Development of microstructures in Fe−15Ni−15Cr single crystal electron beam welds

A detailed analysis of the microstructures produced in an autogenously welded single crystal of Fc−15Ni−15Cr was performed in order to investigate the relationship between growth crystallography and solidification behavior. Electron beam welds were made at various speeds on the (001) surface of single crystals in either the 100] or 110] directions. A geometrical analysis was carried out in order to relate the dendrite growth velocities in the three 〈100〉 directions to the weld velocities for the different crystallographic orientations examined. From this analysis, the preferred dendrite trunk directions were determined as a function of the solidification front orientation based upon a minimum velocity or minimum undercooling criterion. A thorough examination of the weld microstructures and a comparison with the geometrical relationships developed in this work permitted a three-dimensional reconstruction of the weld pool shape to be performed. In addition, the dendrite spacings were measured, and the variation in spacings as a function of growth velocity was compared with theoretical predictions. It was found that the range of velocities over which dendritic growth is expected agreed with the experimental findings, and, furthermore, the change in dendrite spacing with growth velocity varied as predicted by theory. These results clearly demonstrate the effect of crystallography on the micro-structural development during weld pool solidification. The results also show that the resultant microstructures and pool shapes can be explained by geometrical analysis in conjunction with existing solidification models. M. R{upappaz}, formerly Visiting Scientist with the Metals and Ceramics Division, Oak Ridge National Laboratory, is permanently affiliated with the Laboratoire de Métallurgie Physique, Ecole Polytechnique Fédérale de Lausanne