Microstructure and tensile properties of DD32 single crystal Ni-base superalloy repaired by laser metal forming

In this work, the microstructure and tensile properties of DD32 single-crystal (SC) superalloy repaired by laser metal forming (LMF) using pulsed laser have been studied in detail. The microstructures of the deposited samples and the tensile-ruptured samples were characterized by optical microscopy (OM), transmission electron microscope (TEM) and scanning electron microscope (SEM). Due to high cooling rate, the primary dendrite spacing in the deposited area (17.2 mu m) was apparently smaller than that in the substrate area (307 mu m), and the carbides in the deposited samples were also smaller compared with that in the substrate area. The formation of (gamma+gamma') eutectic in the initial layer of repaired SC was inhibited because of the high cooling rate. As the deposition proceeded, the cooling rate decreased, and the (gamma+gamma') eutectic increased gradually. The (gamma+gamma') eutectic at heat-affected zone (HAZ) in the molten pool dissolved partly because of the high temperature at HAZ, but there were still residual eutectics. Tensile test results showed that tensile behavior of repaired SC at different temperatures was closely related to the MC carbides, solidification porosity, gamma' phase, and (gamma+gamma') eutectic. At moderate temperature, the samples tested fractured preferentially at the substrate area due to the fragmentation of the coarse MC carbide in the substrate area. At elevated temperature, the (gamma+gamma') eutectic and solidification porosity in the deposited area became the source of cracks, which deteriorated the high-temperature properties and made the samples rupture at the deposited area preferentially. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.