Simulation of laser cutting
Laser cutting is a thermal separation process widely used in shaping and contour cutting applications. There are, however, gaps in understanding the dynamics of the process, especially issues related to cut quality. This work describes the advances in fundamental physical modelling and process monitoring of laser cutting, as well as time varying processes such as contour cutting. Diagnosis of ripple and dross formation is advanced to observe the melt flow and its separation simultaneously as well as the spatial shape of the cut kerf. The cutting process is described with a spatial three-dimensional Free Boundary Problem for the motion of one phase boundary. In such dissipative dynamical systems a finite dimensional inertial manifold exists which contains the attractor of the system. The existence of a finite dimensional inertial manifold means that the motion of a finite set of degrees of freedom can give a good approximation to the complete solution. Asymptotic methods are used to identify the degrees of freedom, and integral methods are applied to derive their equations of motion. Experimental findings about the morphology of ripple formation guide the modelling approach and motivate the investigation of what is known as the one phase problem. Solving inverse problems and the properties of the thermal boundary layers are discussed. The model reproduces details of the U-shaped ripples evolving at the cut surface. In discussion of what is known as the two phase problem the properties of the melt flow are included. The additional degrees of freedom are the melt film thickness, the mass flow and the temperature at the melt film surface. The onset of evaporation and the increase in capillary forces are the two physical phenomena relevant to the build-up of adherent dross. The dynamic model predicts a modulation frequency for the laser power that leads to almost complete suppression of adherent dross in contour cutting. Heat transport in thin film flow is investigated demonstrating how to control the error of reduced models by spectral methods. To find the properties of the gas flow leading to melt ejection is a fundamental task in cutting. The interaction of the gas flow with the condensed phase is mediated by two quantities, namely the pressure gradient and the shear stress along the liquid surface. Results of a detailed analysis of the momentum boundary layer of the gas flow is compared with numerical calculations using the Euler equations as well as the viscous effects described by the compressible Navier-Stokes equations. Deflection and separation of a supersonic gas jet emanating from a nozzle and propagating into the cut kerf is investigated using Schlieren photography and theoretical analysis. Looking for the different situations present in cutting and trepanning, the formation of horizontal structures in the ripple pattern in the cut is discussed. The effect of design and alignment parameters on nozzle performance in cutting are investigated and two dominant effects are discussed, namely the feedback of the gas flow into the nozzle and deflection of the gas flow away from the cutting front. Discussion of the onset of dross formation is extended to include compressible gas flow in the simulation such that the nozzle pressure enters the calculation of the processing domain for a dross-free cut.