Stray Grain Formation in Welds of Single-Crystal Ni-Base Superalloy CMSX-4

Autogenous welds on the single-crystal (SX) alloy CMSX-4 were prepared over a wide range of welding parameters and processes to investigate the formation and behavior of stray grains (SGs). The quantity and location of SGs in the welds were analyzed by orientation imaging microscopy (OIM). Heat- and fluid-flow modeling was conducted to understand the influence of welding parameters on the local solidification conditions and resultant SG formation tendency. The results indicate that constitutional supercooling and SG formation are generally 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 in alloy CMSX-4 for the conditions examined here occur at intermediate travel speeds of ~6 mm/s. These findings were corroborated with heat-transfer/fluid-flow modeling simulations that were coupled with SG predictions. These calculations also indicate that SG formation will be greatest where different regions of dendrite growth intersect, due to the so-called off-axis heat flow. For a given set of welding conditions, the amount of SGs will also vary with substrate orientation. This effect is attributed to differences in the number and location of dendrite growth intersection regions within the melt pool that occur with changes in substrate orientation.