Enhancing the microstructure and mechanical performance of laser weld-brazed aluminum alloy/steel joints by introducing a multi-element interlayer

• A multi-element interlayer of Al 80 Zn 14 Li 2 Mg 2 Cu 2 was designed to enhance the joining of aluminum alloy and steel. • The interlayer reduced the reaction layer thickness from 11 µm to 6 µm and promoted uniform, less brittle phase formation at the interface. • The joint tensile strength increased from 91 MPa to 181 MPa, and the fracture mode changed from brittle to ductile. • Lattice mismatch at the interface decreased from 66.21 % to 5.97 %, improving bonding strength and joint toughness. Replacing steel with aluminum alloys is increasingly important in the automotive industry. However, the formation of Fe-Al intermetallic compounds (IMCs) in welded joints weakens their strength and limits practical applications. In this study, we demonstrate that using an Al 80 Zn 14 Li 2 Mg 2 Cu 2 multi-element interlayer significantly enhances the quality and performance of laser weld-brazed Al alloy/steel joints. The maximum tensile strength of joint reaches 181 MPa, which is 90 % higher than that of direct welding without an interlayer. This improvement is attributed to the reduced formation of brittle η-Fe 2 Al 5 and θ-Fe 4 Al 13 phases, and the newly formed interface structure consisting of ƞ-Fe 2 Al 5-x Zn x , α-Al, and θ-Fe 4 Al 13 phases. Three-point bending tests reveal improved joint plasticity, with no cracks observed at the maximum bending angle of 106°. Furthermore, the addition of the interlayer alters both the fracture mode and location: the tensile fracture shifts from a brittle failure at the IMC layer/steel interface to a ductile failure in the aluminum alloy’s heat-affected zone (HAZ). Interface analysis shows that the lattice mismatch of the ƞ-Fe 2 Al 5-x Zn x /Fe interface is 5.97 %, significantly lower than the 66.21 % mismatch at the η-Fe 2 Al 5 /θ-Fe 4 Al 13 interface. This indicates that cracks preferentially accumulate at the η-Fe 2 Al 5 /θ-Fe 4 Al 13 interface, leading to joint failure.