Additive manufacturing of heat-treatment-free high-strength CNT/Al6Mg nanocomposites using emerging additive friction extrusion deposition

Abstract The application of post-processing solid-solution and quenching to complex structures made of heat-treatable aluminum alloys often leads to distortions and cracks. Solid-state additive manufacturing of commercial Al–Mg alloys can eliminate the need for solid-solution and quenching, but resulted in insufficient yield strength (YS). To address the challenges, a new strategy that employs the emerging additive friction extrusion deposition (AFED) to prepare carbon nanotube (CNT) reinforced nanocomposites was explored. Fine-grained AFED CNT/Al6Mg nanocomposites with dispersed nanoparticles of CNTs, MgO and MgAl 2 O 4 were successfully fabricated. Compared to the AFED 5083Al that was characterized by a low YS of ~ 170 MPa, the AFED CNT/Al6Mg exhibited significantly enhanced YS of ~ 303 MPa, which is the highest YS among all reported non-heat-treatable Al–Mg based materials produced by friction stir based additive manufacturing methods. Precision microstructural analyses and theoretical models were adopted to gain deep insights into the formation mechanism of the unique microstructure of the AFED materials and establish the relationship between the microstructure and mechanical properties. By quantifying the contributions of different mechanisms to strengthening, it was found that the remarkable 78% increase of YS in the AFED CNT/Al6Mg was mainly ascribed to the grain boundary strengthening and Orowan strengthening.