Mechanistic understanding of enhanced mechanical strength of Type 304 stainless steel brazed joints

: microscopic and macroscopic considerations

  • Yao Li

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

High-temperature brazing is a joining technique being widely applied in the fabrication of engine components for the aerospace and automotive industries. Experimental evidences showed that brazed joints had enhanced mechanical strength when compared to the bulk filler metal. The strength enhancement was almost exclusively attributed to the stress triaxiality induced by the mechanical constraint of the base metal. In other words, strain heterogeneity would present in a brazed joint and is always associated with the enhanced mechanical strength. However, previous investigations provided neither a quantitative evaluation of stress triaxiality nor the influence of joint microstructures. Thus, the strengthening mechanism of brazed joints deserve further investigation and experimental verification.

The aim of this research is to identify the strengthening mechanism of brazed joints and quantify the effect of joint microstructure and mechanical constraint. The primary objectives are: 1) to determine the fracture strength and fatigue life of the Type 304 stainless steel brazed joints processed by pure copper; 2) to characterise the microstructure of the brazed joint and evaluate its contribution to the overall joint strength; 3) to quantify the stress triaxiality level and to estimate the influence of mechanical constraint; 4) to provide an experimental evidence for the presence of mechanical constraint. Key findings are summarised as follows.

Firstly, the mechanical strength of brazed joints as a function of the joint interface roughness was determined through uniaxial tensile and fatigue testing. This was to investigate whether the joint mechanical strength could vary with different interface roughness conditions. Key findings have indicated that the brazed joints showed enhanced mechanical strength when compared to the filler metal. In addition, the interface roughness levels had little influence on the mechanical strength. This is because all the brazed joints failed entirely within the joint centre (i.e. inside the filler metal) rather than at the interfacial region, as revealed by SEM based fractography study.

Secondly, microstructural characterisation has revealed a two-phase microstructure within the joint region: the star-shaped Fe-Cu-rich precipitates and the copper matrix. Theoretical evaluation of the collected microstructural data has suggested that Cu-Mn solid-solution dominated the overall strengthening, whereas contributions from precipitation hardening as well as grain-size strengthening were negligible.

Finally, the mechanical constraint was revealed by comparing the fracture strengths of two identical joints but with their interfaces orientated at either 90° or 45° with respect to the loading direction. The 45° joint configuration had a lower fracture strength as compared to the 90° counterpart, as a result of the reduced mechanical constraint level. The Bridgman necking criteria was then applied to derive the longitudinal flow stress at sample fracture for the 90° brazed joint. The discrepancy between the theoretically calculated and experimentally determined strengths was judged as the influence of mechanical constraint. Thus, the enhanced mechanical strength of brazed joints is a concurrent consequence of (i) microscopic Cu-Mn solid-solution strengthening and (ii) macroscopic mechanical constraint.

In addition, geometrically necessary dislocation (GND) distribution was mapped by using electron backscatter diffraction (EBSD). The pile-up of GNDs was observed at the base-filler metal interface for the 90° joint. This observation suggests that GNDs were introduced to accommodate deformation incompatibility imposed by the mechanical constraint. This finding is thus considered as an experimental (microscopic) evidence for strain inhomogeneity due to the presence of mechanical constraint.
Date of AwardJul 2020
Original languageEnglish
Awarding Institution
  • Coventry University
SponsorsCoventry University
SupervisorXiang Zhang (Supervisor), David Parfitt (Supervisor) & Bo Chen (Supervisor)

Cite this

Mechanistic understanding of enhanced mechanical strength of Type 304 stainless steel brazed joints: microscopic and macroscopic considerations
Li, Y. (Author). Jul 2020

Student thesis: Doctoral ThesisDoctor of Philosophy