Effect of heat treatment on interfacial microstructure and mechanical properties of the aluminum/steel joints with copper transition layer

In this study, aluminum/steel joints with a copper transition layer are successfully fabricated using wire-arc directed energy deposition (DED) based on the cold metal transfer (CMT) process. The influence of heat treatment on aluminum/steel joints is investigated, focusing on the evolution mechanisms of interfacial microstructures and the regulation of the mechanical properties. The interfacial compound (IMC) layer at the copper-aluminum interface is predominantly composed of CuAl, Cu₉Al₄, and CuAl₂. The transition layer above the IMC layer is composed of α-Al, CuAl₂, Al₇Cu₂Fe, and Al-Si eutectic phase. As the annealing temperature increases and the holding time prolongs, a substantial number of IMCs are generated within the copper-aluminum interface layer, leading to an augmentation in the interfacial layer thickness and an enhancement in the interfacial hardness. When the heat treatment temperature reaches 480 °C, the interfacial hardness increases to 447 HV_0.2, while the tensile strength decreases to 22.2 MPa. Conversely, with a solution temperature of 180 °C for 1 h, the tensile strength reaches 74.3 MPa, marking a 39 % improvement over the untreated aluminum/steel joint. The results provide guidance for the fabrication of aluminum/steel joints.