Free surface flow and heat transfer in conduction mode laser welding
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INTRODUCTION
D U R I N G laser welding of alloys, the structure and properties of the welds are influenced by the simultaneous occurrence of several important physical processes. These include the absorption of the laser beam by the weld pool surrounded by the hot plasma, heat exchange between the weld pool surface and the surroundings, vigorous circulation of molten metal resulting primarily from the spatial variation of surface tension at the weld pool surface, convective heat transfer in the molten metal, and its solidification. Much of the earlier work on these component processes was motivated by the appreciation that the improved understanding would ultimately lead to better welds. Indeed, if the weld structure and properties can be predicted from first principles of transport phenomena, it would be possible, at least in principle, to adjust welding parameters so as to achieve a desired combination of microstructure and mechanical properties. Because of the small size of the weld pool and the presence of plasma in the vicinity of the weld pool, physical measurements of important parameters such as the temperature and velocity fields in the weld pools are not easy tasks. Therefore, much of the pr evious work I~-4] on the transport processes in the weld pool was based on the mathematical modeling of the essential physical features of the welding process, and the technique has been successful in revealing detailed insight about certain aspects of the welding process that cannot be obtained otherwise. However, in this approach, our inability of making important measurements is traded, to a large extent, with the difficulties involved in adequately understanding and modeling the complex component physical processes of welding. In addition, certain experimental data are required for the calculations. For example, although we can account for the attenuation of the beam energy due to the presence of the plasma by providing an experimentally determined value of absorptivity, this must be viewed as a compromise of the predictive capability of the model in our quest for accuracy. A more fundamental limitation is imposed by the lack of necessary data pertinent to the plasma-weld pool system. The primary driving force for fluid flow in the laser melted pools is the surface tension gradient. The interfacial tension between the molten metals and plasma I~1 has received very little atA. PAUL, Graduate Student, and T. DEBROY, Associate Professor of Metallurgy, are with the Department of Materials Science and Engineering, The Pennsylvania State University, 212 Steidle Building, University Park, PA 16802. Manuscript submitted February 17, 1988. METALLURGICAL TRANSACTIONS B
tention in the past and systematic investigations of surface tension of binary metal-solute systems ~' at temperatures much above the melting point are just beginning. Furthermore, the heat exchange between the weld pool surface and the plasma cannot be currently estimated with a reasonable degree of trustworthiness. At high laser powers, additional com
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