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Lightweight Design worldwide

, Volume 10, Issue 5, pp 26–31 | Cite as

Multi-functional battery housing for electric vehicles

  • Rico Schmerler
  • Tobias Gebken
  • Sebastian Kalka
  • Tobias Reincke
Construction Multi-Material Construction
  • 625 Downloads

In the current research project a lightweight part with integrated functions for electric vehicles is developed. With the approach of a multi-material mix consisting out of fibre reinforced plastics (FRP), aluminium foam and solid aluminium a reference battery housing made of steel is replaced. Here the focus is on the integration of mechanical functions, the thermal management as well as crash and intrusion protection.

Motivation

Electro mobility offers a huge potential to realise sustainable transportation. The low range of current electric vehicles is one of the main reasons for the actual insufficient establishment on the automotive market. Weight reduction of car components in combination with structural optimisation are essential approaches to increase ranges. Due to increasing battery weights as a consequence of higher storage capacities and ranges those lightweight measures on their own are not satisfying. Only by integrating several functions in innovative material systems increasing demands in regard to efficiency, safety, design space and comfort can be fulfilled.

Project Goal and Solution Approach

The goal of the research activities is to develop a battery housing for an electric vehicle, which combines inter alia mechanical and thermal functions in innovative material combinations. Therefore a conventional steel housing of an existing electric vehicle locally reinforced with bars is replaced by a sandwich structure shown in Figure 1. This structure consists out of an aluminium top layer, an aluminium foam core and an endless fibre reinforced thermoplastic (organo sheet).
Figure 1

Demonstrator of the battery housing subshell with aluminium top sheet, organo sheet as outer shape and several core materials (© TU BS IWF | Fraunhofer IWU@OHLF)

The organo sheet is representing the outer shape of the battery subshell and prevents corrosion in addition to its mechanical functions. In combination with the aluminium foam core, which displays excellent energy absorption, the battery modules are protected from chip damage and intrusion [1]. The subshell is attached to the upper shell made out of a glass fibre reinforced polyamide 6 (PA6) thermoplastic.

A thermal management function is realised by the infiltration of phase change material (PCM) into the aluminium foam. PCM has got a high thermal capacity in the range of its phase change solid-liquid. This enables to smooth thermal peaks and to buffer high heat quantities [2]. The heat storage capacity of the used PCM from Rubitherm is 250 kJ/kg and 70 Wh/kg, respectively [3]. Due to the protection of battery cells from temporary thermal overcharge the cell life can be increased. During the manufacturing process a firmly bonded connection between the aluminium sheet and the aluminium foam is realised. The aluminium top layer of the sandwich enables optimal heat conduction from the cells into the foam. With this setup an excellent heat transfer into the PCM, which thermal conductivity is 100 times less than that of aluminium foam, is assured [1, 2]. A further essential advantage is the abandonment of adhesive. Beside cost and process time additional weight can be reduced referring to dimensions of 1000 x 1700 mm. Referring to CAD-data a mass reduction of 26.6 kg of the new multi- material battery housing subshell was calculated, already including the weight of PCM. In comparison to the reference model with about 35 kg a considerably mass reduction could be achieved.

Manufacturing and Characterisation of Semi-finished Products

Fraunhofer WKI carried out the manufacturing of fibre-reinforced thermoplastic composites (FRTC). In order to follow a sustainable approach a completely bio-based PA11 has been chosen as the matrix polymer. For the present project, commercially available PA11 pellets were processed to a flat sheet film by jura-plast. This film was combined with unidirectional, non-woven fabrics comprising glass, carbon or flax fibres to determine mechanical properties. To produce the respective laminates a film stacking process was employed which involves alternating stacking of thermoplastic sheets and reinforcement fabric layers and their impregnation as well as consolidation using a hot press followed by a cooling cycle. The resulting tensile properties of the different laminates are displayed in Figure 2.
Figure 2

Selection of tensile test parameters. Fibre volume fractions: 42 % FFVT, 33 % GFVT and 34 % CFVT (© Fraunhofer WKI@OHLF)

The carbon fibre-reinforced thermoplastic (CFRT) yields a distinctly higher mean Young’s modulus (60 GPa) than the glass fibre-reinforced (GFRT) or flax fibre-reinforced thermoplastic (FFRT) which exhibited 21 GPa and 19 GPa, respectively. Likewise, the CFRT also displayed the highest mean tensile strength (490 MPa) followed by GFRT (320 MPa) and FFRT (200 MPa). While the mean elongation at break of GFRT (1.5 %) and FFRT (1.3 %) is quite similar, the CFRT features a more brittle behavior with only 0.8 % elongation at break. For the FFRT as well as CFRT fibre volume fractions (FVF) of 42 % and 34 %, respectively could be achieved and the resulting mechanical properties are in good accordance with those found in similar works [4, 5]. The GFRT displays a FVF of 33 % and the resulting Young’s modulus is notably lower than in comparable literature [6] where, however, a powder impregnation was applied. Especially regarding GFRT it can be assumed that a better fibre wetting can be realised using a powder impregnation method when compared to the present film stacking. This issue will be further examined during the future course of the project.

Manufacturing closed cell aluminium foam semi-finished products near-net-shaped with only one metal top layer was a considerable challenge. Therefore extensive tests at Fraunhofer IWU were performed. The aspired products with densities of 0.5–0.7 g/cm3 were manufactured successfully. At Fraunhofer IFAM foam structures with aluminium foam spheres were manufactured (Advanced Pore Morphology, APM). Those spheres were coated with polyamide 12 (PA12) operating as glue during the component production. The various foam structures are illustrated in Figure 1.

The battery housing construction on scales of 1:1 as well as the simulation of draping was performed at the IK. Mechanical characterisations of all metallic materials and simulations concerning structural mechanics basing on this data were run at Fraunhofer IWU. For instance the intrusion damage tolerance was investigated in drop tower tests. In these tests a sandwich structure with equivalent weight as the reinforced steel sheet was determined that shows a 25 % reduction of the maximum impact force on the battery modules. Furthermore demands on sealing and the absence of cracks were proved.

A thermal management function is realised by the infiltration of phase change material into the aluminium foam.

Economical Process Technology

An efficient process chain reduces component costs significantly in the industrial value creation. Next to standardisation, the automation and the efficiency of the individual manufacturing steps as well as the reduction of process steps are promising solutions. By using integral production processes, which involve forming and joining of different materials, several process steps can be combined in one production step. This enables economical production of hybrid components. The production process outlined in Figure 3 was developed for manufacturing the described battery housing subshell. The process technology is examined using a scaled demonstrator.
Figure 3

Manufacturing process chain for the battery housing subshell (©TU BS IWF@OHLF)

The aluminium foam structure can be foamed directly into the final geometry, whereas the organic sheet is prepared close to the final contour, for example by water jet cutting, for the forming process. The forming tool produced at the IWF is equipped with the heated organo sheet as well as the aluminium foam with a residual heat of approximately 230 °C from the semi-finished manufacturing process. In the consolidation process, the aluminium foam is placed under the stamp through a vacuum system and is pressed onto the organic sheet. Within this process, the foam is forming the organic sheet into its final contour. By using the residual heat in the aluminium foam, a material closure between aluminium and thermoplastics can be achieved. By using an aluminium foam with an open pore structure on the side of the connection, form-fitting undercuts can be generated in the boundary surface, while for the variant with a closed oxide layer, a material closure is predominantly produced in the boundary surface. The forming process is designed in such a way that the aluminium foam is not damaged yielding a compressive strength of 10 MPa at a density of 0.5 g/cm3 or 20 MPa at a density of 0.7 g/cm3. The integral shaping and joining process provides a further advantage with respect to function integration. Between the molten thermoplastic and the aluminium cover plate, a sealing can be produced during the pressing process, so the PCM infiltrated battery housing subshell is protected against possible leakage.

By using the residual heat in the aluminium foam, a material closure between aluminium and thermoplastics can be achieved.

The integrated production process uses foaming, forming (organo sheet) and integrated merging operations with resulting fluid tightness functions providing a promising and economical process chain.

Quality Management

Computed tomography (CT) is used to test and evaluate the quality of semi-finished and finished parts. Measured parameters such as wall thickness of pores, pore size and its distribution as well as fibre orientation of the FRP give information about the quality of the process. On the left side in Figure 4 an infiltrated metal foam sample can be seen. It shows a high local infiltration rate of the aluminium foam with PCM in solid state. An infiltration rate of 87 % is aspired, due to thermal expansion of the PCM during its phase change. Figure 4, right side, depicts a CT view of the produced material compound.
Figure 4

CT view of closed cell aluminium foam infiltrated with PCM (left), 3-D-CT-view of the aluminium FRP material compound (right) (© Fraunhofer IWU | Fraunhofer WKI@OHLF)

The optical 3-D-measuring-system Atos from GOM is used to analyze the dimensional accuracy. Figure 5 shows the graphical evaluation of the target-performance comparison exemplarily in which the quantitative deviation is shown. In this case, it is produced by a different temperature control of the upper tool (80 °C) and the lower tool (160 °C). The evaluation shows that due to time-offset solidification on the upper and lower side of the organo sheet a deflection of about 0.4 mm is produced. Geometrical evaluation can be used to draw conclusions on form tool accuracy and semi-finished product dimensions as well as to the pressing parameters.
Figure 5

Deviation of a formed organo sheet structure from the CAD geometry determined by optical 3-D-measurement (© TU BS IWF@OHLF)

Joining Technology

In consideration of high safety requirements of electric vehicles, joining of the battery housing upper shell and subshell as well as joining of separate battery units to the lower battery shell is of great importance.

The construction of battery units to form the subshell was carried out for crash requirements of 35 G. To fulfill these requirements each of the four joints between battery module and sub-shell has to withstand a pull-out force of 1.7 kN. Based on developed material and component concepts different joining technologies were evaluated and compared under consideration of various manufacturing and costs criteria. In addition, examinations of bonded onserts as well as different insert technologies were carried out with the aim of load adapted assembly of battery modules on an aluminium sandwich.

Selected joining technologies as flow drill forming with subsequent thread forming, bonding of inserts and onserts as well as integration of inserts within the foaming process were chosen, due to its advantages of process integration or transferability of various material combinations. Pull-out investigations were carried out for selected joining technologies with M6 and M8 threads for sandwiches with 2 mm aluminium top layers. The results of the pull-out tests for M8 threads are shown in Figure 6.
Figure 6

Pull-out forces of various joints with M8 threads for joining battery modules and battery housing subshell (© Fraunhofer IWU/TU BS ifs@OHLF)

Threading inserts were used in variant 1 whereas inserts in variant 2 were additionally bonded with a two component epoxy based adhesive. In variant 3 and 4, round respectively quadratic aluminium was locally integrated into the sandwich. The foaming process leads to a firmly adhesive bonded joint between positioned inserts and aluminium foam and combines joining and manufacturing in one step. The flow drill forming with subsequent threading was used in variant 5. The surface of the onserts in variant 6 were grit blasted with corundum whereas onserts in variant 7 were pre-treated by SACO (sandblast coating) prior to bonding.

Pull-out forces requirements are fulfilled by all joining technologies without non-bonded threading inserts. Variant 5 is most promising because of its lowest cycle time and lowest amount of cycle times process steps. Furthermore, variant 3 and 4 show highest pull-out forces which could be important by stacking battery modules. The application of inserts could be considered by using top layers consisting of fibre reinforced plastics to prevent fibre damage.

For the new battery housing subshell a mass reduction of 23 % was calculated in comparison with the reference model.

Design-adapted joining of subshell and upper shell was examined at the ifs by using epoxy based adhesives as well as rubber elastic polyurethanes in regards to multi-material bonds. Based on experimental identification of material data, material models were developed and implemented in Finite Element (FE) simulations. Cohesive zone models have been found to be valid constitutive laws for epoxy based structural adhesives. Consequently, material properties for cohesive zone models were determined, i.e. opening mode fracture toughness by examining Tapered-Double-Cantilever-Beam (TDCB) specimens. After successful simulations of these tests, material data and models were transferred into component simulations. Determination of material data of polyurethane based adhesives and simulative verification as well as implementation into component simulation were carried out by using hyper-elastic material models, which require adapted test methods such as planar tensile tests.

Recycling

For the final component the IWF and Fraunhofer WKI are developing different recycling strategies which also encompass the possible influences of upscaling effects. Herein, mechanical and thermal recycling concepts are examined both targeting the separation of the different material classes used in this project e.g. aluminium foam and FRTCs. Once the individual fractions of the battery housing are reclaimed, they can be transferred to their respective recycling routes: the aluminium for instance can be recycled directly. Regarding the FRTCs joint fibres and matrix can be separated using pyrolysis. This approach allows for the re-utilisation of most carbon fibres in a cascading manner.

Summary and Outlook

In the current research project a multi-functional battery housing combining mechanical and thermal features was developed including the related process technology. For the new battery housing subshell a mass reduction of 23 % was calculated in comparison with the reference model. This already includes the weight of PCM for thermal management.

The construction of battery units to form the subshell was carried out for crash requirements.

Prospectively load path and draping adjusted textile semi-finished products with local endless fibre reinforcement shell replace the current layer stack consisting out of woven and unidirectional fabrics. Furthermore higher fibre volume fractions are aspired. The production of organo sheets will be optimised by adjusting temperatures and consolidation times in addition to adapted tools to improve the impregnation of fibres with matrix material.

In the future semi-finished textile parts will be produced on a multi-axial textile machine with a warp thread offset module and impregnation on an omega calendar afterwards. Besides investigations with hybrid fabrics produced on a double gripper weaving machine are planned. All thermal investigations are performed in a further OHLF-project. Additionally material and product investigations will be extended to dynamic loads and transmitted to real part dimensions.

Open Hybrid Lab Factory

Goal of the »Open Hybrid Lab Factory« (OHLF) is the development and mass-production orientated testing of the whole process chain for manufacturing lightweight structures. Within the presented project Fraunhofer-Projektzentrum as a consortium of Fraunhofer Institutes for Machine Tools and Forming Technology IWU, for Wood Research WKI and for Manufacturing Technology and Advanced Materials IFAM is developing the new battery housing together with the Institutes of Machine Tools and Production Technology (IWF), of Joining and Welding (ifs) and for Engineering Design (IK) of the Technical University Braunschweig, Germany.

Notes

Thanks

All authors and the whole project consortium thank the Niedersächsisches Ministerium für Wissenschaft und Kultur (MWK) for the supplied funds to perform the investigations (allocation number: VWZN2990).

References

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    Rubitherm Technologies GmbH. Online: https://www.rubitherm.eu/, access: 16 June 2017Google Scholar
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Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Rico Schmerler
    • 1
  • Tobias Gebken
    • 2
  • Sebastian Kalka
    • 3
  • Tobias Reincke
    • 2
  1. 1.Fraunhofer Institute for Machine Tools and Forming Technology IWUWolfsburgGermany
  2. 2.Technische Universität BraunschweigGermany
  3. 3.Fraunhofer Institute for Wood Research WKIHannoverGermany

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