Function Integrated, Bionic Optimised Vehicle Lightweight Structure in Flexible Production

Transcription

1 DEVELOPMENT Materials Function Integrated, Bionic Optimised Vehicle Lightweight Structure in Flexible Production Edag AUTHORS Jörg Ohlsen is CEO of the Edag Engineering GmbH in Wiesbaden (Germany). Frank Herzog is President & CEO of Concept Laser in Lichtenfels (Germany). Sergio Raso is Strategic Marketing Director Laser Products of the BLM Group in Levico (Italy). Prof. Dr.-Ing. Claus Emmelmann is CEO of the Laser Zentrum Nord, University Professor and Director of the Institute of Laser and System Technologies (ilas) in Hamburg (Germany). 34

2 Automotive development is confronted now more than ever with the integration of various drive concepts and energy storage systems. The resulting load categories and package situations have different effects on the performance of the vehicle body and the distribution of weight in the complete vehicle. Furthermore, growing numbers of additional vehicle body variations of a basic type are being produced. In the innovation project involving the partners Edag, BLM Group, Concept Laser and Laser Zentrum Nord, a solution is presented, which illustrates the high variation vehicle body concepts on a highly flexible and low tool production line On Demand. SPACEFRAME CUTOUT IN THE A-PILLAR SECTION Growing numbers of additional vehicle body variations place increasingly complex demands on the vehicle body together with the call for lightweight design to save resources [1, 2]. Edag, BLM Group, Concept Laser and Laser Zentrum Nord have developed a Spaceframe cutout in the A-pillar section on the basis of the concept car Edag Light Cocoon, FIGURE 1, which takes the following requirements into account: profiles, which can be optimised to the required functions with adjustment of the wall thickness and intelligent laser cutting bionic optimised and additive manufactured nodes load path optimised Spaceframe equipment arm assembled by laser welding. Production can be adapted extremely flexibly to the fluctuating volume requirements of vehicle variants with little logistical effort: the nodes are produced on-site for the respective just in sequence (JIS) variant and the profiles are also only cut accordingly just before production. It must also be highlighted that we can quickly and simply react to new functional requirements, for example due to structural parts requiring update, during the vehicle s life cycle. Using maximal cutting forces in a full frontal crash load case from an equivalent structure, a substitute load case has been defined with which the nodes have been bionic optimised. The front main chassis beam has been optimised with respect to folding in a crash load case by adding holes using laser cutting. The full deformation is realised in the front half of the profile, FIGURE 2. HYBRID DESIGN Additive Manufacturing is a manufacturing process, which, on the strength of the high geometric freedom of design, offers entirely new design options, and potential for functional integration and FIGURE 1 Spaceframe in the A-pillar area ( Edag) lightweight design. The limits of this technology at present are the limited installation space and the slow process speed, which makes its use for high-volume production uneconomical. The use of hybrid structures (combination with sheet, cast and milled parts etc.) is appropriate to get around these restrictions. This is not being used industrially yet and is currently only being researched by ilas and TUHH [3, 4]. The focus is on joining individual components to form a hybrid structure in order to manufacture bionic optimised structures, which is not possible at the moment. Laser welding is used, which is characterised by delicate welding seams and low heat levels and is therefore perfectly suited to the function integrated lightweight application aimed for. MATERIAL AND JOINING CONCEPT The materials engineering basis of this project is a highly developed steel design. In comparison with aluminium, steel still offers higher strengths while offering lower manufacturing costs. The bionic node shape cannot currently be shown in cast steel, because the cast nodes display restrictions in the area of 10I2015 Volume

3 DEVELOPMENT Materials geometric complexity as well as in the minimal wall thicknesses. This constraint of cast steel can be worked around with the use of additive manufacturing. Additive manufactured components can display a higher geometric complexity while also displaying lower wall thicknesses of approximately 1 mm minimum. Typical high-strength steel alloys in automobile construction are currently not qualified for additive manufacturing, but are investigated in R&D departments. Inconel 718 is used as a material in this project, because extensive experience in the additive manufacturing process as well as very good welding suitability, including in combination with the profile steel alloys, is available. For the steel profiles, material S235 for low load structures and Autoval 390 for structures with high loads is used [1]. The components are welded with a fillet weld at the overlap join. The geometric basis is the fully circumferential enclosure of profiles in the nodes, each with a depth of 10 mm. This connection allows circumferential welding for a large connection length with simultaneous good pre-positioning of components. The profiles are automatically aligned and fixed by the nodes. PREPARATION OF WELDS AND COMPONENTS The laser cut and bent profiles can be directly processed further in final assembly after bending and cutting. Only the interface areas need to be cleaned of dirt accumulation. However, due to the fact that the profiles display deviations from the target contour, adjustment of the nodes to the profiles is required to maintain the authorised clearance of max. 0.2 mm for laser welding. The profiles are scanned for this. Due to the fact that the tolerance deviations are not mirror-symmetrical for right angled profiles, the weld seam is identified with a marker in the scan. The scanned data is taken into the CAD design of the nodes so that clear allocation of the profiles and their orientation is provided in final assembly. Max. plastic deformation mm 565 mm FIGURE 2 Maximal plastic deformation shown on the non-deformed component, basis above, variant below ( Edag) the welding process development is therefore on determining an appropriate parameter framework and mastering the geometrics for complex 3-D application. A disc laser with a robot-controlled optical system is used. The following welding parameters are determined in the course of basic welding process development: laser output: 2.5 kw focus diameter: 300 µm defocussing: + 2 mm feed rate: 2 m/min lateral welding angle: 15. Defocussing of + 2 mm is used to widen the welding seam and therefore optimise both the connection width and the gap bridging. With the selected laser power of 2.5 kw results a welding depth of 2.5 mm, FIGURE 3, which displays a sufficient reserve for the transfer of applied loads. Furthermore, a very homogeneous upper bead results. In further welding tests, the maximum gap bridging capacity of 0.2 mm was determined without the use of additional wire. With larger gaps of up to 0.4 mm, a connection is still possible but a significant seam shrinkage is inevitable. On account of the complex bionic component geometrics, accessibility for the laser head is limited in many areas. However, parameter studies show that good welding results can still be achieved with a piercing or dragging arrangement of the laser beam of up to 45 and therefore difficult to access geometries can be achieved. The process is good natured and considerably extends the selected limits of the beam angle otherwise 4.0 mm 2.5 mm WELDING PROCESS DEVELOPMENT The general welding suitability of the selected material combination is known from basic investigations. The focus of 36 FIGURE 3 Results of the welding process development, left even upper bead, right seam cross-section ( Laser Zentrum Nord)

4 selected in literature by ± 20 while still achieving good seam qualities. PRODUCTION OF EXHIBITION DEMONSTRATORS AND PROFILES The final welding of the overall assembly is simulated in advance using offline programming. The optimum plugging and joining sequence is simulated and therefore allows collision-free production. With the profile plugging principle in the nodes, the clamping technique work in welding processing is minimised. The setup of a structure with profiles offers several advantages with respect to lightweight design and crash performance. The wall thickness can also be varied while maintaining the outer dimensions in order to set various load categories in the structure. The profiles can be bent and cut with little tool investment. In the BLM Group, the bending process and the cutting process, FIGURE 4, are carried out on electronically operated and monitored machines and can therefore be fully integrated in highly flexible production. FIGURE 4 Example of a highly flexible laser cut on a steel profile ( BLM Group) PRODUCTION OF NODES The so-called Laser Cusing process generates components in layers directly from 3-D CAD data. The method allows the production of complex geometric components without tools. Components, which are very difficult or impossible to make with conventional manufacturing, can be produced. In order to ensure fault-free setup, a support structure should be provided on surfaces with an of less than 45 to the construction platform. Alongside providing a purely supporting function, the support primarily absorbs internal stress and prevents distortion. Due to the complex node geometry, clean preparation of the support is the foundation for successful production. The component is virtually cut into individual layers, FIGURE 5, after preparation of the support. After data transfer to the Laser Cusing system, the corresponding process parameters are allocated and the building process started. PRODUCTION CONCEPT The following is a conceptual description of how such a Spaceframe can effectively FIGURE 5 Layers from machine software: 1500 th layer (top), 3000 th layer (central), 5000 th layer (bottom) dark areas component, light areas support structure ( Concept Laser) 10I2015 Volume

5 DEVELOPMENT Materials The option of producing the smart parts on-site simplifies the intralogistics considerably, meaning fewer different kinds of semi-finished products and components This cell set-up will be sufficient for small Spaceframe volumes. The system can easily be extended with further, identically constructed cells even during ongoing production. The cell groups are then programmed and tested offline by means of virtual commissioning. Operating and maintenance staff are also trained to handle this combination of real control and a virtual system. The horizontal networking of components described here, for example measuring technology with the control software of the 3-D printers and the management of the supplier network, reflects a systematic implementation of Industry 4.0 concepts. FIGURE 6 Production cell ( Edag) be produced. The multi-stage production system is based on high flexibility with respect to volumes (standardised modular set-up), high standardisation of tools and equipment (standardised grippers) and a consistent quality and logistics concept. The system, FIGURE 6, consists of identical cells, each with three collaborating industrial robots (IR), a repository for profiles and the option of attaching a production line for steel nodes. All additive manufactured nodes include an identical docking point, the aim being to achieve low-equipment production. This means that the entire line has only one component interface for all robot grippers that have to handle nodes: an important element when it comes to the scalability of the production system, especially with respect to volumes and stocking spare parts. Nodes and intermediate assemblies are held by the large industrial robots. A lightweight design robot picks up the individual profiles and places them in 38 the scheduled fixtures on the nodes. This is particularly interesting because these are able, by slightly changing the position of the components to be inserted relative to one another, to apply the profile in areas where there is a tight fit. Finally, the third robot guides the laser welding head. A further innovative approach can be found in this cell s QM concept, which is based on an integrated and dynamic SPC (statistical process control) method, which means the immediate, regulating feedback of assessment findings into the process. Profiles and nodes are manufactured as smart parts, that is tolerance fluctuations in the profiles and nodes are compensated for by means of customised remodelling or by adjusting the 3-D print data. The Spaceframe is measured when all joins have been completed. In the next step ( best match ), the contact surfaces for assembly and add-on parts (for example cockpit or trim elements) are additive manufactured onto the profiles, to suit the respective component. REFERENCES [1] Hillebrecht, M., Begert, M., Reul, W., Kiel, B.: Technologies for Hybrid Design from 2020 Onwards. In: ATZworldwide 116 (2014), No. 5, pp [2] Hillebrecht, M.: Potenzialbewertung von neuen generativen Fertigungsmethoden für Leichtbaukonstruktionen. Conference materials ATZ Werkstoffe im Automobilbau, , Stuttgart [3] Emmelmann, C., Petersen, M., Kranz, J., Wycisk, E.: Bionic lightweight design by laser additive manufacturing (LAM) for aircraft industry, Proc. SPIE 8065, 80650L, 2011 [4] Emmelmann, C., Sander, P., Kranz, J., Wycisk, E.: Laser Additive Manufacturing and Bionics: Redefining Lightweight Design. Physics Procedia 12, Part A, 2011 THANKS Thanks go to the extended project team: Mr. Michael Schmidt, Mr. Eric Fritzsche and Dr.-Ing. Martin Hillebrecht of the Competence Center Lightweight Design (Edag Engineering GmbH), Dr.-Ing. Frank Breitenbach (Edag Production Solutions GmbH & Co. KG), Dr. Florian Bechmann and Mr. Peter Appel (Concept Laser GmbH), Mr. Frank Beckmann (Laser Zentrum Nord GmbH) and Mr. Daniele Colombo, Mr. Maurizio Sbetti and Mr. Stefano Farina (BLM Group).

6 75 YEARS AT THE CUTTING EDGE OF ENGINE TECHNOLOGY. IT S OUR BIRTHDAY! Every month for 75 years, MTZ Motortechnische Zeitschrift has been examining the key issues driving our world: the internal combustion engine and other powertrains. Throughout all those years, our magazine has pulled off the miraculous feat of staying young and fresh while keeping its finger on the pulse of engine technology. And in one point in particular, MTZ has always remained true to itself: in its aspiration to offer our readers the ultimate in quality technical journalism. 10I2015 Volume