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

The inter-relationship between the weld pool shape and the weld microstructural is a critical factor that determines the physical integrity and other important properties of fusion welds. In the present work, large single crystals of an Fe-15Ni-15Cr alloy have been used to increase basic understanding of the factors that influence the development of weld microstructures. Oriented ternary alloy single crystals were used to make electron beam welds along various principal directions lying in different principal crystallographic planes. Using oriented single crystals it was possible to obtain crucial microstructural information that is ordinarily lost when welds are made on normal polycrystalline specimens. This quantitative information regarding the microstructural properties of electron beam welds has provided valuable new insight into the fundamentals of the relationships between weld pool shapes and weld microstructures. A new three-dimensional geometrical analytical method has been developed to interpret the microstructural information resulting from welds made using oriented single crystals. This analytical method establishes a direct correlation between the three-dimensional weld pool shape and the dendritic microstructures that are observed in two-dimensional transverse micrographs, and can be used to reconstruct the three-dimensional weld pool shape. Single crystal multipass and single pass bicrystal welds have also been examined. Overlapping multipass single crystal welds showed remarkable reproducibility from pass to pass and replicated the microstructural patterns observed in single pass welds. The microstructure of butt welds joining two single crystals with different orientations showed a one to one correspondence with that associated with each individual crystallographic orientation, and the microstructure essentially represented a composite of two single pass microstructures corresponding to the individual crystal orientations. (C) 1995 The Institute of Materials.