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Forming Processes

Metal Forming Technology for Electromobility

| Author/ Editor: Stéphane Itasse / Alexander Stark

Electric mobility is keeping forming technicians on their toes. The changes of the automotive market and their effects on the industry were discussed at the 13th Forming Technology Colloquium in Darmstadt.

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Press plants will have to increase their flexibility when manufacturing automobiles with different drives on the same line.
Press plants will have to increase their flexibility when manufacturing automobiles with different drives on the same line.
( Source: BMW )

Dr. Johannes Staeves, Project Manager Hybrid Construction at BMW, reports that complex system integration in the automotive industry is necessary in order to cope with current changes. Firstly, there are new customer requirements must be integrated into a body concept — in addition to electromobility this includes autonomous driving or digitization. Secondly, the new bodies must be integrated into the 60-s cycle of production; and thirdly, new products must be integrated into the existing plants. "If we want to produce internal combustion engines, hybrid and battery vehicles on the same line, we must align the work in terms of time," says Staeves.

Making Room for Batteries in the Bodywork

According to the BMW project manager, the most important change for body construction is the integration of battery storage. Only for small vehicles, plug-in hybrids or flat sports vehicles, the Munich-based manufacturer is focusing on placing the battery in the previous position of the tank or — as with the i8 — in the area of the tunnel. For greater electrical ranges, BMW integrates the batteries as flat accumulators in the underbody. These batteries are modular and scalable, which is why future body concepts will have to be flexible to accommodate them.

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With the increasing number of variants, more flexibility is also required in the press shop, as Staeves explains. On the one hand, this results in the requirement to further reduce the service life of the presses, for example through automatic tool changers. On the other hand, the presses must take over functions that were previously fulfilled by individual tools. As an example, the BMW manager mentions crossbar feeders that can swivel a component around all axes, which in turn eliminates expensive slides in the forming tools. "If we don't know exactly how many components we're going to produce with one tool, we'd rather invest in the press," he says.

Cold Forming Industry Afraid of Changes Caused by Electromobility

Prof. Mathias Liewald, Director of the Institute for Forming Technology (IFU Stuttgart), gave a lecture on how the demand for cold formed products is developing with regard to combustion engines and electric mobility. He assumes that, from a global perspective, sales of cars with combustion engines will continue to grow until 2020 for diesel and until 2025 for gasoline engines. "We have not yet reached the world's top number of sold combustion engines, even if the percentage is declining," says Liewald. The value added by combustion engine components is also continuing to rise, although the percentage is also declining.

However, Liewald expects the number of electric vehicles to rise and thus the demand for components for combustion engines and conventional drive trains to decrease in the longer term. This means that solid formed components such as pistons, connecting rods, valves, camshafts, crankshafts and others are no longer required. Other solid formed products such as axles, gear wheels, shafts, chassis components, gear stages with constant transmission ratios or even shiftable gear stages will remain in purely electrically driven vehicles. In addition, electric engines and their drive trains offer potential for innovative components that could also be manufactured using solid forming processes. In order to continue to be successful, companies must become more flexible in their strategy, recommends the institute's director.

Forging Companies Must Adapt Processes to New Markets

There are basically three options available to forging companies: technical improvements, the opening up of new business fields or the addition of new manufacturing processes to address new markets. With technical improvements, companies can differentiate themselves from their competitors by, for example, improving existing processes and expanding areas of application. Products resulting from technical improvements are, for example, toothed hollow components with a special preform geometry or lightweight gears, in which the gear body is joined in a single stroke with a prefabricated gear rim. The forming process is a combination of backward extrusion, cup forward extrusion and transverse extrusion.

Liewald sees a greater challenge in the development of new business fields for solid forming. This is because production structures in this sector are particularly capital-intensive and therefore require particularly large batch sizes. In order for smaller quantities to become more profitable for the solid formers, they must make their production more efficient, more flexible and increase the level of automation. Liewald illustrated this at the colloquium with a concept for digitally supported production in the forging shop: First, the CAD drawing of a customer's target geometry is entered into a deep learning software. This software automatically creates a stage plan for the production of the product.

Digitalization Can Open up New Markets for Solid Formers

After an employee has checked, possibly adjusted and approved the stage plan, the software creates a process simulation, which in turn is approved by another employee. This is followed by the partially automated design of the tool. For components that have already been produced in a similar way, the software can design the tool independently. With each design, it learns better how to include the processes and designs — especially when a person has previously intervened and adjusted the process.

After approval of the tool design by an employee, a decision is made as to whether the tool is to be manufactured conventionally or using additive processes — including reworking. Additive processes enable a faster and more flexible tool design, but may lead to a shorter tool life. During the production of the customer's product, the tools are automatically transported to the press and exchanged.

According to Liewald, this overall concept allows costs to be reduced, orders to be processed more quickly and even smaller batch sizes to be produced economically. As disadvantages he mentions the development effort for the alignment of all process steps in line with the digital twin and the high initial investments required for automation.

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The director of the institute emphasized that new components for electromobility would be an opportunity for the industry if they were manufactured with innovative solid forming processes. However, these components must first be identified and analyzed. One possible application is the production of components for asynchronous motors: for example, sheet bundles in the stator could be produced by compacted rolling and cutting. Compacted rolling takes place after insulation and the joining of several electrical sheets to form a stack of sheets. Rolling reduces the thickness of the individual sheet layers, which increases the engine’s efficiency. Compacted cutting also begins with insulating and joining the electrical sheets into smaller sheet packs, then they are cut to the required size. In this way, thin sheets that cannot be cut individually can be cut together. These thin sheets in turn lead to the increased efficiency of the electric motor.

Electromobility also Offers Opportunities for Solid Forming

Another example for innovative components are cage rotors. Currently, they are manufactured as an assembled version or by die-casting aluminum onto the metal packet — die-casting with copper is not economical. To simplify assembly and production, the cage can be manufactured in a new concept by full-forward extrusion. In a further concept, the cage is pressed directly onto the sheet packs, which improves die filling and thus efficiency.

In their presentation, Stefan Köhler from the Institute for Production Technology and Forming Machines (PTU) in Darmstadt and Martin Heckmann from Läpple Automotive showed how lightweight construction with sheet metal, for example for electromobility, can be implemented in practice. Reinforcement with ribs or webs is a lightweight construction principle found in nature, Köhler explains. "This allows conventional sheet metal to be reinforced by a factor of up to 43," says the scientist. The webs are applied by laser welding, and in a second step the sheet is formed. The experiments at PTU initially started with high-pressure sheet metal forming, but the researchers are now working with rigid tools. "This increases the yield and there is the possibility of integrating web structures, which slightly shifts the process limits," reports Köhler.

In a transfer project with Läpple, he created a first prototype; an underride guard was selected as the component — the structural component has no visible surface, a high sheet thickness, and offers the possibility of integrating reinforcements with a small deformation of the webs. "We have been able to triple the stiffening factor with web plates or achieved weight savings of 20 % with thinner plates," says Köhler.

"The stiffening and reinforcement of the sheets results in lightweight body construction with established materials at moderate costs," adds Heckmann. Looking at the cost-effectiveness of the prototype, the estimated lightweight construction costs are € 2.28 per kilogram for a production of 5,000 units per year, € 1.07 per kilogram for 25,000 units per year and € 0.66 per kilogram for 120,000 units per year. This is within the limits that the automotive industry accepts as lightweight construction costs.

This article was first published by MM MaschinenMarkt

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