Transient liquid phase bonding of commercially pure iron using Cu and Au–12Ge interlayer

Commercially pure Fe was diffusion bonded using Cu (25 μm) and Au–12Ge eutectic interlayer (100 μm). At 1000–1090 °C bonding temperature and 10–30 h bonding time, Cu interlayer did not diffuse completely into the base metal. The compositions of residual interlayer and base metal at the interface were limited by the solubility range of ɛ-Cu and γ-Fe. Residual Cu interlayer was also observed after bonding at 1100 °C for 3–4 h. No residual Cu was found after 5 h bonding time at 1100 °C; however, porous microstructure was observed along the bond centerline. Cu diffused predominantly along austenite grain boundaries in all bonding conditions. Fe was diffusion bonded using Au–12Ge interlayer at 900–990 °C for 1–15 h. At 900 °C for 1 h the residual interlayer thickness was 12.6 μm; the thickness decreased to 6 μm when bonding time was increased to 10 h. Fingerprint-like microstructure was observed in the residual interlayer. At 920 °C for 15 h, the interlayer completely diffused into the base metal in some areas and was retained in some areas. The thickness of the residual interlayer was 1.25–3.8 μm. Dispersed Au-rich particles were observed in the base metal near interface. The highest ultimate tensile strengths obtained for the bonded Fe were 291 ± 2 MPa using a Cu interlayer at 1030 °C for 10 h and 315 ± 4 MPa using a Au–12Ge interlayer at 950 °C for 15 h.