This investigation applies sonic and supersonic coaxial and off-axis nozzles to the cutting of mild steel sheet of 1–4 mm thickness. A 1.5 kW CW CO2 laser is used with nitrogen as the assist gas. Sonic coaxial cutting is compared to that of off-axis nozzles, which vary in orientation from 20–60° behind the laser axis and in exit Mach number from 1 to 2.4. Results show a 50% increase in maximum cutting speed at approximately a 40° off-axis nozzle angle. In comparison, variations in off-axis nozzle exit Mach number have little effect. A scale-model kerf was used to visualize the kerf gas dynamics, revealing that nozzle angles of 0–20° cause a shock wave/boundary layer interaction with flow separation inside the kerf. Angles of 20–45° alleviate this interaction, producing a uniform supersonic flow throughout the kerf, which yields high cutting speeds due to uniformly high shear forces. For nozzle angles greater than 45°, the assist gas is diverted away from the kerf, reducing cutting speed. Cut edge quality observations were also conducted to verify the results of the model tests.
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Brandt et al. (1997) studied this question.