Effects of melt-pool geometry on crystal growth and microstructure development in laser surface-melted superalloy single crystals. Mathematical modeling of single-crystal growth in a melt pool (part I)

The effects of melt-pool geometrical parameters on crystal growth and microstructure development during laser surface melting of single-crystal alloys were studied by means of mathematical modeling and experiments. A mathematical model was developed for the three-dimensional (3-D) melt-pool geometry and single-crystalline melt-pool solidification in laser surface melting. The 3-D meltpool geometry corresponding to the solidification interface is described by four geometrical parameters (w, l, h, a). The model was used to study the effects of variations in the geometrical parameters on crystal growth and microstructure development in the melt pool. Laser surface melting experiments with single-crystal nickel-base superalloys were conducted to verify the computational results of microstructure development in the melt pool. Results indicate that the melt-pool geometrical parameters have profound influences on the dendrite growth velocity and growth pattern in the melt pool. For the (001)/[100] substrate orientation, variations in l/w and a can influence both the number and the relative sizes of growth regions while the variation in h/w can only influence the relative sizes of the growth regions. Unidirectional dendrite growth along the [001] crystallographic direction can be achieved for an a value of 45 or below. The maximum ratio of dendrite-growth velocity to the beam velocity in the melt pool is related to a and l/w. Experimental microstructure observations agreed well with the computational results. These findings show that the desired dendrite growth velocity and microstructure can be obtained through proper control of the 3-D melt-pool geometry. 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.