EWI has a 600-watt fiber laser with a 9-micron delivery fiber. With our beam delivery and motion system we can achieve theoretical laser spot sizes of around 17-microns. We frequently weld lap joints of thin metal at high speeds approaching 1-meter/second. With this small spot size the laser welds on thin sheet metal resemble electron beam welds in cross section. Traditionally the acceptable gap between layers should not exceed 10% the thickness of the thinnest part. For thin materials that is not much gap. However, the narrow and fast fiber laser welds show a surprising tendency to bridge the gap. The images below show cross sections of fiber laser welds on lap joints of 0.004-inch stainless steel. The objective was to evaluate the change in welding speed. However, because of inadequate squeezing of the fixturing there are unintentional gaps that the laser weld bridges. If the gap is excessive the laser can cut a trough in the top sheet. The laser beam can also melt both layers and they won’t stick together. You can tell if there is a gap by looking at the top surface of the weld. If it is undercut there is probably a gap between the parts. Still, these cross sections show that there is a better tolerance to gaps with the narrow laser welds.