The edge-effect in induction welding of thermoplastic composites

A shift in material and manufacturing methodology is needed to cope with the increasing demand for Urban Air Mobility (UAM) and Cargo Air Vehicles (CAVs), which require both light-weight structures capable of handling aerospace-loading conditions, and a high rate production throughput. To address these requirements, thermoplastic composites are explored to utilize their weight, mechanical, toughness and durability properties to comply with structural requirements. In addition, thermoplastic composites open the possibility to fastener-free assembly technologies, such as induction welding, allowing composite structures to be assembled through local melting and re-solidifying, without the need for mechanical fasteners or adhesives. However, during the induction welding process undesired localized hot-spots can occur near the edges of a part, resulting in subpar quality of the joint and an unwanted surface finish (thermal based delamination). To better understand how the hotspots occur, a detailed investigation by analyzing the current flow through the thermoplastic composite laminate layers is performed. The analysis was done using an in-house development mixed boundary element (BE) / finite element (FE) software code, WelDone. For a laminated thermoplastic composite, it is shown that the current flow is influenced by the electromagnetic (EM) field generated by the induction coil. The induction coil shape dictates the current flow in a specified direction, and together with the material architecture and anisotropic conductivity of the thermoplastic composite, a build-up of current near an edge can occur, matching in experimentally observed hot-spots during the induction welding process. At last, the current build-up at edges in a metal is investigated as well, and the differences between a metallic monolithic plate and a composite laminate are discussed.