Disentangling the Failure Response of Various Intermetallic Compounds with Surrounded Interface Formed During Laser-Weld-Brazing Process

This study elucidates the effect of surrounded interface intermetallic compounds (SI-IMCs), i.e., IMCs formed within the braze seam, on the mechanical performance and fracture behavior of the joints. Laser weld brazing (LWB) was performed on a Zn-coated Gen 3 Q&P980 steel to produce braze joints containing various SI-IMCs that were dependent on the heat input of the LWB process. SI-IMCs formed at a lower heat input (ΔHI_Relative range of 2.3–7.5 J/mm) significantly improved the mechanical performance, with a 91.7 pct increase in the peak load, a 319.84 pct increase in the displacement, and a 1200 pct improvement in toughness compared to the degraded properties observed in SI-IMCs formed at higher heat inputs (ΔHI_Relative range of 13–17.5 J/mm). Electron backscatter diffraction (EBSD) was used to examine the fracture microstructure and texture of the SI-IMCs and the Cu-rich matrix of the braze. Five types of SI-IMC structures with different morphologies and compositions were identified, each affecting crack behavior. Four distinct SI-IMC structures were found in the braze, noted as α, β, γ, and δ in this study. Fine α-, β-, and γ-SI-IMCs delayed failure, while coarse α- and δ-SI-IMCs accelerated failure. It was found that a strong {100} texture alignment between SI-IMCs and the braze matrix can enhance strain accommodation that improves crack resistance. A unitless GND.Ratio_(Cu/Fe) was introduced to measure dislocation density and to control joint toughness in the Cu-rich braze joints. At a GND.Ratio_(Cu/Fe) > 0.67 with high braze/SI-IMC texture alignment, a “SI-IMC-induced twinning” mechanism was activated, locally rotating the Cu-rich matrix orientation to < 112 > and forming twins near the crack propagation path, enhancing strain accommodation, and shifting the fracture mode from brittle to ductile, which improved the mechanical performance.