Britta Laux

Fast Epitaxial High Temperature Brazing of Single Crystalline Nickel Based Superalloys

A new high temperature brazing technology for the repair of turbine components made of single crystalline nickel based superalloys has been developed. It allows the repair of single crystalline parts by producing an epitaxially grown braze gap within very short times. In contrast to commonly used brazing technologies, the process is not diffusion based but works with consolute systems, particularly nickel-manganese alloys. Brazing experiments with 300 μm wide parallel braze gaps, as well as V-shaped gaps with a maximum width of 250 μm, were conducted. Furthermore, thermodynamic simulations, with the help of THERMOCALC software, Version TCR, were carried out to identify compositions with a suitable melting behavior and phase formation. With the new alloys complete, epitaxial bridging of both gap shapes has been achieved within brazing times as short as 10 min.

Development of Ni-Mn-based alloys for the fast epitaxial braze-repair of single-crystalline nickel-based superalloys

Abstract Diffusion brazing is a widely-used technology for the repair of cracks in hot section turbine components, mostly fabricated from nickel-based superalloys. However, the filling of wide cracks in the range of 100–300 μm is difficult since the precipitation of brittle secondary phases, which are formed by the conventionally used melting point depressants B and Si, leads to deteriorating mechanical properties. Therefore, new Ni – Mn-based braze alloys were developed which allow a very fast epitaxial healing of particularly wide cracks in single-crystalline components. As B and Si are replaced by Mn, the repair process can be significantly shortened since the epitaxial solidification is not completely controlled by diffusion but can also be controlled by cooling. Ni – Mn-based systems enhanced by Al, Cr and Ti were investigated. In this work an improved brazing cycle for the minimization of porosity within the braze gap as well as an enhanced heat treatment, which produces a γ/γ′ microstructure very similar to the parent material, are presented. Results from tensile tests at room temperature and at 900 °C conducted on 300 μm gap width samples are discussed.