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.

Advancement in Fast Epitaxial High Temperature Brazing of Single Crystalline Nickel Based Superalloys

“…Elements such as phosphorus, hafnium and zirconium are also possible to lower the melting point in nickel based materials, however, the use of these elements is of lesser extent [16]. It is observed that an epitaxial bridging of braze gaps in single crystalline alloys can be achieved within long hold times in the range of several hours in combination with high temperatures.…”

“…In conventional transient liquid phase bonding (TLP) technologies usually boron and/or silicon are used as melting point depressants [7,16]. Elements such as phosphorus, hafnium and zirconium are also possible to lower the melting point in nickel based materials, however, the use of these elements is of lesser extent [16].…”

“…The most serious problem of the current technologies are the very long hold times that are necessary, since the solidification is diffusion controlled [4,[12][13][14][15][16]. However, silicon and boron have a very low solubility in the nickel matrix, hence, a complete diffusion out of the braze gap is essential for isothermal solidification.…”

Transient Liquid Phase Bonding of Pairings of Parent Superalloy Material with Different Composition and Grain Structure

“…The crystal orientation of the solidified joint will equal the parent material, because the liquid will grow epitaxially at the solid-liquid interface, and the nucleation of new grains in front of the solidification front is suppressed. There is one problem reported in literature [4][5][6][7] regarding boron as MPD: As the solubility of boron in nickel-base alloys is very limited, brittle borides precipitate in the parent alloy around the joint during the brazing process. These precipitations will deteriorate the mechanical properties of the joint [8].…”

Mechanical Properties of New Ni-Mn-Based Braze Alloys for the Fast Epitaxial Braze Repair of Single-Crystalline Ni-Base Superalloys

“…However, as the whole solidification runs diffusion controlled, the epitaxial repair of wide cracks in the range of several hundred microns requires very long hold times. If the temperature is lowered before the epitaxial solidification is completed, the poor solubility of B within Ni leads to the formation of brittle secondary phases serving as nucleation sites for stray grains and therefore deteriorating mechanical properties [4,5]. Thus, a completely different approach for the development of high-temperature braze alloys has been chosen: B as MPD has been replaced by Mn since the Ni-Mn-system provides almost a complete miscibility.…”