The columnar-to-equiaxed transition (CET) or the formation of stray grains in the laser melting deposition is the least desirable for the repair of single-crystal blades.In this work,the forced water-cooling was conducted on a single-crystal René N5 substrate during the direct energy deposition (DED).The single track remelting,one-layer,two-layer,and eight-layer depositions were investigated to explore the grain growth mechanism.The solidification conditions of the DED process,including temperature field,temperature gradient,and solidification speed,were numerically analyzed by a finite element model.The single-track remelting results showed that the fraction of columnar crystal regions increases from 55.81％ in the air-cooled sample to 77.14 ％ in the water-cooled one.The single-track deposits of one-and two-layer have the same trend,where the proportion of columnar crystal height was higher under the forced water-cooled condition.The electron backscattered diffraction (EBSD) grain-structure maps of an eight-layer deposit show that the epitaxial growth height increases from 1 mm in the air-cooling sample to 1.5 mm in the water-cooling one.The numerical results showed that the temperature gradient in[001]direction was significantly increased by using forced water-cooling.In conclusion,the in-situ substrate cooling can become a potential method to promote epitaxial growth during DED via the influence on CET occurrence.