The penetration depth is one of the fundamental process parameters in laser beam welding. On-line monitoring and control of the penetration depth is a significant contribution to meet todays quality requirements and enhance the integration of laser technology into manufacturing.
An experimental set-up to monitor the penetration depth by detecting the optical emission of the metal vapour is presented.
For a wide range of welding parameters the radiation emitted coaxially with respect to the laser beam axis depends linearly on the penetration depth. The advantage of the chosen coaxial set-up compared with an off-axis arrangement is demonstrated. Spatially integrated spectroscopic measurements of the density and temperature of the metal vapour/plasma are carried out. Coaxial and off-axis observing positions of the spectrometer arrangement are compared. For a wide range of the welding parameters the coaxial alignment leads to elevated values for the vapour density and almost unchanged temperatures compared with the off-axis set-up.
Using a CCD-camera the radiation signal can be monitored with a lateral resolution of 63.64 pixels.
Taking into account the linear relation between the optical radiation signal and the penetration depth the geometrical shape of the keyhole is deduced from the CCD-frames/1 /.
An additional well known monitoring signal is the acoustic emission of the welding process. The features with respect to the penetration depth of both signals are compared.