JPS587393B2 - Resistance welding method for steel plates - Google Patents

Description

Detailed Description of the Invention The present invention produces a nugget (fusion weld) by applying Joule heat to the overlapping portion of steel plates while applying pressure and clamping them with electrodes.
In resistance spot welding, which forms and welds steel plates, it is possible to suppress the nugget from growing more than necessary in the direction of the plate thickness, and to form a welded part with the necessary strength with relatively little heat input. This is a resistance welding force method for steel plates with the purpose of
This method is suitable for resistance welding of steel plates of 2 mm or more.

Needless to say, the bonding mechanism of resistance spot welding is fusion welding using resistance heat generation (Joule heat).

This heat generation is greatly influenced by welding conditions such as the shape of the electrode tip, pressing force, welding current, and energization time.

Therefore, as a rational welding condition, the Japanese Industrial Standards (JIS)
), the Resistance Welding Machine Manufacturers Association (RWMA), or the American Welding Society (AWS).

When welding under these recommended conditions, the resulting nugget generally has a so-called foundation stone-like form.

The nugget grows in the diameter direction and also grows sufficiently in the thickness direction.

Figure 1 shows the relationship between the ratio of the thickness of the nugget to the nugget diameter and the plate thickness produced under these recommended conditions.

When the plate thickness to be spot welded is thin (about 1 mm), the above ratio is relatively small, but as the plate thickness increases, this ratio becomes large.

The strength of a spot weld has a very good correlation with the diameter of the nugget, as shown in FIG.

The nugget thickness at this time was approximately 1.6 mm in each case, and the nugget thickness was not directly related to static strength.

On the contrary, if the nugget thickness becomes larger than necessary, the heat-affected zone around the fusion weld will expand excessively, which is not preferable in terms of fatigue strength.

Therefore, it is necessary to pay attention to the ratio of the nugget thickness to the nugget diameter.

According to the research conducted by the present inventors, when spot welding steel plates with a relatively thin thickness of 1 mm or less, a current-carrying path corresponding to the required nugget diameter is formed immediately after welding starts, and after that, the nugget mainly In contrast, when the thickness of a steel plate exceeds 1 mm and increases further, a fusion weld region, that is, a current-carrying path, whose area is smaller than the required nugget diameter is formed immediately after welding starts. As a result of subsequent energization, the nugget increases in thickness and also grows in the diametrical direction, so that when the required nugget diameter is reached, the nugget thickness also grows to a considerable thickness.

From these experimental studies, the present inventors found that the reason why there is a large difference in the nugget formation process between thin plates and thick plates is that in spot welding of thin plates8, due to the thinness of the plates, pressure is applied to the stacked steel plates using an electrode. When clamped, the ratio of the contact area between the steel plates to the electrode area is large, so a current-carrying path close to the required nugget diameter can be obtained immediately after welding starts, and the nugget thickness can be adjusted by adjusting the current-carrying time. When the plates to be welded become thicker, they are more rigid and less flexible than thin plates when clamped under pressure by electrodes, so the ratio of the contact area between the steel plates to the electrode area is small, and therefore the steel plates do not come into contact with each other at the start of welding. Since the fusion welding area is expanded by passing current through the nugget, immediately after welding starts, only a current-carrying path with a small area compared to the required nugget diameter is obtained. As the fusion weld becomes thicker, the dissipation of heat from the fusion weld to the outside air becomes worse.As a result, the fusion weld area is expanded from the fusion weld area, which becomes the current-carrying path immediately after welding starts, to the thickness force direction and the diameter force direction of the electrode. How to get the required nugget diameter? We obtained the knowledge that if the plate is stretched, it will become a nugget that has grown sufficiently in the direction of the plate thickness and force.

The results of the above experimental research indicate that if the welding conditions are controlled so that the ratio of the nugget thickness to the nugget diameter does not exceed 0.25, the thermal effect around the fused part of the spot weld will not adversely affect the strength. In addition, when following the recommended spot welding conditions mentioned above, as shown in Figure 1, when the thickness of the steel plate is 1.1 mm or less, the ratio of the nugget thickness to the nugget diameter is 0.25. However, the plate thickness does not exceed 1. If it exceeds 2 mm, the ratio is 0.25
I found that it exceeds the

Therefore, when welding steel plates with a thickness exceeding 1.2 mm by spot welding, the present inventors determined that at the beginning of welding, the magnitude of the current that causes fusion welding due to Joule heat, which is determined by the material of the steel plates, and the normal Electrode heating is achieved by applying a current that is 40% to 70% of the main welding current used in spot resistance welding until the amount of heat generated by energization and the amount of heat dissipated reach an equilibrium state. Under pressure, current is applied to ensure a current-carrying path with an area corresponding to the required nugget diameter, that is, a fusion weld corresponding to the current value, and then the main welding current mainly causes the fusion weld to expand in the thickness direction. We have developed the technology to do this.

That is, according to the present invention, the main welding current is 4
When applying a current with a magnitude of 0% to 70%, fusion welding starts from the contact area of the steel plate within the area clamped by the electrodes, but since the magnitude of the applied current is small, the contact They found that the softening of the surface takes priority, and the area of the fusion weld zone is expanded by gradually enlarging the current conduction path to form a current conduction path that matches the required nugget diameter before increasing the thickness of the fusion zone.

As time passes after this energization is started, the energization path stops expanding in proportion to the magnitude of the energizing current and reaches a state of equilibrium, but the initial energization time described above to secure the energization path is This minimum value is preferred.

Of course, if the initial energization is continued after the equilibrium is reached, no adverse effects will occur, but it will be a waste of energy and time.

When the above-mentioned equilibrium state is reached, the area of the fusion welded area of the steel plate is still expanding, and the tendency to increase in size in the thickness direction is extremely small. It is observed that it is extremely flat.

Then, when the main welding current is applied to the electrode, the thickness of the fused area and the diameter are expanded by the concentrated resistance, so welding is terminated when the welding area has grown to an appropriate thickness.

The cross-sectional shape of the nugget obtained according to the present invention in the force direction shows a substantially uniform thickness, indicating that the nugget thickness can be controlled by changing the current application time.

The present invention will be explained below in comparison with various welding conditions.

First, using a commonly used spot welding electrode, select the following welding conditions from among various recommended conditions, stack two steel plate specimens approximately 3 mm thick, and weld them under the same suppression conditions to measure the growth of the nugget. The process was observed by cutting specimens.

FIG. 3 is a diagram showing the relationship between the applied current and the applied time under the welding conditions, and the magnitude of the main welding current is the same for each welding condition.

FIG. 3 I shows the most common welding method found in the recommended conditions, and the magnitude of the welding current is constant during the energization time.

As shown in Fig. 4, the nugget made by this welding method generates heat from the contact point at the center of the electrode pressurizing range between the steel plates at the initial stage of energization (Fig. 4a), and then the nugget is The plate thickness grows significantly in the force direction from the middle stage of energization (Fig. 4 b) to the final stage of energization (Fig. 4 c) without progressing in the force direction, and as the diameter approaches the required dimension as the energization time increases, the thickness becomes extremely large. Grow thick.

For this reason, the steel plate is softened due to heat generation throughout the thickness and force direction, and at the end of welding (Fig. 4 d), it is also affected by the pressing force of the electrode, and the indentation caused by the electrode is large.

That is, in this welding method, the welding grows in the direction of the plate thickness and diameter around several heat generation starting points, forming a nugget with a thickness approximately close to the plate thickness.

Fig. 3 (3) shows a welding method in which the magnitude of the applied current is linearly increased in a gradient manner to the magnitude of the welding current at the initial stage of welding F, and then a constant current is applied.

The time for the gradual increase in the initial stage of welding is usually about 10% of the total current application time.

The purpose of the gradual increase is to prevent scattering, and the nugget formation process is not much different from that shown in FIG.

FIG. 3 (■) shows a so-called pulsation welding method in which welding current is repeatedly turned on and off.

In this method, as shown in FIG. 5, melting and solidification are repeated for each pulse, and nuggets are observed for each pulse.

That is, Fig. 5a shows the nugget generated at the beginning of the first pulse application, and as in the initial stage of the welding force method shown in Fig. 3I, heat generation progresses from several steel plate contact points, resulting in the nugget shown in Fig. 4a. They form almost similar nuggets.

At the end of the first pulse energization, a nugget shown in FIG. 5b is formed.

Moreover, in the first pulse, there was a tendency for the nugget to grow faster in the thickness direction because it was not sufficient to secure a current-carrying path that matched the ultimately required nugget diameter.

During the second pulse energization, there is a tendency for heat generation to proceed as the nugget formed during the first pulse energization acts as a energization path, and as a result, as shown in Figure 5C, the plate There is little growth in the thickness direction, and more growth in the diameter direction.

By repeating this process, a melt-solidified zone with overlapping nuggets corresponding to the number of pulses is finally formed as shown in Figure 5d, and as a result, the nuggets grow sufficiently in both the diameter force direction and the plate thickness force direction. nuggets are formed.

From the above facts, when the steel plate to be welded is relatively thin, such as 1 mm or less, when applying pressure and current using the upper and lower electrodes P and Q as shown in Fig. 6, the required nugget can be obtained immediately after welding starts. A current-carrying path corresponding to the diameter is formed, and then the nugget grows mainly in the direction of plate thickness force. However, if the steel plate to be welded is thick, no matter which welding method under the conventional recommended conditions is used, the nugget will grow as shown in Figure 7. As shown, it can be seen that the current-carrying path formed immediately after the start of welding expands less than the required nugget diameter, and thereafter the nugget grows in the diametrical direction while increasing its thickness.

Therefore, in resistance spot welding of steel plates with a thickness exceeding 1.2 mm, it is necessary to first form a thin current-carrying path in the force direction of the plate thickness, commensurate with the required nugget diameter as shown in Figure 6, immediately after welding starts. If successful, it will be possible to adjust the nugget thickness by adjusting the current application time during resistance spot welding of thick steel plates, and the ratio of the nugget thickness to the nugget diameter can be made 0.25 or less.

The present invention succeeds in forming a current-carrying path corresponding to the required nugget diameter immediately after the start of welding by applying a current of 40% to 70% of the main welding current,
For comparison with the recommended conditions, FIG. 8 shows a diagram showing the relationship between the magnitude of the welding current and the current application time, similar to that shown in FIG. 3.

In the present invention, the welding current is controlled in two stages, and in the first stage at the initial stage of welding, the steel plate is fusion welded by Joule heat in order to secure a current-carrying path suitable for the required nugget diameter under a predetermined electrode pressure. A current that is 40% to 70% of the welding current that produces the The welding current is allowed to flow until a thermal equilibrium state is reached, and then the main welding current is applied as a second step.

The current value in the first stage is aimed at achieving close contact over a spread area due to the softening of the surface of the nugget forming part of the steel plate, so there is no need for the temperature of the part to be welded to exceed the melting point, so the main welding current is It is not necessary to have a magnitude of 70% or more, and a minimum magnitude of 40 times the main welding current is required in order to obtain a sufficiently softened contact state between the steel plates with a predetermined pressing force.

When using the welding force method of the present invention, the nugget growth is
As shown in the figure, at the end of the first stage of energization and at the beginning of the second stage of applying the main welding current, a thin fusion weld area with an extremely wide spread and almost uniform thickness appears as shown in Figure 9a. However, the main welding current mainly grows in the thickness force direction of the energizing path from the middle of the energization period (Fig. 9b) to the final energization period, and also grows in the diameter force direction due to concentrated resistance, and at the end of energization (Fig. 9 In d), a nugget with a required nucket diameter and approximately uniform thickness is obtained.

From FIG. 9, it is clear that according to the present invention, the thickness of the nugget can be adjusted by controlling the current application time in the latter half of the second stage main welding current application.

Examples of the present invention will be described below.

The present invention was applied to resistance spot welding of discs and rims of automobile disc wheels.

The disc is made of SHP material with a thickness of 3.2 mm, and the rim is made of SHP material with a thickness of 3.5 mm.

The dimensions of the electrodes used for welding were: the upper electrode had a diameter of 25 mm, the spherical curvature of the pressurizing surface had a radius of 180 mm, the lower electrode had a diameter of 25 mm, and the radius of spherical curvature was 100 mm, and the electrode pressing force was 1.7 tons. It is.

The energization time was determined by the number of cycles of the commercial power source connected to the welding equipment.

Figure 10 is a circuit diagram of the single-phase AC welding device used in the example, in which the welding circuit (■) connects the power supply circuit (■) that supplies current for heating and welding, and the welding part (■). It consists of an opening/closing current control circuit (2) that controls opening/closing as appropriate.

That is, this control circuit (2) sequentially issues a heating control signal to soften the welding member (2) and a welding control signal to weld it.

This circuit (2) includes a current setter 1 that sets the welding current value using a volume or the like, a phase control circuit 2 that determines an advance angle commensurate with the resistance value of the current setter 1 in accordance with the applied frequency,
A thyrisk ignition circuit 3 generates a signal to send a control signal for heating or welding to the gate of the thyristor 6 at a time difference of advance angle (time) determined by this circuit 2, and The welding current setting circuit 4 includes a thyristor 5 and a circuit consisting of two series of an energization time setting circuit 4 that determines whether or not a welding current of an applicable frequency is applied for a predetermined period of time.

Using the above welding equipment, the first stage current is set to 25 kiloamperes, the energization time is set to 50 cycles of the commercial power supply, the second stage energization current is set to 32 kiloamperes, and the energization time is set to 30 to 45 cycles of the commercial power supply. The present invention was carried out in this manner.

In this Example 3, an energization path approximately close to the desired nugget diameter of 18 mm was obtained at the beginning of the second stage of energization, and the desired flat nugget was obtained at all welded portions.

The cross section of these nuggets in which the second stage of energization time was set to 45 cycles is reproduced in FIG.

For reference, if the energization time in the first stage is stopped at 30 cycles during which the energization path in the joint is in the process of being formed, the contact area between the steel plates will not soften sufficiently in the first stage, and the target will not be met. Not only was it not possible to obtain the desired nugget diameter, but the nugget became thick.

A cross-section of one example is shown as a reproduction in FIG.

For comparison, we also attempted pulsation welding under recommended conditions.

In this welding method, the welding current is 32 kiloamperes, the energization time is 36 cycles of the commercial power source as one pulse, and the welding current is 32 kiloamperes.
Three pulses were applied with two cycles of rest time in between.

This result shows that the nugget was formed three times, as shown in Figure 13, which shows an example of a cross-sectional copy of the weld, and that the nugget grew to almost the entire area in the thickness direction during the first and second pulses. It can be seen that

Figure 14 shows an example of the present invention (indicated by a black circle), an example in which the energization time in the first stage was shortened for comparison (indicated by a white circle), and the amount of human heat produced by the pulsation welding method (indicated by a triangle). (KA xsec: is a diagram showing the relationship between the formed nugget diameter.

As can be seen from the figure, according to the method of the present invention, a nugget with a diameter equal to or larger than that of the conventional pulsation welding method can be obtained with about 2/3 the amount of human heat, and the flatness of the nugget is lower than that of the conventional pulsation welding method. It was much better.

In summary, according to the present invention, the joint parts of the steel plates are softened by the first stage of electrical heating, a current-carrying path corresponding to the required nugget diameter is secured under the required electrode pressure, and then the main welding current is applied. Since the second stage of energization heating is performed,
It is possible to prevent the growth or scattering of nuggets in the plate thickness direction, which occurs when a large current is continuously applied without a sufficient current-carrying path as in the conventional method, and after securing a current-carrying path that matches the required nugget diameter. Since nugget growth occurs mainly in the direction of plate thickness force, the thickness of the nugget can be adjusted by adjusting the current application time, which reduces power consumption and also reduces the heat load on the electrode. This is a useful method that can also improve lifespan.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the thickness of the steel plate at the spot weld and the ratio of the nugget thickness to the nugget diameter, and Figure 2 is the nugget diameter at the spot weld and the shear strength of the nugget. Figure 3 is a diagram showing the relationship between energizing current and energizing time using the welding force method based on various recommended conditions, etc. Figures 4 and 5 are created using the welding force method shown in Figure 3. 6 and 7 are schematic diagrams showing the growth of a nugget in a welded part of a steel plate, and FIG. 8 is a line showing the relationship between the applied current and the applied time according to the present invention. FIG. 9 is a sectional view showing a nugget formed according to the present invention, and FIG.
Figure 0 is a circuit diagram of the welding device used to carry out the present invention.
Figure 1 is a reproduction of the cross section of a nugget obtained by implementing the present invention, Figures 12 and 13 are reproductions of the cross section of a nugget of a comparative example, and Figure 14 is the relationship between nugget diameter and human heat amount. FIG. In the figure, P, E and Q are electrodes, 1 is a current setting device, 2 is a phase control direction, 3 is a cyrisk ignition circuit, 4 is an energization time setting circuit, and 5 is a cyrisk, respectively.

Claims (1)

[Claims] 1. In the resistance welding force method for steel plates using Joule heat, when the plate thickness is 1. 40% to 70% of the welding current of a magnitude that causes fusion welding of the steel plates by Joule heat while pressing the overlapped portion of steel plates exceeding 2 mm with electrodes.
A current with a magnitude of A resistance welding force method for steel plates, characterized in that the welding current is applied while maintaining clamping to form a final welded state.