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Simulating the induction spot welding of hybrid material joints

Spot welding is a very common process used to join sheet metal components in the automotive industry mass production environment. Recently, thermoplastic composites as light weight alternatives to metals have begun to make their way into production. The induction spot welding of hybrid materials, in particular aluminum or steel to thermoplastic based composite materials, is one promising method to create the required connections between these dissimilar materials and maintain productivity. As opposed to continuous welding, the spot welding of two materials with different thermal properties can help prevent heat distortion and internal stresses in parts. To improve on the design of the process a deep understanding of the interacting physics of an induction spot welding head is required. A hybrid material spot welding head consists of a stamp, an integrated induction coil and cooling channels. During the process the metal (Aluminum) joining partner on the top surface and the fiber reinforced thermoplastic (CF-PEEK) on the bottom are fusion bonded to one another via induction heating and pressure. The novel idea of locally concentrating pressure in the fusion zone through a preformed dimple in the metallic joining partner, in order to prevent deconsolidation during induction heating in the thermoplastic adherend, led to noticeably improved joints. In order to gain a better understanding of this method, the effects of different dimple geometries in the aluminum sheet were reviewed in a simulation utilizing the LS-DYNA® R7 EM Solver. The model considers the mechanical deformation of both metal and composite, the initial micro stresses in the aluminum adherend, thermal convection and radiation, in addition to the electrical conductivity and contact of both joining partners. The model is then used to investigate the temperature, pressure and stresses at the location of the spot weld in order to assess whether an adequate bond has been achieved.