Background of the invention
The present invention relates'to the general field
of marking mechanical parts.
A field of application of the invention is that of
marking aviation parts, and in particular parts for
10 aviation turbine engines in order to enable them to be
identified and authenticated.
In the aviation field, it is known to put a serial
number on certain engine parts (e.g. in the form of a
hexadecimal base code or in the form of a Datamatrix
15 code), thereby enabling such parts to be identified (they
are said to be "marked"). Using such a number, it is
possible to know exactly that a part is authentic and to
know its origin.
Marking is particularly desired for the blades of
20 turbines and compressors in aviation turbine engines.
Blades are critical replacement parts for which it is
important to know their exact origin in order to take
account of the influence of replacing such parts on the
lifetime of the turbine or compressor in question.
25 Parts, and in particular turbine or compressor
blades for an aviation turbine engine can be marked in
various ways. It is thus known to perform marking by
means of a laser that makes in imprint in the part for
marking over a plurality of passes by removing material.
30 It is also known to perform marking mechanically with the
help of a hammer or a pneumatic piston that, by imparting
successive impacts, enables a serial number of .to be
marked on a part. It is also known to perform marking by
means of a manual or automatic milling machine.
35 Marking techniques that rely on the principle of
removing material or of deforming the part for marking so
as to place the desired serial number thereon present a
2
manifest drawback for the soundness of the material of
the part that is to be marked. Specifically for a part
that is made as a single crystal of metal alloy,
deforming or removing material can lead to localized
5 recrystallization or to an irreversible defect in its
microstructure.
Furthermore, with turbine or compressor blades,
marking by those conventional techniques is generally
performed on the roots of the blades. Marking a portion
10 of the blade that is exposed to gas (e.g. its airfoil)
cannot be envisaged: the gas sweeping over the surface of
such a zone runs the risk of erasing the serial number by
erosion/oxidation, or indeed by tearing out material.
Furthermore, a crack starter at the location of the
15 marking might appear as a result of vibratory fatigue.
Unfortunately, marking a blade on its root raises
the problem of the root of a blade being a zone that is
hidden once the blade is assembled, such that identifying
the blade becomes impossible once it has been mounted in
20 an engine.
Obj ect and summary of the invention
An obj ect of the present invention is thus to
provide a method that enables marking to be performed
25 simply and quickly while not presenting the abovementioned
drawbacks.
In accordance with the invention, this object is
achieved by a method of surface marking a mechanical part
with a predefined graphical representation, the method
30 comprising using a laser source to apply a single laser
pulse to an outside surface of a part for marking, with a
mask being interposed between the laser source and the
outside surface of the part, the mask having a predefined
graphical representation, and the laser pulse having
35 power flux density of at least 20 megawatts per square
centimeter (MW/cm2) and a duration less than or equal to
100 nanoseconds (ns).
3
The Applicant has found that applying a laser pulse
under the above-specified conditions through a mask makes
it possible to make a mark (possibly visible to the naked
eye, depending on the diameter of the impact of the laser
5 pulse) on a mechanical part, and in particular on a gas
turbine engine part, with very little ablation of
material and with excellent ability to withstand
oxidizing conditions at high temperature. In particular,
it has been found that the imprint left by the single
10 laser pulse on the surface of the part for marking is
very superficial in depth (less than one micrometer). It
has also been observed that the imprint made by the laser
pulse is longlasting, even when it is subjected to high
temperature (about 1100°C) in a highly oxidizing
15 atmosphere.
As a result, in an application to marking compressor
or turbine blades, the method of the invention may be
applied to the portion of the blade that is exposed to
gas (i.e. to the airfoil), thereby avoiding all of the
20 drawbacks of marking a zone of the blade that is hidden.
In particular, it is possible to identify blades by means
of their marking, even while the blades are mounted in
the engine.
Furthermore, it has been found that the method of
25 the invention is just as effective (in terms of little
ablation of material and long life) regardless of whether
the part for marking is made of metal (in particular Ni,
Al, Ti, Fe, etc.), of composite material (in particular
carbon fibers with an epoxy matrix), or of ceramic (in
30 particular of zirconia).
Finally, the method of the invention is fast (only
one laser pulse is needed), simple to perform (no
material needs to be applied), and makes it possible to
make marks with shapes that are complex (e.g. a company
35 logo), depending on the selected mask.
A focusing lens may be interposed between the laser
source and the mask in order to change the size of the
I
4
beam emitted by the laser source. Which source may be an
Nd-YAG laser. Furthermore, the laser pulse may have an
impact diameter of at least 0.5 millimeters (mm), thereby
ensuring that the resulting mark is visible to the naked
5 eye.
When the part for marking is made of metal, the
laser pulse preferably has power flux density lying in
the range 0.04 GW/cm2 to 0.55 GW/cm2.
When the part for marking is made of composite
10 material comprising carbon fibers and an epoxy matrix,
the laser pulse preferably has power flux density lying
in the range 0.15 GW/cm2 to 2 GW/cm2.
When the part for marking is made of ceramic, the
laser pulse preferably has power flux density lying in
15 the range 0.10 GW/cm2 to 0.34 GW/cm2.
According to an advantageous provision, the method
further includes interposing an opaque mark between the
laser source and the outside surface of the part, the
opaque mark having a plurality of color gradations so as
20 to obtain multi-contrast marking of the part. Having
recourse to such an opaque mask enables the marks that
are made to be more complex, thereby making them much
more difficult to reproduce.
The invention also provides the use of the method as
25 defined above for marking a fan blade, a turbine blade,
or a compressor blade of an aviation turbine engine.
Brief description of the drawings
Other characteristics and advantages of the present
30 invention appear from the following description made with
reference to the accompanying drawings, which show an
implementation having no limiting character. In the
figures:
• Figure 1 is a diagrammatic view of an example of a
35 setup for performing the method;
5
• Figures 2 to 4 are photos showing different
examples of marking obtained by the method of the
invention; and
• Figure 5 is a diagrammatic view of an example
5 opaque mask for performing a variant implementation of
the invention.
Detailed description of the invention
The invention applies to surface marking any
10 mechanical part with a predefined graphical
representation, and in particular to marking parts for
aviation, and more particularly parts of a gas turbine
engine.
The term "predefined graphical representation" is
15 used to mean any predetermined design or geometrical
shape, e.g. such as a logo, a serial number, a Datamatrix
code, etc.
A non-limiting example application of the invention
is that of surface marking fan blades, turbine blades, or
20 compressor blades for an aviation turbine engine.
The method of the invention comprises applying a
single laser pulse to the outside surface of a part for
marking, with a mask being interposed between the laser
source and the outside surface of the part, which mask
25 has the predefined graphical representation that it is
desired to mark on the part.
In the invention, the laser pulse that is applied to
the outside surface of the part possesses power flux
density of at least 20 MW/cm2, and a duration that is less
30 than or equal to 100 ns.
Figure 1 is a diagram showing an example of a setup
suitable for use in performing the marking method of the
invention.
A part 10 for marking (e.g. a turbine blade) having
35 an outside surface 10a on which the marking is to be
performed is supported by a support stand 12. The
outside surface 10a of the part faces upwards.
6
A laser source 14, e.g. an Nd-YAG type laser
producing radiation at a wavelength of 1.064 micrometers
(pm) and frequency-doubled is positioned over the support
stand 12 and is configured to deliver pulses having power
5 flux density of not less than 20 MW/cm2 for a duration
that is less than or equal to 100 ns.
Furthermore, a mask 16 having a predefined graphical
representation is interposed between the laser source and
the outside surface 10a of the part 10 for marking.
10 Likewise, a focusing lens 18 (convergent or divergent) is
positioned between the laser 14 and the mask 16 in order
to match the size of the beam emitted by the laser to the
dimensions of the mask.
As a result, the laser 14 produces radiation that is
15 focused by means of the focusing lens 18 into a beam that
passes through the mask 16 before illuminating a selected
zone of the outside surface of the part. The laser pulse
produced by the laser 14 generates a plasma in this zone,
and expansion of the plasma gives rise to a large amount
20 of (thermomechanical and acoustic) energy being released,
thereby causing local modification to the surface of the
part for marking. When the laser pulse produced by the
laser is set at specified above (i.e. with power flux
density of at least 20 MW/cm2 and duration less than or
25 equal to 100 ns), this local modification to the surface
of the part gives rise to an imprint that is left in the
surface of the part.
As a result, the marking obtained by the method of
the invention is in the form of an imprint left in the
30 surface of the part, which imprint possesses a design
corresponding to that of the mask used (where the mask
acts as a negative).
This imprint presents dimensions, specifically a
diameter, corresponding to the diameter of the impact of
35 the laser pulse produced by the laser. Thus, with an
impact diameter of at least 0.5 mm for the laser pulse,
the marking that is obtained presents a diameter of at
7
least 0.5 mm (which makes it possible for it to be
visible to the naked eye). This diameter may be up to
150 mm (if necessary by having recourse to a focusing
lens 18 that is divergent).
5 When the laser pulse is applied under such
conditions, it has been found that only one pulse is
needed to mark the part. In particular, there is no need
to apply a plurality of laser pulses to the same zone in
order to obtain such a result.
10 It has also been found using a profile meter that
the imprint left in the outside surface 10a of the part
10 is very superficial in depth (less than one
micrometer) regardless of the impact diameter and
regardless of the material of the part (metal, ceramic,
15 or composite).
It has also been found that the marking that is
obtained, even though it is of very superficial depth,
nevertheless withstands an environment that is highly
oxidizing and at high temperature. In particular, tests
20 have demonstrated that such marking (made on a nickel
part) remains after performing 700 cycles of 1 hour (h),
each in air at 1100°C.
It should be observed that a mark can be marked
without it being necessary to have a confinement medium
25 that is transparent at the laser wavelength used (such as
for example water or glass for an Nd-YAG type laser) in
order to obtain better expansion of the plasma generated
by the laser.
Likewise, it may be observed that the marking can be
30 performed in an ambient atmosphere, providing the power
flux density of the laser pulse produced by the laser is
limited to 10 GW/cm2 (which corresponds to the breakdown
threshold of air). For power flux density at values
higher than 10 GW/cm2, marking needs to be performed in a
35 vacuum.
The marking method of the invention may be applied
to any type of material. In particular, it is well
8
adapted to marking parts made of metal, of ceramic, or of
composite material. It also applies to any surface
coating material for a part.
When the part for marking is made of metal, a power
5 flux density should be applied that preferably lies in
the range 0.04 GW/cm2 to 0.55 GW/cm2 so as to obtain an
imprint that is perfectly intelligible.
More precisely, with a part made of nickel, a power
flux density is advantageously applied that lies in the
10 range 0.10 GW/cm2 to 0.52 GW/cm2. For a part made of
aluminum, a power flux density should be applied that
lies in the range 0.20 GW/cm2 to 0.55 GW/cm2, and for a
part made of steel, a power flux density should be
applied that preferably lies in the range 0.10 GW/cm2 to
15 0.50 GW/cm2.
Furthermore, with a part for marking that is made of
composite material having carbon fibers and an epoxy
matrix, the laser pulse should preferably have power flux
density lying in the range 0.15 GW/cm2 to 2 GW/cm2 so as
20 to obtain an imprint that is perfectly intelligible.
Finally, with a part for marking that is made of
ceramic, the laser pulse should preferably have power
flux density lying in the range 0.10 GW/cm2 to
0.34 GW/cm2, in order to obtain an imprint that is
25 perfectly intelligible.
With reference to Figures 2 to 4, there follows a
description of various examples of marking made using the
method of the invention.
Figure 2 is a photo (scale 4:1) showing the result
30 of applying a laser pulse in accordance with the method
of the invention on a substrate of composite material of
the carbon/carbon type.
The marking 20 in this figure that is visible to the
naked eye and that is circular in shape was obtained by
35 means of a single laser pulse having an impact diameter
of 8.7 mm, power flux density equal to 99 MW/cm2, and a
9
duration of 5.2 ns. The mask used did not possess any
graphical representation.
Likewise, Figure 3 is a photo (scale 4:1) showing
the result of applying a laser pulse in accordance with
5 the method of the invention to a substrate made of
zirconium dioxide (Zr02/Y203).
The marking 20' in this figure that is visible to
the naked eye and that is circular in shape was obtained
by means of a single laser pulse having an impact
10 diameter of 9.1 mm, power flux density equal to
135 MW/cm2, and a duration of 5.2 ns. The mask used did
not have any graphical representation.
Finally, Figure 4 is a photo (scale 4:1) showing the
result of applying a laser pulse in accordance with the
15 method of the invention to a substrate made of aluminum.
The marking 20" in this figure that is visible to
the naked eye and that is circular in shape was obtained
by means of a single laser pulse having an impact
diameter of 13 mm, a power flux density equal to
20 41 MW/cm2, and a duration of 5.2 ns. The mask used did
not possess any graphical representation.
Together these photos show that applying a single
laser pulse under the above-specified power flux density
and duration conditions make it possible to obtain
25 marking that is perfectly intelligible regardless of the
material of the part that is to be marked (or of its
surface coating), this marking being innocuous for the
material of the part and being capable of withstanding an
environment that is both oxidizing and at high
30 temperature.
According to an advantageous provision, it is
possible to interpose an additional mask referred to as
an "opaque" mask between the laser source and the outside
surface of the part for marking. This opaque mask is in
35 addition to the mask 16 having a predefined graphical
representation as described above (the opaque mask may be
10
positioned equally well upstream or downstream from the
mask 16 in the travel direction of the laser beam).
The opaque mask is in the form of a mask comprising
a medium (e.g. a liquid or a glass) with a plurality of
5 color gradations so as to obtain multi-contrast marking
of the part.
In particular, the opaque mask should be selected in
such a manner as to provide controlled attenuation of the
intensity of the laser source as a function of the
0 pattern of the mask for reproducing on the part. Thus,
the zones of the opaque mask with little attenuation pass
greater laser intensity, while the zones with
considerable attenuation pass only very little laser
intensity.
5 Figure 5 shows an example of an opaque mask 22
suitable for use in obtaining contrast gradation for
marking. In this example, the opaque mask 22 has a
central zone 22a with little attenuation and a peripheral
zone 22b with stronger attenuation. Using such an opaque
0 mask thus makes it possible to obtain a graphical
representation having contrast gradation between the
central zone and the peripheral zone of the
representation.

CLAIMS
1. A method of surface marking a mechanical part with a
predefined graphical representation, the method
comprising using a laser source to apply a single laser
5 pulse to an outside surface of a part for marking with a
mask being interposed between the laser source and the
outside surface of the part, the mask having a predefined
graphical representation, and the laser pulse having
power flux density of at least 20 MW/cm2 and a duration
10 less than or equal to 100 ns.
2. A method according to claim 1, wherein a focusing lens
is interposed between the laser source and the mask.
15 3. A method according to claim 1 or claim 2, wherein the
laser source is an Nd-YAG laser.
4. A method according to any one of claims 1 to 3,
wherein the laser pulse has an impact diameter of at
20 least 0.5 mm.
5. A method according to any one of claims 1 to 4,
wherein, when the part for marking is made of metal, the
laser pulse has power flux density lying in the range
25 0.04 GW/cm2 to 0.55 GW/cm2.
6. A method according to any one of claims 1 to 4,
wherein, when the part for marking is made of composite
material comprising carbon fibers and an epoxy matrix,
30 the laser pulse has power flux density lying in the range
0.15 GW/cm2 to 2 GW/cm2.
7. A method according to any one of claims 1 to 4,
wherein, when the part for marking is made of ceramic,
35 the laser pulse has pov/er flux density lying in the range
0.10 GW/cm2 to 0.34 GW/cm2.
12
8. h method according to any one of claims 1 to 7,
further including interposing an opaque mark between the
laser source and the outside surface of the part, the
opaque mark having a plurality of color gradations so as
to obtain multi-contrast marking of the part.
9. The use of the method according to any one of claims
to 8 for marking a fan blade, a turbine blade, or a
compressor blade of an aviation turbine engine.