Investigation of columnar to equiaxial transition criterion and solidification conditions for Ni-based superalloy in laser powder bed fusion

Laser powder bed fusion (L-PBF) is a pioneering additive manufacturing (AM) technology for superalloys. The study of the columnar to equiaxial transition (CET) of a superalloy in the L-PBF process is important for controlling the grain morphology of the superalloy and achieving the production of a superalloy with a single-crystal microstructure. This study aims to reveal the fundamental law of CET of superalloys in the L-PBF process. An algorithm was developed to successfully extend the application of the CET model to cover the solidification rate interval of the L-PBF process, and the accuracy of the model was experimentally verified. The extended CET model indicated that the critical temperature gradient from columnar epitaxial growth to equiaxial growth does not increase monotonically with increasing solidification rate but exhibits a peak. The temperature gradient at the solidification interface of the L-PBF molten pool was greater than this peak. In addition, the solidification mode of the alloy remained epitaxial after adjusting the laser power and scanning rate, because adjusting these two parameters exerted a weak effect on the temperature gradient at the solidification interface of the L-PBF molten pool. The excessively high temperature gradient of L-PBF also makes it difficult to change the solidification mode from epitaxial to equiaxial at preheating temperatures below 1000 K. Changes in the morphology of the molten pool due to the increased temperature of the deposited layer are of more concern in terms of microstructure formation. The results of this study are expected to provide guidance for designing process parameters for optimizing the microstructures of superalloys fabricated using L-PBF.