9500181 Yao This project is aimed at advancing knowledge of material-laser interactions under unsteady state conditions. The laser cutting process has been chosen for investigation because the effects of unsteady state conditions on material-laser interactions are more pronounced in the laser cutting process than other similar processes like laser welding. Specific objectives are (1) advance knowledge of geometric effects on material-laser interactions, (2) understand the mechanism of using variable process parameters to offset the detrimental effects of unsteady-state material-laser interactions, (3) explore the nature of oscillatory phenomena in general and striation formation in particular in the laser cutting process, (4) expand the technology base necessary for improving process reproducibility, eliminating the needs for trial-and-error, and optimizing the process under unsteady state conditions. The project will develop a transient three-dimensional model embracing the coupled continuity, momentum and energy equations for the molten liquid layer, solid workpiece and gas jet. The liquid/solid will be modeled as a single domain with an enthalpy-porosity formulation for the phase change. A control-volume based finite-difference algorithm with power-law scheme for convective terms and pressure correction algorithm will be used to solve the equations. Instability of the molten layer will be numerically studied by introducing perturbations. The model can accurately predict the kerf geometry, energy transport and melt flow. The model will be used to investigate various unsteady state phenomena as mentioned above, the effects of Computer Numerical Control (CNC) interpolations, and optimization strategies under unsteady state conditions. Oxygen assist carbon dioxide laser cutting of mild steel will be the focus. Experimental investigation under various unsteady state conditions will be undertaken, including boundary encroachment, cornering, contour cuts and variable process parameters. Experimental techniques to be used include high speed filming to investigate cutting front mobility and striation formation; close focus pyrometer and thermocouples to investigate cutting front temperature and workpiece temperature distribution under unsteady state conditions. Focused heat delivery capability of lasers makes it ideal for modifying materials in a very selected local zones. Delivery of such high energy concentration also poses difficulties in continuously controlling the unsteady process. Successful demonstration of controlled laser cutting as a result of the predictive model developed in this project will significantly impact the use of lasers in materials processing industries.
