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G. Liedl, G. Humenberger:
"Usage of emitted thermal radiation during laser processing of materials for quality control";
Keynote Lecture: Welding and Failure Analysis of Engineering Materials - WAFA 2018, Aswan (invited); 2018-11-19 - 2018-11-22.

Laser beam welding (LBW) is an attractive technique compared to conventional joining tech-niques of metals since processing speed and precision are high whereas heat input is very low. As a consequence, thermally influenced problems like unwanted microstructural changes or distortions of the workpiece are not as dramatical as with other thermal joining techniques. Nevertheless, LBW faces several challenges, like keyhole instabilities which can contribute to the formation of pores, blowholes as well as other defects. An online defect detection system, which already recognizes defects during processing, would be very desirable.
Two different approaches for the detection of defects are presented here. Both utilise thermal radiation emitted during laser processing of metallic samples. A NIR (near infrared) camera, operating at a wavelength of 1500 nm has been used to detect keyhole instabilities during LBW. By an evaluation of the dynamics of the recorded images, it was possible to detect keyhole instabilities which are responsible for weld imperfections. A second approach uses polarised radiation emitted during laser material processing. The emitted thermal radiation is polarised according to Fresnel´s equations, and the ratio of the polarisation components paral-lel (p-polarised) and perpendicular (s-polarised) to the plane of emission depends on the emis-sion angle. The ratio of these components can then be used to determine the inclination of the emitting surface and hence allow an estimation of the dynamic behaviour of the molten mate-rial. For first experiments, a narrow band pass filter with a center wavelength of 770 nm has been used for the detection of emitted polarised radiation. Following that, the setup has been modified for a wavelength of 1500 nm and individual polarisers have been replaced by a pho-toelastic modulator oscillating at 100 kHz. So far, the setup has been tested on an optical workbench with promising results.

laser welding, NIR, thermal radiation