Product Code: JLA_9_6_269

Authors:
Aaron D. Brandt
Gary S. Settles
Gas Dynamics Laboratory, Mechanical Engineering Department, Penn State University, University Park, PA 16802, U.S.A.

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.