OXIDE-LAYER FORMATION ON A NICKEL BASED ALLOY USING LASER SURFACE MELTING - EXPERIMENTAL AND NUMERICAL STUDY OF THE LASER-METAL INTERACTION

摘要

The nickel based alloy 690 is commonly used for steam generator as heat exchanger tubes in pressurized water reactors. Oxidation of this material in the primary circuit coolant leads to nickel release which is responsible for radioactive contamination in the reactor. It is known that a healing chromium high content layer can prevent the surface of a nickel based alloy from releasing the Ni-containing corrosion products. In a previous study, laser Surface Modification (LSM) by nano-pulsed laser produced oxidation and chromium enrichment of the specimen surface. However, it also produced a roughness which can change the contact area with the primary circuit coolant. In this study, to better understand the roughness formation, the process modelling and simulation have been developed. It was assumed that the morphology of single laser spot depends on the thermal field, hydrodynamic flows in the melted zone and external forces (vapor pressure). The numerical model takes this coupling into account to be realistic. The simulation is made on COMSOL Multiphysics that allows us to represent the thermal field, the flows during and after the laser pulse and the residual surface deformations (Arbitrary Lagrangian Eulerian method). The simulations demonstrate a good agreement with the experiments concerning the induced surface topography and so suggest a scenario to explain the surface irregularities.