Cost-efficient multi-material design requires suitable joining techniques, ideally with low investment cost by re-using existing assembling lines. The recently developed resistance rivet spot welding (RRSW) technique combines mechanical joining with spot welding and enables cost-efficient joining of aluminum (Al) to steel for multi-material body-in-white structures. Here, the static and fatigue strengths of different hybrid Al-steel specimens made by RRSW were measured and compared to other state-of-the-art joining techniques, such as self-piercing riveting (SPR) and RSW. The static strength of RRSW matched or exceeded that of SPR regardless of the sheet thickness, whereas the fatigue strength of the RRSW joints showed a strong dependency on the thickness of the steel sheets. For thinner steel sheets, the fatigue of the RRSW-joined metal sheets was lower in comparison with SPR. Fatigue cracks were initiated in thin steel sheets around the weld nugget. By contrast, for thicker steel sheets, the fatigue strength of RRSW matched or exceeded that of SPR. With a thicker material combination of 1.5 mm steel and 1.0 mm Al, fatigue cracks occurred only in the Al sheet in both SPR and RRSW. For suitable steel sheet thickness, RRSW is thus a durable technique to join steel and Al.
Fiber reinforced plastics (FRPs) are increasingly being used to reinforce the steel or aluminum automotive structure components. Currently, these multi-material applications are realized in general by a multi-stage manufacturing with considerable cost increase. To avoid this, a new hybrid forming method has been proposed and developed. It uses the half liquid FRP as a pressure media to hydro-form the sheet metals, form the reinforcement ribs of FRPs, and create the bonding between metal sheet and FRP in the same time and process step. In this work, the die sealing concepts for two test geometries could be successfully developed. The hybrid forming parameters (Temperature, pressure, speed etc.) are in a reasonable range and can be accepted by the industry. It could be approved that high strength steels with up to 800 MPa tensile strength could be hybrid formed successfully together with a LFT PA6 GF40 for a real car component. 20% weight reduction could be achieved.
<div class="section abstract"><div class="htmlview paragraph">Hybrid structural parts combining aluminum or steel sheets with long glass fiber reinforced thermoplastics (LFT) offer a great opportunity to reduce component weight for automotive applications. But due to high manufacturing cost, metal-LFT hybrid components are still scarcely used in automotive large-scale production. Thus in this work a novel cost- and time efficient manufacturing process for simultaneous metal sheet forming and compression molding of long fiber reinforced thermoplastics to manufacture automotive lightweight components is presented. In this manufacturing process, which is referred to as “Hybrid forming”, a fiber reinforced thermoplastic melt is used as a forming medium in the manner of well-known hydroforming processes. After forming the metal sheet by polymer melt in combination with the rigid die, the melt solidifies and forms a local reinforcement structure in the hybrid component. Since the metal sheet is pre-coated with a bonding agent prior to the forming process, a firmly bonded connection between metal and LFT can be achieved.</div><div class="htmlview paragraph">For proof of concept a longitudinal control arm in a multi-link rear axle is chosen. By utilizing Hybrid forming a hybrid steel-LFT control arm is manufactured with weight savings of 20 % with regard to the metal reference component. Weight savings are derived by reducing the metal thickness and compensate stiffness and strength with local load-conforming LFT ribs. The metal part of the hybrid control arm guaranties the same positive fail-safe behavior of a metal component in contrast to the brittle failure mechanics of pure CFRP/GFRP components.</div><div class="htmlview paragraph">To verify the resilience of the hybrid component and especially the bonding surface between steel and LFT quasi-static tests and fatigue tests were conducted. The results are compared with the FE-simulations to validate the simulation technique, which can be used to design metal-LFT structural parts manufactured by hybrid forming for future applications.</div></div>
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