Influence of Ultrasonic Assistance on the Forming Limits of Steel

Since Blaha discovered the beneficial effect of superimposing high-frequency oscillations to the metal forming process, the occurring force and stress reduction are well-known phenomena. Leading to an immediate flow stress reduction, the so-called ultrasonic assistance is a promising approach to enable forming with reduced process forces. While the achievable force reduction has been investigated intensively, only a few analyses have been conducted in the context of the impact of ultrasonic superposition on the forming limits. The latest investigations state earlier material failure due to localized forming with recurrent high strain rates. Within this paper, the influence of superimposed vibrations on the forming limits of C35E is investigated with regard to distinct oscillation amplitudes and press velocities. It was found that superimposed 20 kHz oscillations of 10 and 20 µm cause considerable earlier material failure. In comparison to conventional testing, the crack initiation, as well as the shear zone formation, is alternated due to the ultrasonic assistance.