JPH06277856A - Spot welding electrodes - Google Patents

Abstract

PURPOSE:To obtain the electrodes suitable for spot welding of metallic sheets having high electric conductivity and heat conductivity such as aluminum and an aluminum alloy. CONSTITUTION:Metallic materials to be welded having high electric conductivity and heat conductivity such as the aluminum alloy are superposed on each other and these are held between a couple of upper and lower electrodes 1. These electrodes 1 are formed by fitting a titanium cup-shaped cap 4 integrally on a thread part 3 of the tip of a Cu-Cr alloy electrode main body 2 with the vertex surface 6 of a vertex wall 5 of the cap 4 as a part of the spherical surface and an indentation 8 having a round hole shape is formed on the central part of the vertex surface 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode used for spot welding a metal material having high electric conductivity and thermal conductivity, and more particularly to an electrode structure suitable for small current welding.

[0002]

2. Description of the Related Art Spot welding is a technique in which two metal plates are superposed, sandwiched between a pair of rod-shaped electrodes and pressed to energize, and the Joule heat generated heats and melts the metal plates. This is a method of pressure welding. When welding a metal plate such as aluminum, aluminum alloy, magnesium alloy, etc. having high electric conductivity and thermal conductivity by this spot welding method, the amount of Joule heat generated is small and the generated heat has a high thermal conductivity. Since it dissipates through a good material to be welded, it requires a large current and a large pressing force (due to the electrode) as compared with the welding of steel plates and the like, and the welding must be completed in a short time. Therefore, a large transformer is required, and a large spot gun that impairs workability in response to a large pressing force must be adopted, which makes it difficult to make the welding machine portable.

Further, even when a large current is used, in performing multi-point spot welding, due to the good electric conductivity of the material to be welded, there is a shunt current passing through the already completed welding point, and There was also a problem that welding at the welded part was not performed properly.

Japanese Patent Application Laid-Open No. 57-56175 discloses an invention invented to solve such a problem. In the invention described in this publication, a metal plate made of a metal material having a lower thermal conductivity and electric conductivity than the material to be welded and a high melting point temperature is interposed between the tip of the electrode and the material to be welded. .

In the invention of the above-mentioned publication, Joule heat is concentrated and generated in a metal material metal plate having lower thermal conductivity and electric conductivity than the material to be welded, and the metal plate is heated to a high temperature to achieve heat conduction. The nugget is formed on the material to be welded, which has a high degree of electrical conductivity, and spot welding is performed.However, since this metal has a uniform thickness, electricity was applied between both electrodes and pressure was applied. In spot welding, the part of the material to be welded immediately below the periphery of the pad metal flows outward and the contact surface pressure at that part decreases, and the contact surface pressure at the center part of the pad becomes the largest and the current concentrates. However, the part of the material to be welded that is in contact with the center of the pad becomes overheated, the diameter of the nugget becomes smaller and the welding strength decreases, and the overheating of the part of the material to be welded causes a pick-up phenomenon Phenomenon) occurs, Continuous RBI of the gold Te is reduced.

[0006]

SUMMARY OF THE INVENTION The present invention relates to an improvement in a spot welding electrode that overcomes the above problems, and it is a material having a lower thermal conductivity and electrical conductivity than the material to be welded and a high melting point temperature. A cup-shaped cap made of a metal is fitted on the tip of the electrode body, and the cap has a top surface close to a spherical surface, and the top wall has a large wall thickness in the central portion and a peripheral wall in the peripheral portion. It is characterized in that it is formed small and a round hole is formed in the center of the top surface of the cap.

According to the present invention, as described above, the cup-shaped cap made of a material having a lower thermal conductivity and a higher electric conductivity than the material to be welded and a higher melting point temperature is fitted on the tip of the electrode body. When a pair of electrode bodies are sandwiched by two pairs of relatively thin materials to be welded, such as a pair of electrodes, and a cap at the tip is sandwiched, and the two materials to be welded are energized through the pair of electrodes, the electrical resistance The resistance heat generation of the cap having a large size and the contact resistance heat generation of the cap and the material to be welded heat the portion of the material to be welded between both caps to a high temperature, and the electrical resistance of the sandwiched portion is large. Then, a high temperature nugget is generated in the welded material portion, and the required spot welding is performed.

Further, according to the present invention, the top surface of the cap is formed into a shape close to a spherical surface, and the wall thickness of the top wall is formed to be large in the central portion and small in the peripheral portion, so that the contact surface pressure with the material to be welded is large. Where the current concentrates in the central portion of the cap top surface and becomes a heating state, by thickening the central portion of the cap top surface, the current density of the portion sandwiched between both caps in the material to be welded is made uniform, You can get the nugget without overheating.

By performing spot welding using the present invention as described above, a high-temperature nugget without local overheating can be generated in the portion of the material to be welded which is sandwiched between the two caps, and moreover than the material to be welded. A cup-shaped cap made of a material with a high melting point temperature was fitted on the tip of the electrode body, so there was a pick-up where the material to be welded adhered to the electrode and an erosion where the material to be welded adhered to the electrode diffused and penetrated into the electrode It is possible to suppress and obtain a good welded portion.

[0010]

EXAMPLE An example of the present invention illustrated in FIGS. 1 to 3 will be described below. 1 and 2 show that the materials 12 and 13 to be welded, which are plate materials made of a metal material having a high electric conductivity and a high thermal conductivity, such as an aluminum alloy, are polymerized, and the upper and lower electrodes 1 and 1 are combined with each other. It shows the sandwiched state. As shown in FIG. 3, the electrode 1 is made of a general-purpose electrode material (eg, Cu-Cr alloy). It is configured by screwing a cup-shaped cap 4 formed of titanium (Ti) having a low melting point temperature and a low electric conductivity and a low thermal conductivity.

Further, the crown surface 6 of the cap 4 is formed as a part of a spherical surface (referred to as R shape), and as a result, the wall thickness of the crown wall 5 is large in the central portion 7 and small in the peripheral portion 9,
Moreover, a round hole-shaped recess 8 is formed on the outer surface of the crown wall 5, that is, in the center of the crown surface 6 (the tip of the electrode body 2 has an F-shaped electrode shape).

Since the embodiment shown in FIGS. 1 to 3 is constructed as described above, the materials 12 and 13 to be welded are energized under the same current condition (small current) as that for welding mild steel plates. As a matter of course, contact resistance heat generation occurs at the contact interface A between the materials 12 and 13 to be welded, but the contact interface B between the caps 4 and 4 and the materials 12 and 13 to be welded which are formed of titanium having a large electric resistance. ，
A large amount of contact resistance heat is generated at B, and the cap 4 itself also generates resistance heat, so that the heat generated at the contact interfaces B and B is
As indicated by arrow C, the material to be welded 1 is a good heat conductor.
Can be efficiently transmitted to 2 and 13. If the material to be welded is a mild steel sheet, the contact interface A generates heat at the contact interface A due to contact resistance heat generation.
A nugget (melted part) should be generated in the vicinity of. However, in this example, since the materials to be welded 12 and 13 are made of a material having a large electric conductivity and a high thermal conductivity such as an aluminum alloy, a nugget does not occur only by the heat generated at the contact interface A, and the contact The heat (arrow C) transmitted from the interface B raises the temperature of the portion sandwiched between the caps 4 and 4, and accordingly the electric resistance of the portion increases, so that the materials to be welded 12 and 13 (on the caps 4 and 4). Along with an increase in the resistance heating amount of the sandwiched portion), the materials to be welded 12 and 13 are melted in a cylindrical shape, and a nugget 14 as shown in FIG. 2 is produced. At this time, since the melting point temperature of the titanium cap 4 is sufficiently higher than that of the materials 12 and 13 to be welded, melting of the cap 4 does not occur.

By the way, an important condition required for the spot welding electrode is that the continuous spotting property is good. Electrical conductivity compared to materials 12 and 13 to be welded such as aluminum alloys,
Carbon steel, stainless steel, titanium (Ti) and the like can be used as the material for the cap 4 having a small thermal conductivity (see Table 1), but titanium is most suitable in view of the continuous hitting property. Titanium has properties such as a high melting point, a small thermal expansion coefficient, and a low thermal conductivity, as well as appropriately high strength and toughness. Titanium has a cap 4 made of titanium. Adhesion to electrodes) and erosion (a phenomenon in which the material to be welded adhered to the electrodes diffuses and permeates into the electrodes to form an alloy), and good continuous dot-forming properties can be secured.

[0014]

[Table 1]

The shape of the cap 4 is important for improving the continuous hitting property. The top surface of the cap 4 has a shape close to a spherical surface, that is, an R shape.
The contact surface pressure with 2 and 13 is the largest in the central part of the crown surface, and when the temperature of the cap 4 and the materials to be welded 12 and 13 rises due to energization and they thermally expand, this tendency becomes more and more remarkable. Therefore, the current is concentrated in the central portion of the top surface of the cap 4, the central portion is overheated, and heat is intensively applied to the welded materials 12 and 13 in a small area range. As a result, the diameter of the nugget 14 is reduced. As the size of the cap 4 becomes smaller, the nugget 14 also becomes overheated and a pickup phenomenon occurs, and as a result, the continuous hitting property of the cap 4 tends to deteriorate. However, in this embodiment, the thickness of the top wall of the titanium cap 4 is set at the center. This problem was solved by forming a large size on the periphery and a small size on the periphery.

This embodiment utilizes the fact that the energization resistance is proportional to the size of the wall thickness. Compared to the case where the wall thickness of the crown wall 5 is uniform, the central portion 7 of the crown wall 5 is larger. The current density decreases and the current density in the peripheral portion 9 increases.
As a result, the electric current that tends to concentrate in the central portion 7 of the crown wall is dispersed in the entire crown wall 5, and uniform contact resistance heat is generated in the entire contact interface between the crown surface 6 and the materials 12 and 13 to be welded. The heat input to 12 and 13 is evenly performed in a large area range, and a large nugget diameter as shown in FIG. 2 can be obtained without local overheating.

Further, by making the current density on the crown surface 6 uniform from the central portion 7 to the peripheral portion 9, local overheating of the materials 12 and 13 to be welded is prevented, and the coefficient of thermal expansion of the material to be welded is higher than that of steel. Phenomenon in which the contact surface pressure that increases with the crown surface 6 of the cap 4 due to the thermal expansion of the materials to be welded 12 and 13 formed of a large aluminum alloy, magnesium alloy, etc. at the central portion 7 of the crown wall becomes excessively large Therefore, the pickup and the erosion accompanying it can be suppressed and a good continuous hitting property can be guaranteed. Furthermore, since a small amount of current is applied, the contact resistance heat generated at the contact interface B between the cap 4 and the materials 12 and 13 to be welded is injected into the materials 12 and 13 to be welded to generate a nugget. Shunting via points does not matter.

Since the recess 8 is formed in the central portion 7 of the parietal wall 5, the thickness of the central portion 7 is larger than that of the peripheral portion 9, so that the current is effectively dispersed to the surroundings when energized. Contact resistance heat is uniformly generated at the contact interface between the crown surface 6 and the materials 12, 13 to be welded, and the crown wall 5 and the materials 12, 13 to be welded are generated.
Nugget 14 with a sufficiently large diameter (Fig. 2)
(Refer to FIG. 3) is formed, pickup and erosion accompanying it are suppressed, and continuous dot performance is improved. Furthermore, the material to be welded formed of an aluminum alloy, a magnesium alloy, or the like has a large coefficient of thermal expansion (the coefficient of thermal expansion of the aluminum alloy is about three times that of steel), and the material to be welded 12 that has undergone large thermal expansion when energized, When the high temperature molten portion of 13 pushes the crown surface 6 and the surface pressure between the crown surface 6 and the materials to be welded 12, 13 becomes excessive, a recess 8 is formed in the central portion 7 of the crown surface 6. Therefore, a part of the hot-melted high-temperature melted portion (the portion located at the center is the highest temperature) that has thermally expanded enters the recess 8, and the rise in the surface pressure is moderated, and the occurrence of pickup and erosion accompanying it is prevented. It

Moreover, when the electrodes 1 and 1 are brought into contact with the materials 12 and 13 to be welded for pressurization and energization, a large load due to the pressurization acts on the cap 4, and thermal shock stress due to a rapid temperature rise is applied to the cap 4. Occurs in It is effective to increase the length (L) of the cap 4 in order to reduce the stress generated in the cap 4 by the action of load and thermal shock, and the heat capacity is increased, and the contact with the electrode body 2 at the screwing portion 11 is increased. Due to the increase of the area, the heat transfer to the water-cooled electrode body 2 is favorably performed, and the coupling area with the electrode body 2 is increased to prevent the cap 4 from loosening during repeated use. When the cap 4 is loosened, the top wall 5 of the cap 4 is
Is separated from the tip surface of the electrode body 2 and the crown wall 5 is retracted and deformed by the applied pressure, and the contact with the materials 12 and 13 to be welded is not correctly performed. This means that its durability can be improved.

In performing spot welding, it is effective to apply silicone oil to the top surface 6 of the cap 4 or the electrode contact surfaces of the materials 12 and 13 to be welded beforehand.
Pickup and erosion accompanying it are suppressed, and continuous hitting property is improved. Silicone oil has a low flash point (172 ° C), and the contact resistance heat generated at the contact interface between the cap 4 and the materials 12 and 13 vaporizes, burns, and carbonizes it under a large pressure to form a hard thin film with high temperature strength. It is formed. This coating protects the crown surface 6 of the cap 4, suppresses pickup and erosion, and can always obtain a stable quality nugget.

An electrode 1 according to another embodiment of the present invention shown in FIG.
In A, the entire surface of the titanium cap 4A is subjected to nitriding treatment, and the entire inner surface is subjected to nitriding treatment with copper plating treatment, and is sandwiched between the electrode body 2A and the crown wall 5A by the silver interposer 15. The structure is different from that of the electrode 1.

The effects obtained by adopting a structure different from that of the electrode 1 are as follows. Nitriding treatment ... The nitriding treatment improves the surface hardness and rigidity of the cap 4A. The hardness of the titanium material that is not nitrided is about Hv200, and the hardness of the titanium material after nitrided is about Hv1000, showing a remarkable increase in hardness. If the surface hardness of the cap 4A is large, it is difficult for the molten material to be welded to adhere, and therefore pickup and erosion are effectively suppressed,
Together with the good wear resistance, it is possible to attempt to improve the continuous hitting point property.

If the rigidity of the cap 4A is large, the distortion caused by the pressure contact with the materials 12 and 13 to be welded and the distortion caused by the thermal shock stress caused by the rapid temperature rise at the time of energization are small, and the deformation prevention effect is obtained. large. In addition, the maximum electric resistivity of titanium that is not nitrided is 50 μΩ · cm, while T
The electrical resistivity of i N is 130 μΩ · cm at maximum, and the nitriding treatment increases the electrical resistance of the cap 4A. Therefore, the contact resistance between the cap 4A and the materials 12 and 13 to be welded increases, heat generation can be promoted, and the generation of the nugget becomes easier.

Copper plating treatment: When no copper plating treatment is performed, Joule heat is concentrated on the central portion 7 of the crown wall 5A of the cap 4A during energization, and the temperature difference between the central portion 7 and the peripheral portion 9 is large. On the other hand (see FIG. 5 (a) temperature distribution curve), in the case where the inner surface of the cap 4A is copper-plated as in the embodiment of FIG. 4, the concentration of Joule heat in the central portion 7 is relaxed, The temperature difference between the central portion 7 and the peripheral portion 9 is small (see the temperature distribution curve in Fig. 5 (b)). Therefore, it is possible to prevent local overheating of the materials to be welded 12 and 13 by performing the copper plating treatment, suppress the occurrence of pickup and erosion, and improve the continuous hitting property.

Further, since the cap 4A is a member that functions as a heat source for the materials to be welded 12, 13, it is preferable to raise the temperature to a certain extent, but an excessive temperature rise promotes its deterioration and must be avoided. I have to. In this sense, it is effective to apply a copper plating film having good thermal conductivity to the inner surface of the cap 4A, and the electrode body 2 cooled by water flow is effective.
Since the heat transfer to A is smoothly performed, the cap 4A
Overheating is prevented.

Use of silver intercalation plate 15 ... The intercalation plate 15 is not limited to silver, and is a metal having better electrical conductivity than the electrode body 2A, better thermal conductivity than the cap 4A, and less hardness than the cap 4A. It may be formed by. The function and effect of interposing the silver interposer 15 between the electrode body 2A and the top wall 5A are similar to those in the case of copper plating, and the interposer 15 is attached to the electrode body 2A and the cap 4A. The Joule heat, which adheres well and improves the current efficiency by reducing the contact resistance between the electrode body 2A and the cap 4A, tends to concentrate on the central portion of the crown wall 5A.
Is diffused to the peripheral portion 9 of the parietal wall 5A, and the temperature difference between the central portion 7 and the peripheral portion 9 becomes small.
It is possible to attempt to increase the nugget diameter by preventing local overheating in Nos. 2 and 13 and to improve continuous hitting property by suppressing pickup and erosion. Further, the interposing plate 15 ensures good heat transfer from the cap 4A to the electrode body 2A, and the cap 4A
Overheating is prevented.

Further, as described above, when a pressure is applied to the cap 4A, a large load is applied to the cap 4A and a thermal shock stress is generated in the cap 4A due to a rapid temperature rise. It is effective to interpose the interposition plate 15 between the parietal wall 5A and the electrode body 2A in order to relax the stress generated in the parietal wall cap 4A by the action of this load and the thermal shock. That is, the insertion plate 15 functions as a buffer against the load acting on the crown wall 5A due to the applied pressure, and as a good heat transfer medium for the water-cooled electrode body 2 against the thermal shock, the insertion plate 15 is used. The function of 15 prevents a rapid temperature rise of the parietal wall 5A and suppresses deterioration of the parietal wall 5A.

<Welding test> Using the electrode 1A capable of obtaining the above-described effects as an example of the present invention, a single-phase AC portable welding machine for welding steel plates was used to produce two aluminum alloy plates (plate thickness 1.0 m).
m) spot welding was performed. For comparison, without using a titanium cap, a single-phase AC portable welder for welding steel sheets (Comparative Example I), a single-phase AC stationary welder (Comparative Example II), a single-phase rectifying type for aluminum welding Spot welding of two aluminum alloy plates (plate thickness 1.0 mm) was performed with a portable welder (Comparative Example III). The welding conditions and welding results (nugget diameter, tensile shear strength (average value)) are shown in Table 2.

[0029]

[Table 2]

<Evaluation of Test Results> From the comparison between the present invention example and the comparative example I, the titanium cap 4
It can be seen that the use of A enables welding with a small current and a short welding time, which has been impossible in the past. From the comparison of the present invention examples and Comparative Examples III, IV, by using a titanium cap, small current, large current even if welding with a small pressurizing force, a nugget diameter equal to or greater than those welded with large pressure, It can be seen that tensile shear strength is obtained.

Further, it was confirmed that in the example of the present invention, continuous 100 welding points could be welded.

When pickup or erosion occurs on the electrode, the portion must be removed. In Comparative Examples III and IV, the removal work was required every 5 welding points, whereas in the present invention example, 10 welding points were used. Removal work was required for each welding. From this, it can be seen that the titanium cap 4A is less likely to cause pickup and erosion, has improved continuous hitting property, and consequently has improved productivity.

[Brief description of drawings]

FIG. 1 is a schematic view illustrating a state in which spot welding is performed using an embodiment of a spot welding electrode of the present invention.

FIG. 2 is a schematic view of a state where a nugget is formed in the above-mentioned embodiment.

FIG. 3 is an enlarged vertical sectional front view of a main part of the embodiment.

FIG. 4 is an enlarged vertical sectional front view of a main part of another embodiment of the present invention.

FIG. 5 is a diagram showing a difference in temperature distribution of the cap top surface at the time of energizing welding between the embodiment shown in FIGS. 1 to 3 and the embodiment shown in FIG. 4, (a) showing the first embodiment, b) shows the second embodiment.

[Explanation of symbols]

1 ... Electrode, 2 ... Electrode body, 3 ... Screw, 4 ... Cap, 5
... parietal wall, 6 ... parietal surface, 7 ... central part, 8 ... dent, 9 ... peripheral part, 10 ... peripheral wall, 11 ... screw, 12,13 ... welded material, 14 ... nugget, 15 ... intercalation plate.

Claims (2)

[Claims] 1. A cup-shaped cap made of a material having a lower thermal conductivity and a higher electric conductivity than the material to be welded and having a high melting point temperature is fitted onto the tip of the electrode body, and the cap has a top surface thereof. Spot welding characterized in that it is formed in a shape close to a spherical surface, the wall thickness of the crown wall is large in the central part and small in the peripheral part, and a round hole recess is formed in the center of the crown surface of the cap. Electrodes. 2. The electrode for spot welding according to claim 1, wherein a nitriding layer is formed on the entire outer surface of the cup-shaped cap.