JP2006029882A - Spot welding part inspection device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device capable of efficiently and precisely inspecting quality of a spot welding part. <P>SOLUTION: This spot welding part inspection device 1 inspects the spot welding part S formed by electrifying a pair of electrodes 2a, 2b arranged opposedly with layered steel sheets M1, M2 therebetween. The spot welding part inspection device 1 is provided with an electromagnet 11 arranged near one electrode 2a to generate a high-frequency magnetic field toward the spot welding part S, a detecting coil 12 arranged near the other electrode 2b to detect the high-frequency magnetic field generated by the electromagnet 11 and transmitted through the spot welding part S, and a determination circuit 13 for measuring the time until an induced voltage induced in the detecting coil 12 goes down to a prescribed value after finishing the electrification for the paired electrodes 2a, 2b. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an apparatus for inspecting the quality of a spot welded portion formed by spot welding a steel plate, and particularly to an apparatus capable of efficiently and accurately inspecting the quality of a spot welded portion in a welding process.

  Spot welding is performed by sandwiching stacked steel plates between a pair of electrodes and melting a part of the steel plate located between the pair of electrodes by heat generated by energizing between the pair of electrodes. This is a method of locally welding by pressing (hereinafter, a locally welded steel plate portion is referred to as a spot welded portion). Such spot welding of steel sheets is widely used as a joining method indispensable for manufacturing automobile bodies, for example.

  Conventionally, as a method for inspecting the quality of a spot welded portion (whether the joint strength of the spot welded portion is sufficient), destructive inspection such as so-called chisel inspection and peeling inspection has been performed. However, since the destructive inspection is performed by appropriately extracting the spot-welded steel plate in the first place, it is impossible to inspect all the steel plates. Furthermore, for example, when manufacturing a car body of an automobile, several thousand spot welds are applied per vehicle. Therefore, destructive inspection of all these spot welds is not practical in terms of inspection efficiency, and is extracted. The actual situation is that the destructive inspection is carried out only on the spot welded part further appropriately selected for the steel plate (car body). Therefore, there is a problem that the reliability of inspection is not always sufficient.

  In order to solve the above problem, it is considered effective to non-destructively inspect the spot weld. Here, it is known that the joint strength of a spot welded part can be evaluated by the dimension of a part (called a nugget part) where a steel sheet is melted and solidified by welding. In other words, it is known that the quality of the spot welded part can be evaluated by the dimensions (diameter and thickness) of the nugget part. FIG. 5 is a diagram schematically showing various forms of the nugget portion generated in the spot welded portion S of the stacked steel plates M1 and M2. If the diameter of the nugget portion N1 is small as shown in FIG. 5 (a) or the thickness of the nugget portion N2 is small as shown in FIG. 5 (b), it is determined that the spot welded portion S is defective. On the other hand, as shown in FIG. 5C, if the diameter and thickness of the nugget portion N3 are sufficient, it can be determined that the spot welded portion S is normal.

  As a method for non-destructively inspecting the dimensions of such a nugget part, a magnetic field is applied to the spot welded part after spot welding is completed, and the nugget part dimension is estimated from the magnetic property change of the spot welded part ( For example, see Patent Document 1). In addition, a method for estimating the size of the nugget part from the propagation state of the ultrasonic wave to the spot welded part (see, for example, Patent Documents 2, 3, and 4) has been proposed.

  However, any of the methods described in Patent Documents 1 to 4 above is a method of performing inspection in a separate process after spot welding is completed, and it is necessary to accurately position and inspect a detection sensor or the like at the spot welded portion. is there. Therefore, there is a problem that the inspection efficiency is extremely low in order to inspect the total number of spot welds as many as several thousand points like the body of an automobile.

On the other hand, as a method of inspecting the total number of spot welds, an attempt is made to measure the change in electrical resistance between the electrodes during spot welding and determine the quality of the nugget part based on the change in electrical resistance accompanying the generation of the nugget part Has also been reported. However, it is necessary to consider the fact that the tip of the electrode wears out during repeated spot welding and its resistance changes sequentially, and that current is diverted to the adjacent spot welds. It is very difficult to judge well. Further, although the electrical resistance between the electrodes strongly depends on the change in the diameter of the nugget portion, it hardly depends on the change in the thickness of the nugget portion. Therefore, the nugget portion in the form shown in FIG. There is a problem that the nugget portion having the form shown in FIG. That is, even when the thickness of the nugget portion is small, it may be determined that the nugget portion is normal.
Japanese Patent No. 3098193 Japanese Patent No. 2849205 Japanese Patent No. 2772895 Japanese Patent No. 2596090

  The present invention has been made to solve such problems of the prior art, and is an apparatus capable of efficiently and accurately inspecting the quality of spot welds formed by spot welding steel plates. The issue is to provide.

  In order to solve the above-mentioned problem, the present invention is an apparatus for inspecting a spot weld formed by energizing between a pair of electrodes arranged opposite to each other with stacked steel plates interposed therebetween, and the spot weld An electromagnet disposed in the vicinity of one electrode so that a high-frequency magnetic field can be generated toward the electrode, and a radio-frequency magnetic field generated by the electromagnet and transmitted through the spot welded portion is disposed in the vicinity of the other electrode Spot welding section inspection comprising: a detection coil; and means for measuring a time from when the induced voltage induced in the detection coil ends to the predetermined value after the energization between the pair of electrodes ends. A device is provided.

  Preferably, the electromagnet and the detection coil are each configured to include a yoke portion that is shaped so as to form a closed magnetic circuit across a steel plate.

  According to the spot welded portion inspection apparatus according to the present invention, since the quality of the spot welded portion formed by spot welding the steel plate can be inspected with high accuracy in the welding process, the total number of spot welded portions is inspected with high accuracy. Therefore, an excellent effect is achieved in that a highly reliable inspection is realized.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic diagram showing a schematic configuration of a spot welded portion inspection apparatus according to an embodiment of the present invention.
As shown in FIG. 1, a spot welded portion inspection apparatus (hereinafter referred to as an inspection apparatus as appropriate) 1 according to the present embodiment is between a pair of electrodes 2 a and 2 b arranged opposite to each other with sandwiched steel plates M1 and M2. Is an apparatus for inspecting a spot welded portion S (more specifically, a nugget portion N) formed by energizing the current.

  The inspection apparatus 1 detects the high frequency magnetic field generated by the electromagnet 11 and transmitted through the spot welded portion S, and the electromagnet 11 disposed in the vicinity of the one electrode 2a so as to generate a high frequency magnetic field toward the spot welded portion S. The detection coil 12 arranged in the vicinity of the other electrode 2b and the time until the induced voltage induced in the detection coil 12 drops to a predetermined value after the energization between the pair of electrodes 2a and 2b is finished. And a determination circuit 13 as means for measuring.

  In addition, the inspection apparatus 1 according to the present embodiment includes a high-frequency power source 14, a filter circuit 15, an amplifier 16, and a detector 17.

  The high frequency power supply 14 supplies a high frequency current to the electromagnet 11, and the electromagnet 11 generates a high frequency magnetic field. The frequency of the high-frequency current supplied from the high-frequency power source 14 is preferably set to 10 kHz or more in consideration of the influence of eddy current described later.

  The high frequency magnetic field generated by the electromagnet 11 and transmitted through the steel plates M1 and M2 including the spot welded portion S induces a high frequency voltage in the detection coil 12 by electromagnetic induction. The filter circuit 15 is provided to remove noise from the induced voltage induced in the detection coil 12. More specifically, the induced voltage induced in the detection coil 12 includes a frequency component of the current flowing through the electrodes 2a and 2b and a frequency component caused by noise. Only the same frequency component as the high frequency current supplied from the power source 14 to the electromagnet 11 is extracted.

  The induced voltage from which noise has been removed by the filter circuit 15 is subjected to predetermined voltage amplification by the amplifier 16, and then the amplitude and phase components are detected by a detector 17 such as a synchronous detector. The determination circuit 13 measures the time from when the energization between the electrodes 2a and 2b ends until the induced voltage drops to a predetermined value based on the amplitude and phase component of the induced voltage detected by the detector 17, The spot welded portion S is judged to be good or bad (more specifically, the size of the nugget portion N is good or bad) according to the measured time.

  Hereinafter, the reason why it is possible to determine the quality of the spot welded portion S (the quality of the nugget portion N) according to the amount of time from when the energization between the electrodes 2a and 2b ends until the induced voltage drops to a predetermined value. explain.

  In general, it is a well-known fact that when a high frequency magnetic field is applied to a steel plate, an eddy current flows through the steel plate so as to cancel the high frequency magnetic field. Also in the case of the inspection apparatus 1 according to the present embodiment, eddy currents flow through the steel plates M1 and M2 so as to cancel out the high-frequency magnetic field generated by the electromagnet 11 (so as to prevent entry into the steel plates). The magnitude of such eddy current depends on the frequency of the high-frequency magnetic field (frequency of the high-frequency current supplied from the high-frequency power source 14 to the electromagnet 11) f, and the magnetic permeability μ and conductivity σ of the steel plates M1 and M2.

  Here, when the temperature of the spot welded part S (particularly the nugget part N and its vicinity) located between the electrodes rises by energizing between the electrodes 2a and 2b, the conductivity σ of the part where the temperature has risen decreases. Will do. However, even if the temperature σ rises to 1000 ° C., the electrical conductivity σ of the steel sheet decreases only to a fraction of 1 to about 1 at a normal temperature.

  On the other hand, with respect to the magnetic permeability μ, although the steel plates M1 and M2 are ferromagnetic at normal temperature, they become non-magnetic at a temperature above the magnetic transformation point (near 780 ° C.). It will drop to about one-hundredth at room temperature.

  In the generation process of the nugget portion N caused by energization between the electrodes 2a and 2b, the nugget portion N and the vicinity thereof rise to a temperature above the magnetic transformation point, and both the conductivity σ and the permeability μ decrease, and the vortex This acts in the direction of decreasing the current. In particular, as described above, the permeability μ that sharply decreases at the boundary of the magnetic transformation point contributes significantly to the reduction of eddy current.

  When the eddy current rapidly decreases, the high-frequency magnetic field generated by the electromagnet 11 becomes more prominently penetrated into the steel plates M1 and M2. As a result, the magnetic field transmitted through the steel plates M1 and M2 increases, and the induced voltage induced in the detection coil 12 Will also increase.

  Among the regions of the steel plates M1 and M2 through which the magnetic field is transmitted, the region where the magnetic permeability μ, which is the main factor of the decrease in eddy current (and thus the increase in induced voltage), is the nugget portion N having a temperature higher than the magnetic transformation point and its Since it is in the vicinity, the magnitude of the magnetic field transmitted through the steel plates M1 and M2 (the magnitude of the induced voltage induced in the detection coil 12) strongly depends on the volume of the nugget portion N (both the diameter and thickness of the nugget portion N). It will be. Therefore, it is considered that the dimensions (diameter and thickness) of the nugget portion N can be estimated by observing the behavior of the induced voltage induced in the detection coil 12.

  FIG. 2 is a diagram schematically showing changes in the magnetic field Ht that passes through the steel plates M1 and M2 including the spot welded portion S before and after welding (before and after energization between the electrodes 2a and 2b). Note that such a change in the transmitted magnetic field Ht corresponds to a change in the induced voltage induced in the detection coil 12.

  As shown in FIG. 2, the temperature of the nugget portion N and the vicinity thereof increases with the start of energization (at the time indicated by A in FIG. 2), and the transmitted magnetic field Ht increases abruptly when the temperature reaches or exceeds the magnetic transformation point. . At this time, the increment of the transmission magnetic field Ht (ΔHt in FIG. 2) is proportional to the volume of the region where the temperature is equal to or higher than the magnetic transformation point including the nugget portion N.

  Thereafter, when energization is completed (at the time indicated by B in FIG. 2), the spot welded portion S begins to cool, and the transmitted magnetic field Ht decreases rapidly as the area below the magnetic transformation point increases. Here, after the energization is completed, it is considered that the cooling starts after the nugget portion N is first solidified. Therefore, depending on the size (volume) of the nugget portion N, all regions of the spot welded portion S are magnetically transformed. It takes a certain time (Δt in FIG. 2) to reach a temperature below the point. That is, if the size of the nugget portion N is large, it is considered that the time until all regions reach a temperature below the magnetic transformation point is long.

  From the above, in determining whether the dimensions of the nugget portion N are good, it is possible to use the increment ΔHt of the transmitted magnetic field or the time Δt until the temperature of the entire region of the spot welded portion S becomes equal to or lower than the magnetic transformation point after energization. Although it is considered possible, in the present embodiment, a configuration using Δt is adopted for the following reasons (1) and (2).

  (1) ΔHt is proportional to the volume of the region where the temperature is equal to or higher than the magnetic transformation point including the nugget portion N as described above. Here, it is known that by repeating spot welding, the tip portions of the electrodes 2a and 2b are consumed (the area of the tip portion in contact with the steel plates M1 and M2 increases). When the tips of the electrodes 2a and 2b are consumed, the current density that flows between the electrodes 2a and 2b decreases (the area that is energized in the steel plates M1 and M2 expands), so even if the dimensions of the nugget portion N are constant. The region where the temperature becomes higher than the magnetic transformation point increases, and if the dimension of the nugget portion N is determined only by the magnitude of ΔHt, an error occurs.

  (2) On the other hand, Δt corresponds to the time until the temperature after the nugget portion N is melted and solidified until it reaches a temperature equal to or lower than the magnetic transformation point. 2b, and the influence of the consumption of the electrodes 2a and 2b is small compared to ΔHt.

  For the above reasons, the inspection apparatus 1 according to the present embodiment employs a configuration for determining whether or not the dimensions of the nugget portion N are good by using Δt, thereby realizing a highly accurate inspection. Specifically, the determination circuit 13 measures the time (the time corresponding to the above Δt) from when the induced voltage induced in the detection coil 12 drops to a predetermined value after the energization ends. The size of the nugget portion N is judged based on the size. More specifically, the induced voltage is compared with a predetermined threshold value, and the time from when the energization is finished until the induced voltage falls below the threshold value is counted. The quality of the nugget portion N is judged to be good (good / bad judgment of the spot welded portion S) by comparing the time with a preset good / bad reference time.

  FIG. 3 is a graph showing an example of the relationship between Δt measured by the inspection apparatus 1 according to this embodiment and the dimensions of the nugget portion N. More specifically, the time from when the induced voltage induced in the detection coil 12 considered to correspond to Δt drops to a predetermined value (approximately the same value as before the start of energization) after the energization ends, and the nugget portion It is a graph which shows an example of the relationship with the dimension of N (the product of the diameter and thickness of the nugget part N measured by destructive inspection). As shown in FIG. 3, since Δt and the dimension of the nugget portion N have a positive correlation, it is possible to determine the quality of the nugget portion N according to the time.

  As described above, according to the spot welded portion inspection apparatus 1 according to the present embodiment, the quality of the spot welded portion S formed by spot welding the steel plates M1 and M2 is determined in the welding process (more specifically, Can be inspected with high accuracy immediately after the end of energization. Therefore, even when a very large number of spot welds S are present as in the case of a car body, it is possible to inspect all of them with high accuracy, and an excellent effect that a highly reliable inspection is realized. Play.

  In addition, since the inspection apparatus 1 according to the present embodiment is configured to detect the transmitted magnetic field (transmitted magnetic flux) by the detection coil 12, the spot welded portion S to be inspected exists in the vicinity of the end faces of the steel plates M1 and M2. In this case, a magnetic flux that wraps around the end surface of the steel plate is generated, and an error due to this occurs. Therefore, in order to suppress the influence of the wraparound of the magnetic flux at the end surface of the steel plate, it is preferable to employ the structure of the electromagnet 11 and the detection coil 12 so that the transmitted magnetic flux becomes a closed magnetic path.

  More specifically, as shown in FIG. 4, the electromagnet 11 is configured to include a yoke portion 112 around which the electromagnet winding 111 is wound, and the detection coil 12 is wound with the detection coil winding 121. What is necessary is just to make it comprise so that the rotated yoke part 122 may be comprised and each yoke part 112,122 may form a closed magnetic circuit across steel plate M1, M2. 4A is a front view, and FIG. 4B is a side view. With such a configuration, the magnetic field (magnetic flux) generated by the electromagnet 11 follows the closed magnetic path mainly including the yoke portion 112, the steel plates M1 and M2, and the yoke portion 122, and is detected by the detection coil winding 121. Therefore, it is possible to minimize the wraparound of the magnetic flux at the end surface of the steel plate. In addition, the yoke parts 112 and 122 can be formed from a ferrite material, for example.

FIG. 1 is a schematic diagram showing a schematic configuration of a spot welded portion inspection apparatus according to an embodiment of the present invention. FIG. 2 is a diagram schematically showing a change in the magnetic field transmitted through the steel plate including the spot weld before and after welding. FIG. 3 is a graph showing an example of the relationship between Δt measured by the spot welded portion inspection apparatus according to one embodiment of the present invention and the dimensions of the nugget portion. FIG. 4 is a schematic diagram schematically showing a configuration example of an electromagnet and a detection coil included in the spot welded portion inspection apparatus according to an embodiment of the present invention. FIG. 5 is a diagram schematically showing various forms of the nugget portion generated in the spot welded portion S. As shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Spot welding part inspection apparatus 2a, 2b ... Electrode 11 ... Electromagnet 12 ... Detection coil 13 ... Determination circuit 14 ... High frequency power supply 15 ... Filter circuit 16 ... Amplifier 17 ... Detectors M1, M2 ... Steel plate S ... Spot weld

Claims (2)

An apparatus for inspecting a spot weld formed by energizing between a pair of electrodes opposed to each other with a stacked steel plate interposed therebetween,
An electromagnet disposed near one of the electrodes so as to generate a high-frequency magnetic field toward the spot weld,
A detection coil disposed in the vicinity of the other electrode so as to detect a high-frequency magnetic field generated by the electromagnet and transmitted through the spot weld,
A spot welded portion inspection apparatus comprising: means for measuring a time from when the induced voltage induced in the detection coil drops to a predetermined value after the energization between the pair of electrodes ends.   2. The spot welded portion inspection apparatus according to claim 1, wherein the electromagnet and the detection coil each include a yoke portion that is shaped so as to form a closed magnetic path across a steel plate.

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