This paper was produced for the 2019 NAFEMS World Congress in Quebec Canada
Hybrid design with the combination of metal and GFRP offers a great opportunity to reduce component weight for automotive applications. Due to high manufacturing cost, hybrid components are 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 (LFT) to manufacture automotive lightweight components is presented. In this manufacturing process which is called “Hybridforming” the molten LFT is used as a forming medium following the methodology of a hydro-mechanical forming. After forming the metal sheet by the LFT in combination with the die, the LFT solidifies and forms a local reinforcement structure in the hybrid component. Since the metal sheet can be pre-coated with a bonding agent prior to the forming process, a firmly bonded connection between metal and LFT can be achieved.
For proof of concept a longitudinal control arm in a multi-link rear axle is chosen. By utilizing Hybridforming 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.
For this novel manufacturing process, the methodology for designing hybrid formed components utilizing finite element method (FEM) is presented in this work. It basically consists of three stages with raising complexity, which is described as follows: Firstly, topology optimization with the FE-Solver Optistruct considering design space and load requirements on the component is conducted and optimization results are used to design metal and LFT parts of the hybrid component. The second step consists of linear finite element analysis (FEA) of the designed component, which is also the basis for fatigue simulation. Concluding step is to perform non-linear simulation in Radioss with focus of modeling failure behavior of the bonding agent. For modeling the connection, a phenomenological failure model is calibrated based on shear and cross tension tests.
By using the common simulation chain for the component design of fiber reinforced plastics local anisotropy of the LFT can be taken into account, by mapping results derived in a process simulation to the structural FEM simulation. Since the molten LFT is used as forming medium for the metal sheet the melt flow varies drastically from a melt flow in common compression molding without forming metal sheets. Due to the unpredictable melt flow in the Hybrid forming process the fiber orientation cannot be approximated with commercial simulation software, thus the LFT is considered as isotropic in the early stage of component design. To validate the FEM simulation of hybrid formed components for designing the control arm a generic cross-member is manufactured in the early stage of the project by Hybridforming. This generic hybrid cross-member is tested in quasi-static torsion and 3-point bending.
The test results in 3-Point bending and torsion of this cross-member demonstrated a very high lightweight potential in comparison to a common steel cross-member consisting of a hat profile with a closing panel. A good correlation between 3-Point bending test results and simulation of the hybrid steel LFT cross-member with the bonding agent modeled utilizing the phenomenological failure model Radioss LAW83 was achieved.
|Date||18th June 2019|