Eigenfrequency characterization and tuning of Ti-6Al-4V ultrasonic horn at high temperatures for glass molding

Glass molding assisted by ultrasonic vibration is a promising yet challenging technique for microoptics fabrication. During glass molding localized high temperatures (300-600 °C) often result in transformed eigenfrequencies of the ultrasonic horn, and hence decreased electroacoustic efficiencies of the ultrasonic system. This study proposes a systematic methodology to optimally tune the objective eigenfrequency of the horn at elevated temperatures. Theoretical and numerical analyses are first performed to characterize the thermally disturbed modal characteristics of the horn. Numerical results indicate that the longitudinal eigenfrequency of the horn decreases significantly with the increasing molding temperature T_m. To compensate for this eigenfrequency decrease, numerical size optimization is then conducted and a two-segment cylindrical horn with an optimized tool (68.62 mm in length) is obtained. In situ eigenfrequency measurements of the optimized horn are further implemented at varying molding temperatures. Experimental results suggest that the tuned eigenfrequencies of the optimized Ti-6Al-4V horn are within the prescribed frequency-tracking range (35 ± 0.5 kHz) over a wide range of molding temperatures (226-641 °C). Thus, by merely pre-adjusting the theoretical eigenlength of the horn, a well-tuned and adaptable high-temperature ultrasonic vibration system can be effectively developed. In addition to glass molding, the proposed methodology applies to design and optimization of ultrasonic horns for diverse thermally involved processes.