Researchers from the Shenyang National Laboratory for Materials Science, Institute of Metal Research at the Chinese Academy of Sciences, working with the University of Science and Technology of China, have demonstrated a solid-state additive manufacturing route for producing aluminum-based nanocomposites that do not require post-heat treatment. Published on Springer Nature, the study introduces additive friction extrusion deposition (AFED) to fabricate carbon nanotube (CNT)-reinforced Al6Mg nanocomposites. The resulting material achieved a yield strength (YS) of approximately 303 MPa, a 78 percent increase over AFED 5083 aluminum alloys and the highest value reported for non-heat-treatable Al–Mg alloys made by friction-stir-based additive manufacturing.
Post-processing heat treatments such as solid-solution and quenching are often required to strengthen aluminum alloys but can cause distortion and cracking in complex geometries. Solid-state additive manufacturing (SSAM) methods such as friction stir-based additive manufacturing (FSBAM) avoid melting and solidification defects by using frictional heat and severe plastic deformation. Yet, Al–Mg alloys made by these processes typically exhibit yield strengths below 270 MPa.
Additive friction extrusion deposition, a newer variant of FSBAM, introduces a pre-deformation stage that generates intense friction between the feedstock and the rotating die before extrusion. This dual-shear deformation step improves particle dispersion and refines microstructure during deposition, producing dense, defect-free layers.
Controlled processing and mechanical response
Feedstock for the CNT/Al6Mg samples was produced via powder metallurgy using Al–6 wt.% Mg powder mixed with 1 wt.% multi-walled CNTs (MWCNTs). The mixture was milled at 200 rpm, compacted, and hot-extruded into cylindrical billets. These billets were used as AFED feedstock, deposited at 700 rpm for CNT/Al6Mg and 300 rpm for 5083Al. Thermocouples embedded in the first layer recorded thermal cycles, showing peak deposition temperatures of about 503 °C for CNT/Al6Mg and 455 °C for 5083Al, confirming higher thermal input at faster tool rotation.
Ten-layer deposits were produced with uniform thickness and no cracks or voids. While 5083Al yielded smooth surfaces at 300 rpm, CNT/Al6Mg required higher speed to eliminate surface pits. Microhardness testing recorded an average of 129 HV for CNT/Al6Mg compared with 90 HV for 5083Al, representing a 43.3 percent increase. Tensile testing showed isotropic behavior across longitudinal, transverse, and build directions. CNT/Al6Mg achieved a yield strength of ~303 MPa and an ultimate tensile strength (UTS) of ~418 MPa, while 5083Al reached ~170 MPa YS and ~314 MPa UTS.
CNT/Al6Mg therefore surpassed all previously reported non-heat-treatable Al–Mg materials produced by friction-stir-based additive manufacturing, as shown by comparative data in Fig. 15 of the study.
Microstructure refinement and strengthening origins
Electron backscatter diffraction (EBSD) revealed that grain refinement was key to the enhanced performance. AFED 5083Al contained equiaxed grains averaging 3.8 μm, whereas CNT/Al6Mg showed elongated grains averaging 1.3 μm, a 66 percent reduction in grain size. The fraction of high-angle grain boundaries reached 94.3 percent in CNT/Al6Mg, compared with 80 percent in 5083Al. Texture analysis indicated a weaker shear component of 2.4 multiples of random distribution (MRD) versus 5.15 MRD in 5083Al, demonstrating a more randomized grain orientation.
Transmission electron microscopy (TEM) confirmed dispersed CNTs surrounded by nanoparticles of MgO, MgAl₂O₄, and Al₄C₃, forming particle aggregation zones (PAZs). These particles limited grain growth and created localized dislocation pinning. Raman spectroscopy supported that CNTs remained largely intact, with the Iᴅ/Iɢ ratio increasing from 0.81 in the feedstock to 0.97–1.06 after AFED, indicating only minor damage.
Quantitative analysis attributed the majority of strengthening to grain-boundary and Orowan mechanisms. Grain-boundary strengthening contributed roughly 182.6 MPa, while Orowan strengthening from dislocation–particle interactions added 51 MPa. Dislocation and solid-solution strengthening made smaller contributions, and load-transfer effects were negligible because CNTs tended to cluster within PAZs.
Comparative performance and solid-state efficiency
CNT/Al6Mg exceeded the yield-strength range of 200–270 MPa typical for prior friction-stir-based Al–Mg systems. Because AFED operates entirely in the solid state, it avoids defects associated with melting and cooling in fusion-based additive manufacturing and removes the need for quenching or aging. The process produced a uniform hardness distribution and consistent tensile behavior in all directions.
Chemical analysis showed that interactions between magnesium atoms and oxygen adsorbed on CNTs formed MgO and MgAl₂O₄ particles during AFED. These nanoparticles acted as diffusion barriers that restricted further reaction between CNT fragments and the aluminum matrix, reducing formation of brittle Al₄C₃ phases and improving interface stability. The combination of refined grains and nanoscale reinforcement produced the observed 78 percent increase in yield strength without additional heat treatment.
Further studies may explore parameter optimization and CNT-content adjustment to refine the strength–ductility balance. Changes in rotation speed and feed rate could tailor grain size and particle dispersion for different structural applications. The demonstrated performance of AFED CNT/Al6Mg confirms that solid-state additive manufacturing can deliver high-strength, heat-treatment-free aluminum nanocomposites suitable for lightweight engineering components.
By combining severe plastic deformation, precise thermal control, and nanoscale reinforcement, this work establishes AFED as an effective route for manufacturing aluminum-based composites with high mechanical stability and minimal distortion.
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Featured image shows Schematic diagram showing the formation of MgO and Al2O3 particles around CNT fragments. Image via Springer Nature.
