FR2870765A1 - Carbon dioxide type laser welding process for welding together one or more metal items - Google Patents

Abstract

The process involves setting up a first protective gas containing helium on the right side of the item(s) to be welded and a second protective gas containing helium on the back of the item(s) to be welded, and making a full penetration weld using the laser beam delivered to the right side without creating plasma on the back in the second gas during the welding operation : The two protective gases are set up simultaneously or concurrently. The second protective gas can have the same composition as the first. The protective gases are helium, argon, or an argon/helium, helium/nitrogen, helium/oxygen, helium/CO2, helium/argon/oxygen, helium/argon/CO2, argon/hydrogen or helium/hydrogen mixture. The welding stage involves using an electric arc and the weld is made between the items using the electric arc and the laser beam as they combine on the right side of the item(s). The item(s) are metal and are flat plates or a tube. They have a thickness of at least 0.2mm, preferably at least 0.5mm. The electric arc/laser combination is a TIG-laser or MIG-laser hybrid process. The second protective gas is preferably helium.

Description

The invention relates to a method of laser or hybrid laser-arc welding

  one or more metal parts, in particular flat panels intended for use on a shipyard, or the longitudinal edges of tubes or pipelines.

  Laser welding is a high-performance welding process that achieves very high welding speeds and penetration depths when compared to other more traditional processes, such as arc welding.

  These performances are obtained thanks to the high power densities implemented during the focusing, by one or more mirrors or lenses, of the laser beam on the part or parts to be welded.

  Indeed, these high densities of laser power cause the surface of the part (or parts) a very strong evaporation which, by expanding outward, induces a progressive digging, called 'rocket effect', of the welding bath and leads to the appearance, in the thickness of the sheet, of a steam capillary or keyhole D. This capillary allows a direct deposit of energy in the core of the sheet as opposed to a more conventional method where the fusion is carried out mainly by thermal propagation.

  Typically, a capillary consists of a mixture of metal vapors and metal vapor plasma whose particularity is to absorb the laser beam and thus to trap energy within the capillary itself.

  When the capillary is opening, it is said that the weld is open, that is to say that it completely crosses the sheet to be welded. This process is accompanied by a loss of energy side up because all the power of the laser beam is not used to melt the sheet. There is therefore a portion of this laser power that is transmitted through the sheet, which is all the more important that the sheet is thin, the laser power is important and the welding speed is low.

  In addition, the laser-arc hybrid welding process is a welding process that combines electric arc welding with laser welding.

  Such a hybrid laser-arc process is described in particular in EP-A-800434, EP-A-1273383, EP-A-1199128, EP-A-1212165, EP-A-1337375, WO-A03 / 11516, WO -A-03/43776, WO-A-03/82511, EP-A-1160048, EP-A-1160046, EP-A1160047 and EP-A-1380380.

  The principle of this method is to generate an electric arc between a fusible or non-fusible electrode and the workpiece or parts to be welded, and to concomitantly focus a power laser beam, of the YAG or CO2 type, for example, in the arc zone. . This method, if it also allows to obtain very high welding speeds and penetration depths thanks to the appearance of a vapor capillary, also makes it possible to considerably increase the positioning tolerances of the parts before welding by It refers to the very precise positioning required in laser welding alone because of the small size of the focal point which is implemented in the latter.

  An existing problem in laser welding and hybrid arc-to-laser welding using a CO2 laser generator is the creation of a cover gas plasma.

  Indeed, the plasma of metal vapor present in the capillary, which is inherent to laser welding alone and which is reinforced in hybrid welding by the presence of an electric arc, by seeding the cover gas in free electrons, can trigger the appearance of a blanket gas plasma that is detrimental to the welding operation.

  The laser beam can then be strongly or completely absorbed and thus lead to a significant reduction in the depth of penetration, or even a loss of coupling between the beam and the material, thus to a momentary interruption of the welding process.

  The onset threshold of this cover gas plasma depends on the cover gas used and the power and focus parameters of the laser beam.

  To remedy this problem, gaseous mixtures that can be used for welding with a CO2 type laser or hybrid welding have been proposed by documents EP-A-1404482, WO-A-03/57398, EP-A-1371444, EP-A 13141445, EPA-1371446 and EP-A-1375054, which make it possible to guard against the appearance of this cover gas plasma on the location side.

  On the other hand, another problem in laser or hybrid laser-arc welding is related to the nitrogen content of the weld bead. Indeed, nitrogen is, in general, a detrimental element to the mechanical properties of welded joints when it is too large in the molten metal.

  In other words, the problem to be solved is to propose an improved laser or hybrid laser-arc welding process, in which the amount of nitrogen incorporated in the molten metal during the production of the welding joint is limited.

  The solution of the invention is then a CO2 laser welding process for joining by welding one or more metal parts, in which: (a) the side of the part or parts to be welded is used, a first helium-containing shielding gas, (b) a second shielding gas containing helium is used on the back side of the weld part (s), and (c) a full weld seal is made penetration by means of at least the laser beam delivered on the side of the part or parts without creating, during the production of the solder joint, plasma on the reverse side in the second shielding gas.

  Depending on the case, the method of the invention may comprise one or more of the following characteristics: steps (a) and (b) are carried out simultaneously or concomitantly.

  said second protective gas is a gas of identical composition to that of the first protective gas.

  the first protective gas is chosen from helium, argon, argon / helium, helium / nitrogen, helium / oxygen, helium / CO2, helium / argon / oxygen, helium / argon / CO2, argon / hydrogen mixtures; or helium / hydrogen.

  the second shielding gas is chosen from helium, argon, argon / helium, helium / nitrogen, helium / oxygen, helium / CO2, helium / argon / oxygen, helium / argon / CO2, argon / hydrogen mixtures; or helium / hydrogen.

  in step (c), an electric arc is also used, and the solder joint is produced between the part (s) to be welded by means of at least the electric arc and the laser beam delivered in combining with each other on the side of the room or rooms.

  the part or parts are made of metallic materials, such as carbon steels, carbon manganese steels, micro-alloyed steels, austenitic stainless steels, ferritic stainless steels, martensitic stainless steels and aluminum alloys, and / or the parts are flat sheets or a tube.

  the part or parts to be welded have a thickness of at least 0.2 mm, preferably at least 0.5 mm.

  it is chosen from the hybrid TIG-laser or MIG-laser methods. the second shielding gas is helium.

  In the context of the invention: io - "CO2-type laser beam" means a laser beam generated by a CO2-type laser generator - "side-side" means the side of the part or parts to be welded located directly opposite the laser or hybrid laser welding head, which receives the beam and / or the arc first, that is to say the side of the upper surface of the sheet or sheets to be welded - by "side towards "means the side of the part or parts opposite to the location side, that is to say the side of the lower surface of the sheet or sheets to be welded, -" keyhole "means the capillary formed of metallic vapor and metal vapor plasma allowing a direct deposit of the energy of the laser beam at the heart of the sheet to be welded, which is created by the high power density of the laser.

  The inventors of the present invention have demonstrated that depending on the welding speed and the laser power used, it was possible to create in the air below the weld bead, a plasma on the reverse side, which, when was triggered, was an important source of nitrogen generation in the weld bead.

  Indeed, the power that leads to the end of the keyhole and which is not absorbed in the sheet or sheets to be welded may be sufficient to create a plasma in the air below the sheet or sheets to be welded.

  This plasma dissociates the nitrogen from the air and thus allows it to penetrate the molten metal and to deteriorate the mechanical and metallurgical properties.

  However, the inventors of the present invention have shown that to guard against the appearance of such a plasma on the reverse side and thus avoid the introduction of nitrogen into the weld bead, it was necessary to use a protective gas on the reverse side which is difficult to ionize, that is to say either helium, or a gas mixture based on helium, which is identical or different in composition to that used for the protection of the side of the joint welding.

  In other words, according to the invention, it prevents the formation of a plasma on the back side of the weld and thus avoids the introduction of nitrogen into the gasket by using the back side of a gaseous protective atmosphere. helium or a gas containing helium.

  The invention will be better understood in view of the following explanations given with reference to the appended figures in which: FIG. 1a represents a CO 2 laser beam welding macrograph with a power of 10.4 kW according to the prior art, of parts 5 mm thick, at a welding speed of 2.5 m / min, with helium gas in place and gas helium on the reverse side, and with laser focusing on the surface of the parts to welded.

  FIG. 1b represents a macrography obtained under the same conditions as those of FIG. 1a with no gas on the reverse side, that is to say that the backside is in the ambient air.

  - Figure 2a shows a transverse photograph illustrating the laser welding process, the result of which is shown in Figure 1b.

  FIG. 2b represents a transverse photograph illustrating the laser welding process, the result of which is indicated in FIG.

  Figure 1b is a macrography of a weld seam whose cord foot has large and large porosities. The molten metal contains an extremely large amount of nitrogen, namely 550 ppm by weight.

  By observing the photograph of Figure 2a, one sees, on the reverse side, an extremely important luminosity which is the signature of the formation of a plasma created in the atmosphere on the back side of the weld bead which is in the ambient air .

  This plasma forming in the air on the back side causes a dissociation of the nitrogen molecules from the air and thus allows their penetration into the weld bead, which deteriorates the metallurgical properties in particular.

  Conversely, as shown in FIG. 2b, according to the invention, when the formation of any plasma on the reverse side is prevented, the cord obtained, as shown in FIG. 1a, has only a low nitrogen inclusions in the molten metal, ie not more than 50 ppm by weight here. This confirms the effectiveness of the process of the invention.

  It should be noted that in FIGS. 2a and 2b, on the site side, there is a plasma of metal vapors escaping from the keyhole during welding.

  It should be noted that when this backside plasma is created in the air, the amount of nitrogen entering the weld bead is much higher than when this plasma on the reverse side is not tripped. Thus, if the welding speed is increased, until it falls below the onset threshold of this plasma on the reverse side, the amount of nitrogen introduced into the weld bead drops significantly. However, there is inevitably some of the nitrogen in the air that enters the cord even if the plasma on the reverse side is not triggered. It is for this reason that helium / nitrogen mixtures can still be used on the reverse side as long as this plasma on the reverse side is not triggered.

  It should also be noted that it is not always possible to increase the welding speeds to get rid of this plasma on the back side, especially for chamfer welding or when a particular form of bead is imposed.

  Preferably, the same gas is selected on the back side and on the side side, that is to say helium or helium / argon or helium / nitrogen mixtures.

2870765 7

Claims (10)

claims   A method of laser welding of the CO2 type for joining by welding one or more metal parts, wherein: (a) is implemented on the side of the part or parts to be welded, a first shielding gas containing helium (b) a second shielding gas containing helium is used on the back side of the workpiece (s) to be welded, and (c) a full penetration weld joint is made by means of at least one laser beam delivered on the side of the part or parts without creating, during the production of the solder joint, plasma on the reverse side in the second shielding gas.   2. Method according to claim 1, characterized in that steps (a) and (b) are carried out simultaneously or concomitantly.   3. Method according to one of claims 1 or 2, characterized in that said second shielding gas is a gas of identical composition to that of the first shielding gas.   4. Method according to one of claims 1 to 3, characterized in that the first shielding gas is selected from helium, argon, argon / helium mixtures, helium / nitrogen, helium / oxygen, helium / CO2 helium / argon / oxygen, helium / argon / CO2, argon / hydrogen or helium / hydrogen.   5. Method according to one of claims 1 to 3, characterized in that the second shielding gas is selected from helium, argon, argon / helium mixtures, helium / nitrogen, helium / oxygen, helium / CO2 helium / argon / oxygen, helium / argon / CO2, argon / hydrogen or helium / hydrogen.   6. Method according to one of claims 1 to 5, characterized in that in step (c), it implements, moreover, an electric arc and the welding joint is made between the or parts to soldering by means of at least the electric arc and the laser beam delivered by combining with each other on the side of the part or parts.   7. Method according to one of claims 1 to 6, characterized in that the 5 or parts are made of metal materials and / or the part or parts are flat sheets or a tube.   8. Method according to one of claims 1 to 7, characterized in that the or parts to be welded have a thickness of at least 0.2 mm, preferably at least 0.5 mm.   9. Method according to claim 6, characterized in that it is selected from hybrid TIG-laser or MIG-laser processes.   10. Method according to one of claims 1 to 9, characterized in that the second shielding gas is helium.