FR2644090A1 - Method and device for ensuring the quality of components produced by arc welding - Google Patents

Abstract

At predetermined time intervals, the welding robot applies the end of the torch 3 in a fluid-tight (sealed) manner at the inlet of a conduit 17 pierced into a workpiece 16 mounted so as to move in a support 21 by virtue of the end position of a return spring 23. The conduit 17 communicates with an apparatus 29 for measuring the flowrate of gas, this apparatus being equipped with a threshold detector 32 which can trigger an alarm and/or act on the device for controlling the torch 3 if the value of the flowrate is not correct. Application to the production of components for the motor-vehicle industry.

Description

The subject of the invention is a method and a device intended to ensure simply and quickly the quality of parts produced by arc welding.

In the automobile industry, many parts such as, for example, the body cradles, are produced by this method. Figure 1 attached schematically recalls the principle.

To make the part 1, for example by joining two elements along a weld line, one can use a filler metal which is in the form of a wire 2 which is brought to a torch 3 and an electric arc is made to flow between this wire and the part 1. This has the effect of melting the metal of the wire and the drops of metal successively deposited ensure the welding.

In the example illustrated in FIG. 1, the wire 2 is wound on a spool 4 and it is brought to the torch 3 by means of a reel 5, which is connected to a control device 6.

The wire is conveyed through a drive plate Sa, equipping the reel 5, then a wire guide sheath 7, then at the center of the torch body to exit at the end thereof.

To generate an electric arc between the wire 2 and the part 1, a current source 8 is also used which is connected to the control device 6. The positive pole of the source 8 is connected to the wire 2 while its negative pole is connected to the part 1. In the example described, the positive pole is in fact connected to the torch body and the negative pole to the support 9 of the part 1, but the electrical contact is ensured between the wire 2 and the torch body. on the one hand, the part 1 and the support 9 on the other hand (see also Figure 2).

In order to protect the weld pool, a gas flow is channeled to the torch. For this, a source of compressed gas 10 is used, for example a bottle, equipped with a regulator 11 and connected to the torch by a pipe 12.

On the latter is interposed a solenoid valve 13 connected to the control device 6. The gas flow is brought to the upper part of the torch and it comes out at its lower part, to the right of the molten bath, as illustrated in the figure 2. This has the effect of protecting the molten bath against atmospheric agents and thus avoiding its pollution.

In the case of MIG (metal inert gas) welding, welding is carried out by melting a metal electrode under a flow of inert gas (argon or argon-helium mixture in particular) which is used only for protection. In the case of MAG (metal active gas) welding, the fusion of a metal electrode is always used, but under an active gas flow (in particular C02 and argon-C02, argon-oxygen, argon-C02-oxygen mixtures) . These gases cause a certain number of chemical reactions on contact with the electric arc, which makes it possible to improve the welding process (for example by providing additional heat due to certain reactions).

The torch 3 can be operated using a robot (not shown). As for the control device 6, it can be associated with the robot cabinet or with a programmable automaton, if one is in automatic welding.

Since the parts thus produced can be important parts for vehicle safety, such as the body cradles, it is necessary to control the quality of the weld. At present, this control is done by different means (ultrasound, etc.) on the finished part and each part is controlled. It is understood that this method lengthens the manufacturing time, which increases the production costs.

The invention aims to remedy this drawback by proposing a method and a device which make it possible to ensure the quality of the parts produced by arc welding in a much faster manner than with the methods of the prior art.

According to the main characteristic of this process, this one, intended to ensure the quality of the parts produced by arc welding using a torch connected to a control device and supplied with protective gas, comprises the following steps consisting in: :
a) - measure, at predetermined time intervals, the
gas flow at the end of the torch;
b) - check whether the value obtained corresponds to
values, previously determined, for which the
weld bead quality is assured; and
c) - act on the control device according to the
value determined in step a).

In fact, the inventors have shown that certain welding parameters, and in particular the gas flow rate, have an influence on the quality of the weld.

The gas flow protects the weld pool against atmospheric agents during welding. On the other hand, the spurting of the arc between the wire 2 and the part 1 corresponds to a short circuit which has the effect of melting part of the wire 2 and depositing a drop of metal. The weld bead therefore consists of a series of solidified metal drops 14 (Figure 2) each of which represents a short circuit. In addition, each short circuit causes projections of particles 15.

A lack of gas flow, in particular an insufficient flow, can lead to a high porosity rate within the weld bead and, thereby, weaken its mechanical characteristics. Insufficient flow can come from various causes, for example a defect in the supply line 12 which can be porous, cut, pinched, etc. It may also be that the end of the torch, c that is to say the gas outlet nozzle, presents an excessive fouling, this could have been caused by a particularly high splash rate or by the non-respect of the cleaning frequencies.

It is understood that the gas flow rate is not the only parameter which influences the quality of the weld, but it is faster and easier to control the welding parameters than the finished parts. It is possible, by preliminary tests, to determine the values of these parameters for which the good quality of the welds can be ensured.

It is therefore unnecessary to check the parts themselves as long as the values of the welding parameters remain within the limits thus determined.

 Preferably, the control of the gas flow is carried out in masked time, that is to say between the end of the production of one piece and the start of the production of the next.

Indeed, there is always a dead time to allow the evacuation of the part which has just been carried out and the implementation of the following: the control of the gas flow can be done very quickly during this dead time, this which does not affect the total time taken to produce the parts.

The invention also relates to a device for implementing this method. According to the main characteristic of this device, it includes:
- a gas flow measurement device;
- means of connection between the torch and this
apparatus; and
- means for bringing the torch to these means of
connection.

According to other characteristics of this device:
- the gas flow meter is of the type
comprising a column in which a float operates
mobile which, under the action of a flowing gas stream
vertically from bottom to top across this column,
stabilizes at a height depending on the gas flow;
- the gas flow meter is equipped with
minus a threshold detector connected to the
ordered;
- the means of connection between the torch and the device
gas flow measurement reil include one piece
rigid pierced with a conduit having a first end
having a seat on which the end of the torch
can be applied tightly and a second
end which can be put in communication with
the input of the gas flow meter;
- the rigid part comprises a seal fixed to said seat
and on which the end of the torch can be
applied;
- the rigid part is mounted mobile inside a
housing provided in a fixed support;
- return means are interposed between the support
and the rigid part, the torch acting against
these recall means when applied to the
seat;
- the torch is mounted on a robot able to bring it in
contact with the connection means, constituting
thus said supply means.

The invention will appear more clearly on reading the description which follows, given solely by way of example, with reference to the appended drawings in which:
- Figure 1 shows schematically the whole of a
arc welding installation;
FIG. 2 is a schematic view on an enlarged scale,
partially sectioned from the tip of the torch
used in the installation of figure 1; and
- Figure 3 is a schematic view partially in
section of the device object of the invention.

When it is desired to control the gas flow rate, the torch 3 is brought to the device which is the subject of the invention, which is illustrated in FIG. 3.

If the torch is activated by a robot, this device is preferably placed near the welding station so that the torch can be brought to this device by the robot.

The device which is the subject of the invention firstly comprises a rigid part 16 pierced with a conduit 17. In the example described here, the part 16 is arranged substantially vertically and the conduit 17 widens at its upper part to be finish with a seat 18 in the form of a truncated cone, to which the end of the torch 3 can be applied, which is also in the form of a truncated cone. A seal 19 provided on the seat 18 seals and guarantees that the entire flow of gas from the torch, and only this flow, passes through the conduit 17.

The part 16 is mounted movable in translation inside a passage 20 provided in a support 21. In the example described here, the part 16 has at its upper part a large part 16a whose cross section corresponds to that passage 20 and a lower part 16b of smaller dimensions capable of passing through a passage 22 of corresponding section provided in the lower part of the support 21. A spring 23 is interposed between the bottom of the passage 20 and the underside of the part 16a , so as to push the part 16 upwards. This movement is limited by the presence of a flange 24 mounted on the lower part of the part 16 and which abuts against the lower face of the support 21. The latter can be fitted with a base 25 allowing it to be fixed on a support. For this purpose, the base 25 can be pierced with holes such as 26 for the passage of screws.

The end of the pipe 17 opposite the seat 18 is in communication with a pipe 27, for example by means of a connector 28. The pipe 27 may be a flexible pipe which leads to a gas flow meter 29 .

In the example described here, this device is of the "debiliter" type, that is to say it comprises a movable float 30 inside a glass column arranged vertically. This float moves under the action of a gaseous current flowing vertically from bottom to top and it stabilizes at a height depending on the gas flow rate. The latter arrives at the lower part of the apparatus 29 and escapes from it at its upper part by manifold 31. The gas can be evacuated either directly into the atmosphere or into a special exhaust circuit.

The device 29 is equipped with a threshold detector 32 connected to the control device 6 in order to be able to trigger an alarm and / or give appropriate orders if the value of the gas flow rate is not correct. For practical reasons, the detector 32 is placed at a height corresponding not to a maximum or minimum value of the gas flow rate, but to the predetermined optimal value. In fact, it is easy to know whether the float 30 is located in front of the detector or not, but if it is not there, it is very difficult to know whether it has passed above or below. This is why it is preferred to act in this way and to trigger the alarm as soon as the float 30 moves away from the detector 32, that is to say as soon as the gas flow deviates from the optimal value.

A gas flow control operation with the device which is the subject of the invention takes place in the following manner:
Once the production of a part has been completed, the robot moves the torch 3 to bring it above the seat 18 of the part 16. The location of the device illustrated in FIG. 3 is preferably chosen so that this movement can to be done easily and quickly. The robot then lowers the torch 3 until its end is in contact with the seal 19 and the movement continues until the part 16 descends slightly in the passage 20 by compressing the spring 23. The effort thus exerted on the torch ensures good contact with the gasket 19 and all the gas flow passes through the device object of the invention.

If the flow rate is correct (float 30 opposite the detector 32), the control device authorizes the continuation of the manufacturing cycle and the robot brings the torch back to the welding station. If the flow rate varies, the float 30 is no longer at the level of the detector 32 (for example, it passes below the latter if the flow rate decreases). The detector then gives appropriate orders to the control device 6 (triggering of an alarm, interruption of the manufacturing cycle, etc.).

 The method and the device which are the subject of the invention have numerous advantages, the main one of which is to reduce the manufacturing time of the parts, since it is not necessary to control them as long as the gas flow rate and, of course , the other welding parameters are good. It is only when the detector 32 detects an abnormal flow rate that we are led to check the part or parts which have been produced since the previous check (and only these) and to check the gas supply system. As the device interrupts the manufacturing cycle, we are sure that we will not manufacture other bad parts or those likely to be. In addition, the control of the gas flow can be done in masked time, in the time interval which separates the production of two consecutive pieces, which in no way modifies the duration of the manufacturing cycle. This check can therefore be carried out as often as necessary, and even at each room if desired.

Finally, it is understood that the invention is not limited to the single embodiment which has just been described, but that many variants can be imagined without departing from the scope of the invention. Thus the person skilled in the art will be able to choose, according to each particular case, the parameters such as the frequency of the controls, the means of supplying the torch to the control device, the constitution and the location of that. -this, the type of gas flow meter, etc.

Claims (10)

1. Method for ensuring the quality of parts (1) produced by arc welding using a torch (3) connected to a control device (6) and supplied with protective gas, characterized in that it includes the following stages consisting in:  a) - measure, at predetermined time intervals, the  torch gas flow (3);  b) - check whether the value obtained corresponds to  values, previously determined, for which the  weld quality is satisfactory; and  c) - act on the control device (6) depending on  the value determined in step a). 2. Method according to claim 1, characterized in that step a) is carried out in masked time, that is to say between the end of the production of one piece and the start of the production of the next. 3. Device for implementing the method according to any one of claims 1 and 2, characterized in that it comprises:  - a gas flow measuring device (29);  - connection means between the torch (3) and this  apparatus; and  - means. to bring the torch (3) to these means of  connection. 4. Device according to claim 3, characterized in that the gas flow measuring device (29) is of the type comprising a column in which evolves a mobile float (30) which, under the action of a gas current flowing vertically from bottom to top through this column, stabilizes at a height depending on the gas flow rate. 5. Device according to any one of claims 3 and 4, characterized in that the gas flow measuring device (29) is equipped with at least one threshold detector (32) connected to the control device (6 ). 6. Device according to any one of claims 3 to 5, characterized in that the connection means between the torch (3) and the gas flow measuring device (29) comprise a rigid part (16) pierced with '' a conduit (17) having a first end having a seat (18) on which the end of the torch (3) can be applied in a sealed manner and a second end which can be put in communication with the inlet of the device for measuring gas flow (29). 7. Device according to claim 6, characterized in that the rigid part (16) comprises a seal (19) fixed on said seat (18) and on which the end of the torch (3) can be applied.  8. Device according to any one of claims 6 and 7, characterized in that the rigid part (16) is movably mounted inside a housing (20) formed in a fixed support (21). 9. Device according to claim 8, characterized in that return means (23) are interposed between the support (21) and the rigid part (16), the torch (3) acting against these return means (23) when applied to the seat (18). 10. Device according to any of claims 3 to 9, characterized in that the torch (3) is mounted on a robot capable of bringing it into contact with the connection means, thus constituting said supply means.