S. Babu, Edison Welding Institute, Columbus, OH; J. M. Vitek, S. A. David, Oak Ridge National Laboratory, Oak Ridge, TN
An overview of scientific and technical issues related to joining of single crystal nickel base superalloys will be presented. These issues will be demonstrated with examples from commercial single crystal alloys that are used in gas turbine industries.
The interaction between crystallography, dendrite grain structure, and weld pool shape was modeled using a geometry model. This geometry model, capable of describing dendrite growth directions in any arbitrary welding direction in the crystal reference coordinate system, was used to interpret the grain structure development in a electron beam and laser welds. The results show close correlation between stray grain formations and weld cracking in these alloys.
The importance of solid-state decomposition of γ phase on final γ + γ' microstructure during rapid cooling conditions will be illustrated with detailed microstructural characterization involving atom probe tomography.
Application of computational thermodynamics and kinetic model to describe solidification characteristics as well as solid-state decomposition of γ phase to γ + γ' microstructure under continuous cooling conditions will be demonstrated.
The paper reviews scientific and technological issues related to modeling of repair weldability of nickel base single crystal superalloys. Models relating the role of weld pool geometry and crystallographic orientation of the single crystals on the solidification grain structure will be presented. The role of high-resolution characterization tools including electron microscopy and atom probe field ion microscopy in modeling nonequilibrium microstructure evolution will be discussed.
