Park, Jun Mo
(2010).
Behaviours of bifilms in A356 alloy during solidification: developing observation techniques with 3-D micro x-ray tomography.
University of Birmingham.
M.Res.
Abstract
The reduction of mechanical properties by oxide films has been documented and studied for decades. Among these studies, Campbell proposed that tangled thin oxides created during the filling of a mould become flattened or acquire a spherical shape because of several processes: the penetration of dissolved hydrogen from the liquid aluminium into the bifilm during solidification, the pushing of oxides by dendrites during solidification, the growth of heterogeneously nucleated intermetallic faces upon the wetted oxide side, and shrinkage that pulls oxides. This proposal has been argued from a theoretical standpoint; however, there is no direct experimental evidence to support it. This work attempts to build an experimental procedure to obtain experimental evidence for the proposal. To obtain aluminium bifilms smaller than 3 mm in 12-mm diameter specimen bars, an oxide generator was designed using a gating system. Micro X-ray tomography (Skyscan 1702) was used to establish the best specimen diameter and scanning conditions, including exposure time, magnification, rotation angle and rotation-step angle. Following scanning, the specimens were re-melted for 1, 2, 3, 4 and 5 minutes at 745 °C, and then quenched in the air until solidified. They were then re-scanned to establish the changes in their defect morphologies. In the experiments, the maximum scanning exposure time, minimum rotation step angle, maximum magnification, minimize specimen size and centring of the specimen on the specimen holder were important to obtain the best detail in the topographical results. In addition, adjusting the threshold level had an important effect on the volumetric analysis. This research could not use a re-melting method. However, in a comparison before and after re-melting processes, the surface connected defects did not change their shape. Hydrogen penetration and intermetallic effects could not be observed in this experiment. However, dendrite pushing and shrinkage could be observed.
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