Effect of tapered-pin tool on defect evolution and mechanical anisotropy in AZ80 magnesium alloy fabricated by wire-based friction stir additive manufacturing

Wire-based friction stir additive manufacturing (W-FSAM) is an emerging solid-state route for fabricating magnesium alloy components through severe plastic deformation and layer-by-layer deposition. However, the role of tool-pin geometry in regulating material flow, interlayer defect formation, crystallographic texture, and mechanical anisotropy remains insufficiently understood. In this study, AZ80 magnesium alloy single-wall deposits were fabricated using a pinless tool and a tapered-pin tool to clarify the effects of tool-pin geometry on defect evolution, texture formation, and mechanical properties. A sound deposit was obtained using a 2.5 mm tapered pin at 350 rpm, with yield strength, ultimate tensile strength, and elongation of 146.9 MPa, 243.9 MPa, and 5.7% along the processing direction, respectively. The corresponding values along the building direction were 129.8 MPa, 188.1 MPa, and 3.2%, giving a relatively low yield-strength anisotropy of 13.2%. Multiscale characterization revealed that the pinless tool primarily produced weakly bonded interlayer interfaces, which acted as preferential crack-propagation paths and reduced ductility. In contrast, the tapered-pin tool generated periodically distributed triangular voids because of the re-stirring of previously deposited layers. Thermo-flow analysis further indicated that the pinless tool provided lower heat input and limited Z-direction material flow, leading to insufficient interlayer mixing, whereas the lower temperature and Z-direction velocity in the re-stirring zone of tapered-pin deposits hindered the timely filling of pin-induced cavities. The sharp tips of these triangular voids promoted strain localization and void coalescence during tensile deformation. Quantitative strengthening analysis demonstrated that the strong <0002> // BD basal texture governed yield-strength anisotropy by altering the Taylor factor and texture-dependent Hall-Petch slope. These findings reveal the coupling among thermo-flow behavior, defect formation, texture evolution, and anisotropic mechanical properties in W-FSAM AZ80 magnesium alloy.