The effects of convection patterns on freckle formation of directionally solidified Nickel-based superalloy sample with abruptly varying cross-sections were investigated experimentally and numerically. The experimental results demonstrate that freckles were only observed at the bottom of larger cross-section. Numerical results indicate that this phenomenon should be attributed to the different convection patterns at front of solidification interface. As the withdrawal rate increased, the primary dendrites spacing has an obvious influence on freckle formation. A more in-depth investigation of the convection patterns can provide a better understanding of freckle formation and perhaps offer methods to minimize freckles in turbine blades. (C) 2017 Elsevier B.V. All rights reserved.