A Study of the Diffusion Brazing Process Applied to the Single Crystal Superalloy CMSX-4

Abstract

The requirement for economic reconditioning of the latest ALSTOM gas turbine generation with Single Crystal (SX) superalloys has lead to the development of advanced repair processes such as Diffusion Brazing or Transient Liquid Phase bonding. Diffusion Brazing (DB) of conventionally cast polycrystalline turbine components has been carried out for many years but the requirement for this joining and repair technique to be applied to DS and SX superalloys has emerged only more recently. The main concern for the use of a braze-repair process for the more highly loaded SX components is the ability to guarantee sufficient thermal and mechanical integrity throughout the component lifetime. Such high strength braze joints in SX superalloys can be achieved by combining a brittle phase-free and high γ′ content microstructure, while maintaining the crystallographical orientation of the SX parent material within the repair zone. Prior to the brazing process, a suitable crack surface preparation is essential, and this is achieved by the employment of specifically optimized Fluoride Ion Cleaning (FIC) process. This guarantees the complete removal of oxide from the crack surfaces and promotes the flow of the braze alloy for complete filling down to the crack tip. This paper presents the development of the DB process which has been specifically tailored for the repair of SX superalloys. The principles of the diffusion brazing process as applied to the CMSX-4 superalloy are discussed and the parameters which control the brazing kinetics are outlined. The optimization of the brazing heat treatment cycle will be presented. This paper also demonstrates the retention of the single crystal micro-structure in the repair zone, and demonstrates the test procedures developed to achieve the required thermal and mechanical integrity of braze repairs for application in SX gas turbine components.