Determination of the flow field in the vicinity of a solidification front by X ray radioscopy

Melt convection significantly affects the solidification of metallic alloys. Therefore, the knowledge of the flow field in the melt is an important issue. Velocity measurements in liquid metals are complicated by the specific material properties. Recently, X-ray radioscopic methods became an important diagnostic tool for real-time and in-situ observation of the solidification front with a spatial resolution of a few microns.
Solidification experiments using Ga-30wt%In alloy (melting point 38°C) have been conducted. The metal alloy was poured in a flat solidification cell (150 µm gap thickness) made from two fused quartz glasses, and was heated above its melting point and was subsequently solidified by cooling of the solidification cell’s bottom. A microfocus X-ray radioscopy setup was used for qualitative real-time visualisation of concentration fluctuations and structure formation within the solidifying melt at a spatial resolution of 10 µm. Image integration times of 440 ms were found to be sufficient to ensure sufficient temporal sampling rate.
The optical flow method has been adopted to determine the velocity of the liquid and the dendritic growth rate from translocations of concentration contour lines appearing during a temporal image sequence. Therefore, Gaussian spatio-temporal low-pass filtering was applied to smooth spurious image brightness fluctuations caused by noise. Then any local image brightness variation was related to a certain amount of change in the local alloy composition. Image pattern deformations due to diffusion processes in the melt are considered to be negligible. Thus, the continuity equation was used as a first constraint to compute local translocation velocities in image regions providing sufficient brightness gradients. The physically justified assumption of a locally smooth velocity field was used as a second constraint.