MAGNESIUMDIC · In-Situ Observation and Control of Microstructure and Surface Defect Evolution during Magnesium Sheet Forming

In-Situ Observation and Control of Microstructure and Surface Defect Evolution during Magnesium Sheet Forming

Small-scale and multi-axial testing of sheet metals, particularly of lightweight aluminum and magnesium alloys are becoming important as these materials exhibit forming behavior sensitive to their unique microstructural features and strain paths. As an alternative to large-scale standard tests, in this project we introduce a novel biaxial tensile test apparatus utilizing miniature cruciform samples. The compact and portable apparatus includes a custom-built optical microscope and high-resolution digital image correlation (DIC) equipment for in-plane and in-situ strain measurements at the microstructure scale. With our cruciform test setup, it is now possible to test the samples until their fracture limit without the formation of local necking and premature fracture. The predetermined strain path stays constant until the fracture. This way the local necking is eliminated and the actual material behavior is observed under large strains. In the literature, most of the micro-scale strain maps are limited to the small strains. Our surface preparation technique and imaging setup, however, allow the strain maps to be plotted under the large strains. Therefore, we are able to capture the strain localizations to the microstructural features and the defects originating from the strain localizations. Strain localizations in aluminum originate from the orientation differences between the individual grains, resulting in strain accumulations at the grain boundaries. They become the first sites to fail in the microstructure and micro-cracks propagate through the grain boundaries leading to the ultimate sample failure. The localizations are also strongly sensitive to the strain path. In magnesium, strain localizes to the grain boundaries under the uniaxial tension, but the twins accumulate strains when the strain path is switched to the biaxial tension. Under uniaxial tension, cracks propagate through the grain boundaries, whereas twinning-induced shear bands form under biaxial tension. Cracks propagate through the shear bands and result in catastrophic fracture. The shear bands also cause a nonuniform microstructure. The results obtained in this project have two major applications: First, is the standardization and commercialization of the cruciform tests. In this project, we showed that the cruciform test is on par with the standard tests when measuring the formability of materials. They also have certain advantages, such as simple sample design, frictionless setup, and in-plane deformation. Second, the results on aluminum and magnesium sheets should lead to a better design of the forming processes and the processed materials for improved defect control. We now know what is responsible from the localizations and how they lead to the defect formations. The starting texture of the formed sheets and the strain paths in the forming processes can be optimized for improved formability and defect-free sheet products. This project also allowed fellow to establish an independent research group consisting of: 2 PhD, 5 Master and 3 undergraduate students. Fellow holds the tenure-track Assistant Professor position and is the sole supervisor of all the students. Fellow is expanding his research facilities and capabilities with the help of domestic grants and collaborations. As a part of this project, 1 PhD and 4 Master students will complete their thesis. Fellow is integrated EU scientific community by participating and publishing in European conferences and journals. Overall, 5 journal and 6 conference papers will be published as part of the project. Lightweight alloys and associated challenges are explained to public through outreach activities. Scientific careers and EU’s support are promoted to high school and first year engineering students.

Data: CORDIS, © European Union