Jan Michel

Dynamics of ripple formation and melt flow in laser beam cutting

The dynamical behaviour of the laser beam fusion cutting process of metals is investigated. Integral methods such as the variational formulation are applied to the partial differential equations for the free boundary problem and a finite dimensional approximation of the dynamical system is obtained. The model describes the shape of the evolving cutting kerf and the melt flow. The analysis is aimed at revealing the characteristic features of the resultant cut, for example, ripple formation and adherent dross. The formation of the ripples in the upper part of the cut, where no resolidified material is detectable, is discussed in detail. A comparison with numerical simulations and experiments is made.

Microholes in zirconia-coated Ni-superalloys for transpiration cooling of turbine blades

Drillings in zirconia coated Ni-superalloys is done by melt extraction with pulsed laser radiation provided by a Nd:YAG slab laser with microsecond pulse duration. This laser system distinguishes itself by a high beam quality and offers the possibility to investigate drilling of holes with a diameter of 200 micrometer by percussion drilling and trepanning. The quality of drilled holes, e.g. the heat affected zone (HAZ), the recast layer and the conicality, are presented. During drilling different process gases are used. The results in drilling velocities, melt thickness and chemical composition of the melting zone are shown for oxygen, argon and nitrogen by SEM and EDX. A numerical simulation of the trepanning process will be presented. The different time scales of the contributing physical processes related, for example, to the small melt film layer during trepanning are described. A coating is distributed on the multilayer system to protect the blade from recast. Aim of the investigation is the production of holes in a multilayer system, consisting of CMSX-4, VPS-MCrAlY and EB-PVD-zirconia. With this used laser system inclined holes up to 60 degrees in this layer system can be drilled. No recast layer and no spalling of the zirconia-layer are observed.

Melt Expulsion by a Coaxial Gas Jet in Trepanning of CMSX-4 with Microsecond Nd:YAG Laser Radiation

Trepanning of 200 μm holes in 2-5mm thick CMSX-4 sheets is done by laser radiation provided by a lamp-pumped Nd:YAG slab laser with pulse durations of 100 - 500 μs. Pulse energies <1J determine the material removal mainly by melt expulsion assisted by a processing gas jet coaxial to the laser beam. Stagnation and deflection of the gas jet at the entrance of the kerf, friction in the molten material, and friction at the liquid/solid interface hinder an efficient melt expulsion. A simulation tool for supersonic gas flow solving Euler equations by the Finite Volume Method is developed in order to investigate the gas flow through the trepanning kerf. Gas pressures above and within the kerfs while trepanning at different inclination angles, geometries and arrangements of nozzles as well nozzle reservoir pressures are presented. The computed gas flow is compared to melt expulsion investigated metallographically by the determination of kerf widths and the thickness of the resolidified melt.

Approximate model for laser welding

The industrial application of laser welding implies a reliable and efficient production process. Thus, modelling and simulation is used to reveal the crucial points of the welding process. The welding process is described by transport phenomena for mass, momentum and energy. The three involved phases (solid, liquid, gaseous) interact over their free moving phase boundaries. The existing boundary layer character allows to reduce the dimension of this Free Boundary Problem. The resulting welding model reproduces the time-dependent spatially 3d-distributed welding process. Technical relevant predication about seam width and depth are compared to experimental results; the possibility to control predictively the welding process is investigated. The reproduction of thermal monitoring signals from the interaction zone between laser and material is approached.

Identification of process domains in cutting

Summary form only given. Laser cutting is an established separation process. Reliable machining at high productivity and quality as well as simplified machine operation are the actual goals. Monitoring and control of the dynamical process is investigated. This work describes the advances in fundamental physical modeling of time dependent processes such as piercing and contour cutting. Singular perturbation and spectral methods are applied to reveal the number and type of characteristic dynamical variables. Refinement of the dynamical model is performed in order to identify the relations between phase space dimension of the model, properties of the monitoring signals and the quality features of the process. As result the dynamical properties of the process are mapped to different domains of the processing parameters.