Theoretical analysis of grain boundary liquation in heat affected zone of inconel 718 alloy study of laser weldability of Ni-base superalloys (3rd report)

摘要

This study focuses on the phenomenon of grain boundary liquation due to local melting of NbC as the base metal product phase as one factor responsible for the occurrence of liquation cracking during laser welding of Inconel 718 alloy. To clarify this phenomenon theoretically, a numerical analysis is performed by a two-dimensional grain boundary liquation model. The calculation procedure involves the grain boundary liquation process due to NbC local melting being divided into the three stages of the NbC solid solution stage during heating, the stage of liquid phase formation and growth at the NbC/matrix interface and of grain boundary infiltration of the liquid phase, and the liquid phase solidification stage during cooling. The results obtained may be summarised as follows. 1 The results obtained during X-ray diffraction analysis of the extracted residues sampled from forged Inconel 718 alloy suggest that massive NbC product phase is intergranularly and transgranularly present in the base metal. The results obtained during microstructural observations of the simulated thermal cycle specimens and observations of the liquation cracking fracture surfaces suggest that a liquid phase is formed by NbC local melting and infiltrates into the grain boundaries. 2 The constitutional liquation mechanism is used to consider NbC local melting and grain boundary infiltration of the liquid phase. The results obtained-during construction of a two-dimensional grain boundary liquation model and numerical analysis of NbC local melting during the heating process suggest that a liquid phase is formed at the NbC/matrix interface at 1439 K, that the amount of this liquid phase increases with a rising temperature and that this liquid phase infiltrates into the grain boundaries.