Planar-cellular-dendritic transformation in the fusion zone of a GH909 superalloy weldment

Solidification behaviors and resultant crystalline growth of primary gamma (gamma) and gamma/Laves phases were systematically investigated in a GH909 superalloy laser weldment. Results showed that horizontal microstructural evolution varied along the thickness direction under Marangoni force and eruption of metal vapor from keyhole. Overall, there existed three distinct microstructures: gamma trunks, eutectic gamma and Laves distinguished by composition and band contrast map. Shoji-Nishiyama orientation relation was verified between gamma and Laves in the eutectic constituents. For gamma trunks in mid-penetration, its morphology displayed a planar-cellular-dendritic growth sequence. Texturally, there displayed an increasingly concentrated tendency of [100]-crystal direction along RD and high Schmidt factor was observed in planar and cellular zones. By recording and analyzing practical thermal field, an increasing solidification rate and a decrease of thermal gradient not only produced the planar-cellular-dendritic growth mode, but also increased gamma primary spacing and grain size from planar zone to the dendritic. As for gamma/Laves constituents, reduced eutectic reaction time resulted in a decreased fraction from planar growth to cellular zone. Tensile test and fracture analysis indicated that high Schmidt factor, relatively large grain size and high fraction of gamma/Laves constituents jointly led to the failure in planar and cellular zones of the GH909 laser weldment. (C) 2020 Published by Elsevier B.V.