In-Situ Defect Spectroscopy of Al Welds During Mechanical Load Using a Scanning Positron Microbeam

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The goal of our project is to contribute to the fundamental understanding of the relation between material defects created on an atomic level and the mechanical stability of welds. For this purpose, we want to investigate in-situ the formation and distribution of defects in laser beam welds and friction stir welds of Al alloys during mechanical load. In particular, age-hardenable alloys – we will focus on AlCu6Mn – play an important role in aerospace industry due to their high tensile strength, excellent corrosion resistance and good weldability. During welding the large heat impact with subsequent rapid cooling results in a complicated interplay of vacancy formation, dissolution of precipitates, and partial defect annealing. This in turn results in a complex microstructure with a local variation of the concentration of vacancies, precipitates and grain refinement. The associated local variation of the strength is crucial for the mechanical resilience and the lifetime of the welded technical component. Spatially resolved positron beam experiments (spatial resolution ~30µm) will be applied because they are especially suited for the non-destructive exploration of lattice defects and their elemental environment in technical alloys. For our experiments, we will use the unique scanning positron-microbeam at the CDB spectrometer at the high-intensity positron source NEPOMUC. In-situ defect spectroscopy of the Al specimens will be performed during tensile and fatigue tests starting at room temperature up to high temperature. We will also apply complementary techniques, e.g. optical microscopy of the micro-sections and measurements of the micro-Vickers hardness By this, we will be able to correlate the formation and distribution of defects such as vacancies, vacancy-solute atom complexes, and precipitates with macroscopic properties like tensile strength and hardness. We expect that the outcome of this project will have a large impact for technical applications not only for welded Al alloys but for metallic alloys general. In the long term, we want to establish spatial resolved positron annihilation spectroscopy as a powerful tool for defect analysis that is of high importance in engineering science for the application of technical materials and components, e.g., in shipbuilding, airplane and automotive industry.