Mechanisms for removing impurities in a single crystal superalloy by electron beam drip melting

Abstract

The development of clean and green manufacturing of superalloys has attracted widespread attention. The returned scrap of superalloys, however, contains a significant amount of oxygen (O), nitrogen (N), and inclusions, which cannot be used effectively, and thus resulting in substantial waste generation. In this study, the electron beam drip melting (EBDM) is used to deeply remove the O, N and inclusions in a single crystal superalloy. The concentration of O and N can be reduced to 0.57 and 0.55 ppmw after drip melting, with removal rates of 97.9 % and 83.8 % respectively. The number density of inclusions experienced a significant decrease, dropping from 12.90 to 0.18 mm−2, representing a reduction of 98.6 %. The intense Marangoni convection will gather the surface inclusions in the central area of the electron beam spot. The inclusions get close to each other to form cavities, and thus the aggregation reaction occurs to form inclusion clusters. Due to the dissolution and decomposition reactions caused by the local overheating and bombardment of electron beam, the inclusions can be removed effectively. The generated O and N are eliminated through mass transfer and evaporation reactions, while the remaining inclusions on the surface can be artificially removed. The electron beam drip melting can be used for the preparation of superalloys with extremely low levels of O, N and inclusions.