The efficiency of the laser cutting process depends on both the rate of melting and rate of melt removal from the cut kerf. The depth of flow separation and the dross attachment on the lower cut edge relate to the efficiency of the melt removal process and can be used to characterize the rate of melt removal from the cut kerf. The melt flow velocity and melt film thickness are formulated in this study by consideration of the fundamentals of viscous incompressible fluid flow. The calculated melt flow velocity and melt film thickness are correlated with the depth of flow separation on the 10 mm stainless steel AISI 304 (EN 1.4301) laser cut edge. The effects of process parameters—including assist gas pressure, nozzle diameter, nozzle standoff, focal point position, and cutting speed—on the depth of flow separation and the dross attachment on the lower cut edge are investigated. The assist gas pressure, nozzle diameter, and focal point position are found to significantly affect the efficiency of melt removal from the cut kerf.

Inert gas assisted laser cutting of 10 mm stainless steel plate and 4 mm aluminum sheet was performed with a 5 kW fiber laser. The effects of laser power, cutting speed, focal point position, and assist gas pressure on the cutting performance and cut quality were investigated. Clean cut surfaces without or with minimal dross were achieved with some combinations of process parameters and attempts were made to define parameter windows in terms of cutting speed and laser power for good quality cutting. The maximum cutting speeds for acceptable cut quality were determined at different power levels. The range at which complete through cutting could be achieved (so-called parameter window) was limited upwards by insufficient power intensity to obtain through cutting at high cutting speeds and downwards by heat conduction at slow cutting speeds. The effects of focal point position and assist gas pressure on the striation pattern (cut surface roughness) were also examined. Low surface roughness was achieved with the focal point position inside the workpiece showing the need for a wider kerf for better melt ejection in thick-section metal cutting. There was also a reduction in surface roughness with increase in assist gas pressure, but there was no significant reduction in surface roughness above the gas pressure of 16 bar, which could be due to the gas flow dynamics inside the narrow cut kerf at high assist-gas pressures.

The high-power fibre laser presents a possibility for the application of solid-state lasers in thick-section metal cutting, a field which has been dominated by high-power CO2 lasers. The current paper presents the lumped-parameter formulation of the laser power requirement as a function of cutting speed for oxygen-assisted laser cutting of mild steel and nitrogen-assisted laser cutting of stainless steel. The calculated laser power requirement is compared with the incident laser power used in the cutting of 15 mm mild steel with oxygen assist gas and 10 mm stainless steel with nitrogen assist gas using a multi-mode 5 kW fibre laser and a 4 kW CO2 laser. The incident laser power required for cutting at a given cutting speed is found to be lower for fibre laser cutting than for the CO2 laser cutting, indicating a higher absorption of the fibre laser beam. The effects of process parameters on the cut kerf quality in mild steel laser cutting with oxygen assist gas using the high-power fibre laser are presented. The critical process parameters affecting the quality of the cutting process and the resulting cut kerf are the cutting speed, oxygen pressure, and nozzle diameter.

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Optimization of the fibre laser cutting parameters for attainment of high cut edge quality in 10 mm stainless steel plate was demonstrated in this study. The tested process parameters included cutting speed, focal position, and focal length. Optimization of these process parameters enhances the melt removal from the cut kerf so as to prevent the undesired dross adherence on the lower cut edge or even incomplete penetration of the workpiece when the incident intensity is not sufficient to penetrate the workpiece. Dross-free cut edges with lower surface roughness and lower deviation of cut edge squareness could be achieved by reducing the cutting speed from the maximum achievable value, using the longer focal length lens for focusing the laser beam, and with focal position located on the bottom workpiece surface. These conditions enhance a high melt removal rate resulting in a high cut edge quality.