High-reliability repair of single-crystal Ni-base superalloy by selective electron beam melting

Here, we report the utilisation of a powder-bed-fusion additive manufacturing to achieve the CMSX-4 single-crystal repair with René N5 powders. The single-crystal repair is characterised by a fine columnar dendritic substructure, limited in-grain misorientation over tens of millimetres in height, and strong deposit-to-substrate bonding. The brighter contrast region of 300 to 400 μm thick, formed by the cellular solidification front, sits above the melted back region with a horizontal low-angle boundary (4° misorientation) in-between. Its higher hardness of 450 HV is attributed to the fine cuboidal primary γ′ of 135 ± 7 nm. Their presence is a combined effect of high cooling rate and limited local compositional difference. The Scheil-Gulliver predicted solute partitioning of Ta and Hf to the inter-dendritic region agrees well with the MC carbides appearing at the columnar dendritic and cellular boundaries. Computational-fluid-dynamics (CFD) simulations, combined with the minimum undercooling criterion, helps to determine the vertical distance between the melt-pool bottom and the oriented-to-misoriented transition borderline, providing a convenient guideline to assess the single-crystal likelihood, in a sense that this distance needs to be larger than the hatch depth. The CFD-calculated solidification velocities and thermal gradients agree with the columnar-to-equiaxed transition curve and the experimental observation.