###### Abstract  
The effects of nickel coating and heat treatment on the interfacial bonds of aluminum-iron (Al/Fe) alloys hybrid structures were investigated using microstructural analysis. The application of a nickel coating successfully suppressed the formation of defects such as gaps and oxide scale, improving the physical bonding of the interface. Optimizing the heat treatment conditions generated superior chemical bonding at the interface and facilitated the formation of a nickel-bearing phase in the Al matrix. Also, the types of nickel-bearing phase were influenced by solution treatment and proximity to the interface. By analyzing the isopleth phase diagram of the aluminum system for the ranges of nickel present in the Al, it was confirmed that the Ni:Cu ratio affected the precipitation characteristics of the system. However, when heated under conditions that were optimized for chemical bonding, the Al matrix decreased by approximately 40% (from 100 HV to 60 HV), due to grain growth. The effect of artificial aging increased the hardness of the Al matrix away from the interface by 35% (from 63 HV to 90 HV). On the other hand, this did not occur in the Al matrix near the interface. These results indicate that the nickel that diffused into the Al matrix interfered with the precipitation hardening effect. nickel coating; nickel-bearing intermetallic phases; heat treatment; diffusion-bonding; interfaces; microstructure +
Footnote †: journal: Applied Sciences  
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of the two metals, such as the melting points and thermal expansion coefficients, induce poor wettability [5; 6; 7; 11; 12]. Therefore, a new process that overcomes these issues must be developed to achieve successful bonding. The intermediate layer between ferrous and aluminum alloys has played a role in mitigating oxidation and improving joint performance, according to much previous research. The formation of the intermediate layer which contributes to obtaining an excellent bonding has been carried out by the hot dipping method. Coating materials have been selected from zinc, aluminum, and copper [2; 5; 7; 11; 13; 14; 15]. These coatings can effectively prevent direct interaction between steel and aluminum alloys and avert the oxidation of steel during the preheating stage, thus improving bonding [5; 6; 7; 13; 14]. There are few studies, however, on the effects of nickel coatings. Nickel coatings can enhance the corrosion resistance of metal objects [16; 17], and nickel atoms can also augment the mechanical properties of aluminum alloys at elevated temperatures [18; 19; 20] due to their \"impurity atom\" behavior. Our research aimed to utilize the unique characteristics of nickel, with the aim of improving the interfacial bonding and mechanical properties of Al/Fe alloys hybrid structures. In this work, we applied a nickel coating on an Al/Fe alloys hybrid structure, which was fabricated using a dissimilar casting technique. Then, various heat treatment conditions were implemented to evaluate the effects of nickel diffusion on the interfacial bonding and the microstructure of the Al matrix. We aimed to determine the optimized conditions to achieve superior chemical bonding. The chemical bonding mechanism under optimized heat treatment conditions was investigated and compared with thermodynamic calculations for an aluminum alloy system with a 1-10% nickel concentration range.