Multi-Material Concept For A Battery Electric Vehicle

Technology September 2018 Multi-Material Concept Battery Electric Vehicle
Multi-Material Concept For A Battery Electric Vehicle

Due to big and heavy energy storages and the electromotive drive, Battery Electric Vehicles (BEV) require modified body concepts than the ones used for Internal Combustion Engine (ICE) vehicles. Lightweight design and crash safety are significant criteria in this case. For the implementation, the main goal of the research project Alive, coordinated by Volkswagen Group Research, was to achieve an appropriate requirement and optimised load path-oriented material use.

ECONOMICAL LIGHTWEIGHT DESIGN

In the Alive (Advanced High Volume Affordable Lightweighting for Future Electric Vehicles) research project, funded by the European Union, a concept for a battery electric vehicle (BEV) was developed and built up. One of the main objectives of the project was to reduce the weight of the Body-in-White (BIW), closures, chassis components and interior components (seats) up to 30 to 40 %. Furthermore, the economical realisation of these concepts for mass production was an important goal of the project.

In order to achieve these objectives, various lightweight materials and simulation methods based on Finite Element Method (FEM) have been enhanced. Additionally material characteristics have been recorded and joining technology models have been created. A lifecycle analysis has been carried out in parallel. A particular challenge in this project was to realise three BIW prototypes for demonstration. This was necessary to validate and proof the crash simulation results by using the results of the real crash tests. A front, a side pole and a rear impact crash were conducted in the Alive project.

Twenty three European partners, including seven OEMs, seven suppliers and five research facilities had worked on the project together from October 2012 to September 2016. Alive was part of the Seam cluster, an association of several EU projects that pursued the common objective of developing advanced vehicle structures and advanced materials for future vehicle generations, in particular for electric vehicles. The projects of the Seam cluster are based on the successfully completed projects Elva [1], SuperLight-Car [2] and SmartBatt [3].

(1) Material distribution of the Alive body in white concept (© Alive)

BODY-IN-WHITE CONCEPT

The before-mentioned weight targets are very ambitious. For achieving these targets various multi-material concepts have been compiled and balanced. As a result, a body concept was created using different aluminium alloys, high-strength steel and Fibre Reinforced Plastics (FRP), (1). The main load paths have been designed by using high strength steels with strengths of up to 1,800 MPa combined with aluminium extrusions. The share of the weight of the aluminium alloys in the BIW is about 65 %. The roof and tailgate are made of FRP and metal reinforcement elements. The integration of the battery case into the body including a wiring concept of the battery modules was another critical issue of the development. Lightweight concepts that have 44 % lightweight design potential in comparison to the present technologies have been developed for the doors using different aluminium alloys (5000, 6000 and 7000 series).

A particularly challenging aspect was the design of the load transfer in the event of a frontal crash. The loads are transferred via a deformable aluminium engine mount through the almost rigid longitudinal beam, which has a strength of more than 1,800 MPa, outwards into the aluminium sills. Special attention also had to be paid to the design of the side structure, which protects the battery from damage during a side impact.

(2) Joining techniques for different material combinations (© Alive)

JOINING TECHNOLOGY

The realisation of the concept concerning mass production had the focus on utilising a limited amount of affordable joining methods. This is a demanding task, especially in the area of multi-material design. In the first step, several joining technologies for each possible combination have been analysed and prioritised considering the material concept. Furthermore, three methods for the BIW concept have been defined: a prioritised method with focus on feasibility in mass production, an alternative and a prototypical method. All three methods included the necessary boundary conditions and process limits. (2) shows an example of joining techniques for different material combinations.

Based on the body concept and the preliminary work, the joining technology planning has been carried out parallel to the vehicle design process in several loops. The assembly sequence is decisively determined by the solution chosen for the area of A-pillar and firewall. The connections between the aluminium firewall and the hot-formed steel of the A-pillar presented a challenge regarding the accessibility. For this reason, the total assembly starts in the front area of the vehicle with the firewall and the inner side of the A-pillar. This assembly is connected to the central floor and to the rear longitudinal member afterwards. The body is closed with the floor components, the tailgate frame and the roof frame. Outer structural parts, outer B-pillar and frontend structure are added subsequently. Finally the outer skin components are added. (3) shows the assembly sequence of the Alive body in white schematically.

(3) Assembly sequence (© Alive)

Enhanced simulation methods have been developed in order to achieve higher predictive accuracy in the calculation of body stiffness, crash and fatigue performance from the beginning of the project. The characterisations of the joints have been tracked with special attention in the Alive consortium. Joining technologies that are used in the areas of high loads have been selected for this purpose. During this research, Renault, Volkswagen and Benteler have produced samples for flow form drilling, aluminium spot welding, MIG welding, resistance element welding and adhesive bonding in order to conduct tests for the specification of the stiffness, the dynamic strength, the fatigue behaviour and the crash resistance. The particular process parameters have been initially defined. The partners of KU Leuven, Fraunhofer LBF Darmstadt and Renault were involved in this task. (4) shows the load displacement curves/ characteristics of different joining technologies under tensile shear loading, as example. In addition, the failure behaviour has been analysed. These results form the basis for evaluation of the suitability of the methods for high load zones and have been used as input data for the design of the substitute model for the simulation.

The structural optimisation of the BIW regarding the joining has been supported by creating virtual joining models. This task has been done by Renault and Volvo each with two different types of models (volumetric elements and cohesive elements).

(4) Comparison of force-displacement curves for different connection types in tensile test (© Alive)

Special attention was needed at the material mix in the connection area of the main load paths. An example is a connection of the front longitudinal member support made of phs-ultraform 1800 to the sill made from high strength aluminium as well as the connection of the B-pillar from the same steel material to the sill and the aluminium roof rail. For these areas flow form drills have been chosen for connections with one-sided accessibility and resistance element welds for connections with two-sided accessibility both in combination with the usage of the structural adhesive. Both methods are characterised by their high strength characteristics and their high cost-effectiveness.

(5) shows a comparison of the maximum strengths under shear and normal loading from tests and simulations for the two types of connections in the multi-material design.

A very good accordance for the force-deformation behaviour between the test and the simulation has been achieved for the samples. These models were afterwards used for load path optimisation and the adjustment of the structure of the connecting areas. In this project, the same approach was successfully used also for the resistant spot welding and the arc welding for aluminium.

Additional to the adjustment of existing connection technologies, new concepts were developed as part of the Alive project: on the one hand, a process for the point joining of zink coated/ galvanised cold-formed steel sheet to aluminium [4] and, on the other hand, a method for the joining of various materials in the chassis [5]. The representation of these developments is not included in this publication.

(5) Comparison of strengths in test and simulation for the material combination phs-ultraform 1800 with EN AW6082-T6 (© Benteler)

COMPARISON OF THE SIMULATION RESULTS ON TOTAL VEHICLE MODEL LEVEL

In order to validate the simulation results, three BIW for demonstration have been built and selected load cases were used for testing. One of the load cases is the pole impact, which was done according to the Euro-NCAP protocol called Oblique Pole Side Impact Test Protocol v7.0.2. In this test, the vehicle strikes against a fixed rigid pole at 32 km/h and an angle of 75°. The test was filmed by high-speed video cameras and the deceleration of the vehicle was measured. A laser scanner was also used to measure the deformation after the test. These test results were compared with the simulation results afterwards. The total global behaviour was in accordance to the one from simulation. The intrusion and acceleration values also correspond to the test values and confirm the results of the simulation.

Also, the fracture behaviour of particular components in the test and the simulation is the same. There are differences only in crack growth. However, this was not considered in detail in the simulation. In (6), a visual comparison of the impact area is shown by way of example. The deformations occurring in the test correspond to those from the simulation. The set target values regard to remaining seat width and protection of the drive battery against deformation were confirmed by the test results.

(6) Deformation comparison of simulation (left) and test (right) for side pole crash, top view (© Alive)

SUMMARY

The other vehicle tests carried out in the Alive project show a similar picture, so that the virtual lightweight concepts with regard to the crash performance could be confirmed by the tests.

Overall, the development of the body concept showed a considerable influence of the failure of joint connections on the crash performance. For this reason, the tensile and shear forces were evaluated at critical points and interpreted with regard to the limits determined from the component tests. Finally, the objectives set in the consortium were examined by means of overall vehicle tests. In reality and in simulation, joining technology is a key to lightweight concepts in multi-material design, which must meet safety and cost objectives.

REFERENCES

[1] RWTH Aachen University: Elva – Advanced Electric Vehicle Architectures. Online: www.elva-project.eu, access: 18 August 2017

[2] SLC – Superlight-Car Project. Innovative Development for Lightweight Vehicle Structure. International Conference Super Light Car, Wolfsburg, 2009

[3] AIT Austrian Institute of Technology GmbH: SmartBatt – Smart and Safe Integration of Batteries in Electric Vehicles. Online: www.smartbatt.eu, access: 18 August 2017

[4] Potthast, S.; Tölle, J.: Möglichkeiten zur Realisierung thermisch gefügter Stahl-Aluminium-Verbindungen. EFB-Kolloquium Blechverarbeitung, 28-29 March 2017

[5] Reimann, T.; Albrecht, P.; Kleinhans, R.: Verbundbauteil und Verfahren zur Herstellung eines Verbundbauteils. Patent DE102005119396A1

AUTHORS

DR-ING JENS MESCHKE is Head of Lightweight Design and Structure Optimisation in the Vehicle Technology Department within the Volkswagen Group Research in Wolfsburg (Germany).

DR-ING JÖRN TÖLLE is Teamleader of the R&D Team Hybrid Systems at Benteler Automotive in Paderborn (Germany).

DIPL-ING LUTZ BERGER is Manager Simulation Passive Safety in the Body Department of fka Forschungsgesellschaft Kraftfahrwesen mbH Aachen (Germany).

THANKS

The authors would like to thank all project partners for their cooperation in the Alive project. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 314234 (Alive).