Laser fusion cutting: how laser wavelength affects cut edge quality

Laser cutting is an established industrial process for a wide variety of materials and thicknesses, but the underlying physical mechanisms are not yet fully understood. Particularly the difference in quality between CO₂ and solid-state laser cutting remains an open question. Several explanatory approaches have been proposed but no consistent and reasonable physical mechanism could be identified until now. Recently, the authors developed a CFD model for the cutting gas flow characteristics, clarifying the cut edge quality and structure formation during fiber laser cutting. For cutting with CO₂ laser, the absorption of radiation at the partly vaporized material is significantly increased. This causes an additional heat source in the cutting gas and may lead to the formation of a gas plasma. The established cutting gas CFD model was modified for the additional effects in CO₂ laser cutting, in particular vapor distribution, radiation absorption and local heating. The improved CFD model is now capable of reproducing the altered gas flow and boundary layer development, which lead to the characteristic cut edge structure with horizontal stripes in cutting with CO₂ laser. Of particular importance is a high-temperature and high-viscosity layer between cut front surface and core gas flow, which weakens the vertical vortices responsible for striation generation in cutting with fiber laser. In addition, when cutting with CO₂ laser, there is a significantly higher momentum transfer to the melt and the heat transfer is directed from the gas phase to the material. The model can therefore explain the improved melt ejection and the structure of the cut edges when cutting with CO₂ laser as a result of the modified gas flow.