Product Code: ICAL05_2307

Advantages of Coaxial External Illumination for Monitoring and Control of Laser Materials Processing
Authors:
Boris Regaard, Fraunhofer ILT; Aachen Germany
Stefan Kaierle, Fraunhofer ILT; Aachen NRW Germany
Wolfgang Schulz, Fraunhofer ILT; Aachen Germany
Anas Moalem, Lehrstuhl fr Lasertechnik, Aachen University; Aachen Germany
Presented at ICALEO 2005

Due to the progressive use of laser materials processing in industrial mass production and the continuous expansion of process limits, the need of sensor systems for process monitoring increases. Sensor systems are used in process development for gathering process information as well as in the production process for quality assuring process monitoring and quality improving process control.
There are a large number of different methods and sensor types used to measure process variables and to detect defects in laser materials processing. The most common sensor systems measure and analyze secondary radiation from the laser machining process using spatially integrating detectors like photodiodes or spatially resolving detectors like CMOS-cameras. These methods gather information either about the plasma radiation (at wavelengths of 400nm to 650nm) or the heat radiation (900nm to 2300nm), but only limited information about the material surface structure.
At the Fraunhofer ILT a different measuring method has been developed. It uses an additional laser beam coaxial to the working laser beam to illuminate the molten material surface, where the intensity of the reflected illumination is higher than the secondary radiation of the process. The reflected light is observed likewise coaxial by a CMOS camera. This method makes it possible to measure the pool crater geometry with spatially and temporal high resolution. This information can be used e.g. to on-line measure welding seam width and depth, to evaluate cut edges or to detect surface pores. A further application is the precise on-line measurement of the relative displacement between laser processing head and work piece during laser processing.