The main drawback of friction stir welding (FSW) dissimilar metals is the formation of intermetallic compounds (IMCs), which are brittle and affect the strength of the joint. The formation of these compounds is inevitable due to their low enthalpy of formation; however, their emergence is an indication of metallurgical bonding between dissimilar metals. This means that the determining factors of intermetallics should be optimal to ensure the formation of the joint and, at the same time, the performance of the joint. It is known that various parameters such as welding parameters, joint configuration, and tool geometry have an influence on the formation of these compounds. However, the influence of the base metal is not adequately addressed in the literature. The current review paper focuses on intermetallic formation during the friction stir welding of aluminum/steel (Al/St) alloys to explore how the types of alloys affect the thicknesses and morphologies of the intermetallics. Different structural steels and stainless steels were considered to see how they affect intermetallic formation when welded to different types of aluminum alloys. The thicknesses of the IMCs in the FSW of different aluminum/steel alloys were taken from the literature and averaged to provide insight into the contribution of the elements to IMC formation. Thermodynamic and kinetic analyses were used to explain this effect. Finally, the mechanism of intermetallic formation is explained to provide a useful guide for selecting dissimilar metals for welding using friction stir welding.

Adhesive bonding is widely seen as the most optimal method for joining composite materials, bringing significant benefits over mechanical joining, such as lower weight and reduced stress concentrations. Adhesively bonded composite joints find extensive applications where cyclic fatigue loading takes place, but this might ultimately lead to crack damage and safety issues. Consequently, it has become essential to study how these structures behave under fatigue loads and identify the remaining gaps in knowledge to give insights into new possibilities. The fatigue life of adhesively bonded composite joints is influenced by various parameters, including joint configuration and material properties of adherends and adhesive. Numerous studies with varying outcomes have been documented in the literature. However, due to the multitude of influential factors, deriving conclusive insights from these studies for practical design purposes has proven to be challenging. Hence, this review aims to address this challenge by discussing different methods to enhance the fatigue performance of adhesively bonded composite joints. Additionally, it provides a comprehensive overview of the existing literature on adhesively bonded composite joints under cyclic fatigue loading, focusing on three main aspects: Adherends modification, adhesive modification, and joint configurations. Since the effect of modifying the adhesive, adherends, and joint configurations on fatigue performance has not been comprehensively studied in the literature, this review aims to fill this gap by compiling and comparing the relevant experimental data. Furthermore, this review discusses the challenges and limitations associated with the methods that can be used to monitor the initiation and propagation of fatigue cracks.

Adhesive bonding is a constantly developing technique, and the volume of its industrial applications is rapidly increasing, which, in turn, requires improving the compatibility and performance of joining methods in specific applications. The industrial growth of adhesive bonding has also been linked to the broader implementation of composite materials and the increasing number of applications requiring joining dissimilar materials. Compared to traditional joining methods, adhesive bonding does not require local heating of the substrates or introducing holes or notches to them. It is instead a continuous joining method that promotes fewer regions of discontinuities and uniformity of the stress fields. Due to the industrial interest in this method, a substantial effort has been made to expand its range of applications and to provide the design tools that ensure it is a safe, reproducible, reliable, and durable process. The adhesive bonding research field is broad, ranging from adhesive formulation to evaluation of the final bonded structure. The present paper collects the relevant literature and discusses fundamental concepts and recent developments in the adhesive bonding technology, covering three essential topics: adhesive materials, joint designs and joint manufacturing methods, and joint modelling methods. Citing a wealth of relevant review papers, original papers, and book chapters, the paper intends to provide a coherent view of the state of the art, so the reader can identify the opportunities originating from the recent progresses in adhesive bonding.