JP6287232B2 - Current waveform determination method and resistance spot welding method in resistance spot welding - Google Patents

Description

  The present invention relates to a method for determining a current waveform in resistance spot welding, and a resistance spot welding method for energizing with a waveform determined by this method. More specifically, a method of determining a current waveform of resistance spot welding that can obtain a desired nugget diameter without generating dust, and a waveform determined by the above method to a member to be welded that is pressed through an electrode The present invention relates to a resistance spot welding method in which resistance spot welding is performed by energization.

  As one method for joining materials in various industrial members such as automobiles, resistance spot welding is used. In resistance spot welding, two or more members to be welded are sandwiched between electrodes, and electricity is applied while pressing the members to be welded by the electrodes. Thereby, a part of contact interface of the to-be-welded two or more to-be-welded members is melted and joined.

  In resistance spot welding, dust generated during resistance spot welding and splash of molten metal from a molten portion called spatter (hereinafter, the phenomenon of molten metal scattering is sometimes referred to as “chilli”) is a work environment. Worsen. Further, if dust adheres to the product surface, the product quality deteriorates. Therefore, when there is a concern about the generation of dust, the product is previously subjected to processing for facilitating removal of the adhered dust. Since such processing contributes to high costs, it is desired to suppress the generation of dust.

  The joining strength of the welded members joined by resistance spot welding changes according to the diameter of the melted part melted by energization (hereinafter, sometimes referred to as “nugget diameter”), and the larger the nugget diameter is, the more the joining strength increases. Easy to increase strength. For this reason, it is preferable to increase the nugget diameter from the viewpoint of securing a desired bonding strength. However, if the nugget diameter is too large, dust is generated. Therefore, when performing resistance spot welding, the nugget diameter is equal to or larger than the minimum nugget diameter necessary for ensuring the desired joint strength (hereinafter, sometimes referred to as “minimum nugget diameter”), and The welding conditions are determined so as to be less than the size at which dust starts to occur.

As a technique related to such resistance spot welding, for example, in Patent Document 1, the surface of the workpiece is softened by maintaining a current value at a level that can suppress the occurrence of spatter at the initial stage of energization time, and then the current is An energization control method for spot welding is disclosed in which the nugget is grown while maintaining the value high for a predetermined time to suppress the occurrence of spatter. Patent Document 2 discloses a time zone for maintaining a high current value at which a nugget is grown on a steel sheet to be welded within a current-carrying time for resistance welding, and a degree for softening the steel sheet without causing spattering. An energization control method for spot resistance welding in which energization is performed so as to alternately repeat time zones for maintaining a low current value is disclosed. Patent Document 3 discloses a first step of generating nuggets by gradually increasing an energization current to a high-tensile steel plate, a second step of decreasing the current after the first step, A high-strength steel spot welding method is disclosed in which spot welding is performed by a process including a third step of increasing the current after two steps and performing main welding while gradually decreasing the energization current.
Furthermore, as a conventional technology, non-patent literature "Studies of Fukumoto, Kazuo Okamura, Kiyoyuki Fukui," Development of high-precision spot welding CAE technology for thin steel sheets for automobiles ", Preprint of Academic Lectures of the Society of Automotive Engineers of Japan, 2006, No. 74-06, p. 9-12 ".

JP 2006-43731 A JP 2006-181621 A JP 2003-236684 A JP 2007-283328 A

In recent years, the use of new steel sheets such as high tensile steel and super high tensile steel has been increasingly oriented. These high-strength steel sheets are likely to generate heat because, as a measure for increasing the strength, generally, an alloy such as Si is added more than mild steel, and as a result, the specific resistance is high. Therefore, when high-strength steel plates are resistance spot welded, the temperature rise at the contact interface is severe and it is easy to obtain a melted part (nugget), but the contact area including the non-melted part is It becomes difficult to maintain a state larger than the area, that is, to avoid generation of dust. Therefore, the appropriate current range defined by the current value (lower limit current) necessary to obtain the minimum nugget diameter and the current value (upper limit current) immediately before the occurrence of dust is narrow. When the appropriate current range is narrow, the welding conditions that can suppress the generation of dust while ensuring the minimum nugget diameter are limited. Therefore, the wear of the electrode tip caused by continuous multiple times of welding, The margin for various disturbances that may occur in actual production, such as a change in the state of superposition due to gaps, etc., is small, and in some cases, welding is performed under conditions that deviate from the appropriate condition range. Therefore, it is desired to expand the appropriate current range by maximizing the upper limit current.
So far, techniques such as Patent Documents 1 to 3 have been developed, but these documents do not describe whether or not the appropriate current range is expanded. Therefore, even if these techniques are used, there is a possibility that the appropriate current range cannot be expanded. Further, since energization is performed to such an extent that the steel plate is softened, the total energization time becomes long. As a result, when the welding time is limited due to production restrictions, it may be difficult to apply to actual production.

  Accordingly, the present invention provides a current waveform determination method in resistance spot welding that can determine a current waveform that expands an appropriate current range that can suppress generation of dust while ensuring a minimum nugget diameter, and is determined by this method. It is an object of the present invention to provide a resistance spot welding method for performing resistance spot welding with a current waveform.

The present inventor considered that the aspect of “the wide appropriate current range” is that the curve of the curve described on the coordinate plane with the current as the horizontal axis and the nugget diameter as the vertical axis, the so-called weld lobe gradient becomes gentle. More specifically, when an energization waveform representing the energization amount at time t such that the time average value of the energization amount with respect to the energization time τ becomes the upper limit current is IS (t), it is obtained by IS (t). When the nugget diameter DS and the lower limit current are expressed by IS (t) −ΔI (ΔI> 0), the difference between the nugget diameter D0 obtained by the energization waveform IS (t) −ΔI is expressed as ΔD = DS−D0 (> 0), it was considered that the appropriate current range can be expanded by selecting an energization waveform IS (t) that reduces ΔD / ΔI.
As a result of earnest research based on this idea, the present inventor found that the relationship between the nugget diameter and the energization time when energized with a waveform having such characteristics is a coordinate plane with the energization time as the horizontal axis and the nugget diameter as the vertical axis. It was found that the growth rate of the nugget diameter immediately after the start of energization is high and the nugget diameter gradually increases after the nugget diameter reaches a predetermined value. Further, the present inventor gives, as a constraint condition, a mathematical expression expressing this feature and a mathematical expression representing that the nugget diameter is equal to or larger than the minimum nugget diameter as an objective function and a condition representing a condition to be satisfied in order to prevent generation of dust. In addition, it has been found that it is possible to determine a current waveform that expands an appropriate current range by solving an optimization problem using a current waveform (current pattern) as a design variable. The present invention has been completed based on such findings. The present invention will be described below.

According to a first aspect of the present invention, a laminate composed of a plurality of materials to be welded is sandwiched between a pair of electrodes, and the laminate is energized while pressing the laminate with the pair of electrodes. A method for determining a current waveform in resistance spot welding for joining two or more of the plurality of materials to be welded through the process of forming a melted portion at the interface of the plurality of materials to be welded, comprising: The applied pressure applied to P is P, the energization time for passing the current between the pair of electrodes is τ, and the nugget diameter at time t taking any value from 0 to τ is DN (t), 0 to 1 The coefficient taking the value of k is a non-melting part (referred to as corona bond) that is an interface where any two workpieces that are in contact with each other included in the laminate contact each other without melting during resistance spot welding F (t), the force at time t acting on And the objective function is that S, which is the integral value of DN (t) at time t = 0 to τ, and that DN (τ) is maximal, and F (t) Design the current waveform IS (t) at time t with the relation that F _min, which is the minimum value after the start of nugget formation at time t = 0 to τ, is F _min > (1−k) · P. A method for determining a current waveform in resistance spot welding, which includes a step of solving an optimization problem as a variable.

Here, in the present invention, “current” means effective current in the case of alternating current. The objective function “S is the maximum” corresponds to a current waveform that easily expands the appropriate current range. The objective function “DN (τ) is maximum” corresponds to maximizing the nugget diameter, which corresponds to maximizing the upper limit current of the appropriate current range. By increasing the maximum value of the nugget diameter as much as possible, it becomes easier to make the nugget diameter larger than the minimum nugget diameter. Furthermore, in the present invention, the constraint condition “F _min > (1-k) · P” corresponds to the absence of dust.
In the optimization problem in the present invention, since “S is maximum” and “DN (τ) is maximum” are objective functions, the current waveform obtained by solving the optimization problem is an appropriate current. It is a current waveform that makes it easy to expand the range and maximizes the upper limit current of the appropriate current range. Further, since “F _min > (1−k) · P” is a constraint, the obtained current waveform is a current waveform in which no dust is generated. In the present invention, the current waveform is determined by solving the optimization problem in which such conditions are set. Therefore, according to the present invention, it is possible to determine a current waveform that expands the appropriate current range.

  In the first aspect of the present invention, the coefficient k is preferably 0.85 to 0.95. By adopting such a form, it becomes easy to improve the prediction accuracy of occurrence / non-occurrence of dust, so that it becomes easy to determine a current waveform that expands the appropriate current range.

  According to a second aspect of the present invention, the resistance spot welding is performed with the current waveform IS (t) determined by the current waveform determination method in the resistance spot welding according to the first aspect of the present invention. This is a spot welding method.

  In the second aspect of the present invention, the resistance spot welding is performed with the current waveform IS (t) determined by the current waveform determination method in the resistance spot welding according to the first aspect of the present invention. It is possible to provide a resistance spot welding method capable of manufacturing a material at a low cost and with high accuracy in a favorable working environment.

  According to the present invention, there are provided a current waveform determination method in resistance spot welding capable of determining a current waveform that expands an appropriate current range, and a resistance spot welding method of energizing with a waveform determined by this method. Can do.

It is a schematic diagram explaining the relationship between the force F which acts on a corona bond, and the time t. It is a schematic diagram explaining the relationship between the nugget diameter D and the electric current I. FIG. 2A is a schematic diagram for explaining the relationship between the nugget diameter D and the current I when the appropriate current range is narrow, and FIG. 2B is a schematic diagram illustrating the nugget diameter D and the current I when the appropriate current range is wide. It is a schematic diagram explaining the relationship. It is a schematic diagram explaining the relationship between the nugget diameter D and the time t. FIG. 3A is a schematic diagram for explaining the relationship between the nugget diameter D and time t as a typical form of the energization waveform when the appropriate current range is narrow, and FIG. 3B is a case where the appropriate current range is wide. It is a schematic diagram explaining the relationship between the nugget diameter D and the time t as a mode of a typical energization waveform in FIG. It is a figure explaining S. FIG. It is a figure which shows the 5-step electricity supply pattern obtained by the optimization calculation. It is a figure which shows the relationship between the nugget diameter and energization time in the case of the 5 step | paragraph energization pattern obtained by optimization calculation, and single energization. It is a figure which shows the relationship between the force and the energization time which act on the non-melting part in the case of the 5-stage energization pattern obtained by optimization calculation, and single energization. It is a figure explaining the appropriate electric current range of an Example. It is a figure explaining the appropriate current range of a comparative example.

  Embodiments of the present invention will be described below. In addition, the following description is an illustration of this invention and this invention is not limited to the form illustrated below.

1. Current Waveform Determination Method in Resistance Spot Welding Considering the state of resistance spot welding at the manufacturing site, the three major welding conditions (pressure P applied from the electrode to the workpiece, current I passed between the pair of electrodes, energization Among the time τ), the applied pressure P is not large in the adjustment range and the degree of freedom of adjustment due to the performance of the welder and the control mechanism, and the energization time τ is restricted by the production tact. Although it is difficult to secure the degree of freedom of adjustment, the current I has a large degree of freedom of adjustment within the range of the performance of the control device (timer). Thus, the welding condition that is most easily changed among the three major welding conditions is the current I. Therefore, when determining the current waveform according to the present invention, the pressure I and energization time τ are kept constant, and the current I pattern is optimized. In general, the welded material joined by resistance spot welding is required to have a predetermined strength or more. While increasing the nugget diameter D makes it possible to increase the strength of the welding material, if the nugget diameter is too large, dust is generated. For this reason, in the present invention, the current I pattern that maximizes the nugget diameter D within a range in which no dust is generated is searched. Here, k is a coefficient taking any value of 0 to 1, and F is a force acting on a non-melting portion which is an interface where two workpieces that are in contact with each other are not melted during resistance spot welding and are in contact with each other. Then, the present inventor has found that “no dust is generated” can be expressed as F _min > (1−k) · P. Therefore, in the present invention, the condition that “no dust is generated” is expressed by F _min > (1-k) · P. The relationship between F (t) and time t is schematically shown in FIG. In FIG. 1, F _crit is the value of F just before dust occurs. As shown in FIG. 1, generation of dust can be avoided by energizing under a condition satisfying F _min > (1−k) · P = F _crit .

Further, the present inventor has shown that a graph set described on a coordinate plane having a current I as a horizontal axis and a nugget diameter D as a vertical axis in a plate set with a narrow appropriate current range and a plate set with a wide appropriate current range. I found it different. The relationship between the nugget diameter D and the current I is schematically shown in FIG. FIG. 2A is a schematic diagram for explaining a case where the appropriate current range is narrow, and FIG. 2B is a schematic diagram for explaining a case where the appropriate current range is wide. 2 (a) and 2 (b), IS is the current immediately before the occurrence of dust (upper limit current of the appropriate current range), D _min is the minimum nugget diameter, and the range specified by the arrow is appropriate Current range.

  As shown in FIG. 2A and FIG. 2B, the board assembly having a wider appropriate current range (FIG. 2B) is a graph than the board assembly having a narrower appropriate current range (FIG. 2A). The slope of was gentle. Therefore, the current shape having a wide appropriate current range is considered to satisfy the condition that “the change amount of the nugget diameter D when the current I changes is small”.

  As discussed so far, the current shape having a wide appropriate current range determined by the present invention satisfies the condition that “the change amount of the nugget diameter D when the current I changes is small”, and further, “the occurrence of dust is generated. The condition of “maximizing the nugget diameter D within the range not to be satisfied” is satisfied. As a result of earnest research, the present inventor conducted resistance spot welding with a current shape that satisfies these conditions, and a graph described on a coordinate plane with time t after the start of welding as the horizontal axis and the nugget diameter D as the vertical axis. Has become a peculiar form. The relationship between the nugget diameter D and the time t is schematically shown in FIG. FIG. 3A is a schematic diagram for explaining a case where the change ΔD of the nugget diameter when the current I changes is large. FIG. 3B shows the change ΔD of the nugget diameter when the current I changes. It is a schematic diagram explaining the case where it is small. 3A and 3B, ΔI is a current reduced from the current IS at which dust starts to be generated, and ΔD is a nugget diameter obtained when I = IS and I = IS−ΔI. This is the difference from the nugget diameter obtained in this case.

When it is attempted to obtain a nugget diameter having a certain size by changing the energization amount with the applied pressure and the energization time being constant, the following two extreme cases can be considered as the nugget growth history. That is, the relationship between the nugget diameter D and the time t after the start of energization is such that the nugget growth after the start of resistance spot welding is slow and the nugget diameter grows abruptly before the end of resistance spot welding (FIG. 3 ( a)), and the nugget diameter grows rapidly immediately after the start of resistance spot welding, and the growth of the nugget diameter saturates before the end of resistance spot welding (FIG. 3B).
Comparing FIG. 3A and FIG. 3B, ΔD when the current is reduced by the same ΔI is larger in FIG. 3A than in FIG. Since ΔD / ΔI corresponds to the slope of the graphs shown in FIGS. 2A and 2B, FIG. 3A corresponds to the case where the appropriate current range is narrow, and FIG. 3B shows the appropriate current. This corresponds to a wide range.

  FIG. 4 shows S. In FIG. 4, DS is the nugget diameter when I = IS. When FIG. 3A, FIG. 3B, and FIG. 4 are compared, S is larger in FIG. 3B than in FIG. Therefore, it can be considered that the form as shown in FIG. 3B can be expressed as “maximizing S”.

In order to optimize the current waveform, an optimization problem using the current I pattern as a design variable may be solved. As described above, in order not to generate dust, the condition “F _min > (1−k) · P = F _crit ” may be satisfied. Therefore, in the present invention, all of “F _min > (1−k) · P = F _crit ”, “maximum nugget diameter D”, “maximum S” and satisfy the pattern of current I as design variables The current waveform can be determined by solving the optimization problem. Specifically, the current waveform that expands the appropriate current range is determined by using resistance spot welding simulation software that can accurately predict the occurrence of nugget diameter and dust and optimization software that performs optimization calculations. be able to. The current waveform determined by this process satisfies all of “F _min > (1−k) · P = F _crit ”, “maximum nugget diameter D”, and “S is maximum”. A current waveform through the IS is determined.

  When resistance spot welding is performed on a material to be welded that is a cold rolled steel sheet of base material strength class 980 MPa class, it is possible to expand the optimum current range compared to single energization in which a constant current is passed from the start of energization to the end of energization. Attempt to determine the correct current waveform. The present invention will be described more specifically by describing the contents below. Here, a cold-rolled steel sheet having a base material strength class of 980 MPa is used as the material to be welded, but the material to be welded to which the present invention is applicable is not limited to this.

  When determining the current waveform according to the present invention, it is not necessary to set the basic form of the determined current waveform in advance, but in consideration of the functional restrictions of the control device (timer) of the welding machine, We searched for an optimal solution for five-stage energization in which the pattern can be set to any five stages. In addition, this time, a software that incorporates a function that makes it possible to accurately predict the occurrence of nugget diameter and dust is added to the commercially available finite element method analysis software. It was used as “welding simulation software”, and commercially available optimization software was used as “optimization software”. The analysis conditions are shown in Table 1. The present invention can also search for other forms of optimal solutions such as three-stage energization in which the current pattern can be set at any three stages and seven-stage energization that can be set at any seven stages.

First, the current pattern that becomes the upper limit of the appropriate current range was optimized. The number of trials for optimization calculation is 50. (1) Input data of pressure P, welding current I, and energization time τ. (2) Resistance spot that can accurately predict the occurrence of nugget diameter and dust. Finite element method analysis by welding simulation software, (3) Output nugget diameter DN (t) and force F (t) acting on non-melted part, (4) S maximum and DN (τ) maximum by optimization software An optimization calculation using an objective function, F _min > (1−k) · P as a constraint, and a current pattern as a design variable was repeated. In this optimization calculation, k = 0.9. FIG. 5 shows the obtained 5-stage energization pattern (hereinafter sometimes referred to as “optimal 5-stage energization”).

  As shown in FIG. 5, in the optimal five-stage energization, the current value gradually increased / decreased at each stage and gradually increased gradually.

  FIG. 6 shows the relationship between the nugget diameter and the energization time, and FIG. 7 shows the relationship between the force acting on the non-melted portion and the energization time. Each is shown. 6 and 7 show the results obtained by performing a finite element method analysis in which resistance spot welding is performed under a single energization condition (I = 7.5 kA) in which a nugget diameter equivalent to the optimum 5-stage energization is obtained. Also shown.

As shown in FIG. 6, in the growth of the nugget diameter, there was no significant difference between the optimum five-stage energization and the single energization condition. On the other hand, as shown in FIG. 7, in the history of the force F acting on the non-melted portion, a large difference was observed between the optimum five-stage energization and the single energization condition. Since there is a time zone in which F is less than F _crit under the single energization condition, dust occurs, but there is no time zone in which F is less than F _crit (= 0.1 P) in the optimal 5-stage energization, so generation of dust We were able to prevent.

Next, the average current value Im (= (I ₁ + I ₂ + I ₃ + I ₄ + I ₅ ) / 5) is changed at 0.5 kA intervals by moving the current waveform up and down while maintaining the current waveform of the optimum five-stage energization. By performing the finite element method analysis, the appropriate current range of the current waveform (example) obtained by the optimization calculation was examined. In addition, by conducting a finite element method analysis in the case of a single energization condition in which the current value was changed at intervals of 0.5 kA, the appropriate current range in the case of the single energization condition (comparative example) was also investigated. The results of the example are shown in FIG. 8, and the results of the comparative example are shown in FIG. In FIG. 8 and FIG. 9, the ■ connected by the line that rises to the right is the result of the nugget diameter D, and the circle that is connected by the line that descends to the right is the result of F. The _Dmin shown in FIGS. 8 and 9 was 4.73 mm.

  As shown in FIG. 8, the appropriate current range of the example was about 1.9 kA. On the other hand, as shown in FIG. 9, the appropriate current range of the comparative example was about 1.3 kA. From the above results, it was confirmed that the optimum current range can be expanded by about 0.6 kA in the optimum five-stage energization from the single energization condition. In addition, since this result is a result obtained by analysis using a high-strength steel plate as a material to be welded, the present invention also applies to an appropriate current range for a plate assembly including a high-strength steel plate whose proper current range is considered to be narrow. It was confirmed that can be expanded.

Claims (3)

A laminate composed of two or more welded materials is sandwiched between a pair of electrodes, and energized while pressing the laminate with the pair of electrodes. A method for determining a current waveform in resistance spot welding for joining a plurality of welded materials of two or more through a process of forming a melted portion at an interface,
The pressure applied from the electrodes to the laminate is P, the energization time for passing a current between the pair of electrodes is τ, and the nugget diameter at time t taking any value from 0 to τ is DN (t) , K is a coefficient that takes a value of 0 to 1, and any two of the welded materials that are in contact with each other and are included in the laminate are interfaces that do not melt during the resistance spot welding and contact each other When the force at the time t acting on the non-melting part is F (t),
The DN (t) is obtained from the current waveform IS (t) at time t by finite element analysis using simulation software capable of predicting the occurrence of nugget diameter and dust.
The objective function is that S, which is an integrated value of DN (t) at time t = 0 to τ , is maximum, and that the nugget diameter DN (τ) at time τ is maximum, and F (t of), the optimal time t = the minimum value after the nugget formation starting at 0～Tau Fmin is the Fmin> (1-k) · P the relationship constraints, and, to design variables the iS (t) Current waveform determination method in resistance spot welding, which includes a step of solving the optimization problem using optimization software . The current waveform determination method in resistance spot welding according to claim 1, wherein the coefficient k is 0.85 to 0.95. A resistance spot welding method, wherein resistance spot welding is performed with a current waveform IS (t) determined by the current waveform determination method in resistance spot welding according to claim 1 or 2.