High-energy X-ray computed tomography of the progression of the solidification front in pure aluminum

An X-ray computed tomography (CT) system was developed for monitoring the s olidification front in metal casting. The X-ray source was a 6 MeV linear a ccelerator (linac) emitting photons in 5 mu s pulses at a rate of 180 Hz. T he source intensity was 300 R/min at 1 m. The X-ray beam was collimated in a 30 deg fan shape with a 10-mm height. A detector array comprising 128 ele ments was located 845 mm from the source. Pure aluminum in a clay-graphite crucible (178-mm o.d., 146-mm i.d.) was melted in a resistance heater furna ce, and a cooling tube at the center of the crucible solidified the molten aluminum to simulate the casting process. A solidification front formed aro und the tube and progressed outward over an hour until the aluminum was com pletely solidified. X-ray attenuation measurements were taken every minute during this time. Density images were later reconstructed from these measur ements using CT. From these images, the progression of the solidification f ront was determined with a planar resolution of 1.3 mm and a sensitivity of 3.7 pct. The density maps agree with expected values and correlate well wi th temperature measurements obtained independently by thermocouples.