Researchers at the Fraunhofer Institut für Lasertechnik (ILT; Aachen, Germany) have, in what they believe is a first, succeeded in deep-penetration welding a 2-mm stainless-steel sheet. Until now, laser-diode welding has been restricted to ordinary welding by melting, which leads only to superficial, pot-shaped seams with an aspect ratio (depth/width) of typically 0.5. Conversely, deep-penetration welding, as achieved by CO2 and Nd:YAG lasers, is very useful for generating deeply welded connections with improved mechanical solidity and, in comparison to welding by mere melting, a reduced heat-affected zone.
This deep-penetration welding effect is hard to induce with a laser diode because of its lower beam quality. For deep-penetration welding, the beam must be narrowly focused to supply sufficient power density to generate a plasma. Once this state is reached, the absorption of the laser radiation increases rapidly so that a plasma channel is formed that burns into the depth of the material. The critical power density required to induce this effect in small spots is 105 W/cm2, which is not yet attainable by standard laser diodes.
Laser diodes are considered the tools of choice for most applications because of their high lasing efficiency, compactness, low weight, and reliability. Therefore, it's worth some effort to study their potential for deep-penetration welding.
At ILT, the researchers improved the laser-beam quality and, more important, improved the process control, which resulted in power densities lower than the critical value of 105 W/cm2. Using a 1-kW, 808-nm laser diode (Jenoptik; Jena, Germany), a 2-mm stainless-steel sheet was welded together, with the seam almost uniform across the sheet (see figure). The welding speed was 0.5 m/min.
Welding depths up to 6 mm were reported in more recent samples, achieved by applying a 2.5-kW, broadband 900-nm laser diode (Rofin-Sinar; Hamburg, Germany). Thus, the ILT team expects rapid development in laser-diode deep welding. Improvements should come in the form of better beam shape and controlled processes, including control of the gas supply.