U.S. patent application number 13/768875 was filed with the patent office on 2013-08-15 for method and apparatus for reliably laser marking articles.
This patent application is currently assigned to ELECTRO SCIENTIFIC, INDUSTRIES, INC.. The applicant listed for this patent is ELECTRO SCIENTIFIC, INDUSTRIES, INC.. Invention is credited to Patrick Leonard, Glenn Simenson, Haibin Zhang.
Application Number | 20130208074 13/768875 |
Document ID | / |
Family ID | 44353400 |
Filed Date | 2013-08-15 |
United States Patent
Application |
20130208074 |
Kind Code |
A1 |
Zhang; Haibin ; et
al. |
August 15, 2013 |
METHOD AND APPARATUS FOR RELIABLY LASER MARKING ARTICLES
Abstract
The invention is a method and apparatus for creating marks 194
on an anodized aluminum specimen 190 with selectable color and
optical density. The method includes providing a laser marking
system 148 having a laser 150, laser optics 154 and a controller
160 operatively connected to said laser 150 to control laser pulse
parameters. The laser marking system 148 is directed to produce
laser pulses 152 having laser pulse parameters associated with the
desired color and optical density in the presence of a fluid 168
directed to the surface of the anodized aluminum specimen 158 while
marking.
Inventors: |
Zhang; Haibin; (Portland,
OR) ; Simenson; Glenn; (Portland, OR) ;
Leonard; Patrick; (Ann Arbor, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ELECTRO SCIENTIFIC, INDUSTRIES, INC.; |
|
|
US |
|
|
Assignee: |
ELECTRO SCIENTIFIC, INDUSTRIES,
INC.
Portland
OR
|
Family ID: |
44353400 |
Appl. No.: |
13/768875 |
Filed: |
February 15, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12871619 |
Aug 30, 2010 |
8379678 |
|
|
13768875 |
|
|
|
|
12704293 |
Feb 11, 2010 |
8379679 |
|
|
12871619 |
|
|
|
|
Current U.S.
Class: |
347/264 |
Current CPC
Class: |
B23K 26/40 20130101;
B23K 2101/34 20180801; B23K 2101/35 20180801; B23K 2103/10
20180801; B23K 26/0622 20151001; B23K 26/14 20130101; B41J 2/442
20130101; B23K 26/0006 20130101; B41M 5/262 20130101; B41M 5/24
20130101 |
Class at
Publication: |
347/264 |
International
Class: |
B41J 2/44 20060101
B41J002/44 |
Claims
1. A method for creating a mark on an article, said method
comprising: providing said article; cooling at least a portion of
the article; and directing a beam of laser pulses onto said article
thereby creating said mark.
2. The method of claim 45 wherein said article is an anodized
aluminum article.
3-4. (canceled)
5. The method of claim 49 wherein said fluid is water.
6. The method of claim 49 wherein said fluid is a flowing
fluid.
7. The method of claim 6 further comprising moving said fluid in
relation to said anodized metal article to maintain a relationship
between said fluid and said laser pulses.
8-17. (canceled)
18. The method of claim 1 wherein said mark has an optical density
equal to or less than about L*=40, a*=5, and b*=10.
19. (canceled)
20. A laser marking apparatus adapted to produce marks on an
article, said apparatus comprising: a laser operative to produce
laser pulses; laser optics operative to modify and direct said
laser pulses; a stage operative to hold and position said article;
a fluid operative to absorb heat from said article; and a
controller operative to access predetermined laser pulse parameters
and in cooperation with said laser, laser optics and stage, create
and direct said laser pulses according to said predetermined laser
pulse parameters to impinge upon said article while said fluid
absorbs heat created by said laser pulses from said article thereby
producing said marks.
21-23. (canceled)
24. The apparatus of claim 20 wherein said fluid is water.
25. The apparatus of claim 20 further comprising a nozzle
configured to direct a flow of said fluid onto said anodized
article.
26. The apparatus of claim 25 wherein said fluid flow is moveable
in relation to said article to maintain a relationship between said
fluid and said laser pulses.
27-41. (canceled)
42. The method of claim 1, therein said article includes a
substrate.
43. The method at claim 42, wherein said substrate is a metal
substrate.
44. The method of claim 42, wherein said article includes a layer
on a surface of the substrate.
45. The method of claim 1, wherein said article includes a layer
having, at a first temperature, a fluence threshold above which
said layer tends to become damaged, and said cooling at least said
portion of said article comprises cooling at least said portion of
said layer to a second temperature lower than the first
temperature.
46. The method of claim 45, wherein directing said beam of laser
pulses onto said article comprises directing said beam of laser
pulses onto said article at a fluence level above said fluence
threshold.
47. The method of claim 44, wherein said layer includes an oxide
layer.
48. The method of claim 47, wherein said layer includes an anodic
oxide layer.
49. The method of claim 1, wherein cooling at least a portion of
said article comprises thermally contacting said article with a
fluid.
50. The method of claim 1, wherein cooling at least said portion of
said article comprises cooling at least said portion of said
article while directing said beam of laser pulses onto said
article.
51. The apparatus of claim 25, further comprising a fluid supply
coupled to said nozzle, said fluid supply configured to supply said
fluid to said nozzle.
52. An article having a mark created thereon, said mark being
created according to a process comprising: providing said article;
cooling at least a portion of the article; and directing a beam of
laser pulses onto said article thereby creating said mark.
53. The article of claim 52, wherein said article includes a
substrate.
54. The article of claim 53, wherein said substrate includes a
metal.
55. The article of claim 53, wherein said article includes a layer
on a surface of the substrate.
56. The article of claim 55, wherein said layer includes an oxide
layer.
57. The article of claim 56, wherein said layer includes an anodic
oxide layer.
58. The article of claim 52, wherein said article includes a layer
having, at a first temperature, a fluence threshold above which
said layer tends to become damaged, and said cooling at least said
portion of said anodized metal article comprises cooling at least
said portion of said anodic oxide layer to a second temperature
lower than the first temperature.
59. The article of claim 56, wherein directing said beam of laser
pulses onto said anodized metal article comprises directing said
beam of laser pulses onto said anodized metal article at a fluence
level above said fluence threshold.
60. The article of claim 52, wherein said article is an anodized
aluminum article.
61. The article of claim 52, wherein said mark has an optical
density equal to or less than about L*=40, a*=5, and b*=10.
Description
[0001] This application is a continuation of prior application Ser.
No. 12/871,619, filed Aug. 30, 2010, which is a
continuation-in-part of prior application Ser. No. 12/704,293,
filed Feb. 11, 2010.
TECHNICAL FIELD
[0002] The present invention relates to laser marking of anodized
articles with a laser processing system. More particularly it
relates to marking anodized articles in a durable and commercially
desirable fashion with a laser processing system. Specifically it
relates to characterizing the interaction between ultraviolet,
visible and infrared wavelength laser pulses and the anodized
articles to reliably and repeatably create durable marks with a
desired color and optical density in the presence of a fluid.
BACKGROUND OF THE INVENTION
[0003] Marketed products commonly require some type of marking on
the product for commercial, regulatory, cosmetic or functional
purposes. Desirable attributes for marking include consistent
appearance, durability, and ease of application. Appearance refers
to the ability to reliably and repeatably render a mark with a
selected shape, color and optical density. Durability is the
quality of remaining unchanged in spite of abrasion to the marked
surface. Ease of application refers to the cost in materials, time
and resources of producing a mark including programmability.
Programmability refers to the ability to program the marking device
with a new pattern to be marked by changing software as opposed to
changing hardware such as screens or masks.
[0004] Anodized metallic articles, which are lightweight, strong,
easily shaped, and have a durable surface finish, have many
applications in industrial and commercial goods. Anodization
describes any one of a number of electrolytic passivation processes
in which a natural oxide layer is increased on surfaces of metals
such as aluminum, titanium, zinc, magnesium, niobium or tantalum in
order to increase resistance to corrosion or wear and for cosmetic
purposes. These surface layers (also known as "anodic oxide
layers") can be colored or dyed virtually any color, making a
permanent, colorfast, durable surface on the metal. Many of these
metals can be advantageously marked using aspects of the instant
invention. In addition, metals such as stainless steel which resist
corrosion can be marked in this fashion. Many articles manufactured
out of metals such these as are in need of permanent, visible,
commercially desirable marking. Anodized aluminum is an exemplary
material that has such needs. Marking anodized aluminum with laser
pulses produced by a laser processing system can make durable marks
quickly at extremely low cost per mark in a programmable
fashion.
[0005] Creating color changes on the surface of anodized aluminum
with laser pulses has been known for several years. An article
titled "Dry laser cleaning of anodized aluminum" by P. Maja, M.
Autric, P. Delaporte, P. Alloncle, COLA'99-5th International
Conference on Laser Ablation, Jul. 19-23, 1999, Gottingen, Germany,
published in Appl. Phys. A 69 [Suppl.], S343-S346 (1999), pp
S43-S346, describes removing anodization from aluminum surfaces,
however, note is taken of color changes which occur at laser
energies below that required for removal of anodization from the
surface.
[0006] One mechanism which has been put forth to explain the change
in optical density or color of metallic surfaces is the creation of
laser-induced periodic surface structures (LIPSS). The article
"Colorizing metals with femtosecond laser pulses" by A. Y. Vorobyev
and Chunlei Guo, Applied Physics Letters 92, (041914) 2008, pp
41914-1 to 141914-3 describes various colors which may be created
on aluminum or aluminum-like metals using femtosecond laser pulses.
This article describes making black or gray marks on metal and
creating a gold color on metal. Some other colors are mentioned but
no further description is made. LIPSS is the only explanation
offered for the creation of marks on metallic surfaces. Further,
only laser pulses having temporal pulse widths of 65 femtoseconds
are taught or suggested to create these structures. In addition, no
mention is made as to whether the aluminum samples are anodized or
have had the surface cleaned prior to laser processing. Further the
article does not discuss possible damage to the oxide layer.
[0007] When discussing laser pulse duration, the method of
measuring pulse duration should be defined. Temporal pulse shape
can range from simple Gaussian pulses to more complex shapes
depending upon the task. Exemplary non-Gaussian laser pulses
advantageous for certain types of processing are described in U.S.
Pat. No. 7,126,746 GENERATING SETS OF TAILORED LASER PULSES,
inventors Sun et al., which patent has been assigned to the
assignees of the instant invention and is hereby incorporated by
reference. This patent discloses methods and apparatus to create
laser pulses with temporal profiles that vary from the typical
Gaussian temporal profiles produced by diode pumped solid state
(DPSS) lasers. These non-Gaussian pulses are called "tailored"
pulses because their temporal profile is altered from the typical
Gaussian profile by combining more than one pulse to create a
single pulse and/or modulating the pulse electro-optically. This
creates a pulse which the pulse energy varies as a function of
time, often including one or more power peaks wherein the
instantaneous power increases to a value greater than the average
power of the pulse for a fraction of the pulse duration. This type
of tailored pulse can be effective in processing materials at high
rates without causing problems with debris or excessive heating of
surrounding material. An issue is that measuring the duration of
complex pulses such as these using standard methods typically
applied to Gaussian pulses can yield anomalous results. Gaussian
pulse durations are typically measured using the full width at half
maximum (FWHM) measure of duration. In contrast to this, using the
integral square method, as described in U.S. Pat. No. 6,058,739
LONG LIFE FUSED SILICA ULTRAVIOLET OPTICAL ELEMENTS, inventors
Morton et al., allows complex pulse temporal shapes to be measured
and compared in a more meaningful manner. In this patent, pulse
duration is measured using the formula
t = ( .intg. T ( t ) t ) 2 .intg. T 2 ( t ) t ##EQU00001##
where T(t) is a function which represents the temporal shape of the
laser pulse.
[0008] Another problem with reliably and repeatably producing marks
with desired color and optical density in anodized aluminum is that
the energy required to create very dark marks with readily
available nanosecond pulse width solid state lasers is enough to
cause damage to the anodization, an undesirable result. "Darkness"
or "lightness" or color names are relative terms. A standard method
of quantifying color is by reference to the CIE system of
colorimetry. This system is described in "CIE Fundamentals for
Color Measurements", Ohno, Y., IS&T NIP16 Conf, Vancouver,
Conn., Oct. 16-20, 2000, pp 540-545. In this system of measurement,
achieving a commercially desirable black mark requires parameters
less than or equal to L*=40, a*=5, and b*=10. This results in a
neutral colored black mark with no visible grayness or coloration.
In U.S. Pat. No. 6,777,098 MARKING OF AN ANODIZED LAYER OF AN
ALUMINIUM OBJECT, inventor Keng Kit Yeo describes a method of
marking anodized aluminum articles with black marks which occur in
a layer between the anodization and the aluminum and therefore are
as durable as the anodized surface. The marks described therein are
described as being dark grey or black in hue and somewhat less
shiny than unmarked portion using nanosecond range infrared laser
pulses. In addition, the aluminum is required to be cleaned of all
surface particles, for instance particles remaining after
polishing, prior to anodization. Making marks according to the
methods claimed in this patent are disadvantageous for two reasons:
first, creating commercially desirable black marks with
nanosecond-range pulses tends to cause destruction of the oxide
layer and secondly, cleaning of the aluminum following polishing or
other processing adds another step in the process, with associated
expense, and possibly disturbs a desired surface finish by further
processing.
[0009] What is desired but undisclosed by the art is a reliable and
repeatable method of making marks on anodized aluminum in both
black or grey or in color that does not require an expensive
femtosecond laser or disturb the oxide layer in the process or
require cleaning following surface preparation. In addition, no
information is supplied on how to repeatably create various colors
on anodized aluminum surfaces, nor has the effects of bleaching or
damage to the anodization layer been thoroughly investigated. What
is needed then is a method for reliably and repeatably creating
marks having a desired optical density or grayscale and color on
anodized aluminum using a lower cost laser, without causing
undesired damage to the overlaying oxide or requiring cleaning
prior to anodization.
SUMMARY OF THE INVENTION
[0010] An aspect of this invention is to mark anodized aluminum
articles with visible marks of various optical densities or
grayscale and colors. These marks should be durable and have
commercially desirable appearance. This is achieved by using laser
pulses to create the marks. These marks are created at the surface
of the aluminum underneath the oxide layer and are therefore
protected by the oxide. The laser pulses create commercially
desirable marks without causing significant damage to the oxide
layer, thereby making the marks durable. Durable, commercially
desirable marks are created on anodized aluminum by controlling the
laser parameters with which create and direct laser pulses. In one
aspect of this invention a laser processing system is adapted to
produce laser pulses with appropriate parameters in a programmable
fashion. Using a fluid flow while laser marking inhibits thermal
damage in the oxide layer during marking, permitting higher
energies to be used which yield greater range of colors and optical
densities and higher throughput.
[0011] Exemplary laser pulse parameters which may be selected to
improve the reliability and repeatability of laser marking anodized
aluminum include laser type, wavelength, pulse duration, pulse
repletion rate, number of pulses, pulse energy, pulse temporal
shape, pulse spatial shape and focal spot size and shape.
Additional laser pulse parameters include specifying the location
of the focal spot relative to the surface of the article and
directing the relative motion of the laser pulses with respect to
the article.
[0012] Aspects of this invention create durable, commercially
desirable marks by darkening the surface of the aluminum beneath
the anodization with optical densities which range from nearly
undetectable with the unaided eye to black depending upon the
particular laser pulse parameters employed. Other aspects of this
invention create colors in various optical densities in shades of
tan or gold, likewise depending upon the particular laser pulse
parameters employed. Other aspects of this invention create
durable, commercially desirable marks on anodized aluminum by
bleaching or partially bleaching dyed or colored anodization with
or without marking the aluminum beneath. Other aspects use a fluid
flow during laser processing to reduce oxide damage.
[0013] To achieve the foregoing with these and other aspects in
accordance with the purposes of the present invention, as embodied
and broadly described herein, a method for creating a color and
optical density selectable visible mark on an anodized aluminum
specimen and apparatus adapted to perform the method is disclosed
herein. The invention is a method and apparatus for creating a
color and optical density selectable visible mark on an anodized
aluminum specimen. The method includes providing a laser marking
system having a laser, laser optics and a controller operatively
connected to said laser to control laser pulse parameters and a
controller with stored laser pulse parameters, selecting the stored
laser pulse parameters associated with the desired color and
optical density, directing the laser marking system to produce
laser pulses having laser pulse parameters associated with the
desired color and optical density while directing a fluid flow at
the article being marked.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1. Laser processing system
[0015] FIG. 2. Mark made with prior art nanosecond pulses
[0016] FIG. 3. Mark made with picosecond pulses
[0017] FIG. 4. Beam waist
[0018] FIG. 5. Grayscale marks on anodized aluminum
[0019] FIG. 6. Marks on anodized aluminum
[0020] FIG. 7. Dyed, visible marked anodized aluminum
[0021] FIG. 8. Dyed, IR marked anodized aluminum
[0022] FIG. 9. Graph showing visible laser pulse thresholds
[0023] FIG. 10. Graph showing IR laser pulse thresholds
[0024] FIG. 11. Image data converted to laser parameters
[0025] FIG. 12a-i Color anodization being applied to an aluminum
article
[0026] FIG. 13. Laser marking system with fluid flow
[0027] FIG. 14a. Anodization bleaching showing cracking
[0028] FIG. 14b. Anodization bleaching showing no cracking with
fluid
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0029] Aspects of this invention mark anodized aluminum articles
with visible marks of various optical densities and colors,
durably, selectably, predictably, and repeatably. It is
advantageous for these marks to appear on or near the surface of
the aluminum and leave the anodization layer substantially intact
to protect both the surface and the marks. Marks made in this
fashion are referred to as interlayer marks since they are made at
or on the surface of the aluminum beneath the oxide layer that
forms the anodization. Ideally the oxide remains intact following
marking in order to protect the marks and provide a surface that is
mechanically contiguous between adjacent marked and non-marked
regions. Further, these marks should be able to be produced
reliably and repeatably, meaning that if a mark with a specific
color and optical density is desired, a set of laser parameters is
known which will produce the desired result when the anodized
aluminum is processed by a laser processing system. It is also
contemplated that some such marks created with a laser processing
system be invisible. In this aspect, the laser processing system
creates marks which are not visible under ordinary viewing
conditions, but which become visible under other conditions, for
example when illuminated by ultraviolet light. It is contemplated
that these marks be used to provide anti-theft marking or other
special marks.
[0030] An embodiment of the instant invention uses an adapted laser
processing system to mark anodized aluminum articles. An exemplary
laser processing system which can be adapted to mark anodized
aluminum articles is the ESI MM5330 micromachining system,
manufactured by Electro Scientific Industries, Inc., Portland,
Oreg. 97229. This system is a micromachining system employing a
diode-pumped Q-switched solid state laser with an average power of
5.7 W at 30 K Hz pulse repetition rate, second harmonic doubled to
532 nm wavelength. Another exemplary laser processing system which
may be adapted to mark anodized aluminum articles is the ESI ML5900
micromachining system, also manufactured by Electro Scientific
Industries, Inc., Portland, Oreg. 97229. This system employs a
solid state diode-pumped laser which can be configured to emit
wavelengths from about 355 nm (UV) to about 1064 nm (IR) at pulse
repetition rates up to 5 MHz. Either system may be adapted by the
addition of appropriate laser, laser optics, parts handling
equipment and control software to reliably and repeatably produce
marks in anodized aluminum surfaces according to the methods
disclosed herein. These modifications permit the laser processing
system to direct laser pulses with the appropriate laser parameters
to the desired places on an appropriately positioned and held
anodized aluminum article at the desired rate and pitch to create
the desired mark with desired color and optical density. A diagram
of such an adapted system is shown in FIG. 1.
[0031] FIG. 1 shows a diagram of an ESI MM5330 micromachining
system adapted for marking articles according to an embodiment of
the instant invention. Adaptations include the laser 10, which, in
an embodiment of this invention is a diode pumped Nd:YVO.sub.4
solid state laser operating at 1064 nm wavelength, model Rapid
manufactured by Lumera Laser GmbH, Kaiserslautern, Germany. This
laser is optionally frequency doubled using a solid state harmonic
frequency generator to reduce the wavelength to 532 nm or tripled
to about 355 nm, thereby creating visible (green) or ultraviolet
(UV) laser pulses, respectively. This laser 10 is rated to produce
6 Watts of continuous power and has a maximum pulse repetition rate
of 1000 KHz. This laser 10 produces laser pulses 12 with duration
of 1 picosecond to 1,000 nanoseconds in cooperation with controller
20. These laser pulses 12 may be Gaussian or specially shaped or
tailored by the laser optics 14 to permit desired marking. The
laser optics 14, in cooperation with the controller 20, direct
laser pulses 12 to form a laser spot 16 on or near article 18.
Article 18 is fixtured upon stage 22, which includes motion control
elements which, in cooperation with the controller 20 and laser
optics 14 provides compound beam positioning capability. Compound
beam positioning is the capability to mark shapes on an article 18
while the article 18 is in relative motion to the laser spot 16 by
having the controller 20 direct steering elements in the laser
optics 14 to compensate for the relative motion induced by motion
of the stage 22, the laser spot 16 or both.
[0032] The laser pulses 12 are also shaped by the laser optics 14
in cooperation with controller 20, as they are directed to form a
laser spot 16 on or near article 18. The laser optics 14 directs
the laser pulses' 12 spatial shape, which may be Gaussian or
specially shaped. For example, a "top hat" spatial profile may be
used which delivers a laser pulse 12 having an even dose of
radiation over the entire spot which impinges the article being
marked. Specially shaped spatial profiles such as this may be
created using diffractive optical elements. Laser pulses 12 also
may be shuttered or directed by electro-optical elements, steerable
mirror elements or galvanometer elements of the laser optics
14.
[0033] The laser spot 16 refers to the focal spot of the laser beam
formed by the laser pulses 12. As mentioned above the distribution
of laser energy at the laser spot 16 depends upon the laser optics
14. In addition the laser optics 14 control the depth of focus of
the laser spot 16, or how quickly the spot goes out of focus as the
plane of measurement moves away from the focal plane. By
controlling the depth of focus, the controller 20 can direct the
laser optics 14 and the stage 22 to position the laser spot 16
either at or near the surface of the article 18 repeatably with
high precision. Making marks by positioning the focal spot above or
below the surface of the article allows the laser beam to defocus
by a specified amount and thereby increase the area illuminated by
the laser pulse and decrease the laser fluence at the surface.
Since the geometry of the beam waist is known, precisely
positioning the focal spot above or below the actual surface of the
article will provide additional precision control over the spot
size and fluence.
[0034] FIG. 2 is a microphotograph showing a mark created on
anodized aluminum 30 using prior art laser with greater than one
nanosecond pulses without fluid flow. The anodization shows clear
signs of cracking 32 in the mark area 34, an undesirable result.
FIG. 3 shows the same color and optical density mark 38 on the same
type of anodized aluminum 36 made with a picosecond laser showing
no cracking. Picosecond lasers mark anodized aluminum articles with
a commercially desirable black without causing damage to the oxide
layer. Commercially acceptable black is defined as a mark having
CIE chromaticity of L*=40, a*=5, and b*=10 or less. Another
advantage of using picosecond lasers is that they are much less
expensive, require much less maintenance, and typically have much
longer operating lifetimes than prior art femtosecond lasers. In
addition, aspects of the instant invention do not require cleaning
of the aluminum surface prior to anodization to create commercially
desirable marks.
[0035] An embodiment of the instant invention performs marking on
anodized aluminum under the anodization. For the interlayer marking
to happen, the laser fluence, defined by:
F=E/s
where E is laser pulse energy and s is the laser spot area, must
satisfy
F.sub.u<F<F.sub.s
where F.sub.u is the laser modification threshold of the
substrate/coating interface, aluminum/aluminum oxide in this case,
and F.sub.s is the damaging threshold for the surface layer, or
anodization. F.sub.u and F.sub.s have been obtained by experiments
and represent the fluence of the selected laser at which the
substrate and surface layer start to get damaged. For 10 ps pulses,
our experiments show that F.sub.u for Al is .about.0.13 J/cm.sup.2
for ps green and .about.0.2 J/cm.sup.2 for ps IR, and the F.sub.s
is .about.0.18 J/cm.sup.2 for ps green and .about.1 J/cm.sup.2 for
ps IR. Varying the laser fluence between these values creates marks
of varying color and optical density. Different pulse durations and
laser wavelengths would each have corresponding values of F.sub.u
and F.sub.s. The actual thresholds for a given set of laser
parameters and anodized article are determined experimentally. The
advantage of using a fluid flow while marking is that the fluid
flow increases the damage threshold F.sub.s thereby permitting
higher energies to be used to mark the articles which permits
higher throughput and a wider range of marking densities.
[0036] An embodiment of this invention precisely controls the laser
fluence at the surface of the aluminum article by adjusting the
location of the laser spot from being on the surface of the
aluminum article to being located a precise distance above or below
the surface of the aluminum. FIG. 4 shows a diagram of a laser
pulse focal spot 40 and the beam waist in its vicinity. The beam
waist is represented by a surface 42 which is the diameter of the
spatial energy distribution of a laser pulse as measured by the
FWHM method on the optical axis 44 along which the laser pulses
travel. The diameter 48 represents the laser pulse spot size on the
surface of the aluminum when the laser processing system focuses
the laser pulse at a distance (A-O) above the surface. Diameter 46
represents the laser pulse spot size on the surface of the aluminum
when the laser processing system focuses the laser pulses at a
distance (O-B) below the surface.
[0037] In addition to commercially desirable black, marking
articles with grayscale values is also useful. FIGS. 5 and 6 show a
series of grayscale marks made on anodized aluminum made by an
embodiment of this invention. The optical density of the marks
range from nearly indistinguishable from the background to fully
black. According to an aspect of the instant invention, each
grayscale mark can be identified by a unique triplet of CIE
colorimetry values. L*, a* and b*. An aspect of the instant
invention associates each desired grayscale value with a set of
laser parameters that reliably and repeatably produce the desired
grayscale value mark on anodized aluminum upon command. Note also
that the marks which may seem indistinguishable to the naked eye
can become visible when illuminated with other than broad spectrum
visible light, for example ultraviolet light.
[0038] FIG. 5 shows black marks 60, 62, 64, and 66 made on anodized
aluminum 70 by an embodiment of this invention. These marks 60, 62,
64, and 66 have CIE chromaticities ranging from less than L*=40,
a*=5 and b*=10, to totally transparent making them commercial
desirable marks. Another feature of these marks is that since they
are underneath undamaged anodization, they have uniform appearance
over a wide range of viewing angles. Marks made using prior art
methods tend to have wide variation in appearance depending upon
viewing angle due to damage to the anodization layer. In
particular, when marking with prior art nanosecond pulses, applying
enough laser pulse energy to the surface to make dark marks causes
damage to the anodization which causes the appearance of the marks
to change with viewing angle. Marks made by an aspect of the
instant invention do not damage anodization regardless of how dark
the marks are, nor do they change in appearance with viewing angle.
These improved marks were made with the following laser
parameters:
TABLE-US-00001 TABLE 1 Laser parameters for color and grayscale
marking Laser Type DPSS Nd:YVO.sub.4 Wavelength 532 nm Pulse
duration 10 ps Pulse temporal Gaussian Laser power 4 W Rep Rate 500
KHz Speed 25 mm/s Pitch 10 microns Spot size 10-400 microns Spot
shape Gaussian Focal Height 0-5 mm with 0.5 mm step
[0039] The marks 60, 62, 64, 66 range in optical density from
virtually unnoticeable 60 against the unmarked aluminum to full
black 62. Grayscale optical densities 64, 66 between the two
extremes are created by moving the focal spot closer to the
article, increasing the fluence and thereby creating darker marks.
The height of the focal spot above the surface of the aluminum
varies from zero, in the case of the darkest optical density mark
62, increasing by 500 micron increments for each mark 64, 66 from
right to left in FIG. 5, ending at 5 mm above the surface for the
lightest mark 60. Note that marks 64 created with focal spot
located 4.5 to 1.5 mm above the surface of the aluminum show tan or
golden colors and marks created with focal spot one mm 62 and 66 or
less appear gray or black. Maintaining this precise control over
the laser focal spot distance from the work surface in addition to
maintaining other laser parameters within normal laser processing
tolerances permits laser marks with desired color and optical
density to be made on anodized aluminum. In addition, the darkest
mark exhibits a CIE chromaticity of less than L*=40, a*=5, and
b*=10, making it a commercially desirable black mark.
[0040] Another aspect of the instant invention determines the
relationship between marks with colors other than grayscale and
picosecond laser pulse parameters. Colors other than grayscale can
be produced on anodized aluminum in two different ways. In the
first, a gold tone can be produced in a range of optical densities.
This color is produced by changes made at the interface between the
aluminum and the oxide coating. Careful choice of laser pulse
parameters will produce the desired golden color without damaging
the oxide coating. FIG. 5 also shows various shades of gold or tan
created by an aspect of the instant invention.
[0041] Laser marking of anodized aluminum can also be achieved by
an aspect of the instant invention which uses IR wavelength laser
pulses to mark the aluminum. This aspect creates marks of varying
grayscale densities by varying the laser fluence at the surface of
the aluminum in two different manners. As discussed above, grey
scale can be achieved by varying the fluence at the surface by
positioning the focal spot above or below the surface of the
aluminum. The second manner of controlling grey scale is to vary
the total dose at the surface of the aluminum by changing the bite
sizes or line pitches when marking the desired patterns. Changing
bite sizes refers to adjusting the rate at which the laser pulse
beam is moved relative to the surface of the aluminum or changing
the pulse repetition rate or both, which results in changing the
distance between successive laser pulse impact sites on the
aluminum. Varying line pitches refers to adjusting the distance
between marked lines to achieve various degrees of overlapping.
FIG. 6 shows an aluminum article 74 with an array of marks 72.
These marks 72 are arranged in an array of six columns and four
rows. The six columns represent six Z-heights of the focal spot
above the surface of the aluminum ranging from 0 (top row) to 5 mm
(bottom row). The four rows represent pitches of 5, 10, 20 and 50
microns reading from top to bottom. As can be seen from FIG. 6,
varying the Z-height of the focal spot and varying the pitch of the
laser pulses can predictably produce graylevels of any desired
optical density from less than CIE L*=40, a*=5, and b*=10 to nearly
transparent, thereby producing commercially desirable marks on
anodized aluminum.
TABLE-US-00002 TABLE 2 Laser pulse parameters for grayscale IR
marking Laser Type DPSS Nd:YVO.sub.4 Wavelength 1064 nm Pulse
duration 10 ps Pulse temporal Gaussian Laser power 2.5 W Rep Rate
500 KHz Speed 50 mm/s Pitch 5, 10, 20, 50 microns Spot size 55-130
microns Spot shape Gaussian Focal Height 0-5 mm with 1 mm step
[0042] A second type of marking which may be applied to anodized
aluminum using picosecond laser pulses is alterations in color
contrast caused by bleaching of dyed anodization. On a microscopic
scale, anodization is porous, and will readily accept dyes of many
types. Referring again to FIG. 3, this microphotograph of anodized
aluminum shows the porous nature of surface. Laser pulses used to
mark dyed anodized aluminum can, depending upon the wavelength and
pulse energy, bleach the dye as it marks the aluminum, making the
anodization transparent and thereby reveals the marks on the
aluminum underneath. With higher fluence, simultaneous dye
bleaching and marking of the aluminum beneath the anodization layer
with black, grey scale, or colors presented in previous section is
possible. Less energetic pulses can partially bleach the
anodization dyes rendering it translucent and thereby partially
coloring the underlying aluminum marks. Finally, longer wavelength
pulses can mark the aluminum with commercially desirable black or
grey scale colors without bleaching the anodization. FIG. 7 shows a
dyed anodized aluminum article with marks made with visible (532
nm) laser pulses. Note that the dye in the anodization is bleached
in the areas subjected to laser pulses. FIG. 8 shows the same type
of dyed anodized aluminum article with marks made with IR (1064 nm)
laser pulses. Note that the anodization is not bleached by the IR
laser pulses and therefore does not reveal the aluminum color
beneath beyond the translucency of the original oxide.
[0043] Another aspect of this invention relates to laser marking
anodized aluminum with colored anodization using picosecond lasers.
Since anodization typically forms a porous surface, dyes can be
introduced which alter the appearance of the aluminum. These dyes
can either be opaque or translucent, allowing varying amounts of
incident light to reach the aluminum and be reflected back through
the anodization. FIG. 7 shows an anodized aluminum article 80 with
pink dye in the anodization and an array of marks 82 produced
according to an aspect of the instant invention. Colors are created
by bleaching the dye in the oxide layer as the aluminum underneath
showed native (silver) color to a range of laser-marked colors from
shades of tan, to gray and finally black. These shades are created
by varying the fluence of the laser pulses at the surface of the
aluminum. The four rows represent varying the pitch of the laser
pulses from 10 to 50 microns and the columns represent varying the
focal spot distance from the surface from 0.0 to 5.0 mm. These
laser parameters in all cases bleach the dye in the oxide
overlaying the aluminum allowing the marks on the aluminum to show
through. The laser marks optical density range from transparent to
CIE chromaticity less than L*=40, a*=5, b*=10. Laser parameters
used to create these marks are given in Table 3.
TABLE-US-00003 TABLE 3 Laser parameters for visible oxide bleaching
Laser Type DPSS Nd:YOV.sub.4 Wavelength 532 nm Pulse duration 10 ps
Pulse temporal Gaussian Laser power 4 W Rep Rate 500 KHz Speed 50
mm/s Pitch 10 microns Spot size 10-400 microns Spot shape Gaussian
Focal Height 0-5 mm
[0044] Bleaching of anodization dye is frequency dependent. As
shown in FIG. 7, 532 nm laser pulses bleach anodization dye even at
the lowest fluence. IR laser wavelengths, on the other hand, create
marks on dyed anodized aluminum without bleaching the dye for most
translucent dye colors. FIG. 8 shows an anodized aluminum article
100 with pink dye with marks 102 made with IR laser pulses. The
marks range from translucent to black and were made by altering the
laser fluence by both changing the distance from the focal spot to
the surface and by changing the pitch. The six columns represent
changing the distance between the focal spot of the laser pulses
and the surface of the aluminum from 5.5 mm (right) to zero (left).
The four rows represent changing the laser pulse pitch from 10 to
50 microns. Laser parameters used to create these marks is shown in
Table 4.
TABLE-US-00004 TABLE 4 Laser parameters for IR colored anodization
marking Laser Type DPSS Nd:YOV.sub.4 Wavelength 1064 nm Pulse
duration 10 ps Pulse temporal Gaussian Laser power 4 W Rep Rate 500
KHz Speed 50 mm/s Pitch 10 microns Spot size 10-400 microns Spot
shape Gaussian Focal Height 0-5 mm
[0045] The relationship between bleaching anodization dye, marking
aluminum and causing surface ablation for 532 nm (green) laser
wavelengths is shown in FIG. 9. For 532 nm (green) laser pulses
with parameters within those given in Tables 1 and 3, FIG. 8 shows
the fluence thresholds in Joules/cm.sup.2 for bleaching anodization
(Fb), marking aluminum under the anodization (F.sub.u), and surface
ablation (F.sub.s). For an aspect of the instant invention 532 nm
laser pulses yield the values are Fb=0.1 J/cm.sup.2, F.sub.u=0.13
J/cm.sup.2, and F.sub.s=0.18 J/cm.sup.2. FIG. 10 shows the fluence
thresholds in Joules/cm.sup.2 for 1064 nm (IR) laser pulses with
parameters within those given in Tables 2 and 4. For an aspect of
the instant invention the fluence threshold values for 1064 nm
laser pulses in Joules/cm.sup.2 are F.sub.u=0.2 J/cm.sup.2 and
F.sub.s=1.0 J/cm.sup.2. Note that no threshold for bleaching
anodization is available since IR wavelength laser pulses do not
begin to bleach anodization until laser fluence is great enough to
cause damage to the overlaying anodization. Note that the exact
values for Fb, F.sub.u and F.sub.s will depend upon the particular
laser and optics used. They must be determined experimentally for a
given processing setup and article to be marked and stored in the
controller for later use.
[0046] In another embodiment of this invention, the programmable
nature of the adapted laser processing system permits the marking
of anodized aluminum articles with commercially desirable marks in
patterns. As shown in FIG. 11, in this aspect a pattern 110 is
converted into a digital representation 112, which is decomposed
into a list 114, where each entry 116 in the list 114 contains a
representation of a location or locations, with a color and optical
density associated with each location. The list 114 is stored in
the controller 20. The controller 20 associates laser parameters
with each entry 116 in the list 114, which laser parameters, when
sent as commands to the laser 10, optics 14 and motion control
stage 22 will cause the laser 10 to generate one or more laser
pulses 12 which impinge aluminum article 18 at or near the surface
16. These pulses will create a mark with the desired color and
optical density. By moving the laser pulses 12 in relation to the
aluminum article 18 according to the locations stored in the list
as the marks are being created, marks of the desired range of
colors and optical density are made on the anodized aluminum
surface in the desired pattern.
[0047] In another embodiment of this invention colored anodization
is patterned over previously patterned marks to present additional
colors and optical densities. In this aspect, a grayscale pattern
is created on an anodized aluminum article. The article is then
coated with a photoresist coating that can be developed by exposure
to laser pulses. The grayscale patterned, photoresist coated
article is placed into the laser processing system and aligned so
that the system can apply laser pulses in registration with the
pattern already applied to the article. The photoresist used is a
type known as "negative" photoresist, where areas exposed to laser
radiation will be removed and the unexposed areas will remain on
the article following subsequent processing. The remaining
photoresist protects the surface of the article from introduction
of dyes, while the areas of the anodization which had been exposed
and subsequently removed will be dyed the desired color. This
anodization layer is designed to be translucent in order to allow
light to pass through the anodization to the pattern below and be
reflected back through the anodization and thereby create color
patterns with selected color and optical density. This color
anodization can also be bleached if necessary using techniques
disclosed by other aspects of this invention to create a desired
color with desired transparency. This color can be applied over
areas of the underlying pattern or applied on a point-by-point
basis down to the limits of resolution of the laser system,
typically in the 10 to 400 micron range. This operation can be
repeated to create multiple color overlays. In one aspect of this
invention, the anodization color overlay is applied in a multiple
color overlay grid, such as Bayer pattern. By designing the
grayscale pattern to work with the color overlay grid, a durable,
commercially desirable full color image can be created on the
anodized aluminum article.
[0048] FIGS. 12a through 12i show a sequence of steps used to
create this color overlay for two colors. In FIG. 12a, an aluminum
article 118 has a transparent anodization layer 120 and marks 122
previously applied according to other aspects of this invention. A
negative photoresist 124 is applied to the surface of the
transparent anodization 120. In FIG. 12b, laser pulses 126 expose
areas 128, 130 of the photoresist 124. In FIG. 12c the unexposed
resist 134 remains following resist processing, but the exposed
resist has been removed leaving voids 132 in the processed resist
layer 134. FIG. 12d shows the base anodization layer 120 with
sections 136 where the anodization has been dyed with color beneath
the voids 132 in the processed resist layer 134. The intact
processed resist 134 prevents the anodization from acquiring color
anywhere except where the processed resist 134 has been removed
132. FIG. 12e shows the article 118 with base anodization 120 with
color portions of anodization 136 in relation to previously applied
marks 122 following removal of processed resist.
[0049] FIG. 12f shows an article 118 with base anodization 120
including colored portions 136 and a second resist layer 138. FIG.
12g shows this second layer of resist 138 impinged by laser pulses
142 to cause area 140 to become exposed. FIG. 12h shows the article
118 with base anodization 120 following processing to, dye the
anodization beneath the removed resist 140, and removal of the
remaining resist 138. This leaves the intact base anodization layer
with colored areas 136, 144 over the previously marked areas 122.
FIG. 12i shows subsequent laser pulses 146 being used to optionally
bleach portions of the previously anodized and dyed portions of the
aluminum article to create additional desired colors or optical
densities. The processing described by this aspect of this
invention results in a colored pattern being overlaid over a
grayscale pattern, yielding marks with a wide range of durable,
commercially desirable colors and optical densities in patterns
which are programmable.
[0050] In another embodiment of this invention, the color
anodization may be created on the anodized aluminum article in
particular patterns which yield the appearance of full color images
when viewed. In this aspect, a pattern representative of an image
is applied to the surface using techniques described herein. The
color dyes are introduced in the manner illustrated in FIGS. 12a
through 12i, except that the pattern with which these dyes are
introduced into the base layer of anodization is designed to
convert the grayscale representation into full color. An example of
such a pattern is a Bayer filter (not shown), which juxtaposes red,
green and blue filter elements in a pattern such that the eye
perceives the red, green and blue elements fusing into a single
color with optical density related to the grayscale mark underneath
the color anodization filters, thereby creating the appearance of a
full color image or pattern. The resist may be negative or positive
resist, and the patterns which expose the resist may be created by
masks, such as used in circuit or semiconductor applications, or
directly written by a electronic means or directly deposited by
technologies such as inkjet or directly ablated by laser.
[0051] In another embodiment of this invention, the adapted laser
marking system 148 is shown in FIG. 13 to include a nozzle 164 and
fluid supply 166. FIG. 13 shows an adapted laser marking system 148
including a laser 150 which emits laser pulses which travel along a
laser beam path 152, which travels through beam steering optics 154
where it is directed to impinge an article 158 which is fixtured on
a motion stage 162, all under control of a controller 160. The
nozzle 164 is supplied with fluid by fluid supply 166 and directs a
fluid flow 168 to the article 158 at or near the location being
impinged by the laser beam 152 at or near the time the laser 150 is
energized and emitting pulses along the laser beam 152. In some
embodiments, the nozzle 164 is attached to motion control equipment
170 which, under direction of the controller 160, moves the nozzle
164 and hence the fluid flow 168 in relation to the article 158
thereby directing the fluid flow 168 to the vicinity of the
location on the article 158 where the laser beam 152 impinges the
article. In other embodiments the surface of the article 158 can be
flooded with fluid while laser machining, eliminating the need to
move the nozzle 164 in conjunction with the laser beam 152.
[0052] This fluid flow 168 cools the surface of the article and
increases the amount of fluence that can be applied to a location
on the article 158. This increases F.sub.s for the particular
anodized aluminum article being marked therefore allowing more
fluence to be used to alter the surface of the aluminum at the
interface between aluminum and oxide, but also permits greater
fluence and therefore greater throughput. In this embodiment water
is used as the fluid however, air or other gasses such as nitrogen
or argon or other fluids could be used. The purpose of the fluid
flow on the surface is to keep the temperature of the anodization
from reaching the temperature at which significant damage starts.
Fluid flow rates which reduce temperatures adequately for given
laser parameters are determined empirically and will differ
depending upon the fluid used and the heat transfer-related
properties of the anodization and metal article.
[0053] The ways in which fluid is delivered to the surface of the
article while the laser marking is occurring depends upon the fluid
used. Where the fluid flow is a small stream of relatively high
velocity fluid, such as air or inert gas, the nozzle 164 may have
to be mechanically coupled to the beam steering optics 154 to
maintain the alignment of the fluid flow 168 and the laser beam
path 152. In the case of a fluid such as water, the surface of the
article may be flooded, thereby providing thermal protection over a
large area without requiring that the nozzle 164 be made to move
while the article 158 is being laser marked.
[0054] This cooling effect allows the laser parameters used to
create marks to change to permit more intense color marking,
greater anodization bleaching and increased throughput while
limiting damage to the anodization caused by thermal stress. FIG.
14a shows an anodized article 180 with dyed anodization 182. A
portion of the anodization has been laser bleached 184, which has
resulted in cracking of the anodization 186. Laser parameters used
are listed in Table 5.
TABLE-US-00005 TABLE 5 Laser bleaching parameters Laser Type DPSS
Nd:YOV.sub.4 Wavelength 532 nm Pulse duration 10 ps Pulse temporal
Gaussian Laser power 2 W Rep Rate 200 KHz Speed 100 mm/s Pitch 10
microns Spot size 10-400 microns Spot shape Gaussian Focal Height
0-5 mm
[0055] In FIG. 14a, the dyed anodization is bleached using laser
parameters listed in Table 5. In this embodiment laser parameters
are selected which result in high power, stable operation of the
laser and a marking rate which provides good system throughput. The
focal height is then adjusted to provide fine control over the
laser fluence. In this case a laser fluence of 0.38 J/cm.sup.2 is
used to bleach the anodization, which also creates cracks 186,
which are undesirable. For this particular sample, all fluences
above 0.13 J/cm.sup.2 results in cracking of the anodization. Thus,
for this particular sample, the anodization cannot be efficiently
bleached without cracking the anodization. FIG. 14b shows the
results of employing an embodiment of this invention to bleach
anodization also using laser parameters listed in Table 5. An
anodized article 190 has been dyed 192 and a portion 194 has been
bleached in the presence of a fluid. Note that no cracks are
visible in this sample in spite of being bleached using laser
fluence of 0.25 J/cm.sup.2. Cracking was prevented by flooding the
article with 2-3 mm of water during laser beaching.
[0056] Laser parameters which may be advantageously employed by
embodiments of this invention include using lasers with wavelengths
which range from IR through UV, or more particularly from about
10.6 microns down to about 355 nm. The laser operates at 2 W, being
in the range of 1 W to 100 W, or more preferably 1 W to 12 W. Pulse
durations range from 1 ps to 1000 ns, or more preferably from 1 ps
to 200 ns. The laser rep rate is in the range from 1 K Hz to 100 M
Hz, or more preferably from 10 KHz to 1 MHz. Laser fluence ranges
from about 0.1.times.10.sup.-6 J/cm.sup.2 to 100.0 J/cm.sup.2 or
more particularly from 1.0.times.10.sup.-2 J/cm.sup.2 to 10.0
J/cm.sup.2. The speed with which the laser beam moves with respect
to the article being marked ranges from 1 mm/s to 10 m/s, or more
preferably from 100 mm/s to 1 m/s. The pitch or spacing between
adjacent rows of laser pulses on the surface of the article ranges
from 1 micron to 1000 microns or more preferably from 10 microns to
100 microns. The spot size of the laser pulses measured at the
surface of the article ranges from 10 microns to 1000 microns or
more preferably from 50 microns to 500 microns. The location of the
focal spot of the laser pulses with respect to the surface of the
article ranges from -10 mm to +10 mm or more particularly from 0 to
+5 mm.
[0057] It will be apparent to those of ordinary skill in the art
that many changes may be made to the details of the above-described
embodiments of this invention without departing from the underlying
principles thereof. The scope of the present invention should,
therefore, be determined only by the following claims.
* * * * *