Effects of simultaneous rotational ultrasonication and vortex-induced casting technique on particle distribution and grain refinement in AA7075/h-BN nanocomposites

A simultaneous rotational ultrasonication with mechanical stirring (RUMS) technique was applied to successfully produce aluminum alloy/hexagonal boron nitride (AA7075/h-BN) metal matrix nanocomposites (MMNCs). The rotating ultrasonic probe with a mechanical stirring impeller arrangement was employed in the casting process. The ultrasonic probe with 20 kHz frequency, 2.5 kW power, and 400 rpm rotational speed was used for the experimental work. The five variations of MMNCs (AA7075 – 0 wt.%, 0.15 wt.%, 0.3 wt.%, 0.6 wt.% and 1.0 wt% h-BN) were fabricated by RUMS process. The MMNCs were solutionized and artificially aged as per ASTM T6 heat-treatment conditions. The tensile test, Vickers microhardness, and porosity measurements were conducted. The tensile strength of RUMS-fabricated AA7075/0.6 wt% h-BN MMNC was improved by 29.46% for as-cast, and 15.90% for T6 heat-treated condition when compared with AA7075/0 wt.% h-BN as-cast and T6 heat-treated MMNCs, respectively. The microstructures and morphologies of the fractured surfaces were evaluated through optical microscopy and SEM. The elemental composition of the MMNCs was studied using SEM-EDS. The XRD was used to investigate the phase composition of the MMNCs. The dislocation density and the interfacial bonding between the matrix and reinforcements were reviewed by TEM. The presence of nano h-BN particles was confirmed by TEM-SAED and XPS techniques. The rheological effects of rotational ultrasonication and combined mechanical stirring mechanism were explained schematically. The RUMS-fabricated AA7075/0.6 wt.% h-BN MMNC showed good improvement in the mechanical properties when compared with AA7075/0.6 wt.% h-BN MMNCs synthesized by conventional mechanical stir (MS) and ultrasonic (UT) assisted casting process. The enhancement in mechanical properties of RUMS-synthesized MMNCs is attributed to the amalgamated effect of ultrasonic transient cavitation and rotational acoustic streaming along with vortex motion in the novel RUMS fabrication process, which have assisted in dendritic fragmentation, and accelerated the refinement of grains to achieve consistent scattering of nano h-BN particulates without any agglomeration.