The development of a laser reflectometer diagnostic technique for laser keyhole welding studies of stainless steel SS 304

It is generally understood that in deep penetration laser welding, a keyhole is formed in the weld pool through which the laser energy is transported far inside the workpiece. However, no detailed, universally accepted physical explanations are available.

To provide further insight into the phenomena, a diagnostic technique called a laser reflectometer for monitoring the surface behavior of a sample and the keyhole during CO₂ laser welding was developed. A low power Argon probe laser beam was focused on a weld pool created in stainless steel SS 304 by a RS3000 CO₂ laser operated in single pulse mode at peak powers from 500 W to 2500 W.

The laser reflectometer data allow to distinguish between reflections from a conduction limited weld and a deep penetration weld. The technique was applied to monitor the weld pool on the upper sample surface as well as the point of penetration on the lower surface. Information about the phase transition time of the material on the upper surface and the penetration time of the laser energy through the sample could be gained. Variations in the probe beam reflected intensity from two different sample thicknesses were detected, identifying the surface waves and providing information about their frequency and velocity. The results identify strong oscillations of the weld pool surface during the welding process around its resonance frequency of 200 Hz to 350 Hz and around 800 Hz. The surface wave velocity was calculated as 0.4 m/s. The amplitude of these waves is estimated as 40 µm.