The effect of welding conditions on stray grain formation in single crystal welds - theoretical analysis

Stray grain formation during solidification of nickel-based single crystal superalloy welds leads to a degradation of mechanical properties and cracking. Based on the mechanism of constitutional supercooling ahead of the advancing dendritic growth front, the effect of welding conditions (weld speed and power) on the tendency to form stray grains during solidification was evaluated. A simple 3D thermal model was combined with a geometric model to determine the extent of stray grain formation as a function of welding conditions and position in the weld pool, taking into account the influence of dendrite growth orientation. A parameter describing the degree of stray grain formation, averaged over the entire solidification front, was calculated. Processing maps that show the severity of stray grain formation as a function of weld speed, power and orientation were developed. It was found that low power and high weld speed were optimal for minimizing the stray grain formation potential. The effect of crystallographic orientation of the weld on the overall tendency to form stray grains was minimal, although local variations based on orientation were quantified. The theoretical analysis was compared to earlier experimental work on laser welded nickel-based single crystals and all of the experimental observations could be reproduced in the calculations.