High-performance lightweight structures with Fiber Reinforced Thermoplastics and Structured Metal Thin Sheets

A heavy-duty multi-material-design (MMD) can be realized through the combined use of structured sheet metal and reinforced plastics (FRP). To exploit the high lightweight potential of the various material groups within a multi-material system as efficient as possible, a material-adapted and particularly fiber adjusted joining method must be applied. The present paper primarily focuses on the manufacturing and mechanical testing of novel multi-material joints with structured sheet metals and carbon fiber reinforced thermoplastics (CFRP). For this purpose the applicability of the new Flow Drill Joining (FDJ) method, which was developed for joining of heavy-duty metal/composite hybrids, was investigated.


Introduction
Load-path adapted multi material designs initiate significant savings of energy and material in automotive industry.Of all the material groups, the combined use of metal sheets and fiber-reinforced plastic (FRP) offers a very high lightweight potential (Klein, 1997;Rosato, 2005;Goede et al., 2010).With respect to this, metal components are usually applied for a uniform initiation of concentrated loads.In contrast to that FRPs are more suitable for load transmission along large distances (Schürmann, 2005;Kroll, 2009).
In particular, the use of thermoplastic FRPs with high strength and stiffness properties is raised sharply within the last decade in automotive industry (Ind.Exp., 2013;Witten, 2013).Primarily this is due to comparatively short cycle times when manufacturing components and the use of inexpensive matrix systems such as polypropylene, but also higher-quality plastics such as polyamide or polyether ether ketone.Consequently they are predestined for large scale production of complex lightweight structures (Döhler et al., 2014).
The use of structured metal sheets also offers a high lightweight potential for multi-material designs (Sterzing, 2005;Malikova et al., 2013).In Particular three dimensional structured sheets with embossed secondary design elements, vault structures or beadings are characterized by much higher bending stiffness-values than plane sheets.On the one hand this is caused by a higher moment of inertia and on the other by the strain hardening effect, which is evoked by the forming process (Viehweger et al., 2002).Compared to a plane sheet metal construction, the wall thickness can be consequently reduced and ultimately also the component weight.
Through the combined use of thermoplastic FRPs and thin-walled structured metal sheets, weight-optimized components of a body in white can be developed for vehicle construction.For instance the described multi-material approach can contribute to increase the rigidity of a body in white structure while reducing weight.For this the required joining technology takes a central role within the manufacturing process.
The joining of thin-walled structural components, made of FRP and metal sheets, is considered to be an extreme challenge.Usually -due to the strongly different thermo-mechanical behavior and the divergent material composition and morphology -primarily mechanical methods such as screwing, blind and punch riveting as well as hybrid techniques are applicable (Simon, 2005).Regarding this, high and strong local acting joining forces cause a process-related smoothing of the structured cross-section.This reduces the excellent bending stiffness of the thin-walled and structured sheet metal components considerably (Viehweger et al., 2005

Flow Drill Joining Concept
The flow drill joining (FDJ) concept, which was developed at the Institute of Lightweight Construction at Chemnitz University of Technology, primarily serves to join comparatively thicker thermoplastic FRPs with metal sheets.The process is characterized by short processing times, high-strength joining properties appropriate for the force fluxes as well as the high degree of lightweight construction and can be utilized in various industrial sectors.
The essential advantage towards typical joining technologies relates to the fact that no additional auxiliary joining elements are needed for joining the components.
In order to manufacture a FDJ joint, the plastic flow properties of the metal component are exploited in a targeted way.With the aid of a rotating mandrel, a bushing is thermo-mechanically shaped from the metallic base material.
While doing so, this bushing is pushed directly through the also locally plasticized thermoplastic FRP component during the shaping operation and is subsequently pulled over in a positive-locking form when the mandrel is retracted or in an additional working step (Figure 1).The rotation of the mandrel and the associated friction serve to release thermal energy which is locally introduced into the FRP component as a result of the process and thus initiates the partial melting of the thermoplastic matrix.Therefore, the continuous fibers arranged in the plastic become "moveable" and can be aligned towards the force fluxes during the shaping of the bushing (Figure 2a).
The described FDJ process was carried out by an automated joining jig (Figure 2b).According to DIN EN ISO 14273, the machine serves to manufacture tensile shear test specimens with a length of 175 mm, width of 50 mm and an overlapping length of 35 mm.Joining of the comparatively thick walled GFRP and CFRP components can www.ccsenbe support processing For the inv of realigne the galvan of an elect The applie white struc exemplaril Table 3. M

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Figure 1 .
Figure 1.Process scheme for manufacturing FDJ joints

Table 2 .
). 's with orthotropic glass and carbon fiber reinforcement, embedded in a polyamide 6 (PA 6) matrix system.Table2gives an overview referring to the applied unidirectional (UD) prepreg systems, lamina lay-up and further specific properties.Properties of GFRP and CFRP joining partners