paper presents a detailed experimental assessment of the effect of the thickness of flat energy directors (ED) on heat generation at the interface during ultrasonic welding. Power and displacement data showed clear differences caused by the change of thickness, related to heat concentration at the weld line during the process. The extent of the heat-affected zone was assessed by welding specimens without consolidation at different stages of the process. It was confirmed through optical microscopy that heat is generated at the interface and transferred to the bulk adherends earlier in the process for thinner ED. The analysis of their fracture surface under optimum welding conditions revealed signs of matrix degradation, leading to less consistent quality, likely due to faster heat generation rate in both the ED and the substrates, and incidentally, higher temperatures surrounding the energy director.

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Aiming faster and more reliable end products, the composite material industry is nowadays an active research topic. Innovative composite forming processes are actively designed and tested. For example, ultrasonic welding of composite thermoplastic materials is being investigated, since it shows many advantages over classical methods. In fact, energy directors allow a preferential heating of the manufactured part through the propagation of mechanical waves in a composite laminate, without including any foreign material in the welded region. However, ultrasonic welding of composite materials is not mastered yet because of the coupled and complex behavior of such materials. Thus, simulation of ultrasonic heating becomes compulsory for understanding the complex multi-physics coupled problem. In this work, we propose to model the ultrasonic welding process using a dynamic viscoelastic model in the frequency domain. Later on, this model is coupled to the transient heat equation, giving the temperature field as well as the heat flux in the simulated part. However, the result depends on the chosen experimental and material parameters such as the thickness of the part, its viscosity, its modulus of elasticity, the imposed frequency and displacement... Which makes the optimization of the process a tricky issue requiring a new set of solutions of the problem for each choice of the process parameters. Using the proper generalized decomposition (PGD), along with a coupled viscoelastic/thermal model, where all the parameters mentioned above are included as extra coordinates of the problem, appears to be a suitable solution for the optimization problem. Moreover, the PGD multidimensional solution considering all the process parameters as extra coordinates is obtained within a realistic timeframe. In fact, by using the PGD, we alleviate the curse of dimensionality since the PGD performs a separation of variables which reduces the problem dimensionality [1]. The result is therefore a computational vademecum that can be used to explore in real time the solution of the problem for any choice of the process parameters, speeding up its optimization [2].@en

This paper mainly investigates the impact of changes in boundary conditions on the welding process and the variability of the created spot-welded joints. The experimental results provides a good base for manufacturing and investigation of the ultrasonic multi-spot welded joints in thermoplastic composites. @en

The in-plane and out-of-plane mechanical behaviour of both ultrasonically spot-welded and mechanically fastened joints was investigated by double-lap shear and pull-through tests, respectively. Spot-welded specimens showed comparable onset failure load and significantly higher joint stiffness compared to mechanical fasteners when carrying shear load. The failure modes and the damage within specimens were analysed after mechanical tests. Intralaminar failure and very limited damage on the out-most ply were found for welded specimens, whereas catastrophic through-the-thickness failure was observed for mechanically fastened joints. Based on the experimental outcomes, the mechanical performance and failure mechanisms of spot-welded joints were critically assessed in comparison to the mechanical fasteners . @en

Continuous ultrasonic welding of plastic films, fabrics, and even thermoplastic composite prepreg tape is a common industrial practice. However, continuous ultrasonic welding of stiff thermoplastic composite plates is challenging due to squeeze flow of resin at the welding interface, and significant local deformation of the welding stack, that are generally needed to achieve strong welds. This paper presents a novel approach to continuous ultrasonic welding of thermoplastic composite plates based on zero-flow welding. The proposed technique can create strong welds before any squeeze flow takes place at the interface. It is enabled by the use of very thin flat energy directors, owing to simultaneous melting of both energy director and adherends’ matrix. The results prove the feasibility and indicate the potential for high-strength welds between thermoplastic composite plates at very high speed.@en

Ultrasonic welding is a promising assembly technique for thermoplastic composites and it is well-suited for spot welding. In this paper, the in-plane and out-of-plane mechanical behaviour of ultrasonically spot-welded and mechanically fastened joints are investigated by double-lap shear and pull-through tests, respectively. Special attention was paid to the comparison of onset failure load and joint stiffness of both types of joints in either type of test. Fractography was utilized to analyse the teste specimens. Failure of welded specimens was found limited in the joints but catastrophic damage was introduced into composite substrates of mechanically fastened specimens. The possibility of the substitution for mechanically fastened joints by spot-welded joints is discussed. @en

This paper presents an overview of the latest research on ultrasonic welding of composites performed at the Delft University of Technology. Firstly, we showed that for thermoplastic composites, a simple flat energy director shape, made of a loose film of neat resin, can be used to produce welds of high quality. Furthermore, for single lap shear coupons, it was shown that the use of a microprocessor-controlled welder allows for in-situ monitoring through power and sonotrode displacement data. As a result, a smart ultrasonic welding procedure was designed in which the feedback from the ultrasonic welder was used to define the processing parameters that yield optimum weld quality, significantly decreasing development times. Based on the knowledge developed at a lab-scale level, a welding strategy was developed to demonstrate the assembly of small and medium-sized components. Experimental comparison between ultrasonically spot-welded and mechanically fastened joints into double-lap shear and pull-through configurations further outlined the potential application of this technology, as well as its limitations. While ultrasonic welding is an efficient technique to join thermoplastic composites components, another potential application was shown to be the welding of thermoplastic and thermoset composites, enabled through the very short heating times in the ultrasonic welding process. This opens up new possibilities for ultrasonic welding where optimum design and manufacturing of aircraft parts call for the assembly of dissimilar materials, and could lead to significant costs and weight reduction as compared to mechanical fastening.@en