Gas Dynamic Effects On Laser Cut Quality

Cutting efficiency and quality are very sensitive to gas jet pressure and nozzle standoff distance. Do a high gas pressure and a small standoff distance necessarily give better material removal capability and cutting quality? Simple experiments to measure the mass flow rate were carried out for two types of geometry: a circular hole and a linear cut directly underneath an axisymmetric nozzle. The mass flow rate for the three dimensional case shows the same behavior (i.e., discontinuity as gas pressure and standoff change) as that of the axisymmetric case, indicating the basic shock structures of the axisymmetric case are applicable to the real cutting cases. The two important forces exerted by the gas jet for melt ejection, namely, shear force and pressure gradient are examined. Laser cutting experiments under the corresponding conditions were performed and the cut quality characterized by roughness, dross attachment, recast layer thickness was analyzed. The deterioration of cut quality is found to occur when the mass flow rate reaches bottom, which could occur at a high gas pressure or at a not so large standoff distance. It is thus indicative of the fact that the gas pressure and standoff distance affects cutting efficiency and quality through the underlying shock structure of the gas jet. Nomenclature Ah hole cross section area (mm) As slot cross section area (mm) B slot width (m) D workpiece thickness (m) d hole diameter (m) F vector of x-directed fluxes H nozzle standoff distance (m) L slot length (m) mh through-hole mass flow rate (kg/sec) ms through-slot mass flow rate (kg/sec) p static pressure (Pa) p0 total pressure (Pa) pt total pressure (Pa) Pe total pressure at delivery nozzle (Pa) r radial coordinate (m) U average velocity inside hole (m/s) x axial coordinate (m) y distance normal to the wall (m)  density (kg/m)  stress tensor xr shear stress (Pa) xx normal stress (Pa) rr normal stress (Pa) a average shear stress inside hole (Pa) r molten layer thickness (m) p pressure drop through hole