U.S. patent application number 11/560287 was filed with the patent office on 2007-05-31 for method for cutting c-mn steel with a fiber laser.
This patent application is currently assigned to L'Air Liquide Societe Anonyme pour I'Etude et I'Exploitation des Procedes Georges Claude. Invention is credited to Francis Briand, Karim Chouf, Hakim Maazaoui.
Application Number | 20070119833 11/560287 |
Document ID | / |
Family ID | 36636156 |
Filed Date | 2007-05-31 |
United States Patent
Application |
20070119833 |
Kind Code |
A1 |
Briand; Francis ; et
al. |
May 31, 2007 |
METHOD FOR CUTTING C-Mn STEEL WITH A FIBER LASER
Abstract
The invention relates to a laser cutting method for cutting a
C-Mn steel workpiece, characterized in that laser beam generation
means comprising at least one silica fiber with an ytterbium-doped
core is used to generate the laser beam. Preferably, the
ytterbium-based fiber has a wavelength between 1.07 and 1.1 .mu.m,
preferably 1.07 .mu.m, the quality factor of the laser beam is
between 0.33 and 8 mm.mrad, and the laser beam has a power of
between 0.1 and 25 kW. The assistance gas for the laser beam is
chosen from nitrogen, helium, argon, oxygen, CO.sub.2 and mixtures
thereof, and, optionally, it further contains one or more
additional compounds chosen from H.sub.2 and CH.sub.4.
Inventors: |
Briand; Francis; (Paris,
FR) ; Chouf; Karim; (Levallois Perret, FR) ;
Maazaoui; Hakim; (Cergy St. Christophe, FR) |
Correspondence
Address: |
AIR LIQUIDE
2700 POST OAK BOULEVARD, SUITE 1800
HOUSTON
TX
77056
US
|
Assignee: |
L'Air Liquide Societe Anonyme pour
I'Etude et I'Exploitation des Procedes Georges Claude
Paris
FR
Air Liquide Welding France (La Soudure Autogene
Francaise)
Paris
FR
|
Family ID: |
36636156 |
Appl. No.: |
11/560287 |
Filed: |
November 15, 2006 |
Current U.S.
Class: |
219/121.72 ;
219/121.73; 219/121.84 |
Current CPC
Class: |
B23K 2103/04 20180801;
B23K 26/1436 20151001; B23K 2103/50 20180801; B23K 26/06 20130101;
B23K 26/08 20130101; B23K 26/38 20130101; B23K 26/40 20130101; B23K
26/0648 20130101; B23K 26/0665 20130101 |
Class at
Publication: |
219/121.72 ;
219/121.73; 219/121.84 |
International
Class: |
B23K 26/38 20060101
B23K026/38; B23K 26/06 20060101 B23K026/06 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 25, 2005 |
FR |
0553605 |
Claims
1. A laser cutting method for cutting a C-Mn steel workpiece,
wherein laser beam generation means comprising at least one
ytterbium-containing fiber for generating a laser beam and in that
the quality factor of the laser beam is between 0.33 and 8
mm.mrad.
2. The method of claim 1, wherein said fiber is formed from an
ytterbium-doped core clad with silica.
3. The method of claim 1, wherein said laser beam generated by the
ytterbium-based fiber has a wavelength between 1 and 5 .mu.m,
preferably between 1.04 and 3 .mu.m.
4. The method of claim 1, wherein said laser beam generated by the
ytterbium-based fiber has a wavelength between 1.07 and 1.1 .mu.m,
preferably of 1.07 .mu.m.
5. The method of claim 1, wherein said laser beam has a power of
between 0.1 and 25 kW, preferably between 0.5 and 15 kW.
6. The method of claim 1, wherein said laser beam is a continuous
or pulsed laser beam, preferably a continuous laser beam.
7. The method of claim 1, wherein the workpiece to be cut has a
thickness between 0.25 and 30 mm, preferably between 0.40 and 20
mm.
8. The method of claim 1, wherein said cutting speed is between 0.1
and 20 m/min, preferably from 1 to 15 m/min.
9. The method of claim 1, wherein the assistance gas for said laser
beam is selected from the following group: a) nitrogen; b) helium;
c) argon; d) oxygen; and e) CO.sub.2 and mixtures thereof; and f)
optionally, said gas further contains one or more additional
compounds chosen from H.sub.2 and CH.sub.4.
10. The method of claim 1, wherein the quality factor of said laser
beam is between 1 and 8 mm.mrad, preferably greater than 2 mm.mrad,
even more preferably greater than 3 mm.mrad and/or preferably less
than 7 mm.mrad and even more preferably less than 5 mm.mrad.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn. 119 (a) and (b) French Application No. 0553605, filed
Nov. 25, 2005, the entire contents of which are incorporated herein
by reference.
BACKGROUND
[0002] The invention relates to a laser cutting method for cutting
carbon-manganese (C-Mn) steel using a laser source of the
ytterbium-based fiber type.
[0003] At the present time, laser cutting using a laser source of
the CO.sub.2 type to generate a laser beam, with a wavelength of
10.6 .mu.m and a power ranging up to 6 kW, is widely used in
industry.
[0004] This method is used in particular for cutting C-Mn steels.
Within the context of the invention, the term "C-Mn steel" is
understood to mean any non-alloy steel or low-alloy steel, the
carbon and manganese contents of which are less than 2% by weight
and the contents of the other alloying elements optionally present
are less than 5% by weight.
[0005] However, the cutting speeds that can be achieved and the
cutting quality that results therefrom are very variable, depending
on the material to be cut and, moreover, depending on the cutting
method parameters adopted, such as the nature of the assistance
gas, the diameter of the focused beam, the power of the incident
laser, etc.
[0006] Thus, CO.sub.2 lasers cannot be used with assistance gases
of low-ionization potential, for example such as argon, without the
risk of generating parasitic plasmas that could impair the
method.
[0007] Furthermore, CO.sub.2 lasers are limited in terms of power,
thereby directly impacting the cutting speed.
[0008] In addition, the fact of having to guide the laser beam from
the laser generator right to the focusing head, that is to say the
cutting head, has drawbacks, especially as regards alignment of the
optics in the optical path. This is because guiding optics are
generally polished and/or coated copper mirrors and their positions
determine the path followed by the laser beam. Therefore, the
alignment of the mirrors must be perfect in order to ensure optimum
entry of the laser beam into the focusing head or cutting head.
Now, the position of these mirrors is generally adjusted by
mechanical means, which may easily go out of alignment according to
the wear of parts and the environmental conditions, such as the
ambient temperature, moisture content, etc.
[0009] In addition, the optical path of the beam must necessarily
be kept in an inert atmosphere in order to avoid any contamination
and to maintain a medium with a constant optical index, which is
necessary for good propagation of the beam. These conditions make
it possible for the properties relating to the beam diameter and
the transverse distribution of the beam energy, and also the beam
quality properties, to remain satisfactory for the method, the
quality factor for beam parameter product (BPP) of the high-power
CO.sub.2 laser beams used in cutting generally being between 3
mm.mrad and 6 mm.mrad. Such an atmosphere also makes it possible to
preserve the guiding optics and to prevent them from
deteriorating.
[0010] However, this is not practical in an industrial situation,
as it complicates the installation and incurs additional costs.
[0011] In an attempt to alleviate these problems, it has been
proposed to carry out the cutting with a laser device of the Nd:YAG
type within which the beam is generated by a resonator containing a
solid amplifying medium, namely a neodymium(Nd)-doped YAG rod, and
then sent via an optical fiber to the focusing head.
[0012] However, this solution is not satisfactory from the
industrial standpoint either.
[0013] More precisely, it has been found that cutting with a laser
beam output by an Nd:YAG laser source with a wavelength length of
1.06 .mu.m gives poor results in terms of cutting quality and
cutting speed, in particular when cutting a workpiece made of C-Mn
steel.
[0014] This is because Nd:YAG lasers have quality factors (BPP
values) unsuitable for the laser cutting process hence their range
from around 15 mm.mrad to 30 mm.mrad, depending on the laser
source.
[0015] Now, it should be understood that the higher the quality
factor of a laser, i.e. the higher the product of the focused beam
waist multiplied by the beam divergence, the less effective the
laser beam for the laser cutting process.
[0016] In addition, the transverse energy distribution in a focused
Nd:YAG laser beam is not Gaussian but has a top-hat profile, while
beyond the focal point the transverse energy distribution is
random.
[0017] The limits on using Nd:YAG lasers in laser cutting, in
particular for C-Mn steel, are therefore immediately
understood.
[0018] More generally, to cut a C-Mn workpiece by laser cutting
with an Nd:YAG laser is far from being obvious when it is desired
to achieve cutting speeds and cutting qualities that are acceptable
from the industrial standpoint.
[0019] The problem that arises is therefore how to provide an
improved and industrially acceptable method for cutting C-Mn steels
with a laser beam, which can achieve, depending on the thickness in
question, speeds ranging up to 15 to 20 m/min, or even higher, and
good cutting quality, that is to say straight cutting faces, no
burrs, limited roughness, etc.
[0020] The solution provided by the invention is therefore a laser
cutting method for cutting a C-Mn steel workpiece, characterized in
that laser beam generation means comprising at least one
ytterbium-containing fiber for generating a laser beam used to melt
the workpiece and thereby perform the actual cutting, and in that
the quality factor of the laser beam is between 0.33 and 8
mm.mrad.
[0021] The laser beam generation means comprise an exciter,
preferably several exciters, which cooperate with at least one
excited element, also called amplifying medium, in order to
generate the laser beam. The exciters are preferably several laser
diodes, while the excited elements are fibers, preferably silica
fibers with an ytterbium-doped core.
[0022] Furthermore, for the purpose of the invention, the terms
"laser beam generation means" and "resonator" will be used
indiscriminately.
[0023] Depending on the case, the method of the invention may
include one or more of the following features: [0024] the fiber(s)
is(are) formed from an ytterbium-doped core clad with silica;
[0025] the laser beam generated by the ytterbium-based fiber has a
wavelength between 1 and 5 .mu.m, preferably between 1.04 and 3
.mu.m; [0026] the laser beam generated by the ytterbium-based fiber
has a wavelength between 1.07 and 1.1 .mu.m, preferably of 1.07
.mu.m; [0027] the laser beam has a power of between 0.1 and 25 kW,
preferably between 0.5 and 15 kW; [0028] the laser beam is a
continuous or pulsed laser beam, preferably a continuous laser
beam; [0029] the workpiece to be cut has a thickness between 0.25
and 30 mm, preferably between 0.40 and 20 mm; [0030] the cutting
speed is between 0.1 and 20 m/min, preferably from 1 to 15 m/min;
[0031] the assistance gas for the laser beam is chosen from
nitrogen, helium, argon, oxygen, CO.sub.2 and mixtures thereof,
and, optionally, it further contains one or more additional
compounds chosen from H.sub.2 and CH.sub.4; [0032] the quality
factor of the laser beam is between 1 and 8 mm.mrad, preferably
greater than 2 mm.mrad, even more preferably greater than 3 mm.mrad
and/or preferably less than 7 mm.mrad and even more preferably less
than 5 mm.mrad; [0033] the cutting speed for a steel workpiece with
a thickness between 0.4 mm and 3 mm, using oxygen as assistance gas
at a pressure of between 0.2 and 6 bar, is between 6 and 15 m/min;
[0034] the cutting speed for a steel workpiece with a thickness of
between 3 mm and 6 mm, using oxygen as assistance gas at a pressure
of between 0.2 and 6 bar, is between 2 and 6 m/min; [0035] the
cutting speed for a steel workpiece with a thickness of between 6
mm and 10 mm, using oxygen as assistance gas at a pressure of
between 0.2 and 6 bar, is between 1 and 3 m/min; [0036] the cutting
speed for a steel workpiece with a thickness of between 10 mm and
20 mm, using oxygen as assistance gas at a pressure of between 0.2
and 6 bar, is between 0.1 and 2 m/min; [0037] more generally, the
assistance gas pressure is between about 0.1 bar and 10 bar, and is
chosen according to the thickness that is to be cut; and [0038] the
diameter of the gas injection orifice is between 0.5 and 5 mm,
typically between 1 and 3 mm.
[0039] FIG. 1 appended hereto is a diagram showing the principle of
an installation for implementing a laser cutting method using a
laser beam 3 to cut a C-Mn steel workpiece 10, employing a laser
source 1 with a resonator 2 or laser beam generation means
comprising a silica fiber with an ytterbium-doped core to generate
the laser beam 3.
[0040] The laser source 1 is used to generate a laser beam 3 with a
wavelength between 1 .mu.m and 5 .mu.m, more precisely, at 1.07
.mu.m.
[0041] The beam 3 propagates through beam-conveying means 4, such
as an optical fiber made of fused silica with a diameter of between
20 .mu.m and 300 .mu.m, as far as the zone 11 of interaction
between the beam 3 and the workpiece 10 where the beam strikes the
C-Mn steel workpiece and melts the constituent material of said
workpiece, thus forming the kerf.
[0042] On exiting from this fiber 4, the laser beam 3 possesses
particular optical characteristics and a quality factor (BPP) of
between 1 and 8 mm.mrad. The beam 3 is then collimated using an
optical collimator 5 equipped with a collimation doublet made of
fused silica coated so as to limit the divergence of the beam
exiting the fiber and to make the laser beam parallel.
[0043] The parallel beam 3, the divergence of which has been
considerably limited by the collimator, is then focused onto or
into the workpiece 10 to be cut by a coated, fused-silica lens 6
having a focal length of between 80 mm and 510 mm, preferably
between 100 mm and 250 mm.
[0044] Before striking the workpiece 10, the beam 3 passes axially
through the laser head 6, which is equipped with a nozzle 7 having
an axial exit orifice 8 located facing the workpiece 10 to be cut,
the beam 3 and the assistance gas passing through said nozzle. The
orifice of the nozzle may be between 0.5 mm and 5 mm, preferably
between 1 mm and 3 mm.
[0045] The laser head 6 itself is fed with assistance gas via a gas
inlet 9, for example for an inert gas such as nitrogen, argon,
helium or a mixture of several of these gases, or else an active
gas, for example, such as oxygen, or even active gas/inert gas
mixtures.
[0046] The pressurized assistance gas is used to remove the molten
metal from the kerf 12 being formed in the workpiece 10, as the
workpiece undergoes relative displacement with respect to the laser
head 6 along the desired cutting path. The reverse situation,
consisting in moving the cutting head while keeping the workpiece
stationary gives the same result.
[0047] FIG. 3 is a diagram illustrating the configuration during
cutting at the kerf (material of thickness e), where the angle of
divergence .theta. of the laser beam after focusing, the diameter
2Wo of the focused beam and the angle .alpha. of the cutting front
have been indicated.
[0048] The beam quality factor or BPP is defined as the product of
the divergence angle .theta. multiplied by its radius Wo.
[0049] The cutting process is governed by the absorption of energy
from the laser beam in the material during cutting. Depending on
the wavelength of the laser beam employed, there therefore exists
an optimum angle for energy absorption by the material. Outside
this optimum angle, some of the energy is reflected and/or
lost.
[0050] FIG. 3 illustrates the fact that, in the optimum cutting
condition, the angle .alpha. of the cutting front corresponds to
exposure of the entire thickness e of the material to the beam with
a diameter 2Wo.
[0051] FIG. 4 shows the variation in the optimum angle .alpha. of
the cutting front as a function of the cutting thickness. The upper
curve corresponds to that obtained with a 4 kW CO.sub.2 laser in
TEM 01* mode, while the lower curve is that obtained with a 2 kW
ytterbium-based fiber laser according to the invention. The two
curves are not coincident because of the difference in optimum
energy absorption angle at 10.6 .mu.m, which is the wavelength of
the CO.sub.2 laser, and at 1.07 .mu.m, which is the wavelength of
the ytterbium-based fiber laser.
[0052] It is clearly apparent from these curves that, for small
thicknesses, the optimum angle of the cutting front is higher than
for larger thicknesses. The maximum angle for transmitting the
laser energy into the material is obtained geometrically, and is
the sum of the angles, namely .alpha.+.theta..
[0053] It will therefore be understood that, when small thicknesses
(a few mm) are being cut, it is necessary to use a low beam
divergence angle, that is to say a small BPP, since the spot
diameter is set by the fiber diameter used, in order to keep the
optimum energy absorption angle constant.
[0054] It is also deduced therefrom that the transmission of the
energy from the beam to the material becomes less efficient beyond
a value of 8 mm.mrad.
[0055] Therefore, for the purpose of the invention, a laser beam
having a quality factor preferably between 1 and 8 mm.mrad,
preferably between 2 and 8 mm.mrad, is used.
EXAMPLE
[0056] To demonstrate the effectiveness of the method of the
invention, several cutting trials on C-Mn steel workpieces were
carried out using a resonator to generate the laser beam, which
contained an amplifying medium composed of silica optical fibers
with an ytterbium-doped core, which were excited by diodes
according to the method of the invention. The results of these
trials are given in the example below.
[0057] More precisely, the laser source used in the example below
consisted of an amplifying medium formed from ytterbium-doped
silica fibers, generating a laser beam of 2 kW power and 1.07 .mu.m
wavelength, propagated in a 100 .mu.m coated fused-silica optical
fiber, possessing a quality factor (BPP) on exiting the fiber of 4
mm.mrad. The collimator located at the exit of the fiber was
equipped with a doublet of 55 mm focal length.
[0058] To determine the speed ranges that could be achieved with
the method of the invention according to the thicknesses of the
workpieces to be cut and the pressure and composition of the
assistance gas employed, cutting trials were carried out on C-Mn
steel workpieces having thicknesses of between 2 mm and 20 mm.
[0059] The gas used was injected into the interaction zone where
the beam interacts with the workpiece at pressures varying between
0.6 and 20 bar depending on the gas used, through laser cutting
nozzles having orifices with diameters ranging between 0.5 and 3 mm
depending on the case.
[0060] When an active gas, such as oxygen, was used, the working
pressure was 0.2 to 6 bar, whereas with an inert gas, such as
nitrogen, higher pressures were generally required, namely
pressures of around 8 to 20 bar. Of course, intermediate pressures
could be used with inert gas/active gas mixtures, for example, with
oxygen/nitrogen mixtures, or even with air.
[0061] In the present case, the trials were carried out with oxygen
at pressures between 0.4 and 1 bar, typically 0.7 bar, for nozzles
with a diameter ranging from 1 mm to 2.5 mm. The greater the
thickness to be cut, the larger the nozzle diameter to be used, the
diameter adopted being chosen empirically by carrying out routine
tests.
[0062] Focusing lenses with a focal length of between 127 mm and
190.5 mm were used to focus the laser beam generated by a resonator
based on ytterbium-doped fibers. More precisely, for a 2 mm
thickness to be cut, a lens with a focal length of 127 mm was used,
while for the other thicknesses, a focal length of 190.5 mm was
used. This beam was conveyed to the focusing lens of the cutting
head by optical conveying means, such as a 100 .mu.m-diameter
optical fiber.
[0063] The results obtained are given in the appended FIG. 2, which
shows the speed obtained (plotted on the y-axis) as a function of
the thickness to be cut (plotted on the x-axis).
[0064] This figure shows that, on a 2-mm thick plate, under the
abovementioned conditions, a speed of 10 m/min was achieved and
that, logically, the cutting speed decreased with an increase in
thickness of the material cut.
[0065] Moreover, it should be emphasized that, after visual
examination, for all cut thicknesses between 2 and 15 mm, the
quality of the cut, in terms of burrs, oxide edge and striations,
was considered to be very satisfactory from an industrial
standpoint. The maximum thickness cut under these conditions and
giving good results was about 20 mm.
[0066] In other words, the method of the invention has demonstrated
its effectiveness in terms of cutting speed and cut quality, in
particular for thicknesses of less than 20 mm.
[0067] It will be understood that many additional changes in the
details, materials, steps and arrangement of parts, which have been
herein described in order to explain the nature of the invention,
may be made by those skilled in the art within the principle and
scope of the invention as expressed in the appended claims. Thus,
the present invention is not intended to be limited to the specific
embodiments in the examples given above.
* * * * *