Solid-state additive manufacturing of carbon nanotube reinforced Al-Cu-Mg composites with enhanced grain thermal stability and strength-ductility synergy

Additive friction extrusion deposition (AFED) has emerged as a promising solid-state manufacturing method for 3D printing of high-strength aluminum-based alloys and composites. However, the occurrence of abnormal grain growth in the AFED Al-Cu-Mg alloys during post-processing solid solution heat treatment often leads to significant degradation in tensile strength and ductility. To address this issue, 1.5 vol% carbon nanotubes (CNTs) were incorporated into 2009Al to form composite feedstocks for AFED. Microstructural characterization revealed that, in addition to CNTs, dispersed MgAl 2 O 4 nanoparticles formed by the reaction between the introduced oxygen and matrix alloy were also observed in the AFED CNT/2009Al. Quantitative analysis showed that the presence of 3.3 vol% dispersed nanoparticles led to a high pinning parameter of 0.71, situating the AFED CNT/2009Al within the no grain growth zone of the Humphreys’ grain growth model. As a result, the AFED CNT/2009Al exhibited significantly enhanced grain thermal stability and tensile strength. Different from the conventional strategy that increases strength by grain refinement, the strength of the AFED CNT/2009Al was further enhanced by increasing the tool rotation rate during AFED for obtaining a moderately coarser grain structure with a higher proportion of intragranular nanoparticles. The ultimate tensile strength and ductility of the AFED CNT/2009Al were increased from 562 MPa and 5.7% to 573 MPa and 13.5%, respectively, attributable to significantly increased strain hardening rates and alleviated strain concentration. These findings offer a new pathway for developing advanced nanocomposites with an exceptional strength-ductility synergy.