New Lightweight Construction Concepts in Sheet Metal Forming
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KeywordsMagnesium Alloy Sheet Metal Folding Axis Folding Structure Lightweight Material
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.
▸ “Lightweight and stiff“: components made of lightweight materials with high stiffness.
▸ “Thin walled and high-strength“: components made of thin and high-strength materials.
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 .
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.
▸ Definition of folding axes, the so called pre-creases.
▸ Unfolding of the flat structured plate to a 3-D-structure.
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.
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 .
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 . 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.
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.
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