Note: Descriptions are shown in the official language in which they were submitted.
METHOD AND APPARATUS FOR
REPAIRING METAL IN AN ARTICLE
This invention relates to the repair of a
metallic portion of an article and, more particularly, to
such a repair using a metal powder and a laser beam.
BACKGROUND OF THE INVENTION
Modern gas turbine engines, particularly of the
type designed primarily for aircraft applications, include
components which are expensive to manufacture because of
their complex design and materials of construction.
Durin~ normal operation of such apparatus, some components
can experience events such as normal wear or thermal
damage. In addition, mishaps such as machining errors or
other inadvertent damage can occur during initial
manufacture. For example/ such events or damage can occur
in connection with compressor or turbine spools or disks.
It has been found that, in respect to localized
repair of a portion of an article rather than replacement
of the portion, ordinary known means of repair such as
electron beam welding and gas tungsten arc welding can
adversely affect properties of the material of construction
because of relatively high heat input which results in
distortion and a relatively deep heat affected zone. Such
an excessive heat distribution has been found to cause
component cracking which is particularly critical in
rotating components such as wheels, drums, and spools of
gas turbine engines.
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SUMMARY OF T~E INVENTION
It is a principal object of the present
invention to provide an improved method -for repairing a
damaged metallic portion of an article to recreate its
original configuration and material while reducing adverse
effects on the article resulting from the method by limiting
the extent of the heat affected zone.
Another object is to provide such a repair
method which combines the use of metal repair powder
of substantially the same composition as a metal portion
of an article and a laser beam for the consolidation of
a repair layer or plurality of superimposed repair
layers on the article.
Still another object is to provide apparatus
of components which in combination provide a consistent
continuous powder flow to enable practice of the repair
method.
These and other objects and advantages will be
more fully understood from the following detailed
drawing, description, and examples, all of which are
intended to be representative of, rather than in any way
limiting on, the scope of the present invention.
Briefly, one form of the method of the present
invention for repairing an article having an article
portion of a metallic material includes the steps of
concurrently applying to the article portion a metal
powder, alone or in combination with another powder such
as an abrasive, and a laser beam. The metal powder is oE
a composition substantially the same as that of the
metallic material of the article portion and preferably
is applied at a rate in the range about 2-15 grams per
minute. The laser is applied, in a laser beam spot, to
the powder and the article portion beneath the powder
within a power density range of 10 to 106 watts per
square centimeter to generate a molten interaction zone
from the metal powder and the metallic material of the
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article portion. Inorder to further control the
resultant heat affected zone, an interaction time is
maintained in the range of 0.005-2 seconds between the
laser beam spot and the powder along wi-th the article
portion beneath the powder, for example by relative
lateral movement, to enable progressive movement, cooling,
and solidification of the molten interaction zone. This
controls and reduces the extent of the heat affected zone
in the article portion and provides a repair layer on the
article portion consolidated from the applied powder and
metallic material of the portion. Other forms of the
invention contemplate repeating such steps, if desired,
to provide a plurality of superimposed, bonded-together
repair layers using the preceding repair layer as the
"article portion" for the concurrent application of metal
powder and laser beam.
One form of the apparatus of the present
invention comprises a unique combination o~ components
including an enclosed, preferably heated powder reservoir,
means to introduce inert gas under pressure into the
reservoir, a mechanical volumetric powder feed means, a
powder vibrator, and a fluid-cooled delivery nozzle.
Together, such components deliver a consistent, continuous
flow of powder.
B~IEF DESCRIPTION OF THE DRAWING
FIGURE 1 is a fragmentary, partially sectional,
diagrammatic view of a portion of a compressor spool wi-th
which the present invention can be used.
FIGURE 2 is an enlarged fragmentary portion of
spool 10 of Figure 1.
FIGURES 3 and 4 are forms of specimens used in
the evaluation of the present invention.
FIGURE 5 is a diagrammatic, partially sectional
view of the apparatus practicing the method of the
present invention.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the initial manufacture as well as in -the
operation of aircraft gas turbine engine components,
damage from manufacturing error as well as from
operational thermal cycling, wear, part interference,
etc. can occur. Because of the relative high cost of
such components, there are many potential benefits which
can be obtained by their repair rather than their
replacement. Although such damage can occur to stationary
as well as rotating parts, damage is particularly critical
in the rotating components because of the stresses
generated in operation, during rotation. Because damaged
portions of components can be relatively thin, known
methods of repair, for example to build up undersized
portions, can produce heat affected zones inordinately
large and detrimental to the operation capabili-ty of the
component.
An example of one gas turbine engine component
with which the present invention has been used is a
compressor rotor including a plurality of disk-type
members disposed radially inwardly from a drum or spool
surface. One such compressor rotor is shown in U.S.
Patent No. 3,765,795, issued October 16, 1973, and
assigned to the assignee of the present invention. A
portion of such a spool is shown in the fragmentary,
partially sectional, diagrammatic view of Figure 1
including a four-stage segment of a larger compressor
rotor joined such as through bolting to an adjacent
member.
With reference to Figure 1, such a spool or
drum member is shown generally at 10 as comprising four
disk portions having a central opening and disposed for
rotation about engine axis 12. Drum or spool 10 is joined
to adjacen-t rotating member 14 through bolt means 16 which
also connects member 14 to an adjacent spool 18. Rotating
drum 10 and member 14 carry rotating blading members 20
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which cooperate with stator vanes 22, as is well known in
the axial flow compressor art.
In respect to the present invention, a lip
portion 24 of spool 10, enlarged in Figure 2, cooperates
with an adjacent portion of member 14 during operation.
Portion 24 can be subject to operation damage such as
through wear, erosion, thermal fatigue, etc. Enlarged
Figure 2 showing an end portion of spool 10 which
includes lip 24 shows, in phantom, material 23 worn from
lip 24. ~ecause it is desired in such a component to
maintain a smooth airflow therethrough and minimize
leakage, excessive clearances generated by such damage
can affect performance of the component. As can be
appreciated from the complexity of the structure involved,
it is far more attractive to repair rather than to replace
such a member. However, because of the relatively thin
sectional area involved, ordinary repair methods have been
found to be undesirable and detrimental to the metal from
which the member has been constructed.
During the evaluation of the present invention,
a variety of specimens were prepared including those of
the types shown in Figures 3 and 4. Such specimens were
intended to evaluate the effect of a laser beam in melting
powder to be consolidated with a substrate or a metallic
material of an article portion. The object of such
consolida-tion and rebuilding, sometimes called "Reverse
Maching" me-thod, is -to provide a repair layer or layers
of the same material as that from which the article was
made, while minimizing the heat affected zone as an
undesirable result of any such method. In general, the
heat affected zone is that portion of the material of an
article which has experienced relatively high localized
heat, for example as in brazing, welding, and other types
of processes, including repair processes, in which heat
energy is applied to a material surface. As the result of
such application of heat, the mechanical properties of
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the material can be significantly reduced, sometimes
to the point at which the article is no longer suitable
for its intended purpose without some subsequent restrengthen-
ing procedure. Because of the complexity of an article
such as the compressor rotor portion shown in Figure 1,
it is not economically attractive to restreng-then the
member after a repair process which generates an inordinately
large heat affected zone, particularly in such a highly
stressed member.
During the evaluation of the present invention,
coupons shown in the perspective, partially sectional
views of Figures 3 and 4 were used. Test repair layers
25 and 27 in Figure 3, of approximately 1.5 inches in
length, were deposited along the top surface 29 of the
specimen. In one series of tests, metal powder and the
laser beam spot were applied concurrently in two passes
made along the test surface: the first in which the
central axis of the laster beam spot was at the edge 26
of the specimen in Figure 3; and the second in which the
axis was along a line identified as broken line 28 and
0.065 inches from edge 26.
In these evaluations, the powder feed rate was
in the range of about 5-10 grams per minute, within
the broad range of 1-30 grams per minute according to
the present invention, concurrently with the application
of the laser beam from a 5000 watt continuous wave CO2
laser manufactured by Spectra Physics as Model 975 and
maintained at a power setting of 1.8-2.0 kilowatts which
corresponds to power densities in the range of 3.5-3.9 x 104
watts per s~uare centimeter. The beam spot size was in
the range of 0.05-0.15 inches. In these examples, a molten
interaction zone was created from the metal powder and
the metallic material of the coupon described above. The
coupons were made from a nickel base alloy commercially
available as IN718 alloy and the powder was IN718 alloy
having a composition substantially -the same as that of the
cUPn material
In all tests in which the first pass laser beam
axis was at edge 26, an internal or interdendritic crack
occurred in the repair layer. ~owever, in o-ther tests
using the same materials and apparatus and in which the
location of the axis of the laser beam spot in the first
pass was moved about 0.01-0.02" (nominally 0.15") away from
edge 26, no such cracks were observed. Therefore, -the
method of the present invention recognizes that in the
method of repairing an article portion including an edge
of the article, it is critical to direct the central axis
of a laser beam spot away from the edge of the article in
order to avoid cracking of the repair layer deposits. In
the evaluation in which the first pass beam spot axis was
away from the edge of the article, three vertical layers,
each including two side-by-side passes as described above
and displaced 0.06 inches, did not result in cracking of
the deposit. Therefore, a plurality of repair layers
can be superimposed one upon the other according to the
method of the present invention without generating cracks
in the deposit upon cooling. As used herein, the
designation "edge of an article" means the intersection
of surfaces or planes which, when examined in planar
cross section, results in an abrupt change in angle.
For example, an edge of the article can be a relatively
sharp outside corner of a radius of about 0.1 inch or
less.
In another series of evaluations, the
arrangement shown in Figure 4 was used. Although the
materials and conditions were the same, additional IN718
alloy powder 30 was preplaced in a groove along the side
of the coupon. Such groove was defined by the coupon and
sheet 32 held with the coupon body 33 as shown in Figure
4. This was intended to create a more favourable heat
transfer condition with the extra powder absorbing
photon energy and providing additional protection to the
edge from excessive melting. The additional power 30 can
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assist in reducing peak temperature by absorbing energy
required for melting (heat of fusion).
A ~urther evaluation of the present invention
involved a portion of the repair of a gas turbine engine
fan blade airfoil interlock surface which had been damaged
during manufacture. The blade was made of a titanium alloy
commercially available as Ti-6-4 alloy. The powder,
employed in the above-described method and apparatus to
repair the blade, was -35 mesh titanium powder conforming
to Aerospace Material Specification (AMS) 4928H, ELI grade.
In this example, the metal powder feed rate, assisted by
pressurized argon gas, was 4.5-5.5 grams per minute, with
the powder feed nozzle to workpiece at an angle of about
45-50. The above-described 5000 watt laser was used in
the power range of 3.5-4 kilowatts which corresponds to
power densities in the range o~ 6.9-7.9 x 104 watts per
square centimeter. The beam spot size was about 0.100
inch at an interaction time of about 0.27 seconds. In
this example, multiple layers were superimposed on the
workpiece surface, as described above, each layer with
a powder thickness of about 0.018-0.020 inch with a
total weld buildup not exceeding 0.140 inch.
As a result of these tests and evaluations, the
power density for practice of the method o the present
invention was defined as being in the range of 104 to 106
watts per square centimeter. Below the lower limit,
energy is reflected and the beam will not be sufficiently
absorbed by the substrate metallic material of the article,
leading to lack of fusion or consolidation between the
powder and the article substrate material. It is below
the threshold required for effective coupling of the
beam to the material. In addition, power densities
higher than 106 watts per square centimeter can produce
excessive vaporization at the article surface and
provide a deeper molten zone. From such higher power
densities, a wider heat affected zone is created with
B
resultant higher residual stresses between the metallic
material of the article portion and the added resolidi-
fication cast structure of the repair layer after cooling
of the molten zone.
The method of the present invention recognizes
the need for controlling the interaction time between the
laser beam and the article material with its superimposed
powder. Therefore, at the power densities defined for
the present invention, it has been recognized that, in a
preferred form, an interaction time of 0.1-2 seconds is
necessary for such control. Interaction time is defined
as the ratio of beam spot size to relative lateral
movement between the laser beam spot and the article
portion.
It has been recognized that the method of the
present invention can be operated in an interaction time
of as low as 0.005 second provided that the power density
domain is maintained within the range of 10~ to 106
wat-ts per square centimeter. The following table presents
typical, calculated relationships between laser beam spot
size, relative lateral movement represented by table speed,
and interaction time, within the scope of the method of
the present invention.
'TA~LE
25SPOT SIZETABLE SPEED INTERACTION TI~E
(inches')'('inches per-sec.) (seconds)
0.005 0.5 0.01
0.200 0.5 0.~
0.005 0.1 0.05
0.200 0.1 2.0
300.005 1.0 0.005
0.200 1.0 0.2
Therefore, the method of the present invention
minimizes the extent of the heat affected zone resulting
from the application of the laser beam to the combination
of a powder superimposed on the metallic material of an
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article portion by defining the limits of the power
density and -the interaction -time: the power density is
in the range of 104 to 106 watts per square centimeter
in combination with the interaction time in the range of
0.005-2 seconds between the beam spot and the powder and
article portion beneath the powder.
The apparatus of the present inven-tion and that
used in the evaluation of the method is shown in the
diagrammatic sectional view of Figure 5. The apparatus
includes an enclosed powder reservoir shown generally at
40, heated by heating coils 42 for the purpose of controlling
the moisture content at a low level in the powder. Also
included is a gas inlet port 44 through which a preferably
dry inert gas such as argon, represented by arrow 46, is
introduced to maintain powder 48 in reservoir 40 under
pressure and to assist in powder transport. Associated
with the powder reservoir is a mechanical, volumetric
powder feed mechanism such as powder feed wheel 50 of a
type commercially available. For e~ample, the type used
in one form of the apparatus of the present invention was
a modified ~etco powder feed "L" type wheel.
Downstream of wheel 50 is a vibrator such as
air actuated vibrator 52 associated with conduit 54 to
inhibit powder particles moving in conduit 54 from adhering
one to the other or to walls of the condui-t 54. Conduit
54 terminates in a water-cooled powder delivery nozzle 56
which directs -the powder, assisted by the pressurized
inert gas, in a consis-tent flow, such as toward an ar-ticle
or workpiece surface. It has been found -that reflection
from -the laser beam can resul-t in clogging of powder
passing through nozzle 56. Therefore, such a nozzle,
preferably having at least a -tip portion made of a
material, such as copper or aluminum, which is highly
reflective to the wave length of the laser used, is fluid
cooled, such as by water, to avoid such problem and to
assist in a consistent flow of powder. Such consistent
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flow of powder results from the combination of use of
powder maintained in a low moisture condition, under a
positive inert gas pressure, being fed by a mechanical,
volumetric powder feed mechanism along with a powder
vibrator, and a cooled nozzle through which the powder
passes toward the article surface in the laser beam spot.
Associated with -the apparatus of the present
invention is a laser 58 emitting a beam 60 having a beam
axis 62. Beam 60 has a focal plane 64 beneath article
portion or workpiece surface 66 to provide at the surface
a beam spot 68 of a size in the range of 0.005-0.2 inches,
and preferably in the range of 0O05-0.15 inches. As was
described above, the laser beam was applied in the power
density range of 104 to 106 watts per square centimeter
while controlling the interaction time between the beam
spot and the powder and article portion beneath the
powder in the range of 0.005-2 seconds. Such control
is obtained by relative lateral motion shown by arrow 70,
for example from a table moving the article in the
direction shown at a rate to provide the desired inter-
action time.
The powder is fed from nozzle 56 at an angle
in the range of about 35-60 degrees from the article
surface and preferably in the range of about 40-55 degrees.
Greater than about 60 degrees makes it difficul-t for the
nozzle and powder to avoid premature interaction with the
laser beam, and less than about 35 degrees makes it
difficult to deliver the powder concurrently with the
laser beam at the spot desired on the article surface.
As the powder and laser beam are applied concurrently to
the article surface, a molten interaction zone is
generated in the area of the beam spot. As relative
lateral movement is provided between the laser beam spot
and the article carrying its superimposed powder,
progressive movement, cooling and solidification of the
molten interaction zone occurs. This provides consolidated
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repair layer 72 as shown in Flgure 5. Such a consolidated
layer resulting from the practice of the present invention
also is shown in phantom at 23 in Figure 2.
After application and consolidation, excess of
the repair layer material added can be shaped to a desired
configuration, such as by machining, grinding, dressing
of abrasive particles, etc. In this way, there is provided
a repaired article having a minimized heat affected zone
and desired mechanical properties.
Although the present invention has been described
in connection with specific examples and embodiments, it
will be unders-tood by those skilled in the arts involved,
for example metallurgy, metal joining, and laser technology,
that the present invention is capable of modification
without departing from its scope as represented by the
appended claims.
