Removal of oxygen, nitrogen, and inclusions in powder superalloy scraps by electron beam smelting and induced solidification and the purification mechanisms

Abstract

More and more attention has been paid to re-use of the scraps of superalloy ingots with the development of green manufacturing. However, the high content of O and N as well as inclusions are difficult to remove completely. In this research, the electron beam smelting (EBS) combined with induced solidification technology is used to deeply remove O, N and low-density inclusions from the scraps of powder superalloy ingots. It is found that the concentration of O and N can be reduced to 2.75 and 0.25 ppmw, and the number density of the inclusions (number of inclusions per square millimeter) can be reduced to 7.45 mm−2. O is removed mainly from O-rich inclusions, whereas N is removed from N-rich inclusions and through interface evaporation, in which the mass transfer of N in the liquid boundary layer is the controlling step. The intense natural and Marangoni convection contributes greatly to inclusions movement. The natural convection plays the major role in the melt center, whereas Marangoni convection works dominantly in the edge. The flow reversal will agglomerate inclusions at the vicinity of the surface center during electron beam smelting. The in-situ removal of the inclusions is achievable through dissolution and decomposition reactions by local overheating of the melt and bombardment of the electron beam in extremely high vacuum. By means of induced solidification, the inclusions can be collected at the ingot edge, and form inclusion raft under the effect of the Marangoni flow and surface tension of the melt. A 200 kg ingot with total O and N of less than 9 ppmw is prepared by a scaled-up electron beam smelting test, and a yield of 97.7 % is reached. The results indicate that electron beam smelting plus induced solidification is an effective way to remove both O, N, and inclusions, and permit the preparation of high-purity superalloys.