Semi-Solid Processing of Metal Matrix Nanocomposites

Metal matrix nanocomposites (MMNCs) can significantly improve mechanical properties of light alloys such as aluminium alloys beyond the properties of conventional metal matrix composites (where the reinforcement particles are micronsized). Therefore, MMNCs are potentially strong candidates for use in the automotive industry, where the mechanical performance and energy conservation are highly demanded. However, the challenge is to incorporate ceramic nanoparticles into liquid metals due to their large surface – to – volume ratio and poor wettability. In the present study, several nanoparticle feeding mechanisms (the most critical factor in the fabrication of nanocomposites by the ultrasonic method) were explored. SiC and TiB2 nanoparticles with an average diameter between 20nm and 30nm were dispersed through liquid A356 alloy with a green compact nanoparticle incorporation method under ultrasonic cavitation and streaming. The green compact method which has been developed during this project was found to be a promising mechanism achieving the engulfment and relatively effective distribution of the nanoparticles into the melt. Advanced FEGSEM and TEM techniques were used for the microstructural characterisation of the nanocomposites. The microstructural studies reveal that the nanoparticles were embedded into A356 alloy without any observed intermediate phase between the particles and matrix. It has been shown that with only 0.8 wt.% addition of the nanoparticles, the hardness was considerably improved. The nanocomposite billets were reheated into the semi-solid state to be thixoformed at a solid fraction between 0.65 and 0.70 for near net shape components with reduced porosity. The feasibility of thixoforming for aluminium nanocomposites was demonstrated. The microstructures, hardness and tensile mechanical properties of the thixoformed nanocomposites were investigated and compared with those of the asreceived A356 and thixoformed A356 alloys. The tensile properties of the thixoformed nanocomposites were enhanced compared to thixoformed A356 alloy without reinforcement, indicating the strengthening effects of the nanoparticles.