Ultrasonic-Assisted Upsetting of Steel with 15 kHz Oscillation Frequency

摘要

Common limitations in bulk metal forming processes are the maximum available press force and forming characteristics of metallic materials. Conventional measures to overcome those limitations, such as forming at elevated temperature, are not always applicable. An alternative approach is the use of a superimposed axial tool vibration with ultrasonic frequency. This enables a considerable reduction of required forming forces. The underlying mechanisms of this phenomenon have so far mainly been investigated for frequencies above 20 kHz and easily deformable materials, like copper. Due to limitations concerning the system technology, materials with higher strength have been considered only to a very limited extent. In order to allow investigations on the deformation behavior of materials with higher strength during ultrasonic-assisted upsetting, a tool setup containing a 15 kHz oscillating system has been developed. It offers a larger loading capacity in comparison to industrially available systems with higher frequency. Ultrasonic-assisted upsetting experiments with varying amplitude and press velocity are carried out to examine vibration-induced changes in the flow behavior of steel S235JR. Changes in the material characteristics are analyzed by evaluating the mean upsetting force as well as the microstructure of the upsetting specimen before and after forming. The resulting forces show a strong process dependence regarding the oscillation amplitude. The static press velocity and contact status between tool and specimen also influence the process forces, but to a much lower extent than the amplitude. Concerning the microstructure a rising oscillation amplitude leads to an increased radial elongation and axial compression of ferrite grains at the strongly deformed specimen center.