“RECAST”
The Repair of Airfoils When Welding is Unacceptable.
Abstract
The age-old problem of fixing a flaw in an airfoil of a vane or
bucket that is otherwise serviceable, has been resolved by with
the careful blending of two known technologies, HVOF (High Velocity
Oxy Fuel) and HIP (Hot Isostatic Pressing). This recently patented
[1], [2]. process facilitates the successful replacement of parent
material as to allow a component which would otherwise be taken
out of service to be reused in a “like new” condition.
HVOF spraying of an airfoil employ’s the same material as
the parent part. This process by itself will not provide sufficient
adhesion to cause a complete marriage of materials. HIP’ing
completes the union. Resulting “RECAST” repaired components
display the same mechanical and physical properties as the parent
material.
Introduction
Even though both HVOF and HIP have been utilized in industry for
over 30 years, their combination will now produce unique and dramatically
better results.
HVOF application equipment such as Metco’s Diamond Jet, Setllite’s
Jet-Kote, Miller’s Topgun, and TAFA’s JP-5000 are all
well known and widely accepted within the industry. Each has its
unique properties, but any of them can successfully apply RECAST.
It is widely accepted that spraying metals using HVOF on various
configurations is a proprietary portion of the technology. Each
spray facility has developed the parameters needed to successfully
spray in response to market demand. Essentially there remains a
significant art form that is applied when these processes are performed.
None of these processes apply material that will perform as well
as the parent metal to which it is being added. HIP is also a process
practiced for many years. It is less of an art form than HVOF, but
more of a high capital equipment cost process. The process is well
documented and detail data is readily available for most alloys.
HIP is a high-pressure heat treatment usually in an argon environment.
One of its more significant uses is to heal cracks in new castings
manufactured out of super alloys and other materials. Super alloys
are prone to cracking as they are cast. Micro-porosity and shrinkage
cavities create unacceptable flaws. HIP’ing the parts “heals”
these flaws. Although HIP resolves this problem, it degrades the
mechanical and physical properties of the super alloys. This condition
is resolved with additional metallurgical processing, utilizing
controlled cooling rates, in post HIP heat treatments.
“RECAST” PROCESS
See Fig. 1. which diagrams the process steps.
Figure 1: Process Diagram.
The five steps identified above are the sequential methods needed
to RECAST a part. Prior to initiating the process, the part could
have been welded, stress relieved, drilled, machined or altered
in any way that is normally acceptable for the part’s application.
Generally, the HVOF can be applied in any desired thickness although
greater thickness, in excess of 0.060” (1.5mm), will require
specialized design. Specialized design is required because there
is less experience with thicknesses in excess of 0.060” (1.5mm).
The spray parameters may be somewhat unique when compared to traditional
HVOF spraying but are known for most conditions and an experienced
spray facility can deal with them. All of the normal spray evaluation
and normal approval testing still apply.
Sintering is a specialized heat treat used in the application of
powdered metals. Its use is well known and again is common in industry.
The process is basically a vacuum heat treat with the cool down
occurring only after argon is inserted into the chamber. Its purpose
in the RECAST process is to effectively seal the as sprayed RECAST
prior to HIP. Without sintering, the densification that occurs during
HIP would entrap gasses, which would eventually cause bubbles on
the surface of the part during an elevated temperature exposure.
HIP is the process that increases the density of the sprayed material
to match that of the parent metal. It is a high-pressure argon heat
treatment. Typically pressures are between 15 and 45 ksi (103-310
MPa) with temperatures ranging from 2,200 to 2,500 oF (1204-1371
oC) respectively. The use of the HIP process in RECAST results in
the dense coating becoming part of the parent metal joined so well
to the original metal that the interface is difficult to discern.
The first post-HIP heat treat process is mandatory for all metals.
It is a solution annealing treatment, which restores the original
mechanical and physical properties to the integrated metal part.
Solution annealing a common heat treatment for metals whose temperature
has been raised above their normal operating temperature during
a repair or alteration process.
Precipitation heat treatment is a requirement for some alloys as
part of their normal repair process. These alloys contain chemical
elements that enhance the mechanical and physical properties.
See Fig. 2. It is a photomicrograph which shows the parent metal
/ Recast interface. It is difficult to see because the two metals
have become one.
Figure 2: Successful RECAST application (20 to 1).
Traditional HVOF vs. HVOF for RECAST
The use of HVOF technology is readily available within the industry.
There are sufficient numbers of suppliers of this service to accommodate
the demand. The process generally requires the application of a
“bond slug“ test in which the tensile strength of the
sprayed material is verified and is always well below the yield
or tensile strengths of the parent metal. These test methods are
published by ASTM, DIN, and other recognized standards organizations.
The use of the RECAST process produces an integral bond with the
parent material. This bond and all of the deposited material displays
all of the mechanical properties of the parent metal. This unique
condition can be explained through acceptance of the fact that the
sprayed material is actually the same as the parent metal and that
its density is the same as the parent metal. Strict application
of HVOF without HIP produces a less dense, more porous product that
does not replicate the original metal.
Application parameters for HVOF become unique and specific to the
part being re-crafted. Specific conditions of stresses resulting
from the application of the material become paramount during the
application of material. During the 5-year development of this product,
it has become apparent that the implicit tensile or compressive
stresses in the sprayed material can have a deleterious effect on
the final product. Accordingly, HVOF spraying of the part now must
consider the additional impact of residual stress. Without this
consideration, excellent RECAST / parent Metal bonds can separate
internally. These conditions can become more difficult to resolve
if the residual stress is ignored and the material applied exceeds
minimal values of .010” to .020” [254-508?m]. Fig. 3
shows the effect of uncontrolled tensile stresses at the leading
edge of a vane
Figure 3: Effect of Uncontrolled Tensile Stresses.
Applications
The ability to correct localized erosion or sulfidation effects
provides an extremely cost effective method to re-use buckets or
vanes that would otherwise be scrap. The process is presently being
utilized in both ground based and flying gas turbine engines. The
aircraft engine parts have all of the special provisions and approvals
required by the Federal Aviation Administration (FAA).
Turbine and compressor blades in both worlds experience erosion
and sulfidation that locally destroys the part’s ability to
be repaired and reused. (See Fig.’s 4 and 5, a frame 7 first
stage bucket.) This part is badly eroded in areas that cannot be
repair welded. RECAST can be applied to bring the wall thickness
back into specification. Note that this technology can be used on
any airfoil configuration. Accordingly, single vanes or cluster
vanes can also benefit from its utilization.
Figure 4: Badly Sulfidated Bucket.
Figure 5: Close UP View of Sulfidation Shown on Fig. 4.
The pitting shown on Fig. 5 can easily be RECAST. Fig. 6 shows
a frame 6 first stage blade after RECAST application. Note that
the surface can be blended, machined, etc as necessary to produce
a smooth surface. The use of traditional blending / machining techniques
is as acceptable now as it was before RECAST’ing the part.
Figure 6: First Stage Blade with RECAST, as Deposited.
References
1. United States Patent, Arnold, Number 6,049,978, Apr. 18, 2000.
2. United States Patent, Arnold, Patent Number: 5,956,845, Sep.
28, 1999.
For further more detailed technical information please
contact:
James E. Arnold
Director Engineering & Technical Support
Flight Support, Incorporated
30 Eaton Street
North Haven, CT 16473
Telephone 203-562-1415
Fax 203-865-5694
E-Mail jim@flightsupport.net |
