Fabrication of a nanostructured high strength steel tube by friction-forging tubular additive manufacturing (FFTAM) technology

A newly developed solid-state 3D-printing route of friction-forging tubular additive manufacturing (FFTAM) is applied to produce a low-carbon steel tube with superior mechanical properties. FFTAM technology is based on high-temperature severe plastic deformation and consolidation of metal chips via the friction stir welding/processing (FSW/FSP) mechanism to achieve a fully-dense tubular shape in a layer-upon-layer manner. This achieve metallurgical bonding between the layers using a rotating mandrel to maintain radial friction followed by press-die forging under hydrostatic pressure. After FFTAM deposition, the initial ferrite-pearlite microstructure of the steel chips is transformed to a triple-phase alloy consisting of ferrite, martensite, and austenite grains. The formation of martensitic phases by rapid cooling during layer-upon-layer deposition led to significant material hardening, and this further modified by the thermo-mechanical history upon deposition of subsequent layers. Transmission electron microscopy (TEM) observations revealed the microstructural refinement of the manufactured steel tube at the nano-scale range (<10 nm) to characterize the martensitic shear deformation caused by rapid cooling and severe plastic friction straining during the FFTAM process. The tensile, compression and forming response were evaluated in the additive manufactured steel tube, and attributed to the refined and triple-phases microstructure. A favorable combination of tensile strength (up to similar to 675 MPa) and elongation to failure (similar to 15 %) with a high indentation hardness of similar to 400 HV was demonstrated, along with a combination of ductile-brittle fractography features in related fracture surfaces.