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Abstract
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The novel combination of friction stir processing (FSP) and additive manufacturing (AM) was studied in present work. Laser-based powder bed fusion of metals (PBF-LB/M) was employed to establish 316L stainless steel with a bimodal microstructure. Upon FSP, the as-built bimodal microstructure with an average grain size of 179 μm was transformed into a unimodal microstructure containing ultra-fine grains with an average grain size of 1.2 μm. Results obtained by mechanical testing revealed that after FSP; the hardness, the yield point and the ultimate strength of additively manufactured 316L were enhanced by 45%, 77% and 62%, respectively. Microstructure assessment revealed that such a unique improvement in the mechanical properties was due to considerable structural refinement leading to grain boundary strengthening. Energy-dispersive X-ray diffraction analysis revealed that phase transformation did not occur upon FSP. Fracture analysis further indicated that severe plastic deformation (SPD) during FSP can promote the transformation of coarse voids to fine voids and, hence, densification of as-built parts.
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