Method for arc welding with a dual gas flow, with a central flow containing argon and hydrogen, and with a sheath flow including argon and either carbon dioxide or oxygen

Abstract

The invention relates to an arc welding method that implements an arc welding torch provided with an electrode, in which a central gas flow is supplied so as to contact the electrode and an annular gas flow is supplied on the periphery of said first gas flow. The central gas flow contains only argon and hydrogen (H2), the hydrogen content being 2 to 8 vol %. The sheath gas flow only contains argon and 1.8 to 3 vol % of carbon dioxide (CO2) or 0.9 to 1.5 vol % of oxygen (O2). The method is used for welding parts made of steel, in particular stainless steel or carbon steel, as well as steel coated with zinc or aluminium or any other material for preventing the corrosion of said steel.

Claims

1. An electric arc welding process using an arc welding torch equipped with an electrode comprising the steps of a) delivering a central gas flow in contact with the electrode and b) delivering an annular gas flow peripherally to said central gas flow, and wherein: i) the central gas flow consists essentially of argon and hydrogen (H.sub.2), the hydrogen content being between 2 and 8% by volume, and ii) the annular gas flow consists essentially of argon and 1.8 to 3% by volume of carbon dioxide (CO.sub.2), or 0.9 to 1.5% by volume of oxygen (O.sub.2).

2. The process of claim 1, wherein the central gas flow is between 3% and 8% by volume of hydrogen.

3. The process of claim 1, wherein the central gas flow is between 4% and 8% by volume of hydrogen.

4. The process of claim 1, wherein the central gas flow is between 2% and 7% by volume of hydrogen.

5. The process of claim 1, wherein the annular gas flow consists of argon and of 1.8 to 2.5% by volume of carbon dioxide (CO.sub.2) or of argon and of 0.9 to 1.3% by volume of oxygen (O.sub.2).

6. The process of claim 1, wherein the arc welding torch is equipped with an infusible electrode made of tungsten.

7. The process of claim 1, further comprising the step of welding one or more parts made of steel.

8. The process of claim 7, wherein the one or more parts made of steel comprise a zinc or aluminum surface coating.

9. The process of claim 1, further comprising the step of welding tailored blanks.

10. The process of claim 1, wherein the welding is carried out at a rate of at least 2 m/min.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

(2) FIG. 1, which is taken from the document by W. Savage, et al., Effect of arc force on defect formation in GTA welding, Welding Journal, 1979, vol. 58, no. 7, pp. 212-224, represents the welding speed (on the y-axis, expressed in cm/min) as a function of the welding current (on the x-axis, expressed in A).

(3) FIG. 2 illustrates the thermal conductivities (on the y axis) of the components of the protective gas mixtures as a function of the temperature (on the x axis).

EXAMPLES

(4) To try to solve the problem of humping, within the context of the present invention, metal flows and weld bead formation have been studied in the presence of several different gases or gas mixtures in automatic dual-flow TIG welding.

(5) In particular, the combined action of a central gas flow formed of an Ar/H.sub.2 mixture and of an annular gas flow formed of an Ar/CO.sub.2 mixture for increasing the welding speed were tested. This combination was carefully chosen to benefit from the features of each component in order to obtain the desired effects.

(6) Thus, argon facilitates the initiation of the arc and stabilizes it owing to its low ionization energy of 15.8 eV compared with 24.6 eV needed for helium.

(7) Hydrogen is added to increase the energy of the arc. Since H.sub.2 is a diatomic molecule, its presence in the gas mixture will increase the supply of heat to the sheet owing to the energy released during the recombination (4.5 eV/molecule). Moreover, the higher thermal conductivity of the hydrogen also increases the temperature of the pool.

(8) At high temperatures, that is to say temperatures of more than 5000 K, 97 mol % of CO.sub.2 of the annular gas flow is dissociated in the arc according to the equation: CO.sub.2.fwdarw.CO+ O.sub.2, which leads to the formation of oxygen. Oxygen is a surfactant element, its presence in the molten metal lowers the surface tension of the metal. Thus, the document by B. Keene, Review of data for the surface tension of iron and its binary alloys, International Materials Reviews, 1988, vol. 33, no. 1, pp. 1-37, gives, for a binary FeO mixture, the equation: .sub.Fe-O=.sub.Fe7490[at.-% O]. The oxygen formed will therefore fluidize the pool.

(9) In an alternative manner to the CO.sub.2, it is possible to use oxygen but at a content two times lower in the annular gas flow, as explained below.

(10) FIG. 2 illustrates the thermal conductivities (on the y-axis) of the components of the protective gas mixtures as a function of the temperature (on the x-axis). The thermal conductivity is defined as being the speed at which heat propagates by conduction across a unit surface area, normal to the direction of the heat flow, and this per unit of length and unit of temperature difference. It therefore controls the transfer of heat by conduction and has an impact on the morphology of the bead, the temperature of the weld pool and the wetting.

(11) It can be seen in FIG. 2 that helium and hydrogen have higher thermal conductivities than that of argon and will therefore make it possible to obtain more energetic arcs.

(12) Table 1 below gives the compositions (% by volume) of the various gases G1 to G9 which were tested in dual-flow TIG welding.

(13) TABLE-US-00001 TABLE 1 % by volume Ar H.sub.2 He CO.sub.2 O.sub.2 G1 100 / / / / G2 98 2 / / / G3 95 5 / / / G4 99 / / 1 / G5 98 / / 2 / G6 95 / / 8 / G7 99 / / / 1 G8 98.5 / / / 1.5 G9 80 / 20 / /

(14) Table 2 below records the results obtained in terms of maximum welding speed (Vs) before appearance of the humping phenomenon on A42 steel parts having a thickness of 1.5 mm, using a conventional automatic dual-flow TIG welding torch equipped with a pointed infusible electrode having a diameter of 3.2 mm made of ceriated tungsten of W-2% CeO.sub.2 type, with an arc height of around 2 mm and a welding current of around 200 A and by using various combinations of the gases G1 to G9.

(15) TABLE-US-00002 TABLE 2 Test No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Central gas G1 G2 G5 G9 G1 G1 G1 G1 G1 G3 G3 G3 G3 G3 Annular gas G1 G1 G1 G1 G4 G5 G6 G7 G8 G4 G5 G6 G7 G8 Vs max (cm/min) 200 200 230 220 220 250 220 250 240 250 300 220 300 240

(16) It emerges from table 2 that the pairs G3/G5 (test 11) and G3/G7 (test 13) according to the invention offer the best performances both in terms of increase in speed and of limitation of the oxides present at the surface. In fact, with these particular combinations of gases, there is a surfactant effect coupled with an increase in the volume of molten metal and with a rise in the temperature of the pool, which goes toward confirming Marangoni convection since this is then directed toward the center of the bead. Consequently, an equilibrium of the pressures forms in the transition zone which is modified since the pressure of the weld pool is increased. The pool has a better aptitude for wetting, therefore the dry zones are covered more rapidly. Finally, the hotter pool solidifies more slowly.

(17) These tests 11 and 13 therefore confirm the advantage of working with a CO.sub.2 content of much less than around 3% or an O.sub.2 content of much less than around 1.5% (the rest being argon), preferably a CO.sub.2 content of less than 2.5% or an O.sub.2 content of less than 1.2%, advantageously contents of the order of 2% of CO.sub.2 or of 1% of O.sub.2, combined with a central flow of argon and of hydrogen between 2 to 8%, preferably a H.sub.2 content between 4 and 7%, advantageously around 5% of H.sub.2.

(18) Indeed, as tests 12 and 14 show, the use of annular gases that are too highly charged with oxidizer, lead to a degradation in the performances by the appearance of an oxide film on the welded parts. This film of oxides thwarts the surfactant effect via an increase in the viscosity, or even leads to an appearance of larger or smaller black marks on the weld beads.

(19) To prevent these damaging phenomena, the use of a too highly oxidizing gas mixture, as annular gas, is avoided.

(20) Hence, in accordance with the invention, the CO.sub.2 content is limited to 3% and the O.sub.2 content to 1.5%, or even to less than 2.5% of CO.sub.2 and to less than 1.3% of O.sub.2.

(21) Likewise, it is important to carefully choose the composition of the gas mixture containing hydrogen and argon used as central gas.

(22) Indeed, hydrogen is one of the factors that gives rise to cold cracking (ferritic grades) and may also be the cause of porosities. It is therefore necessary to establish the metallurgical compatibility (non-hardening steels and austenitic stainless steels) of the gas mixture used and therefore to strictly control the H.sub.2 content present in said mixture.

(23) In order to do this, additional comparative tests were carried out, under the same operating conditions as before, but with different pairs of gases (cf. table 3 below) so as, in particular, to determine the best H.sub.2 content to use and to confirm whether or not the oxygen had an effect equivalent to the CO.sub.2 in the annular gas by comparison with the results obtained for the G3/G4 pair from table 3.

(24) TABLE-US-00003 TABLE 3 Central gas Ar + 2% H.sub.2 Ar + 5% H.sub.2 Ar + 5% H.sub.2 Annular gas Ar + 1% CO.sub.2 Ar + 1% O.sub.2 Ar + 1.5% O.sub.2 Maximum Vs Lower than Approx. equal to Lower than that obtained that obtained that obtained with G3 with G4 with G4 (% by volume)

(25) The results obtained show that: a central flow containing only 2% by volume of H.sub.2 in argon (instead of 5% of H.sub.2 for G3) leads to a lower maximum speed being obtained relative to that obtained with the G3/G4 pair. By limiting the hydrogen content, the effect on the provision of heat to the sheet is limited and both pool volume and temperature gradient are lost. Hence, it will be preferred to use H.sub.2 contents of at least 2% by volume, more preferably greater than 3%, or even greater than 3.5%, advantageously between 4 and 8%. when the central flow is G3 (Ar+5% H.sub.2) and when the peripheral annular flow is replaced by a mixture of argon and oxygen, then a volume content of 1% of O.sub.2 leads to results that are substantially equivalent in terms of speed to G4 Ar+1% CO.sub.2), whereas an increase in the O.sub.2 content up to 1.5% in the annular gas distributed at the periphery, leads to a reduction in the speed performances and the appearance of the bead is also degraded. Indeed, all the available sites are already occupied by the soluble oxygen atoms and the additional soluble atoms only serve to create oxides. Therefore there is no positive effect on the Marangoni convection and conversely, an appearance of damaging oxide plates is observed, which oxide plates go toward increasing surface tensions and are detrimental to the appearance of the bead. At the temperatures that exist at the periphery of the arc, the CO.sub.2 dissociates completely and working with a gas containing 1% of O.sub.2 is equivalent to working with a gas containing 2% of CO.sub.2. Hence, the O.sub.2 content in the annular gas flow must not exceed 1.5% by volume, preferably an O.sub.2 content of less than or equal to 1.3%, or even less than or equal to 1.2% will be used. An oxygen content of the order of 1% is particularly suitable.

(26) The process of the invention is particularly suitable for welding motor vehicle sheets, such as tailored blanks.

(27) 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.