Improvement in the Quality of Superalloy VAR Ingots

摘要

White spots are defects that occur in VAR melted ingots which appear as non-etched bright areas especially in the superalloy macro-structure. In the present study, 'dirty' discrete white spots which are associated with inclusions were surveyed. It was made clear that the 'shelf' is an important source of this type of white spot. It was thought that the large fill ratio may allow the arc to reach the circumference of the molten metal pool enough to prevent the growth of a 'shelf'. To investigate this, VAR ingots of Alloy 718 were remelted at various fill ratios. All the materials made from these ingots were then tested by ultrasonic inspection and the results Showed that the larger the fill ratio, the fewer the number of defects in each ingot. Moreover, we found two kinds of inclusions associated with this type of white spots: one, made of fine Al_2O_3 particles and TiN particles mixed and agglomerated to form a large scale cluster; and the second, made of a relatively small number of TiN particles. The reason for the difference of these morphologies was discussed.