Train Front Module in Aluminium-foam-sandwich Design
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Rail vehicles shall be lightweight but also meet the high requirements for crash resistance, long life fatigue strength and fire protection. A developed front module made of aluminium foam composites fulfills these demanding requirements. In addition to its construction, the project team developed manufacturing concepts for this composite material.
Lightweight Front Module
Not only automobiles shall become lighter, more efficient, more ecological and affordable. Due to customer’s increasing environmental awareness and social responsibility in dealing with resources, issues such as electro-mobility or the use of high-performance materials are more and more in the focus of developmental work. People’s need for mobility, high demands on environmental compatibility, crash behaviour, fire protection as well as comfort, thus, create also completely new demands for rail vehicles.
Until now, two material strategies have been used — the fibre composite construction and the metallic construction. Both fulfilled the above mentioned requirements only to a limited extent. Fiber composites (FRCs) are lightweight and superior to conventional materials in terms of their fatigue strength and thermal expansion. However, a lack of concepts concerning their reparability and recycability can be considered disadvantages. An essential contribution for the initiation of this development was the poor impact behaviour of glass fibre reinforced plastics. The weaknesses of a purely metallic outer skin are quite different. In order to meet the extraordinary stress in the high-speed range, complex stiffening structures in the form of extruded sections form a substructure. Relatively thick-walled sheets have to be used additionally. Consequently, this design results in an increased total mass, which must be accelerated and decelerated at all times.
The front module of the traction unit has the most demanding requirements.
Aluminium foam as sandwich material is not completely new. Since 2004, aluminium foam has been implemented standardly in combination with steel cover sheets in a tool slide by Niles-Simmons . In other areas, too, for example, in shipbuilding, the material has mainly been processed as planar plates so far.
To find out whether the aluminium-foam sandwich is a suitable construction material for high-speed trains, it had to be verified. Usually, the front module of the traction unit of a rail vehicle has the most demanding requirements. This is why this module has been chosen. Hence, the objective was to build a technology demonstrator of such a front module appropriate for the high-speed sector in aluminium-foam-sandwich design, see info box, including all components on a scale 1:1.
Fraunhofer IWU was supported by Voith Engineering Services (now: Leadec Engineering) and by Kuka Systems (now: Porsche Werkzeugbau). The material characterisation was done by the material testing institute MFPA Leipzig.
With the “Lightweight Front Module for High-Speed Trains in aluminium-foam-sandwich Design”, the researchers developed a lightweight construction that extends the state of the art in the area of rail vehicle construction.
The sandwich material consists of aluminium foam and aluminium cover layers, which have a metallic bond. The composite is produced by rolling cover sheets with foamable precursor material (aluminium powder with blowing agent) before the foaming process. It can be processed similarly to sheet metal. The pressure that is created by the liberation of the propellant during the foaming process can be used for shaping.
Construction and Design
In the first step, it was necessary to design a front module for high-speed trains. The designers decided to imply the design features of existing high-speed train generations in their concept, in order to achieve a certain recognition effect without having to explicitly define a potential end-user beforehand. Constructively, the design differed significantly from conventional constructions in that the substructure with its ribs was omitted and the outer skin was supported solely by the sandwich structure.
In order to verify the material, the calculation of the design model involved comprehensive mechanical tests for the determination of basic parameters such as Young’s modulus, tensile strength, bending strength and compressive strength. Furthermore, fire tests, fatigue tests and impact tests as well as welding tests were carried out and physical parameters such as heat transition and thermal expansion were calculated.
Conceptual Technology Development
For shaping the segments, three technology variants have been developed in close cooperation with the project partner Kuka Systems. Two of these variants differ fundamentally in the question of whether the foamable pre-product is pre-shaped and foamed afterwards or whether the ready foamed sandwich is shaped subsequently. The third variant proposes a realisation of the form by foaming alone. After a systematic examination of the concepts, the decision was made on the variant “first forming, then foaming”. The combination of sheet metal forming and forming by foaming offered the potential to use technological synergy effects in order to develop cost-optimised tools.
These established technology concepts provided the basis for a detailed process development. In the further course of the project, innovative concepts were designed, which, in addition to the proof of the fundamental feasibility, made an efficient production possible. The adaptation of conventional forming technologies tailored to the semi-finished product, clearly simplified the complexity of the forming tools and, furthermore, brought a significant cost reduction. Very light and flexible adjustable system-foam-forms have been developed, which can be produced with minimal material use and production effort and which do not require almost any storage space. In addition, an assembly strategy has been implemented with which quick, simple and precise positioning of the individual sandwich segments is possible. Last but not least, it was possible to produce large-scale, three-dimensional aluminium- foam-sandwiches and to realise a full-sized front module.
The objective of developing forming tools for the shaping of the aluminium-foam-sandwich pre-product was to achieve an adequate accuracy in the forming process of the preformed foamable pre-products. The biggest challenge here was to transform the 10 mm thick pre-product. In comparison to a conventional deep-drawing sheet this pre-product is much thicker and furthermore, multilayered. No conventional software for the calculation of forming processes could deal with these input data. Therefore, the design of a testing tool for the first forming tests was based on the long-term experience of Kuka Systems.
In the first test, the component still lay fixed on the sheet metal holder, which resulted in a limited material flow. Consequently, cracking defects occurred at positions with a locally high deformation degree. Adjusting the shape of the circuit board and lubricating the critical positions sufficed to prevent the crack formation. However, it seemed obvious, that thick foamable boards have a low plastic deformation capability. The flow properties and the sheet thickness of the precursor material were not suitable for a deep-drawing process. Design edges could be created with adequate accuracy, but there was also a clear elastic rebound. Calculated over-bending would have been a possibility to counteract this effect. Nevertheless, it was much more important to realise that a sufficiently precise geometry could be achieved during the foaming process, despite surface deviations of the preformed pre-products. This gave the developers the idea to simplify the tools so far that this effect could be used as an advantage. Pre-products, which were formed by significantly simplified stamping tools, did not negatively affect the foaming result, despite major shape deviations before the foaming. Thus, compared to reference deep drawing tools, the development of different tool types helped to reduce the tool costs by up to 60 % depending on their quantity. The researchers even went so far as to transform the pre-non-tool-based. Large-scale, curved sheets with sheet thicknesses of up to 80 mm are formed in a container construction by 3-roll bending machines. The altogether 14 slightly curved roof segments and the side panels were successfully transformed by this alternative technology. For each segment, gages gave the curvature to be achieved. The big advantage of this method was that forming represented less than 0,01 % of the tool costs compared to stamping.
The decision was made on the variant “First forming, then foaming”.
3-D Aluminium Sandwich Form Foaming
The biggest challenge was to transform the 10 mm thick pre-product.
Vacuum Clamping Device
Results and Forecast
The comprehensive process development in the project for the production of large-scale, three-dimensionally curved aluminium-foam-sandwich components and the production of the technology demonstrator created the basis for converting aluminium-foam-sandwich constructions into applications under economically attractive conditions. This is illustrated by the cross-technological project innovations, such as the versatile design possibilities of the forming tools, the high flexibility by the use of simple mold inserts in foaming tools and efficient assembly possibilities with high positioning accuracy and very low positioning effort.
Based on this fundament, the technology can be transferred to many other technical fields. This can also be other applications in rail vehicle construction, such as side panels, roofs and floors of passenger cars. Naturally, the advantages of an aluminium-foam-sandwich construction can be utilised to other commercial vehicles in agricultural machinery, in shipbuilding or in any other technological field.
The results presented here are the result of a joint project entitled “Development of a Front Module for High-Speed Trains in aluminium-foam-sandwich Design” with the project number 100067610. It was funded under the European Regional Development Fund (ERDF) by the European Union and the Free State of Saxony. The authors thank for the financial support.
- Hohlfeld, J.; Hipke, T.: Aluminiumschaum Eigenschaften, Herstellung, Anwendungen. In: Volkova, O. (Hrsg.); Metallurgisches Kolloquium zu Ehren von Prof. Dieter Janke; Freiberger Forschungshefte; B 366 Werkstofftechnologie; 67. Berg- und Hüttenmännischer Tag 2016; pp. 92–102Google Scholar