Roll up for lighter parts

A new system for making tubular steel components could help reduce the weight of cars and improve fuel efficiency, according to German steel maker ThyssenKrupp Steel. Combining pressing and laser-welding, the system can be used to make a variety of tubular shapes and could cut out time-consuming steps in component production.

Most steel components for car chassis and bodies are made by stamping and welding steel sheets. Many of the parts of the chassis and car body — including the side and transverse rails of the chassis, roof bars and impact protection beams — are made from tubes, but these are not made in a single piece. Instead, the parts are formed from two halves which are spot-welded together by robots.

The robots weld by clamping their jaws onto a flange running the length of both halves of the piece. This can account for more than 15 per cent of its total weight.

As part of a project called New Steel Body (NSB), ThyssenKrupp has calculated that switching from these welded forms to purpose-made tubes could reduce the weight of an average car by 24 per cent, with an increase in production costs of around 2 per cent. Tubular components, made by bending a flat blank into a tube and welding the seam using a laser, are much lighter than flange-welded forms.

According to ThyssenKrupp: ‘Compared with conventional stamped and welded parts, tubular components are at least cost-neutral and offer a weight saving of up to 26 per cent.’

Moreover, because they are made from a single piece with a continuous seam, the tubes are more resistant to torsion than conventional welded parts, which are held together with a few spot-welds along their length.

The new ThyssenKrupp machine, developed in-house and built in collaboration with Karl Eugen Fischer, a machine tool specialist based in Burgkunstadt, works like a four-column press. Starting from a flat blank up to three metres long, which can have a variety of strengths, thicknesses and coatings, the machine presses the sheet into at least two dies to curl it into the required shape.

The cross-section can vary over the length of the piece; for example, it can change from cylindrical to conical to triangular. The integral laser welding head then runs the length of the tube to seal the edge.

The tubes can also incorporate secondary design features such as recesses or projections; these can be formed in the blank before curling, or made during the curling process by pressing the part into a shaped core.

‘The parts are so close to net shape that car manufacturers can eliminate complete manufacturing steps,’ the company claims. In particular, it says, the process usually eliminates the need for hydroforming, a complex, multi-step process where features are formed on a tube by placing it inside a mould and filling it with a high-pressure fluid to force it into shape.

The pilot press can develop a total tonnage of 1,600 tonnes, including 1,000 tonnes on a single axis, which allows it to process even high-strength steels. Parts can be positioned in four directions simultaneously with high accuracy.

According to a ThyssenKrupp spokesman, the machine is the only one of its type and is being used for low to medium volumes. ‘The development time for a new car is around five years, so we can’t say when the system might be used for full-scale production,’ he added, ‘but we need to be able to handle the technology very well, so that we could set up the machinery as soon as there is interest in it.’