The formation of microstructures during solidification is strongly affected by the interaction that occurs between factors, such as heat field, melt flow, solute distribution, and number of effective nucleation cores. In this study, the microstructure evolution of a high-purity hypoeutectic Al-Cu alloy at its region of variable cross-section was studied using synchrotron radiation imaging. The characteristics of the thermal field, solute field, and flow field were analyzed according to these radiographs. The results showed that the region of variable cross-section is the site that is more prone to dendrite arms fracture. These dendrites spread radially from the center of the thin wall and the isotherms distribute as an arc-shape. These are the main reasons for the uneven distribution of the microstructure, which lead to undesirable phase formation and aggregation at the region of variable cross-section. The microstnicture can be significantly refined and solute enrichment can be eliminated by adding refiners to alloys. Comparing these two conditions, it can be concluded that the solute and flow field distribution at the variable cross-section region is significantly affected, uniformity of the solidification microstructure is improved, and undesirable phases are eliminated by increase in effective heterogeneous nucleation cores. (C) 2015 Elsevier B.V. All rights reserved.