Fundamental investigation of ultrasonic-assisted forming of metallic materials under compression and shear loading

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The increasing use of high strength materials in cold forging and the associated rising process forces as well as the reduced formability present a major challenge for conventional forming processes. A promising approach to reduce forming forces and expand the forming limits is the superposition of oscillations in the ultrasonic range to the tool motion. Due to the so-called ultrasonic-assistance the material is softened temporarily and process forces are reduced significantly. This phenomenon was first discovered by Blaha and Langenecker and has been confirmed in several investigations for various forming processes, such as ultrasonic-assisted wire drawing, deep drawing and extrusion. Despite the proposition of numerous hypotheses regarding the causes of this phenomenon, such as stress superposition, reduced friction and heating, the cause-effect relationships remain unsettled. Therefore, the objective of this research project is the comprehensive investigation of the flow behavior and forming limits of metallic materials during compression, tensile and shear loading in combination with ultrasonic excitation. Within the first project phase the effects of various oscillation frequencies, amplitudes, press velocities and oscillation durations were analyzed with regard to the occurring process force reduction. Besides the temporary force reduction, which is strongly amplitude dependent, a modified hardening state was determined after ultrasonic excitation. Metallographic analyses as well as micro hardness measurements confirmed these findings due to more pronounced shear bands and more inhomogeneous hardness distribution. Thus, the investigation of ultrasonic-based influences on the work-hardening as well as the determination underlying cause-effect relationships are objectives of the second project phase. In this context, ultrasonic-assisted compression tests with various true strain are carried out for the materials C35, Cu-OFE and CuZn30. This way, the hardening process is examined gradually. Based on the measurements for the different materials transferable knowledge is acquired. Moreover, the investigations regarding the ultrasonic-based material softening are extended by tensile tests. Excluding any tribological effect, the understanding of the underlying softening mechanisms is increased. Finally, the identified ultrasonic-based effects will be evaluated with respect to the investigated materials and stress states.