The FAA has reviewed the evaluation performed by GE and concurs
with the following conclusions regarding the nature of the casting
defect and the manufacturing process:
- The casting anomaly is dross-like in nature. A dross inclusion is
defined as a string or cluster of oxidized particles that form in
metal during melting. Dross is an oxide that forms as a result of
oxygen combining with the reactive elements in the alloy while in a
molten state.
- Scanning Electron Microscopy, Optical Microscopy and Energy
Dispersive Spectroscopy revealed Titanium and Aluminum-rich oxides
present. Both of these elements are typically found in oxides of
the HPT S2 blade alloy, Rene'80.
- Dross formation is possible during production of the ingot master
heat manufacture and/or the blade casting process.
- Due to the unusual, internal location of this defect (i.e. the
center of an internal shank rib - most dross-like inclusion are
connected to an external surface) FPI was not able to detect this
anomaly. X-ray inspection was also not capable of detecting this
particular defect due to the significant mass in the rib
location.
- Cleanliness of the manufacturing process is relied on to prevent
defects of this nature.
- High time blades without internal coatings are susceptible to
IGO.
Audits of the current ingot master heat and blade casting processes
have been conducted at the two HPT S2 blade manufacturers. All
current procedures were found to conform to GE specifications to
minimize the likelihood of blades containing casting defects being
released to service.
GE also released blades with internal coatings in 1992 to reduce
the likelihood of IGO.
The Engine & Propeller Directorate believes that no further
action is necessary regarding this issue and asks that FAA Safety
Recommendation 00.058 be closed.
