Local microstructural evolution, interfacial integrity and rate effects associated with modern joining processes using plastic deformation

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This project considers microstructural evolution and the corresponding changes of properties in joining zones after collision joining. This process is associated with high degrees of deformation. Moreover, joining occurs at very high velocities. Therefore, the material behavior needs to be characterized systematically under high strain rate conditions as a basis for the simulation of the joining process itself. The project builds on close collaborative work with processing-oriented projects of the Priority Programme, where the collision joining process is studied both in a custom-made, simplified experimental setup, and under more realistic processing conditions close to the industrial process. These collaborations provide a wealth of samples that allow for detailed microstructural and micromechanical investigations. Using joints of (technically) pure aluminum and a combination of steel and aluminum, respectively, the effect of different initial microstructures and surface conditions is investigated. Recent results document in detail the formation of characteristic, wavy joining zones. Moreover, the dynamic material behavior of the base materials is characterized to support numerical simulations in the partner projects. Careful microstructural investigations indicate that the joining zones typically consist of thin regions that may even have briefly melted during joining. The subsequent, very rapid cooling results in recrystallized, rapidly solidified regions. The focus of current and future research is placed on understanding how the special morphology of these joining regions is affected by different processing parameters, how it influences strength and integrity of the joints, and how the typical processing windows are in turn limited by the corresponding fundamental microstructural processes. The project will continue to provide materials with well-defined initial microstructures for collision joining, and to characterize the dynamic material behavior of these base materials. The research of this project will be primarily focused on microstructural characterization of the joining zones by electron microscopy, and on complementary nanoindentation investigations for local mechanical characterization. The investigations propsed here will also contribute to an analysis of two main topics (bonding mechanisms during joining by plastic deformation and high strain rate material behavior) of the Priority Programme.