Laser weld repair of single crystal Ni-base superalloys

The weld repair of single-crystal (SX) gas turbine blades can be hampered by the loss of the SX structure due to the development of stray grains in the liquid, severely reducing creep resistance. This Columnar-to-Equiaxed Transition (CET) has been investigated extensively in casting processes, and more recent studies have focused on its occurrence during welding. These results have shown that weld pool shape and thermal conditions significantly affect the stray grain formation tendency. Heat transfer/fluid flow models were used to predict weld pool shape. The results of these simulations were compared with the experimental weld passes on the SX Nibased superalloy CMSX-4. Laser welds were conducted under various processing conditions and crystallographic orientations. The weld pool dimensions were measured, and the stray grains were revealed in the microstructure using orientational imaging microscopy in the SEM. The fraction of stray grains was shown to be a function of processing parameters. The simulated weld pools could provide critical information about local solidification conditions that could ultimately assist in the construction of a tool for prediction of the CET.