An exploratory study on miniaturized additive friction stir deposition

Without melting and rapid solidification, additive friction stir deposition offers a deformation processing route to additive manufacturing and repair of high-performance alloys with full density and desirable properties. However, this approach has traditionally relied on expensive and bulky equipment, posing challenges for smallfeature creation and portability. Here, we explore miniaturized additive friction stir deposition by modifying an affordable benchtop mill and introducing innovative tool design. With in situ monitoring of temperature and force evolution, we demonstrate the efficacy of this approach by dissimilar metal printing, depositing highstrength martensite steel (AISI 4140) onto mild steel (AISI 1018). Compared to conventional facilities, the present approach reduces the in-plane dimension of the feed-rod by a factor of three and narrows the deposition tracks to a few millimeters in width. The as-deposited steel exhibits full density and retains martensitic characteristics without forming new phases at the interfacial region. The as-deposited steel shows significant microstructure refinement with increased hardness because of the extreme thermomechanical conditions during deposition. We discuss the influences of downsizing on the thermal and mechanical aspects of deposition. Owing to the size effects in heat generation and heat loss, the rotation rate of the print head needs to be significantly increased in miniaturized additive friction stir deposition to ensure good deposition quality. To prevent buckling, the unsupported length of the feed-rod should be reduced in proportion to its in-plane dimension.