Nugget diameter estimation method

Abstract

Provided is a method for estimating the nugget diameter of a welded material bonded by resistance spot welding, the method comprising: a reference welding step for measuring the amount of expansion of a test material and bonding the test material; a nugget diameter measuring step for measuring the nugget diameter of the test material that has been bonded; a relational expression determining step for determining a relational expression between the measured nugget diameter of the test material and an amount of expansion of the test material; a main welding step for measuring the amount of expansion of the welded material and bonding the welded material while energizing the same under pressure by means of a pair of electrodes; and a nugget diameter estimation step for estimating the nugget diameter of the welded material that has been bonded, using the amount of expansion of the welded material and the relational expression.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Japanese Application No. 2022-84205, filed on May 24, 2022, the contents of which are incorporated herein by reference. [Technical Field]

[0002] The present disclosure relates to a method for estimating nugget diameter. [Background technology]

[0003] There is known a welding device that stores inter-electrode displacement data, which consists of the time-dependent change in inter-electrode displacement obtained when there is no gap between the workpieces and both electrodes press the workpieces vertically and the maximum displacement, as reference data (for example, JP 2000-005882 A). This welding device compares the obtained inter-electrode displacement data with the stored reference data to determine the reliability of the welded state, such as the nugget diameter, and quality abnormalities. Summary of the Invention [Problem to be solved by the invention]

[0004] The correlation between the inter-electrode displacement and the nugget diameter may change depending on the welding condition of the workpieces, and in such cases, the conventional techniques may not be able to accurately estimate the nugget diameter. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms. A first aspect of the present disclosure is a method for estimating a nugget diameter of workpieces joined by resistance spot welding, the method including: a reference welding process in which test materials corresponding to the workpieces are joined by applying current while being pressurized by a pair of electrodes, the reference welding process measuring an amount of expansion of the test material and joining the test material; a nugget diameter measurement process measuring a nugget diameter of the joined test material; a relational expression determination process determining a relational expression between the measured nugget diameter of the test material and the measured amount of expansion of the test material; a main welding process in which the amount of expansion of the workpieces is measured and the workpieces are joined by applying current while being pressurized by the pair of electrodes; and a process of determining the acceptability of the joined welded materials based on the estimated nugget diameter after the main welding process and after the nugget diameter estimation process, wherein the reference welding process joins the test materials using reference welding conditions that increase the amount of expansion from a current start time to a current end time, and the current end time is set so that the expansion amount does not peak out and the expansion amount can be gradually increased to an amount at which a desired nugget diameter is obtained, and the main welding process joins the welded materials using main welding conditions that increase the amount of expansion from a current start time to a current end time.

[0006] (1) According to one aspect of the present disclosure, there is provided a method for estimating a nugget diameter of workpieces joined by resistance spot welding. The nugget diameter estimation method includes a reference welding process for joining test materials corresponding to the workpieces by applying current while applying pressure between a pair of electrodes, the reference welding process measuring the amount of expansion of the test materials and joining the test materials, a nugget diameter measurement process measuring a nugget diameter of the joined test materials, a relational expression determination process determining a relational expression between the measured nugget diameter of the test materials and the amount of expansion of the test materials, a main welding process measuring the amount of expansion of the workpieces and joining the workpieces by applying current while applying pressure between the pair of electrodes, and a nugget diameter estimation process estimating a nugget diameter of the joined workpieces using the amount of expansion of the workpieces and the relational expression. According to this form of nugget diameter estimation method, by using the measured value of the expansion amount and the measured value of the nugget diameter in the reference welding process, it is possible to determine a relational equation that has a strong correlation between the expansion amount and the nugget diameter, thereby improving the estimation accuracy of the nugget diameter of the welded material obtained in this welding process.

[0007] (2) In the nugget diameter estimation method of the above aspect, the reference welding step may join the test material using reference welding conditions in which the amount of expansion gradually increases from the start time of current application to the end time of current application. According to the nugget diameter estimation method of this aspect, it is possible to suppress changes in the correlation between the expansion amount during welding and the nugget diameter, thereby improving the accuracy of estimating the nugget diameter.

[0008] (3) In the nugget diameter estimation method of the above embodiment, the reference welding condition may be to gradually increase the welding current value from the start of energization to a predetermined first current value, and then to reach a predetermined second current value at a predetermined end time of energization. According to this embodiment of the nugget diameter estimation method, the expansion amount of the molten portion of the test material can be increased more gradually than under the reference welding conditions in which the current is increased to the second current value immediately after the start of current flow and maintained at the second current value until the end of current flow, thereby suppressing changes in the correlation between the expansion amount and the nugget diameter during welding and improving the accuracy of nugget diameter estimation.

[0009] (4) In the nugget diameter estimation method of the above aspect, the reference welding condition may be to gradually increase the welding current value from the start of current flow to a predetermined first current value, and then to gradually increase the welding current value until one cycle of the power supply frequency before the predetermined current flow end time. According to this aspect of the nugget diameter estimation method, the range of conditions under which the accuracy of estimating the nugget diameter can be improved can be expanded compared to when the expansion amount is increased until the end of current supply.

[0010] (5) In the nugget diameter estimation method of the above aspect, the reference welding step may include a step of measuring the expansion amounts of the plurality of test materials and joining the plurality of test materials using a plurality of levels of welding current values. The nugget diameter measurement step may include a step of measuring the nugget diameters of the joined plurality of test materials. The relational equation determination step may include a step of determining the relational equation using the measured nugget diameters of the plurality of test materials and the measured expansion amounts of the plurality of test materials. According to the nugget diameter estimation method of this aspect, the accuracy of estimating the nugget diameter can be improved by increasing the number of samples.

[0011] (6) In the nugget diameter estimation method of the above aspect, the relational expression determining step may include a step of determining the relational expression by regression analysis using the measured nugget diameters of the plurality of test materials and the measured expansion amounts of the plurality of test materials. According to the nugget diameter estimation method of this embodiment, the nugget diameter can be estimated by a simple method using a regression equation.

[0012] (7) In the nugget diameter estimation method of the above embodiment, the main welding process may include a process of joining the workpieces using a welding current value included in the plurality of levels of welding current values ​​used in the reference welding process. According to the nugget diameter estimation method of this aspect, by matching the conditions of the reference welding process and the actual welding process, it is possible to improve the accuracy of estimating the nugget diameter by regression analysis. The present disclosure may be realized in various forms other than a nugget diameter estimation method, such as a resistance spot welding method, a nugget diameter estimation device, a resistance spot welding device, a control method for these devices, a computer program for implementing the control method, or a non-transitory recording medium on which the computer program is recorded. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is an explanatory diagram showing a schematic configuration of a resistance spot welding device. [Figure 2] FIG. 2 is a block diagram showing the internal functional configuration of the control device. [Figure 3] 1 is a flowchart of a resistance spot welding method according to a first embodiment of the present disclosure. [Figure 4] 10 is a flowchart showing details of a reference welding process. [Figure 5] 10 is a flowchart showing details of the actual welding. [Figure 6] 10 is a graph showing an example of a change in the amount of expansion of a test material in a reference welding process. [Figure 7] FIG. 3 is an explanatory diagram showing patterns of welding current values ​​set under reference welding conditions and actual welding conditions of the resistance spot welding device according to the present embodiment. [Figure 8] FIG. 2 is an explanatory diagram showing a method for deriving a nugget diameter calculation formula. [Figure 9] 6 is a graph showing changes in the amount of expansion of materials to be welded in a main welding process. [Figure 10] 10 is a graph showing the change in the amount of expansion of a comparative sample. [Figure 11]FIG. 10 is an explanatory diagram showing the evaluation results of the estimation accuracy of the nugget diameter of the welded material and the comparative sample produced under the present welding conditions. DETAILED DESCRIPTION OF THE INVENTION

[0014] A. First embodiment: FIG. 1 is an explanatory diagram showing a schematic configuration of a resistance spot welding apparatus 100. The resistance spot welding apparatus 100 joins multiple metal members by resistance spot welding. The resistance spot welding apparatus 100 also executes a method for estimating the nugget diameter of a workpiece WK according to a first embodiment of the present disclosure. In this embodiment, the multiple metal members to be joined are two plate materials W1, W2 made of galvannealed steel sheets (also called "GA steel sheets") having a thickness of approximately 1 to 2 millimeters.

[0015] Resistance spot welding apparatus 100 includes a welding gun 10, a robot arm RA, and a control device 80. Welding gun 10 includes a movable electrode side arm 10T, a pair of electrodes, namely, a movable electrode 20 and a fixed electrode 30, a fixed electrode side arm 10B connected to movable electrode side arm 10T, an electrode lifting device 40, and a current adjusting device 50.

[0016] The welding gun 10 is held by a robot arm RA. The movable electrode 20 is attached to a movable electrode-side arm 10T of the welding gun 10 via an electrode lifting device 40. The fixed electrode 30 is attached to a fixed electrode-side arm 10B of the welding gun 10. The movable electrode 20 and the fixed electrode 30 are arranged so that their tips face each other. When welding the workpieces WK, the workpieces WK are clamped between the movable electrode 20 and the fixed electrode 30 and current is applied while pressure is applied. This causes resistance heating to melt the workpieces WK, which then solidify, joining the plate materials W1 and W2. When current is applied to the workpieces WK from the movable electrode 20 and the fixed electrode 30, the workpieces WK expand as they melt. In this disclosure, for ease of explanation, the joined plate materials W1 and W2 and the overlapping plate materials W1 and W2 before joining are also referred to as the "workpieces WK."

[0017] The electrode lifting device 40 is an electric device that raises and lowers the movable electrode 20. The electrode lifting device 40 is attached to the tip of the movable electrode side arm 10T of the welding gun 10. The electrode lifting device 40 includes a servo motor 41 and a lifting member 42. The lifting member 42 is connected to the drive shaft of the servo motor 41 via a gear (not shown). The electrode lifting device 40 raises and lowers the lifting member 42 by operating the servo motor 41 in accordance with a command signal from the control device 80. When the lifting member 42 is lowered, the workpiece WK is clamped between the movable electrode 20 and the fixed electrode 30.

[0018] The current adjusting device 50 adjusts the current value of the welding current flowing between the movable electrode 20 and the fixed electrode 30 in response to a current command signal transmitted from the control device 80. As the current adjusting device 50, for example, a device equipped with a variable resistor or a device equipped with a converter is applied.

[0019] Resistance spot welding apparatus 100 further includes measuring devices, namely, a welding force measuring unit 92, a current measuring unit 94, a movable electrode displacement measuring unit 96, and a fixed electrode displacement measuring unit 98. These measuring devices are electrically connected to control device 80, and the measurement results of each measuring device are transmitted to control device 80.

[0020] The pressure measuring unit 92 measures the pressure applied to the workpiece WK by the movable electrode 20 and the fixed electrode 30. The pressure measuring unit 92 is, for example, a load cell housed inside the electrode lifting device 40. The current measuring unit 94 is a current sensor and measures the value of the welding current flowing between the movable electrode 20 and the fixed electrode 30.

[0021] The movable electrode displacement measuring unit 96 measures the elevation position of the movable electrode 20. The movable electrode displacement measuring unit 96 is an encoder housed inside the electrode elevation device 40. The movable electrode displacement measuring unit 96 acquires the position of the movable electrode 20 by detecting the rotation angle position of the output shaft of the servo motor 41 and measuring the elevation position of the movable electrode 20. The movable electrode displacement measuring unit 96 acquires the position of the movable electrode 20 relative to the initial position as a displacement, using the initial position of the movable electrode 20 at the start of current application for reference welding or main welding as a reference.

[0022] The fixed electrode displacement amount measuring unit 98 is a strain sensor. The fixed electrode displacement amount measuring unit 98 measures the amount of strain of the fixed electrode side arm 10B. In this embodiment, the fixed electrode displacement amount measuring unit 98 measures the amount of strain of the fixed electrode side arm 10B when pressure is applied to the workpiece WK. The fixed electrode displacement amount measuring unit 98 obtains the position of the fixed electrode 30 by multiplying the measured amount of strain by a coefficient that is predetermined as a characteristic of the welding gun 10. The fixed electrode displacement amount measuring unit 98 obtains, as the amount of displacement, the position of the fixed electrode 30 relative to the initial position of the fixed electrode 30 at the start of current application for reference welding or main welding.

[0023] 2 is a block diagram showing the internal functional configuration of the control device 80. The control device 80 includes a CPU 60 as a central processing unit, a storage device 70, a timer (not shown) for measuring time, and an input / output interface (not shown). These are connected to each other via an internal bus so that they can communicate with each other.

[0024] CPU 60 executes a control program stored in advance in storage device 70, thereby functioning as control unit 62, expansion amount calculation unit 64, relational equation determination unit 65, nugget diameter estimation unit 66, and determination unit 68. Control unit 62 comprehensively controls each operation of resistance spot welding apparatus 100, specifically, the current value, current application time, electrode pressure, current application timing, pressure application timing, etc.

[0025] The expansion amount calculation unit 64 calculates the expansion amount of the workpiece WK using an expansion amount calculation formula 76 stored in the storage device 70. The relational formula determination unit 65 determines a nugget diameter calculation formula 78, which is a relational formula between the nugget diameter of the test material and the expansion amount of the test material, using the expansion amount and nugget result of the reference welding. The nugget diameter estimation unit 66 calculates an estimated value of the nugget diameter of the workpiece WK joined by the main welding process using the nugget diameter calculation formula 78 stored in the storage device 70. The judgment unit 68 uses the estimated value of the nugget diameter calculated by the nugget diameter estimation unit 66 to judge whether the workpiece WK after the main welding is a non-defective product.

[0026] The storage device 70 is, for example, a RAM, a ROM, or a hard disk drive (HDD). The HDD or ROM stores various programs for realizing the functions provided in this embodiment. The various programs read from the HDD or ROM are expanded on the RAM and executed by the CPU 60. The readable / writable area of ​​the storage device 70 is provided with a storage unit for storing reference welding conditions 72, actual welding conditions 74, an expansion amount calculation formula 76, and a nugget diameter calculation formula 78.

[0027] The reference welding conditions 72 are processing conditions for the test material used in the reference welding process. The actual welding conditions 74 are processing conditions for the workpiece WK used in the actual welding process. The reference welding conditions 72 and the actual welding conditions 74 vary depending on the type and combination of test material and workpiece WK, and a corresponding number of reference welding conditions 72 and actual welding conditions 74 are stored. As described below, when regression analysis is used to calculate the relationship between the nugget diameter of the test material used in the reference welding and the expansion amount of the test material, it is preferable that the reference welding conditions 72 further include multiple levels of conditions for one type of test material. In this embodiment, the reference welding conditions 72 include three levels of welding current conditions: "high," "medium," and "low," set for each type of test material.

[0028] The expansion amount calculation formula 76 is used to calculate the expansion amounts of the workpiece WK and the test material. For example, the expansion amount calculation formula 76 can be the following formula (1). En = ΔD + ΔP Equation (1) En is the amount of expansion of the workpiece WK or the test material. The amount of expansion En corresponds to the amount of expansion in the direction perpendicular to the surface direction of the workpiece WK and the test material. ΔD is the amount of displacement of the movable electrode 20, and corresponds to the amount of displacement of the movable electrode 20 acquired by the movable electrode displacement measurement unit 96. ΔP is the amount of displacement of the fixed electrode 30, and corresponds to the amount of displacement of the fixed electrode 30 acquired by the fixed electrode displacement measurement unit 98.

[0029] The nugget diameter calculation formula 78 is used to estimate the nugget diameter of the workpiece WK joined by the actual welding. In this embodiment, the nugget diameter calculation formula 78 is obtained by regression analysis such as the least squares method using the actual measurement value of the nugget diameter of the test material welded by the reference welding and the expansion amount of the test material during the reference welding, as will be described later. The nugget diameter calculation formula 78 differs for each type and combination of workpiece WK, and a corresponding number of formulas are stored.

[0030] 3 is a flowchart of a resistance spot welding method according to a first embodiment of the present disclosure. In step S10, a reference welding process is performed by the resistance spot welding apparatus 100. In the reference welding process, a nugget diameter calculation formula 78 is determined and stored in the storage device 70. In step S20, a main joining process is performed by the resistance spot welding apparatus 100. In the main joining process, workpieces WK are joined by welding, and an estimated value of the nugget diameter of the joined workpieces WK is calculated.

[0031] FIG. 4 is a flowchart showing the details of the reference welding process. In step S102, information on the test material is acquired. The test material may be the workpiece WK, or may be another member having similar factors affecting welding, such as material and thickness, to the workpiece WK. The control unit 62 acquires identification information, such as the type of test material, by, for example, recognizing the test material using a captured image, or by receiving or reading identification information assigned to the test material, such as an RF tag or two-dimensional code. In step S104, the control unit 62 selects reference welding conditions 72 corresponding to the acquired identification information.

[0032] In step S106, the control unit 62 starts applying current to the test material. Specifically, the control unit 62 sends an electrode position command signal to the electrode lifting device 40 to move the movable electrode 20 to the electrode position of the selected reference welding conditions 72. The control unit 62 sends a current command signal to the current adjusting device 50 to apply a welding current to the test material according to the welding current value of the reference welding conditions 72. In this embodiment, the reference welding conditions 72 include three levels of welding current conditions set for each type of test material. Steps S106 to S112 are then repeated for each level of welding current. In this embodiment, the number of samples is increased to improve the estimation accuracy of the nugget diameter calculation formula 78. Specifically, three samples are processed at each level. That is, in the reference welding process, three samples are processed at each of the three levels of welding current, resulting in a total of nine samples. Note that different test materials may be welded for each welding current condition, or different positions on a single test material may be welded for each welding current condition.

[0033] In step S108, the expansion amount calculation unit 64 acquires the displacement amount ΔD of the movable electrode 20 from the movable electrode displacement amount measurement unit 96. In step S110, the expansion amount calculation unit 64 acquires the displacement amount ΔP of the fixed electrode 30, which is the displacement amount of the fixed electrode 30, from the fixed electrode displacement amount measurement unit 98. Note that steps S108 and S110 are repeatedly executed, for example, every 2 msec during the reference welding so that the change in the expansion amount over time can be confirmed.

[0034] In step S112, the control unit 62 terminates the application of current to the test material in accordance with the current application termination time of the reference welding conditions 72. The current application time may be set in terms of time or the number of cycles of the power supply frequency. In step S114, the expansion amount calculation unit 64 calculates the expansion amount En of the workpiece WK according to the expansion amount calculation formula 76 using the displacement amount ΔD of the movable electrode 20 and the displacement amount ΔP of the fixed electrode 30 acquired during the current application. Step S114 may be executed in real time during the current application from step S106 to step S112.

[0035] Step S116 is a nugget diameter measurement step in which the nugget diameter of the test material joined by the reference welding is measured. In this embodiment, the welded portion of each test material produced under three levels of welding current conditions is cut, and the nugget diameter is measured. To facilitate measurement of the nugget diameter, it is preferable to polish the cut surface and then etch it with, for example, a saturated aqueous solution of picric acid. The nugget diameter may be measured by various methods, or non-destructively using any measuring device.

[0036] Step S118 is a relational expression determination step. The relational expression determination unit 65 determines a nugget diameter calculation formula 78 using the expansion amount En calculated in step S114 and the nugget diameter measured in step S116. In this embodiment, the nugget diameter calculation formula 78 is determined by regression analysis using the expansion amount En and the measured nugget diameter values ​​for each of three levels of welding current conditions. The determined nugget diameter calculation formula 78 is stored in the storage device 70. Note that, as will be described later, the relational expression determination unit 65 uses the peak value of the expansion amount of the test material up to the end of current application as the expansion amount En. The reference welding is terminated when the nugget diameter calculation formula 78 is determined.

[0037] FIG. 5 is a flow diagram showing details of the main welding. In step S202, identification information of the workpiece WK is acquired using a method similar to that of step S102 described above. In step S204, main welding conditions 74 corresponding to the identification information of the workpiece WK are selected and acquired. The main welding conditions 74 preferably match one of the conditions included in the reference welding conditions 72. This strengthens the correlation between the reference welding and the main welding, and improves the estimation accuracy using the nugget diameter calculation formula 78. In this embodiment, the welding current of the main welding conditions 74 matches the "middle" level of the welding current among the three levels of welding current included in the reference welding conditions 72.

[0038] In step S206, control unit 62 starts applying current to workpiece WK. Steps S206 to S214 are the same as steps S106 to S114 of the reference welding process except that one workpiece WK is produced using main welding conditions 74 having one level of welding current value, and therefore a description thereof will be omitted.

[0039] Step S216 is a nugget diameter estimation step. The nugget diameter estimation unit 66 estimates the nugget diameter of the workpieces WK joined in this welding step. The nugget diameter estimation unit 66 uses the expansion amount calculated in step S214 to calculate an estimated value of the nugget diameter from the nugget diameter calculation formula 78 determined in step S118 of the reference welding.

[0040] In step S218, the determination unit 68 determines whether the calculated estimated value of the nugget diameter is within a predetermined range stored in the storage device 70. If the nugget diameter is within the predetermined range (S218: YES), the process proceeds to step S220, where the determination unit 68 determines that the workpiece WK is a non-defective product and ends the process. If the nugget diameter is not within the predetermined range (S218: NO), the process proceeds to step S222, where the determination unit 68 determines that the workpiece WK is defective and ends the process.

[0041] FIG. 6 is a graph showing an example of the change in the amount of expansion of the test material during the reference welding process. The horizontal axis of the graph represents the elapsed time (unit: msec) from the start of current application. The vertical axis represents the amount of expansion En (unit: millimeters) of the test material calculated by the expansion amount calculation unit 64 using the expansion amount calculation formula 76. As described above, in this embodiment, the reference welding conditions 72 include three levels of welding current: "high," "medium," and "low." Graph G11 represents the calculation result of the amount of expansion En under the "high" welding current condition, graph G12 represents the result under the "medium" welding current condition, and graph G13 represents the calculation result of the amount of expansion En under the "low" welding current condition. Note that, in reality, there are three samples of measurement results of the amount of expansion En for each level of welding current, but these are omitted from the illustration for ease of explanation.

[0042] Time T1 in FIG. 6 indicates the time when current distribution ends. The relational equation determination unit 65 obtains the peak value of the expansion amount En up to time T1 in each of graphs G11 to G13. In this embodiment, as shown in FIG. 6, in each of graphs G11 to G13, the expansion amount En shows an increasing trend from the start of current distribution to time T1. The peak values ​​of the expansion amount En are expansion amounts E11 to E13 at time T1. Note that if the expansion amount peaks out between the start and end of current distribution, the relational equation determination unit 65 uses the expansion amount at the peak regardless of the time when current distribution ends.

[0043] In this embodiment, the direction of expansion of the workpiece WK calculated as the expansion amount En is perpendicular to the surface direction of the workpiece WK. In contrast, the nugget diameter estimated in this embodiment is a diameter in a direction different from the direction of expansion of the expansion amount En, specifically, along the surface direction of the workpiece WK. Here, for example, if the molten portion of the workpiece WK expands significantly due to resistance spot welding, the welded portion near the fixed electrode 30 and the movable electrode 20 may be cooled by heat dissipation to these electrodes. The inventors have newly discovered that in this case, the molten portion is less likely to grow in the perpendicular direction of the workpiece WK and more likely to grow in the direction along the surface direction of the workpiece WK. If the molten portion continues to grow along the surface direction, it will be pushed away from the welding position and more likely to flow between the plate materials W1 and W2 of the workpiece WK. In such a case, the expansion amount of the molten portion in the perpendicular direction decreases, while the expansion amount in the surface direction increases. As a result, the correlation between the acquired expansion amount En and the nugget diameter changes, and it may not be possible to accurately estimate the nugget diameter using the expansion amount En.

[0044] In this embodiment, in order to stabilize the correlation between the expansion amount En and the nugget diameter, welding conditions were selected under which the expansion amount En increases until the end of current application. This is because it is believed that the expansion amount in the planar direction of the molten part is unlikely to increase while the expansion amount increases in the perpendicular direction. However, the expansion amount does not necessarily have to continue to increase until the end of current application. For example, experimental results have shown that if the expansion amount En increases until one cycle of the power supply frequency before the end of current application, the same level of accuracy can be obtained in estimating the nugget diameter.

[0045] FIG. 7 is an explanatory diagram showing patterns of welding current values ​​set under reference welding conditions 72 and actual welding conditions 74 for resistance spot welding apparatus 100 according to this embodiment. The horizontal axis of the graph represents elapsed time (unit: msec) from the start of current application, and the vertical axis represents welding current value (unit: amperes). Graph GA, shown by a solid line in FIG. 7, is a master pattern of welding current values. In reference welding and actual welding, control unit 62 operates current adjusting device 50 so that a current having the pattern shown in graph GA flows between fixed electrode 30 and movable electrode 20.

[0046] As shown in graph GA, the welding current is adjusted, for example, by so-called upslope control. In upslope control, the welding current is increased to a first current value IA of several thousand amperes immediately after the start of current application, and then gradually increased to a second current value IB at the end of current application. This allows for a more gradual increase in the expansion of the molten portion of the workpiece WK compared to, for example, control in which the welding current is increased to the second current value IB immediately after the start of current application and maintained at the second current value IB until the end of current application. In this embodiment, the reference welding conditions 72 and the actual welding conditions 74 are set to welding times that allow the expansion amount to gradually increase to the amount required to obtain the desired nugget diameter without peaking. It is preferable that the first current value IA and the second current value IB are not close to each other.

[0047] Fig. 8 is an explanatory diagram showing a method for deriving the nugget diameter calculation formula 78. Fig. 8 shows the correspondence between the nugget diameters measured after the reference welding process and the peak values ​​of the expansion amounts En corresponding to the graphs G11 to G13 shown in Fig. 6. The vertical axis of the graph represents the nugget diameter Dn (unit: millimeters), and the horizontal axis represents the expansion amount En (unit: millimeters). Fig. 8 plots the results of a total of nine samples, three samples for each of three levels of melting current values ​​as the reference welding conditions 72.

[0048] The relational expression determining unit 65 determines the nugget diameter calculation formula 78 by regression analysis using the nine plots shown in Fig. 8. In this embodiment, the relational expression determining unit 65 determines the following formula (2) as the nugget diameter calculation formula 78. Dn=14.779 · En-0.138 · · Formula (2) Dn is the measurement result of the nugget diameter after the reference welding, and En is the peak value of the expansion amount En. The nugget diameter calculation formula 78 differs depending on the type and combination of the test material and the workpiece material WK. The accuracy of the nugget diameter estimation may differ depending on the type and combination of the test material and the workpiece material WK. In this case, the nugget diameter calculation formula 78 may use different parameters depending on the type and combination of the test material and the workpiece material WK to improve the estimation accuracy. For example, the expansion amount En may be the integral value of the expansion amount instead of the peak value. The integral value of the expansion amount may be the integral value from the start time of the current application to the end time of the current application. The integral value of the expansion amount may be the integral value from the start time of the current application to a time after the end time of the current application, which is arbitrarily set. The integral value of the expansion amount may be the integral value from the start time of the current application to one cycle of the power supply frequency before the end time of the current application. Alternatively, both the peak value of the expansion amount and the integral value of the expansion amount may be obtained, and the one with the higher accuracy of the nugget diameter estimation may be used.

[0049] FIG. 9 is a graph showing the change in the expansion amount of the workpiece WK during the actual welding process. The graph shown in FIG. 9 shows the results obtained from an experiment conducted using the same workpiece WK and actual welding conditions 74 as those used in the actual welding process. The horizontal axis of the graph represents the elapsed time (unit: msec) from the start of current application. The vertical axis represents the expansion amount En (unit: millimeters) of the workpiece WK calculated by the expansion amount calculation unit 64 using the expansion amount calculation formula 76. In this embodiment, the welding current value of the actual welding conditions 74 is the "middle" welding current value among the three levels of welding current included in the reference welding conditions 72. The welding current value of the actual welding conditions 74 is set so that it is included in the reference welding conditions 72 in order to improve the accuracy of deriving the nugget diameter calculation formula 78 using regression analysis.

[0050] In the experiment, changes in the amount of expansion were obtained using 10 types of samples. For ease of explanation, only graph G2 showing the results of one of the samples is shown, and the other results are not shown. The 10 types of samples were produced under the same production conditions as the actual welding conditions 74, such as pressure and current duration, and were also produced under conditions that included a certain amount of noise in order to achieve a so-called robust design that takes noise into account. Specifically, the samples included two samples produced under noise-free conditions and two samples each produced under the four types of noise conditions shown below, for a total of eight samples. (1) A sample prepared with a gap of 1 millimeter between the plates W1 and W2 of the workpiece WK. (2) A sample prepared with the electrode tilted by 3 degrees from the perpendicular position of the electrode to the workpiece WK. (3) A sample prepared with the initial position of the workpiece WK, also known as the teaching position deviation, shifted by 3 mm. (4) A sample made using an electrode that was in a certain state of wear.

[0051] As shown in graph G2, the expansion amount En shows an increasing trend until time T1 when the current supply is terminated. That is, in this welding process, the expansion amount En does not peak out and a peak value of expansion amount E2 is obtained at time T1, the same as in the reference welding. Note that time T1 is the same as the current supply termination time in reference welding condition 72.

[0052] FIG. 10 is a graph showing the change in the amount of expansion of the comparative sample. The number of comparative samples is three, and three graphs are shown in FIG. 10. The conditions for creating the comparative sample are the same as those for the workpiece WK, except for the master pattern for the welding current. The master pattern for the welding current used to create the comparative sample differs from the pattern shown in FIG. 7 in that the current is not gradually increased until the end of current application, but is instead increased to a predetermined current value immediately after the start of current application and then maintained at that current value until the end of current application. As shown in FIG. 10, in the comparative sample, the expansion amount En peaks at an expansion amount ER before the end of current application, and time T1 elapses after the peak. In this case, the relational expression determining unit 65 determines the nugget diameter calculation formula 78 using the expansion amount ER.

[0053] FIG. 11 is an explanatory diagram showing the evaluation results of the nugget diameter estimation accuracy for the workpiece WK fabricated using the present welding conditions 74 and the comparative sample. The evaluation results shown in FIG. 11 were obtained by cutting the workpiece WK and the comparative sample and measuring the actual nugget diameter. The horizontal axis of the graph in FIG. 11 represents the error of the estimated nugget diameter calculated using nugget diameter calculation formula 78 relative to the actual measured nugget diameter. Specifically, this is the result of subtracting the actual measured value from the estimated value and then dividing by the actual measured value. The closer the value is to zero, the higher the estimation accuracy. The circular plots in FIG. 11 represent the evaluation results for the workpiece WK fabricated using the present welding conditions 74, and the triangular plots represent the evaluation results for the comparative sample shown in FIG. 10. The workpiece WK fabricated using the present welding conditions 74 was a sample fabricated under the 10 conditions, including the noise shown in FIG. 9.

[0054] As shown in Figure 11, the workpiece WK created using this welding condition 74 had an error of 10% between the measured and estimated nugget diameter, regardless of whether noise was present or not, which is better than the error in the comparison sample. This is thought to be because the use of upslope control in the master pattern of the welding current prevented the expansion amount En from peaking out within the welding time, improving the accuracy of estimating the expansion amount En.

[0055] As described above, the method for estimating the nugget diameter of welded materials WK according to this embodiment includes a reference welding process for measuring the expansion amount En of test materials and joining the test materials, a nugget diameter measurement process for measuring the nugget diameter Dn of the joined test materials, a relational expression determination process for determining a relational expression between the measured nugget diameter Dn of the test materials and the expansion amount En of the test materials, a main welding process for measuring the expansion amount En of the welded materials WK and joining the welded materials WK, and a nugget diameter estimation process for estimating the nugget diameter Dn of the joined welded materials WK using the expansion amount En of the welded materials WK and the determined relational expression. In the reference welding process, by using the actual measured values ​​of the expansion amount En and the nugget diameter Dn, it is possible to determine a relational expression that has a strong correlation between the expansion amount En and the nugget diameter Dn, thereby improving the accuracy of estimating the nugget diameter of the welded materials WK obtained in the main welding process.

[0056] According to the method for estimating the nugget diameter of the workpiece WK of this embodiment, the nugget diameter calculation formula 78 is determined using a setting condition in which the expansion amount En during welding does not peak out and gradually increases until the end of current application. Therefore, it is possible to suppress or prevent a problem in which the correlation between the expansion amount En and the nugget diameter Dn changes and the accuracy of estimating the nugget diameter decreases, and it is possible to further improve the accuracy of estimating the nugget diameter.

[0057] B. Other Embodiments: (B1) In the above embodiment, an example was shown in which the reference welding conditions 72, the actual welding conditions 74, the expansion amount calculation formula 76, and the nugget diameter calculation formula 78 are stored in the storage device 70 of the control device 80. In contrast, the reference welding conditions 72, the actual welding conditions 74, the expansion amount calculation formula 76, and the nugget diameter calculation formula 78 may be stored, for example, in a server that can communicate with the control device 80. In this case, the control device 80 can connect to the server via a wide area network (WAN) such as the Internet or a local area network (LAN) to acquire and store various conditions.

[0058] (B2) In the above embodiment, an example was shown in which reference welding conditions 72 are set with three levels of welding current conditions, "high," "medium," and "low," for each type of test material. In contrast, reference welding conditions 72 may be set with two levels of welding current with different magnitudes, or may be set with any number of levels of welding current, including three or more. Furthermore, although an example was shown in which the welding current value of actual welding conditions 74 is included in the three levels of welding current value of reference welding conditions 72, the welding current value of actual welding conditions 74 and the welding current value of reference welding conditions 72 may be set to different values.

[0059] (B3) In the above embodiment, an example was shown in which the workpiece WK was produced by welding two GA plate materials W1 and W2 together. However, the number of plate materials may be three or more, and the plate materials may be various metal materials such as iron (steel), aluminum alloy, magnesium, titanium, copper, etc. Also, welding of dissimilar metals may be performed.

[0060] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted.

Claims

1. A method for estimating a nugget diameter of welded materials joined by resistance spot welding, comprising: a reference welding process in which a test material corresponding to the workpiece is welded by applying current while being pressurized by a pair of electrodes, and the expansion amount of the test material is measured while the test material is being welded; a nugget diameter measuring step of measuring a nugget diameter of the joined test materials; a relational expression determining step of determining a relational expression between the measured nugget diameter of the test material and the measured expansion amount of the test material; a main welding process in which the expansion amount of the workpieces is measured and the workpieces are joined by applying current while being pressed by a pair of electrodes; a nugget diameter estimation step of estimating a nugget diameter of the joined workpieces by using the measured expansion amount of the workpieces and the determined relational expression; a step of determining whether the joined welded materials are good or bad based on the estimated nugget diameter after the main welding step and after the nugget diameter estimating step, the reference welding step is to join the test material using reference welding conditions that increase the amount of expansion from the start time of current application to the end time of current application, the current application end time is set so that the expansion amount can be gradually increased to an expansion amount at which a desired nugget diameter can be obtained without the expansion amount peaking out, the main welding step is to join the workpieces using main welding conditions that increase the amount of expansion from a current-flow start time to a current-flow end time; Nugget diameter estimation method.

2. 2. The nugget diameter estimation method according to claim 1, wherein the reference welding condition is to gradually increase the welding current value so that the welding current value reaches a predetermined first current value after the start of current flow and then reaches a predetermined second current value at the current flow end time.

3. 2. The nugget diameter estimation method according to claim 1, wherein the reference welding condition is to gradually increase the welding current value from the start of energization to a predetermined first current value, and then increase the welding current value until one cycle of a power supply frequency before the energization end time.

4. 2. The nugget diameter estimation method according to claim 1, the reference welding step includes a step of measuring the expansion amounts of the plurality of test materials using a plurality of levels of welding current values ​​and joining the plurality of test materials; the nugget diameter measuring step includes a step of measuring nugget diameters of the joined plurality of test materials, the determining step of the relational expression includes a step of determining the relational expression using the measured nugget diameters of the plurality of test materials and the measured expansion amounts of the plurality of test materials. Nugget diameter estimation method.

5. 5. The nugget diameter estimation method according to claim 4, the determining step of the relational expression includes a step of determining the relational expression by regression analysis using the measured nugget diameters of the plurality of test materials and the measured expansion amounts of the plurality of test materials. Nugget diameter estimation method.

6. 6. The nugget diameter estimation method according to claim 5, the main welding step includes a step of joining the workpieces using one welding current value included in the plurality of levels of welding current values ​​used in the reference welding step, Nugget diameter estimation method.

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