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Seeing flaws with a laser beam
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Aachen, October 13, 2006
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Lasers have become an indispensable tool in the metal-processing industry. They are routinely used for cutting and welding metals and plastics on the production line. Controlling the quality of work processes is a number-one priority for manufacturing companies. At the Euroblech trade fair in Hanover from October 24-28 (Hall 11, Stand F44), Fraunhofer researchers are presenting a system for Coaxial Process Control (CPC) during material processing.
Appearances can be deceptive, even in welding processes. “When sheets of metal have been welded together with a laser beam, you may sometimes see a weld seam on top and underneath, yet the overlapping metal sheets have not actually fused at all. Car manufacturers dread this hard-to-detect flaw,” states Peter Abels of the Fraunhofer Institute for Laser Technology ILT in Aachen. “Our goal with the Coaxial Process Control system is to track down these and other defects more easily in future.”
Quality assurance for materials processing with lasers is an important issue for the Fraunhofer researchers. Laser welding is a complex process that is influenced by a number of factors. Companies must subject laser-welded seams to a quality inspection either during, or very soon after, the welding process in order to avoid a costly search for defects later on and to prevent damage to objects that could ultimately cause injury to human beings. Often, quality control can only be done by means of elaborate inspection procedures. “Our observation system is located directly in the laser head. The advantage of this arrangement is that it can ‘see’ directly through the beam. The radiation emitted by the process – the luminosity – is uncoupled from the beam path via specially developed optical components, monitored with a high-speed camera, and recorded. The input process images are interpreted on a computer, in real time wherever possible,” says Abels, explaining the process.
The Fraunhofer researchers set their quality control system to work for the first time in an industrial-scale welding process on the premises of the Visotek company. The welded joints of a fan controller for a six-cylinder engine were tested for gas leakages. The experts used the CPC to monitor the production process. Visotek produced 510,000 fan controllers during the 15-month warm-up phase, around 20,000 of which the system rejected as faulty. This eliminated the need for time-consuming leakage tests that the employees had previously been obliged to perform manually. All they now had to do was to check the automatically rejected parts, which saved over 95 percent of the originally planned inspection capacity. Other advantages were that the required handling time dropped from 20 to 14 seconds and that no defective parts were, or will ever be, delivered to the customer. Incidentally, human inspectors failed to recognize 10 percent of the defects in a benchmark test. The process itself has now been better adjusted on the basis of the CPC results, cutting the error rate to less than one percent.
CPC offers other advantages for companies besides quality control. Since the system is able to process the data in real time, it offers – within certain limits – the possibility of direct feedback on the laser process. The process can then be so adjusted as to produce fewer faulty parts. What is more, the coaxial process control system can meanwhile be used not only in cases where the work process itself ‘glows’. “We plan to use extraneous lighting in future. The idea is that an integrated light source illuminates the welding process and thus outshines the laser luminosity. This will make it possible to distinguish between melted and non-melted material, for instance, and to incorporate this information in the quality assessment,” says Abels. The new control system will be of particular interest to the metal-processing industry, not least because it can be retrofitted to almost any existing laser plant with or without extraneous lighting.
Contact
Dipl.-Ing. Peter Abels
Steinbachstraße 15
52074 Aachen
Phone +49 (0) 2 41/89 06 –428
Fax +49 (0) 2 41/89 06 -121
Email: peter.abels@ilt.fraunhofer.de
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