| Abstract Scope |
The earliest nickel-base superalloys used for turbine engine blades were air melted and direct forged. Subsequent alloy and process development proceeded rapidly, leading to the introduction of alloys with higher strength and temperature capability, but with reduced damage tolerance. The development and commercial implementation of vacuum melting, consumable electrode remelting, inert gas atomization and fine grain forging have enabled the industry to use these materials with increasing safety and reliability. These processes resulted in a greatly improved cleanliness and facilitated more sensitive sonic inspection. The triple melt VIM/ESR/VAR process provided another improvement in quality. More recent engineering developments have focused on process modeling, in situ monitoring, and sophisticated control schemes. This has provided a more precise understanding of the mechanisms of defect formation and defined conditions that favor their occurrence. Never-the-less, several classes of anomalies are still routinely detected during manufacturing and some escape detection. The recorded anomaly frequency and size distribution inform engine lifing and inspection rules with high aircraft operating cost implications. Although the alloy melters and engine OEMs continually strive to improve process controls and inspection methods, it would be better to prevent their formation. Some of these anomalies are generated by the remelting process at a time and length scale below current detection capability. This understanding has initiated a discussion about whether a zero defects scenario is possible with current technology. This paper will consider the anomaly mechanisms and the means to detect them and discuss prospects to go the final mile on “the road to zero defects”. |