Abstract: The emerging additive friction extrusion deposition (AFED) enables the 3D-printing of aluminum-based materials with deformed microstructure and wrought-like mechanical properties. However, the process optimization and application were delayed by the limited understanding of the process. To bridge this knowledge gap, the microstructure evolution of aluminum alloy (Al6061) and carbon nanotube reinforced aluminum matrix composites (CNT/Al6061) throughout the whole process of AFED was investigated comprehensively. For both materials, extensive dynamic recrystallization (DRX) has occurred before they entered the die orifice. After the pre-plasticized materials were extruded out of the die orifice, further grain refinement occurred due to the enhanced DRX at higher strain rates. Compared to AFED of Al6061, AFED of CNT/Al6061 exhibited significantly refined grains (∼1/10 of grain size of Al6061) and reduced texture density at each AFED stage. A high density of rod-shaped Q precipitates formed in the AFED Al6061, while Mg₂Si and MgAl₂O₄ particles formed in the AFED CNT/Al6061. After the T6 heat treatment, the AFED CNT/Al6061 retained the fine grains (∼1.2 µm) and exhibited a high density of β'' precipitates and dispersed CNTs and MgAl₂O₄ particles. As a result, the T6-treated AFED CNT/Al6061 exhibited a 38% increase in tensile strength without sacrificing ductility compared to the T6-treated AFED Al6061. This work clarified the microstructure evolution details throughout the complex AFED process for the first time, establishing a crucial foundation for advancing the emerging AFED solid-state AM technology.

Key words: Additive manufacturing, Additive friction extrusion deposition, Additive friction stir deposition, Aluminum matrix composite, Carbon nanotube