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Aluminium alloys have evolved as suitable materials for automotive and aircraft industries due to their reduced weight, excellent fatigue properties, high-strength to weight ratio, high workability/formability, and corrosion resistance. Recently, the joining of similar and dissimilar metals have achieved huge success in various sectors. The processing of soft metals like aluminium, copper, iron and nickel have been fabricated using friction stir processing. Friction stir processing (FSP) is a microstructural modifying technique that uses the same principles as the friction stir welding technique. In the majority of studies on FSP, the effect of process parameters on the microstructure was characterized after a single pass. However, multiple passes of FSP is another method to further modify the microstructure in aluminium castings. This study is aimed at reviewing the impact of multi-pass friction stir processed joints of aluminium alloys and to identify a knowledge gap. From the literature that is available on multi-pass FSP, it has been observed that the majority of the literature focused on the processing of plates than the joints. There is limited literature reporting on multi-pass friction stir processed joints. This then creates a need to study further on multi-pass friction stir processing on dissimilar aluminium alloys.

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Metal matrix composites (MMCs) are now one of the most significant groups of modern engineering materials as a result of the increased attention they have received in recent years. MMCs have recently been manufactured using a variety of technical specifications and techniques, with properties such as the ability to withstand thermal stability at the lowest possible cost, reduced weight and density, increased strength and toughness, and improved wear resistance. It is crucial to homogenize the distribution of the reinforcing phase during composite processing in order to generate particulate or fibrous solid microstructures, depending on the form of the reinforcing phase of the composite. This implies that new procedures must be employed to enhance the mechanical and microstructural properties of metal products. One of the answers to the above challenges is friction stir processing (FSP). FSP improves the surface quality, ductility, formability, strength, hardness, and fatigue life of metal alloys without altering the properties of metals in bulk. This study aims to review MMCs suitable for FSP-designed marine structures and identify knowledge gaps. According to the literature, MMCs are advanced materials capable of exhibiting microstructure, increased hardness, strength, excellent damping, wear, and reduced thermal expansion, making them suitable for a wide range of applications. Although FSP is recognized as a new secondary processing approach to enhance the microstructure and properties of MMCs, few studies have reported the production of MMCs suitable for marine applications. Therefore, this opens a large gap that needs to be filled and requires further investigation of MMCs development.