Production of Semi-Solid Slurry through Heterogeneous Nucleation in Metal Matrix Nanocomposites (MMNC) Using Nano-Scale Ultrasonically Dispersed Inoculants

Abstract

Ever since copious nucleation was shown to be an efficient, cost effective method for producing semi-solid slurry, many processes have been developed to take advantage of the cost savings inherent in this method of slurry production. Despite great advances in various aspects of semi-solid processing, the cost competitive nature of the industry, most noticeably the auto industry, has prevented a wider adoption of semi-solid casting technology. This research aims to realize a more industrial appealing process by combining the synergistic benefits of semi-solid casting technology with metal matrix nanocomposite (MMNC) technology, thus creating higher value products with superior properties cost-effectively. To do this, a process that produces a semi-solid slurry though the nucleation catalysis induced by nanoparticle additions as small as 1 wt. % to alloys is proposed and the results are presented in this paper. Examination of the potential for nano-scale inoculants to catalyze nucleation of solidification showed that despite their small sizes, inoculants on the scale of tens of nanometers are capable of catalyzing nucleation in the zinc and aluminum alloys studied. Employing the differential scanning calorimetry (DSC), differential thermal analysis (DTA), and droplet emulsion techniques with nanocomposite samples showed a significant reduction in undercooling owing to the homogeneous distribution of nanoparticles by ultrasonic mixing and the potency of those nanoparticles to catalyze nucleation. Comparison of undercoolings between different types of nanoparticles, such as silicon carbide (SiC), gamma and alpha alumina (Al2O3), and titanium carbide (TiC), to relative potencies predicted by minimum lattice disregistry showed a strong correlation. Results were also examined in light of free growth and nucleation controlled grain initiation. For nanoparticles predicted to be potent nucleation catalysts by lattice disregistry, the undercoolings observed fell into the free growth controlled grain initiation regime.