WO2003090966A1

WO2003090966A1 - Welding method and welding apparatus

Info

Publication number: WO2003090966A1
Application number: PCT/NL2003/000301
Authority: WO
Inventors: Wilhelmus Anthonius Johannes Brabander
Original assignee: Pol Elektronica
Priority date: 2002-04-24
Filing date: 2003-04-23
Publication date: 2003-11-06
Also published as: EP1507622A1; NL1020461C2; AU2003224508A1

Abstract

A method for executing a GMAW-welding process is described, using a welding torch (1) with a tube-shaped contact electrode (3) which is provided with an auxiliary electrode (8). Initially, it is assured that the end (l0a) of a welding wire (10) is inside the tube-shaped electrode (3). Then, a preheating arc (30A) is ignited between the auxiliary electrode (8) and a workpiece (W) concerned, while a current source (100) is operated in a TIG-mode. Sometime after ignition of the preheating arc (30A), wire transport of the welding wire (10) is set in motion, and the current source (100) is operated in a GMAW-mode in this case, such that a welding arc (30B) is going to burn between the end (l0a) of the welding wire (10) and the workpiece (W) concerned. After the welding process has ended, the end (10a) of the welding wire (10) is withdrawn to within the contact electrode.

Description

TITLE : Welding method and welding apparatus

The invention relates in general to the GMAW-welding process (Gas Metal Arc Welding) , also indicated as MIG-welding process (Metal Inert Gas) .

The GMAW-welding process is commonly known, and an elaborate description thereof can be omitted here. By way of example, reference is made to the international patent publication WO-02/04162, where a description is given of the GMAW-welding process and of an apparatus usable for this process. Suffice it to note that use is made of a welding torch with a hollow contact tube, through which a continuous supply wire is guided. The contact tube makes electrical contact with the wire. An electrical voltage is applied between the contact tube and a metal workpiece, resulting in an electrical arc between the end of the wire and this workpiece, causing the wire to melt. Feeding means continuously feed new wire, which compensates the melting of the wire .

In conventional welding torches, the contact tube is made of copper or the like. In the above-mentioned patent publication, the present inventor proposes an improvement, consisting in providing a contact tube with an auxiliary electrode of an arc-resistant material, for instance carbon or graphite, preferably in the form of a mantle which extends beyond the contact tube. This achieves inter alia the advantage that the arc can burn for some time between the auxiliary electrode and the workpiece in order to thus preheat the workpiece without melting of the welding wire. Further, this offers a protection against damage as a consequence of a welding wire getting stuck. In the above-mentioned patent application, the present inventor describes a starting procedure, wherein first a primary GMAW-arc is started between the workpiece and the end of the GMAW-wire, to which end the GMAW-wire extends from the contact tube during the starting procedure. In this case, the wire feeding means are switched off, such that the extending GMAW-wire part melts away by the primary GMAW-arc, until the primary GMAW-arc reaches the end of the auxiliary electrode. After a sufficient preheating time, the wire feeding means are switched on and a secondary GMAW-arc can burn between the workpiece at the end of the GMAW-wire, with which the actual melting process starts.

The present invention aims to provide a further improvement for the method proposed in WO02/04162. Further, the present invention aims to provide a current source which is adapted to implementing this method.

In the known method as described above, wherein the GMAW- wire extends from the contact tube during the starting procedure and is then withdrawn into the contact tube, it may happen that the fluid end of the GMAW-wire causes problems in the contact tube. In order to prevent this problem, it is now proposed to withdraw the GMAW-wire into the contact tube before the arc is started. Thus, the primary arc is started between the auxiliary electrode and the workpiece. After the welding process has ended, when the arc is switched off, the welding feeding means are driven in the reverse direction in order to withdraw the GMAW-wire into the contact tube, such that the system is ready for a new start.

These and other aspects, features and advantages of the present invention will be further clarified by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: figure 1 schematically shows a welding torch; figures 2A-C illustrate successive stages of a welding process proposed by the present invention; figures 3A-B illustrate several characteristics of a current source .

Figure 1 schematically shows a welding torch 1, which may be equal to the welding torch as proposed in WO02/04162. The welding torch 1 comprises a contact tube holder 2 with a contact tube 3 screwed therein. The contact tube holder 2 and the contact tube 3 are made from an electrically well- conducting material, such as for instance brass. Around the contact tube 3, a tube-shaped auxiliary electrode 8 is arranged, made from carbon or graphite, which is attached to the contact tube 3 by a U-shaped clamping bracket 9.

Cooling water channels 4 and shielding gas channels 5 are arranged in the contact tube holder 2. A gas cup 6 is mounted concentrically with the contact tube 3, the gas cup 6 also being made from electrically conductive material such as for instance brass, but electrically insulated from the contact tube holder 2 by an insulation mantle 7.

A GMAW-welding wire 10 is transported through the contact tube 3 by controllable wire feeding means 20, the welding wire 10 making electrical contact with the contact tube 3. The controllable wire feeding means 20 may be substantially equal to the wire feeding means 20 known per se, and for that reason they are only schematically shown in the figure for sake of simplicity. It is important that the wire feeding means 20 are suitable for pushing the wire 10 through the contact tube 3, but also for transporting the wire 10 in the opposite direction. This property of the wire feeding means 20 will be indicated by the phrase "reversible" wire feeding means.

A controllable current source 100 has a positive terminal 101 which is connected to the contact tube 102, and a negative terminal 102 which is connected to a workpiece W. The controllable current source 100 is adapted to be able to operate in at least two different operative conditions, as will be explained more extensively later. A control device 50 is provided with one or more control buttons 51, which is/are actuated by a welder. In the case of welding by hand, the control buttons 51 may be attached to or may be part of the welding torch 1, as is known per se .

The control device 50 has a first control output 52 which is coupled to the wire feeding means 20; the control device 50 is adapted for generating a first control signal SI at its first control output 52, for controlling the operation of the wire feeding means 20. The control device 50 has a second control output 53 which is coupled to the current source 100; the control device 50 is adapted to generate a second control signal S2 at its second control output 53, for controlling the operation of the current source 100.

The GMAW-welding process is considered known per se, and will not be explained extensively here; suffice it to refer to the discussion in WO02/04162. Further, for an elaborate discussion of the construction and operation of the welding torch 1, reference is made to the discussion in WO02/04162. The details described in WO02/04162 are considered incorporated herein by reference.

Now, the method proposed by the present invention will be explained with reference to the figures 2A-C, which illustrate successive stages of the method. In these figures only the contact tube 3, the auxiliary electrode 8, and the free end 10a of the welding wire 10 are always shown, as well as a part of a welding arc 30, in a larger scale than in figure 1. As a first step, it is assured that the welding wire 10 is withdrawn in the contact tube 3, as illustrated in figure 2A. Further it is assured that cooling water and shielding gas with the correct composition is supplied in sufficient quantities to the welding torch 1, as will be clear to a person skilled in the art.

Then, as second step, a preheating arc 30A is ignited between the auxiliary electrode 8 and the workpiece (not shown in figure 2) . This ignition can take place in the usual manner, by short-circuiting between the auxiliary electrode 8 and the workpiece or by applying high voltage ignition pulses between the auxiliary electrode 8 and the workpiece. High voltage ignition pulses can be applied by a separate auxiliary ignition source, switched in parallel to the current source 100, or the current source 100 itself may be adapted for generating such pulses, as described in said publication. In this case, the wire feeding means 20 are still switched off. In the case of control by hand, a welder will switch on the current source 100 in a first operative condition without switching on the wire feeding means 20. In the case of control by the control device 50, the welder will actuate a start button 51, to which the control device 50 responds by sending a control signal S2 to the current source 100 in order to switch on this current source in a first operative condition, without sending a control signal SI to the wire feeding means 20.

In the first operative condition, the current source 100 has a decreasing characteristic 31, as illustrated in figure 3A. Figure 3A is a graph of output voltage (vertical) against output current (horizontal) of the current source 100; for different arc lengths, a work point sets on the line 31 shown in the graph. This line 31 makes an angle with the horizontal axis, which may be very large, up to approximately 90°, in which case this is also indicated as a vertical characteristic. As known to persons skilled in the art, and as explained in said publication, the source 100 then behaves like a current source with almost constant current intensity, as used in the TIG-welding process. Thus, in this first operative condition, the welding torch behaves like a TIG-torch, for which reason this first operative condition is also indicated as TIG-mode, with the understanding that the plus is preferably connected to the electrode.

Depending on circumstances and wishes of the welder, the current intensity in this first operative condition can be set to a value in the order of 100-300 A.

In this condition, the workpiece is heated by the preheating arc 30A, without material melting off of the welding wire 10. When the workpiece has been preheated sufficiently, a transition is made to the second operative condition of the current source 100, and the wire feeding means 20 are switched on to feed the welding wire 10.

The transition moment can be chosen depending on the temperature of the workpiece, or the size of the welding pool formed, or the like, either by using suitable detectors, or on the basis of visual observation by a welder. In the case of control by hand, a welder will switch the current source 100 to the second operative condition and will switch on the welding feeding means 20. In the case of control by the control device 50, the welder will actuate a switch-over button 51, to which the control device 50 responds by sending a control signal S2 to the current source 100 in order to switch this current source to the second operative condition, and also sending a control signal SI to the wire feeding means

20 in order to switch on the wire feeding means in the direction "feeding".

However, the switch-over moment can also be selected as a predetermined time after the switch-on moment, or as a predetermined time after ignition of the preheating arc 30A. Suitable values for this are for instance in the range of 10 ms to 1 sec, depending on the conditions, such as for instance the thickness of the workpiece. In the case of control by the control device 50, the welder will then only have to actuate a start button 51. The control device 50 responds by sending a control signal S2 to the current source 100 in order to switch on this current source in a first operative condition, will maintain this situation during the said predetermined time, and will then send a control signal S2 to the current source 100 in order to switch this current source to the second operative condition and will also send a control signal SI to the wire feeding means 20 in order to switch-on these wire feeding means in the direction "feeding".

If the ignition time is taken as starting point for said predetermined time, the control device 50 is provided with a sense input 54 for connecting to a arc current detector 55; since such arc current detectors are known per se, and discussed by way of example in the said patent publication, the arc current detector 55 is shown only schematically in figure 1, and it is not necessary here to discuss the design and operation thereof elaborately.

Figure 2B illustrates the second operative condition. The welding wire 10 extends from the contact tube 2 and from the auxiliary electrode 8, a welding arc 30B is burning between the end 10a of the welding wire 10 and the workpiece, and the end 10a of the welding wire 10 is melting away, such that droplets 10b are formed which are supplied to the welding pool. The melting of the welding wire 10 is compensated by the feeding of the welding wire 10 by the wire feed means 20.

In this second operative condition, the current source 100 has a substantially horizontal characteristic 32, as illustrated in figure 3B. As known to persons skilled in the art, and as explained in the said publication, the source 100 then behaves as a current source with a substantially constant arc voltage, as conventionally used in the GMAW-welding process. Thus, in this second operative condition, the welding torch behaves as a conventional GMAW-torch, for which reason this second operative condition is also indicated as GMAW-mode .

In this second operative condition, the current magnitude can be in the order of about 125 A (for instance in the case of thin sheet) to in the order of about 400 A (quickly filling seams in heavy workpieces) , depending on the circumstances.

Thus, the current source 100 is a controllable current source with switchable characteristic. The current source 100 has a control input 103 for receiving a control signal S2 from a control device 50. The current source 100 is adapted to switch-on in a mode determined by the control signal S2 , or to switch-off, in response to receiving a suitable control signal S2. Such controllable current source with switchable characteristic is known per se, and therefore the design and operation thereof will not be explained in more detail here.

When the end of the welding process is reached, the arc is switched-off as third step. According to an important aspect of the present invention, the end 10a of the wire 10 is also withdrawn into the contact tube 3, as indicated by arrow 11 in figure 2C. In the case of control by hand, a welder will switch-off the current source 100 and will, during a brief time, switch the wire feeding means 20 in reverse direction. In the case of control by the control device 50, the welder will actuate a stop button 51, to which the control device 50 responds by sending a control signal S2 to the current source 10 for switching this off, and by also sending a control signal SI to the wire feeding means 20 for switching this briefly in the direction "withdraw", and then switch-off the wire feeding means 20.

Herein, the wire feeding means 20 can be switched in the direction "withdraw" during a predetermined time. Preferably, however, the wire feeding means 20 are designed to withdraw the wire over a predetermined distance in response to receiving a withdraw command signal SI. This predetermined withdrawal distance has been chosen such that, in normal use and the associated normal extension length D of the wire 10 out of the contact tube 3, the end 10a of the wire 10 is with certainty withdrawn into the contact tube 3, while on the other hand it is prevented that the wire 10 is withdrawn from the torch 1.

Withdrawing can be done slowly, and/or set in motion sometime after the arc 30B has been stopped by switching off the current source 100. However, the possibility exists that a ball-shaped droplet with a diameter larger than the wire diameter solidifies at the end of the welding wire 10, such that withdrawing of the welding wire 10 into the contact tube 3 is prevented. Starting a next weld can then only happen after the end of the welding wire 10 has been cut off, which is not easy and takes time. Therefore, withdrawing is preferably done relatively fast, immediately when the current source is switched off, such that such droplet is still sufficiently fluid for being deformed by the contact tube 3 and/or the auxiliary electrode 8 on withdrawal.

After the welding process has ended in the described manner, the apparatus is immediately ready for starting a new welding process.

It will be clear to a person skilled in the art that the invention is not limited to the exemplary embodiments discussed in the above, but that several variations and modifications are possible within the protective scope of the invention as defined in the attached claims.

For instance, it is possible that the control device and the current source are integrated into a single unit. It is even possible that the control device, the wire feeding means and the current source are integrated into a single unit . In the context of the present invention, it is further possible that the control device is absent, and that the method proposed by the present invention is executed by a welder by manual control of the wire feeding means and the current source .

Further, it is possible that the auxiliary electrode 8 that protects the contact electrode 3 from the arc 30 is implemented as a sleeve arranged around the contact electrode 3, as illustrated, or as a protective layer arranged on the contact electrode 3, or as a separate electrode arranged adjacent the contact electrode 3, connected electrically with the contact electrode 3.

Claims

1. Welding method for executing a GMAW-welding process, wherein a welding wire (10) is fed through a welding torch (1) with a tube-shaped contact electrode (3) which is provided with an auxiliary electrode (8) ; and wherein a welding arc (30B) is generated between an end (10a) of the welding wire (10) and a workpiece (W) ; the method comprising the steps of: initially assuring that the end (10a) of the welding wire (10) is within the tube-shaped contact electrode (3) ; - igniting a preheating arc (30A) between the auxiliary electrode (8) and the workpiece (W) concerned; some time after igniting the preheating arc (30A) , feeding welding wire (10) such that a welding arc (30B) starts to burn between the end (10a) of the welding wire (10) and the workpiece (W) concerned.

2. Method according to claim 1, wherein igniting the preheating arc (30A) is implemented by means of short- circuiting or by means of high voltage pulses.

3. Method according to claim 1 or 2 , wherein a current source (100) for igniting and maintaining the preheating arc (30A) is operated in a first operative condition with a vertical or TIG-characteristic (31) , and wherein said current source (100) for maintaining the welding arc (30B) is operated in a second operative condition with a horizontal or GMAW-characteristic (32) .

4. Method according to claim 3, wherein the current source (100) is switched from the first operative condition to the second operative condition simultaneously with the start of the wire transport, or a short time later.

5. Method according to any of the previous claim, wherein the welding process is ended by switching off the welding arc (30B) and withdrawing the end (10a) of the welding wire (10) to within the contact electrode (3) .

6. Method according to claim 5, wherein the welding wire (10) is withdrawn immediately when the welding arc (30B) is switched off.

7. Apparatus for executing the method according to any of claims 1-6, comprising: a GMAW welding torch (1) with a tube-shaped contact electrode (3) that is provided with an auxiliary electrode (8) ; a controllable current source (100) with switchable characteristic ; controllable wire transport means (20) , which are adapted to keep a welding wire (10) still in a still operative condition, to feed the welding wire (10) to the welding torch (1) in a feeding operative condition, and to withdraw the welding wire (10) from the welding torch (1) in a withdrawal operative condition; control means (50; 51) coupled to the current source (100) and the wire transport means (20) for controlling the operation of the current source (100) and the wire transport means (20) ; wherein the control means (50; 51) , in a preheating phase, in response to receiving a start command, are adapted to switch on the current source (100) in a first operative condition with a vertical or TIG-characteristic (31) and to hold the wire transport means (20) in the still operative condition.

8. Apparatus according to claim 7, wherein the control means (50; 51), in a welding phase following the preheating phase, are adapted to switch the current source (100) to a second operative condition with a horizontal or GMAW-characteristic (32) and to switch the wire feeding means (20) to the feeding operative condition.

9. Apparatus according to claim 8, wherein the control means (50; 51) are adapted to switch-over from the preheating phase to the welding phase in response to receiving a switch-over command having this extent.

10. Apparatus according to claim 8, wherein the control means (50; 51) are adapted to switch-over from the preheating phase to the welding phase at a predetermined time after receiving the start command.

11. Apparatus according to claim 8, provided with a current detector (55) coupled to the control means (50) , and wherein the control means (50; 51) are adapted to switch-over from the preheating phase to the welding phase at a predetermined time after receiving a detector signal which is indicative for starting of the preheating arc (30A) .

12. Apparatus according to any of the previous claims 7-11, wherein the control means (50; 51) are adapted, in response to receiving a stop command, to switch off the current source (100) and to switch-over the wire transport means (20) to the withdrawal operative condition.

13. Apparatus according to claim 12, adapted to let the withdrawal operative condition last sufficiently long in order to assure that the end (10a) of the welding wire (10) is withdrawn to within the contact electrode, and to then switch off the wire transport means (20) .