Analysis of solidification microstructures in Fe-Ni-Cr single-crystal welds
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INTRODUCTION
T H E development of the fusion-zone grain structure in weld metal is a complex phenomenon. Although it is well established that initial solidification within the fusion zone occurs epitaxially at the partially melted grains lying along the fusion line, t~-4] it is difficult to identify the extent to which growth crystallography and the direction of the thermal gradient influence the grain growth selection process and the associated microstructural growth behavior in a polycrystalline weld metal. While it is known that for cubic metals the six (100) crystallographic directions are preferred dendritic growth directions (known as easy growth directions), tl,3,5-71 in a polycrystalline cubic weld metal it is impossible to determine which of these six variants for dendritic growth is active or how they influence the development of weld-metal grain structure. However, by making welds along specific crystallographic directions on a well-characterized single crystal, it is possible to relate the combined influence of the growth crystallography and the heat flow direction to the development o f weld-metal microstructures. A detailed analysis of the microstructural development
M. RAPPAZ, formerly Visiting Scientist, Solid State Division, Oak Ridge National Laboratory, is Group Leader, Modeling of Solidification Processes Group, with the Laboratoire de Mrtallurgie Physique, Ecole Polytechnique Frdrrale de Lausanne, Lausanne, Switzerland. S.A. DAVID, Group Leader, Materials Joining Group, Metals and Ceramics Division, J.M. VITEK, Research Staff Member, Microscopy and Microanalytical Sciences ~3roup, Metals and Ceramics Division, and L.A. BOATNER, Section Head, Ceramics and Interfaces Section, Solid State Division, are with Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted August 14, 1989. METALLURGICAL TRANSACTIONS A
of autogenously welded single crystals was reported previously for welds made along a few principal crystallographic orientations, tal In particular, the microstructures which formed in Fe-15Ni-15Cr single-crystal electron beam welds were analyzed for two different situations, i.e., welds made along either the [100] or [110] crystallographic directions on a (001) surface. The dendritic growth model of Kurz, Giovanola, and Trivedi (KGT) t91 was applied to the ternary alloy system with the assumption that dendrite formation was controlled by the diffusion of Ni. Selection of the preferred dendrite trunk orientation among the six (100) crystallographic directions was determined based on a minimum velocity (i.e., minimum undercooling) criterion. The relationship between the moving heat source velocity, ~b, the solidification front velocity, Cs, and the dendrite tip velocity, ~hkt, was derived assuming that the shape of the pool is constant (steady state). Earlier work by others t~~ had also established the relationship between these three velocities for the limiting case that these velocities were all coplanar, but this restriction was removed in the previous paper by the presen
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