ENHANCED IMAGING OF A PULSED ND-YAG METAL VAPOR PLUME

Understanding the physical processes that occur in laser material proc essing is necessary for developing computational models that simulate the laser/material interaction, acid for real-time process monitoring and control. This work was performed to gain an increased understandin g of the dynamics of the vapor ejected from a metal surface during the laser welding process. The plume is imaged using a laser schlieren te chnique that detects the refraction of a probe beam. A computer-genera ted, graded filter replaces the traditional knife-edge schlieren filte r, and yields images with enhanced resolution and sensitivity. Images show that intense vaporization begins within the first 10 mu s after t he laser pulse is incident on the metal surface, and that the vapor pr opagates as two distinct waves. The first wave is a spherical wave, an d is initiated just after the beginning of the laser pulse. For typica l laser welding surface intensities, the second, mon energetic wave, i s initiated approximately 0.6 to 1.5 ms after the beginning of the las er pulse, and propagates away from the surface in a focused trajectory . The results suggest that a simple optical method of detecting the pr oduction of a keyhole weld may be performed using laser schlieren imag ing. (C) 1998 Laser Institute of America. [S1042-346X(98)00502-6].