Enhanced imaging of a pulsed Nd:YAG metal vapor plume

Abstract

Understanding the physical processes that occur in laser material processing is necessary for developing computational models that simulate the laser/material interaction, and for real-time process monitoring and control. This work was performed to gain an increased understanding of the dynamics of the vapor ejected from a metal surface during the laser welding process. The plume is imaged using a laser schlieren technique that detects the refraction of a probe beam. A computer-generated, graded filter replaces the traditional knife-edge schlieren filter, and yields images with enhanced resolution and sensitivity. Images show that intense vaporization begins within the first 10 μs after the laser pulse is incident on the metal surface, and that the vapor propagates as two distinct waves. The first wave is a spherical wave, and is initiated just after the beginning of the laser pulse. For typical laser welding surface intensities, the second, more energetic wave, is initiated approximately 0.6 to 1.5 ms after the beginning of the laser pulse, and propagates away from the surface in a focused trajectory. The results suggest that a simple optical method of detecting the production of a keyhole weld may be performed using laser schlieren imaging.