Unsteady marangoni flow in a molten pool when welding dissimilar metals
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I. INTRODUCTION
THE joining of dissimilar metals is commonly done in welding in order to satisfy different requirements for performance. Dissimilar joints can be made successfully if there is mutual solubility between the two metals. Otherwise, using a third metal that is soluble with each of the others is required to produce a joint. For dissimilar metals having widely different thermal-expansion coefficients, the joint may fail either at solidification or soon thereafter, due to thermal fatigue. This is because internal stresses will be set up in the intermetallic zone, which tends to be extremely brittle. In the case of two metals having different melting temperatures or thermal conductivities, the welding process is also complicated because one metal will be molten before the other. The joining of dissimilar metals can be carried out using both a variety of different metals and several welding processes.[1,2] Understanding the mass and heat processes in welding dissimilar metals, therefore, is crucial. Recently, the computed results[3] showed the different flow and thermal fields and the molten regions of the dissimilar metals as functions of both the surface-tension coefficients, viscosities, melting temperatures, and thermal conductivities of dissimilar metals and different phases, and the working variables of beam power, welding speed, and energy-distribution parameter. However, the transport processes were presented at the instant when the cross section of the molten pool was maximal; that is, the welding was in a quasi-steady state. It should be noted that in order to acquire a clearer description of the phenomena that occur during the welding of dissimilar metals, an in-depth understanding of the unsteady transport processes of the dissimilar molten pools is needed. This is the objective of this study.
P.S WEI, Professor, is with the Mechanical Engineering Department, National Sun Yat-Sen University, Kaohsiung, Taiwan, Republic of China. F.K. CHUNG, formerly Graduate Student, Mechanical Engineering Department, National Sun Yat-Sen University, is Associate Professor, Department of Mold and Die Engineering, National Kaohsiung University of Applied Science, Kaohsiung, Taiwan, Republic of China. Manuscript submitted March 23, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
Seretsky and Ryba[4] observed a frozen swirling convection pattern from poorly mixed liquids of different composition in the pool for laser-welded titanium to nickel. The flow pattern was similar to those obtained by Unger[5] and by Nippes et al.[6] in their studies of the spot-resistance welding of Cor-Ten to 18-8 stainless steel and of monel to steel. The boundary between aluminum and iron in electron-beam welding was also observed to be sharp by Wei et al.[7] Dissimilar molten metals, therefore, can be considered as immiscible fluids experiencing circulations in weld pools. Tinkler et al.[8] found that when similar steels had the same sulfur levels, either good penetration (at 90 ppm) or poor penetration (below 30 ppm) was obtained.
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