Weld repair of single-crystal (SX) Ni-base superalloys used in gas turbine blades is hampered by the nucleation of equiaxed, or stray, grains in the liquid weld pool which compromise the superior creep resistance of the SX.  Current theories hold that constitutional supercooling is the root cause for stray grain formation, but a complete understanding of the threshold solidification parameters for this process is not yet available.  An accurate assessment of local solidification parameters requires advanced techniques for modeling the weld pool.  A heat transfer and fluid flow finite difference model (FDM) was used to simulate high energy density (HED) beam weld pools in alloy CMSX-4.  The simulated weld pool dimensions aligned closely with those measured from experimental autogenous HED weld passes conducted over a wide range of processing parameters.  The liquidus location and local temperature gradients were collected by inputting the FDM data into a FORTRAN program.  The solidification velocity and temperature gradient parallel to the dendrite axes along the entire solid/liquid interface were then calculated. 

           Stray grain formation in the experimental welds was measured using Orientational Imaging Microscopy.  OIM mapping of multiple cross-sections from each weld was used to determine a stray grain volume fraction.  This value was observed to decrease with increasing travel speed, due to its concomitant effect on temperature gradient.  The number of crystal growth regions was also found to be more significant than the symmetry of the welding direction with respect to the SX lattice.  The location of stray grains in the experimental weld passes were compared with the local solidification parameters calculated from the FDM models in order to evaluate critical conditions for stray grain formation.  The results will be useful in constructing an effective tool for predicting the onset of the stray grain formation, thus allowing proper processing parameter selection during weld repair.