Spot welding method and welding material

Abstract

The primary goal is to enable faster spot welding. [Solution] This invention relates to a spot welding method in which two metal base materials 1 and 2 are overlapped and sandwiched between electrodes 3 and 4, and while pressure 5 is applied between electrodes 3 and 4, an electric current 6 is passed through electrodes 3 and 4, causing the clamped portion 8 of the base materials 1 and 2 to melt and join due to the heat generated by the electrical resistance of the base materials 1 and 2. A localized undulation 12 having multiple protrusions 11 is provided on one of the superimposed base materials 1 and 2. With the protruding portion 11 in contact with the other base material 1, 2 at multiple points, the undulating portion 12 is joined by sandwiching it between electrodes 3, 4.

Description

Technical Field

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[0001] This invention relates to a spot welding method and a welding material.

Background Art

[0002] Spot welding is a welding method in which metal base materials are overlapped and sandwiched between electrodes, and while applying pressure with the electrodes, an electric current is passed through the electrodes, and the sandwiched portion of the base materials by the electrodes is melted and joined by heat generated due to the electrical resistance of the base materials.

[0003] Spot welding is performed, for example, using a spot welding apparatus as in Patent Document 1. Note that Patent Document 1 is an example for specifically showing spot welding, and the object and field for performing spot welding and the configuration of the apparatus for performing spot welding are not limited to the content of Patent Document 1 and may be widely general.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] ​​​​For example, spot welding can be sped up by increasing the electrode current to increase the amount of heat generated in the base material, thereby shortening the welding time. However, increasing the electrode current reduces the cooling effect of the cooling water circulating inside the electrode. As a result, electrodes with reduced cooling effect wear down more rapidly at the tip, shortening their lifespan. Therefore, electrodes need to be replaced frequently. Electrode replacement requires stopping the spot welding equipment, which reduces overall production efficiency. For this reason, there are limits to how much the current can be increased, and speeds beyond these limits cannot be achieved.

[0007] Therefore, the main objective of this invention is to contribute to increasing the speed of spot welding. [Means for solving the problem]

[0008] In response to the above problems, the present invention provides: A spot welding method is used in which metal base materials are stacked and sandwiched between electrodes, and while pressure is applied to the electrodes, an electric current is passed through the electrodes, thereby melting and joining the sandwiched portion of the base materials due to the heat generated by the electrical resistance of the base materials, The present invention is characterized by providing a localized undulation with multiple protrusions on one of the superimposed base materials, and joining the undulation by sandwiching the position of the undulation between electrodes while the protrusions are in contact with the other base material at multiple locations. [Effects of the Invention]

[0009] The present invention, with the above configuration, makes it possible to increase the speed of spot welding, among other things. [Brief explanation of the drawing]

[0010] [Figure 1] (a) is a side view showing the spot welding method and welding material according to this embodiment, and (b) is a partially enlarged view of (a). [Figure 2] This is a perspective view of a base material with partially uneven surfaces. [Figure 3A]This is a side view showing examples of knurling patterns. Of these, (a) is a flat pattern and (b) is a diagonal pattern. [Figure 3B] This is a plan view showing the raised areas formed by knurling. Of these, (a) is a flat pattern, (b) is a diagonal pattern, and (c) is a modified example of the raised area. [Figure 4] This is a perspective view showing a base material (test specimen) joined by spot welding. [Figure 5] This is a perspective view showing a tensile test being performed on the joined base materials. [Figure 6] This graph shows the results of a tensile test, with the horizontal axis representing the energizing time (cycle) and the vertical axis representing the welding strength (load (kN)). [Figure 7] These are side views illustrating a standard spot welding method for a comparative example. (a) shows the pressurized state before energization, (b) shows the state at the start of energization, (c) shows the state during energization, and (d) shows the state after energization has been stopped. [Figure 8] These are side views illustrating the projection welding method for the comparative example. (a) shows the pressurized state before energization, and (b) shows the state at the start of energization. [Modes for carrying out the invention]

[0011] This embodiment will be described in detail with reference to Figures 1 to 8. Of these, Figures 1 to 6 are examples, and Figures 7 and 8 are comparative examples. [Examples]

[0012] <Configuration>This embodiment has the following configuration.

[0013] Ordinary spot welding is as shown in the comparative example of Fig. 7. That is, spot welding is a welding method in which metal base materials 1 and 2 are overlapped and sandwiched by electrodes 3 and 4 (clamping process), pressure 5 is applied by electrodes 3 and 4 (pressurizing process, as shown in Fig. 7(a) above), and current 6 is passed through electrodes 3 and 4 (energizing process, Fig. 7(b)). As a result, the base materials 1 and 2 are melted at the clamped portion 8 sandwiched by the electrodes 3 and 4 (melting part 9a, Fig. 7(c)) due to heat generation by the electrical resistance of the base materials 1 and 2 (heat generation center 7a, heat generation part 7b), and are joined (welded part 9b, Fig. 7(d)).

[0014] Here, spot welding can be performed as long as the base materials 1 and 2 to be joined can be sandwiched by the electrodes 3 and 4. Therefore, for example, compared with arc welding and the like, it is less affected by the accuracy of the base materials 1 and 2 and is an easily automated welding method.

[0015] The base materials 1 and 2 are metal members (welding materials) that are the objects (welding targets) for welding. In the figure, the flat base materials 1 and 2 are overlapped and welded. The two flat base materials 1 and 2 are joined in a state where their opposing ends are overlapped and extend in opposite directions. The base materials 1 and 2 can be of any material and can be used in any field or application.

[0016] The electrodes 3 and 4 are energizing terminals for passing current 6 through the base materials 1 and 2 during spot welding, and their tips are contacted with the base materials 1 and 2. The electrodes 3 and 4 are also used for pressurizing the base materials 1 and 2. Electrodes 3 and 4 (spot welding electrodes) for performing spot welding are commercially available in various types and can be of any type. Since the electrodes 3 and 4 wear out during use, they need to be replaced. In order to improve productivity, it is better to have fewer replacements of the electrodes 3 and 4.

[0017] In the figure, a pair of rod-shaped or columnar electrodes 3 and 4 are arranged on both sides of the overlapped base materials 1 and 2 (the upper and lower non-opposing surfaces in the figure). The pair of electrodes 3 and 4 are arranged so as to be substantially aligned in a straight line with respect to the substantially perpendicular direction of the base materials 1 and 2, and thus face each other across the same position of the base materials 1 and 2. In the figure, the tips of the electrodes 3 and 4 are curved, but the tip shape of the electrodes 3 and 4 is not limited to this.

[0018] The pressure 5 is the force (pressing force by the electrodes 3 and 4) for the pair of electrodes 3 and 4 to clamp the overlapped portion of the base materials 1 and 2. By making the overlapped portion of the base materials 1 and 2 adhere closely, the pressure 5 serves to prevent the generation of burrs, supply the current 6 from the electrodes 3 and 4 to the base materials 1 and 2, and ensure and improve the mechanical properties such as the welding strength of the welded portion 9b after energization.

[0019] The current 6 flows from the electrodes 3 and 4 to the base materials 1 and 2, causing heat generation in the base materials 1 and 2 due to the electrical resistance of the base materials 1 and 2. When the current 6 is increased to increase the heat generation amount of the base materials 1 and 2, the spot welding speeds up with a shorter energization time. However, accordingly, the electrodes 3 and 4 are consumed more severely and their lifespan becomes shorter.

[0020] Since there is a correlation among the pressure 5, the current 6, and the energization time, it is important to set these parameters in a well-balanced manner based on long-term experience and achievements for improving productivity by achieving both high speed and long lifespan of the electrodes 3 and 4.

[0021] The clamping portion 8 is a local portion where the overlapped portion of the base materials 1 and 2 is clamped by the electrodes 3 and 4. The clamping portion 8 becomes a portion having approximately the size and shape of the cross-section of the electrodes 3 and 4 (or the tips of the electrodes 3 and 4). In the case of the columnar electrodes 3 and 4, the clamping portion 8 is approximately circular. The melted portion 9a formed in the clamping portion 8 solidifies after spot welding (after energization stops) to become the welded portion 9b.

[0022] Electrical resistance is the degree to which the flow of electricity is hindered in the base materials 1 and 2. Heat generation is the rise in temperature of the base materials 1 and 2 due to their resistance when electricity is passed through them. Due to the heat generation, the clamping portion 8 of electrodes 3 and 4 of the superimposed base materials 1 and 2 melts, forming a molten portion 9a, where their structures mix and fuse together, eliminating the boundary and joining them as a single unit.

[0023] In addition to the basic configuration described above, this embodiment may also have the following configuration.

[0024] (1) The spot welding method of this embodiment is as shown in Figure 1 above, A localized undulation 12 having multiple protrusions 11 is provided on one of the overlapping base materials 1 and 2. The undulation 12 is then joined by sandwiching it between electrodes 3 and 4 (or forming a clamping portion 8) while the protrusions 11 are in contact with the other base material 1 or 2 at multiple points. This results in the undulation 12 being spot-welded.

[0025] Here, the protrusions 11 are mountain-shaped portions that project out of the plane from the surface of the base materials 1 and 2. Multiple protrusions 11 are formed on the surface of one of the base materials 1 and 2 (for example, base material 1) opposite to the electrodes 3 and 4 (for example, electrode 3) (for example, the surface 13 facing the other base material 2), so as to protrude toward the other base material 2 and 1. The multiple protrusions 11 are all of almost the same height. The undulations 12 may also be formed toward the other base material 2 and 1 (so as to protrude toward the base material 1 and 2). There may be two or more or three or more protrusions 11, and there is no upper limit to the number as long as they can be formed on the clamping portion 8 of the base materials 1 and 2. A recess 14 is formed between the multiple protrusions 11.

[0026] The undulating portion 12 is a region in one of the base materials 1, 2 where multiple protrusions 11 are formed together. The undulating portion 12 is formed to a size such that no protrusions 11 remain after spot welding. The undulating portion 12 is formed to a size such that it has, for example, several to several dozen (preferably four to fifty) protrusions 11.

[0027] As shown in Figure 2, the raised portions 12 are formed in one or more locations on one of the base materials 1 and 2, at the locations where spot welding is performed, so as to be approximately the same size and shape as the welded portion 9b formed by the spot welding. The multiple raised portions 12 are formed with the required spacing between them so that the necessary welding strength is obtained overall. In the figure, the raised portions 12 are formed in three locations: near both sides in the width direction near the ends of the base materials 1 and 2 and in the center, but the locations where the raised portions 12 are formed are not limited to these.

[0028] The location where spot welding is performed is the clamping portion 8 by electrodes 3 and 4. The size and shape of the welded portion 9b will be approximately the same as the cross-section (outer diameter) of electrodes 3 and 4 (or the tips of electrodes 3 and 4). For example, the undulating portion 12 will be a localized circular area. Multiple protrusions 11 will be formed inside this localized circular area. The undulating portion 12 will be, for example, a circle with a diameter of several millimeters to several tens of millimeters.

[0029] Multiple locations are the positions (contact areas) of contact points or contact lines formed when the tips of multiple protrusions 11 simultaneously contact the other base material 2,1. To make the contact points or contact lines smaller, it is preferable to sharpen the tips of the protrusions 11. To obtain ideal point contact or line contact, it is preferable to sharpen the tips of the protrusions 11 by making them acute-angled, but the tips may also be rounded or left as flat surfaces of a small area (in this case, it will be a surface contact of a small area that can be considered substantially as point contact or line contact).

[0030] Before welding, when the base materials 1 and 2 are stacked on top of each other, the contact of the tips of the multiple protrusions 11 causes the base materials 1 and 2 to be in a non-contact state, with a small gap between them approximately the height of the protrusions 11, except for the parts with raised sections 12. Then, during the welding process, the raised sections 12 change from the molten section 9a to the welded section 9b, and the protrusions 11 melt away, so the base materials 1 and 2 become in contact with each other as a whole. The raised sections 12 contribute to the formation of the molten section 9a, resulting in a welded section 9b that is in the same state as if there were no raised sections 12.

[0031] (2) The welding material used in the above spot welding method is Before being superimposed, one of the base materials 1 and 2 has a localized undulation 12 with multiple protrusions 11 on the surface 13 facing the other base material 2 and 1. During joining, the overlapping base materials 1 and 2 are clamped between electrodes 3 and 4 at the positions of the undulating portion 12, and the multiple protrusions 11 are in contact with the other base material 2 and 1 at multiple points.

[0032] Here, it is preferable that the multiple protrusions 11 are all the same size and shape. Furthermore, it is preferable that the multiple protrusions 11 are formed almost uniformly over the entire area of ​​the undulating portion 12 and make almost uniform contact with the portion of the base material 2,1 that faces the other undulating portion 12. However, if the planar shape of the undulating portion 12 is circular or the like, the protrusions 11 located on the periphery of the undulating portion 12 may differ slightly in size and shape from the others.

[0033] (3) In the spot welding method and welding material described above, as shown in Figure 3A, The undulating portion 12 may have a plurality of the above-mentioned protrusions 11 locally formed on one of the base materials 1 and 2 by knurling (knurled portion 21).

[0034] Here, knurling is a process that creates a raised and recessed pattern on the surface of metal for purposes such as anti-slip or decorative purposes. Through knurling, the metal surface is given a pattern of cuts such as a flat pattern 22 (Figure 3A(a)) or a diagonal pattern 23 (Figure 3A(b)). The flat pattern 22 is a straight, linear cut pattern that makes line contact. The diagonal pattern 23 is a diamond-shaped cut pattern that makes point contact. In the case of the diagonal pattern 23, each protrusion 11 is in the shape of a rhombus-shaped square pyramid. Both the flat pattern 22 and the diagonal pattern 23 are regular patterns. Therefore, the raised portion 12 formed on the base material 2,1, when viewed in plan, is a nearly circular region in which multiple protrusions 11 formed by the flat pattern 22 or diagonal pattern 23 are arranged regularly at a certain interval from one another, as shown in Figures 3B(a) and (b). Note that Figure 3A is an example of a knurled annular member (side view) and is unrelated to spot welding.

[0035] There are two types of knurling processes: cutting and rolling. Cutting knurling is a process in which a cutting knurling tool is pressed firmly against the base material 1, 2 to cut a pattern. Rolling knurling is a process in which a rolling knurling tool is pressed firmly against the base material 1, 2 to transfer a pattern. Of these, rolling knurling is suitable for forming localized undulations 12 on the surface of the base material 1, 2 because it allows for localized processing. Rolling knurling creates convex portions 11 that protrude from the surface of the base material 1, 2, and concave portions 14 that are recessed from the surface of the base material 1, 2.

[0036] Then, by knurling, localized notches are formed on the base materials 1 and 2, thereby creating raised areas 12 on the base materials 1 and 2. The shape, size, height, and number of each protrusion 11 of the raised areas 12 formed by knurling can be set in various ways by selecting tools such as cutting knurling or rolling knurling. In knurling, the shape, size, and number of the protrusions 11 are selected within a range where the height is greater than 0 mm and 0.5 mm or less.

[0037] Figure 3B shows the raised portion 12 formed on the base materials 1 and 2 by knurling, where (a) the raised portion 11 forms a flat pattern 22, and (b) the raised portion 11 forms a diagonal pattern 23; either is acceptable. By making the raised portion 11 of the raised portion 12 a flat pattern 22, the processing load on the knurling machine can be reduced compared to a diagonal pattern 23.

[0038] The raised portion 12 may also be formed on the base materials 1 and 2 by a processing method other than knurling.

[0039] (4) In the spot welding method and welding material described above, as shown in Figure 1, The base materials 1 and 2 may also be plated steel sheets 33, which are steel sheets 31 with a plating 32 applied to their surface. The plated steel sheet 33 may be configured such that the area around at least the top 34 of the multiple protrusions 11 of the uneven portion 12 has less plating 32 attached compared to the area other than the uneven portion 12 (or it may be a plated area 35).

[0040] Here, the steel plate 31 is steel that has been processed into a plate shape. The material and type of steel used in the base materials 1 and 2 can be anything.

[0041] Plating 32 is a process that forms a thin metallic film on the surface of metals or nonmetals. The material and type of plating 32 can be anything relative to the base materials 1 and 2. For example, plating 32 can be made of a zinc-based material or something with high rust-preventive properties.

[0042] The plated steel sheet 33 is a steel sheet 31 with a plating 32 applied to almost the entire surface and edges. The plating 32 is formed on the surface and edges of the steel sheet 31 with a nearly uniform thickness. The base materials 1 and 2 may be plated steel sheets 33 of the same type or different types.

[0043] The portion other than the undulating portion 12 is the rest of the surface and end face of the steel plate 31 that does not have the undulating portion 12 (general portion). The rest portion has a layer of plating 32 of substantially uniform thickness.

[0044] The area around the top 34 of the protrusion 11 is the tip of the protrusion 11 and its surrounding area.

[0045] The amount of plating is the amount of plating 32 adhering to the surface of the plated steel sheet 33 during spot welding. The plating reduction area 35 is formed on the surface of the plated steel sheet 33 before or when forming the uneven area 12. For example, the base materials 1 and 2 are processed to completely remove the plating 32 from the uneven area 12 or to remove a portion of the plating 32 from the uneven area 12, thereby reducing the amount of plating adhering to the entire uneven area 12, including the recessed area 14, or at least around the top 34 of the convex area 11.

[0046] Furthermore, when knurling is performed to form the raised portion 12 of the plated steel sheet 33, a physical force is applied to the raised portion 12, which reduces the amount of plating 32 adhering to the raised portion 12. Therefore, the plated portion 12 may be formed by reducing the plating 32 only through knurling, without removing the plating 32 in pre-processing, thereby creating a plated portion 35.

[0047] "Less" means reducing the amount of plating 32 that adheres, and includes eliminating the amount of plating altogether. In the plated area 35, the amount of plating 32 is less than in the general area, or there is almost no plating 32.

[0048] Furthermore, if necessary, the plating reduction portion 35 may also be formed in the region of the other base material 2,1 that faces the undulating portion 12 of the clamping portion 8 (or in the portion where multiple protrusions 11 come into contact).

[0049] (5) In the spot welding method and welding material described above, as shown in the modified example in Figure 3B(c), The localized undulations 12 may be formed in a ring shape without a protrusion 11 in the central part 42 (or they may be formed as ring-shaped undulations 41).

[0050] Here, forming the undulation portion 12 in a ring shape means that the undulation portion 12 has a shape having two regions: a flat central part 42 (flat part) and a ring portion 43 surrounding the central part 42. Therefore, the ring-shaped undulation portion 12 is different from the circular undulation portion 12 shown in Figure 3B(b), which has multiple protrusions 11 formed on the entire circular region. The central part 42 is a flat circular region without protrusions 11. The ring portion 43 is an annular region of a certain width on which multiple protrusions 11 are formed. The central part 42 and the ring portion 43 are arranged and formed concentrically. It is preferable that the diameter of the central part 42 is smaller than the outer diameter of the circular undulation portion 12, or the outer diameter of the electrodes 3, 4 (or the outer diameter of the tips of the electrodes 3, 4). It is preferable that the area of ​​the ring portion 43 is equal to the area of ​​the circular undulation portion 12.

[0051] <Effect> The effect of this embodiment is as follows:

[0052] Conventional spot welding is performed as shown in the comparative example in Figure 7. First, two metal base materials 1 and 2 are placed on top of each other, and the overlapping portion of the two base materials 1 and 2 is sandwiched between a pair of electrodes 3 and 4 (clamping step). Next, pressure 5 is applied to the overlapping portion of the base materials 1 and 2 with electrodes 3 and 4 (pressurization step, as shown in Figure 7(a)). Then, while applying pressure 5 with electrodes 3 and 4, an electric current 6 is passed through electrodes 3 and 4 to spot weld the sandwiched portion 8 of the base materials 1 and 2 (current application step, Figure 7(b)). As a result, the sandwiched portion 8 of the two base materials 1 and 2 melts due to the heat generated by electrical resistance, becoming a molten portion 9a, and the structures of the two materials mix and fuse together in the molten portion 9a, eliminating the boundary (Figure 7(c)). After a predetermined current application time has elapsed, the current is stopped, the molten portion 9a solidifies into a welded portion 9b, and the two base materials 1 and 2 are joined together (Figure 7(d)).

[0053] In this process, pressure 5, current 6, and energizing time are correlated. Therefore, based on long-term experience and proven results, spot welding is performed by setting these parameters in a balanced manner so that both high speed and extended lifespan of electrodes 3 and 4 can be achieved, thereby improving productivity. As a result, these parameters have already been optimized to almost their maximum potential, and there is currently little room to improve production efficiency through parameter adjustments.

[0054] Therefore, in order to overcome the current limitations of productivity improvement, this embodiment, as shown in Figure 1, provides a localized undulation 12 with multiple protrusions 11 on one of the superimposed base materials 1 and 2. Then, with the protrusions 11 of the undulation 12 in contact with the other base material 1 and 2 at multiple points, the undulation 12 is clamped between electrodes 3 and 4 and pressurized, and a current 6 is passed through to spot weld the base materials 1 and 2.

[0055] As a result, multiple protrusions 11 on one of the base materials 1 and 2 become multiple contact points (contact points or contact lines) with the other base material 1 and 2. Current 6 flows through these multiple contact points, causing the base materials 1 and 2 to melt simultaneously at these points and connect, resulting in rapid and smooth melting of the base materials 1 and 2. Therefore, it is possible to shorten the energizing time compared to the parameters of normal spot welding.

[0056] The base materials 1 and 2, which are joined by spot welding, may also be, for example, components of a building unit used in the unit construction method. The unit construction method is a method of constructing a building (unit building) in a short period of time by transporting rectangular building units, which are manufactured in advance in a factory, to the construction site and assembling them there. The rectangular building unit has a metal unit frame inside which forms the skeletal structure. The unit frame is a three-dimensional frame in the form of a box-frame structure, with the upper ends of four columns connected by four ceiling beams and the lower ends of four columns connected by four floor beams. The columns have connecting fittings called joint pieces integrally attached to their upper and lower ends. The joint pieces are joined to the ceiling beams and to the floor beams by spot welding. In this case, it is preferable to provide the undulating portion 12 on the side of the joint piece.

[0057] Furthermore, the base materials 1 and 2 to be spot-welded are not limited to building unit components. Spot welding is widely used in all fields of architecture and other industries, and the spot welding in this embodiment has no limitations on its application field.

[0058] The effectiveness of spot welding in this embodiment was confirmed by the following experiment.

[0059] The experiment used plated steel plates 33 as base materials 1 and 2. These plates consisted of two structural steel plates 31 (structural steel plates) with thicknesses of 4.0 mm and 6.0 mm, each having a tensile strength equivalent to JIS standard SS400, and each plated with a layer of hot-dip zinc-aluminum-magnesium alloy plating 32.

[0060] Then, the base material 1 (the thinner of the two sheets), which was 4.0 mm thick, had a knurled pattern 23 formed on a portion of its surface by rolling to create a raised section 12. The raised section 12 was formed on the surface 13 opposite to the base material 2, which was 6.0 mm thick. Due to the knurling process, the raised section 12 became a plated area 35.

[0061] The knurled surface 12 has a raised portion 11 with a height of 0.5 mm, which is the highest within the JIS standard. However, the height of the raised portion 11 is not limited to 0.5 mm.

[0062] Spot welding was performed with a nearly constant pressure of 5 and a fixed current of 6 (current value kA) (e.g., 18kA), while varying the energizing time (cycle: 1 second = 50 cycles). The change in weld strength (kN) during this process was confirmed by shear testing.

[0063] Furthermore, when spot welding is performed at multiple locations on base materials 1 and 2, the weld strength decreases from the second spot onward due to the effect of flow diversion to the first spot, etc. Therefore, shear tests were performed on the second spot onward, excluding the first spot where the weld strength is strongest.

[0064] Specifically, as shown in Figure 4, the base materials 1 and 2, which served as test specimens, were spot-welded at two locations: the end of the width and the center of the width. Then, the first welded section 9b at the end of the width was cut out to form a hole X. The shear test was performed on the remaining second welded section 9b(Y) at the center of the width using a shear testing machine M as shown in Figure 5.

[0065] The shear test was performed using unprocessed base materials 1 and 2 without the formation of the undulation 12, and base materials 1 and 2 with the undulation 12 formed. The unprocessed base materials 1 and 2 were spot-welded for the usual energizing time (e.g., 170 cycles). The base materials 1 and 2 with the undulation 12 were spot-welded while varying the energizing time (e.g., 169 cycles, 150 cycles, 130 cycles) to be shorter than the normal energizing time of the unprocessed base materials 1 and 2. Each of the above base materials 1 and 2 was spot-welded a total of five times for each energizing time.

[0066] The shear test results were evaluated using the mean -3σ (σ: standard deviation). The mean -3σ represents a normal distribution, meaning that 97% of the results fall between the mean -3σ and mean +3σ. The mean -3σ value represents the lower limit of the weld strength. Therefore, a higher mean -3σ value indicates higher weld strength.

[0067] The results of the shear test were as follows:

[0068] Base material specifications, current application time, average intensity, standard deviation, average -3σ Unprocessed 170 63.6 2.07 57.3 Knurling A 169 63.6 1.26 59.8 Knurling B 150 64.5 1.11 61.2 Knurling C 130 58.9 0.68 56.9 (In all cases, current value = 18kA (constant), pressure 5: constant)

[0069] Figure 6 is a graph interpolated by plotting the results of the shear test with straight lines. It was confirmed that the specimens with knurled ridges 12 (knurled A and B) could achieve welding strength (average value) equivalent to or greater than that of the unprocessed specimens, even when the energizing time was shortened to a certain extent (170 cycles → 150 cycles) (unprocessed and knurled A: 63.6kN, knurled B: 64.5kN). Furthermore, it was confirmed that the specimen with knurled ridges 12 (knurled C) would have welding strength (average value) lower than that of the unprocessed specimens when the energizing time was shortened beyond a certain point (130 cycles) (58.9kN).

[0070] Furthermore, the lower limit of the weld strength (average -3σ) was the same as described above. Specifically, knurled material A achieved 59.8kN, knurled material B achieved 61.2kN, both higher than the untreated material's 57.3kN, while knurled material C achieved 56.9kN, lower than the untreated material.

[0071] The graph shows that the welding strength (average and lower limit) becomes equivalent to that of the unprocessed material around 134 to 135 cycles. It can be reasonably estimated that for the midpoint of each energizing time mentioned above, the average and lower limit values ​​of the welding strength, which are continuous (approximately interpolated with a straight line) with each energizing time, can be obtained.

[0072] In summary, the above experiment showed that, under the above conditions, spot welding with raised sections 12 formed on base materials 1 and 2 allowed for a reduction in energizing time within the following range, and that the highest weld strength was achieved around 150 cycles, which is in the middle of the range. Specifically, the energizing time could be shortened within a range that was longer than 130 cycles (e.g., 135 cycles or more) and shorter than 170 cycles (e.g., 169 cycles or less).

[0073] In this case, the average welding strength within the above energizing time range is in the range of 63.6kN to 64.5kN, and the lower limit of the welding strength is in the range of 57.3kN to 61.2kN. In particular, from the above graph, it was confirmed that setting the energizing time within the range of 145 cycles to 164 cycles is preferable because the lower limit of the welding strength is approximately greater than 60.0kN, and it is even more preferable to set the energizing time within the range of 146 cycles to 160 cycles.

[0074] Furthermore, all of the materials with the raised section 12 formed had a smaller standard deviation than the unprocessed material (unprocessed: 2.07, knurled A: 1.26, knurled B: 1.11, knurled C: 0.68), confirming that the variation in welding strength was less and the welding quality was more stable compared to the unprocessed material.

[0075] This is thought to be because forming the raised section 12 on the test specimen before spot welding causes the melting phenomenon of the base materials 1 and 2 to occur simultaneously at multiple contact points, resulting in smoother melting and thus a shorter energizing time. Furthermore, it is believed that smoother melting also reduces variations in weld strength. In addition, the test specimen after spot welding showed no significant visual changes compared to the specimen without the raised section 12, and the appearance quality was also good.

[0076] <Effects> The effects of this embodiment are as follows:

[0077] (Effect 1) In this embodiment, the spot welding method may be performed by providing a localized undulation 12 on one of the base materials 1 and 2, and joining the undulation 12 by sandwiching it between electrodes 3 and 4 (forming a clamping portion 8) while the multiple protrusions 11 of the undulation 12 are in contact with the other base material 1 and 2 at multiple locations. As a result, the contact points between the base materials 1 and 2 are distributed to multiple locations, so that the current density 6 in the welding area becomes higher than in a normal spot welding (without the undulation 12) with full contact between flat surfaces, as shown in Figure 7. Therefore, the welding time can be shortened in spot welding.

[0078] Furthermore, as described above, joining by contact between the multiple protrusions 11 of the undulating portion 12 results in a higher density of current 6 during energization compared to normal spot welding, which makes it possible to reduce the value of the current 6 applied to electrodes 3 and 4. By reducing the value of the current 6, sufficient cooling effect from the cooling water circulating inside the electrodes 3 and 4 is ensured, thereby suppressing wear on electrodes 3 and 4 and extending their lifespan. Extending the lifespan of electrodes 3 and 4 reduces the frequency of replacement.

[0079] Therefore, by providing a localized undulation 12 on one of the base materials 1 and 2, spot welding can be performed at a higher speed, and productivity can be improved by extending the lifespan of the electrodes 3 and 4, thus exceeding previous limitations.

[0080] On the other hand, existing spot welding methods include projection welding, as shown in Figure 8. In this method, a partially spherical projection T (projection) is provided on one of the clamping portions 8 of the base materials 1 and 2, which is approximately the same size as or slightly larger than the clamping portion 8. The joint is then made by bringing the projection T into contact with the other base material 1 or 2 at a single point.

[0081] However, in projection welding, the contact point is concentrated in one location, which can lead to an excessively high current density (6). Increasing the current to speed up the process may cause the molten portion (9a) of the base materials (1,2) to be blown away by the heat generated during joining, resulting in spatter. Spatter reduces the weld strength and quality. Furthermore, the cooling effect of the cooling water becomes insufficient due to the heat generated during joining. Therefore, there are limits to how much the welding time can be shortened by increasing the current (6).

[0082] In contrast, in this embodiment, spot welding using multiple contact points due to the undulations 12 does not concentrate the contact points at a single point, thus preventing the current density 6 from becoming too high, compared to projection welding. Therefore, spot welding in this embodiment can suppress or prevent the phenomenon that is likely to occur in projection welding, where the molten portion 9a of the base materials 1 and 2 is blown away by the heat during joining, generating spatter. As a result, this embodiment can prevent or suppress the reduction in welding strength due to spatter, and thus improve welding strength and welding quality. Consequently, the welding time can be shortened without increasing or even while lowering the value of the current 6. Furthermore, since the cooling effect of the cooling water is ensured against the heat generated during joining, the electrodes 3 and 4 can be made to have a longer lifespan. For these reasons, spot welding in this embodiment is completely different from projection welding.

[0083] (Effect 2) In the welding material used in the spot welding method of this embodiment, one of the base materials 1 and 2 has a localized undulation 12 with multiple protrusions 11 on the surface 13 facing the other base material 2 and 1 before they are joined together. When joining, the overlapping base materials 1 and 2 are clamped between electrodes 3 and 4 at the positions of the undulation 12, and the multiple protrusions 11 come into contact with the other base material 2 and 1 at multiple points. As a result, the same effects as (Effect 1) above can be obtained.

[0084] (Effect 3) In the spot welding method and welding material of this embodiment, the raised portion 12 may be a plurality of protrusions 11 locally formed on one of the base materials 1 and 2 by knurling. This makes it easy to form the raised portion 12 on one of the base materials 1 and 2, as existing knurling equipment and technology can be used as is, and facilitates mass production of base materials 1 and 2 having the raised portion 12. The raised portion 12 may be provided on either side of the two base materials 1 and 2, but if the two base materials 1 and 2 are different in size, it is preferable to provide it on the side of the smaller member (smaller member or member with thinner plate thickness) as this makes processing easier.

[0085] Furthermore, knurling allows for relatively arbitrary selection of the processing area, size, and shape of the irregularities within the scope of existing knurling equipment, enabling the formation of uneven surfaces 12 that are of a size and shape suitable for spot welding. Additionally, since knurling is a process that generates little to no dust, it can prevent or suppress the reduction in welding strength caused by dust during spot welding.

[0086] (Effect 4) In the spot welding method and welding material of this embodiment, plated steel sheet 33 may be used as the base materials 1 and 2. This allows the base materials 1 and 2 to be rust-prevented by the surface plating 32. The plated steel sheet 33 may have a plate reduction area 35 where the amount of plating 32 is reduced around at least the tops 34 of the multiple protrusions 11 of the uneven portion 12. By reducing the amount of plating 32 around the tops 34 of the protrusions 11, the (low melting point) plating 32 evaporates rapidly due to the heat during spot welding, and the evaporated plating 32 blows away the molten portion 9a of the base materials 1 and 2, thereby suppressing or preventing the generation of dust. Therefore, by reducing the generation of dust due to the evaporation of plating 32 during spot welding, the decrease in welding strength due to dust can be prevented or suppressed.

[0087] (Effect 5) In the spot welding method and welding material of this embodiment, the localized undulation 12 may be formed in a ring shape having a flat portion in the center 42 without any protrusions 11. In a circular undulation 12 with numerous protrusions 11, the molten portion 9a tends to grow from the center to the outer circumference during welding. Therefore, in a ring-shaped undulation 12, the molten portion 9a grows from the inner circumference to the outer circumference of the ring portion 43, and at the same time, the heat of the ring portion 43 causes the molten portion 9a to spread to the flat portion in the center 42 that does not have any protrusions 11. In this way, the outer ring portion 43 melts before (preferentially) the center 42, which increases the area of ​​the formed molten portion 9a and welded portion 9b and increases the welding strength. In this case, if the area of ​​the ring portion 42 is made equal to the area of ​​the circular undulation portion 12, the processing load on the processing machine (for example, a knurling machine) that processes the undulation portion 12 can be kept to the same extent. [Explanation of Symbols]

[0088] 1 Base material 2 Base material 3 electrodes 4 electrodes 5. Pressure 6 current 7b Heating element 8 Clamping part 11 Convex part 12 Relief 13 Opposing surfaces 21 Knurled section 31 Steel plate 32 Plating 33 Plated steel sheet 34 Top 35 Plating reduction area 41 Ring-shaped undulation 42 Center 43 Ring section

Claims

1. A spot welding method is used in which metal base materials are stacked and sandwiched between electrodes, and while pressure is applied to the electrodes, an electric current is passed through the electrodes, thereby melting and joining the sandwiched portion of the base materials due to the heat generated by the electrical resistance of the base materials, A spot welding method characterized by providing a localized undulation having a plurality of protrusions on one of the superimposed base materials, and joining the base materials by sandwiching the position of the undulation with the electrodes while the protrusions are in contact with the other base material at multiple locations.

2. A welding material used in a spot welding method in which metal base materials are stacked and sandwiched between electrodes, and while pressure is applied to the electrodes, an electric current is passed through the electrodes, thereby melting and joining the sandwiched portion of the base materials by the heat generated due to the electrical resistance of the base materials, One of the base materials, before being superimposed, has a localized undulation with multiple protrusions on the surface facing the other base material. A welding material characterized in that, during joining, the overlapping base materials are clamped at the positions of the undulating portions by the electrodes, and the multiple protrusions are in contact with the other base material at multiple locations.

3. The welding material according to claim 2, The welding material is characterized in that the undulating portion has a plurality of protrusions locally formed on one of the base materials by knurling.

4. A welding material according to claim 2 or claim 3, The base material is a plated steel sheet, which has a plating applied to the surface of a steel sheet. The plated steel sheet is a welding material characterized in that at least the area around the tops of the multiple protrusions of the undulating portion is a plated reduction portion where the amount of plating adhered is less compared to the portion other than the undulating portion.

5. A welding material according to claim 2 or claim 3, The welding material is characterized in that the localized undulations are formed in a ring shape having a portion in the center that does not have the convex portion.

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