APPLICATION OF SINGLE-CRYSTALS TO ACHIEVE QUANTITATIVE UNDERSTANDING OF WELD MICROSTRUCTURES

The inter-relationship between the weld pool shape and the weld micros tructural is a critical factor that determines the physical integrity and other important properties of fusion welds. In the present work, l arge single crystals of an Fe-15Ni-15Cr alloy have been used to increa se basic understanding of the factors that influence the development o f weld microstructures. Oriented ternary alloy single crystals were us ed to make electron beam welds along various principal directions lyin g in different principal crystallographic planes. Using oriented singl e crystals it was possible to obtain crucial microstructural informati on that is ordinarily lost when welds are made on normal polycrystalli ne specimens. This quantitative information regarding the microstructu ral properties of electron beam welds has provided valuable new insigh t into the fundamentals of the relationships between weld pool shapes and weld microstructures. A new three-dimensional geometrical analytic al method has been developed to interpret the microstructural informat ion resulting from welds made using oriented single crystals. This ana lytical method establishes a direct correlation between the three-dime nsional weld pool shape and the dendritic microstructures that are obs erved in two-dimensional transverse micrographs, and can be used to re construct the three-dimensional weld pool shape. Single crystal multip ass and single pass bicrystal welds have also been examined. Overlappi ng multipass single crystal welds showed remarkable reproducibility fr om pass to pass and replicated the microstructural patterns observed i n single pass welds. The microstructure of butt welds joining two sing le crystals with different orientations showed a one to one correspond ence with that associated with each individual crystallographic orient ation, and the microstructure essentially represented a composite of t wo single pass microstructures corresponding to the individual crystal orientations. (C) 1995 The Institute of Materials.