Wishbone made of hybrid aluminum foam sandwich
Fraunhofer IWU and the TU Chemnitz have developed a wishbone made of a high-strength sandwich with a metal foam core and hybrid laminate cover layer. The potential is shown in the form of a lightweight wishbone as technology demonstrator, in which the weight reduction leads to a reduction of unsprung masses and thus to a direct improvement of the driving properties.
High Damage Tolerance
Hybrid components made of fiber-reinforced plastic (FRP) composites and metal components make it possible to open up lightweight design applications for complex loads in large-scale production by significantly extending the range of properties compared to monolithic metal structures. Metal foam is therefore very well suited as a sandwich core due to its properties. This applies especially for components that are subject to bending stress, for which flat sandwiches with metallic top layers of aluminum or steel are used almost exclusively. This implies that hybrid core composites and complex geometries can only be implemented with considerable effort. Within the subprojects A, B and C of the Federal Cluster of Excellence MERGE the fusion of near-series processes for the production of metallic basic structures is being researched in order to produce complex sandwich components with a high potential for lightweight applications. The shaping and functionalization of the components is achieved by a technology fusion, not by isolated joining processes in optimized assemblies. The scientific challenge lies on the one hand in determining the optimum point in time for the production of the composite material and on the other hand in the in-situ forming of the components, which differ greatly in their mechanical material behavior. Technologies for the integration of semi-finished products with subsequent shaping, but also technologies for hybrid production and shaping conducted in one process is part of the research. Potential applications are function-integrated high-performance components for new component concepts requiring low mass combined with high stiffness and strength.
Hybrid Laminates as Top Layer
The foam core enables a high damage tolerance and a more benign failure behavior.
The aluminum layers in the outer area protect the fiber-plastic composite from external influences such as moisture and thermal stress. Another advantage is the protection of thermoplastic FRP in abrasive environments. In particular, the aluminum layer prevents the thermoplastic FRP from absorbing moisture. In case of larger damages, which are not or only poorly visible in the fiber composite, the aluminum outer skin can also give an indication of the damage. Due to the metallic surface layers, mechanical processing (sawing, drilling) can be carried out almost in the same way as purely metal components. At the same time, the metallic outer layer allows a better paintability . A final coating seals the open edges of the component and prevents a stress-free delamination that occurs due to swelling .
The supplementary intermediate layer of glass fiber-reinforced plastic (GFRP) serves on the one hand to avoid galvanic contact corrosion in the sense of decoupling of carbon fibers and aluminum and on the other hand to grade thermally induced residual stresses caused by the different Young’s moduli and thermal expansion coefficients .
Due to the high degree of deformation, the hybrid laminates for the cover layers were pre-formed and then compression molded in the first demonstrators. Further experiments without aluminum cover layers were carried out by combining the foaming and joining processes. An aluminum tool for the compression molding was designed and manufactured. Previous to the consolidation, a modification of the surface and the metallic layers was implemented. Through mechanical blasting and flame treatment the aluminum thin sheet was enhanced regarding its roughness and structure to increase the bonding strength with the FRP. Due to the pre-treatment, the surface area is enlarged and interlocking options for the plastic are gained . Furthermore, the fiber-matrix semi-finished products were dried before processing. Subsequently, they were heated by infrared heating along with the metallic components and transferred into the pre-heated pressing tool. The consolidation process is characterized by a defined regime of pressure, time and temperature. In the melting process the thermoplastic matrix bonds to the aluminum components. During the cooling phase, the thermoplastic forms a firm bond with the pre-treated aluminum surface. Using thermoplastics as matrix material allows for the realization of comparatively short cycle times.
Aluminum Foam as Core Material
Metal foams are characterized, due to their cellular structure, by a low specific density, high stiffness, good energy absorption abilities and a good damping behavior. Therefore, metal foams are an excellent choice for diverse lightweight applications, especially when dealing with superimposed loads. The main focus of studies concerning the foaming process of sandwich cores is the research of process restrictions, particularly with regard to the geometric complexity of structural components. Due to the high shear stiffness and shear strength as well as the high pressure stability of metal foams compared to polymer foams, they are also suitable as core material for highly stressed sandwich structures. Thus sandwich systems with aluminum foam as core material have a great potential for lightweight applications.
Using thermoplastics as matrix material allows for the realization of comparatively short cycle times.
The aluminum foam core is foamed in near net shape.
The multidimensional thermal expansion has to be taken into account for the demolding of the core as well. Otherwise the foam could shrink on the tool and would be damaged. This leads to the necessity of demolding the core while still in a hot stage. In the process, which is performed manually, the time frame between complete foaming, collapse of the foam structure and demolding at a suitable temperature is very small. The parameters, such as temperatures, preheating and foaming time, adapted cooling and time for demolding, were therefore determined in several experiments.
Hybrid Sandwich Design
A structural component that combines the lightweight strategies of materials and design in the form of a mixed composite or hybrid composite can withstand compressive, shear and torsional loads and especially loads caused by bending. In addition to simple components, which are designed in a cross-section similar to bending beams, a sandwich construction is particularly suitable for components which are designed as sheet metal, plates or panes and are subject to bending loads. In the case of core composites that are distinguished from layered design methods, such as laminates or layered composites, a material of low density and performance has to be provided for the core, which is thicker than the surface layers, and a high-performance material system must be provided for the typically thin surface layers. In order to extend property characteristics and increase geometric complexity, such core composites can be combined with hybrid laminates that are used as top layers. This results in extremely high specific bending stiffness and strength.
Because of the high thermal stability, the foam core can be used directly for producing a hybrid compound without additional components in the joining zone. By means of compression molding, the heated top layers are applied over the softened thermoplastic matrix material. The closed skin of the foam was pre-treated to enhance the bonding properties . Previous research showed a very good suitability of mechanical blasting with additional silane coating as an appropriate foundation.
The results of the basic investigations are suitable for the design of generic hybrid technology demonstrators that take into account various restrictions and aspects. These include increasing the degree of lightweight design by using core composites with aluminum foam and hybrid laminates, the integration of inserts as interface elements and a complex multi-dimensional sandwich structure with a variable thickness cross-section.
Federal Cluster of Excellence “MERGE”
The wishbone was designed as part of the Federal Cluster of Excellence „Technologiefusion für multifunktionale Leichtbaustrukturen“ (MERGE). The main focus of the cluster is the fusion of basic technologies suitable for mass production for energy and resource-efficient production of lightweight structures and their weight reduction. Lightweight design is regarded as a key technology, particularly in the case of moving masses such as automobiles, aviation and rail transport, and contributes to the sustainable reduction of climate-damaging greenhouse gases.
In the implementation of the lightweight wishbone, research domain A (semi-finished products and preform technologies), led by Professor D. Nestler, and research domainB (metal-intensive technologies), led by Professor W.-G. Drossel, were mainly involved. The focus of research domain A is on the production and processing of semi-finished products and preform technologies based on textiles, plastics and metals in an in-line manufacturing process for mass production applications. Field B deals with the functional expansion of metal-intensive technologies for the production of hybrid metal-plastic composites.
Through close cooperation with the interacting research domains, the expertise from the respective areas could be combined into a resource-efficient technology platform with high performance and function density, which has already been integrated into the system demonstrator of the “Chemnitz Car Concept.”
This work was performed within the Federal Cluster of Excellence EXC 1075 “MERGE Technologies for Multifunctional Lightweight Structures” and supported by the German Research Foundation (DFG). Financial support is gratefully acknowledged.
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