Hot Extrusion of A356 Aluminum Metal Matrix Composite with Carbon Nanotube/Al₂O₃ Hybrid Reinforcement

Abstract

Over the years, the attention of material scientists and engineers has shifted from conventional composite materials to nanocomposite materials for the development of light weight and high-performance devices. Since the discovery of carbon nanotubes (CNTs), many researchers have tried to fabricate metal matrix composites (MMCs) with CNT reinforcements. However, CNTs exhibit low dispersibility in metal melts owing to their poor wettability and large surface-to-volume ratio. The use of an array of short fibers or hybrid reinforcements in a preform could overcome this problem and enhance the dispersion of CNTs in the matrix. In this study, multi-walled CNT/Al₂O₃ preform-based aluminum hybrid composites were fabricated using the infiltration method. Then, the composites were extruded to evaluate changes in its mechanical properties. In addition, the dispersion of reinforcements was investigated using a hardness test. The required extrusion pressure of hybrid MMCs increased as the Al₂O₃/CNT fraction increased. The deformation resistance of hybrid material was over two times that of the original A356 aluminum alloy material due to strengthening by the Al₂O₃/CNTs reinforcements. In addition, an unusual trend was detected; primary transition was induced by the hybrid reinforcements, as can be observed in the pressure–displacement curve. Increasing temperature of the material can help increase formability. In particular, temperatures under 623 K (350 °C) and over-incorporating reinforcements (Al₂O₃ 20 pct, CNTs 3 pct) are not recommended owing to a significant increase in the brittleness of the hybrid material.