Note: Descriptions are shown in the official language in which they were submitted.
CA 02735302 2011-03-25
BLADE AND METHOD OF REPAIR AND MANUFACTURING THE SAME
The invent ion is related to the airfoil of turbine engine and can be used for
a manufacturing and
repair of blisks, fan and compressor blades made of near alpha and alpha-beta
titanium alloys.
It is well known that titanium and it alloys undergo the allotropic phase
transformation also
known as a polymorphous transformation at a temperature of 885 C that results
in changing of a
crystallite lattice from a close-package hexagonal (alpha) to a body centered
cubic know also as
beta phase.
The temperature of a polymorphous transformation know also as a beta transus
temperature
depends on interstitial impurities and alloying elements that either increase
or reduce a
temperature of a polymorphous transformation.
Based on structure and phase compositions titanium alloys belong to one of
following classes:
alpha, near alpha, alpha-beta and metastable beta. These classes denote
peculiarities of
microstructure, mechanical and corrosion properties of all titanium alloys
after welding and heat
treatment.
Alpha alloys usually do not contain beta phase after cooling from a high
temperature neither
after welding nor after heat treatment.
Near alpha alloys contain limited amount of beta phase occupying a transition
place between
alpha and alpha-beta alloys that can produce a large amount of equilibrium
alpha, further (a) or
martensitic a'-phase, which is an oversaturated solid solution of alloying
elements in a close-
package hexagonala-phase and beta, furtherf3-phase.
A metastable beta alloys tend to retain the high temperaturef3-phase at
ambient temperature.
This invention is applicable for blades manufactured of near alpha and toa-
f3titanium alloys.
Therefore, it might be useful to discuss a structure transformation of this
group of alloys in more
details using as an example the well know Ti-6%AI-4%V, further 6-4-Ti
(titanium) alloy, after
cooling from temperature exceeding fi- transus that represents the welding
metal and heat
affected zone (HAZ).
During welding and heat treatment depending on a cooling rate, welds and HAZ
of Ti-6V-4V
alloy can from three different types of a micro structure.
The water quench results in a formation of a martensitica'-phase with a
precipitation off3-phase
prior to beta grain boundaries.
The martensitica' +fstructure microstructure forms also during air cooling,
which is close to a
cooling rate of welding joints that are produced by a conventional fusion
welding such as gas
tungsten arc welding, plasma, electron beam or laser welding. However, the
content of
CA 02735302 2011-03-25
vanadium in the oversaturated solid solution of martensitic a'-phase in this
case is usually less
than after a water quenching.
The equilibrium plate-like a-phase and retained C3-phase prior grain
boundaries is usually formed
during a furnace cooling in vacuum or argon at a very low cooling rate.
As it was shown in the `Fatigue Data Book: Light Structural Alloys, ASM
International , 2008,
397 p', near alpha Ti-8%Al-1%Mo-1%V alloy, also known as 8-1-1 titanium alloy,
after beta
anneal has impact toughness of 104 MPax while after alpha anneal just 50 MPax
\Im-.
The fan blades are most critical engine components. Therefore, they are
manufactured of Ti-6Al-
4V or 8-1-1 titanium alloys by multi steps forging and heat treatment cycles
to produce the
equiaxed globular structure of the equilibriums alpha phase with fine
precipitation of beta phase
to enhance high rupture and low cycle fatigue properties. However, this
structure does not have
the best impact toughness that is extremely important for the leading edge of
fan blades that are
in a permanent risk of a FOD such as bird strike, ice pellets and stones.
Also, erosion reduces a
chord width making eminent a weld repair of fan blades to restore the leading
edge geometry and
chord width using the electron beam patch and other types of weld repairs that
were disclosed in
prior arts US 4,118,147; 5,092,942; 5,142,778; 5,479,704 ; 5,795,412 and
6,339,878. However,
the electron beam patch weld repair is a low productivity and high cost
process.
A laser cladding that was disclosed in patents US 5,897,801; 6,269,540;
6,659,332, 6,972,390
and 7,137,544 is more cost effective process. However, it has not been used
for a full leading
edge (LE) repair because it has been believed that casting structure of fusion
welds produced by
a conventional gas tungsten arc welding (GTAW), plasma and laser will not be
able to withstand
a potential FOD without fan blade failure or fracture.
Therefore, the development of a new blade that can combine a high impact
fracture of the
leading edge to withstand the probable foreign object damage (FOD) and high
rapture and tensile
properties in most stressed section of the airfoil and blade retainer will be
of a great commercial
value to the aviation industry.
A further objective of this invention is a development of a new method for a
repair of these
blades using a laser cladding aiming to produce the required structure
composition in "as
welded" condition.
Another objective of the invention is a development of a new method aiming to
produce the
desirable structure of blade by a heat treatment during a manufacturing cycle.
We have found that due to achieve these objectives a blade should comprise a
stressed section of
airfoil and a blade retainer manufactured of a near alpha or alpha-beta
titanium alloy having a
structure of equiaxed alpha and intergranular beta phases or a similar
structure composition,
which is produced by a consecutive forging and heat treatment cycles to
enhance high rapture
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and tensile properties, and a leading edge that is made of the same alpha-beta
titanium alloy
having a plate-like alpha phase structure with a retained beta phase within
and along alpha plates
and prior grain boundaries having high impact toughness, and wherein a leading
edge is bonded
to an airfoil via a transition layer having a variable morphology of alpha and
beta titanium
phases.
In accordance with another preferable embodiment, a trailing edge and a tip of
the airfoil are
produced with the similar to the leading edge structure and phase composition.
This invention can be used also to produce shrouded blades.
Manufacturing or repair of blades as per the preferable embodiment includes
step of a multi pass
laser cladding to a stub of a defect free airfoil to produce at least the
leading edge using powder
with similar chemical composition, laser beam power of 200-400 watt per each 1
mm width of
the airfoil, laser beam diameter of 0.75-1.8 mm, welding speed of 12.5-100 mm
per minute and
powder feed rate of 0.4-2 grams per minute, which results in a formation of a
microstructure
consisting of a plate-like alpha phase with a retained beta phase along alpha
plates and prior
grain boundaries.
To allow cladding of blades with a variable thickness, a welding head is
oscillated across a
cladding direction with a traveling speed of 8-12 greater than the welding
speed at an amplitude
of maximum of three times greater that the laser beam spot diameter,
proportionally increasing
the laser beam power to maintain a ratio of the beam power to the width of the
oscillation from 1
to 3.
As per other preferable embodiment due to produce the same microstructure
throughout the
leading edge at least one technological pass should be deposited to the top of
the repaired leading
edge followed by a machining off the latter to restore required geometry of
the blade.
To perform a stress relief without affecting the desirable microstructure in
the leading edge
blades should be subjected to a post weld heat treatment below of the transus
temperature, that is
selected with range of 475 - 650 C in accordance with a relevant OEM
standards.
And finally, to produce the desirable micro structure in the leading edge
preserving a forging
equiaxed structure in most stressed sections of the airfoil and blade retainer
during preferably in
manufacturing cycle as well as in repair, the fan blade might be subjected to
a differential heat
treatment, wherein the leading edge is heated to a temperature above the beta
transus at least for
3 minutes while maintaining a temperature of stressed sections of the blade at
a temperature
below of 680 C followed by a controlled cooling of the leading edge with at a
rate of 0.1-2 C
per min during the polymorphous 0 -* a transformation.
Figure 1 is the schematic presentation of the blade, wherein 1 is airfoil, 2-
shroud, 3-blade
retainer also known as a dove tail, 4-leading edge, 5-transition layer, 6-tip,
7-trailing edge.
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Figure 2 depicts the laser cladding of the fan blade in the Liburdi LAW5000
laser system.
Figure 3 shows the microstructure of the laser cladding using optimal
parameters prior to a heat
treatment, further in `as welded' condition.
Figure 4 is the micrograph of the transition layer that is also known as a
heat affected zone
(HAZ).
Figure 5 depicts the typical microstructure of the base material also known as
a parent material.
Figure 6 is a distribution of microhardness from the airfoil parent material
via HAZ and 10
passes of clad welds that were produce on the leading edge of a fan blade.
The fan blades are most critical engine components. Therefore, they are
manufactured of Ti-6A1-
4V or Ti-8A1-1 Mo-1 V titanium alloys by multi steps forging and heat
treatment cycles to
produce the equiaxed globular structure of the equilibriums alpha phase with
fine precipitation of
beta phase to enhance high rupture and low cycle fatigue properties.
The depicted in Figure 1 fan blade have the complex of microstructure and
phase compositions
that guaranty the highest level of impact toughness at the leading edge (4)
and maximum rupture
properties in the most stressed areas of airfoil (1) and blade retainer (3).
The invented method allows manufacturing of new blades during manufacturing
and
modification of serviceable ones during repair.
Undersized or rejected due to other reasons serviceable fan blades are usually
routed to OEM or
FAA approved repair stations for further inspection and repairs that creates
an excellent
opportunity for their modification as per the invented method.
The typical repair work scope includes the sequence of the following below
standard steps that
might be performed based on the applicable OEM Standard Process Manual (SPM).
The first and obvious step after a visual inspection of fan blades is
degreasing using either the
applicable SPM alkaline process or aqueous cleaning followed by a fluorescent
penetration
inspection (FPI) also known as non destructive testing.
In case if no rejectable defects are found in the highly stressed area, fan
blades are routed further
for machining to remove defective materials at the leading or trailing edge or
tip to a standard
dimension leaving a stub of a defect free airfoil material as shown in Figure
2.
To remove machining residues and prepare fan blades for a laser cladding parts
should undergo
alkaline degreasing followed by cold and hot water rinsing and air draying. To
prevent blades
from contamination it is essential to vacuum seal parts using plastic bags.
CA 02735302 2011-03-25
Restoration of tip, leading and trailing edges is done by a laser cladding
using either filler wire or
powder having chemical composition similar to a parent material.
Usually, laser cladding is performed using a laser beam power from 200 to 1400
watt depending
on material thickness to maintain a thermal conductive mode and prevent plasma
formation. The
combination of a thermal conductive mode and low traveling speed also known as
welding speed
produces an excellent boding of clad welds with minimum penetration of a
substrate. Also, it
slows down a cooling rate, which is critical for a formation of a desirable
microstructure.
Due to a high affinity of titanium and it alloy for oxygen and nitrogen,
argon, helium, and
mixture of these gasses may be used for a protection of a welding or repair
area from a
contamination. However, taking into consideration a sensitivity of a
microstructure of near alpha
and a - 0 titanium alloys to cooling rate argon with low heat conductivity is
a preferable inert
gas.
The laser cladding can be made using filler wire or powder made of similar
filler materials. In
both cases a laser beam is focused onto a repair area of a fan blade to create
a molten paddle of a
parent material. After that, filler material in a form of powder or filler
wire is introduced into the
welding puddle to produce clad welds. The laser head is moved forward and
oscillated across the
repair are wherein the thickness of latter is exceeding the laser spot
diameter. This process
produces a near net shape repair minimizing final machining and polishing.
The key element of the invented method is laser cladding parameters. Usually,
a conventional
laser cladding and welding result in a formation of a typical for air cooling
martensitic like
structure on near alpha and a - (3 titanium alloys. In the invented method,
the multi pass laser
cladding with a relatively high heat input, which is produced by a combination
of a low power
laser beam and low welding speed, as well as multiple reheating of previous
weld deposits above
a beta transus temperature and multiple polymorphous 13 -p a transformation
and
recrystallization during deposition consecutive layers results in a formation
of a plate-like alpha
phase structure with a retained beta phase along alpha plates and prior grain
boundaries as shown
in Figure 3 with a superior impact toughness. As it was found by experiments,
deviation of laser
cladding parameters from the specified range results in a formation of a
martensitic structure that
has low fracture and impact toughness typical for welds produced by the
conventional fusion
welding. Also, the last layer has a transition structure between typical
martensitic like and
furnace cooled. Therefore, the preferable embodiment should include the
deposition of at least
one technological layer for a heat treatment of the previous layer followed by
removing of latter
by machining or other means due to produce desirable structure throughout the
leading edge.
After laser cladding, the fan blade is subjected to non destructive testing
(NDT) by the FPI as per
AMS 2647 or relevant OEM standards. The laser cladding area is also subjected
to x-ray
inspection for weld discontinuities as per applicable OEM standards. Usually,
no linear
indications and pores exceeding of 0.15 mm in diameter are allowed.
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After NDT accepted parts are degreased using alkaline and subjected to a heat
treatment in the
alpha region at a temperature from 475 to 650 C that does not result in a
polymorphous
transformation and recrystallization of near alpha and alpha-beta titanium
alloys. This heat
treatment reduce residual stresses and results in a decomposition of
martensitic a'-phase. This
process is also known as an aging that is accompanied by a precipitation of
fine particles of (--
phase along boundaries and within a-plates without changing of morphology of
last ones. Aging
also increase hardness of clad deposits as shown in Figure 6 and tensile
properties of Ti-6A1-4V
alloy.
After heat treatment repaired areas of fan blades are machined to final
dimensions followed by
polishing and ball burnishing to restore a surface roughness.
The final steps of a modification and repair of fan blades as per the
preferable embodiment is
NDT and dimensional inspection to relevant OEM standards followed by a shot
peening of blade
retainers and glass bead peening of airfoils to induce compressive stresses
increasing a high
cycle fatigue (HCF).
The described above method was developed mostly for a modification of
serviceable fan blades
during a repair cycle.
The differential heat treatment of fan blades is more cost effective process
for a manufacturing of
fan blades, wherein the leading edge of forged blades may be subjected to a
heat treatment at a
temperature exceeding beta transus, usually from 890 to 950 C at least for 3
minutes, at the
leading edge while maintaining a temperature of stressed sections of the
airfoil and blade
retainer at a temperature below of 650 C followed by a controlled cooling of
the leading edge
with at a rate of 0.1-2 C per min during a polymorphous (3 -* a
transformation.
The differential heat treatment of fan blades as per another preferable
embodiment can be made
in a vacuum furnace using the press form with a provision of a temperature
control of the blade
in stressed area below of 650 C that is shielded from the heating elements
that are situated either
side of the leading edge by several rows of heat shields. Titanium has very
low heat conductivity.
So, it is feasible to preheat the leading edge to a temperature from 890 to
1000 C for more than
3 minutes followed by a slow cooling of fan blades with the furnace at a range
of 0.1-2 C per
min during at least a polymorphous (3 -* a transformation to produce a plate-
like alpha phase
structure with a retained beta phase along alpha plates and prior grain
boundaries, while stressed
areas will have a the typical for forging globular equiaxed a-phase with the
intergranular beta
phase precipitations.
Therefore, as follows from above, the new and unique combination of
microstructures in the
leading and trailing edges and stressed sections of blades, which was not
known from prior arts,
was produced in different areas of the same fan blade manufactured of the same
alloy having the
same chemical composition by a laser cladding using the developed parameters
or during
manufacturing using a differential heat treatment.
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The demonstration of a feasibility of this invention was made by a laser
cladding repair on the
leading edge and tip of fan blades manufacture of 6-4-Ti alloy by forging
followed by a standard
post weld heat treatment.
The laser cladding was made using Liburdi LAW 5000 system shown in Figure 2,
IPG fiber
laser, 6-4-Ti filler powder and following below cladding (welding) parameters:
Min laser beam power: 225 Watt (at a tin section of an article)
Max laser beam power: 400 Watt
Welding speed: 76.2 mm/min
Oscillation speed: 762 mm/min
Powder: 45-75 m
Powder flow rate: 0.8 gram/min
Focus distance: 150 mm
Nozzle to the leading edge
standoff distance: 9 mm
Flow rate of argon to
The laser head and trailer: 21 1/min
Flow rate of a carrier
Gas (argon): 1 1/min
After cladding one fan blade was subjected to the post weld heat treatment at
a temperature of
580 C for four (4) hours.
Second one was subjected to a metallographic evaluation of a thin section
produced without
oscillation and thickest section that was restored to required dimensions
using the oscillation
above.
Typical microstructures of Ti-6%Al-4%V clad welds in "as welded" condition are
shown in
Figures 3 and 4.
It was found that a laser cladding using the specified parameters produced a
microstructure that
constituted of plate-like a-phase and retained p-phase prior beta grain
boundaries typical for a
furnace cooling of alpha-beta titanium alloys from a temperature exceeding
beta transus. This
fact was not known before from prior arts. As a result, microhardness of laser
clad deposits in
"as welded" condition was at the level of a parent material as shown in Figure
6. The parent
material had the typical forging equiaxed structure as shown in Figure 5.
The post weld heat treatment slightly increase microhardness of clad welds due
to aging and
precipitation of fine particles of beta phase within sub grains of alpha phase
without affecting the
desirable morphology of alpha phase. The transition area also knows as the HAZ
constituted of
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combination of equilibrium forging structure of alpha phase adjacent to a
parent material to plate
like alpha phase with retained beta adjacent to a fusion line.
The key element of the invented method is laser cladding parameters.
Increasing the welding
speed and reducing the laser beam power had resulted in a formation or
martensitic a'-phase
typical for a conventional fusion welding.
Increasing the heat input was essential from a metallurgical point of view but
it affected a
formation of a net shape clad welds.
Therefore, as follows from the example above and description of the invention,
objectives of
latter were achieved by a creating of a plate-like alpha phase structure with
a retained beta phase
within and along alpha plates and prior grain boundaries having high impact
and fracture
toughness in the tip of the airfoil, leading and trailing edges and equiaxed
alpha and intergranular
beta phases or a similar forging structure in the highly stressed sections of
the airfoil and blade
retainer, as well as method of repair and modification of these blades by a
laser cladding using
developed parameters and manufacturing of new blades by a differential heat
treatment.
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