Microstructure, Residual Stresses, and Strain-Rate-Dependent Deformation and Fracture Behavior of AISI 304L Joints Brazed with NiCrSiB Filler Metals

Johannes L Otto,Lars Gerdes,Kerstin Möhring,Milena Penyaz,Alexander Ivannikov,Thorge Schaum,Frank Walther,Boris Kalin,Anke Schmiedt-Kalenborn

doi:10.3390/met11040593

Johannes L Otto, Lars Gerdes + Show 7 more

Open Access

https://doi.org/10.3390/met11040593

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Abstract

The knowledge of alloy–process–structure–property relationships is of particular interest for several safety-critical brazed components and requires a detailed characterization. Thus, three different nickel-based brazing filler metals were produced with varying chromium and molybdenum content and were used to braze butt joints of the austenitic stainless steel AISI 304L under vacuum. Two holding times were used to evaluate diffusion-related differences, resulting in six specimen variations. Significant microstructural changes due to the formation and location of borides and silicides were demonstrated. Using X-ray diffraction, alloy-dependent residual stress gradients from the brazing seam to the base material were determined and the thermal-induced residual stresses were shown through simulations. For mechanical characterization, impact tests were carried out to determine the impact toughness, as well as tensile tests at low and high strain rates to evaluate the strain-rate-dependent tensile strength of the brazed joints. Further thermal, electrical, and magnetic measurements enabled an understanding of the deformation mechanisms. The negative influence of brittle phases in the seam center could be quantified and showed the most significant effects under impact loading. Fractographic investigations subsequently enabled an enhanced understanding of the fracture mechanisms.

Highlights

One of the main goals of engineering is to prevent the failure of components during their operation
The exact knowledge of these properties is necessary to ensure structural integrity throughout the whole operational life of an aircraft or helicopter. This is true for gas turbine engine blades [3] and other turbine components that can be made of corrosion-resistant austenitic steels and joined by high-temperature brazing based on transient liquid phase bonding (TLP bonding) with various types of filler metals, including nickel-based ones [4]
The brazing seam can microstructurally be divided into three main zones [27,28]: the isothermal solidification zone (ISZ), the athermal solidification zone (ASZ), and the diffusion zone (DZ)

Summary

Introduction

One of the main goals of engineering is to prevent the failure of components during their operation. High strengths must be achieved for constructions that require high resistance against failure. The materials used for the latter require high strength and resistance to impact loads, which can be caused by bird strikes or hail [2]. The exact knowledge of these properties is necessary to ensure structural integrity throughout the whole operational life of an aircraft or helicopter. This is true for gas turbine engine blades [3] and other turbine components that can be made of corrosion-resistant austenitic steels and joined by high-temperature brazing based on transient liquid phase bonding (TLP bonding) with various types of filler metals, including nickel-based ones [4]

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