Ni基単結晶超合金の表面溶融部の結晶成長に関する理論的解析

Abstract

Crystal growth behavior in the laser surface-melted region of Ni-base single crystal superalloy CMSX-4 have been investigated to clarify the effect of heat input on the conditions for achieving single-crystallization and/or preventing the formation of stray crystals. The crystal growth in the surface-melted region was numerically analyzed based on the geometric model and CET (columnar to equiaxed transition) theory in combination with an inverse heat conduction analysis. A theoretical analysis of the dendritic growth direction suggested that the surface-melted region consisted of [001], [100] and [010] dendrites, when the laser beam was scanned along the [100] or [110] direction on the (001) surface. Furthermore, it was predicted that the occurrence of the [001] dendrite could be observed at the bottom of the melted region and the [100] and [010] dendrites existed only at the upper part of the melted region. On the other hand, theoretical analysis of the stray crystal formation indicated that stray crystals tended to form under conditions of higher laser power and lower scanning speed, i.e. higher heat input conditions. These calculation results agreed with the metallographic observation. As the results, it can be said that the conditions needed to suppress the formation of stray crystals and to promote single-crystallization in the surface-melted region with epitaxial dendritic growth of [001] orientation and/or the dendritic growth of [100] and [010] orientations is predictable by calculation.