Analysis of stray grain formation in single-crystal nickel-based superalloy welds

The formation of stray grains during weld solidification of Rene N5, a single-crystal nickel-based superalloy, was studied. Experimental laser and electron-beam welds showed the extent of stray grain formation was sensitive to the welding conditions. It was also found that cracking is associated with the presence of stray grains, and cracks follow along the stray-grain high angle boundaries. Modeling was carried out to investigate the mechanism of stray grain formation and to predict the extent of stray grains as a function of welding conditions and location within the weld. The effect of crystallographic orientation was also taken into account. It was found that the mechanism of constitutional supercooling for stray grain formation explained all of the experimental results. Modeling based on this mechanism indicated that welding conditions would have a very important influence on the extent of stray grain formation while crystallographic orientation had only a minor influence.