A Numerical Investigation into Cutting Front Mobility in Co2 Laser Cutting

-The technologically important case of reactive gas laser cutting is examined. A transient twodimensional (2D) model is developed specifically to investigate the effect of various CNC velocity profiles on the resulting cutting front temperature. Co-ordinated motion systems must ramp up and down to their target speeds, and therefore such accelerations must be considered. In doing so, the dynamics of the cutting front cannot be neglected. The implication of this mobility is that a net acceleration will result in greater, more efficient beam coupling to the workpiece, whilst a net deceleration results in a reduction, with the transmitted power simply falling through the generated kerf. The presence of such a kerf is considered and nodal points within it became part of the convective environment. Boundary encroachment and bulk heating issues, due to workpiece geometry, are also studied for their effect on the front temperature. Results show that even under non-accelerating conditions, cutting front mobility plays a significant role in temperature determination. A non-linear velocity profile is also evaluated via an optimization strategy in order to stabilize cutting front temperatures. The motivation being that quality can therefore be assured for intricate workpieces, which inherently have pre-cut sections and boundaries. Results on front mobility and temperature show similar trends as experimental and numerical results found elsewhere, whilst their direct verification is currently under investigation. Due to the hostile environment encountered in the interaction zone, the observation of such p]henomena is extremely difficult. NOMENCLATURE A area Ab absorptivity a net acceleration magnitude amu atomic mass units b kerf width Bi mesh-size Biot number cv heat capacity D workpiece thickness d duct diameter H net energy input hc mixed convective heat transfer coefficient hr forced convective heat transfer coefficient h. natural or free convective heat transfer coefficient hr radiative heat transfer coefficient I(r) radial intensity distribution I(0) peak intensity K thermal[ conductivity L workpiece length Lf latent heat of fusion rh melt removal rate n outward normal co-ordinate Nua Nusselt number Pb(r) radial absorbed beam power Pex,, exothermic power Pi,c incidenlE beam power Pmett melting power P, .. . . transmitted power Pr Prandtl number heat generation qr radiative heat loss r radial distance from beam centre ratio ratio of FeO:Fe tSchool of Mechanical and Manufacturing Engineering, The University of New South Wales, P.O. Box 1, Kensington, NSW 2033, Australia. :~Author to whom all correspondence should be addressed.