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Microstructural origins of strain heterogeneity in formability of aluminum alloys

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2019
Güler, Bara
In forming of sheet metals, strain localizations and subsequent necking are generally induced by geometrical imperfections, such as local thickness variations. Microstructural heterogeneities also take role in strain localizations, yet their influence is neglected when concerning the macro-scale formability. This thesis investigated the effects of microstructural features on the strain localization behavior of aluminum alloys. A small-scale biaxial test device and sample were utilized for this purpose. The custom cruciform sample design and the thickness consistency provided by meticulous sample preparation suppressed necking and allowed deformation until fracture under a constant strain path. In the absence of local necking, determination of the true fracture limits were possible together with the forming limits measured by a method developed in this thesis. Multi-scale strain maps revealed the sources of strain localizations at the global scale as the strain heterogeneities at the microstructure scale, confirming that the localizations originating from the microstructural features could indeed limit the formability of the samples. Then, the microstructural features causing the strain heterogeneities were identified, and their effects were determined both qualitatively and quantitatively. The results showed that the major effect belonged to the orientation of grains followed by the grain size. Grain boundary misorientation had inconclusive effects on the strain distribution. Two separate models were developed in this work in order to quantify the location and magnitude of strain heterogeneities. The models used global strain, grain size and orientation as the only parameters, which were easy to obtain by relatively simple characterization techniques. When compared with experiments performed at large strains and large field-of-views consisting of many grains, the models successfully predicted both the location of heterogeneities and the strain accumulated in them.