Electron microscopy study of laser welded GH909 superalloy joint
Zhengwu Zhu, Xiuquan Ma, Gaoyang Mi, Chunming Wang
Abstract
The formation process of gamma/Laves eutectic constituents were systematically investigated in both heat-affected zone (HAZ) and fusion zone (FZ) of laser-welded GH909 superalloy. Solidification sequences and eutectic reactions were proposed by morphologically and chemically analyzing phase and chemical composition by utilizing various characterization methods. In FZ, the ultimate eutectic constituents experienced three stages: 1) liquid (L)→Primary gamma (γ) (1360 °C), forming a γ dendritic morphology; 2) following L→γ+(Nb, Ti)C precipitation at mushy zone; 3) terminal L→γ+Laves (1184 °C) at γ interdendritic regions. Enrichment of Nb in Laves and surrounding γ produced a high dislocation density in eutectic constituents. Continuous distribution of such γ/Laves structures promoted the formation of micro interdendritic solidification cracks under accumulated tensile stress. In HAZ, however, the fraction of γ/Laves constituents was heat input depended and increased as the reduction of distance from fusion line. An interactionally and simultaneously integrated process NbC+γ→Laves (~1323 °C) caused the formation of eutectic constituents with NbC decomposition, Laves formation, grain boundary liquation and subsequently eutectic reactions.