Simulation-based optimization of ultrasonic horns for improved ultrasonic melt treatment in magnesium nanocomposite stir casting

• Nine ultrasonic horns optimized for 700°C magnesium UMT via COMSOL. • Ti-6Al-4V Horn No. 2: 3.89× gain, 1.8% detuning, 15.4 MPa stress. • UMT validation: 46.3% porosity reduction in ZK60-Al₂O₃. • Experimental f=19985 Hz vs. FEM=19995 Hz (0.05% error). Ultrasonic melt treatment (UMT) is an effective approach to reducing casting defects such as gas porosity in the stir casting of magnesium based nanocomposites; however, its performance at elevated temperatures is hindered by acoustic attenuation caused by thermal detuning, which leads to drift in the resonant frequency of ultrasonic horns. This study addresses this limitation through simulation-based design and optimization of nine ultrasonic horns with stepped, conical, and arcuate profiles, made of Ti-6Al-4V, Monel K-500, and C-103 niobium alloys for high-temperature UMT applications. Modal, harmonic, and thermo-mechanical analyses were conducted in COMSOL Multiphysics at room temperature and 700°C. The results showed that higher temperatures resulted in a reduced resonant frequency and an elevated amplification factor. Among the investigated materials, Ti-6Al-4V horns exhibited the most stable thermo-mechanical performance under elevated temperature conditions. The optimized Ti-6Al-4V arcuate horn was then fabricated and integrated into a UMT system for ultrasonic-assisted stir casting of ZK60-Al 2 O 3 nanocomposites. Introducing 20 kHz vibrations for 10 minutes at 700°C reduced porosity from 3.52 ± 0.35 (mechanical stirring only) to 1.45 ± 0.15, corresponding to a 58.8% reduction, and increased bulk density by 2.2%. These findings validate the effectiveness of the thermally optimized horn design in improving melt quality under high-temperature processing conditions.