Considerations on Assist Gas Jet Optimization in Laser Cutting with Direct Diode Laser

The emergence of new generations of laser sources for cutting of metal sheets results in the reopening of research topics that were long closed for established technologies such as CO2 lasers. After the success of the fiber/disk generation, we now see a third generation of Direct Diode Lasers (DDLs) taking their chance. As the diode laser technology matures and the process boundaries are shifted to meet metal sheet laser cutting requirements, new challenges arise. This paper deals with gas jet optimization in laser cutting with a linearly polarized DDL setup with an output power of 750W. Different concepts were investigated for improving the nitrogen assist gas jet when processing 3mm thick 304 L stainless steel sheets. After performing a set of tests with conventional conical nozzles, several efforts were made in order to test possible improvements in cutting performance. This included accelerated gas jets designed for an exit Mach number of 2 (both a linearized version of the Laval nozzle and a Minimum length nozzle were tested) and non-axis symmetric nozzle configurations, such as laser to nozzle center offset or nozzles with obround exit shapes. A general discussion on the design methods and procedures for these nozzle shapes is also given. Results of this exercise show the importance of well-designed nozzles for cutting with DDL's and specifically the need to take the beam shape and laser quality constraints into consideration.