Mathematical Modelling of Weld Phenomena 12 1 COMPLETE 3D HEAT AND FLUID FLOW MODELING OF KEYHOLE LASER WELDING AND METHODS TO REDUCE CALCULATION TIMES M. COURTOIS*, M. CARIN*, P. LE MASSON* and S. GAIE** *Univ. Bretagne Sud, UMR CNRS 6027, IRDL, F-56100 Lorient, France **ArcelorMittal Global R&D Montataire, F-60160 Montataire, France DOI 10.3217/978-3-85125-615-4-01 ABSTRACT The fluid flow calculation inside the melt pool in welding processes is a complex challenge. It can be useful for defects prediction in the weld seam or to study the influence of some process parameters. The cost in time of these calculations makes these models not widely used, although they are rich in information. The aim of this paper is to present a complete model solving the heat and fluid flow equations in all the states (solid – liquid - gas) of the metal during keyhole laser welding. In order to track dynamically the keyhole shape, a level set function is employed and the momentum equations are solved to predict accurately the melt pool behaviour. The particularity of this model is a drastic optimization to reduce computation time (less than 1 day on a commercial workstation without reducing significantly the accuracy of the fluid flow computation. To achieve this goal, a simplified approach of the recoil pressure effect is employed and an original method with three different meshes adapted for each equation and physics problem is proposed and discussed. Finally, to validate the model simplification, a complete experimental validation is added with longitudinal and transverse micrograph cuts for different welding configurations, micro-thermocouples measurements in solid phase [20-1200°C], pyrometer measurements in liquid phase [1500-3000°C]. Keywords: Laser welding; heat and fluid flow; keyhole, numerical simulation INTRODUCTION In automotive industry, laser welding is widely used for its high production rate and its capacity to produce clean and discreet welded beams. Moreover, the cost of the laser sources and the increased available power offer always more applications. Laser welding of steel is currently employed in industry for 20 years. Nevertheless, the use of new materials or the necessity to weld new configurations raises sometimes difficulties to achieve a satisfying welded joint. In most cases, questions come when the weld seam presents some defects like porosity, lake of materials or partial penetration. In