Enhanced stainless steel cutting quality through optimized air-assisted fiber laser cutting with a novel supersonic nozzle

Abstract

• MOC-designed nozzle generating shock-free, quasi-isentropic expansion supersonic jet. • The developed nozzle enables cost-effective and efficient air-assisted laser cutting. • Jet length extended 1.6 times with Mach number fluctuations ≤±0.2, boosting stability. • Kerf width reduced by 19.95% (284.97 to 228.12 µm), for narrower and cleaner cuts. • Taper angle decreased by 69.23% (2.49° to 0.77°), yielding straighter and precise sidewalls. • Max dross height dropped by 54.61% and HAZ width by 10.24% for superior thermal control. This study presents the design, simulation, and experimental validation of a novel supersonic nozzle developed using the Method of Characteristics (MOC) to enhance air-assisted fibre laser cutting of AISI 304 stainless steel. While compressed air offers economic and environmental advantages over nitrogen, its industrial use is often limited by unstable jet behaviour, excessive dross formation, and poor surface finish, particularly when conventional nozzles are employed. To address these limitations, a minimum-length nozzle (MLZ) was designed featuring a sharp-corner throat and optimized divergent section to generate a quasi-isentropic, shock-free supersonic jet. Computational Fluid Dynamics (CFD) simulations showed that at the design pressure of 4.87 bar, the MLZ achieved Mach number fluctuations of ±0.2 and extended the effective jet length by 1.6 times compared to a conventional nozzle. Schlieren imaging confirmed the absence of Mach discs and oblique shocks, validating the predicted flow uniformity. Experimental cutting trials using a 3.3 kW fibre laser demonstrated that the MLZ reduced kerf width by 19.95% (284.97 µm to 228.12 µm), taper angle by 69.23% (2.49° to 0.77°), maximum dross height by 54.61% (102.46 µm to 46.49 µm), surface roughness (Sa) by 32.67% (10.63 µm to 7.16 µm), and HAZ width by 10.24% (155.2 µm to 139.3 µm). These enhancements are attributed to the MLZ’s ability to maintain a stable, high-thrust jet that improves melt ejection and confines thermal effects. The MLZ provides a technically robust and economically viable alternative to nitrogen-assisted cutting, with strong potential in precision manufacturing sectors including aerospace, microfabrication, and automotive industries.