(19)
(11)EP 1 140 410 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
03.11.2004 Bulletin 2004/45

(21)Application number: 99962629.4

(22)Date of filing:  19.11.1999
(51)International Patent Classification (IPC)7B23K 9/095, B23K 9/10
(86)International application number:
PCT/SE1999/002141
(87)International publication number:
WO 2000/033999 (15.06.2000 Gazette  2000/24)

(54)

METHOD AND ARRANGEMENT FOR GAS METAL ARC WELDING

VERFAHREN UND VORRICHTUNG ZUM GAS-METALL-LICHTBOGENSCHWEISSEN

PROCEDE ET DISPOSITIF DE SOUDAGE A L'ARC MIG


(84)Designated Contracting States:
AT BE CH CY DE DK ES FI FR GB GR IE IT LI LU MC NL PT SE

(30)Priority: 19.11.1998 SE 9803971

(43)Date of publication of application:
10.10.2001 Bulletin 2001/41

(73)Proprietor: Esab AB
402 77 Göteborg (SE)

(72)Inventors:
  • ABERG, Per
    S-695 30 Laxa (SE)
  • BENGTSSON, Lars
    S-695 32 Laxa (SE)
  • ALLANSSON, Anders
    S-692 33 Kumla (SE)


(56)References cited: : 
WO-A1-98/39130
US-A- 4 443 687
DE-A1- 3 234 002
US-A- 5 514 851
  
      
    Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


    Description


    [0001] This invention relates to a method for gas metal arc welding with an inverter power source, and a welding power source of the inverter type with properties associated with terminating welding which prevent the welding wire from sticking in the contact nozzle on the welding burner connected to the welding power source.

    Background



    [0002] In gas metal arc welding, in the following called MIG/MAG welding, the equipment comprises a gas container, reel of wire, wire feeder, welding power source, welding cable and welding burner which comprises a first pipe for taking the protective gas to the molten pool and a second pipe situated inside the first to take the welding wire to the molten pool and to transmit the welding current to the said welding wire. In the following this second pipe is called the contact nozzle.

    [0003] There has always been a certain tendency for the wire to stick in the contact nozzle. The solutions have concentrated upon reducing the friction in all parts which take the wire from the wire feeder to the workpiece. Wire guides of Teflon or similar material are an example of such a solution, and great care has been taken to manufacture the wire with as smooth a surface as possible.

    [0004] Other measures have regarded the main problem as being deposits from the welding wire, either from its thin copper layer or from residues from the wire drawing process.

    [0005] Solutions have been of a mechanical nature - better feeders, greater tolerances between the wire and the hole in the contact nozzle or self-cleaning contact nozzles.

    [0006] However, wider tolerances in the contact nozzle result in a poorer electrical contact and thereby poorer current transmission.

    [0007] This has been solved by ingenious contact nozzles such as spiral-shaped or parrot-beaked nozzles in order to achieve better contact points. Contact nozzles of the normal pipe model have sometimes been given improved characteristics by a little mechanical impact in order to create a distinct contact point. However, such solutions result in increased friction which is not desirable for even wire feeding. In addition, the mechanical wear and tear on such nozzles is greater, which reduces their working life.

    [0008] Since the 1980's welding power sources of the inverter type have been used. The weight and size of the power source is reduced, which is an advantage. In particular, however, the power sources are much quicker to regulate which has meant that the emphasis for the regulation of the welding process has moved from the wire feeder to the power sources. During continual welding the object of the regulation is to supply melting power at a corresponding rate to the feed rate of the electrode.

    [0009] Problems associated with commencing welding have been observed with these power sources, particularly when welding with stainless electrodes. The wire can then stick in the contact nozzle at the actual moment of commencing welding. In the work leading up to this invention it was also studied how the wire behaved during the concluding stage, that is just when the welding current is about to be shut off.

    [0010] The surprising result of these investigations was that the wire did not become wedged in the contact nozzle in connection with the wire feeder motor coming to a stop. On the contrary, at the moment the welding current stopped, the wire was brought to an immediate stop in the contact nozzle, while the wire feeder fed out further wire which was taken up by the play in the wire guide which connects the feeder and the contact nozzle.

    Aim of the invention



    [0011] The aim of the invention is to produce a method or a welding power source of the inverter type, which provides a solution to the problem of electrodes sticking in the contact nozzle during MIG/MAG welding without the disadvantages from which the known technique suffers.

    [0012] The invention solves the problem in the way specified in the characteristic portions of the independent claims.

    Description of preferred embodiments



    [0013] Examples of embodiments of the invention will now be described.

    [0014] Welding power sources of the inverter type have a control unit which sends commands to the power unit concerning the welding current which at each moment is to be transmitted to the welding cable which connects the contact nozzle with the power source. In the contact nozzle the current passes to the welding wire which then passes the current to the workpiece via the arc and from there back to the power source via the return lead. The welding wire is continually fed forwards at a certain speed by the wire feeder and while this is continuing the process regulation is carried out via the control unit so that sufficient current is supplied to the arc for the wire which has been fed forward to be melted at the same rate as it is fed forwards.

    [0015] At the termination of the welding, the feeder is stopped first while the controlling of the power source continues during an after-burning time, so adjusted that the wire will not set fast in the molten pool on the workpiece and nor will it burn up in the contact nozzle and damage this.

    [0016] Sometimes there is a gradual reduction in the power in order to fill craters in the molten pool on the workpiece. This crater-filling time is 0.2-3 seconds.

    [0017] The invention causes the control unit in the power source to carry out a fading of the welding current immediately before the current is cut off.

    [0018] It has proved very effective to insert a fading time of 0.5 - 100 milliseconds, and an even better interval is 1 - 15 milliseconds.

    [0019] This has almost completely eliminated the problem of the wire sticking in the contact nozzle when the welding is terminated and in fact has also almost eliminated the occasions when the wire became stuck in connection with the commencement of welding. Thus after this measure is utilized, a contact nozzle can be used for commencing welding on up to ten times more occasions before the problem arises. The solution is simple to implement in the control unit's microcomputer. Of course solutions can also be implemented in discrete hardware in both digital and analogue form - the construction of the control unit determines the form of implementation.

    [0020] The reason that the invention works is probably that a very small molten mass forms between the welding wire and the contact nozzle as a result partly of resistive heating and partly of spark formation inside the contact nczzle, particularly when it has become somewhat worn. If the current is stopped abruptly the wire sets fast in this molten mass and the remaining pushing force from the feeder is not sufficient to release the wire. As the molten mass is reduced in size due to the fading of the current, the adherence will be less and the pushing force is able to release the wire immediately.

    [0021] The problem connected with the wire sticking at the commencement of welding is probably connected with remaining residues on the front of the contact nozzle of welding. At the commencement of welding the arc often burns right up to the contact nozzle momentarily. Residues from the termination of welding on the front opening of the contact nozzle then melt but solidify again when the arc returns immediately to its normal position (between the electrode projection and the workpiece), so that there is a danger of the wire sticking. As the residues from the termination of welding are so much less when the invention is applied, this solution also works in this case.

    [0022] The invention works best with a number of uniform stages and a fading of the current of approximately 10 milliseconds length in total. However, an obvious benefit is obtained, even if fewer stages are used and also if they vary in size. Of course an embodiment with a purely analogue fading slope can be used.

    [0023] WO-A-98/39130 deals with fading but for a totally different purpose, namely crater-filling. Furtermore fading of the welding current is not mentioned. In DE-A-32 34 002 the current is switched off abruptly at a measured level as is also the case in US-A-4443687.


    Claims

    1. Method for reducing the adherence of the welding wire to the contact nozzle at the termination of gas metal arc welding with a power source of the inverter type, characterized in that immediately before the welding current is shut off the control unit for the welding current in the power source is caused to fade the said welding current over a period of time of between 0.5 and 100 milliseconds so that the power development in the current transition between the welding wire and the contact nozzle is so low that the molten mass which has formed between the welding wire and the contact nozzle is so small that when it has set when the current is shut off it is not able to hold fast the wire against the pushing force on the welding wire from a feeder coming to a stop.
     
    2. Method according to Claim 1, characterized in that the time for the fading of the current is 1 to 15 milliseconds.
     
    3. Method according to Claim 1 or 2, characterized in that the fading of the current takes place in at least two stages.
     
    4. Method according to any of the preceding claims, characterized in that the fading of the current takes place in principally equally large stages.
     
    5. welding power source of the inverter type intended for gas metal arc welding comprising a control unit and power unit where the control unit gives commands to the power unit regarding what welding current is to be supplied at each moment, characterized in that the control unit is arranged so that immediately before it gives the command to shut off the welding current, it initiates a fading of the welding current over a period of time lasting between 0.5 and 100 milliseconds so that the power development in the current transition between the welding wire and the contact nozzle is so low that the molten mass which has formed between the welding wire and the contact nozzle is so small that when it has set when the current is shut off it is not able to hold fast the wire against the pushing force on the welding wire from a feeder coming to a stop.
     
    6. Welding power source according to Claim 5, characterized in that the control unit is arranged to initiate a fading of the current lasting between 1 and 15 milliseconds.
     
    7. Welding power source according to Claim 5 or 6, characterized in that the control unit is arranged to initiate a fading of the current in principally equally large stages and where the number of stages is at least two.
     


    Ansprüche

    1. Verfahren zum Reduzieren der Haftung des Schweißdrahts an der Kontaktdüse am Ende des Gas-Metall-Lichtbogenschweißens mit einer Stromquelle vom Invertertyp, dadurch gekennzeichnet, daß bewirkt wird, daß unmittelbar vor dem Abschalten des Schweißstroms die Steuereinheit für den Schweißstrom in der Stromquelle den Schweißstrom über einen Zeitraum zwischen 0,5 und 100 ms abklingen läßt, so daß die Leistungsentwicklung im Stromübergang zwischen dem Schweißdraht und der Kontaktdüse so gering ist, daß die geschmolzene Masse, die zwischen dem Schweißdraht und der Kontaktdüse entsteht, beim Erstarren beim Abschalten des Stroms so klein ist, nicht in der Lage ist, den Draht gegen die Druckkraft auf den Schweißdraht von einer zu einem Halt kommenden Zuführungseinrichtung festzuhalten.
     
    2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die Zeit für das Abklingen des Stroms 1 bis 15 ms beträgt.
     
    3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß das Abklingen des Stroms in mindestens zwei Stufen stattfindet.
     
    4. Verfahren nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß das Abklingen des Stroms prinzipiell in gleichgroßen Stufen stattfindet.
     
    5. Schweißstromquelle vom Invertertyp für Gas-Metall-Lichtbogenschweißen, umfassend eine Steuereinheit und eine Stromeinheit, wobei die Steuereinheit Befehle an die Stromeinheit dahingehend abgibt, welcher Schweißstrom in jedem Augenblick zugeführt werden soll, dadurch gekennzeichnet, daß die Steuereinheit so ausgelegt ist, daß sie, unmittelbar bevor sie den Befehl gibt, dem Schweißstrom abzuschalten, ein Abklingen des Schweißstroms über einen Zeitraum einleitet, der zwischen 0,5 und 100 ms dauert, so daß die Leistungsentwicklung im Stromübergang zwischen dem Schweißdraht und der Kontaktdüse so gering ist, daß die geschmolzene Masse, die zwischen dem Schweißdraht und der Kontaktdüse entsteht, beim Erstarren beim Abschalten des Stroms so klein ist, nicht in der Lage ist, den Draht gegen die Druckkraft auf den Schweißdraht von einer zu einem Halt kommenden Zuführungseinrichtung festzuhalten.
     
    6. Schweißstromquelle nach Anspruch 5, dadurch gekennzeichnet, daß die Steuereinheit so ausgelegt ist, daß sie ein Abklingen des Stroms einleitet, daß zwischen 1 und 15 ms dauert.
     
    7. Schweißstromquelle nach Anspruch 5 oder 6, dadurch gekennzeichnet, daß die Steuereinheit so ausgelegt ist, daß sie ein Abklingen des Stroms in prinzipiell gleichgroßen Stufen einleitet, wobei die Anzahl der Stufen mindestens zwei beträgt.
     


    Revendications

    1. Procédé de réduction de l'adhérence du fil de soudage sur la buse de contact à la fin d'un soudage à l'arc MIG, avec un générateur de courant du type onduleur, caractérisé en ce que, immédiatement avant la coupure du courant de soudage, le bloc de commande du courant de soudage dans le générateur de courant est amené à diminuer graduellement ledit courant de soudage sur un intervalle de temps compris entre 0,5 et 100 millisecondes, de telle sorte que la formation d'énergie dans la transition de courant entre le fil de soudage et la buse de contact soit si faible que la masse fondue qui s'est formée entre le fil de soudage et la buse de contact soit si petite que, lorsqu'elle s'est solidifiée lors de la coupure du courant, elle soit incapable de maintenir fixement le fil face à la force de poussée s'exerçant sur le fil de soudage suite à l'arrêt d'un dispositif d'alimentation en fil.
     
    2. Procédé selon la revendication 1, caractérisé en ce que la durée de la diminution graduelle du courant est comprise entre 1 et 15 millisecondes.
     
    3. Procédé selon la revendication 1 ou 2, caractérisé en ce que la diminution graduelle du courant se produit en deux étapes au moins.
     
    4. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que la diminution graduelle du courant se produit en des étapes essentiellement d'une même durée.
     
    5. Générateur de courant de soudage du type onduleur destiné au soudage à l'arc MIG, comprenant un bloc de commande et un bloc d'alimentation, le bloc de commande donnant des instructions au bloc d'alimentation quant à la nature du courant de soudage à fournir à tout moment, caractérisé en ce que le bloc de commande est conçu de telle sorte que, immédiatement avant de donner l'instruction de couper le courant de soudage, il déclenche une diminution graduelle du courant de soudage sur un intervalle de temps compris entre 0,5 et 100 millisecondes, de telle sorte que la formation d'énergie dans la transition de courant entre le fil de soudage et la buse de contact soit si faible que la masse fondue qui s'est formée entre le fil de soudage et la buse de contact soit si petite que, lorsqu'elle s'est solidifiée lors de la coupure du courant, elle soit incapable de maintenir fixement le fil face à la force de poussée s'exerçant sur le fil de soudage suite à l'arrêt d'un dispositif d'alimentation en fil.
     
    6. Générateur de courant de soudage selon la revendication 5, caractérisé en ce que le bloc de commande est conçu pour déclencher une diminution graduelle du courant dont la durée est comprise entre 1 et 15 millisecondes.
     
    7. Générateur de courant de soudage selon la revendication 5 ou 6, caractérisé en ce que le bloc de commande est conçu pour déclencher une diminution graduelle du courant en des étapes essentiellement d'une même durée, les étapes étant au moins au nombre de deux.