Publications

The production of metal vapor as a consequence of high intensity laser irradiation is a serious concern in laser welding. Despite the widespread use of lasers in manufacturing, little fundamental understanding of laser/material interaction in the weld pool exists. Laser welding experiments on 304 stainless steel have been completed which have advanced our fundamental understanding of the magnitude and the parameter dependence of metal vaporization in laser spot welding. Calculations using a three-dimensional, transient, numerical model were used to compare with the experimental results. Convection played a very important role in the heat transfer especially towards the end of the laser pulse. The peak temperatures and velocities increased significantly with the laser power density. The liquid flow is mainly driven by the surface tension and to a much less extent, by the buoyancy force. Heat transfer by conduction is important when the liquid velocity is small at the beginning of the pulse and during weld pool solidification. The effective temperature determined from the vapor composition was found to be close to the numerically computed peak temperature at the weld pool surface. At very high power densities, the computed temperatures at the weld pool surface were found to be higher than the boiling point of 304 stainless steel. As a result, vaporization of alloying elements resulted from both total pressure and concentration gradients. The calculations showed that the vaporization was concentrated in a small region under the laser beam where the temperature was very high.

An experimental study of laser spotweld variability for 0.10 mm thick Kovar fillet lap joints has been completed. A fixture was fabricated to vary weld joint gap continuously between 0.0 and 0.25 mm. The maximum gap bridged was determined from many samples and used to evaluate the effect of changes in the independent process variables. An array of process parameters including pulse energy, duration, temporal shaping, beam diameter, and shielding gas were selected for the experiment. It was observed that changes in most process parameters did not improve gap bridging. Images of successful and unsuccessful fillet lap welds are presented. It is thought that due to surface tension effects, increasing or decreasing weld size did not provide additional molten metal to close the gap between the top and bottom plates. Some improvement in gap bridging was observed when the incident beam was angled to the weld joint at 30°, and when the beam was significantly offset towards the top plate side of the joint.