Horizontal directional solidification (HDS) and horizontal directional melting (HDM) of pure Al and Al–4.5%wt Cu alloy were studied in real-time with in situ X-ray radioscopic visualization technology. Cu into pure Al and the stratification and mixing of Cu in Al–Cu alloy melt were monitored. Some characteristics of the dynamics of the solid–liquid interface profile during HDS of a rectangular Al and Al–Cu alloy specimen were exposed. The experimental results obtained with pure Al provided confidence in the experimental procedures. However, for the HDS of Al–4.5%wt Cu alloy, several phenomena were observed that may require more specific numerical modeling efforts.