JP7565669B2 - How to judge the quality of spot welding - Google Patents

Description

The present invention relates to a method for determining the quality of spot welding.

For example, in the assembly process of automobile bodies, spot welding is performed by bringing a pair of electrodes into contact with multiple steel plates and passing an electric current through them, melting the contact points between the steel plates through resistance heating and forming a nugget.

In addition, in spot welding, various current patterns have been proposed to form an appropriate nugget between multiple metal sheets. For example, Patent Document 1 proposes a current pattern in which the current value is gradually increased while alternating between periods of time in which a high current value is maintained to grow the nugget and periods of time in which a low current value is maintained to soften the steel sheet without generating spatter.

Patent Document 2 also proposes a method for stably obtaining an appropriate nugget by passing current while changing the applied pressure in a predetermined pattern in addition to the current pattern.

JP 2006-181621 A JP 2020-99937 A

In recent years, attempts have been made to combine high-tensile steel sheets, known as high-tensile or ultra-high-tensile steel sheets, with mild steel sheets to use as structural materials for vehicle bodies, with the aim of improving collision safety and fuel efficiency. In this case, it is necessary to ensure the quality of the welds in the plate assembly in which high-tensile steel sheets and mild steel sheets are overlapped. However, in plate assembly in which steel sheets with significantly different rigidity are sandwiched and pressurized by a pair of electrodes, there is a problem that gaps (also called plate gaps) are likely to occur between the steel sheets. If the plate gaps cannot be sufficiently closed by softening the steel sheets due to pressure and electrical heating, the contact area between the steel sheets is small and the molten part that becomes the nugget cannot be sufficiently grown. In addition, there is also the problem that spatter is likely to occur because the contact state between the steel sheets is unstable.

In order to prevent spatter from occurring in the above-mentioned plate assemblies, it is necessary to optimize the current pattern by lengthening the time period during which current is passed through the plate for heat softening, or to optimize the pressure pattern by increasing the pressure applied by the pair of electrodes, as described in Patent Documents 1 and 2, for example. However, whether or not a good nugget has been formed cannot be confirmed without cutting the plate assembly after welding. Therefore, in order to optimize the welding conditions, it is necessary to repeat numerous welding tests and cut the plate assembly each time to determine whether the welding is good or bad. This causes a problem of increasing costs by greatly increasing the number of steps required to optimize the welding conditions. In particular, there is insufficient data on welding of plate assemblies including high tensile steel, because it has been only a short time since high tensile steel began to be used, and the above-mentioned problem becomes more pronounced.

In view of the above, the technical problem to be solved in this specification is to provide a method for easily determining the quality of spot welding without cutting the plate assembly after spot welding.

The above problem is solved by the method of determining the quality of spot welding according to the present invention. That is, this method is characterized in that when spot welding is performed on multiple metal plates by clamping the multiple overlapping metal plates between a pair of electrodes and passing a current through them in a predetermined current pattern, the predetermined current pattern includes a preliminary current and, after the preliminary current, a main current that aims to form a molten part at the contact part between the multiple metal plates, and the current resistance of the multiple metal plates is measured during the current period, and the quality of the spot welding is determined based on whether the peak of the measured current resistance occurs during the preliminary current period.

Note that the term "main current" refers to current passed with the purpose of forming a molten zone at the contact point between the metal plates, and "preliminary current" refers to current passed with the purpose of something other than forming a molten zone, such as current passed with the purpose of heating and softening the metal plates. Therefore, current passed with the purpose of forming a joint other than a molten zone, such as solid-state welding, is included in preliminary current.

The inventors of the present invention have focused on the current resistance during the current flow period when spot welding is performed by current flow in a current flow pattern having a preliminary current and a main current flow to form a molten part after the preliminary current flow, and as a result of intensively studying the effect of the peak time of the current resistance on the welding quality, have found that the quality of the resulting joint differs between when the peak of the current resistance occurs during the preliminary current flow period and when it occurs during the main current flow period. The present invention has been made based on the above findings, and when spot welding is performed by current flow in a current flow pattern having a preliminary current and a main current flow to form a molten part after the preliminary current flow, the current resistance during the current flow period is measured, and the quality of the spot welding is judged based on the peak time of the measured current resistance. In this way, the quality of the spot welding can be evaluated during the spot welding (so-called in-process) by judging the quality of the spot welding based on the peak time of the current resistance measured during the current flow period of the spot welding. Therefore, the number of steps can be reduced compared to when the quality of the welding is judged by cutting the plate assembly after welding, and the operating costs can be reduced. Furthermore, if the quality of welding can be determined in-process, improved welding conditions (e.g., increasing the pressure, increasing the pre-energization time, etc.) can be applied to the plate assembly immediately after, making it possible to change the welding conditions efficiently and in a short time. Of course, because the quality of spot welding can be determined in-process, it is possible to reliably obtain only plate assemblies that meet the specified welding quality. This makes it possible to ensure stable welding quality.

As described above, the method for determining whether spot welding is good or bad according to the present invention makes it possible to easily determine whether spot welding is good or bad without cutting the plate assembly after spot welding.

1 is a diagram for explaining an overview of a method for determining quality of spot welding according to an embodiment of the present invention; 4 is a graph showing an example of a current pattern during spot welding. 10 is a graph showing the relationship between a current flow pattern and current flow resistance history when a joint with good welding quality is obtained. 1A is a cross-sectional view of a main portion of a plate assembly when the welding quality is good, and FIG. 1B is a cross-sectional view of a main portion of the plate assembly when a preliminary current is applied and a main current is applied. 13 is a graph showing the relationship between a current flow pattern and a current flow resistance history when a joint having poor weld quality is obtained. 1A is a cross-sectional view of a main part of a plate assembly when the welding quality is not good, and FIG. 1B is a cross-sectional view of a main part at the end of a preliminary current supply. FIG.

The following describes the method for determining whether spot welding is good or bad according to one embodiment of the present invention, with reference to the drawings.

Figure 1 shows the overall configuration of a spot welding device 10 according to this embodiment. This welding device 10 includes a pair of electrodes 11, 12, a transformer 13, an ammeter 14, and a pressure means 15. In addition, a plate assembly to be welded (here, two metal plates 1, 2) is placed between the pair of electrodes 11, 12.

Here, as an example of the metal plates 1 and 2 that make up the plate assembly, one metal plate 1 is a mild steel plate with a tensile strength of 300 MPa or less and a plate thickness of 1.2 mm or less, and the other metal plate 2 is a high-tensile material with a tensile strength of 550 MPa or more and a plate thickness of 0.7 mm or less.

The pair of electrodes 11, 12 are both connected to a transformer 13 as a current supply, and an ammeter 14 is provided on this current path. At least one of the electrodes (here, the upper electrode 11 in Figure 1) is attached to a pressure means 15 such as a cylinder. Therefore, by driving one of the electrodes 11 with the pressure means 15 and pressing it against one of the metal plates 1, the metal plates 1, 2 are clamped between the pair of electrodes 11, 12 and a predetermined pressure force is applied. In addition, a current of a predetermined current pattern is supplied between the pair of electrodes 11, 12 by the transformer 13.

At this time, the magnitude of the current flowing between the pair of electrodes 11, 12 is measured by an ammeter 6 provided on the current path. In this embodiment, the magnitude of the voltage when current is applied is measured as the voltage generated in the secondary coil of the transformer 13. In this embodiment, the magnitude of the pressure force is measured as the reaction force of the cylinder serving as the pressure means 15. In this case, the current resistance (see FIG. 3), which will be described later, is measured as the voltage value measured by the transformer 13 divided by the current value measured by the ammeter 6.

Figure 2 shows an example of a current pattern according to this embodiment. This current pattern includes preliminary current and main current. The period indicated by the symbol S1 in Figure 2 is the period of preliminary current, and the period indicated by the symbol S2 in Figure 2 is the period of main current. In this illustrated example, the preliminary current is a current pattern in which the current value is increased at a constant gradient. Also, the main current is a current pattern in which the current value at the end of the preliminary current is maintained.

In addition, FIG. 2 also shows an example of a pressure pattern according to this embodiment. In this illustrated example, the pressure starts to be applied earlier than the start of the preliminary current application, and is increased at a constant gradient and then maintained.

Below, we will explain an example of a method for determining the quality of spot welding when spot welding is performed using the spot welding device 10 configured as described above.

First, as shown in FIG. 1, the plate assembly to be welded (here, two overlapping metal plates 1, 2) is placed between a pair of electrodes 11, 12. More precisely, the pair of electrodes 11, 12 are placed so that the portion of the plate assembly to be welded is located between the pair of electrodes 11, 12. Then, one electrode 11 is driven by the pressure means 15 to press the electrode 11 against the adjacent metal plate 1. This causes the two metal plates 1, 2 to be sandwiched between the pair of electrodes 11, 12. Therefore, if there is a gap (plate gap) between the two metal plates 1, 2 before pressure is applied, this gap is closed.

The transformer 13, which serves as a current supply unit, starts energizing in a predetermined energizing pattern (for example, the energizing pattern shown in FIG. 2). First, preliminary energization is performed to heat the two metal plates 1 and 2, and to soften the metal plates 1 and 2. As a result, as shown in FIG. 4(a), for example, at least one of the two metal plates 1 and 2, the metal plate 1, is deformed, and the contact area A1 between the two metal plates 1 and 2 increases compared to when preliminary energization began.

After the preliminary current application is completed in this manner, the main current application is started and the current application is continued. As a result, as shown in FIG. 4(b), for example, a molten part 4a that will become a nugget is formed at the contact part 3a between the two metal plates 1 and 2. This molten part 4a grows to a predetermined size by continuing the main current application.

The current flow resistance is also measured during the above-mentioned current flow period (at least the entire preliminary current flow period S1 and several cycles after the start of the main current flow period S2). In this embodiment, as described above, the voltage and current values during the current flow period are measured at predetermined time intervals, and the measured voltage value is divided by the current value to obtain the current flow resistance at that time. Figure 3 shows an example of the history (graph) of the current flow resistance during the current flow period. In this illustrated example, the current flow resistance begins to rise with the start of preliminary current flow, reaches a peak P during the preliminary current flow period S1, and then continues to decrease even after the main current flow period S2 begins.

In this embodiment, the current resistance is measured as described above, and the quality of the spot welding is judged based on the time when the current resistance value reaches its peak P. This quality judgment is automatically performed, for example, based on the current resistance data measured by the control unit of the transformer 13 (or a separate calculation unit not shown). For example, as shown in FIG. 3, if the measured current resistance peak P is within the preliminary current period S1, the control unit of the transformer 13 judges that the spot welding has been performed well. In this case, the joint obtained by the spot welding is transported to the subsequent process as a non-defective product.

On the other hand, as shown in FIG. 5, if the measured peak P of the current flow resistance is within the main current flow period S2, the control unit of the transformer 13 determines that the main spot welding is not performed well. In this case, as shown in FIG. 6, for example, the contact area A2 between the metal plates 1 and 2 at the end of the preliminary current flow is smaller than the contact area A1 (see FIG. 4(a)) when a good welding is performed, or the contact state between the metal plates 1 and 2 is likely to be unstable. Therefore, the size of the molten part 4b formed at the contact part 3b between the metal plates 1 and 2 by the main current flow is also smaller than the molten part 4a (see FIG. 4(b)) when a good welding is performed, or sparks 5 including spatters are likely to occur. Therefore, in this case, the joint obtained by the main spot welding is discarded as a defective product, or is transported to a specified location to perform the welding process again.

In addition, if the above-mentioned poor welding judgment results occur at a rate above a certain level, the control unit of the transformer 13 makes a change, for example, to increase the maximum pressure applied by the pressure means 15. This improves the filling of the gap between the plate set (two metal plates 1, 2) that is immediately brought into the welding position. Specifically, the contact area between the metal plates 1, 2 increases, or the contact state stabilizes in the state shown in Figure 4 (a). This makes it possible to increase the rate of non-defective products and improve the yield.

As described above, in the method for determining whether or not spot welding is performed according to the present embodiment, when spot welding is performed by applying current in a current pattern including a preliminary current and a main current for forming a molten portion 4a after the preliminary current, the current resistance during the current application periods S1 and S2 is measured, and the quality of the spot welding is determined based on the timing of the peak P of the measured current resistance. In this way, the quality of the spot welding can be evaluated during spot welding by determining whether or not the spot welding is performed based on the timing of the peak P of the current application resistance measured during the current application periods S1 and S2. Therefore, compared to the case where the plate assembly after welding is cut to determine whether or not the welding is performed, the number of steps can be reduced, and thus the operating costs can be reduced. In addition, if the quality of the welding can be determined in-process, the improved welding conditions (in this embodiment, an increase in the pressing force) can be applied to the plate assembly immediately after, so that the welding conditions can be changed efficiently and in a short time. Of course, since the quality of the spot welding can be determined in-process, only plate assemblies that meet the specified welding quality can be reliably obtained. Therefore, it is possible to ensure stable welding quality.

In addition, since the resistance during the current flow period can be measured using the transformer 13 and ammeter 6 as in this embodiment, there is no need to provide dedicated measuring equipment or facilities separately. This makes it possible to determine the quality of the welding at low cost.

Although one embodiment of the present invention has been described above, the method for determining the quality of spot welding according to the present invention can have configurations other than those described above without departing from the spirit of the method.

For example, in the above embodiment, an increase in the pressing force is applied to the immediately following welding target (plate assembly) as a response after a poor weld is determined, but other responses are also possible. For example, if the current resistance is measured until the end of the preliminary current period S1 and peak current resistance P does not appear (the current resistance reaches its maximum value at the end of the preliminary current), it is possible to assume that peak P is within the main current period S2 and immediately increase the pressing force. This makes it possible to modify (optimize) the welding conditions in-process for each individual plate assembly, further improving the yield rate.

In the above embodiment, the case where the value of the electrical resistance is obtained (measured) using the voltage value measured by the transformer 13 and the current value measured by the ammeter 6 (by dividing the voltage value by the current value) is illustrated, but of course this is not limited to this. The measurement means is arbitrary as long as it can be considered to be the electrical resistance between the pair of electrodes 11, 12.

In the above embodiment, the current pattern in which the current value increases at a constant gradient is exemplified as the preliminary current, but of course, the present invention is not limited to this. For example, although not shown, a so-called multi-stage current pattern (see Patent Document 1) may be adopted as the current pattern, and the first one-stage current or multiple multi-stage current periods that do not correspond to the main current may be the preliminary current period S1. Similarly, in the above embodiment, the current pattern in which the current value is maintained at a constant magnitude is exemplified as the main current, but of course other current patterns can be adopted. For example, although not shown, a current pattern that includes at least a current period in which the current value increases or decreases at a predetermined gradient may be adopted, or, as with the preliminary current, multiple multi-stage current periods that do not correspond to the preliminary current may be the main current period S2 among the multi-stage current pattern. In short, the current pattern of the main current is arbitrary as long as it is possible to form a molten portion 4a of an appropriate size to become a nugget at the contact portion 3a between the metal plates 1 and 2.

In addition, in the above embodiment, a combination of high tensile steel plate and mild steel plate is exemplified as the plate pair (two metal plates 1, 2) to be welded, but of course this is not limited to this. For example, both metal plates 1, 2 may be high tensile steel. Alternatively, the plate pair to be welded may be a combination of ultra-high tensile steel and mild steel plate, ultra-high tensile steel and high tensile steel, or ultra-high tensile steel and ultra-high tensile steel. In any combination, two or more of each type of metal plate may be used to form a plate pair of three or more plates in total, and this plate pair may be applied to the present invention.

Reference Signs List 1, 2 Metal plates 3a, 3b Contact parts 4a, 4b Welding parts 6 Ammeter 10 Spot welding device 11, 12 Electrode 13 Transformer 14 Ammeter 15 Pressurizing means A1, A2 Contact area P Peak S1 Preliminary current application period S2 Main current application period

Claims (1)

When spot welding a plurality of overlapping metal sheets by clamping the plurality of overlapping metal sheets between a pair of electrodes and passing a current through the plurality of overlapping metal sheets in a predetermined current pattern,
The predetermined current passing pattern includes a preliminary current passing and a main current passing to form a molten portion at a contact portion between the plurality of metal plates after the preliminary current passing,
A method for determining quality of spot welding, comprising measuring the current resistance of the plurality of metal plates during the current flow period, and determining quality of the spot welding based on whether a peak of the measured current resistance occurs during the preliminary current flow period.

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