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

冷锻造中越来越多地使用高强度材料，伴随着工艺作用力的增加以及成形性的降低，这对传统的成形工艺提出了重大挑战。减小成形力和扩大成形极限的一个很有前途的方法是将超声范围内的振动叠加到刀具运动上。由于所谓的超声波辅助，材料被暂时软化，加工力显着降低。这一现象最早是由Blaha和Langenecker发现的，并已在多种成形工艺的研究中得到证实，如超声波辅助拉丝、拉深和挤压。尽管关于这一现象的原因提出了许多假说，如应力叠加、减少摩擦和加热，但因果关系仍未确定。因此，本研究项目的目的是全面研究金属材料在压缩、拉伸和剪切加载过程中的流动行为和成形极限，并结合超声激励。在第一个项目阶段，分析了不同的振荡频率、振幅、压机速度和振荡持续时间对过程力降低的影响。在超声激励下，除了具有很强的幅值依赖性的暂时性的力降低外，还发现了一种修正的硬化状态。金相分析和显微硬度测量证实了这些发现，这是因为更明显的剪切带和更不均匀的硬度分布。因此，调查超声波对加工硬化的影响以及确定潜在的因果关系是第二项目阶段的目标。在此背景下，对C35、CuOFE和CuZn30材料进行了不同真应变下的超声辅助压缩试验。通过这种方式，逐渐检查硬化过程。基于对不同材料的测量，获得了可转移的知识。此外，通过拉伸试验扩展了对超声基材料软化的研究。排除任何摩擦学效应，人们对软化的基本机制的了解有所增加。最后，将根据所研究的材料和应力状态对已识别的基于超声波的效应进行评估。

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.