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Lightweight Design worldwide

, Volume 10, Issue 2, pp 6–11 | Cite as

New Lightweight Construction Concepts in Sheet Metal Forming

  • Mathias Liewald
  • Matthias Schneider
  • Stefan Walzer
Cover Story Lightweight Metals
  • 305 Downloads

Keywords

Magnesium Alloy Sheet Metal Folding Axis Folding Structure Lightweight Material 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Constant increasing demands on the lightweight construction of thin-walled, structurally stiff and functional components are influencing the construction of sheet metal parts for vehicle bodies. Therefore at the Institute of Forming Technology at the University of Stuttgart, new solution approaches for the specific use and processing of lightweight materials like modern magnesium und aluminium alloys are being pursued and also implemented prototypically.

The trend towards saving resources and lightweight design is still the dominant motivation for the majority of development projects in industry as well as in research at IFU. Future sheet metal components are designed to have higher stiffness and toughness properties and reduced weight at the same time. In addition to the research efforts in the fields of classical lightweight construction, topics of an increased functional integration into the component made of sheet metal are also pursued in a constructive sense and, more recently, even with regard to the expected digitisation of goods.

Various research projects at the Institute of Forming Technology (IFU) at the University of Stuttgart focused on the development of new lightweight construction potentials, improved accuracy or the implementation of a higher component complexity. Finally, improvement of process and tool development is also part of the process to increase the efficiency of process chains in sheet metal forming. The following solution methods from the combination of construction, forming methods and lightweight materials are defined by the numerous research topics and approaches:
  • ▸ “Lightweight and stiff“: components made of lightweight materials with high stiffness.

  • ▸ “Thin walled and high-strength“: components made of thin and high-strength materials.

In recent years in industry various new materials were developed and introduced into the market. Today’s examples for “lightweight and stiff” materials are composites (sandwich) or fibre reinforced materials which offer promising material properties, but unfortunately manufacturing these hybrid materials in a large scale production is too expensive up to the present day. The other trend of increasingly using high strength materials with reduced thicknesses is represented by the development of advanced high-strength (AHS) and ultra-high-strength (UHS) steels such as DP1000, CP1000 or TWIP materials. Both composite and high strength materials pose various questions concerning material characterisation and modelling, tribological issues, tool making, and forming processes and new dimensions in terms of quality assurance.

Forming of Magnesium und Aluminium Alloys

The objective of the project SMiLE “System Integrative Multimaterial Lightweight design for Electromobility” (BMBF project, number 03X3041, term 01.09.14 – 31.08.17) is the development of a new, material- and technology oriented car body lightweight concept, which is aligned to the special requirements of the electric mobility. This project is subdivided into various working groups, which concentrate on different material combinations and joining techniques. Eventually, the final results of each individual working group will be assembled and used for a demonstrator car body (see lead picture), to show advances of the developed concepts and materials in an innovative and functional car body. Within this project, IFU is concentrating on improving the forming process of magnesium and high-strength aluminium alloys. Due to their bead structure at room temperature these materials have a much lower forming capacity than the materials established in the industry. From these conditions, the existing processes are further developed in order to enable them to be shaped. This can be implemented with a heated tool concept, because of the thermal activation of further sliding systems in the material the deformability can significantly increase [1].

The aim of the project is to produce demonstrator components, such as the model of a car door plate, Figure 1 a, made from AZ31 magnesium alloy and to evaluate its use within future vehicle concepts from technical and economic point of view. At the IFU, the FEM simulation system AutoForm R6 is used to simulate the forming process of the car door panel. The simulation software provides tools for detecting wrinkles, splits and springback as well as for thermal simulation and strain measurement. Furthermore, a method planning designed for the car door panel was carried out and a heated tool concept, Figure 1 b, was developed.
Figure 1

CAD-Model of the car door panel (a) and model of the heated deep drawing tool (b) (© IFU)

Figure 2

Folding structures depending on the axes orientation [2] (© IFU)

Folded Structures

Folded structures made of different sheet materials can be advantageously used in many applications, e.g. as a visually appealing façade, as a heat exchanger plate or as a core structure for sandwich semi-finished products. Particularly the application in the sandwich compound seems quite promising from today’s point of view. These structures can be unfolded stably and relatively high. As a result, the structural thickness and stiffness of the composite in terms of transverse forces can be increased enormously. Due to the multidirectional folding axis orientation of the structure, the mechanical properties of the composite are surface isotropic and can therefore be adapted individually to specific stress profiles.

In principle, the folding structure offers a higher potential of a more economical production compared to honeycomb. While the honeycombs must always be joined by gluing, the folded combs are created from the folding of a previously planar plate and thus form a cohesive component without additional joining points. In addition, the folded honeycombs have the advantage of their open structure, which allows the integration of additional functional elements, e.g. ventilation to prevent condensation (and thus corrosion), integration of heating elements, cable trees or other sensors without any additional processing steps. In Figure 2 various folding structures are shown as a function of possible folding or bending axes.

The industrial production of these folding structures is still the subject of research, why the convolutions are still mostly carried out manually for initial patterns and principle studies. The manufacture of such folding structures can now only take place in a very simplified form and in special cases (cooler construction). The production concepts, which are known in industry and research, currently focus on materials such as plastics, aramid papers and thin-walled fibre composites with a negligibly small material thickness. Only a few investigations are based on metallic cores with a maximum initial thickness of 0.2 mm (pure aluminium) [3] or 0.1 mm (steel materials). The reasons for these current deficits are the complexity of the kinematic 3-dimensional fold on the sharp-edged folding edges, which increases with the number of bending axis orientations. The folding of complex structures, Figure 2, is therefore not possible from a certain sheet thickness with the current production approach. In most cases, the production of folding cores is achieved by two process steps:
  • ▸ Definition of folding axes, the so called pre-creases.

  • ▸ Unfolding of the flat structured plate to a 3-D-structure.

For applying the pre-creases, different technologies like cutting, embossing, or perforating can be used [4]. In Figure 3, pre-creasing and unfolding are depicted schematically for a geometry with two folding axes.
Figure 3

Pre-creasing (right) and different stages of folding of a structure with two bending axis (© IFU, according to [5])

Actual objective of investigations at IFU is to put a folded structure with a sheet thickness of more than 0.2 mm into effect to make it possible to use sandwich panels with very high strengths in the core (e.g. for ships, trains, construction vehicles, parking lots, etc.). These panels are weldable and therefore they do have a better recyclability. Goals of current research are the investigation of folding mechanisms of sheet metals in relation to pre-crease design and production influences as well as determination and characterisation of process limits of folding (or rather bending) structured sheet metals. Based on these investigations processes and tools are developed for discontinuous and continuous manufacturing technologies in the future.

In order to investigate the influence of pre-crease were design on folding of blank a FEA-Model for a mono cell was implemented. For this model the run of pre-crease and pre-crease shape (resulting from different manufacturing processes like cutting or embossing) could be varied and influences on the folding behaviour investigated during the last months. Additionally the folding kinematics can be evaluated as depicted in Figure (same pre-crease design, different folding kinematic). In Figure 4 (a), a non-beneficial kinematic is shown, illustrating that not only pre-creases are bended, but the plane regions of the cell as well. This is because of an appearing torsion which has to be avoided. In Figure 4 (b) a beneficial folding kinematic is shown with bending process at the pre-creases only.
Figure 4

Non-beneficial (a) and beneficial (b) folding kinematics (© IFU)

In Figure 5 (a), a folded mono cell of Figure 4 (b) is depicted showing von-Mises stress distribution. As can be seen, stress is only present at the defined pre-creases, which enables a defined folding. In addition to this investigation, a FEA model also can be enhanced by use of bending limit curves to apply process limits for cracking at the pre-structured bending lines (pre-creases). In Figure 5 (b) unstructured and structured strips (by machining) were investigated by optical patterns, which can be used for determine limits for cracking due to over bending via optical measurement systems.
Figure 5

Stress distribution at a folded mono cell (a) [2] and specimens for determining bending limits (b) (upper specimen: unstructured; lower specimen: structured) (© IFU)

Based on the latest research, processes and tools for discontinuous and continuous production technologies are developed and made of thin sheet metal using these innovative folding structures. The component properties are examined and determined after their production.

Forming of Sheet Metal Composites

During recent years layered composite materials with metallic cover sheets have been used in industrial applications based on their lightweight potential. Further requirements such as high stiffness of components, vibration damping, strength, and formability are only partially met by today’s advanced composites. As a consequence, great efforts are made in current research to develop sheet metal properties which meet the requirements mentioned above and achieve greater stiffness as well as a reduction of weight.

The material composites of sheet metal materials considered here are in principle composed of the layers of the two cover plates and of the polymer layer lying between them, this layer also effecting the two-dimensional bonding of the two cover plates. An increase in the layer thickness of the adhesive leads to an increased stiffness of the composite, but the transmission of shear stresses due to the forming process is possible only within certain limits. Therefore, only a few combinations of sheet metal composites have been established due to the current state of research in the field of adhesives, transfer correspondingly high shear stresses. This is the main reason for the low use of these composite materials in the automotive industry [6].

In order to increase the stiffness of the component, this research investigation presents a new sheet metal composite material which consists of a plane sheet, a thin intermediate damping-layer, and a cover sheet layer having locally formed elements, such as beads, Figure 6. The different layers are necessary for individual functions, e.g. the plane side can be used as the visible, anticorrosive or pained side of the part. Shape elements incorporated on the other side of the workpiece increase material strength and stiffness due to work hardening and physical dimensions. Furthermore, the geometric variation can be used as design or functional elements. In order to use these layered composites, metal forming characteristics (e.g. deep drawing) must be determined before applying them to industrial products.
Figure 6

Sheet metal composite having stiffness increasing beads on one side (© IFU)

Sandwich material systems, especially multi material hybrids represent an interdisciplinary concept by combining suitable material properties, material composites, design and functionality for fulfilling the high demands regarding modern materials and structures [7]. Sheet metal composites combine advantages of contrary mechanical properties (e.g. damping factor, stiffness, strength) in order to match new fields of application in the automotive industry. Increasing the layer thickness leads to advanced stiffness, but the transmission of shear stress in this case is limited. Hence, only a few combinations of regular sheet metal composites are able to transmit shear stress in the adhesive layer, which is the main reason for a non-comprehensive use of adhesives in the automotive industry.

Sandwich material systems represent an interdisciplinary concept by combining suitable material properties.

Conclusion

As shown in the overview of the current research trends at IFU, optimised forming processes, new materials and improved simulation models in sheet metal processing are becoming increasingly important for the successful development of sheet metal parts. The use of high-strength aluminium and magnesium alloys, folding structures and composite materials has high potential for the development of new lightweight concepts. However, with regard to the shaping of these materials and their combinations the production technology presents great challenges. For this reason, the IFU is striving to advance the research in the field of lightweight construction concepts in sheet metal forming and to develop processes, material models and materials in order to meet the customer requirements of the future.

References

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    Liewald M., Schneider, M., Schmid P.: Origami in der Umformtechnik — Bleche falten, In: blechnet 5 (2016), pp. 126–127Google Scholar
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    Gattas, J. M.; You, Z.: Quasi-static impact response of single-curved foldcore shells. ASME 2014 International Design Engineering Technical Conferences & Computers and Information in Engineering Conference, Buffalo, 2014Google Scholar
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    Fischer, S.: Numerische Simulation der mechanischen Eigenschaften von Faltkern-Sandwichstrukturen. University of Stuttgart, 2012Google Scholar
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    Liewald M., Hofmann D.: Außen edel, innen leicht, Umformtechnik.net, 2015Google Scholar
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    Hofmann D., Liewald, M.: Material characterisation and properties of newly developed sheet metal composites with stiffness increasing layers. IDDRG 2015Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Mathias Liewald
    • 1
  • Matthias Schneider
    • 1
  • Stefan Walzer
    • 1
  1. 1.Institute for Metal Forming Technology (IFU)University of StuttgartStuttgartGermany

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