| Summary: | Additive friction stir deposition (AFSD) is a relatively new additive manufacturing technique to fabricate parts in solid state below the melting temperature. Due to the solid-state nature, AFSD benefits from lower residual stresses as well as significantly lower susceptibility to porosity, hot-cracking, and other defects compared to conventional fusion-based metallic additive manufacturing. These unique features make AFSD a promising alternative to conventional forging for fabricating large structures for aerospace, naval, nuclear, and automotive applications. This review comprehensively summarizes the advances in AFSD, as well as the microstructure and properties of the final part. Given the rapidly growing research in AFSD, we focus on fundamental questions and issues, with a particular emphasis on the underlying relationship between AFSD-based processing, microstructure, and mechanical properties. The implications of the experimental and modeling research in AFSD will be discussed in detail. Unlike the columnar structure in fusion-based additive manufacturing, fully dense material with a fine, fully-equiaxed microstructure can be fabricated in AFSD. The as-wrought structure brings the as-printed parts with comparable properties to wrought parts. The fundamental difference between AFSD and fusion-based metallic additive manufacturing will be summarized. Furthermore, the existing challenges and possible future research directions are explored.
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