Lightweight Design worldwide

, Volume 11, Issue 5, pp 36–41 | Cite as

Media-tight Hybrid Composites for Fluid-bearing Housing Structures

  • Birte  von der Beeke
  • Angela Ries
  • Klaus Dröder
  • Georg-Friedrich Lührs

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.

The HL 600 rear axle gear cover, Figure 1, which is used in all-wheel-drive vehicles, was chosen as a close-to-test prototype. For the production of bearing seats, flange sealing surfaces and bolting domes, the component is CNC-machined. For the production of the prototypes, a window was inserted into the component and subsequently covered with plastic. For further development steps and other components, the notches can be applied directly in the metal die-casting process. The lightweight potential illustrated by the example of the gear cover can also be applied to other large-area cases and component shells.

Figure 1 Design state of the modified rear axle gear cover HL 600 (left) and direct injection molded prototype filled with fiber-reinforced plastic (Altech PA6.6) (right) (© left: Ries | Volkswagen, right: IWF)

Testing and Parameter Study

To examine the media tightness and joint strength of light-metal plastic hybrids for case structures, differently scaled test specimens have been produced and analyzed within the scope of the presented investigations. The basic investigations were carried out at model test level on tensile shear test specimens and flat plate test specimens. The plate test piece has a simple, circular gating geometry on which leak tests can be carried out much more efficiently and effectively than on the prototype itself. In order to ensure comparability with other scientific work, the joint strengths were referenced by means of tensile shear tests in accordance with DIN 1465 [7]. The metal inserts were laser-structured to mechanically interlock the plastic into the metal semi-finished insert, Figure 2 (left). Validation of the tightness-tested variants is performed on further test specimens close to the component in order to transfer the results to the prototype and from there onto a target component in perspective, Figure 1 (right).

Figure 2 Cross section of a laser-structured hybrid composite test piece with interface between metal and plastic (microstructure of 450 µm structure depth) (left) and schematic representation of a snap-in lug geometry for clamping the fiber-reinforced plastic into the metallic insert structure as a macrostructure (right, representation not to scale) (© OHLF)

Testing Method

The test equipment used for leak testing has a modular design in order to be able to test all used shapes with the same measurement setup and to ensure comparability of the tests. In each case, one test specimen is fixed in the test chamber and subjected to compressed air at an excess pressure of up to 0.4 bar, Figure 3. The respective hybrid structure is tested in the pressure difference method at room temperature and relative to atmospheric pressure. Any pressure losses are determined by the sensors and recorded via a data acquisition card interface. The test volume is constant for all test specimen geometries in order to keep thermodynamic effects as low as possible. A leakage test is passed when the component-related limit value is not exceeded. The various modules allow the examination of model- and component-related test specimens to validate the results as well as the testing of the component itself.

Figure 3 Leakage test method, schematic measurement setup (© OHLF)

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

In addition to mechanical interlocking, bonding agent concepts were pursued as a material-locking sealing concept, which were to make a decisive contribution to improving the locking of the different materials. In this case, laser structuring was dispensed with. Instead, three bonding agent systems were applied to the injection molding specimens. This is an unfilled 1-component (1-C) epoxy resin, which is heat-curing and high-strength (bonding agent A), a phenolic resin (bonding agent B) and an ultra-high-temperature high-performance epoxy resin (bonding agent C). The bonding agents were tested on the notched gear box covers, which were then injected with a modified polyamide (PA) 6.6 type with 35 % fiber glass from Albis Plastics. After storing the media in temperature-controlled gear oil and measuring the leakage, only the 1-C epoxy resin (bonding agent A) proved to be leak-proof within the specified component-related limit value, Figure 4. The reference prototypes without bonding agent show comparable leakages to the phenolic resin-based bonding agent B. The most significant leakages were detected for the high-performance epoxy resin (bonding agent C).

Figure 4 Results of the leakage measurement at constant pressure difference on structurally identical prototype components for the three adhesion promoter concepts used, compared to the reference without adhesion promoter and without structuring (© Ries | Volkswagen)

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.

Bond Strength

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 [5]. 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.

However, it has already been shown that adequate joint strength using mechanical interlocking is also possible without expensive pretreatments and even on uncleaned, laser-structured surfaces [7]. The best media-tightness was achieved with a laser-structured surface with a bond strength of about 14 MPa. Figure 5 shows the results for specimens with an overlap area of 12.5 mm × 25 mm on a 100 kN test station according to DIN 1465, using the following materials:

Figure 5 Tensile shear strength of uncleaned and alkaline-cleaned, laser structured test specimen (left) and test arrangement in a tensile testing machine according to DIN 1465 [7] (right) (© IWF)

  • 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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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.

Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2018

Authors and Affiliations

  • Birte  von der Beeke
    • 1
  • Angela Ries
  • Klaus Dröder
  • Georg-Friedrich Lührs
  1. 1.Technischen Universität BraunschweigBraunschweigGermany

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