Thermal–Modal Frequency Drift and High-Temperature Resonance Tuning of Ti–6Al–4V Ultrasonic Horns During Magnesium Nanocomposite Stir Casting
摘要
Ultrasonically assisted stir casting (USC) is a promising, yet challenging, technological approach for the fabrication of magnesium matrix nanocomposites. In this process, the amount of ultrasonic energy transferred to the molten metal is directly related to the stability of the ultrasonic horn resonance at high casting temperatures. However, as the horn temperature gradually increases due to contact with the molten metal, its elastic modulus decreases and its effective length increases. These changes lead to a decrease in the natural frequency and process efficiency. This study presents a comprehensive approach to investigate and tune the resonant frequency of a cylindrical Ti–6Al–4V horn at the casting temperature conditions of magnesium nanocomposites. Analytical modeling, along with coupled thermo-mechanical finite element simulation, was used to track the changes in modal frequency with temperature up to 700 °C. With increasing temperature up to 700 °C, the natural frequency decreased by about 9%. To compensate for this reduction in eigenfrequency, an optimal horn geometry was designed by combining analytical and numerical methods to maintain the desired longitudinal vibration amplitude under casting conditions. The designed horn was fabricated and applied under real USC conditions to produce ZK60-1 vol.% Al2O3 nanocomposites. Experimental results showed that at 700 °C, the resonant frequency of the ultrasonic assembly decreased from about 19858 to 17995 Hz within 600 s, in agreement with the model prediction, with a deviation of about 3%. The stable thermo-modal behavior of the horn at elevated temperatures was reflected in pronounced microstructural refinement, with the average grain size decreasing from 247 µm in the nanocomposite without ultrasonic melt treatment (UMT) to 107 µm in the nanocomposite with UMT, monomore uniform nanoparticle dispersion, and fragmentation of intermetallic networks in the cast magnesium nanocomposites, highlighting the potential of the proposed approach for high-performance lightweight components in automotive and aerospace applications.