Multiscale Microstructural Evolution and Mechanical Properties of Copper–Iron Alloys in Steel Weld-Brazing

Abstract

Weld-brazing is an advanced joining technology characterized by low heat input and reduced susceptibility to fusion-welding defects such as cracking, distortion, and substrate microstructural degradation, enabling the production of reliable joints with improved metallurgical compatibility. Despite increasing industrial adoption for coated steels and dissimilar material systems, the fundamental relationships between processing conditions, joint characteristics, and mechanical performance remain insufficiently understood. This thesis presents a comprehensive multiscale investigation of the factors governing the mechanical properties and failure behavior of weld-brazed joints, with particular emphasis on bead geometry, interfacial characteristics, and microstructure. Advanced multiscale characterization techniques combined with thermodynamic analysis and numerical modeling were employed to establish robust process–structure–property relationships. The influence of heat source was first examined in laser- and arc-brazed ZnAlMg-coated steel, revealing distinct differences in particle formation, elemental distribution, and solidification morphology. Arc brazing produced a high number density of FeSi(Cu) particles accompanied by pronounced Zn and Cu concentration gradients, whereas laser brazing resulted in heterogeneous microstructures comprising eutectic structures, a dendritic transition zone, and a Cu-rich matrix. Following clarification of elemental redistribution and microstructural characteristics within the brazed bead and interfacial layer, this study further correlated particle characteristics with the local mechanical behavior of the joint. It was shown that increasing heat input during gas metal arc brazing promoted dilution of the filler material by the steel substrate, leading to the formation of a high number density of FeSi(Cu) particles. Cyclic load–depth indentation measurements demonstrated that these particles enhanced the local mechanical properties of the brazed bead and improved the strength of the joint under shear–tensile loading. Extending this analysis, a detailed multiscale investigation was conducted to elucidate the nature of particles formed within the brazed bead of Cu–Fe immiscible alloys. The results showed that hierarchical or homogeneous microstructures can be tailored through control of Fe dilution during non-equilibrium solidification. High Fe dilution promoted liquid-state phase separation and Marangoni-driven flow, resulting in hierarchical DO₃-ordered Fe-rich particles with embedded Cu-rich regions and uniformly distributed L1₂ nanoprecipitates, whereas lower Fe dilution resulted in finer, more uniformly dispersed Fe-rich particles and L1₂ phases. These microstructures enhanced local hardness and elastic modulus, revealing that particle number density plays a dominant role in the enhancement of mechanical properties. Finally, the combined effects of bead geometry, interfacial layer, bead microstructure, and loading conditions on joint strength and failure behavior were evaluated. The results demonstrated that joint mechanical performance is governed by the coupled interaction of these factors. In addition to weld-brazed joint characteristics, joint rotation and bending during loading were shown to control strain localization, indicating that bead characteristics alone are insufficient predictors of joint performance. Overall, this work establishes a process–structure–performance framework for weld-brazing, linking processing conditions to multiscale microstructural evolution and joint performance. The insights gained support the broader industrial adoption of weld-brazing in automotive lightweight structures and other advanced steel applications.