Effect of substrate orientation on the columnar-to-equiaxed transition in laser surface remelted single crystal superalloys

For a successful repair of single-crystal (SX) components, an epitaxial growth of columnar dendrites is required, i.e. a columnar to equiaxed transition (CET) has to be avoided. In this study it was found that changing the angles of laser treatment with respect to the standard orientation [1 0 0](0 0 1) can have a strong and positive effect on the quality of the part. By rotating the SX substrate around x-, y-, and z-axis, which coincide with the [1 0 0], [0 1 0], and [0 0 1] crystallographic directions, respectively, the effect of the substrate orientation for different rotation angles on the columnar-to-equiaxed transition (CET) in laser remelting process was determined. It was found that from the [1 0 0] (0 0 1) initial state, the CET varies strongly with the substrate orientation when the rotation is made around the y-axis, whereas there is nearly no change when the rotation is performed around the x- or z-axis. This drastic difference is caused by the fact that for rotation around the y-axis by 45° the melt pool contains no intersection point where dendrite domains with three preferred 〈1 0 0〉 growth directions meet. For the rotation about the x or z-axis, at least one intersection point appears. The intersection point is the most vulnerable place for the CET since the thermal gradient tends to reach its minimal value there. The number and location of the intersection points jointly control the overall CET tendency. Reducing those points and/or moving them to the positions with high thermal gradient are beneficial for avoiding CET generally. Our results clearly demonstrate that the laser processing window for the repair of the single crystal superalloy can be enlarged if the substrate orientation is rotated correspondingly.