Coaxial process control during laser beam welding of tailored blanks

Abstract

Production of laser beam welded tailored blanks requires both, high quality processing as well as a quality assurance by reliable monitoring systems for each welded part. Actual quality monitoring systems for tailored blank applications make use of different sensors for seam tracking, seam shape detection and process control. The suitable process diagnostic device is a plasma sensor which detects the optical emission of the weld plasma. Experimental evidence show that the reliability of the seam quality prediction can significantly be improved by using a camera system with a two-dimensional spatial resolution instead of an integrating plasma detector. The improvement is achieved by exploiting the information provided by the spatially distributed intensity of the plasma emission. In particular, by coaxial arrangement of the camera with respect to the laser beam axis, the direction of observation allows to detect significant process characteristics. Based upon these results a coaxial process control system was developed that can be adapted for different laser materials processing applications like welding, cutting and surface treatment. The system consists of a high speed camera mounted directly at the welding head. The optical path of the camera goes coaxial with the laser beam path through the focusing optics. The camera images taken from the process are analyzed using image processing algorithms. The algorithms are chosen depending on the type of application to be monitored. In the case of welding tailored blanks the system can monitor a full penetration of the workpiece, deviations from the desired welding path, seam width, stability of the capillary shape and defects of the seam caused by spatter and ejection of molten material. The camera system offers the ability to perform simultaneously different quality monitoring tasks like determination of seam and capillary shape, seam tracking and process control. Thus the number of sensors required for quality monitoring is reduced to one single system.