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In this work, the interface characteristics and resulting bond strength were investigated for roll bonded steel-aluminum composites with nickel interlayers, both after rolling and after post-rolling heat treatments at 400 °C−550 °C. After rolling, only mechanical interlocking was achieved between the steel and nickel layers, which resulted in delamination. Post-rolling heat treatments resulted in sufficient metallurgical bonding between the steel and nickel layers, and a significant increase in the bond strength. An intermetallic phase layer formed during the heat treatments, which below 500 °C consisted of Al₃Ni and above, Al₃Ni and Al₃Ni₂. With increasing temperature and time, the Al₃Ni₂ phase consumed the Al₃Ni layer, voids developed along the Al₃Ni₂-aluminum interface, and a duplex morphology developed inside the Al₃Ni₂ layer, in accordance with the Kirkendall effect. The highest bond strength was obtained for the composites that only had an Al₃Ni layer along the interface, and the optimal thickness was found to be 3−5 µm. The bond strength decreased with increasing temperature and time, due to increasing Al-Ni layer thickness, an increase in the fraction of Al₃Ni₂ relative to Al₃Ni, and the development of voids. The results show that nickel can be used as an interlayer in steel-aluminum joints, and a high bond strength can be obtained through post-rolling heat treatments.