Convection in Pulsed Laser Formed Melts
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CONVECTION IN PULSED LASER FORMED MELTS
G. E. POSSIN, H. G. PARKS AND S. W. CHIANG General Electric Research and Development Center Schenectady, New York 12301 USA
ABSTRACT In this paper we treat surface tension driven convection effects in pulsed laser formed melts. Mass transport is determined from an approximate solution of the Navier Stokes equation. It is shown that for small laser spot diameters the characteristic mixing times are on the order of 100's of ns. The dependence of the convection mechanism on material and laser parameters is discussed and extended to thin metal films on Si. Experimental results substantiating the theoretical considerations are presented. INTRODUCTION Anthony and Cline(I) have accounted for the surface rippling observed in CW laser formed melts by a convection mechanism driven by thermally induced surface tension gradients. In this model the return flow is driven by gravitational pressure gradients due to material build up near the edge of the melt pool. This mechanism requires equilibrium times on the order of tenths of milliseconds and hence is not applicable for pulsed laser formed melts. Nevertheless, Van Gurp et al.(2) have observed cellular structure on the order of 1 to 2vlm which they attribute to convection in pulsed laser formed melts for thin 3 Co films on Si. Parks and Rose( ) have also observed an annular ring structure for the near surface penetration of aluminum which they attribute to convection effects in pulsed laser alloying of thin Al films on Si. In this paper we will review the surface tension gradient driven convection mechanism and show that capillary pressure can produce the return flow driving force. A simplified model based on an approximate solution of the Navier Stokes equation shows that this mechanism can account for convection effects on a time scale commensurate with pulsed laser formed melts for small melt pools. The underlying concepts of this model will then be extended for thin metal films on Si. As a result of this extension, we propose a mechanism for the cellular structure observed in some metal Si systems and the annular ring structure observed in other metal Si systems. Experimental data supporting the mechanism for these two types of convective flows will be presented for pulsed laser melts of a thin film of Ni on Si and a thin film of Ag on Si. CONVECTION IN PULSED LASER MELTS Figure 1 shows the temperature profile across the surface of a pulsed laser formed melt which we approximate as gaussian. As a result of this variation in temperature and the negative temperature coefficient of surface tension for liquids, the surface tension, y, across the melt pool varies as indicated in the figure. As a result, a shear stress equal to the gradient in surface tension pulls material from the center of the spot causing it to pile up at the edge of the melt pool. Given enough time, a large melt diameter, and a deep enough melt pool, the buildup of material at the edqe would cause a static pressure head, pgAh, forcing a return flow as indicated
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