Dendrite Fragmentation Behavior and Mechanism during the Solidification of GH4742 Superalloy under Permanent Magnetic Stirring

The effects of permanent magnetic stirring (PMS) on the dendrite fragmentation behavior during the solidification of GH4742 superalloy were in situ investigated and the mechanism was also discussed. The results reveal that the columnar zone size is remarkably reduced with the PMS, because the fragment number at the columnar front increases, resulting in inhibiting the growth of columnar grains. But the columnar zone size shows no significant changing with the increasing PMS rotation speed from 100 to 300 rpm, which is ascribed to the following two aspects. On one hand, the fragment movement is greatly accelerated under the forced melt flow generated by PMS, and the penetration of forced melt flow in the mushy zone has been promoted with the increasing PMS rotation speed. On the other hand, the fragment size gradually increases with the increasing PMS rotation speed, so the fragment number transported to the columnar front by the forced melt flow is almost the similar. Based on the in-situ experiment and theoretical analysis, the dendrite fragmentation during the solidification of GH4742 superalloy is attributed to the remelting of dendrite arm necks because of the local enrichment of Nb, which decreases the equilibrium interface temperature and induces remelting when the superheating exceeds a critical value of 2.8 ℃.