On the Formation of White‐Spot Defects in a Superalloy VAR Ingot

Abstract

For the production of nickel‐based superalloys for the aerospace industry, strict control of the macrostructure of the product is necessary to avoid the appearance of potentially fatal defects. Our study focuses on the prevention of “white spots” in the alloy IN 718. These defects, which are small volumes of a few millimeters of characteristic length, are depleted in niobium. They are known to result from the fall of metal fragments in the liquid pool during VAR processing. According to their history in the liquid metal, these fragments could not being remelted before being trapped in the mushy zone and then give rise to defects. A model calculates the heat transfer in such a precursor to simulate its melting during his stay in the bath. The validation of the predicted melting kinetics requires a series of immersive experiences of synthetic defects in a metal bath. The model and experiments have demonstrated the initial solidification of a layer of metal around the precursor.

Effects of Different Melting Technologies on the Purity of Superalloy GH4738

“…In VAR, uneven heating of the electrode (ingot) may cause it to unexpectedly fall into the molten metal pool. Since the electrode has high density, it sinks to the bottom of the molten pool without fully melting and remains in the solidified ingot to form white spots [ 32 ]. Uneven heating of the electrode can occur because of electrode material pouring from above during VIM casting, which produces open and closed shrinkage defects in the electrode.…”

Numerical Simulation of White Spot Formation in Vacuum Arc Remelting

“…To validate the particle dissolution behavior, a laboratory experiment was conducted by Grignard, [ 6 ] as shown in Figure 2 a. The GH4169 (IN 718) alloy is placed in the Al 2 O 3 crucible and it melts in a vacuum induction melting furnace.…”

“…Many models simulated fluid flow, solidification, segregation, and particle dissolution, but they only considered the ingot solidification phenomena. [5][6][7][8][9][10][11] The consumable electrode remelting behavior received little attention, and only a few works applied a uniform heat source to the liquid and gas interface. [12,13] In this work, the particle dissolution from the electrode and crown is numerically investigated.…”

“…Mitchell [ 5 ] also categorized white spots into three types: the first type was dendrite white spot, which originated from the dendrite clusters of the consumable electrode; the second was a discrete type, and it was the fall‐in from the shelf or crown; the third was solidification white spot, due to the local perturbation during the solidification process. Due to the short dropping time, low initial temperature, and strong solute depletion, Grignard et al [ 6 ] believed that the white spot is mainly from the shelf fragments, which were not remelted before being trapped in the mushy zone. Patel et al [ 7 ] indicated that a larger current intensity and liquid pool depth promoted fragment dissolution.…”