JP2020019030A - Evaluation method of junction of spot welding - Google Patents

Abstract

To heighten evaluation accuracy of a junction of spot welding.SOLUTION: A parameter (for example, a heating density D) contributing to the quality of a junction (nugget N) is acquired, while energizing a pair of electrodes 10, 20. An energization period between the pair of electrodes 10, 20 is divided into a plurality of sections C1-C10, and an evaluation formula using the heating density D of each section C1-C10 is created in consideration of contribution to nugget N quality of the heating density D of each section C1-C10. The nugget N quality is evaluated by using the evaluation formula.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for evaluating the quality of a joint formed by spot welding.

  As a method for evaluating the quality of a joint (nugget) formed by spot welding, a chisel test is known (for example, see Patent Document 1).

  For example, when welding is performed on a bag structure (see FIG. 1) covered with a metal plate having a hat-shaped cross section, a nugget may be formed in a place where a chisel cannot be inserted. Since a nugget formed in such a place cannot be subjected to a chisel test, for example, an ultrasonic inspection is applied as another evaluation method (for example, see Patent Document 2). The ultrasonic inspection is to measure the size of the nugget by oscillating an ultrasonic wave toward the nugget and receiving the reflected wave.

JP 2011-047738 A JP 2008-203082 A

  In spot welding, a metal plate is pressed by an electrode, so that a concave dent is formed on the metal plate. In particular, in indirect spot welding, the metal plate to be pressed by the electrode is not supported from the opposite side, so that the dent becomes deep. When ultrasonic waves are oscillated on such a deep concave dent, the ultrasonic waves are scattered when passing through the surface of the dent, and the reflected ultrasonic waves cannot be received normally, and the size of the nugget May not be able to measure accurately.

  The present inventors have obtained a parameter that contributes to the quality of a joint while performing spot welding, and tried to evaluate the quality of the joint based on this parameter. Specifically, as such parameters, when a current value flowing between a pair of electrodes is I, a voltage between both electrodes is V, and a contact area between a plurality of metal plates to be joined is S, D = I · V The heat generation density D represented by / S was used. Then, indirect spot welding is performed on a large number of samples while obtaining the heat generation density D, the state of the formed joint points (nuggets) is confirmed, and the range of the heat generation density D at which a good nugget is formed is set. did. Using this, the quality of the joints of the indirect spot welding of the actual product was evaluated, but the accuracy of the evaluation was not so high.

  Therefore, an object of the present invention is to improve the accuracy of the evaluation of the joining points of spot welding.

  In order to solve the above-described problems, the present invention provides a spot that joins the plurality of metal plates by applying a current between the pair of electrodes in a state where the pair of electrodes is in contact with the plurality of stacked metal plates. In welding, a method for evaluating the quality of a joint, a step of obtaining a parameter contributing to the quality of the joint while energizing between the pair of electrodes, and a period of energizing between the pair of electrodes. Is divided into a plurality of sections, a step of creating an evaluation formula using the parameters of each section in consideration of the contribution of each parameter to the quality of the junction point, and And a method for evaluating the joining point of spot welding, comprising the steps of:

  As described above, when a parameter that contributes to the quality of the junction is obtained while energizing between the pair of electrodes, the parameter value is clearly determined for the entire energization period when the junction is good and when it is defective. Even if there is no significant difference, a clear difference may appear in the parameter values between the case where the junction is good and the case where the junction is bad during part of the energization period. Therefore, in the present invention, as described above, the period for energizing between the pair of electrodes is divided into a plurality of sections, and the parameter of each section is considered in consideration of the degree of contribution of the parameter to the quality of the junction. An evaluation formula was created using. By using this evaluation formula, the quality of the junction can be accurately evaluated.

  As described above, according to the present invention, the joining points of spot welding can be accurately evaluated.

It is sectional drawing which shows a mode that indirect spot welding is performed with respect to a workpiece. It is a graph which shows the electric current value during welding of the said indirect spot welding, pressing force, and heat generation density. (A)-(E) is sectional drawing of the periphery of the workpiece | work joining planned part at the time of completion | finish of each step S1-S5 of FIG. It is a graph which shows the relationship between the displacement of a welding electrode and the contact area of metal plates.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  In the present embodiment, an indirect spot welding method performed in an assembling process of a vehicle body of an automobile is shown, and more specifically, a case where a workpiece 100 (a frame part of a vehicle body) as shown in FIG. 1 is welded. The workpiece 100 is a frame-shaped component extending in a direction perpendicular to the plane of the drawing, and includes a first metal plate 1 having a substantially flat plate shape, a second metal plate 2 having a hat-shaped cross section, and a first metal plate 1. And a third metal plate 3 having a hat-shaped cross section and disposed in a hollow portion formed by the second metal plate 2. As the metal plates 1 to 3, for example, steel plates are used, and specifically, mild steel plates, high-tensile steel plates (tensile strength of 490 MPa or more), ultra-high-tensile steel plates (tensile strength of 980 MPa or more), and the like are used.

  The first metal plate 1 and the flange portion 2a of the second metal plate 2 are joined via an already joined point Q1 which has been welded in advance by direct spot welding. The bottom portion 2b of the second metal plate 2 and the flange portion 3a of the third metal plate 3 are joined via an already joined point Q2 which has been welded in advance by direct spot welding.

  Then, the portion P to be joined between the first metal plate 1 and the top plate portion 3b of the third metal plate 3 is joined by indirect spot welding. The indirect spot welding apparatus includes a welding electrode 10 and a ground electrode 20, pressing means (an air cylinder, an electric cylinder, or the like) for pressing the metal plate by driving the welding electrode 10 in the axial direction, and a welding electrode using the pressing means. And a control unit (not shown) for controlling the pressing force of 10 and the current value between both electrodes 10 and 20.

  The indirect spot welding to the joining target portion P is performed in the following procedure. First, the ground electrode 20 is brought into contact with a portion of the workpiece 100 different from the portion P to be joined. In the illustrated example, the ground electrode 20 is connected from below to the bottom 2b of the second metal plate 2, in particular, to the already joined point Q2 between the bottom 2b of the second metal plate 2 and the flange 3a of the third metal plate 3. Abutting. In this state, the welding electrode 10 presses the portion P to be joined between the first metal plate 1 and the top plate 3b of the third metal plate 3 from one side in the thickness direction (upper side in the drawing), By energizing between the portions 10 and 20, the portion P to be joined is welded.

  In the present embodiment, welding is performed while changing one or both of the pressure applied by the welding electrode 10 and the current value between the electrodes 10 and 20. Specifically, welding is performed in accordance with the pressurization energizing pattern shown in FIG. Hereinafter, this pressurization energizing pattern will be described in detail.

[First Step S1]
In the first step S1, the portion P to be joined is pressurized by the welding electrode 10 with a relatively high first pressing force F1. As a result, the gap between the metal plates 1 and 3 is reduced so that the two metal plates 1 and 3 are surely in contact with each other, and the contact area between the welding electrode 10 and the first metal plate 1 and the first metal plate 1 And the contact area with the third metal plate 3 can be secured. In this state, a relatively low first current value I1 is applied between the electrodes 10 and 20 to suppress the current density and prevent the metal plates 1 and 3 from melting and scattering. By softening, the contact area between the welding electrode 10 and the first metal plate 1 and the contact area between the first metal plate 1 and the third metal plate 3 can be enlarged. {FIG. reference}. In addition, the area shown by the dotted points in FIG. 3 is the heat affected zone.

[Second Step S2]
In the second step S2, first, a command to reduce the pressing force is issued to the pressurizing means for applying the pressing force to the welding electrode 10 (see FIG. 2). At this time, due to the structure of the pressurizing means, a transition period in which the actual pressurizing force is not instantaneously dropped from F1 to F2 at the same time as receiving the command but is gradually reduced from F1 to F2 is necessarily provided. In this way, while gradually decreasing the pressing force from F1 to F2, the current is supplied at a current value I2 lower than the current value I1 of the first step. Thus, by suppressing the amount of heat input in a state where the pressing force is unstable, the heat balance can be adjusted by appropriately cooling or keeping the temperature of the welding electrode 10 and the metal plates 1 and 3. In the second step S2, the state around the joining portion P of the metal plates 1 and 3 hardly changes (see FIG. 3B).

[Third Step S3]
Thereafter, when a pressing force detecting unit (not shown) that detects the pressing force of the pressing unit detects that the pressing force has dropped to F2, the current value is increased. In the present embodiment, the current value is increased at the same time as the pressure decreases to F2 (see FIG. 2). At this time, if the current is increased at a stretch from the low current value I2 of the second step S2 to the current value of the main energization for forming the nugget (the current value I4 of the next fourth step S4), spatter may occur. Therefore, in the third step S3, the metal plates 1 and 3 are first softened by applying a current value I3 lower than the main current value I4 while pressurizing with the low pressing force F2, thereby softening the metal plates 1 and 3 and contacting them. Can be enlarged {see FIG. 3 (C)}.

[Fourth Step S4]
In the state where the contact area between the metal plates 1 and 3 is secured in this way, in the subsequent fourth step S4, the current is increased to the main current value I4 and energized (see FIG. 2). This makes it possible to reliably form a nugget seed (a nugget that does not reach a desired size) without generating spatters (see FIG. 3D). In the illustrated example, in a fourth step S4, an annular nugget N is formed at the portion P to be joined between the metal plates 1 and 3.

[Fifth Step S5]
After forming the nugget seeds in step S4, in a fifth step S5, a fourth current value I4 is applied between the two electrodes 10, 20 while applying a second pressure F2 by the welding electrode 10. A fifth current value I5, which is also low, is supplied (see FIG. 2). This makes it possible to stabilize the state of the nugget using the preheating of the metal plates 1 and 3 heated in the fourth step S4 while suppressing the amount of heat input to the metal plates 1 and 3. {FIG. See E) II. In the illustrated example, the annular nugget N formed in the fourth step S4 grows on the inner diameter side in the fifth step S5, and the hollow portion is filled to form a substantially disk shape.

  As described above, a nugget N as a joining point having a desired size and shape is formed at a portion P to be joined between the metal plate 1 and the top plate portion 3b of the metal plate 3. 1, 3 are joined.

  The quality of the joint formed by the above-described indirect spot welding is evaluated using the heat generation density D during welding. Specifically, a current value I flowing between the electrodes 10 and 20, a voltage V between the electrodes 10 and 20, a metal plate The contact area S between 1 and 3 is measured.

  At this time, since it is difficult to directly measure the contact area S between the metal plates 1 and 3, the displacement x of the welding electrode 10 is substituted. In the present embodiment, the displacement of the welding electrode 10 in the axial direction (pressing direction) is defined as the displacement x with reference to the position of the welding electrode 10 at the start of energization. The specific method of obtaining the contact area S during welding is as follows. First, the correlation between the displacement x of the welding electrode 10 and the contact area S of the metal plates 1 and 3 is obtained in advance. For example, indirect spot welding is performed on a portion to be joined of a sample similar to the above-described workpiece 100, and a plurality of samples in which energization is stopped at a plurality of stages in the middle are produced. Then, the contact area S of the metal plates 1 and 3 is measured from the cut surface of each sample, the displacement x of the welding electrode 10 at that time is recorded, and (x, S) is plotted on a graph (FIG. 4). reference). From this graph, the correlation between the displacement x and the contact area S is calculated. In the illustrated example, the contact area S and the displacement amount x are roughly in a proportional relationship, and are represented by S = a × x + b (a and b are constants). By using this relational expression, when performing indirect spot welding on an actual product, the contact area between the metal plates 1 and 3 that is difficult to directly measure from the displacement x of the welding electrode 10 that is easy to measure. S can be obtained.

  From these current value I, voltage V, and contact area S (displacement amount x of welding electrode 10), heat generation density D represented by D = VI / S = VI / (ax + b) is calculated. (See the chain line in FIG. 2). Using this heat generation density D, the quality of the nugget N is evaluated. Hereinafter, an example of a specific procedure of a method for evaluating the nugget N using the heat generation density D will be described.

  First, a period from the start to the end of energization is divided into a plurality of sections. In the present embodiment, at least one of the steps S1 to S5 having a constant current value is divided into a plurality of sections. Specifically, as shown in FIG. 2, the first to fifth steps S5 are each divided into two sections. Sections C1 to C10 are formed by equally dividing the section. Then, a large number of samples prepared by performing indirect spot welding under various conditions are prepared in advance, and the values of the heat densities of the sections C1 to C10 at this time (for example, the average value and the integral value of the heat densities of the sections) ) Is obtained, and the presence or absence of a defective joint is confirmed from the cut surface of each sample. Then, a section where the correlation between the heat generation density and the quality of the junction is high, that is, one or a plurality of sections where a clear difference occurs in the value of the heat density between when the junction is good and when it is bad. A selection is made, and an allowable range of the value of the heat generation density is set in the selected section.

  Then, in the actual product, the quality of the joints subjected to indirect spot welding is evaluated. That is, if the value of the heat density in the selected section is within the allowable range, it is determined that a good nugget N is formed in the joint P of the metal plates 1 and 3, and the value of the heat density in the selected section is determined. Is outside the allowable range, it is determined that the nugget N has some defect (insufficient nugget diameter, cracked metal plate, blowhole, etc.).

In evaluating the joining points, instead of directly using the heat density values (average value, integral value, etc.) of the sections selected above, a statistical method (for example, a discriminant analysis method) is used based on the heat densities of these sections. May be used. For example, assuming that the values of the heat densities in the sections C1 to C10 are c1 to c10, an evaluation formula F expressed by F = k1 · c1 + k2 · c2 +... + K10 · c10 may be used. k1 to k10 are coefficients based on the contribution ratio of each term. By using such an evaluation formula, it is possible to more accurately evaluate the quality of the joining point. The evaluation formula, other sections of the value of the heat generation density of each section as described above, terms and the power of the value of the heat generation density of any interval (e.g., c1 ^2, etc.), the heat generation density of the plurality of sections A term of a product of values (for example, c1, c2, etc.) and a ratio (for example, c1 / c2, etc.) may be added.

  As described above, the quality of the nugget N is determined by using the heating value D representing the relationship between the current value I during welding, the voltage V, and the contact area S (displacement x of the welding electrode 10) between the metal plates 1 and 3. By evaluating, even the quality of the nugget N to which the tip test or the ultrasonic inspection cannot be applied can be evaluated.

Further, the heat generation density D (= I.V / S = I ² .R / S) is constantly changed according to not only the current density (= I / S) but also the temperature of the metal plates 1 and 3 and the contact area S. And the resistance value R that changes. By using this heat generation density D, the heat generation state due to resistance heat generation at the contact portions of the metal plates 1 and 3 can be monitored, so that the quality of the nugget N can be accurately evaluated.

  Incidentally, the quality evaluation of the nugget N as described above may be performed in an inspection process provided separately from the indirect spot welding process, or may be performed in the indirect spot welding process. In the latter case, for example, the calculation of the heat generation density D and the quality judgment of the nugget N can be automatically performed at the same time as the completion of the indirect spot welding. In this case, the cycle time can be reduced, and the manufacturing cost can be reduced.

  The present invention is not limited to the above embodiment. For example, in the above-described embodiment, the case where the parameter that contributes to the quality of the bonding point is the heat generation density D has been described, but the present invention is not limited to this. For example, as such parameters, the contact area S between the metal plates, the voltage V between the electrodes 10 and 20, the resistance value R of the current path between the electrodes 10 and 20, or the current density at the contact portion between the metal plates (= I / S) can be used.

  Further, the evaluation method according to the present invention is not limited to the joint points of indirect spot welding, and can be applied to the evaluation of other spot welding joint points such as direct spot welding and series welding.

1-3 Metal plate 10 Welding electrode 20 Earth electrode 100 Work N Nugget (junction point)
P Planned joint Q1, Q2 Existing joint

Claims (1)

In a state where a pair of electrodes are brought into contact with a plurality of superposed metal plates, spot welding for joining the plurality of metal plates by applying a current between the pair of electrodes is to evaluate the quality of a joint point. The method
While energizing between the pair of electrodes, obtaining a parameter that contributes to the quality of the joint,
A step of dividing the period of current supply between the pair of electrodes into a plurality of sections, taking into account the degree of contribution of the parameters of each section to the quality of the junction, and creating an evaluation formula using the parameters of each section; and ,
Evaluating the quality of the joint using the evaluation formula.

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