Punching holes in high-strength steel without wear parts
The bodywork on motor vehicles must be adequately strong and stiff, but processing today's higher-strength steels – for example, punching holes – can be difficult. However, researchers at the Fraunhofer Institute have developed a novel steel-cutting process that could save time, energy and money.Struggling to pierce hard steels, mechanical cutting tools rapidly wear out. And because they also leave a burr, additional time has to be spent on finishing. One possible alternative is to use lasers as cutters, but they require a great deal of energy.
Working together with a number of partners including Volkswagen, researchers at the Fraunhofer Institute for Machine Tools and Forming Technology (IWU) in Chemnitz, Germany, have developed another way to make holes in press-hardened steel bodywork. Dr Verena Kräusel, head of department at the IWU, explains: "The new method is based on electromagnetic pulse technology (EMPT), which was previously used primarily to expand or neck aluminium tubes. We have modified it to cut even hard steels. Whereas a laser takes around 1.4 seconds to cut a hole, EMPT can do the job in approximately 200 milliseconds – our method is up to seven times faster." Another advantage is that it produces no burr, which eliminates the need for finishing. Stamping presses become superfluous, and no costs arise from the need to replace worn parts.
The pulse generators comprise a coil, a capacitor battery, a charging device and high-current switches. When the switch closes, the capacitors discharge via the coil within a matter of microseconds, producing a high pulsed current. The coil converts the energy stored in the capacitors into magnetic energy. To be able to use this process to cut steel, the researchers simply had to modify the coil to ensure the resulting electromagnetic field is strong enough: the pressure with which the field hits the steel must be so high that it forcibly expels the material from the sheet. Kräusel says: "The impact pressure on the steel is approximately 3500bar, which equates to the weight of three small cars on a single fingernail."
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