Potentials and strategies of solid-state additive friction-stir manufacturing technology: A critical review

In this assessment, novel applications of solid-state technology based on friction-stir processing (FSP) principles are appraised for use in additive manufacturing (AM) of metals/alloys and metal matrix composites, as variations of so-called additive friction-stir manufacturing (AFSM). Multiple variants of this technique involve layer-by-layer deposition of materials by thermo-mechanical stirring of a rotating tool to induce high temperature via severe plastic deformation (SPD) and may involve: (i) feeding of new material from powder/wire, (ii) surface cladding, (iii) functionally graded modification of composition, (iv) friction-surfacing, and (v) FSP as modification to the other techniques. This emerging material synthesis technique incorporates high strain rates (1 to 1000 s−1) and high temperature (∼0.8 Tm) through intense plastic deformation while imposing equivalent plastic strains of up to ∼40, leading to rapid thermal cycles. Materials mixing during deposition of each layer forms fully dense and homogenous structures with significant microstructural refinement which is free of any melting and re-solidification defects such as shrinkage micro-voids, porosities, and cracks. The manufactured components by this solid-state technology may enable lower levels of residual stress and distortion as compared to the fusion-based additive processes, and the potential to achieve superior and isotropic strength and ductility, while drastically increasing material production rates. These features are reviewed here while also providing an outlook on future areas and ongoing challenges for this emerging technology.