Alloying- and microstructure-depended fatigue life characterisation and improvement of vacuum brazed AISI 304L joints with Ni-based filler metals in corrosive environments

• Applicant: Professor Dr.-Ing. Frank Walther
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Joining and Separation Technology
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 408904168

The project focuses on unravelling the complex relationships between alloy, process, microstructure and properties of high-temperature vacuum-brazed joints of metastable austenite under superimposed stresses such as corrosion fatigue. NiCrSiB filler metals with additions of Fe and Mo, which are varied together with the Cr content, are investigated. Depending on the manufacturing parameters and specimen geometry, the melting point depressant elements B and Si lead to the formation of intermetallic brittle phases in the brazed joint, such as Cr-rich borides in the diffusion zone or Ni-rich silicides in the center of the brazed joint. The formation of these intermetallic brittle phases mostly dominates the failure mechanisms under corrosion fatigue, since both the formation of microgalvanic elements and microstructural stress concentrations occur, which promote crack initiation and propagation under fatigue. Whereby the effect depends on the stress amplitude. The phase formation varies strongly with the alloy composition and the process parameters. Therefore, image segmentation is used to calculate characteristic values for the microstructures, which consider the size, number, location and morphology of the brittle phases and thus allow a statistical evaluation and estimation of the corrosive mechanical properties. KPFM measurements will also be performed to understand the effects of the filler composition variations on the surface potentials, which are important for the corrosion mechanisms. The aim of the project is to improve the corrosion fatigue properties through a tailored microstructure design. This will be achieved by targeted adjustments to the alloying elements of the filler metal and on the other hand by extended holding times to reduce the fraction of brittle phases by diffusion processes. Since this results in undesirable grain growth, a post-process will be developed and used to reduce the grain size again without damaging the brazed joints. Simulations with FEM will be used among others to gain a detailed understanding of critical stress concentrations. The knowledge gained and methods developed will contribute to improve the service life of brazed components and thus contribute to resource efficiency.

• DFG Programme: Research Grants