A Comprehensive Review of Solid-phase Additive Techniques: Insights into Friction Stir Additive Manufacturing (FSAM) and Additive Friction Stir Deposition (AFSD)

Additive Manufacturing (AM) is increasingly being used in the aerospace and automotive sectors because of its ability to produce lightweight, high-performance components while minimising material waste and lowering buy-to-fly ratios. Among the several AM techniques, metal additive manufacturing (MAM) using alloys such as aluminium, magnesium, and titanium is especially appealing for fabricating geometrically intricate components. Conventional fusion-based MAM approaches often face challenges such as porosity, residual stress, and solidification-related defects, which may undermine mechanical characteristics and dependability. Solid-state alternatives, such as Friction Stir Additive Manufacturing (FSAM) and Additive Friction Stir Deposition (AFSD), have arisen as viable remedies to these constraints. By operating below the melting point, these techniques circumvent fusion-related defects and promote the formation of fine-grained, wrought-like microstructures. Their capacity to manage heterogeneous materials and sustain robust interlayer adhesion expands their application possibilities. Although several studies have examined FSAM and AFSD independently, a comprehensive and comparative analysis, specifically emphasising process parameters, microstructural control, and resultant mechanical properties is lacking in the existing literature. This paper fills the gap by rigorously analysing FSAM and AFSD, highlighting their operational benefits compared to fusion-based approaches, and exploring how changes in parameters like tool rotational speed, traverse speed, and material flow influence build quality and structural integrity. Recent investigations indicate quantitative benefits, including tensile strength increases of 30–40% and hardness improvements of 15–25% relative to fusion-based methods. This research seeks to contribute to the ongoing development and optimisation of solid-state MAM processes by giving a thorough knowledge of relationships between processing parameters, resultant microstructures, and mechanical characteristics. This study provides useful insights for enhancing the performance, reliability, and wider acceptance of FSAM and AFSD in challenging industrial applications by analysing structure–property correlations and identifying unresolved technical issues.