J. T. Norris, Sandia National Laboratories, Albuquerque, NM; M. J. Perricone, R.J. Lee Group, Monroeville, PA; C. M. Ellison, Honeywell FM&T, Kansas City, MO; K. M. Faraone, BWXT Technologies, Inc, Lynchburg, VA
In a laser beam spot weld, weld strength is commensurate with weld cross sectional area. Any loss in weld area due to an undefined or unexpected pore can cause undefined or unexpected loss in strength. For this reason, a better understanding of spot weld porosity is needed. In this study, porosity is examined for both long- and short- pulse welds. Long-pulse spot welds are defined and limited by the slow shutter speed of most high output continuous power lasers. Continuous lasers typically ramp up to a simmer power before reaching the high power needed to produce the desired weld. A post-pulse ramp down time is usually present as well. The result is a pulse length tenths of a second long as opposed to the typical millisecond regime of the short-pulse pulse welding lasers. The effect of pulse shaping is also of interest. In this study, long-pulse pulse shaping was produced by high frequency square wave modulation of the pulse over a constant simmer power. Both square (or top-hat) and constant-ramp down pulse shapes were investigated for short-pulse spot welds. Porosity was evaluated by its size, frequency, and its impact on weld volume. This study revealed that square wave modulation of the long-pulse has processing advantages such as increased transfer efficiency and smaller spot diameters; but has adverse effects due to increasing weld porosity. The severity of the porosity was found to be highly dependent upon weld penetration. This behavior holds true for both long- and short- pulse welds. Evaluation of short-pulse spot welds showed lens focal length and constant-ramp down pulse shaping to assist in minimizing the occurrence of porosity. Through careful selection of processing parameters, porosity in laser beam spot welds can be greatly minimized and often eliminated.
Summary: Weld porosity has been evaluated for long- and short- pulse spot welds along with the effects of pulse shaping. Porosity was evaluated by size, frequency, and its impact on weld volume. Careful selection of processing parameters was found to greatly minimized and often eliminated porosity in laser beam spot welds.
