Media-tight Hybrid Composites for Fluid-bearing Housing Structures
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The use of lightweight plastic-metal-hybrids is interesting for hybrid lightweight construction of fluid-carrying cases, for example in powertrain components. However, the tightness of the plastic-metal compounds varies depending on the aging treatment. Scientists at the Open Hybrid Lab Factory and Volkswagen show how the material classes can be bonded in a media-tight manner with joint strength.
Process Requirements for Plastic Metal Hybrids
Hybrid composites of metals and plastics combine the material advantages of different material classes and thus provide efficient lightweight solutions for the automotive industry. In general, a metallic material is partially substituted by a fiber-reinforced plastic. In the developed plastic-metal compound, the different materials are materially bonded. It is also possible to combine bonding mechanisms, for example by using bonding agents or micro/macro form-locking. Depending on component requirement this results in challenging development of proven large-scale production technologies, such as conventional injection molding or thermopressing. For the former, for example, a semi-finished metal insert can be injected directly within the cavity.
For the hybridization of case structures, the media-tight design of the boundary surface architecture between the two material classes is particularly important, since it must be tight against vehicle-relevant fluids. The production technology used for this must be aligned and optimized along the entire process chain to meet the requirements of both materials used in plastic-metal hybrids. [1, 2]
In addition to process engineering issues, the component-specific operating points must also be taken into account. Here it is necessary to define the thermal and mechanical loads acting on the prototype and to take these requirements into account also in the boundary surface design of the plastic-metal hybrid structure. Although fiber-reinforced plastics are often used for an increase of stiffness and for a reduction of thermal expansion, the uneven expansion of the joint partners caused by thermal influences and the resulting leaks against fluids and gases represent the main challenge, as shown in further studies [3-6]. The mechanical load situations resulting from operation under bearing forces, housing suspension and vibrations are also relevant, which is why increasing the bond strength of plastic-metal hybrid structures is another core objective.
The research campus Open Hybrid Lab Factory, in close cooperation with Volkswagen, has made the production of media-tight joint zones between plastic and metal in a multi-material compound a research goal. In addition, the developed hybrid structures had to be tested taking conventional production technologies into account, and considering acoustic advantages by the use of plastics for noise absorption.
Testing and Parameter Study
Table 1 Overview of specimen geometries (representations, not to scale, units in [mm]) (top, middle: © IWF; bottom: © IfW | Uni Kassel)
Tensile shear test specimen according to DIN 1465
Directly molded leak test specimen as hybrid composite of light metal die-cast and fiber-reinforced plastic with 50 mm diameter of the plastic component
Near-net shaped specimen
Tightness through Bonding Agent
Tightness through Micro- and Macrostructure
Conventional sealings of fluid-carrying case structures are, for example, flat sealings or sealing cords embedded in the case. Their sealing effect is achieved by the subsequent assembly step. By means of a micro- or macrostructure in the material transition zone, media-tight multi-material compounds can be produced by form-locking without the need for subsequent assembly steps. It is used to clamp the fiber-reinforced plastic in or behind a scaled structure of the metallic insert, Figure 2 (left). Here, the problem of different thermal expansion coefficients is specifically utilized in favor of the media tightness: The plastic shrinks on the metallic structures. In the injection molding process, the notched, preheated metal insert is inserted into the temperature-controlled injection mold. During mold filling, the hot polymer melt is pressed against the pre-structured metal insert entering the injection mold, fills the undercuts of the metal structures and clamps to the metal shape to form a hybrid compound.
The cooling of the hybrid compound in the mold results in internal stresses of the plastic due to shrinkage. The resulting combination of form-locking and force-locking may be the fundament for reliable sealing effects at the boundary surface in multi-material compounds, as initial tests suggest. Subsequent work will be aimed at taking up this research question in detail.
For mass production, the reliability of a process is one of the most important aspects in order to guarantee reproducible quality and to keep the number of rejects in production as low as possible. Pretreatments and cleaning of the semi-finished products are often indispensable for adequate wettability with bonding agents . Figure 2 (right) schematically shows an interface architecture with snap-nose geometry in the cross section of the test specimen. The contour may cause problems on complex component topologies with notches in order to evenly apply bonding agents.
PA6.6 Dupont Zytel 70G35HSLX
PPA Dupont Zytel HTN54G35HSLR
PA6.6 Albis Altech PA6.6 A 2035/507 GF35.
These results can also be verified after aging treatments (climatic change tests and storage in gear oil) in accordance with the relevant test standards and guidelines for aging treatments in industrial applications.
Proved Media-tightness without Bonding Agents
The media tightness of the tested specimens could also be proven for the target component. For the present solution approach for the production of media-tight hybrid structures, the comprehensive consideration of the process route, most of all the manufacturing process control, is inevitable. Through further considerations of automation and industrialization, this work makes an approach to the integrated production of hybrid composites for automotive lightweight construction possible. Future work will demonstrate that media tightness can be achieved through the use of micro- or macrostructuring without the use of bonding agents or cleaning work in multi-material compounds. Industrialization of the production technology for the presented plastic metal hybrids is possible and can be combined with short process times based on the use of laser structuring and the elimination of expensive pretreatments.
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Prill, T.: Beitrag zur Gestaltung von Leichtbau-Getriebegehäusen und deren Abdichtung. Stuttgart, Universität Stuttgart, thesis, 2013
Horn, B.; Ries, A.; Junior, W. S.; Kühn, M.; Müller, A.; Arend, C.; Lührs, G.-F.; Dröder, K.: Effect of Untreated and Alkaline-Cleaned Surfaces on the Joint Strength of Plastic-Metal-Hybrids. In: International Journal of Materials Science and Engineering, 2018
Demes, M.; Kühn, M.; Gebken, T.; Dröder, K.; Thermal behavior of polymer metal hybrids of hot stamped steel and fiber-reinforced thermoplastics. In: 4th Brazilian Conference on Composite Materials. Rio de Janeiro, July 22 to 25, 2018
As part of this research project of the Open Hybrid Lab Factory and Volkswagen, laser structuring was provided by Trumpf Laser- und Systemtechnik. We would also like to thank Albis Plastics for supporting our research work with material samples.