Alloying- and microstructure-based fatigue life characterisation and prediction of vacuum brazed AISI 304L/NiFeCrSiB joints in corrosive environments

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The characterisation of the fatigue life of brazed joints under superimposed corrosive loads is of major importance for many industrial applications, such as turbine components or exhaust gas recirculation coolers, and will help to improve the reliable and economical operation of these. Therefore, the knowledge of alloy-process-structure-property relationships for different corrosive environments in terms of chemical composition and temperature is of great interest. Thus, the project aims to generate an extensive characterisation of the alloying-dependent corrosion fatigue mechanisms of nickel-based brazed joints. An artificial neuronal network modelling approach for brazed joints will be applied and trained with destructive testing results, microstructure investigations, and non-destructive measurements to enable lifetime predictions considering the complex superimposition of environmental influencing parameters. This approach will further be used to calculate optimized alloying compositions of the filler metals regarding the iron and molybdenum content. Based on this calculated modifications, optimised specimens will be produced and tested iteratively for further characterisation. Thus, it will be evaluated whether an improvement of corrosion fatigue properties through a well-adjusted molybdenum content can be reached. Further, an understanding of the influence of iron in nickel-based alloys will be gained. Since microstructure analysis is the key element for an understanding of corrosion fatigue properties, the influence of both elements on the formation of brittle phases will be determined by EDX and EBSD scans. In addition, the effect of alloying elements on residual stresses will be investigated at high resolution. This allows the analysis of different brazed zones regarding correlations to crack initiation and propagation in relation of fatigue properties. Eddy current and DC potential drop measurements will be applied additionally to allow non-destructive conclusions about brazing defects and a general statement about the joint quality. The combination, adaption and further development of these selected analysis and testing methods aims to enable a detailed understanding of alloy-process-structure-property relationships for brazed joints even beyond the corrosion fatigue behaviour, and new approaches for brazing shall be transferable to other alloy systems and application conditions.