Tracing plastic deformation path and concurrent grain refinement during additive friction stir deposition

Owing to melting and solidification, serious issues arise in fusion-based metal additive manufacturing, such as solidification porosity, columnar grains, and large grain sizes. Recently, additive friction stir deposition has been demonstrated to overcome these issues via high-temperature, rapid plastic deformation, which can result in fully-dense as-printed material with equiaxed, fine grains. However, the deformation fundamentals underlying this process—e.g., the strain magnitude, its influence on dynamic microstructure evolution, and material flow details—remain poorly understood. Here, we explore the deformation fundamentals of additive friction stir deposition by employing tracer-based feed material (Al-Cu tracer embedded in Al-Mg-Si matrix). This allows us to unravel: (i) the path of plastic deformation, and (ii) concurrent grain structure evolution along the deformation path. X-ray computed tomography is used to directly observe the plastic deformation paths of center and edge tracers. In both cases, the millimeter-scale cylindrical tracer undergoes extrusion- and torsion-like deformation followed by shear-induced thinning, which eventually results in micro-ribbons piling up along the deposition track. Microstructure mapping along the deformation path reveals significant grain refinement during initial material feeding via geometric dynamic recrystallization but no further grain refinement during steady-state deposition. By analyzing the strain components associated with extrusion, torsion, and shear-induced thinning, we estimate the total strain to be on the order of 101 and establish a quantitative relationship between the strain and tracer grain size. While this work focuses on a specific process, the methodology and findings may provide the basis for developing future deformation processing-based additive technologies.