Tailoring intermetallic compound formation within laser weld brazed joints using a novel heat input approach

Abstract

The challenge of intermetallic compound (IMC) embrittlement, resulting from thick IMC layers at the braze/substrate interface, and the lack of a clear strategy to manipulate IMC formation, has hindered the development of reliable laser weld brazing (LWB) processes. This study addresses these challenges by introducing a novel approach to IMC manipulation during LWB of thin-gauge Zn-coated steel with Si-bronze filler on a double-flanged lap joint. By shifting IMC formation from the interface towards the interior region of the braze, the research mitigates embrittlement by developing a new IMC category, termed surrounded interface-IMCs (SI-IMCs), distinct from traditional interface-IMCs (I-IMCs). The study proposes a Wire-adjusted heat input strategy to optimize brazing conditions, introducing a relative heat input equation (HI_Relative) that correlates with various brazing defects and IMC formation. The generic scientific contribution of this work lies in identifying a critical HI_Relative value of 32 J/mm for defect-free brazing, with an additional threshold of 12.44 J/mm above this level to promote a high density of SI-IMCs, occupying up to 38.2 ± 16.9 % of the braze cross-sectional area. These SI-IMCs, characterized by a shell-like Fe-Si layer and a bulky (Fe-rich)-Cu eutectic phase, enhance the mechanical performance of the brazed joints. Furthermore, this study reveals the novel role of Mn segregation in creating diffusion channels for Fe-Si IMC development, advancing the scientific understanding of IMC formation. Visualization through digital image correlation (DIC) during tensile testing showed that increasing the SI-IMC area fraction from 1.2 ± 2.4 % to 38.2 ± 16.9 % resulted in a 14 % increase in tensile peak load and a 350 % increase in ductility. This highlights the critical role of SI-IMCs in improving the strength and ductility of LWB joints, offering a new pathway for enhancing the performance of brazed structures.