ADDITIVE MANUFACTURING OF A HYBRID GRAPHENE-ALUMINUM CARBIDE REINFORCED ALUMINUM NANOCOMPOSITE FOR HIGH TEMPERATURE APPLICATIONS: EVALUATION OF MICROSTRUCTURE AND MECHANICAL PROPERTIES

Abstract

In this work, we demonstrate the synthesis and characterization of an Al- 8wt% Ce- 10wt% Mg hybrid nanocomposite reinforced with graphene (0.6 vol%) and aluminum carbide (Al4C3), synthesized via intermittent attritor milling followed by laser powder bed fusion (LPBF). The hybrid reinforcement comes from the reinforcing approach of ex-situ addition of graphene and in-situ formation of a limited amount of Al4C3 during laser processing. The composite exhibits high structural integrity with a relative volumetric density exceeding 99% (~99.86-99.89%), along with a refined cellular microstructure of α-Al solid solution containing ~6-8 at.% Mg, an interconnected network of Al11Ce3 intermetallic skeleton (~5-12 vol.%) at the intercellular spacing, and globular Al13Mg6Ce intermetallic phases at the boundaries of Al11Ce3 intermetallic skeleton. The cooling rates achieved were on the order of ~105-106 K/s, resulting in an ultrafine microstructure with cellular spacing of ~200-350 nm and grain refinement to ~5-20 μm in the hybrid composite compared to ~20-30 μm in the laser-processed monolithic base alloy.Mechanical testing revealed that the LPBF-processed hybrid composite achieved a yield strength of ~464 MPa, ultimate tensile strength of ~538 MPa, elastic modulus of ~73 GPa, and ~1.45% elongation at room temperature, compared to ~377 MPa yield strength, ~468 MPa UTS, and ~64 GPa elastic modulus for the LPBF processed monolithic base Al- 8wt% Ce- 10wt% Mg alloy processed under identical parameters. The composite retained relatively high strength at elevated temperatures, achieving ~481 MPa UTS at 200°C (10% drop) and ~303 MPa at 300°C (44% drop), indicating the onset of thermal softening, dislocation recovery and reduced effectiveness of strengthening mechanisms at high temperatures. The ~15-16% increase in UTS and ~23% increase in yield strength of the LPBF processed hybrid composite compared to that of the LPBF processed monolithic alloy were attributed to several strengthening mechanisms, including solid-solution strengthening from Mg, grain refinement resulted from extremely rapid solidification, high dislocation density due to CTE mismatch and residual stress during rapid solidification, load transfer from reinforcing phases, e.g. graphene flakes and Al4C3 nanorods, Orowan strengthening from Al4C3 and Al13Mg6Ce particles, and the Al11Ce3/Al13Mg6Ce intermetallic network that impedes dislocation motion even at elevated temperatures. Microstructural evolution, elemental composition, and phase characterization were investigated using polarized optical microscopy/Keyence, FE-SEM/EDS, EBSD/EDS, HR-TEM/STEM/EDS, Raman spectroscopy, and XRD. HRTEM confirmed the formation of Al4C3 nanorods with lengths below ~200 nm, while Raman spectroscopy confirmed retention of multilayer graphene after laser processing. Defect distribution, volumetric fraction and morphology were further investigated using X-ray computed tomography (CT), revealing low porosity levels (~0.11-0.14%) with 40% of defects smaller than 50 μm. A theoretical model was also developed to predict the mechanical properties of the composite, including yield strength and elastic modulus. The predicted yield strength range (~388-597 MPa) showed consistency with the experimentally measured value (~464 MPa). In addition, Wilson-Rosenthal thermal modeling and particle-pushing and engulfment calculations were used to determine process-microstructure relationships and the incorporation of reinforcement during LPBF processing. The results showed the potential of incorporating graphene reinforcement with controlled in-situ formation of aluminum carbide nanorods to enhance the mechanical performance and thermal stability of additively manufactured Al-Ce-Mg alloys for advanced lightweight and high-temperature applications.