Fabricating bimetallic structures (BmSs) to reduce weight and improve performance has challenges in automobile, infrastructure, and aerospace applications. This study investigates the effect of heat input, Q_H, on interfacial properties of three different wire-arc additive manufacturing (WAAM)-based BmSs, SS316L-SS308L, EN31–AA4043, and SS316L–In718, through microstructural and mechanical characterisations. The value 250-400 J/mm is the determined heat input range for SS-SS and SS-Inconel, and for steel–aluminium, the range is 35-60 J/mm. At the interface of SS316L-SS308L BmS, both the austenitic (γ) and delta-ferrite (δ-Fe) phases formed. The higher tensile strength and elongation reached 591 MPa and 37.2 %, respectively, at an optimum Q_H of 330 J/mm, as the composition of the interface was close to the mirror composition with filler wire. While the average micro-hardness achieved at the interface is 248.3 HV, due to δ-α phases, the interface hardness is enhanced. But for the SS316L–In718 interface, the formation of IMCs (FeNi and FeNi₃) is proportionally influenced by the variation in Q_H. The SEM-EDX analysis demonstrated the enhancement of interface thickness (IT) and elongation while tensile stress was reduced (optimum: 542 MPa) with increasing Q_H. The average micro-hardness value reduced (194.7 to 174.6 HV) with increasing QH due to the coarse grain structure. Conversely, the optimum Q_H was achieved at 43.55 J/mm for the EN31-Al4043 interface, and the SEM and XRD analyses revealed the brittle binary (Fe-Al) and ternary (Al-Fe-Si) IMC formations at the interface. Under optimal conditions, minimal IT (<4 µm), considerable tensile strength (73.2 MPa), very little elongation (~0.9%), and an average micro-hardness value of 128.1 HV were achieved. This analysis highlights heat input as a crucial factor for developing tailored BmSs using WAAM as it controls interface properties. To develop a multi-material high-performance structure, process parameters should be optimized.