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Delft University of Technology Modeling and experimental investigation of induction welding of thermoplastic composites and comparison with other welding processes Gouin O'shaughnessey, Patrice; Dube, M; Fernandez Villegas, Irene DOI 10.1177/0021998315614991 Publication date 2016 Document Version Final published version Published in Journal of Composite Materials Citation (APA) Gouin O'Shaughnessey, P., Dube, M., & Fernandez Villegas, I. (2016). Modeling and experimental investigation of induction welding of thermoplastic composites and comparison with other welding processes. Journal of Composite Materials, 50(21), 2895-2910. https://doi.org/10.1177/0021998315614991 Important note To cite this publication, please use the final published version (if applicable). Please check the document version above. 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The Author(s) 2015 Modeling and experimental investigation Reprints and permissions: sagepub.co.uk/journalsPermissions.nav of induction welding of thermoplastic DOI: 10.1177/0021998315614991 jcm.sagepub.com composites and comparison with other welding processes Patrice Gouin O’Shaughnessey1, Martine Dube´ 1 and Irene Fernandez Villegas2 Abstract 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. Keywords Thermoplastic composites, joint/joining, finite element analysis, welding Introduction already begun to use welding as an assembly method for parts made of thermoplastic composites. For exam- Joining is inevitable in the design of large and complex ple, the leading edges of the wings of the Airbus A340- composite structures. Structures made of thermosetting 600 and A380 are assembled by resistance welding composites rely mainly on two joining processes: adhe- (RW), and the empennage of the Gulfstream G650 is sive bonding and mechanical fastening. Both of these assembled by induction welding (IW).1 Another weld- processes come with a number of disadvantages such as ing process that shows potential to be used at large a high sensitivity to surface preparation and long curing scale is ultrasonic welding (UW).2 times for adhesive bonding as well as delamination and In the RW process, an electrically conductive heat- stress concentrations due to holes drilling for mechan- ing element (HE) connected to a power supply is placed ical fastening. These two joining processes can be avoided when a structure is made of thermoplastic 1Department of Mechanical Engineering, E´cole de technologie supe´rieure, composites. In effect, thermoplastic composites offer Montre´al, Que´bec, Canada the possibility to be assembled by welding. Welding 2Department of Structural Integrity and Composites, Delft University of consists in heating a thermoplastic composite over its Technology, Delft, The Netherlands glass transition (amorphous polymer) or melting (semi- crystalline polymer) temperature and allowing it to cool Corresponding author: Martine Dube´, Department of Mechanical Engineering, E´cole de tech- down under the application of pressure. It is a fast pro- nologie supe´rieure, 1100 Notre-Dame Street West, Montre´al, Que´bec, cess, of the order of seconds, and is not sensitive to Canada, H3C 1K3. surface preparation. The aerospace industry has Email: [email protected] 2896 Journal of Composite Materials 50(21) at the interface of two thermoplastic composite parts to UD fiber-based adherends helps generate and concen- be welded (adherends). An electrical current is applied trate the heat at the weld interface. With the develop- to the HE which heats up by Joule effect. The polymer ment of new manufacturing methods such as located in the vicinity of the HE softens or melts and automated fiber placement, UD fiber reinforcement is when the current is stopped, the assembly cools down, more and more popular. UD reinforcement also pro- under the application of pressure, to form a welded vides the composite with high strength and stiffness, joint. Carbon fiber fabrics were historically used as making these materials ideal candidates for many aero- HE; however, in the past years, it was shown that HE space applications. As Bayerl et al.6 and Ahmed et al.7 in the form of stainless steel meshes of various sizes reported, very few studies have focused on thermoplas- offer a better process control and a more uniform tem- tic composites IW based on a HE. Therefore, a study perature over the weld area.3 UW is a process in which on welding of UD carbon fiber thermoplastic compos- low amplitude and high-frequency vibrations, in the ites with an HE is needed and relevant. Furthermore, range of kHz, are transmitted to the thermoplastic although such an HE is a foreign material that one may composite adherends by a sonotrode. As opposed to want to avoid, it was shown in studies on RW that it the RW process, heat is generated by surface and inter- does not affect the weld mechanical performance in a molecular friction which occurs due to the high-fre- negative way. Even under fatigue loading, good mech- quency vibrations. Energy directors, i.e. man-made anical performance was reported for joints made by neat polymer protrusions located at the weld interface, RW with a stainless steel mesh HE.8 are used to localize the heating at the weld line. The first numerical works about induction heating of Historically, energy directors were made of rectangular composites were dedicated to the identification of the or triangular shapes. Recently, Villegas4 successfully dominant heating mechanisms. Many authors claimed used flat energy directors to weld thermoplastic com- that Joule heating within the carbon fibers is mainly posite adherends which facilitated the process control. responsible for the temperature increase,9–19 while IW is based on a high-frequency alternating elec- others20–23 believed that heating occurs at the fiber trical current circulating in a coil. The coil generates a junctions. This last heating mechanism relies on dielec- time-variable magnetic field (MF) of the same fre- tric heating or Joule losses caused by contact resistance quency as the current. If an electrical conductor is at fiber junctions. Yarlagadda et al.22 developed a placed in the vicinity of the MF, eddy currents are model that identified the dominant heating mechanism induced, leading to heat generation by Joule losses. as a function of the dielectric junction impedance, fiber This principle is used to weld thermoplastic composites. resistivity, and contact resistance. In all cases, losses at Here again, an electrically conductive HE is placed junctions were dominant over fiber heating unless the between the two adherends. An electrical current is contact resistance between the fibers was very low. On applied to the coil until the polymer located close to the other side, Mitschang et al.11 demonstrated that the HE softens or melts. The current is then stopped, carbon fiber with or without resin heated up equally, allowing the polymer to cool down under the applica- meaning that dielectric heating would be less important tion of pressure. Similarly to the RW process, the HE than fiber Joule heating and contact resistance at junc- remains trapped in the weld after the welding oper- tions. It is interesting to mention that the methodology ation. The HE may consist in a stainless steel mesh of for the measurement of the electrical resistance of the various dimensions or a magnetic susceptor. adherend proposed by Rudolf et al.,24 which was also Alternatively, if the adherends are made of carbon used by Mitschang et al.,11 takes into account both the fiber fabric, no HE is necessary as the fiber architecture fiber resistance and the junction resistance. Thus, by allows for current close loops to exist. These loops may using such a measurement as an input value, fiber be sufficient to generate heat without having to add any Joule losses and junction Joule losses were implicitly foreign material to the weld stack. However, in such a included in the models. Finally, it should be noted case, heat would be concentrated at the surface of the that the heating mechanism depends on several param- top adherend, i.e. the adherend located closest to the eters such as the material type (consolidated or non- coil. Heat then propagates through the thickness of the consolidated plies), fiber architecture (fabric or UD) adherend until it reaches the location where it is needed, and lay-up, matrix, and induction heating process par- i.e.

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