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Lightweight Design worldwide

, Volume 11, Issue 2, pp 6–13 | Cite as

Multi-material bodies for battery-electric vehicles

  • Martin Heinz Kothmann
  • Andreas Hillebrand
  • Günter Deinzer
Cover Story Multi-Material Design
  • 403 Downloads

Audi has collaborated with 19 industrial and scientific partners to examine how multi-material lightweight design canbeefficientlyimplemented in large-scale production. For this purpose, engineers have developed a new technology- oriented lightweight body concept to meet specific requirements imposed by electromobility.

Large-scale Production Processes

The SMiLE project (System-Integrated Multi-Material Lightweight Design for E-Mobility, German: Systemintegrativer Multimaterial-Leichtbau für die Elektromobilität) aims at gaining a better understanding of the problems of multi-material design against a background of creating large-scale production processes for efficient lightweight design. The project centers on developing a new type of technology-oriented body concept using lightweight design to meet special electromobility requirements. The key focus is on using new materials and material combinations to reduce the weight of functionally integrated vehicle components for innovative structural concepts. Material and process development involve optimizing and validating metallic and non-metallic material systems as part of a holistic approach. This also includes developing joining technologies appropriate for the materials, as well as a painting technology with a reduced process temperature; the so-called 165 °C process. This offers great potential from a body design as well as from a production perspective in terms of innovative lightweight design solutions, cutting-edge production methods and total energy footprints. An important feature of the project is that it takes into account the entire value chain, as reflected in the broad lineup of project partners covering raw materials up to the finished component. The project was selected as a lighthouse project by the BMBF (German Federal Ministry of Education and Research) at a national electromobility conference thanks to its holistic approach and its highly relevant development activities. The project partners includeAudi, Volkswagen and VolkswagenGroup Research, Porsche, Voith, BASF, Brökelmann, Clean-Lasersysteme, Dieffenbacher, Fraunhofer ICT, Fraunhofer IWM, Frimo, University of Stuttgart/IFU, TU Bergakademie Freiberg/Institute of Metal Forming, TUBraunschweig/ifs, KIT/Fast, KIT/IAM, KIT/WBK, Thyssenkrupp and the associate partner Novelis.

In multi-material construction, each component is executed with a choice material matching the requirements, allowing the systematic implementation of the lightweight design concept. The only limits in this respect are restrictions due to, for example, joining technology or production feasibility. This type of construction is known as “The right material at the right place.” This permits resource-efficient construction as lightweight design allows for material consumption to be minimized.

Material and process development involve metallic and non-metallic material systems.

Multi-material Body

Current vehicle development focuses on vehicles with electrified powertrains. However, battery-electric vehicles present new challenges for planning body structures. The high-voltage battery systems require special protection for the environment — and above all for people. Accordingly, safety was prioritized when developing the body.

The primary aim of the project is to develop technologies usable in new types of multi-material body construction. For this reason, the process also involved identifying components and assemblies for use in developing, representing and assessing these technologies. For example, the shock absorber mounting was implemented as a pressure-cast component with integrated aluminum flanges for a cost-effective and corrosion-resistant connection with the body. The vehicle floor was divided into “front” and “rear” sections, which were then used to investigate the manufacturing technology instead of deployment of continuous, fiber-reinforced thermosetting plastics. The vehicle sill is a single section made of heavy-duty aluminum extrusion profile. The vehicle has no classical tunnel. Energy storage units offering up to 200 km in range are located below the front seats (forward battery module) and rear seat bench (rear battery module). The shape of the body after the pre-design phase is shown in Figure 1.
FIGURE 1

Body concept with focal areas for technological development (© Audi)

The simulations presented in Table 1 were conducted using the finite element method to design the body structure. The model was also optimized for every load case to comply with specific threshold values. Finally, these modifications were assessed over all load cases and the body weight reduced in an optimization loop. A body weight of 208 kg was then calculated as the final value for the body with structurally integrated battery enclosure — one significantly lower value than for comparable vehicles.
TABLE 1

Overview of crash load cases (MDB: Mobile Deforming Barrier, MRB: Mobile Rigid Barrier, ODB: Offset Deforming Barrier, SOB: Small Overlap Barrier) (© Volkswagen Group Research)

Side Impact

Frontal Impact

Rear Impact

Specification

Legal

Consumer

Legal

Consumer

Legal

Europe

Barrier

MDB 90°

MDB 90°*

Pole 90°

Pole 75°*

ODB 40 %

Wall 0°*

ODB 40 %*

MRB

Velocity [km/h]

50

50

29

32

56

50

64

38

Weight [kg]

950

1300

-

America

Barrier

MDB 27°

Pole 75°

MDB 27°

MDB 90°

Pole 75°

Wall 0°

Wall 30°

ODB 40 %

Wall 0°

ODB 40 %

SOB 25 %*

MRB 70 %*

Velocity [km/h]

54

32

62

50

32

56

48

40

56

64

64

80

Weight [kg]

1368

1368

1500

1368

* Analyzed in project

The deformed bodies for load cases “ODB frontal crash” and “small overlap” are presented as examples in Figure 2. The intrusions into the passenger compartment are below the statutory requirements. The vehicle framework, meanwhile, remains intact, especially the front vehicle floor made from carbon fiber reinforced thermosetting plastics (CFRPs), albeit with minor damage (cracks). Pole side impact was investigated in detail to protect the battery system. Different pole impact positions were examined, but no critical positions were identified.
FIGURE 2

Result of the Euro-NCAP ODB crash simulation (left) and of the small overlap crash simulation (right) (© Volkswagen Group Research)

Figure 3 shows that the battery enclosure (brown) remained undamaged after the column impact. The entire crash energy is dissipated in the sill profile and partially in the vehicle floor. This was achieved by optimizing the components in extensive computational loops. The focus lay on dimensioning the geometry of the aluminum profiles (location and thickness of the inner walls), and on a layer structure of the fiber composite parts appropriate for the load paths and production.
FIGURE 3

Result of the “pole side impact” crash simulation; YZ-sectional view of the column impact position (© Volkswagen Group Research)

CFRP Floor Module with Integrated Design

The development work performed by Audi in the context of its Modular Sportscar System platform (MSS, Audi R8) showed how integrating functions and components constitutes a significant starting point for cost-efficient lightweight design using fiber composites [1]. This concept was further pursued with the CFRP floor module by integrating the seat lower cross member as the upper load path for the structurally integrated battery enclosure. Audi developed the ultra-RTM concept to produce integral fiber-reinforced polymer (FRP) sandwich components in a resin transfer molding (RTM) process. The cavity pressure-controlled injection process [1] allows for the reproducible processing of foam cores with limited pressure stability. A hydrostatic pressure test bed to determine the compressive strength of polymer foam cores under RTM process conditions was developed as part of the project [2]. The ultra-RTM technology was validated using the CFRP floor module and confirmed as fit for purpose.

In technological terms, the project aims at developing the technology and processes of CFRP components using a thermosetting plastic matrix so as to produce 300 parts every working day. The transverse beams posed a particularly difficult challenge with their sandwich construction and integrated load application elements and a layer structure optimized for loads. It is commercially attractive to produce larger modules, considering the potential reduction in investment costs for each component as well as in time-consuming work steps such as component trimming.

The geometry and layer structure of the integral CFRP floor element were derived from calculations for the whole vehicle. A preform layout, Figure 4, optimized for offcut and performance, was developed in an iterative process, which also offers a load-optimized layer structure for each preform. In addition to classical NCF textiles, Voith’s direct fiber placement system Voith Roving Applicator was considered, applying neat CFrovings to realize near-net-shape preforms. The fiber orientation, determined via draping simulation, was fed back into the structure simulation to ensure that the quality of the forecast was as accurate as possible. The characteristics portfolios of various insert concepts for highly stressed connections were also tested as a further aspect of functional integration. During this process, functional integration elicited verifiable performance and cost advantages, particularly for highly stressed inserts like seat connections.
FIGURE 4

Development process of the CFRP floor module with integrated design: from drape simulation to offcut-optimized preforming concept (© Audi)

The body with structurally integrated battery enclosure weighs 208 kg.

The CFRP components were produced using the ultra-RTM process. The production concept calls for multi-layered cutting, stacking and preforming of sub-preforms, as well as preform assembly. A stacking process is necessary following layer cutting due to the partial reinforcement of individual sub-preforms with UD patches. The simple geometry of the UD patches results in a 95 % material utilization level. Offcut for the entire component, including process surfaces, is around 15 %. The RTM tool design was backed by an ultra-RTM mold-filling simulation. For the first time, this process could be described using a mold-filling simulation, Figure 5. This involves consideration of the layer structure, flows around foam cores, local changes in part thicknesses, and changing fiber volume content resulting from the pressure-controlled injection and compression phase with a variable mold gap.
FIGURE 5

Ultra-RTM mold-filling simulation with Open Foam: fiber volume content at 0.7 mm mold gap (left), mold-filling after 15 s injection time at 140 g/s volume flow (center) and calculated pressure and mold gap progress with pressure control at 20 bar in the injection and compression phase (right) (© KIT/FAST)

The project aims at producing 300 CFRP components with a thermosetting plastic matrix parts every working day.

The components were produced using an in-mold pressure peaking at 20 bar, at a cycle time of 5 min. The floor module weighs 12.1kg in total, including both foam cores at 850 g, and a fiber volume content of 50 %.

A virtual process chain was set up to assess component costs. Compared with steel construction, the weight saving of the assembly with sill, CFRP front floor module and the structurally integrated battery support is 44 % .

FRP Hybrid Floor Module

Sustainable fiber-reinforced materials with a thermoplastic matrix and glass fiber reinforcement are a crucial class of materials. The hybrid FRP floor module developed in the project shows great potential in terms of design freedom and functional integration. Continuous fiber-reinforced thermoplastics provide exceptional mechanical properties in the fiber direction. The advantage of long-fiber reinforced thermoplastics (LFT) is their flow behavior and hence the associated design as well as the energy efficiency in case of direct processing (D-LFT) [5]. Combining both material classes using the “Local-advanced-tailored-D-LFT-process” opens up new levels of freedom to develop thermoplastic FRP components.

The rear vehicle module, Figure 6, comprises three basic components: an extensive laminate made from UD tape, a ribbing structure made from D-LFT molding material with metallic inserts and process-integrated joined aluminum profiles. The extensive D-LFT design freedom allows the implementation of an intelligent crash management system made from D-LFT and UD tape by forming a load-path optimized rib structure.
FIGURE 6

Thermoplastic FRP hybrid floor module consisting of process-integrated joined aluminum profiles, a laminate made from continuous, fiber-reinforced UD tape and a ribbing structure made from D-LFT (© Audi, KIT/FAST simulations)

Besides the production process, essentialdevelopment work also included improving the forecast quality of the draping behaviorof hot UD tape in the mold [6] and of the flow behavior of the D-LFT molding material [7]. The process simulations were validatedas part of the component sample acceptance process. The resulting fiber orientation was also taken into account in the component design.

To produce the hybrid FRP rear vehicle module using the “Local-advanced-tailored-D-LFT-process”, the aluminum profiles are first fastened in the upper mold. The pre-heated, consolidated UD tapes and two hot D-LFT strands are then placed and pressed into the mold. The term “local-tailored” comes from the use of tailored, pre-consolidated base laminates made from UD tapes with load-path appropriate fiber orientation and purely local reinforcement using D-LFT ribbing without large-scale flow around the base laminate through the method of hydraulic advancing slides [8]. The process chain allows components to be produced in a cycle time of less than 1 min.

The thermoplastic GFRP/metal composite construction of the hybrid FRP rear vehicle module enables weight savings of 25% compared with a steel-only construction — at an attractive cost for lightweight construction. Although using carbon fibers could roughly halve the weight, glass fibers were used deliberately for cost reasons and the life-cycle assessment.

Focus on Technologies

Successful multi-material lightweight design requires aluminum and magnesium materials to be optimized for the semi-finished products of sheet, profile and cast. The project centered on further developing production processes and tool technologies and the qualification of new metallic lightweight materials for reducing component weight. Work on the subject of sheet alloys concentrated on determining the molding limits for the design of complex component geometries.

New materials and material combinations require the development of new joining technologies. In the project, further development was performed on self-piercing riveting using half-hollow rivets for low-ductile and high-strength alloys. The compressive strength generated by the counterforce during forming permits damage-free joining of less ductile materials. The counterforce also reduces sheet deflection, thus improving material separation of the upper components in the riveting phase. This extends the applicable range considerably compared with conventional self-piercing half-hollow rivets.

The sill, CFRP floor module and structurally integrated battery support weigh 44 % less than when constructed using steel.

Furthermore a low-temperature coating process was developed promising fewer ?? problems thanks to a reduced cathodic dip coating (CDC) temperature of 165 °C, thus paving the way for increased and structurally improved hybrid construction. Simulations were used to investigate the required hardening of the aluminum during the low-temperature CDC process, which requires alloy substitution for some alloys.

Summary

The results of the project show that lightweight bodies for electric vehicles can be realized through an innovative multi-material mix. The body concept meets all current crash requirements for electric vehicles. Compared with conventional steel construction, significant weight savings could be achieved. The chosen strategy of producing large, functionally integrated modules proved a practical way for increasing the cost-effectiveness of the components made from fiber-reinforced materials. The project objectives were achieved espacially by the new Ultra-RTM process and the Advanced-local-tailored-D-LFT process in combination as well as a painting technology with a reduced process temperature and new joining technologies.

Ultra-RTM Process

In the course of the project, the ultra-RTM concept, which was developed by Audi to enable integral CFRP structures to be produced cost-efficiently using thermosetting matrix materials in large-scale production, was further optimized and validated using complex components. The goal is to develop a process usable for large-scale production with the lightweight design and performance potential that integrated sandwich construction of large and complex CFRP structures offers. Ultra-RTM technology is the key to producing integrated large CFRP modules and hence cost-effective lightweight design. It allows for the challenging combination of fast-curing resin systems, a sandwich construction with thin foam cores and functional integration for large CFRP structural components. Producing cost-effective and high-performance integrated components depends on significantly reducing internal mold pressure within a very short time to allow for the complete infiltratation of the component without destroying any existing foam cores. This goal is primarily achieved through innovative process control and optimization of the materials used. During the process, ultra-RTM technology takes advantage of the physical interrelations in accordance with Darcy’s law [3]. High permeability in the semi-finished product and low flow viscosity affect the reduction in mold interior pressure accordingly. An additional positive influence on both can be achieved by opening the mold minimally during the injection phase [1]. During the subsequent compression phase, the mold gap is closed through cavity pressure control.

Local-advanced-tailored-D-LFT-process

The “Local-advanced-tailored-D-LFT-process” combines the local use of long-fiber reinforced thermoplastics and continuous fiber-reinforced UD tapes [8]. In an initial step, the thermoplastic unidirectionally reinforced UD tapes are automatically placed on a lay-up machine (Fiberforge 4.0 from Dieffenbacher) with the desired fiber orientation, and pre-fastened. They are then processed into consolidated blanks in the subsequent vacuum consolidation (Fibercon from Dieffenbacher). In the advanced D-LFT process, technical thermoplastics like polyamide 6 are first mixed with additives and fillers in an inline compounder. The fibers are then introduced into the molten material in the attached twin-screw extruder. The shearing forces cause the fibers to be impregnated, and the desired fiber length is set. The properties intensify significantly with increasing fiber length [9].

In the “Local-advanced-tailored-D-LFT-process”, the preheated tailored blanks are placed inside the mold cavity with the D-LFT strands. A special transfer system was developed by Dieffenbacher as part of the project. Advancing slides in the mold drape the tailored blanks and form the cavity of the ribbing structure, over which D-LFT flows locally. A further closing movement of the mold presses the D-LFT into the ribs.

Notes

Thanks

The authors wish to thank the German Federal Ministry of Education and Research for funding the project SMiLE — Multi-material Lightweight Designs for Electromobility (FKZ 03X3041A) — and all project partners for the successful collaboration and their active support.

References

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Copyright information

© Springer Fachmedien Wiesbaden 2018

Authors and Affiliations

  • Martin Heinz Kothmann
    • 1
  • Andreas Hillebrand
    • 1
  • Günter Deinzer
    • 1
  1. 1.Germany

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