Origin of stray grain formation in single-crystal superalloy weld pools from heat transfer and fluid flow modeling

Stray grain formation in laser and electron beam welds on single-crystal alloy CMSX-4 was investigated through heat transfer and fluid flow simulations. The results were combined with a single-crystal growth model and stray grain calculations to investigate the influence of welding parameters on stray grain formation. Stray grain contents were also experimentally measured on laser and electron beam welds prepared over a wide range of parameters. The experimental and simulation results each demonstrate that stray grain content initially increases and then decreases with increasing travel speed. Increases in beam power produce an increase in the stray grain content. The results also demonstrate that restriction of growth along 〈1 0 0〉 directions decreases the magnitude of the temperature gradient and increases the growth rate along the dendrite growth direction (relative to the solidification interface normal). This effect promotes stray grain formation in the pool by increasing the extent of constitutional supercooling.