Low Temperature Soldering of Laser Structured and Metal Coated Fiber-Reinforced Plastic K. Gustke, D. Kupke, R. Drehmann, T. Lampke, Materials and Surface Engineering Group, Institute of Materials Science and Engineering, Chemnitz University of Technology, D-09107 Chemnitz, Germany J. Gebauer, U. Klotzbach Fraunhofer Institute for Material and Beam Technology, Fraunhofer IWS, D-01277 Dresden, Germany A. F. Lasagni Institute for Manufacturing Technology, Technische Universität Dresden, D-01069 Dresden, Germany Abstract Fiber-reinforced plastics (FRP) exhibit excellent mechanical properties combined with low density. Thermoset FRP and metals are mainly connected by mechanical joining processes or adhesive bonding. Mechanical joining processes deteriorate the mechanical properties of the FRP by cutting the fibers. Adhesives require a long curing time and lead to inseparable material compounds. A new approach of joining FRP and metals is a laser pre-treatment process before functionalization of the plastic based substrate by a wire-arc sprayed coating process. Afterwards, the joining process of the coated FRP with a metallic counterpart is carried out by a low temperature soldering process. Due to the massively enlarged interface area resulting from laser structuring, bond strengths of up to 15.5 MPa could be achieved. Introduction Fiber-reinforced plastics (FRP) and in particular carbon fiber- reinforced plastics (CFRP), are an attractive option for replacing metallic materials and thereby achieving a weight reduction due to their high specific stiffness and strength as well as their low density. To obtain a broader spectrum of applications, there are several key challenges related to advanced FRP that must be overcome. Indeed, the application of FRP remains limited by their physical properties and tribological behavior, which include their low wear resistance and insufficient thermal and electrical conductivity. Nevertheless, there are many applications where metallic components are still essential and can only be partially substituted by FRP. Due to the different thermal expansion behavior and the risk of contact corrosion, it is still a challenge to join these completely different materials. Currently, adhesive joints, mechanical joining elements or inserts are often used to join FRP to metals. Adhesive bonding allows a very homogeneous stress distribution, but also represents an irreversible joining technique [1]. Mechanical joining elements might allow reversible joining of different materials, but for FRP the fiber structure is damaged, so that additional material is needed. Furthermore, local stress concentrations occur [2]. The inclusion of inserts enables the joining of metals and FRP without damaging the fibers. Due to the integration during the manufacturing process, this joint is also non-separable and expensive to produce. Soldering is another way to join different materials. However, it is only suitable for metallic materials. Thus, in order to use it for joining FRP with metallic materials, it is necessary to metalize the composite material with a metallic coating. Currently, the joining of FRP and metals by soldering is still largely unexplored. First investigations carried out by Winkelmann et al., joined a sandwich sheet consisting of two steel cover layers and a core layer of FRP with another steel sheet using the soldering alloy SnCu3. The amount of thermal energy input during the joining process is one of the most important parameters to prevent delamination between the polymer and metal within the sandwich sheet [3]. A very effective method for producing metallic coatings directly onto the polymer surface is demonstrated by thermal spraying technologies [4–6]. In advance of the coating process, the surface undergoes a pre-treatment process. This is usually carried out using grit-blasting in order to roughen the surface and thus ensure adhesion to the substrate. However, this also results in damage to the fibers close to the surface caused by the sharp and highly accelerated blasting particles. In contrast, laser structuring enables a reproducible and defined surface design. There are several studies demonstrating that a laser-structured surface can improve the bond strength. Gebhardt and Fleischer applied a linear laser structure to a metal sheet before joining to a CFRP using the resin transfer molding process. A significant increase of the shear tensile strength was achieved compared to a grit-blasted surface [7]. Heckert and Zaeh used two different laser structures on an aluminum part, which was then joined to a thermoplastic FRP by laser radiation. The shear tensile strength was increased compared to a grit-blasted surface treatment [8]. Schuberth et al. created a lattice structure on steel and aluminum parts with a pulsed fiber laser and joined them with a glass fiber-reinforced polyamide 6 inside a heat press. A significantly higher shear tensile strength was observed compared to reference samples whose surfaces were grit- blasted [9]. Gebauer et al. structured the surface of an aluminum component by means of a laser pre-treatment, which was then over molded with polyamide and thereby achieved a tensile shear strength of 11.9 MPa, which corresponds to 90 % of the properties of the pure polyamide [10]. In other research study, Thermal Spray 2021: Proceedings from the International Thermal Spray Conference May 24–28, 2021 F. Azarmi, X. Chen, J. Cizek, C. Cojocaru, B. Jodoin, H. Koivuluoto, Y. Lau, R. Fernandez, O. Ozdemir, H. Salami Jazi, and F. Toma, editors DOI: 10.31399/asm.cp.itsc2021p0569 Copyright © 2021 ASM International® All rights reserved. www.asminternational.org 569 Downloaded from http://dl.asminternational.org/itsc/proceedings-pdf/ITSC 2021/83881/569/487974/itsc2021p0569.pdf by guest on 23 September 2021