Quality assurance and advanced use of process gas by means of process monitoring and control for laser beam welding

Quality assurance concepts grew out of quality control which, in the stage of industrial development, became a necessity in many industries. In welded construction problems with weld quality have led to a tremendous growth in weld inspection (often on a 100% basis) nevertheless quality can not be inspected into a product - it has to be designed or built in /1/. A production of quality instead of inspection requires control in an early stage of manufacture. Regarding welded components this can be achieved beside other possibilities by on-line control or regulation of the welding process. Already a pallete of monitoring and controlling methods exists concerning laser beam welding /2-5/ Some of them have been integrated into a Modular Process Control System (MPC) for laser manufacturing which also provides modules for laser cutting and surface refinement. This paper will concentrate on the modules for laser beam welding only. In laser beam welding with a deep penetration effect the qualit y of the welding result is strongly influenced by the characteristics of the laser induced plasma at the workpiece surface. Mostly the plasma is influenced by suitable use of the process gas /6/, which often is called plasma control gas. Recently a closed-loop regulation method, which is called plasma shielding control (PSC) has been developed /7/ for an advanced use of process gas and the improvement of seam quality.