Structure, mechanical properties, and wear behavior of functionally graded hybrid AA6061-B4C-CNT composites using combined stir casting and centrifugal casting

A functionally graded hybrid Al-B₄C-CNT composite cylinder was fabricated using combined stir and centrifugal casting to improve the properties of both the outer and inner surfaces. While a dense B 4 C-rich region of approx. 1 mm thickness was achieved at the outermost surface due to the higher density of B 4 C particles over molten AA6061, the areas adjacent to the inner surface were preferentially reinforced by CNTs. Electron backscattered diffraction (EBSD) analysis confirmed grain refinement and increased local strain in reinforcement-rich regions, with the finest grains and highest kernel average misorientation (KAM) value observed in the region, preferentially enriched with the CNTs. Raman spectroscopy confirmed the gradient CNT distribution, and high-resolution transmission electron microscope analysis proved the strong interfacial bonding due to the formation of a thin Al 4 C 3 layer. Due to the presence of high B 4 C content, the highest hardness of 520 ± 30 HV was achieved at the outer periphery, while the presence of CNTs in the inner regions resulted in higher hardness in those regions compared to the reinforcement-free mid-thickness zone of the cylinder. Samples from five regions along the thickness — outer, outer-middle, middle, middle-inner, and inner were subjected to three-point bend and wear tests. The inner-middle part demonstrated the best combination of flexural strength and toughness (flexural strength ∼350 MPa and flexural strain ∼11%) along with the lowest coefficient of friction (= 0.23) and specific wear rate due to the uniformly distributed CNT reinforcement. In conventional centrifugal-cast functionally graded composites, the inner surface generally suffers from inferior properties due to the accumulation of defects. In that respect, this novel composite design with gradient distribution of hybrid reinforcements provides a methodology to reinforce both surfaces simultaneously.