Simulation of time-dependent pool shape during laser spot welding: Transient effects
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11/7/03
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Simulation of Time-Dependent Pool Shape during Laser Spot Welding: Transient Effects GEORG EHLEN, ANDREAS LUDWIG, and PETER R. SAHM The shape and depth of the area molten during a welding process is of immense technical importance. This study investigates how the melt pool shape during laser welding is influenced by Marangoni convection and tries to establish general qualitative rules of melt pool dynamics. A parameter study shows how different welding powers lead to extremely different pool shapes. Special attention is paid to transient effects that occur during the melting process as well as after switching off the laser source. It is shown that the final pool shape can depend strongly on the welding duration. The authors use an axisymmetric two-dimensional (2-D) control-volume-method (CVM) code based on the volumeaveraged two-phase model of alloy solidification by Ni and Beckermann[1] and the SIMPLER algorithm by Patankar.[2] They calculate the transient distribution of temperatures, phase fractions, flow velocities, pressures, and concentrations of alloying elements in the melt and two solid phases (peritectic solidification) for a stationary laser welding process. Marangoni flow is described using a semiempirical model for the temperature-dependent surface tension gradient. The software was parallelized using the shared memory standard OpenMP.
I. INTRODUCTION
IN recent years, great effort has been made to predict the formation of different pool shapes that occur in conduction mode welding. Many research groups have developed sophisticated models that are able to describe the complex interaction of surface tension forces, electromagnetic forces, turbulence, heat losses by radiation, air convection, and evaporation. A short overview is given in Winkler et al.[3] Most welding applications include moving heat sources. For these applications, numerical steadystate solutions yield the most interesting information. For welding problems with stationary heat source instead, the numerical steady-state solutions, e.g., References 4 through 8, only give part of the information that might be useful. Some recent works investigate the transient behavior of weld pools, but do not take into account the transient processes after switching off the heat source, e.g., References 3 and 9. Many experimental and theoretical articles have been published concerning the description and prediction of pool shapes. So the pool shapes shown here, especially V and W shapes, have been experimentally observed and are well known in the literature, e.g., References 3, 8, and 10. But, in general, the formation of the different shapes is explained and interpreted as a function of the concentration of surfaceactive elements such as sulfur or oxygen. Pitscheneder et al.[11] investigate the question why the splitting into flat pools for low sulfur contents and deep pools for high sulfur contents happens at high welding powers and not at low GEORG EHLEN, formerly Ph.D. Student with the Foundry Institute, Aachen Universi
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