|
Ultrasonic Consolidation of Titanium Alloys
Titanium components are used throughout airframe, armament, and
propulsion systems by the U.S. military because of their desirable
properties. However, when these components fail prematurely, the cost
and logistics impact is significant. Not only is it expensive to replace
the titanium components due to the material costs and costly complex
fabrication operations, but the loss of a subsystem or a system has even
greater impact to systems level cost and overall military readiness. For
instance, if the removal of a titanium component results in an “aircraft
on the ground – AOG” while the part is being replaced (or repaired, if
possible) it can result in loss in mission capability and flight
readiness. This program proposes a new solution to the direct repair of
worn components with an ultimate goal of creating a small, portable
field repair tool.
The timely, in situ repair of such components without degrading the
parent material properties offers tremendous opportunities for cost and
schedule savings for AOGs and other platforms where titanium components
are critical to meeting mission requirements. However, a repair method
for titanium alloy components must be highly robust since, in many
cases, the choice of titanium has been predicated on the fracture
critical nature of the application and/or on exposure to severe
environments such as hot gases.
Effective repair approaches have proven difficult to develop as
conventional additive metal deposition technologies include a molten
phase transformation. The large dimensional changes associated with
liquid-solid transformations, and the significant heat input to the
surrounding material volumes, result in high residual stresses in the
repair area following welding. Furthermore, the Heat Affected Zone
associated with traditional joining methods produces a low strength,
non-homogenous region at the joint. These changes in the materials
properties of the repaired parts are detrimental to the fatigue life,
and are a major concern where cyclic loading is experienced. The use of
high power ultrasonic consolidation (UC) technologies bypasses the
liquid-solid transition and creates a solid state weld or a “cold”
joint. This method then allows for strong, homogenous structures to be
manufactured and repaired in the field.
Titanium welding, when not in an inert gas shroud, is highly susceptible
to oxygen embrittlement and contamination in atmospheric environments
above about 400°C. Costly processes such as vacuum melting or inert gas
shielding must be taken to protect the weld metal and adjacent
base-plate heat-affected zone until sufficient cooling has occurred. The
proposed project using UC, an otherwise cold bonding method, mitigates
the need for these expensive precautions, thereby bringing down the cost
of manufacturing and repair of titanium components.
Program Manager: Mike Gnam, (734) 995-4971,
mikeg@ncms.org
|
