Effects of substrate crystallographic orientations on crystal growth and microstructure development in laser surface-melted superalloy single crystals. Mathematical modeling of single-crystal growth in a melt pool (Part II) Original Research Article

The mathematical model developed for single-crystalline solidification in laser surface melting (LSM) described in Part I [Acta Mater 2004;52:4833–4847] was used to compute the dendrite growth pattern and velocity distribution in the 3D melt pool for various substrate orientations. LSM experiments with single-crystal nickel-base superalloys were conducted for different orientations to verify the computational results. Results show that the substrate orientation has a predominant effect on crystal growth pattern, and simultaneously influences the magnitude and distribution of dendrite growth velocity in the melt pool. The selected 〈1 0 0〉 growth variants and the number of the chosen growth variants are dependent on the substrate orientation. The maximum velocity ratio (dendrite growth velocity over the beam velocity, V/Vb) in the melt pool is a function of melt-pool geometrical parameters and the substrate orientation. The largest maximum velocity-ratio among the symmetric orientations is 1.414 for the (0 0 1)/[1 1 0] and image orientations, while that value for asymmetric orientations is 1.732 for the image orientation. Good agreement was obtained between the predicted and experimentally observed microstructures. The results are discussed with the susceptibility to stray grain formation as a function of substrate orientations and melt-pool geometrical parameters. These findings have some important implications for single-crystal surface processing.