| Weld repair of single crystal (SX) Ni-base superalloys can be hindered by the nucleation and growth of stray grains which, when incorporated into the weld metal, do not share the crystallographic orientation of the substrate. Epitaxial growth from the substrate that preserves the SX structure can be promoted by reducing the amount of constitutional supercooling (CS), which is considered the primary root cause for stray grain formation. A large series of autogenous welds on SX superalloy CMSX-4 conducted over a wide range of welding parameters and processes were used to investigate the formation and behavior of stray grains. The quantity and location of stray grains in the welds was analyzed through the use of Orientation Imaging Microscopy (OIM) mapping techniques. The results indicate that constitutional supercooling and stray grain (SG) formation is reduced in low power, high travel speed welds. Because of the complex effect of travel speed on temperature gradient and solidification velocity, the worst conditions for SG formation can occur at intermediate travel speeds (∼ 6 mm/s). Both of these findings were corroborated with heat transfer/fluid flow modeling simulations that were coupled with stray grain predictions. These calculations indicated that SG formation will be greatest where different regions of dendrite growth intersect due to off-axis heat flow. SX substrate orientation controls SG content by controlling the amount and location of dendrite growth junctions along the solidification front. The OIM maps also revealed several characteristic locations for stray grains in the fusion zone, which suggests that the stray grain content is influenced by fluid currents and epitaxial growth. The presence of stray grains is a critical factor towards solidification cracking, the critical conditions for which have not been studied. The same solidification conditions that promoted stray grain formation were found to promote solidification cracking due to the high grain boundary area and high heat input. Direct metal deposition experiments also indicated that the weld geometry for a cladding procedure may assist in the elimination of solidification cracks even in the presence of stray grains. |
