Perfect Laser Welds Created for Car Bodies
FREIBURG, Germany, May 16, 2011 — A new process using a camera that generates temperature images is enabling perfectly controlled surface laser welding, offering car makers a greater benefit in comparison with full-penetration welding.
Together with colleagues from the IFSW Institut für Strahlwerkzeuge at Stuttgart University and the Institut für Grundlagen der Elektrotechnik und Elektronik (IEE) at Dresden University of Technology, Fraunhofer Institute for Physical Measurement Techniques IPM research scientists have developed a prototype that eliminates the expensive process of hiding welding seams resulting from zinc’s inability to vaporize during processing. The corrosion problems encountered on galvanized car bodies are therefore a thing of the past.
In this process, the welding head on a robot's arm races along the sheet metal parts. Where the laser hits, sparks fly, and the metal glows red hot. The process lasts only a few seconds. A thin weld seam extends along the joint but can be seen on only one side. From the other side of the welded car door, the joint is invisible. This is considered a perfect weld because it could be used anywhere on the car body. Expensive work to hide the seam, such as folding the sheet metal or covering with trim, would no longer be necessary.
In the new surface welding process, the laser produces a perfect seam. Bottom left: Weld seam profile — the penetration depth is controlled without damaging the bottom surface. (Image: Fraunhofer IPM)
"Controlled partial penetration welding" is how experts refer to the process in which the laser does not burn right through all the sheets of metal — in contrast to full penetration welding, where a hole briefly forms in the melt pool. Instead, the weld seam is controlled to penetrate the lower sheet without damaging the bottom surface. Up to now, however, it was not possible to precisely control this type of welding and produce a seam that meets the requirements in respect of strength.
"As we do not weld through the sheet, basically we cannot see what we are doing," said Andreas Blug, project manager at Fraunhofer IPM, outlining the problem. But the researchers found the solution using an innovative camera that generates temperature images. This enables the system to recognize how deep the laser has penetrated into the sheets. Where it burns into the metal, causing it to melt, the images show a hot region. If the bottom of the melt pool reaches the gap between the upper and lower sheets, the conduction of heat is interrupted, and a cooler point can be seen. This is referred to as the full penetration hole. From the relative frequency of this full penetration hole, the system calculates the penetration depth into the lower sheet. A software program then adapts the output of the laser to the specific requirements. "The process is closed-loop controlled in real time," Blug said.
An extremely rapid camera system is needed for this, which is the key to Fraunhofer IPM's innovation. The system is based on cellular neural networks. A tiny processor is integrated in each pixel. They all work simultaneously and speed up the analysis of the individual images enormously, whereas in conventional image processing systems, a few processors process the data consecutively.
"In this way, the system analyzes up to 14,000 images per second," Blug said. This compares with the usual rate of only 1000 to 2000 images per second.
For more information, visit: www.fraunhofer.de
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