Method for producing a connected body

Abstract

One aspect of the present disclosure provides a method comprising: clamping a first combination of metal workpieces (P1, P2) between a first electrode pair (21, 22); applying a welding current between the first electrode pair to weld the first combination of metal workpieces, specifying a magnitude of the welding current such that a resistance value between the first electrode pair falls within a specified range; measuring the resistance value between the first electrode pair while welding the first combination of metal workpieces based on (i) the magnitude of the welding current and (ii) a magnitude of a voltage between the first electrode pair; and determining, based on the measured resistance value deviating from a specified permissible range, that a welding defect has occurred in the first combination of metal workpieces.

Description

STATE OF THE ART The present disclosure relates to a process for manufacturing a connected body. The unexamined Japanese patent application, publication no. 2006-055893, discloses a spot welding determination system. This system comprises a resistance sensing unit for detecting a resistance value between electrode tips during spot welding, and a determination unit for determining weld quality based on a variation in the resistance value detected by the sensing unit. In particular, the determination unit calculates: the resistance value at the beginning of the current application; the difference between the maximum resistance value during the current application period and the final resistance value at the end of the current application period; and an average resistance value or an integrated resistance value during the current application period, and determines the weld quality based on each calculated result. SUMMARY The aforementioned determination system requires not only measuring the resistance value during the current application period to determine weld quality, but also calculating the aforementioned difference and the average or integrated resistance value. Furthermore, the aforementioned determination system must compare the measured resistance value, the calculated difference, and the calculated average or integrated resistance value with their respective associated threshold values. For this reason, a large number of calculations must be performed during the application of current in the aforementioned determination system, which may make the welding control excessively complex. In one aspect of the present disclosure, it is desirable that a welding defect can be determined solely based on a resistance value measured between electrodes during welding. One aspect of the present disclosure provides for a method for producing a joined body, wherein the method comprises: clamping (or inserting or arranging or interposing) a first combination of metal workpieces between a first pair of electrodes; applying (or supplying or passing) a welding current between the first pair of electrodes to weld the first combination of metal workpieces, wherein a magnitude of the welding current is specified such that a resistance value between the first pair of electrodes falls within a specified range; measuring the resistance value between the first pair of electrodes while welding the first combination of metal workpieces based on (i) the magnitude of the welding current and (ii) a magnitude of a voltage between the first pair of electrodes;and determine, based on the measured resistance value deviating from a specified permissible range, that a welding defect has occurred on the first combination of metal workpieces. This method allows the determination of the welding defect's occurrence based on the resistance value measured between the first electrode pair during welding. This reduces the processing load required during welding and prevents the welding control from becoming excessively complex. The welding current can be set according to a predetermined current profile over time. The predetermined current profile can be set to correspond to a resistance profile over time. The resistance profile can be measured by (i) clamping (or inserting, arranging, or interposing) a second combination of metal workpieces between a second electrode pair and (ii) applying (or supplying or conducting) a constant measuring current (or a fixed measuring current) between the second electrode pair. The second combination of metal workpieces can be equivalent to the first combination of metal workpieces. By applying a constant measuring current between the second pair of electrodes, it is possible to measure the following properties: the rate at which the second combination of metal workpieces heats up, the time at which the heating begins, the time at which the resistance reaches its maximum value after the heating has begun, and the degree of subsequent current loss (current division) as the resistance profile (i.e., change in resistance over time (waveform of resistance)). Therefore, by applying the welding current in accordance with the specified current profile (i.e., change of the welding current over time (waveform of the welding current)), which corresponds to the resistance profile, it is possible to control the magnitude of the welding current in such a way that the resistance between the first electrode pair is essentially constant. Accordingly, unlike, for example, in the Japanese unexamined patent application, Publication No. H 06-198453, it is unnecessary to control the size of the welding current while measuring the resistance between the first electrode pair in order to reduce the occurrence of weld spatter. This makes it possible to easily define the welding conditions to ensure that the resistance value between the first electrode pair falls within the specified range when welding the first combination of metal workpieces. Furthermore, the processing load required during welding can be reduced. The constant measuring current can be set to a value such that no weld spatter is generated on the second combination of metal workpieces. The resistance profile is designed so that no weld spatter is generated on the second combination of metal workpieces. This allows the welding current to be set so that no weld spatter is generated on the first combination of metal workpieces, resulting in improved weld quality on the first combination. Applying a welding current between the first pair of electrodes to weld the first combination of metal workpieces may involve the use of a welding device capable of controlling the magnitude of the welding current with a time resolution on the order of milliseconds. In such a case, the welding current can be precisely varied according to the resistance profile measured before welding. This allows for improved constant resistance welding. BRIEF DESCRIPTION OF THE DRAWINGS An exemplary embodiment of the present disclosure is described below with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of a welding device according to one embodiment; Fig. 2 shows a welding current profile; Figs. 3A and 3B are schematic representations of workpieces undergoing welding; Fig. 4 is a flowchart showing the operation of a resistance meter; Fig. 5 shows a resistance profile measured by the resistance meter; and Fig. 6 is a flowchart showing the operation of a quality control device. DETAILED DESCRIPTION OF EXAMPLE EXECUTION FORMS The present embodiment provides a welding device 1. [Design of the welding device] As illustrated in Fig. 1, the welding device 1 is designed to perform resistance spot welding (RSW) to weld a workpiece W. The workpiece W comprises a first and second metal workpiece P1 and P2, which are stacked in layers. In particular, the welding device 1 produces a joined body comprising the first and second metal workpieces P1 and P2. The first metal workpiece P1 and / or the second metal workpiece P2 is or are a high-strength steel sheet (or sheet) with a tensile strength of 590 MPa (megapascals) or greater, such as hot-pressed steel and a high-strength material. For example, the first metal workpiece P1 can be a hot-dip galvanized steel sheet or a high-strength steel sheet with a tensile strength of 590 MPa or greater. Alternatively, the first metal workpiece P1 can be a high-strength steel sheet with a tensile strength of 980 MPa or greater (so-called ultra-high-strength material). Just like the first metal workpiece P1, the second metal workpiece P2 can, for example, be a hot-dip galvanized steel sheet, but it is not limited to high-strength steel. In particular, the second metal workpiece P2 can be a steel sheet with a lower tensile strength than high-strength steel sheets. Alternatively, the second metal workpiece P2, like the first metal workpiece P1, can be a high-strength steel sheet or an ultra-high-strength material. In the present embodiment, the reason why at least the first and / or the second metal workpiece P1 and P2 is a high-strength steel sheet is that the greater the tensile strength of the first and second metal workpiece P1 and P2, the higher the electrical resistance (hereinafter simply referred to as resistance) at the overlapping surfaces of the first and second metal workpiece P1 and P2 before welding. Since the welding device 1 is suitable for welding high-strength steel sheets, as described above, at least the first and / or the second metal workpiece P1 and P2 is a high-strength steel sheet. However, the welding device 1 can also be used to weld steel sheets with lower tensile strength than high-strength steel sheets. In another embodiment, the welding device 1 can weld three or more stacked metal workpieces. The welding device 1 includes a resistance welding unit 20. The resistance welding unit 20 performs resistance spot welding to weld the first and second metal workpieces P1 and P2 together in their stacking direction. The stacking direction corresponds to the direction along which the first and second metal workpieces P1 and P2 are arranged, or to the normal direction of the surfaces of the first and second metal workpieces P1 and P2. In other words, the stacking direction runs along the thicknesses of the first and second metal workpieces P1 and P2. The resistance welding device 20 has a first electrode 21 and a second electrode 22. The first electrode 21 is positioned below the workpiece W. The second electrode 22 is positioned above the workpiece W to clamp the workpiece W together with the first electrode 21 along the stacking direction of the first and second metal workpieces P1 and P2. The first electrode 21 is movable in a vertical direction (up and down) relative to the second electrode 22. The first electrode 21 and the second electrode 22 are brought into contact with the workpiece W during welding. The first electrode 21 is brought into contact with the second metal workpiece P2, which is positioned in the lower part of the workpiece W. The second electrode 22 is brought into contact with the first metal workpiece P1, which is positioned in the upper part of the workpiece W. The first and second electrodes 21 and 22 together clamp the workpiece W along the stacking direction while exerting pressure on the workpiece W from both sides. The resistance welding device 20 applies a welding current between the first and second electrodes 21 and 22 in both directions, while the workpiece W is clamped between the first and second electrodes 21 and 22. The workpiece W generates heat through resistance heating, thereby welding it. The resistance welding device 20 is designed to control the size of the welding current with a time resolution on the order of milliseconds in such a way that the size of the welding current is varied in accordance with a predetermined welding current profile. The welding device 1 has a control system for the welding current, in particular a resistance meter 30, a current profile generator 40, a welding control 50 and a quality determiner 60. <Schweißsteuerung> The welding control unit 50 is designed to control the magnitude of the welding current when welding the workpiece W. In particular, the welding control unit 50 varies the value of the welding current over time (especially reducing the value along a curved profile) in accordance with the welding current profile, as illustrated in Fig. 2. The welding current profile is divided into a high-current application period (TA) and a main current application period (TB). The high-current application period (TA) corresponds to the period during which the welding current is applied. The main current application period (TB) corresponds to the period following the high-current application period (TA), during which welding is carried out using the welding current. During the high current application period TA, the welding current is set to a value higher than that required to weld workpiece W. Using a welding current at this value blows away contaminants on the first and second metal workpieces P1 and P2. During the main current application period TB, the welding current is set to the value required to weld workpiece W. In particular, during the high current application period TA, the welding current is rapidly reduced from the maximum value to the required value, and during the main current application period TB it is reduced more gradually than during the high current application period TA. Such a welding current profile makes it possible to blow away contaminants from the first and second metallic workpieces P1 and P2 during the high current application period (TA) and to equalize the surfaces of these workpieces. This reduces the occurrence of weld spatter caused by contaminants during the main current application period (TB). Furthermore, during the high current TA application period, the first and second metal workpieces P1 and P2 generate heat due to the high welding current. This creates a gap GA, as illustrated in Fig. 3A, between the first and second metal workpieces P1 and P2 around a welded section M located between the first and second electrodes 21 and 22. This concentrates the welding current on the welded section M. Therefore, it is possible to reduce the current loss caused by the current flowing between the first and second metal workpieces P1 and P2 around the welded section M. While the magnitude of the welding current decreases along a curved profile during the high-current application period TA and the main-current application period TB, in accordance with the resistance profile illustrated in Fig. 2, the welding current flows, as illustrated in Fig. 3B, through sections around the welded section M where the first and second metal workpieces P1 and P2 are in contact. The reason the welding current flows in this way is that the resistance increases in the center of the welded section M due to the melting of the workpiece W. The flow of the welding current thus eliminates current loss and causes a weld nugget to form on the workpiece W, enabling the first and second metal workpieces P1 and P2 to be properly joined. The diameter of the weld nugget can be adjusted by shifting the welding current profile illustrated in Fig. 2 up or down. <Widerstandsmesser und Stromprofilerzeuger> The current profile generator 40 is designed to generate (or define) the welding current profile. The resistance meter 30 is designed to perform a resistance measurement process illustrated in Fig. 4 in order to measure a resistance profile between the first and second electrodes 21 and 22 over time. The resistance profile is used by the current profile generator 40 to define the welding current profile. The welding current profile is determined at the time when the resistance welding device 20 initiates the production of the joined body under the control of the welding control 50. As illustrated in Fig. 1, the resistance meter 30 and the current profile generator 40 can be provided together with the welding control 50 as functions of the welding device 1. Alternatively, the resistance meter 30 and the current profile generator 40 can be provided as functions of an additional welding device separate from the welding device 1. The additional welding device, like the welding device 1, has the resistance welding unit 20 and the first and second electrodes 21 and 22. The additional welding device can be installed in a production facility for manufacturing the joined body together with welding device 1. Alternatively, the additional welding device can be installed in a production facility separate from welding device 1. However, the additional welding device can also alternatively be installed in a plant control system. The plant control system manages the welding conditions of welding devices installed in one or more production facilities. The welding devices can each be designed in the same way as welding device 1. The functions of the resistance meter 30 and the current profile generator 40 are achieved by performing the resistance measurement process illustrated in Fig. 4 in the welding device 1 or the additional welding device. The resistance measurement process is carried out in a stage where a first combination of first and second metal workpieces P1 and P2 is clamped between the first and second electrodes 21 and 22. The first combination of first and second workpieces P1 and P2 is intended for use in the resistance measurement process. A second combination of first and second workpieces P1 and P2 is provided, which is intended to be welded to form a joined body and is equivalent to the first combination. As illustrated in Fig. 4, in response to the start of the resistance measurement process, a constant measuring current is applied between the first and second electrodes 21 and 22 via the resistance welding device 20 in S110 (S denotes step). The constant measuring current is set to a value such that no weld spatter is generated on the workpiece W. The resistance value between the first and second electrodes 21 and 22 is repeatedly measured in S120 until a specified measurement time has elapsed (i.e., until it is determined in S130 that the measurement is complete). The measurement of the resistance value is achieved by (i) obtaining the value of the voltage between the first and second electrodes 21 and 22 from the resistance welding device 20 and (ii) calculating the resistance value from the value of the voltage and the value of the constant measuring current. When S130 determines that the measurement is complete (S130: YES), the resistance measurement process proceeds to S140, where the application of the constant measuring current is terminated and the welding current profile is generated based on time series data of a series of resistance values ​​repeatedly measured in S120 (see Fig. 5). In S140, the resistance profile is derived from the time-series data of the resistance values ​​measured in S120 and illustrated in Fig. 5. Subsequently, the welding current profile is defined, as illustrated in Fig. 2, such that the welding current magnitude is varied over time with the same slope (or rate of change) as the resistance value. The welding current profile is stored as welding current control data in a storage medium, such as flash memory or a hard drive. In S140, this process serves as a current profile generator 40. When the welding current profile in S140 is stored in the storage medium, the resistance measurement process is complete. The welding current profile stored in the storage medium is used by the welding controller 50 to control the welding current. The welding controller 50 can be designed so that a user can adjust the welding current based on the welding current profile determined during the resistance measurement process. As described above, the constant measuring current is applied between the first and second electrodes 21 and 22, while the first combination of first and second workpieces P1 and P2 is clamped between the first and second electrodes 21 and 22. Accordingly, the resistance meter 30 can measure the rate at which the first combination of first and second workpieces P1 and P2 heats up, the time at which the heating has started, the time at which the resistance reaches its maximum value after the heating has started, and the subsequent degree of current loss (current division) as the resistance profile. The current profile generator 40 determines the welding current profile used to weld the second combination of the first and second metal workpieces P1 and P2, which is equivalent to the first combination of the first and second metal workpieces P1 and P2, in order to match the resistance profile. Thus, with the welding device 1, it is possible to achieve constant resistance welding (or constant resistance control of resistance welding) without monitoring fluctuations in the workpiece W's resistance during welding. In this process, the welding current is controlled such that the resistance between the first and second electrodes 21 and 22 is kept at a substantially constant value, as shown in Fig. 2. This reduces the processing load required in the welding control unit 50 during welding. The welding conditions for constant resistance welding can be easily determined via the resistance measurement process using the resistance meter 30 and the current profile generator 40. Since the resistance welding device 20 is capable of controlling the welding current with a time resolution on the order of milliseconds, the welding controller 50 can precisely control the welding current in accordance with the resistance profile (and thus the welding current profile). This enables improved constant resistance welding. The magnitude of the constant measuring current is set such that no weld spatter is generated. Similarly, the magnitude of the welding current is also set such that no weld spatter is generated. This improves the weld quality of the second combination of the first and second workpieces P1 and P2. <Qualitätsbestimmer> The quality determiner 60 is designed to perform a quality determination process illustrated in Fig. 6, while the welding control 50 applies a welding current to the second combination of the first and second metal workpieces P1 and P2 (i.e. during the welding of the workpiece W) to determine the welding quality of the workpiece W. In response to the welding control 50 initiating the welding of the workpiece W in accordance with the welding current profile illustrated in Fig. 2, the quality determiner 60 in S210 first measures a resistance value between the first and second electrodes 21 and 22 and then proceeds to S220. More precisely, in S210, the quality determiner 60 obtains (i) the voltage value between the first and second electrodes 21 and 22 from the resistance welding device 20, (ii) the welding current value from the welding control 50, and (iii) calculates the resistance value based on the obtained voltage and welding current values. In S220, the quality inspector 60 determines whether the resistance value measured in S210 falls within a specified permissible range. If the measured resistance value falls within the permissible range (S220: YES), the welding of workpiece W may have been carried out correctly. Thus, the quality inspector 60 determines that the weld quality is acceptable, and the process then proceeds to S230. In S230, the quality inspector 60 determines whether the welding of workpiece W, performed by the welding control 50, has been completed. If the welding of workpiece W has not been completed (S230: NO), the quality inspector 60 returns to S210. If the welding of workpiece W has been completed (S230: YES), the quality inspector 60 ends the quality determination process. If the measured resistance value in S220 is outside the permissible range (S220: NO), the weld quality of workpiece W may be defective. Therefore, the quality inspector 60 proceeds to S240 and determines that a welding defect has occurred. In S240, the quality inspector 60 outputs the determination result regarding the welding defect to the welding control 50 or an external quality management system, thereby indicating that a welding defect has occurred on the workpiece W currently being welded, and then terminates the quality determination process. As described above, the quality control unit 60 determines whether the weld quality is normal (or acceptable) based on whether the resistance value measured between the first and second electrodes 21 and 22 during welding of the workpiece W falls within the permissible range. This is because, in the present embodiment, the welding control unit 50 is designed to perform constant resistance welding. In other words, if the welding of workpiece W is carried out properly, the resistance between the first and second electrodes 21 and 22 is maintained at a substantially constant value, as illustrated in Fig. 2. Thus, the quality inspector 60 can determine that the welding defect has occurred by measuring the resistance value between the first and second electrodes 21 and 22 during the welding of workpiece W and determining that the resistance value deviates from the permissible range. Therefore, according to the welding device 1 in the present embodiment, it is not only possible to determine whether the weld quality is acceptable during the welding of the workpiece W, but also to determine the quality based solely on the resistance value. This allows the processing load required during welding to be reduced in the control system comprising the welding control 50. [Modified example] The present disclosure is not limited to the embodiment described above, but can be implemented in various forms. For example, the welding device 1 described above can be used to weld three or more metal workpieces stacked in layers. A function performed by a single element in the aforementioned embodiment may be implemented by two or more elements. A function performed by two or more elements may be implemented by a single element. A part of the design in the aforementioned embodiment may be omitted. At least one part of the design of the aforementioned embodiment may be added to another part or replaced by another part of the design of the same embodiment. It should be noted that any and all modifications encompassed by the technical ideas defined by the language of the appended claims shall be considered embodiments of the present disclosure. QUOTES INCLUDED IN THE DESCRIPTION This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature JP 2006-055893

[0002] JP 06-198453

[0012]

Claims

A method for producing a joined body, comprising: clamping a first combination of metal workpieces (P1, P2) between a first pair of electrodes (21, 22); applying a welding current between the first pair of electrodes to weld the first combination of metal workpieces, specifying a magnitude of the welding current such that a resistance value between the first pair of electrodes falls within a specified range; measuring the resistance value between the first pair of electrodes while welding the first combination of metal workpieces based on (i) the magnitude of the welding current and (ii) a magnitude of a voltage between the first pair of electrodes; and determining, based on the measured resistance value deviating from a specified permissible range, that a welding defect has occurred on the first combination of metal workpieces. The method of claim 1, wherein: the magnitude of the welding current is determined in accordance with a predetermined current profile over time; the predetermined current profile is determined to correspond to a resistance profile over time; the resistance profile is measured by (i) clamping a second combination of metal workpieces (P1, P2) between a second electrode pair (21, 22) and (ii) applying a constant measuring current between the second electrode pair; and the second combination of metal workpieces is equivalent to the first combination of metal workpieces. Method according to claim 2, wherein the constant measuring current is set to a value such that no weld spatter is generated on the second combination of metal workpieces. Method according to any one of claims 1 to 3, wherein applying a welding current between the first pair of electrodes to weld the first combination of metal workpieces comprises using a welding device (1) that is capable of controlling the magnitude of the welding current with a time resolution on the order of milliseconds.

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