A simple but realistic model for laser cladding
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INTRODUCTION
THE steady development of high-power lasers has encouraged the industrial application of laser surface treatments. At present, complex components such as turbine blades or engine valves may be laser processed with either laser remelting or laser-cladding treatments.l 1,2,3] The aim of laser remelting is to induce changes in the microstructure of the surface, and the process has already been exhaustively studied from a modeling point of view. Models based on the analytical resolution of the heat-transfer equation[4,5,61 have been proposed since the middle of this century and allow a rapid evaluation of the processing parameters. In addition, complex modelstTq31 requiring the numerical resolution of both the heat-transfer and hydrodynamic equations have been developed in order to more accurately predict the meltpool shape and the solidification speeds (which are essential if microstructure calculations are to be carried out). The aim of the laser-cladding process is to deposit a protective layer (from 0.1 to 1.5 mm) onto a workpiece and for the two to be joined by a fusion bond. A clad track is obtained by injecting powder particles into the molten pool produced by a moving laser beam. In order to cover areas considerably larger than the diameter of the laser beam, successive partially overlapping tracks are deposited (Figure 1). Unlike laser remelting, laser cladding is a difficult process to control because of the interactions among the laser beam, the powder particles, and the molten region. Indeed, even if most of the laser power reaches the workpiece, a fraction is captured by the powder particles, thus heating them. Moreover, only the powder particles striking the molten pool adhere, whereas particles hitting the solid region ricochet and are lost. Another M. PICASSO, formerly Postdoctoral Fellow, Physical Metallurgy Laboratory, now Research Fellow, is with the Mathematics Department, Ecole Polytechnique Frdrrale de Lausanne. C.F. MARSDEN, Postdoctoral Fellow, J.-D. WAGNII~RE, Technical Engineer, A. FRENK, Ph.D. Student, and M. RAPPAZ, Professor, are with the Physical Metallurgy Laboratory, Ecole Polytechnique Frdrrale de Lausanne, 1015 Lausanne, Switzerland. Manuscript submitted December 7, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS B
complex phenomenon is the change of the workpiece absorption (i.e., the ratio between the laser power available at the surface of the workpiece and the power absorbed by the workpiece) with the shape of the molten region. Finally, owing to the complex geometry of the clad under the laser beam, it is difficult to predict the laser power that minimizes the depth remelted into the workpiece (either the previous track or the substrate) but still allows melting of the incoming powder. The processing parameters are numerous. They concern the laser-beam properties and velocity relative to the workpiece, the distance between two successive tracks, the geometry of the nozzle used to inject the powder, and the injection conditions themselves. These processing paramet
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