Microstructural formation in longitudinal bicrystal welds
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I.
INTRODUCTION
IN order to investigate the grain-selection mechanisms that are operative during the welding o f polycrystalline metals, a number o f investigations have recently been carried out that employed welds o f single crystals o f austenitic stainless steel1~,231 o r aluminum-based alloys. 14t For these systems, it was found that epitaxial growth o f dendrites from the unmelted substrate was the dominant mechanism in determining the microstructural features and that, in fact, the single-crystal nature of the base material was retained in the weld trace. Furthermore, the orientation of the dendrite trunks was found to be relatively independent o f the heat-flow direction and corresponded to the ~100) crystallographic orientations in the two cubic systems investigated. X-ray diffraction measurements 14j showed that the departure from a (100) crystallographic orientation o f the dendrite trunk, which is a maximum when the dendrite trunk grows at an angle o f 45 deg with respect to the heat-flow direction, did not exceed 15 deg. Assuming that the dendrites grow precisely along one o f the (100) crystallographic orientations, a well-defined mechanism o f dendrite trunk selection is operative during the welding o f single crystals, t''2'31 This selection mechanism is based on the principle o f minimum velocity (or minimum undercooling) o f the dendrite tips; i.e., M. R A P P A Z is Visiting Scientist, Solid S t a t e Division, Oak R i d g e National Laboratory, and Research Professor, D6partement des Mat6riaux, E c o l e Polytechnique F6ddrale de Lausanne, Lausanne, Switzerland. J.M. VITEK, Senior Research Staff Member, and S.A. DAVID, Group Leader, the Metals and Ceramics Division, and L.A. BOATNER, Senior Research Staff Member, Solid S t a t e Division, are with Oak R i d g e National Laboratory, Oak Ridge, T N 37831-6376. Manuscript submitted A u g u s t 11, 1992. METALLURGICAL TRANSACTIONS A
the dendrite t r u n k orientation that is aligned closest to the maximum thermal gradient at a given point on the melt-pool interface (solid/liquid surface) is the one that is selected. Taking into account the three-dimensional (3-D) shape o f the melt pool, this selection principle was the basis for an analysis o f the complex microstructure that developed in several single-crystal weld configurations.l,.2.3b By further extending the analysis, it has been possible to carry out a semiquantitative reconstruction o f the 3-D melt-pool shape from the microstructural features observed in transverse-section micrographs, t''31 Multipass single-crystal welds and a butt weld o f two single crystals have also been made previously, f2J In the present work, the dendrite trunk-selection analysis that was developed previously for single-crystal welds is systematically applied to the more complex case o f butt welds o f two single crystals with different crystallographic orientations (i.e., longitudinal bicrystal welds). The bicrystal welding configuration is shown schematically in Figure 1 and corr_esponds to the particular case
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