The effect of single crystal and welded substrates on the development of braze microstructures

To emulate combined welding and brazing techniques, frequently used during the rejuvenation of turbine components, TIG and SWET weld overlays have been applied to the surfaces of a single crystal nickel-based alloy and subsequently overlain with braze and subjected to a typical brazing cycle heat treatment. Both welds comprised of columnar dendritic microstructures with an epitaxial [001] growth orientation near the substrate surface and equiaxed randomly oriented grains at the weld surface. While the TIG weld showed a sharp columnar to equiaxed transition, that for the SWET weld was diffuse and comprised colonies of columnar structures with their [001] misoriented to the epitaxial columnar directions as a result of shallower temperature gradients. Analysis of the microstructures of the brazed materials showed that the extents of boron penetration were significantly greater for the welds due to enhanced diffusion along grain boundaries between the equiaxed structures compared to y channel limited transport in the single crystal . A striking feature of the diffusion zone microstructures was the development of carbo-boride needles which formed by the reaction of boron, refractory metals and TaC. It is the release of Ta from TaC which stabilises y’ envelope formation around the needles. The post-brazed microstructures of TIG welds used in this study showed considerably fewer detrimental boride precipitates than brazed SWET welds and as a result are expected to show superior mechanical properties.