Abstract
In cutting metals with solid-state lasers, a characteristic cutting edge structure is generated whose formation mechanisms still elude a consistent explanation. Several studies suggest a major contribution of the pressurized gas flow. Particular emphasis must be devoted to the gas boundary layer and its developing flow characteristics, since they determine the heat and momentum exchange between the cutting gas and the highly heated melt surface and thus the expulsion of the molten material from the kerf. The present study applies a CFD simulation model to analyze the gas flow during laser cutting with appropriate boundary conditions. Specifically, the gas boundary layer development is considered with a high spatial discretization of this zone in combination with a transition turbulence model. The results of the calculation reveal for the first time that the boundary layer is characterized by a quasi-stationary vortex structure composed of nearly horizontal geometry- and shock-induced separation zones and vertical vortices, which contribute to the transition to turbulent flow. Comparison of the results with the striation structure of experimental cut edges reveals a high agreement of the location, orientation, and size of the characteristic vortices with particular features of the striation structure of cut edges.
Highlights
Laser fusion cutting is an advanced manufacturing process for the non-contact separation of diverse metals and non-metals in a wide range of thicknesses
The current study focused on the gas flow in laser fusion cutting of stainless steel of medium material thicknesses (>6 mm)
The results of this study showed that gas dynamics is a crucial aspect of laser cutting that has not received sufficient attention in previous approaches
Summary
Laser fusion cutting is an advanced manufacturing process for the non-contact separation of diverse metals and non-metals in a wide range of thicknesses. A laser beam is guided along a contour, partially absorbed at the material surface and, as a result, the material is melted locally. A high-pressure gas jet is usually guided coaxially to the laser beam, which expels the melt produced from the resulting kerf (Figure 1a). Because of the gas pressure used, usually above 1.0 MPa, the gas flow exceeds the speed of sound in the processing zone. The performance and quality criteria for the cutting of high-alloy corrosion-resistant steels considered here include the achievable cutting speed, the gap geometry with a desired perpendicularity of the edges, their roughness, and the melt adhesion on the underside of the material. The process principle and the process-determining influencing variables are described in detail in the basic literature [1,2,3]
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