EBSD-Based Microstructural Analysis and Dynamic Mechanical Behaviour of Thixoformed Al–Si–Cu–Mg Alloys

Abstract This research explores the relationship between microstructural characteristics and mechanical damping behaviour in semi-solid processed Al–Si–Cu–Mg alloys. The damping capacity across a temperature range of 25 °C to 450 °C was examined using a damping mechanical analyser (DMA) to assess the loss tangent, tan θ. Microstructural evolution was assessed via electron backscatter diffraction (EBSD), focusing on grain boundary characteristics, crystallographic misorientation, and substructure development. The findings indicate that the α-Al grains of the Al–Si–Cu–Mg cast alloy transformed from coarse dendritic structures to globular morphology as a result of the thixoforming process. A notable reduction in grain size was recorded from as-cast to thixoformed, with T6 heat treatment resulting in sizes of approximately 41.8 ± 73.48 µm and 14.76 ± 2.54 µm, respectively. This refinement increased grain boundary area and high fraction of low-angle grain boundaries (LAGBs), thereby enhancing dislocation activity and strain hardening and improving ductility. The thixoformed subjected to T6 heat treatment demonstrated superior mechanical strength, as evidenced by increased Kernel Average Misorientation (KAM) and Geometrically Necessary Dislocation (GND) density values of 0.23° to 0.80 and 0.69 × 10 −14 m −2 to 2.42 × 10 −14 m −2 , respectively. This increased strain localisation is generally linked to the density of grain boundaries and the shear-induced deformation during the thixoforming process. Notably, the thixoformed alloy exhibited the highest damping capacity. However, subsequent heat treatment led to a reduction in damping, consistent with EBSD-quantified reductions in dislocation substructure, KAM value, and subgrain boundary density that govern internal friction. Overall, microstructural refinement via thixoforming significantly improves mechanical performance and damping behaviour.