Coupling of Acoustic Cavitation with Dem-Based Particle Solvers for Modeling De-agglomeration of Particle Clusters in Li
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SEVERAL studies suggest that the addition of nanoparticle reinforcements to light metals significantly enhances their mechanical properties. A clear increase in aluminum Young’s modulus (by up to 100 pct) and in hardness (by up to 50 pct) with the addition of carbon nanoparticles was reported in Reference 1. Another study indicated a slight enhancement in Brinell hardness of aluminum, magnesium, and copper-based MMNCs with Al2O3 and AlN nanoparticles.[2] The study suggested that a better dispersion of nanoparticles is needed. Other researchers also report agglomerations of nanoparticles made visible using high-definition scanning electron microscopy (SEM).[3] The potential of the technique to enhance material properties was ANTON MANOYLOV, GEORGI DJAMBAZOV, and KOULIS PERICLEOUS are with the Centre of Numerical Modelling and Process Analysis, University of Greenwich, 30 Park Row, London SE10 9LS, UK. Contact e-mail: [email protected] BRUNO LEBON is with the the Centre of Numerical Modelling and Process Analysis, University of Greenwich, 30 Park Row, London SE10 9LS, UK, and also with Brunel Centre for Advanced Solidification Technology, Brunel University London, Uxbridge UB8 3PH, UK. Manuscript submitted May 5, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
nevertheless demonstrated in Reference 4. A dense uniform dispersion of dispersed silicon carbide nanoparticles (15 g, at 14 pct by volume) in magnesium was achieved through evaporation of the matrix alloy, leading to enhancement of strength, stiffness, plasticity, and high-temperature stability. However, on a practical size scale, agglomeration of particles remains a problem. The agglomeration of particles in MMCs is related to the fact that micro- and especially nano-sized inclusions have a large ratio of surface area to volume. This causes surface forces such as van der Waals interaction and adhesive contact to dominate over the volume forces such as, e.g., inertia or elastic repulsion. Various mechanisms of detachment of adhered particles have been reported in the literature,[5] including the effects of turbulent flow. It is expected that drag and shear forces in turbulent flow can improve separation of the particles and thus contribute to de-agglomeration. However, the drag force alone is not sufficient to overcome the adhesion forces. This can be qualitatively illustrated by comparing the Stokes equation for the drag force with the force required to break two spherical particles apart, known as the pull-off force, given by, e.g., Bradley[6]: 6plf Rvf ¼ 4pRcsl ;
½1
where vf and lf are the velocity and dynamic viscosity of the melt and csl is the solid–liquid interfacial energy. For aluminum melt, the dynamic viscosity lf = 0.0013 Pa s. Assuming the interfacial energy csl = 0.2 to 2.0 J/m2, Eq. [1] yields vf =100 to 1000 m/s. Such fluid velocity values can be locally achieved instantaneously as a result of the collapse of cavitation bubbles induced by the ultrasonic field. Ultrasonic melt processing has been long known to improve significantly the quali
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