JP6041374B2 - Spot welding method for steel plates with different thickness - Google Patents

Description

  The present invention relates to a spot welding method for a plate assembly having a plate thickness ratio in which two steel plates having different plate thicknesses are overlapped.

Recently, in the housing construction material field, the automobile field, and the like, spot welding is frequently used from the viewpoint of high efficiency and the like when joining two stacked steel sheets. If the two steel plates to be joined have the same thickness, they are spot-welded with almost no problem (see FIG. 1 (a)), but a plate having a thickness ratio in which two steel plates having different thicknesses are stacked. When a set is spot welded, problems such as failure to obtain a desired joint strength may occur.
If welding is performed with electrodes of the same shape and the same material on the top and bottom as used in normal welding with the same thickness, the cooling effect received from the water-cooled electrodes is the same on both the top and bottom. Since nuggets are formed by heat generation from the center, it is considered that the growth of nuggets on the steel plate interface is delayed (see FIG. 1B).

In order to develop a desired bonding strength, it is necessary to form a nugget at the bonding interface of the stacked steel sheets.
For this reason, when spot welding a plate assembly having a plate thickness ratio in which two steel plates having different plate thicknesses are overlapped, as shown in FIG. An electrode with a large tip diameter is used on the plate side, and an electrode with a small tip diameter is used on the thin plate side. Thereby, the contact area of the electrode is varied, the cooling condition from the electrode is changed, and the nugget formation position is shifted closer to the steel plate interface (for example, Non-Patent Document 1).

  Further, Patent Document 1 discloses a thin plate having the smallest rigidity when spot welding is performed by sandwiching a plate assembly in which two thick plates having high rigidity and one thin plate having low rigidity are stacked between each other. By making the tip diameter of the electrode tip in contact with the plate material side smaller than the tip of the electrode tip in contact with the thick plate material side, the area where the thin plate material and the electrode chip contact is smaller than the area where the thick plate material and the electrode chip contact, resulting in low rigidity A method has been proposed in which a nugget is formed at a contact portion between the other plate material and another plate material, and the welding strength can be improved.

  Furthermore, in Patent Document 2, when a plate assembly in which a thin plate is superimposed on one of two or more thick plates that are overlapped is sandwiched between a pair of electrode tips and spot welding is performed, the tip of the electrode tip on the thin plate side has a predetermined tip. Spot welding with an electrode tip that is a curved surface having a radius of curvature, and an electrode tip that is a curved surface having a radius of curvature larger than the radius of curvature of the tip of the electrode tip whose tip is in contact with a flat or thin plate on the other thick plate side The first stage and the second stage are welded in two stages, the second stage is welded at a higher pressure than the first stage, and the second stage welding current value is the first stage. A spot welding method has been proposed in which the current value is less than or equal to the welding current value.

  Furthermore, in Patent Document 3, when spot welding a plate set having a large plate thickness ratio in which a thin plate is further overlapped on at least one of two thick plates that are overlapped, a part of the thin plate is to be welded. Forming a seat surface that is one step higher than the general part, forming the tip of the electrode facing the thin plate into a spherical surface, and deforming so that the seat surface of the thin plate is crushed into a spherical shape by the spherical electrode at the initial stage of welding. And a thick plate adjacent to each other, and thereafter, the two thick plates are welded together with a high pressure force.

Japan Welding Society Resistance Welding Research Committee “Resistance Welding Phenomenon and Its Applications (Part 1)” August 10, 1982, Japan Welding Society, P. 142-146

Japanese Patent No. 3894544 Japanese Patent No. 4543823 Patent No. 3794300

However, preparing two types of electrodes having different tip diameters and tip shapes as in Non-Patent Document 1, Patent Document 1, and Patent Document 2 not only doubles the management of the electrodes, but also for electrode polishing. Two types of tools are required, resulting in high costs.
Moreover, in patent document 2, since control which changes a pressurizing force or an electric current during welding is essential, it takes time to set the control and also causes an increase in equipment costs.

Furthermore, Patent Document 3 requires a step of forming a seat surface, which is one step higher than the general portion, in advance by press working or the like in the portion to be welded of the thin plate, resulting in a problem that productivity is lowered.
The present invention has been devised in order to solve such problems. When spot welding a plate assembly having a thickness ratio in which two steel plates having different thicknesses are overlapped, the tip diameter is determined. Another object of the present invention is to provide a spot welding method with high joint strength so that the nugget formation position is in the vicinity of the interface of the steel sheet to be welded even when electrodes having the same tip shape are used.

In order to achieve the object, the spot welding method for steel plates having different thicknesses according to the present invention has the same shape when spot welding a plate assembly having a thickness ratio in which a plurality of steel plates having different thicknesses are stacked. And an electrode made of tungsten or molybdenum or an alloy based on them on the thin plate side, and an electrode made of copper alloy on the electrode thick plate side made of copper alloy on the thick plate side.
Further, as the electrode disposed on the thin plate side, it is more preferable to use a copper electrode in which a core material made of tungsten or molybdenum or an alloy based on them is embedded at the tip.

  In the present invention, the cooling effect from the thin plate side electrode can be reduced when a plate assembly having a plate thickness ratio obtained by superposing a plurality of steel plates having different plate thicknesses is spot-welded. For this reason, nugget growth in the plate thickness direction is promoted on the thin plate side, and nugget formation at the steel plate interface can be easily obtained. As a result, spot welding that can produce the desired joint strength, even with a plate assembly in which two steel plates with different thicknesses are overlapped, has different tip diameters and tip shapes of the upper and lower electrodes, pressure force and current. Without complicated control of the value, it can be performed stably under general welding conditions and with high efficiency.

The figure explaining the relationship between the electrode tip diameter and the nugget formation position in the conventional example The figure explaining the relationship between the electrode tip diameter and the nugget formation position in the present invention The figure which shows the test piece shape used in the Example The figure which shows the electrode shape used in the Example The figure which shows the nugget dimension measurement position at the time of evaluating the joining body produced in the Example The figure which shows the tensile shear test result of the conjugate | zygote produced in the Example. The figure which shows the measurement result of the nugget thickness of the conjugate | zygote produced in the Example The figure which shows the measurement result of the steel-plate interface nugget diameter of the joined body produced in the Example.

When performing spot welding of a plate assembly having a thickness ratio in which two steel plates having different thicknesses are overlapped, the present inventors make the nugget formation position an interface between the two steel plates to be welded. We intensively studied how to adjust.
As described above, in the prior art, when spot welding a plate assembly having a thickness ratio in which two steel plates having different thicknesses are overlapped, an electrode having a large tip diameter is provided on the thick plate side, Since an electrode having a reduced tip diameter is used, the cooling rate from the electrode on the thin plate side is delayed to shift the nugget formation position closer to the steel plate interface.
Therefore, even if the tip shape of the electrode is the same, if the cooling rate from the electrode on the thin plate side is slow, it is assumed that the residual heat near the steel plate interface becomes uniform and nuggets are formed near the part. The invention has been reached.

Hereinafter, the present invention will be described in detail.
In ordinary spot welding, a copper-based material is often used as an electrode. Therefore, also in the present invention, a copper material is used as one of the electrodes, specifically, a Cr-Cu alloy containing about 1% of Cr that is widely distributed at low cost. As the other electrode, an electrode made of a material having a lower thermal conductivity than that of the Cr—Cu alloy, specifically, tungsten (W) is used.

An electrode made of a material having a low thermal conductivity, that is, a tungsten electrode is arranged on the thin plate side, and an electrode made of a material having a high thermal conductivity, that is, a Cr-Cu alloy electrode is arranged on the thick plate side. Spot weld.
Table 1 shows the thermal conductivity of the Cr—Cu alloy containing 1% Cr and tungsten (W).

The electrode made of a material having high thermal conductivity may be made of, for example, tungsten as a whole. However, considering the cooling property of the electrode itself, it is preferable that the electrode is made of copper with a core material made of tungsten embedded at the tip.
Since the thick plate side electrode has higher thermal conductivity than the thin plate side electrode, as shown in FIG. 2, after passing the spot welding current, the cooling through the thick plate side electrode proceeds more than from the thin plate side electrode. Since the nugget formation position moves from the center position where the two steel plates are stacked to the vicinity of the bonding interface on the thin plate side, a desired bonding strength can be obtained with a low current.

A galvanized steel sheet having a thickness of 0.6 mm and a plain steel sheet having a thickness of 1.6 mm are cut into the shape shown in FIG. 3 as a sample, and an electrode having the shape shown in FIG. 4 is used and shown in Table 2. Spot welding was performed under the conditions. Note that a Cu electrode in which a φ6 mm tungsten core material was embedded at the tip on the thin plate side and a Cr—Cu alloy electrode on the thick plate side were spot welded. For comparison, spot welding was also performed by arranging Cr-Cu alloy electrodes on both the thin plate side and the thick plate side.
Thereafter, the welded joint was observed for the cross section of the welded portion to measure the size of the nugget shown in FIG.

The results are shown in FIGS.
FIG. 6 shows the tensile shear test results, FIG. 7 shows the nugget thickness measurement results, and FIG. 8 shows the steel plate interface nugget diameter measurement results.
From FIG. 6, when the welding current is increased, the tensile shear load is saturated in the same load region in any electrode combination, but in the low current region (6-7.5 kA), which is the initial stage of nugget formation, the thin plate side The combination of a Cu electrode with a tungsten (W) core material, which is a low thermal conductivity material, embedded in the tip, and a Cr-Cu alloy electrode, which is a high thermal conductivity material, on the thick plate side is higher in tensile shear. A load is obtained, and the fracture state in the tensile shear test is also a base material fracture.

As can be seen from FIG. 7 and FIG. 8, a Cu electrode in which a tungsten (W) core material, which is a low thermal conductivity material, is embedded on the thin plate side on the thin plate side, and a Cr— The combination using a Cu alloy electrode has a smaller cooling effect from the electrode on the thin plate side, so that the growth of the nugget on the thin plate side in the plate thickness direction becomes faster, and accordingly the nugget formed at the steel plate interface This is thought to be due to the increased diameter.
The electrode life in spot welding continuous welding with steel plates with different thicknesses overlaps with the steel material interface near the outer periphery of the nugget when welding with electrodes of the same material and shape (see Fig. 1 (b)). When the current density decreases due to electrode wear due to continuous welding and the increase in tip diameter, the joule heat generation decreases and the nugget shrinks, and the nugget is not formed at the steel sheet interface at an early stage. However, the electrode life is shorter than when the steel plates of the same thickness located at the center of the nugget are overlapped. Further, if the nugget is to be formed at the steel plate interface, the welding current must be increased (see FIG. 7), so that the electrode wears out and the electrode life is shortened.

  On the other hand, in the present invention, by using electrodes having different thermal conductivities during spot welding in which steel plates having different thicknesses are overlapped, nugget growth in the thickness direction is promoted on the thin plate side. Even when the nugget is reduced due to a decrease in the amount of Joule heat generated due to electrode wear, the time when no nugget is formed at the steel plate interface is delayed. Moreover, compared with the case where it welds with the electrode of the same material and the same shape, a welding current can be set low (refer FIG. 7). Therefore, by using the present invention, it is possible to improve the electrode life at the time of spot welding in which steel plates having different thicknesses are overlapped.

In addition, although the present Example was performed with the combination of the galvanized steel plate and the ordinary steel plate, the present invention is not limited to this, and can be applied to any plating type, material, and plate combination.
Further, the number of steel plates to be overlapped is not limited to two. For example, the present invention may be applied to a combination of a plurality of plates, for example, by superimposing one thin plate on two stacked thick plates.
Furthermore, tungsten (W) was used as a material having low thermal conductivity. However, since molybdenum is a metal that exhibits almost the same behavior as tungsten, the results obtained with tungsten can be expected to apply to molybdenum. Therefore, a material having low thermal conductivity and conductivity, such as molybdenum, an alloy based on tungsten or molybdenum, may be used.

Claims (2)

  When spot-welding a plate assembly with a plate thickness ratio in which a plurality of steel plates with different plate thicknesses are overlapped, an electrode made of tungsten or molybdenum or an alloy based on them on the thin plate side has the same shape. A method of spot welding steel plates having different plate thicknesses, wherein an electrode made of a copper alloy is used on the thick plate side.   When spot-welding a plate assembly with a thickness ratio in which a plurality of steel plates having different thicknesses are overlapped, a core material having the same shape and made of tungsten, molybdenum or an alloy based on them on the thin plate side A spot welding method for steel plates having different plate thicknesses, characterized in that an electrode made of copper is embedded in the tip and an electrode made of a copper alloy is used on the thick plate side.

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