U.S. patent application number 11/265647 was filed with the patent office on 2006-04-20 for use of helium/nitrogen gas mixtures in up to 12kw laser welding.
Invention is credited to Francis Briand, Karim Chouf, Philippe Lefebvre, Eric Verna.
Application Number | 20060081568 11/265647 |
Document ID | / |
Family ID | 29559173 |
Filed Date | 2006-04-20 |
United States Patent
Application |
20060081568 |
Kind Code |
A1 |
Briand; Francis ; et
al. |
April 20, 2006 |
Use of helium/nitrogen gas mixtures in up to 12kW laser welding
Abstract
Binary gas mixture for welding using a laser beam of up to 12
kW, consisting of 30% to 60% nitrogen by volume, the remainder (up
to 100%) being helium. Application of this gas mixture to the
welding of steel, stainless steel or titanium.
Inventors: |
Briand; Francis; (Paris,
FR) ; Chouf; Karim; (Epinay s/seine, FR) ;
Lefebvre; Philippe; (Saint Ouen I'Aumone, FR) ;
Verna; Eric; (Boissy l'Aillerie, FR) |
Correspondence
Address: |
Elwood Haynes;Intellectual Property Department
Air Liquide
2700 Post Oak Boulevard, Suite 1800
Houston
TX
77056
US
|
Family ID: |
29559173 |
Appl. No.: |
11/265647 |
Filed: |
November 2, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10457671 |
Jun 9, 2003 |
|
|
|
11265647 |
Nov 2, 2005 |
|
|
|
Current U.S.
Class: |
219/121.63 ;
252/575 |
Current CPC
Class: |
B23K 35/383 20130101;
B23K 2103/14 20180801; B23K 2103/50 20180801; B23K 2103/04
20180801; B23K 26/123 20130101; B23K 2103/08 20180801; B23K 2103/05
20180801; B23K 26/125 20130101; B23K 26/32 20130101 |
Class at
Publication: |
219/121.63 ;
252/575 |
International
Class: |
B23K 26/20 20060101
B23K026/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 14, 2002 |
FR |
0207345 |
Claims
1-10. (canceled)
11. A binary gas mixture for laser beam welding consisting of 30%
to 60% nitrogen by volume, and the rest being helium (up to 100
vol. %).
12. The mixture of claim 1, containing less than 59 vol. %
nitrogen.
13. The mixture of claim 1, containing less than 58 vol. %
nitrogen.
14. The mixture of claim 1, containing more than 50 vol. %
nitrogen.
15. The mixture of claim 1, containing more than 52 vol. %
nitrogen.
16. The mixture of claim 1, containing 53 to 55 vol. % nitrogen.
Description
[0001] This application is a continuation of application Ser. No.
10/457,671 filed on Jun. 9, 2003.
BACKGROUND
[0002] The present invention relates to a gas mixture formed solely
from helium and nitrogen and to its use in a laser welding process
operating at a maximum power of 12 kW.
[0003] Laser beam welding is a very high-performance joining
process as it makes it possible to obtain, at high speeds, very
great penetration depths compared with other more conventional
processes, such as plasma welding, MIG (Metal Inert Gas) welding or
TIG (Tungsten Inert Gas) welding.
[0004] This is explained by the high power densities involved when
focusing the laser beam by one or more mirrors or lenses in the
joint plane of the workpieces to be welded, for example power
densities that may exceed 10.sup.6 W/cm.sup.2.
[0005] These high power densities cause considerable vaporization
at the surface of the workpieces which, expanding to the outside,
induces progressive cratering of the weld pool and results in the
formation of a narrow and deep vapour capillary called a keyhole in
the thickness of the plates, that is to say in the joint plane.
[0006] This capillary allows the energy of the laser beam to be
directly deposited depthwise in the plate, as opposed to the more
conventional welding processes in which the energy deposition is
localized on the surface.
[0007] In this regard, the following documents may be cited: DE-A-2
713 904, DE-A-4 034 745, JP-A-01048692, JP-A-56122690, WO 97/34730,
JP-A-01005692, DE-A-4 123 716, JP-A-02030389, U.S. Pat. No.
4,871,897, JP-A-230389, JP-A-62104693, JP-A-15692, JP-A-15693,
JP-A-15694, JP-A-220681, JP-A-220682, JP-A-220683, WO-A-88/01553,
WO-A-98/14302, DE-A-3 619 513 and DE-A-3 934 920.
[0008] This capillary is formed from a metal vapour/metal vapour
plasma mixture, the particular feature of which is that it absorbs
the laser beam and therefore traps the energy within the actual
capillary.
[0009] One of the problems with laser welding is the formation of a
shielding gas plasma.
[0010] This is because the metal vapour plasma, by seeding the
shielding gas with free electrons, may bring about the appearance
of a shielding gas plasma which is prejudicial to the welding
operation.
[0011] The incident laser beam may therefore be greatly disturbed
by the shielding gas plasma.
[0012] The interaction of the shielding gas plasma with the laser
beam may take various forms but it usually results in an effect
whereby the incident laser beam is absorbed and/or diffracted and
this may lead to a substantial reduction in the effective laser
power density at the surface of the target, resulting in a
reduction in the penetration depth, or even in a loss of coupling
between the beam and the material and therefore a momentary
interruption in the welding process.
[0013] The power density threshold at which the plasma appears
depends on the ionization potential of the shielding gas used and
is inversely proportional to the square of the wavelength of the
laser beam.
[0014] Thus, it is very difficult to weld under pure argon with a
CO.sub.2-type laser, whereas this operation may be carried out with
very much less of a problem with a YAG-type laser.
[0015] In general, in CO.sub.2 laser welding, helium is used as
shielding gas, this being a gas with a high ionization potential
and making it possible to prevent the appearance of the shielding
gas plasma, and to do so up to a laser power of at least 45 kW.
[0016] However, helium has the drawback of being an expensive gas
and many laser users prefer to use other gases or gas mixtures that
are less expensive than helium but which would nevertheless limit
the appearance of the shielding gas plasma and therefore obtain
welding results similar to those obtained with helium, but at a
lower cost.
[0017] Thus, gas mixtures are commercially available that contain
argon and helium, for example the gas mixture containing 30% helium
by volume and the rest being argon, sold under the name LASAL.TM.
2045 by L'Air Liquide.TM., which make it possible to achieve
substantially the same results as helium, for CO.sub.2 laser power
levels below 5 kW and provided that the power densities generated
are not too high, that is to say above about 2000 kW/cm.sup.2.
[0018] However, the problem that arises with this type of Ar/He
mixture is that, for higher laser power densities, it is no longer
suitable as the threshold at which the shielding gas plasma is
created is then exceeded.
[0019] Moreover, it is also paramount for the penetration of the
weld beads to be at least maintained, or even preferably increased,
relative to the same laser welding process using helium.
[0020] Furthermore, yet another problem lies in the formation of
NOx-type species, harmful to the welder, which must be kept as low
as possible.
[0021] This is because the metal plasma temperatures, resulting
from laser/material interactions as strong as those involved in
laser welding, are conducive to the dissociation of nitrogen and
oxygen molecules coming from air contamination and lead to the
formation of harmful NOx-type species.
[0022] Consequently, to avoid or reduce the production of NOx-type
species, it is essential to be able to reduce the temperature of
the metal plasma resulting from laser welding.
[0023] The object of the present invention is therefore to provide
a welding gas mixture based on nitrogen and a laser welding process
using this gas, able to be used with a laser having a power of up
to 12 kW, which gas leads to the formation of a less hot metal
plasma, with a total penetration from 5 to 10% greater than that
obtained with the conventional gases used for such power levels,
namely helium, depending on the power and the nitrogen content of
the gas, and to a reduction in the formation of NOx compared with
helium by itself.
[0024] The solution of the invention is therefore a binary gas
mixture for welding using a laser beam of up to 12 kW, consisting
of 30% to 60% nitrogen by volume, the remainder (up to 100%) being
helium.
SUMMARY
[0025] In one aspect of the present invention a binary gas mixture
for laser beam welding is provided. This binary gas mixture
includes 30% to 60% nitrogen by volume, and the rest being helium
(up to 100 vol. %). In another aspect of the present invention the
binary gas mixture includes less than 59 vol. % nitrogen. In
another aspect of the present invention the binary gas mixture
includes less than 58 vol. % nitrogen. In another aspect of the
present invention, the binary gas mixture includes more than 50
vol. % nitrogen. In another aspect of the present invention, the
binary gas mixture includes more than 52 vol. % nitrogen. In
another aspect of the present invention, the binary gas mixture
includes 53 to 55 vol. % nitrogen.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0026] In one aspect of the present invention a binary gas mixture
for laser beam welding is provided. This binary gas mixture
includes 30% to 60% nitrogen by volume, and the rest being helium
(up to 100 vol. %). In another aspect of the present invention the
binary gas mixture includes less than 59 vol. % nitrogen. In
another aspect of the present invention the binary gas mixture
includes less than 58 vol. % nitrogen. In another aspect of the
present invention, the binary gas mixture includes more than 50
vol. % nitrogen. In another aspect of the present invention, the
binary gas mixture includes more than 52 vol. % nitrogen. In
another aspect of the present invention, the binary gas mixture
includes 53 to 55 vol. % nitrogen.
[0027] Depending on the case, the gas of the invention may include
one or more of the following technical features: [0028] it contains
less than 59% nitrogen by volume, preferably less than 58%
nitrogen, preferably less than 55%; [0029] it contains more than
50% nitrogen, preferably more than 52% nitrogen; [0030] it
preferably contains 53 to 55% nitrogen.
[0031] According to another aspect, the invention also relates to a
welding process using a laser beam having a power ranging up to 12
kW, in which a gas mixture according to the invention is used for
welding steel, stainless steel or titanium workpieces.
[0032] Depending on the case, the process of the invention may
include one or more of the following technical features: [0033] the
laser is of the CO.sub.2 type; [0034] a welding operation is
carried out to join two workpieces to be welded together with at
least partial penetration, preferably full penetration; [0035] a
laser having a power from 0.5 to 12 kW, preferably between 4 and 10
kW, is used; [0036] workpieces having a thickness ranging from 0.4
to 30 mm, preferably from 1 mm to 10 mm, are welded; [0037] the
workpieces are made of HYS (High Yield Strength) steel; [0038] the
workpieces to be welded have a zinc surface coating, particularly
galvanized or electrogalvanized steel plates; [0039] the workpieces
to be welded are placed together and lap or butt welded, by
backside welding or at an angle, and with or without a bevel;
[0040] the welding takes place with a single- or multiple-spot
focal spot (impact); [0041] the focal spot is circular or oblong;
[0042] the gas flow rate is between 5 l/min and 100 l/min; [0043]
the pressure of the gas is between 1 and 5 bar; and [0044] the
nozzle delivering the gas is a lateral or axial nozzle having a
diameter ranging from 3 to 30 mm.
EXAMPLE
Measurement of the Penetration of Lines of Melting Produced with a
CO.sub.2 Laser and Shielding Gases Formed from He/N.sub.2
Mixtures
[0045] The curves in the appended figure show measurements of the
penetration of lines of melting produced with a CO.sub.2-type laser
(for power levels ranging from 8 kW to 12 kW) focused on the
surface of a metal target made of mild steel by a parabolic mirror
possessing a focal length of 200 mm, and for variable helium and
nitrogen contents of the shielding gas.
[0046] More precisely, the shielding gas was formed from He/N.sub.2
mixtures having a progressively increased nitrogen content (the
remainder of the mixture being only helium).
[0047] For each curve, the nitrogen content of the mixture used is
plotted as a percentage by volume on the x-axis.
[0048] The gas was delivered in the interaction zone by a lateral
nozzle of cylindrical shape with a diameter of 12 mm, at a flow
rate of 24 l/min. The welding speed was 3 m/min.
[0049] It may be seen in the curves appended hereto that the
penetration of the weld beads is at least maintained for laser
power levels of between 8 and 12 kW; in some cases, an increase in
the penetration of the beads of around 5 to 10% is even
observed.
[0050] During production of these beads, it was found that a
"plasma" and/or "plume" forms in the shielding gas above the
interaction zone and above the metal plasma plume. The dimensions
of the shielding gas plasma and/or plume depended on the nitrogen
content of the mixture, on the incident laser power density, on the
focal length and on the welding speed. A priori, it may have large
dimensions, ranging up to several centimetres.
[0051] It seems that the formation of this plasma and/or plume in
the shielding gas is associated with the presence of nitrogen
molecules and/or atoms near the interaction zone. The associated
consequences of the presence of this gas plasma and/or plume around
the interaction zone are different from those observed in the case
of He/Ar mixtures.
[0052] This is because, unlike He/Ar mixtures in which the
ionization of the argon atoms during the laser welding process
result in the formation of a plasma in the shielding gas which
could be deleterious to the laser welding process, the gas plasma
and/or plume obtained with He/N.sub.2 mixtures does not impair the
welding process.
[0053] In the case of He/N.sub.2 mixtures, the coupling between the
material and the laser beam is maintained, or sometimes even
improved. Only high nitrogen contents in the He/N.sub.2 mixture
significantly impair the laser/material coupling.
[0054] The improvement in the penetration seems to result from the
cooling of the metal plasma plume induced by the dissociation of
the nitrogen molecules of the mixture in contact with it.
[0055] This would therefore lead to a reduction in the size of the
metal plasma plume at the surface of the plate and to a reduction
in the phenomenon of absorption of the incident laser beam by the
plume and an increase in the amount of laser energy available at
the surface of the plate and in the capillary.
[0056] In addition, the exothermic recombination of the nitrogen
atoms or ions at the surface of the walls of the capillary must
also contribute to improving the process.
[0057] Furthermore, it has also been demonstrated during tests
using He/N.sub.2 mixtures that there is an appreciable reduction in
the amount of NOx generated, depending on the nitrogen contents and
laser power densities involved, compared with the amount of NOx
generated with helium alone.
[0058] This is because the dissociation of nitrogen molecules, when
injected into the laser/material interaction zone, absorbs some of
the energy of the metal plasma and cools it.
[0059] This partly explains the reduction in the formation of NOx
species around the metal plasma plume during laser welding in the
laser welding process.
[0060] The content of NOx emitted during the laser welding process
carried out at a speed of about 3 m/min for laser power levels of 2
kW and 8 kW was measured. The shielding gas was brought in
laterally to the displacement by a nozzle 8 mm in diameter at 20
l/min. Various He/N.sub.2 mixtures were used. The sampling was
effected by a stainless steel probe 3 mm in diameter, which sucked
up all the gases emitted by the welding process. The gases
collected then passed into a standardized analyser capable of
detecting the NOx-type elements and of determining their
proportions. The sampling probe was positioned 2 cm from the
surface of the plate, 1.5 cm from the interaction zone, in the
extension of the gas flow.
[0061] The measurements carried out are given in the following
table. TABLE-US-00001 TABLE He/N.sub.2 mixtures 100%/0 70%/30%
50%/50% 30%/70% 0/100% (kW)\(ppm) NO NOx NO NOx NO NOx NO NOx NO
NOx 2 kW 13.5 13.5 -- -- 2.1 2.4 -- -- 1.3 1.4 8 kW 70.8 70.8 40.6
40.6 33.6 33.5 -- -- 2 2
[0062] It may be seen that there is an appreciable reduction in the
contents of NOx emitted during the laser welding process when the
nitrogen content of the He/N.sub.2 shielding gas mixture
increases.
[0063] 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.
* * * * *