GB2065925A

GB2065925A - Controlling the Welding Time in Resistance Spot Welding

Info

Publication number: GB2065925A
Application number: GB8037764A
Authority: GB
Original assignee: Vyskumny Ustav Zvaracsky VUZ
Current assignee: Vyskumny Ustav Zvaracsky VUZ
Priority date: 1979-12-10
Filing date: 1980-11-25
Publication date: 1981-07-01
Also published as: IT1195768B; HU191003B; ATA597280A; FR2473378B3; DE3046269C2; IT8026528A0; GB2065925B; JPS56105876A; AT379536B; CS209713B1; DE3046269A1; FR2473378A1; CH648782A5

Abstract

The method includes the measurement of the resistance R between the welding electrodes starting with the time tmax, at which the resistance reached its local maximum Rmax, and at the time t at which the integral <IMAGE> reached the preselected and preset value the welding current is switched off.

Description

SPECIFICATION A Method for Controlling the Welding Time in Resistance Spot Welding and a Circuit for Carrying Out the Method The invention relates to a method for controlling the welding time in resistance spot welding.

In industrial application of resistance spot welding, the reproducible and high quality of spot welds has to be achieved even though the input factors affecting the welding process and quality of the joints, e.g. the mains voltage, the cleanness of the surface and the shape of the welded parts, the condition and wear of the welding electrodes, and the air pressure in the pneumatic system, may differ and vary from weld to weld. This task becomes more significant, with increasing complexity of the product being manufactured and with increasing loading of single welds in service conditions expected.

The recently widespread method of control of resistance spot welding process is based on keeping the main parameters controlling the welding process, i.e., welding current, welding time and welding upset force, constant. However, this does not suppress the majority of the external disturbing effects in practice. Its consequence is that the fabricated spot welds do not always exhibit uniform and acceptable properties, and brings about useless and expensive exaggeration of the number of welded spots not really needed with regard to loading of the product in service conditions.

It is known that a considerable number of the above stated insufficiences can be eliminated by welding time control based on signals derived from the welding process itself. One method is known which brakes the welding current at the moment when the thermal expansion of a weld nugget reaches or approaches its maximum. Thus the dimensions of the weld nugget are secured to be near their optimum which can be achieved with the given material, geometry and power conditions. However, this method has a drawback, namely that it measures mechanical value-magnitude of thermal expansion. This must be converted into electrical signals for further processing. This requires a suitable sensor-converter being not only expensive but its location on a welding machine is connected with technical and operational difficulties.Moreover, this method is unsuitable for machines with poor rigidity, worn out or in poor condition with mechanical slackness. It is also unsuitable for pendant gun welders.

Also other methods of welding time control based on the resistance change between the electrodes during welding are known, measuring only electric values and thus eliminate the need of the sensor-converter. One of these methods is based on the storage of the maximum value of resistance course indicating that the desired stage in the fabrication of the welded joint was achieved - corresponding approximately to the start of fusion of the weld nugget. To this maximum value a certain percentage is assessed being within the 5 to 45% range and the current is stopped at that moment when the resistance after maximum had dropped by the value set in advance within the given range of 5 to 45% of the maximum value.This method has certain drawback, namely that due to measurement of the absolute value of the resistance different percentage is needed which has to be reset for different technological applications. If the weldment is composed of different combinations of materials, thicknesses and number of sheets, the equipment has to be reset again and again.

Another known method of welding time control, based on the resistance between the electrodes measures the time t max at which the maximum Rmax of resistance occurs and breaks the welding current at the time t calculated from the equation: t=A+B tmax Similarly, as in the previous case, this method has certain drawback, namely that the coeffibientsA+B change in dependence on the character of the used technology, thus the equipment requires resetting in case of any change. Other drawback is rooted in the fact that the equipment does not respond to splashes of the molten metal caused due to overheat or pressure drop and it does not break the welding current at the splash.

The above-stated insufficiences can be eliminated by this invention to a considerable extent. The principle of the method of welding time control in resistance spot welding is in this case based on the measurement of the resistance R between the electrodes from the time tmax at which the resistance reached its maximum Rmax in time t and when the integral

reached the preselected and preset value, welding current is switched off.

The method of welding time control in resistance spot welding according to the invention facilitates connexion as shown in the invention which uses generator of resistance course between the electrodes connected to the change-over switch the first output of which is connected to the input of memory of the maximum value of resistance. The memory output is connected to the input for the initial condition of integrator and also to the input of the circuit of the reference voltage with adjustable input. The circuit of reference voltage has its output connected to the reference input of the comparator. Then the generator of resistance course between the electrodes is connected also to detector of resistance maximum the output of which is connected to control input of the change-over switch and clearing input of integrator.The second output of the change-over switch is led to the signal input of integrator the output of which is connected to the signal input of the comparator which is connected by its output with the welding process.

As it was found out, the integral

represents a quite suitable invariant of resistance spot welding process and it is such a value which does not vary and remains constant in similar physical processes even though the input values vary and thus it remains the criterion of similarity of the processes. Utilization of such similarity criterion for the welding time control is considerably favourable namely due to the fact that the connexion is self-adjustable in a broad range. In this case it means that the method described in the invention comprises the following favourable features: Using only one and the same setting of the resistance integral, makes possible that the sheets of different thicknesses can be welded without sensitivity to the selected thicknesses.

Further on, different continuations of thicknesses and different numbers of sheets can be welded.

The control system adjusts the welding time in such a way that sheets and blanks with poor fit up can be also welded.

In a certain range the unfavourable effect of shunting of welding current caused e.g. by the adjacent weld and diameter change of the contact surfaces of electrodes, as the consequence of wear is eliminated.

The effect of contamination of the welded sheet surface is compensated automatically.

This method can be utilized for welding of surface treated e.g. sheets without any change of setting. Moreover in this case it was found that the number of welds fabricated without dressing of electrode increased by 25%.

As the resistance between electrodes drops stepwise at the splash, the resistance integral rapidly rises and the control system stops the process earlier. Thus the rate of splashes, which are usually undesired in the production decreases.

It is especially favourable for welding thin rims and in the vicinity of sheet edge where the risk of splash is considerably higher.

Example of application of the invention is shown in the accompanying drawings in which Fig. 1 depicts a typical curve of resistance of a weld divided into zones in connection with different stages of weld fabrication, Fig. 2 shows the course of resistance of a weld at splash, Fig. 3 elucidates the terms of integral of complementary area and the integral of resistance, Fig. 4 represents alternative of the unstable program current course, Fig. 5 shows alternative of controlled current course, and Fig. 6 represents the connection of the equipment in accordance with the invention.

A typical course of resistance R of a weld in welding low-carbon steel sheets is shown in the example of the invention. The resistance R is the resultant of of the transition resistances Rp diminishing during the welding current passage in time t and of material resistance Rm. Based on the time dependence of resistance R, three regions typical for formation of resistance spot weld can be distinguished.

The first region is the zone of diminishing transition resistances, lasting from the start of welding current passage to the time tmin where the local minimum of resistance Rmin is attained as seen in the resistance curve R. In this usually very short time interval, in a typical example of welding thin sheets cca 1 mm in thickness, it lasts about 0.04 s, the transition resistances on the contact surfaces of welding electrodes and material being welded diminish and whereas a certain "standardization" of resistance conditions occurs here.

The second region represents heating zone lying in the interval between time tmin and tmax when the resistance Rmax attains its local minimum on the resistance curve R. In this time the materials being welded are further heated up to reaching the first zone in which the welded material begins to melt and the welded joint initiates.

The third region is the zone of weld nugget growth lying in the interval between the time tmax and the end of current passage in time t z called the welding time. In this interval the welded joint grows from the very start of fusion up to the final dimensions of the nugget. It was found that these dimensions determining prevailingly the welded joint quality with the materials of good weldability, are determined above all by the energy supplied into the joint in the zone of weld nugget growth, i.e., from the fusion initiation up to the end of the process. As in this zone the constant value of setting the welding current I is mostly used for welding, the energy is given in the relation:

and it depends mostly on the integral

This integral can be applied as the measure of weld nugget growth and thus for the welding time control.

It is known that the so-called splashes of molten metal from the weld occurs in welding practice very often, e.g. in car body fabrication due to higher values of supplied energy and due to other reasons as well. In general, these are unfavourable because they depreciate weld nugget by the losses of molten metal causing deep superficial traces, decrease joint quality, the spatter of molten metal bothers the operators and contaminates the material being welded, the machine and also the environment. In case the splash has begun or closely before it, the welding process should be stopped because its further progress is practically useless. Moreover it is known that such splash is characterized by a sudden drop in the course of resistance in the weld.However, if the value of integral

is used for control, welding would continue until this integral reaches the preset value and the splash occurrence would not lead either to stoppage of welding or to reduction of welding time, but to an opposite effect. Therefore the integral

is more favourable while it represents the supplementary area and thus it is the measure of energy supplied into the weld from the time tmax to the end of the process. Moreover, it is also favourable because it rapidly increases in the case of splash causing stoppage of the process or shortening of the welding time.

Further it was found that the value of integral

remained constant in welding sheets of different thicknesses or different number of sheets or in varying combinations under otherwise comparable technological conditions and whereas suitable welding time for fabrication of highquality welds was always selected. These favourable properties of the above-mentioned integral are utilized in the method described in the invention. Referring to this method, the resistance R of the weld is measured during welding. At the moment of tmax, at which the resistance R reaches the value Rmax, this value is stored in the memory and by its help the abovestated integral, the value of which is compared with the previously proved and preset value, is created from the instantaneous value of R.When the value of the integral reaches this preset value, the welding current is switched off and the process is completed.

This method can be used also in an alternative, where the welding current I is not set to a constant value but varies in accordance with the preselected program. In practice it means mostly its slope rising at the start of the welding process, in majority of cases, in the first and second zone of the typical course of resistance R in the weld.

Another alternative of the method described can be applied when current I is controlled from the initial value Spot to the desired value Ipovz at the beginning of the welding process, e.g. based on the information acquired from the output of the process and utilized in the feedback. In general, it is favourable to make use of the first and second zone, i.e., at least from the time tmax, where the third zone begins, the setting of welding current I is kept on a constant value.

A circuit in accordance with the invention comprises a source 1 of the course of resistance between the electrodes connected to a switch 2 a first output 17 of which is connected to the input of a store 3 of the maximum resistancve value. The output of the store 3 is connected to an input 11 for the initial condition of an integrator 6 and also to an input 12 of a unit 4 of reference voltage with a setting input 13, the unit 4 of reference voltage having its output connected to the reference input 14 of a comparator 7.

The source 1 of the course of resistance between the electrodes is connected simultaneously to a detector 5 of the resistance maximum, the output of which is connected to the control input 8 of the switch 2 and the clearing input 9 of the integrator 6. At the same time a second output 18 is connected to a signal input 10 of the integrator 6 which provides of the switch 2 the signal input 15 for the comparator 7 which is connected to the welding process 16 through its output.

The circuit in accordance with the invention, senses the course of resistance in the weld which is achieved at the output of the source 1 of the course of resistance between electrodes. The course of resistance in the weld is led to the detector 5 of the resistance maximum which at maximum switches over the switch 2 and simultaneously the integrator 6 stops clearing.

The switch 2 leads the course of resistance in the weld to the store 3 of the maximum value of resistance till the moment when the resistance maximum is achieved. By switching over the switch 2, the maximum of resistance is stored in the store 3 of the maximum resistance value and at the same time the course of resistance in the weld is led to the integrator 6. The output of the store 3 of the maximum resistance value is led to the input 11 for the initial condition of the integrator 6 ensuring thus integration from the maximum resistance value. The output of the integrator 6 is compared in the comparator 7 with the level created by the unit 4 of the reference voltage related to the maximum resistance value.

The output of the comparator 7 controls the welding process 16, i.e. switches off the welding current.

Claims

1. A method for controlling the welding time in resistance spot welding, the method including the measurement of the resistance R between the welding electrodes starting with the time tmax, at which the resistance reached its local maximum Rmax, and at the time t at which the integral

reached the preselected and preset value the welding current is switched off.

2. A method for controlling the welding time in resistance spot welding substantially as herein described with reference to the accompanying drawings.

3. A circuit for carrying out the method according to Claim 1 or 2 comprising a generator of the resistance course between the welding electrodes connected to a switch, a first output of which is connected to the input of a store of the maximum value of resistance, the output of which is connected to an input for the initial condition of an integrator and simultaneously to an input of a unit for the generation of a reference voltage which has also a presetting input, and the unit has its output connected to a reference input of a comparator, the generator of the course of resistance between the welding electrodes is also connected to a detector of the maximum of resistance, the output of which is connected to a control input of the switch and to a clearing input of the integrator, whereas a second output of the switch is connected to a signal input of the integrator, the output of which is connected to a signal input of the comparator, the output of which is connected to the welding process.

4. A circuit for carrying out a method according to Claim 1 or 2 constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, Figure 6 of the accompanying drawings.