Abstract:
For MIG welding, either in a spray mode without current modulation or in a pulsed mode, of aluminum and aluminum alloys, with the use of a gas shield for at least part of the welding zone, the gas shield is a gas mixture consisting, by volume, of from 0.01% to 1.80% oxygen and from 15% to 98.2% helium, and any balance being argon.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a MIG (Metal Inert Gas) process for the electric-arc welding of aluminum and aluminum alloys, either in a pulsed mode or in a spray mode without modulated current. 
     BACKGROUND OF THE INVENTION 
     The MIG electric-arc welding process is widely used in industry, including that of welding aluminum. 
     Shielding gases play an essential role in the performance of this process. 
     Thus, argon does not allow high welding speeds and may give rise, in automatic welding, to arc instabilities above 500 A. The characteristic shape of the narrow penetrations obtained with argon in spray mode is ill-suited to welding by interpenetration. 
     Moreover, argon/helium (hereafter Ar/He) mixtures and helium by itself make it possible to increase the depth of penetration and its root width, and hence make it possible to dispense with expensive preparations, the more so the higher the helium content in the Ar/He mixtures. 
     In other words, for a constant thickness, helium therefore allows welding speeds which increase with its content. 
     However, although the compactness of the beads is also generally improved by the presence of helium, this is to the detriment of the appearance of the beads, which are less shiny than with pure argon. 
     Ar/He mixtures therefore present an appreciable advantage in terms of quality and productivity, both in manual welding (for example for a mixture of the Ar/20% He type) and in automatic welding (for example for mixtures of the Ar/50% to 70% He type) with, however, a not insignificant cost associated with the helium content. 
     For applications not necessarily having to meet these two criteria, it may be judicious to consider other types of mixtures. 
     Thus, document EP-A-639 423 proposes, for TIG and MIG processes, to use a welding gas of the argon or argon/helium type containing, furthermore, from 100 to 1000 ppm by volume of CO 2  and/or O 2 . 
     Furthermore, document DE-A-4241982 proposes to use argon or an Ar/He mixture to which has also been added from 80 to 250 ppm by volume of nitrogen. 
     Document EP-A-442 475 recommends welding with a consumable electrode using a gas mixture consisting of 0.5 to 1.25 vol % carbon dioxide, 30 to 40 vol % helium and the balance being argon. 
     Document U.S. Pat. No. 4,071,732 relates to a mixture formed from an inert gas containing less than 30% carbon dioxide or less than 5% oxygen; however, the examples in this document describe only mixtures of argon and carbon dioxide with contents of about 5 or 15%. 
     It should be emphasized that in terms of increasing the performance, none of these processes is fully satisfactory from the industrial standpoint. 
     Furthermore, in modulated spray MIG welding, that is to say with the welding current being modulated, it has already been recommended to use a shielding gas or gas mixture formed from argon, helium or mixtures thereof, to which from 0.01% to 1.80% of carbon dioxide and/or oxygen have been added, as described in EP-A-909 604. However, in this case, current modulations at a frequency of less than 60 Hz are applied to the current so as to be able to degas the weld puddle in order to remove gaseous inclusions therefrom, particularly diffusable hydrogen, liable to be found therein. This is because MIG processes in spray mode with current modulations are used when it is desired to obtain a high quality of the welded joint, but without actually needing to achieve a high welding speed. 
     Consequently, the problem that arises is to improve the known MIG welding processes not using modulation of the welding current, particularly MIG processes in unmodulated spray mode, that is to say without modulation of the welding current, and processes in pulsed mode, so as to be able to obtain high performance levels in terms of productivity and welding speed. 
     This is because MIG processes in unmodulated spray mode (without modulation of the current) and those in pulsed mode are much more suitable when it is desired to improve productivity rather than quality, that is to say the appearance of the welds. 
     Hitherto, MIG processes in unmodulated mode or in pulsed mode are used little or not at all for welding aluminum or its alloys when the gas shield contains oxygen. 
     This is because it is usually recognized that the presence of oxygen in the gas shield may have a deleterious effect on the weld given that, when oxygen is incorporated into the gas shield, the oxygen can easily combine with aluminum atoms and result in solid inclusions of alumina (Al 2 O 3 ) in the weld, which have a negative effect on the mechanical properties of said weld. Moreover, this has been confirmed for high oxygen contents, that is to say oxygen contents greater than 2%, and for high carbon dioxide contents, that is to say, again, contents greater than 2%. 
     However, conversely, the presence of oxygen in the shielding gas stream results in productivity performance levels which are acceptable from the industrial standpoint. 
     It therefore follows that the problem that arises is to provide an MIG welding process for aluminum and its alloys resulting both in a high and industrially acceptable productivity and a low level of alumina inclusions in the weld without a major or appreciable impact on the mechanical properties of the welded joints. 
     SUMMARY OF THE INVENTION 
     The solution provided by the present invention therefore relies on a process for MIG welding, in spray mode without current modulation or in pulsed mode, of aluminum and aluminum alloys, with the use of a gas shield for at least part of the welding zone, wherein the gas shield is a gas mixture consisting of from 0.01% to 1.80% oxygen and from 20% to 98.2% helium, any balance consisting of argon. 
     Further characteristics of the process of the invention are given below: 
     the shielding gas contains from 0.9% to 1.80% oxygen and from 15% to 98.20% helium, the balance being argon; 
     the shielding gas mixture contains at least 1% oxygen, preferably at least 1.1% oxygen, more preferably at least 1.2% oxygen; 
     the shielding gas mixture contains at most 1.70% oxygen, preferably at most 1.65% oxygen; 
     a solid meltable wire is used; 
     the welding speed is from 0.25 m/min. to 1.20 m/min., preferably from 0.60 to 1 m/min.; 
     the wire speed is from 2.5 m/min. to 20 m/min., preferably from 4 m/min. to 17 m/min.; 
     the mean welding current is from 40 A to 450 A and/or the mean welding voltage is from 15 V to 40 V; 
     the process is in pulsed mode and/or the welding current is from 120 A to 350 A and/or the mean welding voltage is from 20 V to 30 V; 
     the process is in spray mode and/or the welding current is from 180 A to 450 A and/or the mean welding voltage is from 20 V to 39 V. 
     The present invention therefore relies on precise control of the oxygen content in the helium or an argon/helium mixture, it being necessary for the maximum oxygen content not to exceed about 1.80%, the gas mixture thus formed constituting the gas shield used when implementing the MIG process. 
     It should be emphasized that any MIG process in spray mode with modulation of the welding current is excluded from the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be more clearly explained by means of the following examples, given by way of illustration but implying no limitation, the results of which are shown schematically in the figures appended hereto. 
     FIG. 1 shows the influence of adding oxygen to argon and various argon/helium mixtures with respect to welding speed; 
     FIG. 2 shows the influence of adding oxygen to various argon/helium mixtures with respect to tensile strength; 
     FIG. 3 shows stability of current in spray mode with argon/20% helium mixture achieved by addition of oxygen; and 
     FIG. 4 shows stability of current in spray mode with argon/50% helium achieved by addition of oxygen. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to show the effectiveness of the MIG process according to the invention, several comparative trials were carried out. 
     Within the context of these trials, aluminum workpieces (5000 and 6000 grades according to the NFEN 485, 487, 515 and 573 standards) were welded using an MIG process in unmodulated spray mode and in pulsed mode, using a gas shield consisting of argon to which from 1 to 1.5% O 2  was added. 
     The current generator was a 480 TR16 generator sold by La Soudure Autogene Française. 
     The meltable wire used as filler metal was, in all cases, a wire 1.2 mm in diameter of the 5356 type (according to the AWS A5.10 or NF A50.403 standards). 
     Prior to welding, the aluminum workpieces were prepared by mechanical gouging. 
     The other welding parameters are given in Table I below, in which: 
     V wire  represents the feed speed of the meltable wire; 
     I p  represents the intensity of the peak current; 
     I b  represents the base current; 
     I mean  represents the mean current; 
     U p  represents the peak voltage; 
     U mean  represents the mean voltage; 
     F pulse  represents the current pulse frequency (in pulsed MIG mode); 
     T pulse  represents the current pulse time (in pulsed MIG mode). 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Welding parameters for the two transfer modes 
               
             
          
           
               
                   
                 Mode 
                 Spray 
                 Pulsed 
               
               
                   
                   
               
             
          
           
               
                   
                 Grade of the 
                 5000 
                 5000 
               
               
                   
                 aluminum workpiece 
               
               
                   
                 Thickness (mm) 
                 6 
                 6 
               
               
                   
                 V wire  (m/min.) 
                 12.5 
                 9 
               
               
                   
                 I p  (A) 
                 — 
                 330 
               
               
                   
                 I b  (A) 
                 — 
                 110 
               
               
                   
                 I mean  (A) 
                 220-240 
                 154-162 
               
               
                   
                 U p  (V) 
                 — 
                 28 
               
               
                   
                 U mean  (V) 
                 21-23 
                 20-23 
               
               
                   
                 F pulse  (Hz) 
                 — 
                 155 
               
               
                   
                 T pulse  (ms) 
                 — 
                 1.6 
               
               
                   
                   
               
             
          
         
       
     
     The results obtained are given below, after evaluating the performance in terms of productivity (welding speed) and joint quality (compactness and appearance of the bead) and mechanical properties. 
     In Table II below, the increases in speed given for the 5000 grade were determined with respect to the welding speeds for various argon/helium mixtures and with or without the addition of O 2  and, by way of comparison, the results obtained with argon to which O 2  was added, have also been given. These results are shown diagrammatically in FIG.  1 . 
     It may be seen that for identical O 2  additions, the increase in performance, in terms of welding speed and in terms of penetration, remains equivalent for an Ar/He mixture and for argon alone. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Increase in performance in terms of welding 
               
               
                 speed for 5000 grade with respect to an Ar/He mixture 
               
               
                 without the addition of O 2 . 
               
             
          
           
               
                 Mode 
                 Spray 
                   
                 Pulsed 
               
             
          
           
               
                 Gas 
                 Ar 
                 Ar/20% He 
                 Ar/50% He 
                 Ar/70% He 
                 Ar 
                 Ar/20% He 
                 Ar/50% He 
                 Ar/70% He 
               
               
                   
               
             
          
           
               
                  0% O 2   
                 0.56 
                 0.7 
                 0.65 
                 0.7 
                 0.35 
                 0.4 
                 0.44 
                 0.45 
               
               
                 +1% O 2   
                 0.7 
                 0.88 
                 n.d. 
                 n.d. 
                 0.42 
                 0.49 
                 n.d. 
                 n.d. 
               
               
                   
                 25%  
                 25.7% 
                 n.d. 
                 n.d. 
                 20%  
                 22.5% 
                 n.d. 
                 n.d. 
               
               
                 +1.5% O 2   
                 0.78 
                 0.93 
                 0.86 
                 0.93 
                 0.45 
                 0.52 
                 0.54 
                 0.55 
               
               
                   
                 39.2% 
                 32.8% 
                 32.3% 
                 32.8% 
                 28.5% 
                 30%  
                 22.7% 
                 22.2% 
               
               
                   
               
               
                 Welding speed (V w ): in m/min.;  
               
               
                 Speed increase: in %;  
               
               
                 n.d.:  not determined.  
               
             
          
         
       
     
     Table II shows an increase in the welding speed for a weld with perfect penetration for a thickness of 6 mm. 
     Moreover, in order to evaluate the performance in terms of joint quality (compactness and appearance of the bead), X-ray examinations were carried out on the 5000-grade workpieces, which did not reveal any indications other than those normally encountered in MIG with argon or Ar/He mixtures. 
     Complementarily, macrographic samples made it possible to reveal, by simple polishing, areas of reduced-size (about 0.01 mm) inclusions of different distribution and orientations. 
     These areas were identified in a scanning electron microscope as areas of alumina (Al 2 O 3 ). Furthermore, the appearance of the beads obtained with Ar/He/O 2  mixtures was substantially different from those produced with an Ar/He mixture: the beads have a surface deposit which is blackish depending on the retained O 2  content, which is removed by brushing or with the aid of a rag. 
     Next, the mechanical properties were determined from leveled transverse tensile and transverse bending test pieces. The tensile results are shown in FIG. 2 in which the x-axis represents the O 2  contents in the three Ar/He mixtures of different compositions and the y-axis represents the tensile strength (Rm) values (in MPa) of the 5000-grade assemblies. 
     The corresponding intrinsic values and the joint coefficients (strength of the melted metal/strength of the basis metal) are indicated in Table III. 
     It will be noted that there is no significant degradation in the mechanical properties of the melted zone depending on the increase in the O 2  content. The maximum recorded reduction of 7 N/mm 2  (i.e. −2.6%) in spray mode with an Ar/He mixture remains less than that recorded for the same transfer mode with Ar/O 2 , namely 15 N/mm 2  (i.e. −5.5%). 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 Mechanical properties on 5086-series 
               
               
                 assembles for Ar/He mixtures with addition of O 2   
               
             
          
           
               
                 Mode 
                 Spray 
                   
                 Pulsed 
               
             
          
           
               
                 Gas 
                 Ar/20% He 
                 Ar/50% He 
                 Ar/70% He 
                 Ar/20% He 
                 Ar/50% He 
                 Ar/70% He 
               
               
                   
               
             
          
           
               
                  0% O 2   
                 R m  (MPa) 
                 271.12 
                 263.2 
                 266.47 
                 265.03 
                 266 
                 264.03 
               
               
                   
                 Joint coeff. 
                 0.86 
                 0.83 
                 0.84 
                 0.84 
                 0.84 
                 0.83 
               
               
                 +1% O 2   
                 R m  (MPa) 
                 269.92 
                 n.d. 
                 n.d. 
                 267.05 
                 n.d. 
                 n.d. 
               
               
                   
                 Joint coeff. 
                 0.85 
                 n.d. 
                 n.d. 
                 0.84 
                 n.d. 
                 n.d. 
               
               
                 +1.5% O 2   
                 R m  (MPa) 
                 264.72 
                 260.12 
                 259.71 
                 266.61 
                 266.4 
                 261.68 
               
               
                   
                 Joint coeff. 
                 0.84 
                 0.82 
                 0.82 
                 0.84 
                 0.84 
                 0.83 
               
             
          
           
               
                   
                 BM actual/ 
                 315/275 
               
               
                   
                 guaranteed 
               
               
                   
                 R m  (MPa) 
               
               
                   
               
             
          
         
       
     
     All the tensile test pieces (having a thickness of 6 mm) systematically fractured in the melted metal, which is normal behavior for the 5000 series. Areas of fine dispersed black inclusions were revealed on their fracture surfaces. These observations confirm the results of the above macrographic examinations. They may become larger in size and greater in density according to the O 2  content, but, as mentioned above, their influence is not significant either on the mechanical properties of the assemblies or on the deformability, since the results of the bending trials were satisfactory (complete absence of effects after bending through 180°). 
     It therefore follows (FIG. 1) that, for the same O 2  addition, a gain in performance (increase in the penetration or welding speed) remains comparable on an Ar/He mixture as on argon alone. 
     The addition of O 2  in a controlled amount (less than 2%) also makes it possible to leave out of account any particular preparation, such as grooving and separation, of workpieces up to 6 mm in thickness to be welded together. 
     To these gains in productivity must also be added two other substantial operating advantages, namely the ease of ignition and the stability of the arc in the steady state. A comparative example shown diagrammatically in FIG. 3 demonstrates, in recordings, the substantial improvement obtained in the stability of the current in spray mode with an Ar/He/O 2  mixture over spray mode with Ar/He (20 to 50% helium content in both cases), that is to say without the addition of O 2 , as shown by the comparative examples in FIGS. 3 and 4 demonstrating, in Labview recordings, the improvements in the stability of the current in spray mode for an Ar/He mixture to which oxygen has been added. 
     These advantages have been demonstrated not only on pure aluminum but also on 5000- and 6000-series aluminum alloys.