Repair welding of Ni base single crystals

Single-crystal (SX) casting techniques have conferred excellent creep resistance properties to the Ni-base superalloy components in gas turbine engines. The development of a weld repair strategy for SX parts could extend the service life of these expensive components. Preserving the SX nature within the weld zone is of great concern, but the nucleation and growth of "stray" grains during weld solidification will compromise the SX. Further understanding of SX solidification conditions is therefore necessary. In order to study the conditions which promote complete SX growth during repair, a large series of welding trials were conducted with a wide range of welding parameters. The amount of stray grains in each weld was quantified using backscattered electron diffraction techniques. These welds were simulated using an advanced heat/fluid flow model, the data from which was used to make predictions about stray grain formation. The experimental and calculated results both showed that the amount of stray grains in the welds increases with increasing weld power and generally increases with decreasing travel speed. These effects are attributed to an increased amount of constitutional supercooling that occurs in the liquid weld pool. The addition of fluid flow in the melt pool simulations revealed important effects on the preferred sites for stray grain nucleation and final distribution, based on micro structural observations.