Interfacial Microstructures and Mechanical Properties of Aluminum Alloy 6061 and Steel Diffusion‐Bonded Joints Using a Cu Interlayer

This study investigates the bonding of Al 6061 alloy to mild steel facilitated by a copper (Cu) interlayer of varying thicknesses (50 and 100 μm) under controlled temperature (550–575 °C), pressure (0.2 and 3 MPa), and holding time (30–60 min) in a hydrogen atmosphere. Interfacial microstructure using optical microscopy and scanning electron microscopy reveal continuous interfaces with Cu effectively mediating elemental diffusion and phase evolution. The intermetallic compound (IMC) layer shows noticeable growth with increasing diffusion extent across the joint, primarily influenced by changes in Cu interlayer thickness and bonding pressure. Elemental composition using energy dispersive spectrometer analysis and X‐ray diffraction confirms Fe–Al and ternary Al–Fe–Cu phase formation, moderating brittle Fe–Al growth. At lower bonding pressures, a 50 μm Cu interlayer indicated 120 μm IMC thickness resulting in low hardness and 40 MPa shear strength by promoting ductile FeAl and suppressing brittle FeAl 3 /Fe 2 Al 5 phases on the steel side. A 100 μm Cu interlayer reduces strength to 32 MPa and increases hardness due to tongue‐shaped Fe 2 Al 5 formation despite thinner IMC (≈80 μm) at the interface. Higher bonding pressure increases IMC thickness and brittle phases, leading to higher interfacial hardness and reduced joint strength.