JPH0515424Y2 - - Google Patents

Description

[Detailed explanation of the idea]

[Industrial Application Field] The present invention relates to an electrode used for spot welding metal materials with high electrical conductivity and high thermal conductivity, and particularly to an electrode structure suitable for low current welding. [Prior art and problems to be solved] Spot welding is the process of overlapping two metal plates.
This is a method in which the metal plate is held and pressed between a pair of round rod-shaped electrodes and then energized, and the generated Joule heat heats and melts the metal plate, thereby welding the metal plate under pressure. When welding aluminum alloy or magnesium alloy metal plates with high electrical conductivity and thermal conductivity using this spot welding method, the amount of joule heat generated is small;
Moreover, since the generated heat dissipates through the welded material, which has good thermal conductivity, it requires a higher current and greater pressure (using electrodes) than welding steel plates, etc., and can be welded in a short time. must be completed. Therefore, it was difficult to make the welding machine portable because a large transformer was required and a large spot gun had to be used, which could handle a large pressure and impair work efficiency. Furthermore, even if a large current is used, when performing multi-point spot welding, due to the good electrical conductivity of the material to be welded, a shunt flow occurs that passes through the already welded point, and the current welding point There was also a problem where welding was not done properly. [Means and effects for solving the problem] The present invention relates to an improvement of a spot welding electrode that eliminates the above-mentioned problems. , a thick-walled cap made of titanium with a small electrical conductivity is fitted onto the tip of the electrode body, and the top surface of the thick-walled cap extends from the center of the top surface toward the periphery. It is characterized in that it is formed into a curved convex surface that recedes more gently, and the thickness of the parietal wall of the thick cap gradually decreases from the center toward the periphery. As mentioned above, in this invention, a thick cup-shaped titanium cap with low electrical conductivity is fitted onto the tip of the electrode body, so a pair of aluminum alloy or magnesium alloy metal plates are attached to the tip of the pair of electrode bodies. When electricity is applied to the two metal plates through the pair of electrodes with the cap sandwiched between them, resistance heat generation occurs in the cup-shaped thick titanium cap with high electrical resistance and contact resistance heat generation between the cap and the metal plate. As a result, the part of the metal plate sandwiched between the two caps is heated to a high temperature, and the electrical resistance of the metal plate sandwiched part becomes even larger, and a high temperature nugget is generated in the metal plate sandwiched part, which prevents the required spot welding. carried out. Further, in the present invention, the top surface of the cap is formed into a curved convex surface that gently recedes from the center toward the periphery, and the thickness of the top wall of the thick cap is increased from the center toward the periphery. As a result, the current concentrates at the center of the top surface of the cap, where the contact pressure with the metal plate is large, resulting in an overheating state. By equalizing the current density in the area sandwiched between both caps, a nugget without local overheating can be obtained with a small current. In this way, by performing spot welding using the present invention, it is possible to generate high-temperature nuggets without local overheating in a relatively wide area of the metal plate sandwiched between both caps, and also to have a melting point temperature higher than that of the metal plate. A high-strength titanium cup-shaped cap is fitted to the tip of the electrode body, which suppresses pick-up where the metal plate adheres to the electrode and erosion where the metal plate adheres to the electrode diffuses and penetrates inside the electrode, resulting in good welding. It is possible to reduce power consumption at the same time. [Embodiment] An embodiment of the present invention illustrated in FIGS. 1 to 3 will be described below. In FIGS. 1 and 2, materials to be welded 12 and 13, which are plates made of a metal material with high electrical conductivity and thermal conductivity such as aluminum alloy, are overlapped, and upper and lower electrodes 1 are connected to each other. It shows a pinched state. Electrode 1 is
As illustrated in FIG. 3, general-purpose electrode materials (e.g.
The material to be welded 1 is attached to the threaded part 3 formed at the tip of the F-shaped electrode body 2 made of Cu-Cr alloy).
It is constructed by screwing together a cup-shaped cap made of titanium (Ti), which has lower electrical conductivity and thermal conductivity than those of Nos. 2 and 13, and has a high melting point. In FIG. 1, the materials to be welded 12 and 13, which are plates made of a metal material with high electrical conductivity and thermal conductivity, such as aluminum alloy, are overlapped, and the upper and lower parts are overlapped. electrode 1,
1 shows the sandwiched state. The electrode 1 has a material at the tip of an electrode body 2 made of a general-purpose electrode material (e.g., Cu/Cr alloy) that has lower electrical conductivity and thermal conductivity than the materials to be welded 10 and 11, and has a higher melting point temperature. A cup-shaped cap 4 made of titanium (Ti) is fitted therein. In Fig. 1, when the welded materials 12 and 13 are energized under the same current conditions (small current) as for welding mild steel plates, contact resistance heat generation occurs at the contact interface A of the welded materials 12 and 13. Of course, large contact resistance heat generation occurs at the contact interfaces B and B between the caps 4, 4 formed of titanium, which has a high electrical resistance, and the welded materials 12, 13, and the cap 4 itself also generates resistance heat generation. Heat generated at the contact interfaces B, B, as shown by arrow C, is efficiently transferred to the welded materials 12, 13, which are good heat conductors. If the material to be welded is a mild steel plate, the contact resistance heat generated at the contact interface A should produce a nugget (molten part) near the interface A. However, in this example, the materials to be welded 12 and 13 are made of a material with high electrical conductivity and thermal conductivity, such as aluminum alloy, so the heat generated at the contact interface A alone does not cause nuggets, and the contact The temperature of the portion sandwiched between the caps 4 and 4 increases due to the heat transmitted from the interface B (arrow C), and the electrical resistance of this portion increases accordingly, causing the welded materials 12 and 13 to increase in temperature.
Coupled with the increase in the resistance heat generation of the welded material 12 (the part sandwiched between the caps 4, 4),
13 is melted into a cylindrical shape, and a nugget 14 as shown in FIG. 2 is formed. At this time, the material to be welded 12,1
Since the melting point temperature of the titanium cap 4 is sufficiently higher than that of the titanium cap 4, the cap 4 does not melt. By the way, an important condition required for spot welding electrodes is good connection and spotting properties. Carbon steel, stainless steel, titanium (Ti), etc. can be cited as materials for the cap 4 that have lower electrical conductivity and thermal conductivity than the materials to be welded 12 and 13 such as aluminum alloys (see Table 1). Titanium is the most suitable material when considering the dot performance. In addition to the characteristics of high melting point, low coefficient of thermal expansion, and low thermal conductivity, titanium also has suitably high strength and toughness. (phenomenon in which materials adhere to electrodes),
Erosion (a phenomenon in which the material to be welded attached to the electrode diffuses into the electrode and forms an alloy) can be suppressed, and good continuous dotting performance can be ensured.

[Table] Furthermore, the shape of the cap 4 is important in improving its continuous dot performance. The top surface of the cap 4 has a shape close to a spherical surface, that is, an R-shape, but the contact surface pressure with the materials to be welded 12 and 13 is greatest at the center of the top surface, and when energized, the cap 4 and the materials to be welded 12 , 13 increases in temperature and thermally expands, this tendency becomes more and more noticeable. Therefore, the current concentrates at the center of the top surface of cap 4,
The central portion becomes heated, and heat is intensively input to the welded materials 12 and 13 in a small area. As a result, the diameter of the nugget 14 becomes smaller and the nugget 14 also becomes overheated, causing a pick-up phenomenon. As a result, the continuous dot performance of the cap 4 tends to deteriorate; however, in this embodiment, this problem is solved by making the thickness of the top wall of the titanium cap 4 larger at the center and smaller at the periphery. solved.
This embodiment utilizes the fact that the current conduction resistance is proportional to the wall thickness, and the current density at the center 7 of the parietal surface 5 is lower than that in the case where the parietal wall 5 has a uniform thickness. The current density in the peripheral region 9 increases. As a result, the current that tends to concentrate in the central part 7 of the parietal wall is dispersed throughout the parietal wall 5, and the parietal surface 6
Uniform contact resistance heat is generated over the entire contact interface between the material to be welded 12 and 13, and the heat input to the material to be welded 12 and 13 is uniform over a large area range, with no local overheating as shown in Fig. 2. A large nugget diameter as shown in the figure can be obtained. In addition, by equalizing the current density in the top wall 6 from the center part 7 to the peripheral part 9, local overheating of the welded materials 12 and 13 is prevented, and aluminum alloys with a larger coefficient of thermal expansion than steel can be used. Or welded materials 12, 13 made of magnesium alloy
It is possible to avoid the phenomenon in which the contact pressure, which is caused by thermal expansion of the cap 4 and increases with the crown surface 6 of the cap 4, becomes excessively large at the center portion 7 of the crown surface, and pick-up and accompanying erosion are suppressed, resulting in good continuous hitting points. gender is guaranteed. Furthermore, since a small current is applied and the contact resistance heat generated at the contact interface B between the cap 4 and the welded materials 12, 13 is injected into the welded materials 12, 13, nuggets are generated. Diversion through the weld point is not a problem. Furthermore, since the recess 8 is formed in the central part 7 of the parietal wall 5, the thickness of the central part 7 is larger than that of the peripheral part 9, and when electricity is applied, the current is effectively dispersed to the surrounding area. 6 and materials to be welded 12, 13
Contact resistance heat is generated evenly at the contact interface with the nugget 14 (see Fig. 2), which has a sufficiently large diameter so that local overheating of the crown wall 5 and the welded materials 12, 13 does not occur.
is formed, pick-up and accompanying erosion are suppressed, and continuous hitting performance is improved. Furthermore, the thermal expansion coefficient of the welded material made of aluminum alloy or magnesium alloy is large (the thermal expansion coefficient of aluminum alloy is approximately three times that of steel), and the welded material 1 which thermally expanded significantly when energized
When the high-temperature molten parts 2 and 13 press against the crown surface 6 and the contact pressure between the crown surface 6 and the welded materials 12 and 13 becomes excessive, a depression 8 is formed in the center 7 of the crown surface 6. Therefore, a part of the thermally expanded high-temperature molten part (the central part is the highest temperature) enters the recess 8, which alleviates the increase in surface pressure and prevents the occurrence of pick-up and associated erosion. Prevented. Moreover, when the electrodes 1, 1 are brought into contact with the materials to be welded 12, 13 and pressurized and energized, a large load due to the pressurization acts on the cap 4, and thermal shock stress is generated in the cap 4 due to a rapid temperature rise. The length (L) of the cap 4 is important in alleviating the stress generated in the cap 4 due to the action of load and thermal shock.
It is effective to increase the heat capacity, and by increasing the contact area with the electrode body 2 in the threaded part 11, heat transfer to the water-cooled electrode body 2 is performed well, and the electrode body This increases the bonding area with cap 2 and prevents loosening of cap 4 during repeated use. Furthermore, when the cap 4 becomes loose, the top wall 5 of the cap 4 separates from the tip surface of the electrode main body 2, and the top wall 5 deforms backward due to the pressure, so that proper contact with the materials 12 and 13 to be welded can be prevented. Considering that the cap 4 will not come loose, preventing the cap 4 from loosening means that its durability can be improved. When performing spot welding, it is effective to apply silicone oil in advance to the top surface 6 of the cap 4 or to the electrode contact surfaces of the welded materials 12 and 13, thereby suppressing pick-up and accompanying erosion. , continuous hitting performance is improved. The flash point of silicone oil is low (172°C), and it vaporizes and burns under large pressure due to the contact resistance heat generated at the contact interface between the cap 4 and the materials to be welded 12 and 13.
Carbonization forms a hard thin film with high high temperature strength. This coating protects the top surface 6 of the cap 4,
Pick-up and erosion are suppressed, and nuggets of stable quality can always be obtained. Electrode 1A of another embodiment of the present invention shown in FIG.
Now, the entire surface of the titanium cap 4A is nitrided, the entire inner surface is nitrided and then copper plated, and the silver insert plate 15 is inserted between the electrode body 2A and the top wall 5A. The structure is different from that of electrode 1. The effects of adopting a structure different from that of electrode 1 are as follows. Nitriding treatment: The surface hardness and rigidity of the cap 4A are improved by the nitriding treatment. The hardness of titanium material without nitriding treatment is about Hv200, and the hardness of titanium material after nitriding treatment is about Hv1000, and the increase in hardness is remarkable. If the surface hardness of the cap 4A is large, the molten material to be welded will not easily adhere to it, and therefore pick-up and erosion will be effectively suppressed.Coupled with good wear resistance, it will improve the continuous dot performance. It can be planned. If the rigidity of the cap 4A is large, the distortion caused by pressurized contact with the welded materials 12, 13 and the thermal shock stress caused by the rapid temperature rise during energization will be small, and the deformation prevention effect will be large. Further, while the electrical resistivity of titanium that is not nitrided is 50 μΩ·cm at the maximum, the electrical resistivity of TN is 130 μΩ·cm at the maximum, and the electrical resistance of the cap 4A is increased by the nitriding process. Therefore, the contact resistance between the cap 4 and the welded materials 12, 13 increases, heat generation can be accelerated, and nuggets can be formed more easily. Copper plating treatment...If copper plating treatment is not performed, the top wall 5 of the cap 4A will be damaged when energized.
The Joule heat is concentrated in the central part 7 of A, and the central part 7
In contrast, in the case where the inner surface of the cap 4A is plated with copper as in the embodiment shown in FIG. The concentration of Joule heat on the portion 7 is alleviated, and the temperature difference between the central portion 7 and the peripheral portion 9 is small (see the temperature distribution curve in FIG. 5b). Therefore, by performing copper plating treatment, local overheating of the welded materials 12 and 13 can be prevented,
It is possible to suppress the occurrence of pick-up and erosion, and to improve continuous hitting performance. Furthermore, since the cap 4A is a member that functions as a heat source for the materials to be welded 12 and 13, it is preferable that the temperature rises to some extent;
Excessive temperature rise accelerates its deterioration and must be avoided. In this sense, cap 4A
It is effective to apply a copper plating film with good thermal conductivity to the inner surface of the electrode body 2A, which is cooled by water flow.
Since heat is transferred smoothly to the cap 4A, overheating of the cap 4A is prevented. Use of silver insert plate 15...The insert plate 15 is not limited to silver, but is formed of a metal that has better electrical conductivity than the electrode body 2A, better thermal conductivity than the cap 4A, and less hardness than the cap 4A. It is fine as long as it is something. The effect of inserting the silver insert plate 15 between the electrode body 2A and the parietal wall 5A is similar to that of copper plating treatment, and the insert plate 1
5 is in close contact with the electrode body 2A and the cap 4A, and current efficiency is improved by reducing the contact resistance between the electrode body 2A and the cap 4A. The heat is diffused to the peripheral part 9 of the crown wall 5A through the insertion plate 15, and the temperature difference between the central part 7 and the peripheral part 9 becomes small, so that the welding material 12,
13. Enlargement of nugget diameter by preventing local overheating,
It is also possible to improve the continuous hitting performance by suppressing pick-up and erosion. Further, the interposed plate 15 improves heat transfer from the cap 4A to the electrode body 2A, and prevents the cap 4A from overheating. Furthermore, as described above, when pressurizing and energizing, a large load due to pressurization acts on the cap 4A, and thermal shock stress due to a rapid temperature rise is generated in the cap 4A. It is effective to interpose the interposed plate 15 between the parietal wall 5A and the electrode main body 2A in order to alleviate the stress generated in the parietal wall cap 4A due to the action of this load and thermal shock. That is, the interposed plate 15 functions as a buffer against the load acting on the parietal wall 5A due to pressurizing force, and acts as a good heat transfer medium for the water-cooled electrode body 2 against thermal shock. By functioning the insert plate 15, the parietal wall 5A
A rapid temperature rise is prevented, and deterioration of the parietal wall 5A is suppressed. <Welding test> Electrode 1A, which provides the above effects, was used as an example of the present invention, and two aluminum alloy plates (thickness: 1.0 mm) were welded using a single-phase AC portable welding machine for welding steel plates.
Performed spot welding. In addition, for comparison, we have developed a single-phase AC portable welding machine for steel plate welding (comparative example), a single-phase AC stationary welding machine (comparative example), and a single-phase rectifier portable welding machine for aluminum welding, without using a titanium cap. Two aluminum alloy plates (plate thickness 1.0mm) were made using a machine (comparative example).
spot welding was carried out. The welding conditions and welding results (nugget diameter, tensile shear strength (average value)) are shown in Table 2.

[Table] <Evaluation of test results> From the comparison between the present invention example and the comparative example, it can be seen that by using the titanium cap 4A, it is possible to weld with a small current and short welding time, which was previously impossible. . Comparing the present invention example and the comparative example, it was found that by using a titanium cap, even when welded with a small current and a small pressure, the nugget diameter and tensile shear were equal to or greater than those welded with a large current and a large pressure. It can be seen that strength can be obtained. In addition, it was confirmed that in the case of the example of the present invention, welding of 100 consecutive points could be performed. When pick-up or erosion occurs on the electrode, it is necessary to remove that part. In the comparative example, removal work was required every 5 welding points, whereas in the present example, removal work was required every 10 welding points. was necessary. From this, with titanium cap 4A, pick-up,
It can be seen that erosion is less likely to occur, the continuous dot performance is improved, and productivity can be improved as a result.

[Brief explanation of the drawing]

1 and 2 are schematic diagrams showing a spot welding method using the electrode of the present invention, and FIG. 3 is a cutaway diagram of essential parts showing a spot welding electrode according to an embodiment of the present invention.
Fig. 4 is a cutaway view of the main part showing a spot welding electrode according to another embodiment, and Fig. 5 shows the difference in temperature distribution on the top surface of the cap when welding current is applied depending on the presence or absence of a copper plating film on the inner surface of the titanium cap. This is a graph (a is without a film, b is with a film). 1... Electrode, 2... Electrode body, 3... Screw, 4
... Cap, 5 ... Parietal wall, 6 ... Parietal surface, 7
... central part, 8 ... recess, 9 ... peripheral part, 10 ...
...Peripheral wall, 11...screw, 12, 13...material to be welded, 14...nugget, 15...intervening plate.

Claims (1)

[Claim for Utility Model Registration] A spot welding electrode used for spot welding metal plates made of aluminum alloy or magnesium alloy, in which the tip of the electrode body is fitted with a thick-walled cap made of titanium in the form of a cup with low electrical conductivity. The top surface of the thick cap is
The parietal surface is formed into a curved convex surface that gently recedes from the center toward the periphery, and the thickness of the parietal wall of the thick cap gradually decreases from the center toward the periphery. A spot welding electrode featuring: (2) The spot welding electrode according to claim 1, wherein a nitrided layer is formed on the entire outer surface of the cup-shaped thick titanium cap.