Method for detecting surface cracks in spot welds

Abstract

To provide a spot welding surface crack detection method that can determine in-process whether or not cracks caused by liquid metal brittleness have occurred in the welded area. [Solution] The spot welding surface crack detection method is a method for spot welding in which a workpiece consisting of multiple steel plates stacked together, including at least one plated steel plate, is sandwiched between a pair of electrodes, pressurized, and current is passed between the pair of electrodes to weld the workpiece, and comprises a calculation step of calculating the amount of expansion of the workpiece during welding from the pressure and stroke between the pair of electrodes, and a determination step of determining whether or not a crack caused by liquid metal brittleness has occurred in the welded part of the workpiece by comparing the change in the amount of expansion after the end of current application as a contraction amount with a predetermined threshold, wherein the predetermined threshold is set in advance based on the relationship between the crack depth and the amount of contraction that occurs on the surface of the plated steel plate due to welding.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a surface crack determination method for spot welding, and more particularly to a surface crack determination method for spot welding in which a workpiece formed by stacking a plurality of steel sheets including at least one plated steel sheet is sandwiched between a pair of electrodes, and the pair of electrodes is energized while applying pressure to weld the workpiece.

BACKGROUND ART

[0002] Spot welding is known, in which a workpiece formed by stacking a plurality of metal sheets is sandwiched between a pair of electrodes, and the pair of electrodes is energized while applying pressure to weld the workpiece. In such spot welding, a method has been proposed for determining the welding state of spot welding by utilizing the expansion amount of the workpiece during welding.

[0003] Patent Document 1 discloses a resistance spot welding method comprising: a preparation step of preparing a machine learning model trained on the relationship between the feature values of test data recording time-series changes in the expansion amount of a workpiece in test welding and the quality of the welding state of the test welding; and a determination step of determining the quality of the welding state of main welding by using main welding data recording time-series changes in the expansion amount of the workpiece in main welding and the machine learning model. In this resistance spot welding method, the feature values include a first feature value related to the slope of change in the expansion amount of the workpiece during the expansion period in which the workpiece expands due to energization, and a second feature value related to the slope of change in the expansion amount of the workpiece during the contraction period in which the workpiece contracts after the expansion period.

PRIOR ART DOCUMENTS

PATENT DOCUMENTS

[0004]

PATENT DOCUMENT 1

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] For example, when assembling automobile bodies or attaching parts, spot welding of plated steel sheets, such as galvanized steel sheets, is sometimes performed. When spot welding a workpiece made up of multiple stacked plated steel sheets, cracks due to liquid metal embrittlement may occur in the weld depending on the welding conditions. When such cracks occur in the weld, the strength of the weld decreases. Therefore, in order to suppress the deterioration of welding quality due to cracks caused by liquid metal embrittlement, there is a need for a technology that can determine in-process whether or not cracks due to liquid metal embrittlement have occurred in the weld.

[0006] However, the technology described in Patent Document 1 had the problem that it was difficult to determine in-process whether or not cracks caused by liquid metal brittleness were occurring in the welded joint.

[0007] This disclosure was made to solve such problems and aims to provide a spot welding surface crack detection method that can determine in-process whether or not cracks caused by liquid metal brittleness have occurred in the weld. [Means for solving the problem]

[0008] The surface crack detection method for spot welding according to this disclosure is a method for spot welding in which a workpiece consisting of multiple steel plates stacked together, including at least one plated steel plate, is sandwiched between a pair of electrodes, pressurized, and current is passed between the pair of electrodes to weld the workpiece, and comprises a calculation step of calculating the amount of expansion of the workpiece during welding from the pressure applied between the pair of electrodes and the stroke, and a determination step of determining whether or not a crack caused by liquid metal brittleness has occurred in the welded part of the workpiece by comparing the change in the amount of expansion after the end of current application as a contraction amount with a predetermined threshold, wherein the predetermined threshold is set in advance based on the relationship between the crack depth and the amount of contraction that occurs on the surface of the plated steel plate due to welding. [Effects of the Invention]

[0009] This disclosure provides a method for determining surface cracks in spot welding, which can determine in-process whether or not cracks caused by liquid metal brittleness have occurred in the welded area. [Brief explanation of the drawing]

[0010] [Figure 1] This figure shows the configuration of the spot welding system related to this disclosure. [Figure 2] This graph shows an example of an expansion waveform, and another graph shows an example of the relationship between contraction and crack depth. [Figure 3] This flowchart shows the method for determining surface cracks in spot welds according to this disclosure. [Modes for carrying out the invention]

[0011] Embodiment 1 Embodiments of this disclosure will be described below with reference to the drawings. However, this disclosure is not limited to the embodiments described below. Also, for clarity, the following description and drawings have been simplified as appropriate. What is shown in the drawings is only a part of the whole, and there are actually many other components that are not shown. In the following description, the same or equivalent elements are denoted by the same reference numerals, and redundant descriptions are omitted.

[0012] Figure 1 is a diagram showing the configuration of the spot welding system according to this disclosure. The spot welding system 100 will be described with reference to Figure 1(a). The spot welding system 100 shown in Figure 1(a) includes a spot welding device 10 and a surface crack detection device 50.

[0013] The spot welding apparatus 10 has a welding gun G. The welding gun G is held by a robot arm R. The welding gun G has a pair of electrodes, a movable electrode 11 and a fixed electrode 12, a movable electrode side arm 13, a fixed electrode side arm 14, an electrode lifting device 20, and a current adjustment device 30.

[0014] The movable electrode 11 is mounted on the movable electrode side arm 13 via the electrode lifting device 20. The fixed electrode 12 is mounted on the fixed electrode side arm 14. The movable electrode 11 and the fixed electrode 12 are positioned so that their ends face each other. The movable electrode 11 and the fixed electrode 12 are energized while pressurizing the workpiece W1 between them. The workpiece W1, consisting of stacked steel plates W2, W3, and W4, is placed between the movable electrode 11 and the fixed electrode 12.

[0015] The electrode lifting device 20 includes a servo motor 21 driven by a control signal from a control unit that controls the spot welding apparatus 10, and a lifting member 22 coupled to the output shaft of the servo motor 21 via a gear. The electrode lifting device 20 holds the movable electrode 11 and raises and lowers the movable electrode 11 mounted on the lifting member 22 by the rotational driving force of the servo motor 21. The movable electrode 11 is movable in the vertical direction by the electrode lifting device 20 so that the movable electrode 11 and the fixed electrode 12 face each other.

[0016] The current adjustment device 30 adjusts the current supplied between the movable electrode 11 and the fixed electrode 12 in accordance with a control signal from the control unit that controls the spot welding apparatus 10. For example, the current adjustment device 30 can be a device equipped with a variable resistor or a device equipped with a converter.

[0017] The spot welding apparatus 10 further includes an electrode position measuring unit 41 and a pressure measuring unit 42. The electrode position measuring unit 41 includes, for example, an encoder that detects the rotational angle position of the output shaft of the servo motor 21 to obtain the position of the movable electrode 11. The electrode position measuring unit 41 measures the stroke by detecting the position of the movable electrode 11. The stroke is the amount by which the movable electrode 11 is pushed in and pushed back after contacting the outer surface of the workpiece W1 (specifically, the surface of the steel plate W2).

[0018] The pressing force measurement unit 42 includes a strain sensor that detects the amount of strain of the fixed electrode-side arm 14 when the workpiece W1 is pressed. The pressing force measurement unit 42 measures the pressing force applied by the movable electrode 11 and the fixed electrode 12 using the detected strain amount and a strain conversion coefficient described later.

[0019] Between the movable electrode 11 and the fixed electrode 12, a workpiece W1 formed by stacking steel plates W2, W3, and W4 is arranged. Each of the steel plates W2, W3, and W4 is a plated steel sheet having a metal plating layer such as zinc plating represented by electrogalvanizing and hot-dip galvanizing (the hot-dip galvanizing includes alloyed hot-dip galvanizing), or zinc alloy plating containing elements such as aluminum and magnesium in addition to zinc, on the surface of the base material (base steel sheet). The steel plates W2, W3, and W4 preferably have a metal plating layer with a melting point lower than that of the base material. The steel plates W2, W3, and W4 having a metal plating layer with a melting point lower than that of the base material are prone to cracks described later.

[0020] The steel plates W2, W3, and W4 constituting the workpiece W1 include at least one plated steel sheet. All of the steel plates W2, W3, and W4 may be plated steel sheets, or a combination of plated steel sheets and steel sheets other than plated steel sheets may be used. The steel plates W2, W3, and W4 shown in FIG. 1 are stacked such that the outer surface of the workpiece W1 is a metal plating layer. Further, the number of steel plates to be welded is not limited to three, and may be two, or four or more.

[0021] The spot welding device 10 melts the workpiece W1 by energizing between the movable electrode 11 and the fixed electrode 12 while pressing the workpiece W1 sandwiched between the movable electrode 11 and the fixed electrode 12. Thereafter, by stopping the energization and maintaining the pressurized state, the workpiece W1 is cooled and solidified, whereby the workpiece W1 is welded. Further, the spot welding device 10 transmits the stroke measured by the electrode position measurement unit 41 and the pressing force measured by the pressing force measurement unit 42 to the surface crack determination device 50.

[0022] As shown in FIG. 1(b), at the joint interface of the welded steel sheets W2, W3, and W4, a weld portion N called a nugget is formed. The nugget is a portion where the melted part of the workpiece W1 is cooled and solidified. The weld portion N of the workpiece W1 expands as it melts due to energization, and contracts as it cools after energization is completed.

[0023] Here, when spot welding is performed on the workpiece W1 formed by stacking the steel sheets W2, W3, and W4, cracks C caused by liquid metal brittleness may occur in the weld portion N depending on the welding conditions. Examples of the welding conditions include the pressing force of the movable electrode 11 and the fixed electrode 12, the energization amount, and the energization time. The cracks C are considered to be cracks caused by so-called liquid metal brittleness, which is triggered when the low-melting-point metal plating layer melts during welding, and tensile stress from the pressing force of the movable electrode 11 and fixed electrode 12, as well as thermal expansion and contraction of the steel sheets W2, W3, W4, is applied to the weld portion N, causing the molten low-melting-point metal to penetrate into the grain boundaries of the base material of the steel sheets W2, W3, W4 and reduce the grain boundary strength.

[0024] The crack C shown in FIG. 1(b) occurs on the surface of the steel sheet W2 on the side in contact with the movable electrode 11. The crack C may occur not only on the surface of the steel sheet W2 on the side in contact with the movable electrode 11, but also on the surface of the steel sheet W4 on the side in contact with the fixed electrode 12, or on the surfaces of the steel sheets W2, W3, W4 on the sides where the steel sheets W2, W3, W4 are in contact with each other. When such a crack C occurs in the weld portion N, the strength of the weld portion N decreases, and thus the welding quality is degraded.

[0025] Accordingly, the surface crack determination device 50 according to the present disclosure is capable of in-process determining whether or not the crack C caused by liquid metal brittleness has occurred in the weld portion N.

[0026] The surface crack determination device 50 shown in FIG. 1(a) is constituted by a computer including a CPU (not shown), a memory including ROM, RAM and the like, an input / output interface, and the like. The surface crack determination device 50 is communicably connected to the spot welding device 10. The surface crack determination device 50 may be incorporated as a part of the spot welding device 10.

[0027] The memory of the surface crack detection device 50 stores a surface crack detection program. The memory of the surface crack detection device 50 also stores a predetermined threshold value for determining whether or not a crack C has occurred in the welded area N. Furthermore, the memory of the surface crack detection device 50 temporarily stores information acquired from the spot welding device 10. The CPU of the surface crack detection device 50 functions as a calculation unit 51 and a determination unit 52 by executing the surface crack detection program pre-stored in the memory.

[0028] The calculation unit 51 calculates the amount of expansion of the workpiece W1 during welding from the pressure and stroke between the movable electrode 11 and the fixed electrode 12. Specifically, the calculation unit 51 sequentially acquires information on the pressure and stroke between the movable electrode 11 and the fixed electrode 12 from the spot welding apparatus 10, and calculates the amount of expansion in the thickness direction of the workpiece W1 during welding from said pressure and stroke. In this way, the calculation unit 51 can sequentially calculate the amount of expansion in-process from the pressure and stroke between the movable electrode 11 and the fixed electrode 12.

[0029] In a preferred embodiment, the amount of expansion is calculated by the following formula (1). E = S + a × F ... (1) Here, E is the expansion amount, S is the stroke, a is the strain conversion coefficient, and F is the applied pressure. The strain conversion coefficient a is a coefficient that converts the applied pressure into strain, and can be any constant. This allows the expansion amount to be easily calculated from the applied pressure and stroke between the movable electrode 11 and the fixed electrode 12 using the above equation (1).

[0030] Figure 2(a) is a graph showing an example of an expansion waveform. The expansion waveform shown in Figure 2(a) shows the time variation of the expansion amount calculated from the measured pressure and stroke. The horizontal axis of the graph in Figure 2(a) represents time [ms]. The vertical axis of the graph in Figure 2(a) represents the expansion amount [mm]. The expansion waveform obtained during a single welding cycle includes, for example, the time variation of the expansion amount at the squeeze time T1, energizing time T2, and holding time T4.

[0031] The squeeze time T1 is the time from when the workpiece W1 is sandwiched between the movable electrode 11 and the fixed electrode 12 and pressurization by the movable electrode 11 and the fixed electrode 12 is started until current is passed between the movable electrode 11 and the fixed electrode 12. The current-passing time T2 is the time during which current is passed between the movable electrode 11 and the fixed electrode 12 while the workpiece W1 is sandwiched between the movable electrode 11 and the fixed electrode 12 and pressurized. This current-passing time T2 is the time during which the welded area N expands due to melting caused by the current. The holding time T4 is the time during which the pressurized state by the movable electrode 11 and the fixed electrode 12 is maintained after the current is stopped from being passed to the movable electrode 11 and the fixed electrode 12. This holding time T4 is the time during which the welded area N contracts due to cooling after the current is passed.

[0032] The determination unit 52 determines whether or not a crack C caused by liquid metal brittleness has occurred in the welded part N by comparing the change in the amount of expansion after the end of energization with a predetermined threshold value, which is defined as the amount of contraction X.

[0033] In a preferred embodiment, the contraction amount X is the difference between the maximum value Emax and the minimum value Emini of the expansion amount during the holding time T4 in which the pressurized state by the movable electrode 11 and the fixed electrode 12 is maintained after the end of energization. This improves the accuracy of the determination.

[0034] The predetermined threshold is set in advance based on the relationship between the crack depth and shrinkage amount X that occurs on the surface of steel plates W2, W3, and W4 due to welding. The relationship between crack depth and shrinkage amount X is such that the shrinkage amount X increases as the crack depth increases, and decreases as the crack depth decreases. The predetermined threshold can be determined in advance through experiments or simulations. For example, spot welding can be performed on multiple experimental workpieces W1 under different welding conditions, and the maximum value of shrinkage amount X corresponding to the crack depth at which no crack C occurs in the welded area N can be set as the predetermined threshold.

[0035] Figure 2(b) is a graph showing an example of the relationship between shrinkage X and crack depth. The horizontal axis of the graph in Figure 2(b) represents shrinkage X [μm]. The vertical axis of the graph in Figure 2(b) represents crack depth [mm]. In Figure 2(b), the boundary line of a predetermined threshold L is shown by a dashed line. The predetermined threshold L shown in Figure 2(b) is set to 0.96 mm.

[0036] The determination unit 52 compares the amount of shrinkage X with a predetermined threshold L read from memory and determines that if the amount of shrinkage X is greater than or equal to the predetermined threshold L, a crack C caused by liquid metal brittleness has occurred in the welded part N (NG). On the other hand, the determination unit 52 compares the amount of shrinkage X with a predetermined threshold L read from memory and determines that if the amount of shrinkage X is less than the predetermined threshold L, a crack C caused by liquid metal brittleness has not occurred in the welded part N (OK).

[0037] Next, Figure 3 is a flowchart showing the surface crack detection method for spot welding according to the present disclosure. The surface crack detection method according to the present disclosure is performed by a surface crack detection device 50. The surface crack detection method is a spot welding surface crack detection method in which a workpiece W1, in which steel plates W2, W3, and W4 including at least one plated steel plate are stacked, is welded by sandwiching the workpiece W1 between a movable electrode 11 and a fixed electrode 12, applying pressure, and passing an electric current between the movable electrode 11 and the fixed electrode 12.

[0038] As shown in Figure 3, the surface crack determination method according to this disclosure includes a calculation step (step S1) and a determination step (step S2). The calculation step is a step of calculating the amount of expansion of the workpiece W1 during welding from the pressure and stroke between the movable electrode 11 and the fixed electrode 12. The calculation step is performed by the calculation unit 51. The determination step is a step of determining whether or not a crack C caused by liquid metal brittleness has occurred in the welded part N of the workpiece W1 by comparing the change in the amount of expansion after the end of energization as a shrinkage amount X with a predetermined threshold L. The predetermined threshold L is set in advance based on the relationship between the crack depth and the shrinkage amount X that occurs on the surface of the plated steel sheet (e.g., steel sheet W2) due to welding. The determination step is performed by the determination unit 52.

[0039] In this way, by setting a predetermined threshold L based on the relationship between the crack depth and shrinkage amount X that occurs on the surface of the plated steel sheet due to welding, it is possible to determine whether or not cracks C caused by liquid metal brittleness have occurred in the welded part N.

[0040] As described above, the surface crack determination method according to this disclosure makes it possible to determine in-process whether or not cracks C caused by liquid metal brittleness have occurred in the welded part N.

[0041] This disclosure is not limited to the embodiments described above, and may be modified as appropriate without departing from its spirit. [Explanation of Symbols]

[0042] 10 Spot welding machine 11 Movable electrode 12 Fixed electrode 13 Movable electrode side arm 14 Fixed electrode side arm 20 Electrode lifting device 21 Servo motor 22 Lifting member 30 Current regulator 41 Electrode position measurement unit 42 Pressure measurement unit 50 Surface crack detection device 51 Calculation unit 52 Determination unit 100 Spot Welding System C Crack G Welding gun N Welded area R Robot arm T1 Squeeze time T2 Energizing time T4 Holding time W1 Workpiece W2~W4 Steel Plate

Claims

[Claim 1] A method for determining surface cracks in spot welding, wherein a workpiece consisting of multiple steel plates stacked together, including at least one plated steel plate, is sandwiched between a pair of electrodes, pressurized, and an electric current is passed between the pair of electrodes to weld the workpiece, A calculation step of calculating the amount of expansion of the workpiece during welding from the pressure and stroke between the pair of electrodes, The system includes a determination step of determining whether or not a crack caused by liquid metal brittleness has occurred in the welded portion of the workpiece by comparing the change in the amount of expansion after the end of the energization with a predetermined threshold as the amount of contraction, The predetermined threshold is set in advance based on the relationship between the crack depth generated on the surface of the plated steel sheet by welding and the amount of shrinkage, in a spot welding surface crack determination method.

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