Influence of ultrasound on the strain hardening behaviour of metallic materials

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Ultrasound is used to improve and optimize the results of many manufacturing processes. Current ultrasound applications are often based on the results of work done by Blaha and Langenecker in the 1960s. They showed that the flow stress of metallic materials can be reduced significantly if ultrasound is superimposed during a tensile test. Additionally, the application of ultrasound can in some materials lead to temporary or permanent hardening.The reason for this so-called acoustic softening and hardening is still not completely understood from a materials science point of view. While there are several publications dealing with acoustic softening and attempting to explain the underlying mechanisms there are just a few publications dealing with acoustic hardening. In all those works acoustic softening and hardening are reported mainly for hexagonal (zinc) or face-centred cubic (aluminium) metals, although many technical applications deal mostly with the ultrasonic supported manufacturing processes of steels.Within the current proposal systematic investigations shall be carried out to clarify the circumstances, at which acoustic softening and hardening occurs in ferritic, body-centred cubic and austenitic, face-centred cubic steels, for the first time. Therefore, an experimental setup has to be built up to superimpose ultrasonic pulses at a defined moment for a restricted time during a classical compression test. These experiments will be carried out with the above mentioned steels to analyse how variations of the initial materials state, namely the initial dislocation density, and variations of the power and the duration of the ultrasonic pulses will influence the acoustic softening and hardening. Thereby, the mechanical materials response has to be measured during and after the ultrasonic pulses. In addition, the microstructural changes have to be characterised by light optical microscopy, electron microscopy, and by local indentation experiments.It is envisaged to finally derive a metal physical explanation has to be derived to explain the ultrasonic induced softening and hardening phenomena. This clarification of the acoustic effect is the main goal of the project and is planned to serve as basis for the application of the acoustic softening and hardening also in other metallic materials.