Electric resistance welding method

Abstract

To provide an electric resistance welding method that when electric resistance-welding welded materials to each other at each weld point, cultivates a nugget with uniform spread and size so as to approach a central part from an outer peripheral edge side of an electrode contact surface pressure-welded to the welded material, and welds the welded materials with high quality.SOLUTION: When applying an electric current in a state where an electrode, in which an electric resistance part is provided at a contact surface midship part pressure-welded to a welded material, is pressure-welded to the welded material, after overheating and melting the welded materials in a state where a current density at a part corresponding to a contact surface outer peripheral edge part becomes higher than that of the contact surface midship part, the welded materials are overheated and molten while a current density gradually decreasing by increase in electric resistance in association with temperature rise at the part corresponding to the contact surface outer peripheral edge part, and on the other hand, a current density at a part corresponding to the contact surface midship part gradually increasing, and a nugget is cultivated so as to approach the midship part from the outer peripheral edge part edge side of the contact surface and to be capable of being welded.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to an electric resistance welding method that enables the growth of uniform nuggets and the production of high-quality joints when spot welding multiple welding points by applying current while pressing at least two overlapping workpieces together with an electrode tip. [Background technology]

[0002] In conventional electric resistance welding, as shown in Patent Documents 1 and 2, for example, when a DC or AC current is applied to at least two overlapping welded materials under pressure with an electrode whose tip contact surface with the welded materials protrudes in a spherical shape with a predetermined curvature, the materials are joined by a nugget that is overheated and melted by Joule heat due to the contact resistance between the welded materials.

[0003] It should be noted that although there are known methods of applying current to the electrodes to the workpiece, such as the direct method, indirect method, series method, and parallel method, the method of the present invention can be implemented using any of these methods.

[0004] The welding quality of each weld point on the materials to be welded depends on the size (diameter) and spread of the nugget, but in conventional electric resistance welding, current is applied via the protruding contact surface of the electrode tip that presses against the materials to be welded, so in the early stages of welding, the current density increases at the point corresponding to the center of the electrode relative to the materials to be welded, causing it to overheat and melt, and then the materials to be welded soften due to overheating, forming an indentation and the outer edge of the electrode is pressed against it, and the current density at the corresponding point increases (the electric resistance increases due to the high temperature at the center, causing the current density to decrease), causing the nugget to grow and weld as it overheats and melts. As a result, the nugget grows and spreads from the center of the contact surface of the electrodes between the materials to the outer edge.

[0005] However, in conventional electric resistance welding, as the number of welding points increases, the electrode tip may bulge and deform due to the applied pressure, as shown in Figure 1, or an alloy layer (in the case of galvanized steel sheet, an Fe-Zn layer, a γCr-ZN layer, a βCr-Zn layer, or a Cu-Zn layer) may be formed as an electrical resistance layer due to the metal composition of the molten electrode and the material to be welded over the entire contact surface of the electrode tip with the material to be welded, or some of the alloy layer may penetrate into the metal at the contact surface, and when the penetrated alloy layer reaches a certain amount, it may peel off due to the impact caused by the applied pressure, forming tiny irregularities on the contact surface and forming a rough surface that reduces the contact area with the material to be welded.

[0006] As a result, compared to the initial stage of welding, variations in electrical resistance, thermal conductivity, etc. occurred due to these alloy layers and rough surfaces, and as the welding work progressed, the current density became unstable, making it impossible to grow the nugget uniformly.

[0007] In particular, when an alloy layer or rough surface is formed on the contact surface of the electrode with the workpiece, the electrical resistance to the workpiece increases compared to when these are not formed.As a result, when current is applied between the electrodes, the temperature rise at the contact point becomes higher than inside the workpiece, causing "expulsion," or the nugget spread direction and size vary, resulting in an uneven nugget diameter and unstable welding quality.

[0008] That is, as shown in Figure 2, the nuggets grown at each welding point between the workpieces had different spreading directions and sizes in the radial direction centered on the electrode tip.

[0009] Furthermore, if the tip of the electrode swells and deforms, or if a rough surface or an alloy layer acting as an electrical resistance layer is formed on the contact surface, the above problem can be solved by performing a dressing operation, as shown in Figure 3, to remove the alloy layer by cutting (grinding) the tip of the electrode, or to smooth it and return it to its normal shape (the shape of the tip of an unused electrode).

[0010] However, in order to remove the alloy layer from the contact surface of the electric tip or to make it smooth, the electrode must be cut down by about 0.1 to 0.3 mm in axial length, which shortens the electrode's lifespan and increases the frequency of replacement, resulting in a problem of reduced work efficiency in electric resistance welding. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 6-55278 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-14843 Summary of the Invention [Problem to be solved by the invention]

[0012] The problem to be solved is that when electric resistance welding is performed at each welding point by applying pressure to an electrode that is pressed against the workpiece, as the number of welding points increases, the tip of the electrode may bulge and deform due to the applied pressure, and an alloy layer of the metal composition of the molten electrode and workpiece may be formed on the contact surface of the electrode with the workpiece, or an uneven rough surface may be formed, causing a change in the electrical resistance to the workpiece and resulting in uneven current density, and therefore uneven heat generation, which causes variations in the direction and size of the spread of the nugget that is grown between the workpieces, resulting in poor welding quality. [Means for solving the problem]

[0013] The present invention is an electric resistance welding method in which an electrode is pressed against at least two or more overlapping workpieces and an electric current is applied while the electrodes are pressed against the workpieces to grow a nugget and weld them together, and its most important feature is that when an electrode having an electric resistance portion at the center of the contact surface that is pressed against the workpieces is pressed against the workpieces and an electric current is applied, the current density at the point corresponding to the outer periphery of the contact surface becomes higher than at the center of the contact surface, overheating and melting the workpieces together, and then the current density gradually decreases due to the high electrical resistance caused by the increase in temperature at the corresponding point on the outer periphery of the contact surface, while the current density at the corresponding point at the center of the contact surface gradually increases, overheating and melting the workpieces together, and a nugget is grown from the edge of the outer periphery of the contact surface toward the center, making it possible to weld. [Effects of the Invention]

[0014] When electrically resistance welding workpieces together at each welding point, the present invention allows a nugget to grow with uniform spread and size from the outer periphery of the electrode tip that is pressed against the workpiece toward the center, thereby enabling the workpieces to be welded with high quality. [Brief explanation of the drawings]

[0015] [Figure 1] (A) is an explanatory diagram showing the tip surface when alloyed galvanized steel sheet is welded with an electrode used in a conventional electric resistance welding method, and (B) is an explanatory diagram when electro-galvanized steel sheet is similarly welded. [Figure 2] FIG. 1 is an explanatory diagram showing a planar growth state of a nugget by a conventional electric resistance welding method. [Figure 3] FIG. 1 is an explanatory diagram showing a dressed state of an electrode used in a conventional electric resistance welding method. [Figure 4] 10 is an explanatory diagram showing an example of an alloy layer formed on the contact surface of the electrode tip portion. FIG. [Figure 5] FIG. 2 is an explanatory diagram showing a shaped state of an electrode used in an electric resistance welding method. [Figure 6] FIG. 2 is an explanatory diagram showing the state of nugget growth in the early stage of energization. [Figure 7]FIG. 10 is an explanatory diagram showing the state of nugget growth from the outer peripheral edge of the contact surface toward the center. [Figure 8] FIG. 2 is an explanatory diagram showing the planar growth state of a grown nugget. [Figure 9] FIG. 1 is an explanatory diagram showing the change in electrode resistance value before and after "dressing" of an electrode used in a conventional electric resistance welding method at the beginning of welding, at 900 welding hits, and at 2100 welding hits. [Figure 10] 1 is an explanatory diagram showing the change in the electrode resistance value before and after "shaping" the electrode 3 used in the electric resistance welding method of the present invention at the beginning of welding, at 900 welding points, and at 2100 welding points. [Figure 11] FIG. 1 is an explanatory diagram showing the transition of the average resistance value before and after "dressing" of an electrode used in a conventional electric resistance welding method and before and after "shaping" of an electrode used in an electric resistance welding method of the present invention. [Figure 12] 1A and 1B are explanatory diagrams showing the change in the pressure applied area before and after "dressing" an electrode used in a conventional electric resistance welding method, and the change in the pressure applied area before and after "shaping" an electrode used in an electric resistance welding method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] In its best form, the present invention is such that when an electrode having an electrical resistance portion at the center of the contact surface that is pressed against the material to be welded is pressed against the material to be welded and current is applied, the current density at the point corresponding to the outer periphery of the contact surface becomes higher than at the center of the contact surface, causing the materials to overheat and melt together, and then the current density gradually decreases due to the higher electrical resistance that accompanies the higher temperature at the corresponding point on the outer periphery of the contact surface, while the current density at the corresponding point on the center of the contact surface gradually increases, causing the materials to overheat and melt together, and a nugget is grown from the edge of the outer periphery of the contact surface toward the center, making it possible to weld. [Example]

[0017] The electric resistance welding method according to the present invention will be described below with reference to examples. As is well known, electric resistance welding involves applying pressure to at least the surfaces of at least two overlapping workpieces 1, such as zinc-plated steel sheets (electro-galvanized steel sheet, hot-dip galvanized steel sheet, alloyed zinc-plated steel sheet, Dacromet-coated steel sheet, etc.), with an electrode 3 made of a metal material with low electrical resistance such as pure copper or chromium copper, while applying a direct or alternating current to the electrode 3, thereby overheating and melting the workpieces 1 due to Joule heat caused by the contact resistance between them, thereby growing and joining a nugget 5.

[0018] The method of the present invention is characterized by using an electrode 3 having an electrical resistance portion 5 provided at the center of the contact surface 3a with the workpiece 1. When the applied current is DC, the positive electrode is placed on the movable side, and the negative electrode or a grounded conductive base is placed on the fixed side. When the applied current is AC, the polarity of the electrodes is not limited.

[0019] As shown in Figure 1, the electrical resistance portion 5 is composed of an alloy layer formed in the center of the contact surface 3a by the test welding performed when starting electric resistance welding, and a rough surface in which part of the alloy layer is peeled off from the contact surface 3a due to the impact caused by the test welding, forming minute irregularities and reducing the contact area with the workpiece 1, and the electrical resistance portion 5 has a higher electrical resistance than the outer peripheral edge of the electrode contact surface 3a.

[0020] When the workpiece 1 is a galvanized steel plate, an Fe-Zn layer, a γCr-Zn layer, a βCr-Zn layer, and a Cu-Zn layer are formed as alloy layers as shown in Fig. 4, and the alloy layers have higher electrical resistance than the outer peripheral edge of the contact surface 3a where no alloy layers are formed. Also, in the center of the contact surface of the electrode 3 where a rough surface is formed, the contact area with the workpiece 1 is smaller, and the electrical resistance is higher than in the outer peripheral edge.

[0021] When bare steel plates are used as the workpieces 1, an Fe-Cu alloy layer is formed at the center of the contact surface 3a of the electrode 3 during the test strike. This alloy layer is vulnerable to impact, so part of it breaks down and peels off when pressure is applied during the next test strike. However, the remaining Fe-Cu alloy layer and the rough surface formed at the center of the contact surface 3a during the peeling become the electrical resistance part 5.

[0022] Furthermore, as electric resistance welding progresses (the number of welding points increases), the tip of the electrode 3 may swell and deform due to pressure being applied to the workpiece 1, and an alloy layer or rough surface may form on the central and outer edges of the contact surface 3a. This causes electrical resistance, which changes the current density to the workpiece 1 and becomes a factor that hinders the formation of a uniform nugget.

[0023] In such a case, when the number of welding points reaches a predetermined number, the alloy layer formed on the contact surface 3a of the electrode 3 must be removed except for a portion, and the contact surface 3a must be flattened and shaped to resemble an unused electrode, so that the current density at the workpiece 1 is returned to its initial state.

[0024] The above-mentioned "shaping" refers to a technical concept that differs from the conventional "dressing" (see Figure 3) in that the tip of the electrode 3 is cut or polished to give it the shape of an unused electrode, in that a rotating shaping tool (not shown) is pressed against the tip of the electrode 3 as shown in Figure 5, and of the alloy layer formed on the contact surface 3a, the Fe-Zn layer, γCr-ZN layer and the remaining βCr-Zn layer are removed, leaving only the Cu-Zn layer and some of the βCr-Zn layer in the center, while the outer edge of the contact surface 3a is stretched and plastically deformed into the shape of the tip of an unused electrode, thereby regenerating it.

[0025] In conventional "dressing," the amount of cutting of the electrode 3 is 0.1 to 0.3 mm in its axial length, whereas in the above-mentioned "shaping," the amount of cutting of the electrode 3 can be reduced to 0.006 mm in its axial length, thereby extending the life of the electrode.

[0026] The jig (shaping jig) used in the above-mentioned "shaping" work is described in International Patent Publication WO2023 / 139670, which is related to the applicant's application, so a detailed explanation will be omitted, but the summary is as follows.

[0027] "An electrode tip shaping jig for shaping an electrode tip whose tip has been deformed by electric resistance welding and has an alloy layer formed on its tip surface into a regular shape with a tip surface that is either flat or curved with a required curvature and has a curved outer circumferential surface that gradually increases in diameter from the tip surface toward the base end, comprising: a shaping chamber having a cup-shaped concave inner circumferential surface into which the tip of the electrode tip can enter; a notch portion cut out on the outer circumferential side except for a part of the bottom surface of the shaping chamber; at least two or more shaping portions provided on the inner circumferential surface of the shaping chamber corresponding to at least the base end side and the tip end side shaping regions assumed for the tip of the electrode tip, which press against the corresponding shaping regions to push and stretch the deformed portions to the respective sides; and a bottom surface of the shaping chamber extending in a radial direction, which press against the tip surface of the electrode tip to scrape off part of the alloy layer formed on the tip surface and shape the tip surface shaping region on the tip surface into a flat or curved shape. an electrode tip shaping jig comprising a tip surface shaping section that shapes the tip surface into either a curved surface or a composite surface thereof, and a cutting blade that is provided on the cutout surface of the cutout section and that cuts off the excess pushed-out portion at least on the outer periphery of the tip portion of the electrode tip that has entered the shaping chamber; as the electrode tip enters the shaping chamber, the shaping sections are successively pressed against each shaping region expected on the tip portion of the electrode tip to push and shape the deformed portion towards the base end and the tip end; when the tip surface of the electrode tip reaches the bottom of the shaping chamber, the tip surface shaping section that is pressed against the tip surface scrapes off the alloy layer formed on the tip surface so that the alloy layer remains at a required thickness, while pushing and shaping the tip surface into either a flat surface, a curved surface, or a composite surface thereof; and the cutting blade that is pressed against at least the outer periphery on the tip side to cut off the excess pushed-out portion on the outer periphery on the tip side to shape the electrode tip into a regular shape.

[0028] When pressure is applied to at least two overlapping workpieces 1 by an electrode 3 having an electrical resistance portion 5 at the center of the contact surface 3a, the center of the contact surface 3a is pressed against the workpiece 1 via the electrical resistance portion 5, and the outer peripheral edge is pressed directly against the workpiece 1.

[0029] When DC or AC current is applied to the electrode 3 in this state for the required time, in the early stages of welding, because an electrical resistance portion 5 is provided in the center, current flows through the outer periphery of the contact surface 3a that is directly pressed against the workpiece 1, and a high current density flows at the location corresponding to the outer periphery, causing the corresponding location to be overheated and melted by Joule heat as a result of the current flow, as shown in Figure 6. After that, as shown in Figure 7, the location corresponding to the outer periphery becomes hotter, causing the electrical resistance to be higher than in the center and the current density to be lower, while the current density gradually increases from the outer periphery to the center, causing the workpieces 1 to be overheated and melted from the outer periphery toward the center, thereby growing a nugget.

[0030] That is, at the contact points of the electrodes 3 between the workpieces 1, the current density increases from the outer periphery of the contact surface 3a toward the center, causing the workpieces 1 to overheat and melt. Therefore, as shown in Figure 8, there is little variation in the spread at each welding point, and nuggets with a nearly uniform diameter are grown to weld the workpieces 1.

[0031] Comparative Example 1 FIG. 9 shows the change in electrode resistance value before and after "dressing" the electrode at the beginning of welding, at 900 welding points, and at 2100 welding points for an electrode used in a conventional electric resistance welding method.

[0032] FIG. 10 shows the change in electrode resistance value before and after "shaping" the electrode 3 used in the electric resistance welding method of the present invention at the beginning of welding, at 900 welding hits, and at 2100 welding hits.

[0033] For electrodes used in conventional electric resistance welding methods, the resistance value fluctuates greatly before ``dressing'' at 900 and 2100 welding points, whereas for electrodes used in the electric resistance welding method of the present invention, the resistance value fluctuates little before ``shaping'' at 900 and 2100 welding points.

[0034] Furthermore, there was little change in resistance value after "dressing" the electrode used in the conventional method and after "shaping" the electrode 3 used in the method of the present invention.

[0035] Furthermore, with regard to the welding diameter of the electrode, the electrodes used in the conventional electric resistance welding method have a large displacement before "dressing" at each welding point, whereas the electrodes used in the electric resistance welding method of the present invention have a small displacement before "shaping" at each welding point.

[0036] Furthermore, with regard to the electrode area, the electrodes used in the conventional electric resistance welding method have a large displacement before "dressing" at each welding point, whereas the electrodes used in the electric resistance welding method of the present invention have a small displacement before "shaping" at each welding point.

[0037] From the above facts, it can be considered that the electric resistance welding method of the present invention has less variation in resistance value, welding diameter, and electrode area compared to conventional methods, and therefore the current value applied to the material to be welded is stable.

[0038] Comparative Example 2 Figure 11 shows the changes in the average resistance values ​​before and after "dressing" of electrodes used in conventional electric resistance welding methods and before and after "shaping" of electrodes used in the electric resistance welding method of the present invention, and shows that while there is a large change in the resistance value before and after "dressing" of electrodes used in conventional electric resistance welding methods, there is a small change in the resistance value before and after "shaping" of electrodes used in the electric resistance welding method of the present invention.

[0039] From the above facts, it is considered that in the electric resistance welding method of the present invention, the resistance value is stable even before and after the "shaping" of the electrode, compared to conventional electric resistance welding methods, and therefore the current passed through the material to be welded is stable, and the nugget that is grown can be made uniform.

[0040] Comparative Example 3 Figure 12 shows the changes in the pressure area before and after "dressing" of an electrode used in a conventional electric resistance welding method, and the changes in the pressure area before and after "shaping" of an electrode used in the electric resistance welding method of the present invention. It shows that while there is a large variation in the pressure area before and after "dressing" of an electrode used in a conventional electric resistance welding method, there is little variation in the pressure area before and after "shaping" of an electrode used in the electric resistance welding method of the present invention.

[0041] From the above facts, it is considered that in the electric resistance welding method of the present invention, compared to conventional electric resistance welding methods, the pressure area remains almost constant even before and after the ``shaping'' of the electrode, and therefore the pressure applied to the material to be welded is stable, resulting in the growth of a uniform nugget.

[0042] The weld strength (nugget strength) achieved by electric resistance welding must be controlled by the applied current value; a low current value will not provide sufficient weld strength, while a high current value will generate weld spatter, reducing the quality of the weld and the efficiency of the welding work. Therefore, the applied current value must be determined based on the required weld strength (4√t, where t is the thickness of the material to be welded). However, current value control technology is well known, and detailed explanations will be omitted. [Explanation of symbols]

[0043] 1. Welding material 3 electrodes 3a Contact surface 5 Electrical resistance section

Claims

1. In an electric resistance welding method in which an electrode is pressed against at least two overlapping workpieces, and a current is applied to overheat and melt the workpieces together while growing a nugget, When an electrode having an electrical resistance portion at the center of the contact surface that is pressed against the workpiece is pressed against the workpiece and an electric current is applied, the current density at the point corresponding to the outer periphery of the contact surface becomes higher than that at the center of the contact surface, causing the workpieces to overheat and melt together, and then the current density gradually decreases due to the high electrical resistance caused by the increase in temperature at the corresponding point on the outer periphery of the contact surface, while the current density at the corresponding point on the center of the contact surface gradually increases, causing the workpieces to overheat and melt together, and a nugget is grown from the edge of the outer periphery of the contact surface toward the center, making it possible to weld.

1. An electric resistance welding method comprising:

2. In claim 1, An electric resistance welding method in which the material to be welded is either galvanized steel plate or bare steel plate.

3. In claim 1, An electric resistance welding method in which, when the material to be welded is a zinc-coated steel plate and the electrode is a chromium-copper alloy, the electrical resistance part provided in the center of the electrode contact surface is at least one of a Cr-Zn alloy layer and a roughened surface.

4. In claim 1, An electric resistance welding method in which the electrode is shaped into a regular shape using a shaping jig after the electric resistance welding trial and / or when the electric resistance welding has reached a predetermined number of welding points, with a portion of the electric resistance part remaining in the center of the electrode contact surface.

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