The effect of geometrical parameters on the characteristics of ultrasonic processing for metal matrix nanocomposites (MMNCs)

Metal matrix nanocomposites (MMNCs) can offer significant improvement of properties such as higher specific strength, specific modulus, controlled thermal expansion and higher corrosion resistance, compared to the base metallic materials. However, agglomeration or clustering of nanomaterials makes it very difficult to disperse them in the metal matrix. Non-linear effects of ultrasonic processing, such as acoustic cavitation and acoustic streaming, help in the dispersion and distribution of the nanomaterials. Non-linearity of the ultrasonic processing makes it very hard to measure or characterize the process experimentally. There is very limited knowledge about the interactions between the geometrical parameters of the ultrasonic processing and the extent of cavitation achieved. Numerical modeling offers powerful tools to overcome the experimental difficulties involved. In this study, a non-linear numerical model was developed to resolve the acoustic pressure field, and the cavitation zone size was quantified from the numerical modeling results. The model was then used to study the effect of geometry on the cavitation zone size. Analysis of variance (ANOVA) was used to identify the significant parameters. A parametric analysis involving these parameters was subsequently performed. A configuration of geometrical parameters offering the highest cavitation zone size was determined. It was found out that a probe immersion depth of 25.4 mm produced a maximum cavitation zone in the ultrasonic processing cell with a diameter of 35.9 mm for processing 57 ml of molten Al alloy. An experimental validation has been accomplished by ultrasonically processing an aluminum alloy with carbon nanofibers and silicon carbide microparticles. With selected parameters the area of micro pores in the MMNC was significantly decreased by 50% and a deviation of the hardness was also decreased by 46% due to further dispersion and distribution of the carbon nanofibers.