Microstructure formation mechanism of vacuum-brazed 6061/304 joint using Al–Si–Ge/Cu composite filler

6061 aluminum alloy was successfully vacuum brazed to 304 stainless steel using Al–Si–Ge/Cu composite filler metal. The thermodynamic model was established to analyze the formation mechanism of microstructure in brazed joint and element diffusion behavior between filler metal and substrate. The findings indicated that the microstructure of 6061 aluminum alloy/304 stainless steel joint was a multilayer structure composed of three zones (Zone I, Zone II and Zone III). The free energy ( \(\Delta G\) $\mathrm{\Delta }G$ ) calculation results indicated that Al–Si–M (M was Fe, Cr, Ni and Cu) ternary intermetallic compounds (IMCs) formed, when \(\Delta G\) $\mathrm{\Delta }G$ on M–Al side and M–Si/Ge side was similar. And only Al–M binary IMCs would be generated when there was large difference between \(\Delta G\) $\mathrm{\Delta }G$ on M–Al side and that on M–Si/Ge side. The calculation results of chemical potential of Si ( \({\Delta \mu }_{\text{Si}}\) ${\mathrm{\Delta }\mu }_{\text{Si}}$ ) and Ge ( \({\Delta \mu }_{\text{Ge}}\) ${\mathrm{\Delta }\mu }_{\text{Ge}}$ ) indicated that there was continuous Si and Ge diffusion toward Zone I, forming (Ge, Si) layer. The segregation of Si and Ge hindered the diffusion of Cr toward Zone II and promoted its diffusion toward (Ge, Si) layer, leading to an upward trend of Cr distribution in Al₇(Fe, Cr)₂Si layer. Negative \({\Delta \mu }_{\text{Ni}}\) ${\mathrm{\Delta }\mu }_{\text{Ni}}$ and \({\Delta \mu }_{\text{Fe}}\) ${\mathrm{\Delta }\mu }_{\text{Fe}}$ were responsible for continuous diffusion of Fe and Ni toward Zone II. The small difference between \({\Delta \mu }_{\text{Cu}}\) ${\mathrm{\Delta }\mu }_{\text{Cu}}$ in Zone I and Zone II contributed to distribution of CuAl₂ in Zone II. The formation mechanism of joint could be mainly divided into four steps.