Resistance spot welding method

Abstract

To provide a technology capable of suppressing generation of spatters regardless of quality variation of a member to be joined.SOLUTION: A resistance spot welding method includes: a pre-energization step of forming an alloy layer by bringing a joint interface of a member to be joined into close contact by energization, the member to be joined being formed of two or more overlapped metal plates; and a main energization step of forming a nugget by melting the joint interface by energization after the pre-energization step. A current value at the time of end of the pre-energization step is a critical current value calculated in advance by executing the pre-energization step and the main energization step by using a test piece, and is smaller than a critical current value at which the nugget can be formed without causing generation of spatters.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a resistance spot welding method. [Background technology]

[0002] BACKGROUND ART In the prior art, in resistance spot welding, a technique is known in which current application is divided into a main current application and a pre-current application that precedes the main current application (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2015 / 83381 Summary of the Invention [Problem to be solved by the invention]

[0004] When at least one of the two or more metal plates used as the workpieces is curved, gaps may form between the overlapping metal plates. Furthermore, the workpieces may be contaminated. As a result, variations in the quality of the workpieces during actual production at a factory can prevent the desired molten state from being achieved, resulting in spatter, even when resistance spot welding is performed under predetermined conditions. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms.

[0006] (1) According to one aspect of the present disclosure, a resistance spot welding method is provided. The resistance spot welding method includes a pre-current step in which an alloy layer is formed by applying current to the joining interface of two or more overlapping metal plates to be joined, and a main current step in which, after the pre-current step, an alloy layer is formed by applying current to the joining interface to form a nugget. The current value at the end of the pre-current step is a limiting current value calculated in advance by performing the pre-current step and the main current step using a test piece, and is smaller than the limiting current value at which the nugget can be formed without generating spatter. According to this aspect, by ending the pre-current step at a current value smaller than the limiting current value, a desired molten state that suppresses spatter generation can be achieved even when the quality of the joined members varies. This suppresses spatter generation regardless of the quality variation of the joined members. (2) In the above embodiment, when the metal plates are steel plates, the current value at the end of the pre-current passing step may be greater than a lower limit current value that is 1,500 amperes less than the limit current value and less than an upper limit current value that is 500 amperes less than the limit current value. According to this embodiment, when the metal plates constituting the members to be joined are steel plates, the current value at the end of the pre-current passing step can be set greater than the lower limit current value and less than the upper limit current value. This makes it possible to more reliably suppress the generation of spatter when the metal plates constituting the members to be joined are steel plates. (3) In the above embodiment, the pre-current passing step may include at least one of pulsation current passing, in which a high-current current passing is performed by repeating a high-current current passing at a predetermined time interval, a low-current current passing at a current equal to or less than the threshold current, a slope current passing, in which the current is increased over time, and a constant current passing, in which the current is kept constant. The pre-current passing step may end upon completion of at least one of the slope current passing and the constant current passing. According to this embodiment, at least one of slope current passing and constant current passing can be performed after the pulsation current passing step in the pre-current passing step. This allows a larger alloy layer to be formed while maintaining the ratio between the nugget diameter and the alloy layer diameter, thereby more reliably suppressing the occurrence of spatter in the pre-current passing step. (4) In the above embodiment, the main current application step may include at least a slope current application in which current is applied while increasing with time, and a constant current application in which current is applied at a constant value. According to this embodiment, the nugget can grow in a shorter time than when the main current application step includes only slope current application. (5) In the above embodiment, the main current application process may be started with the ramp current application prior to the constant current application. According to this embodiment, the main current application process can be started with the ramp current application. This reduces the possibility of a sudden increase in temperature at the welding point of the workpieces. This prevents the nugget from growing rapidly, thereby more reliably preventing spatter from occurring during the main current application process. The present disclosure may be realized in various forms other than the above-described resistance spot welding method, such as a resistance spot welding apparatus, a method for manufacturing a resistance spot welding apparatus, a method for controlling a resistance spot welding apparatus, a computer program for implementing the control method, and a non-transitory recording medium on which the computer program is recorded. [Brief explanation of the drawings]

[0007] [Figure 1]FIG. 1 is a schematic diagram showing the configuration of a resistance spot welding device. [Figure 2] 1 is a flowchart illustrating a resistance spot welding method. [Figure 3] 4 is a graph showing an example of a current pattern in resistance spot welding. [Figure 4] 10 is a graph showing another example of a current pattern in resistance spot welding. [Figure 5] FIG. 10 is a diagram showing the occurrence of spatters. DETAILED DESCRIPTION OF THE INVENTION

[0008] A. First embodiment: FIG. 1 is a schematic diagram showing the configuration of a resistance spot welding apparatus 1. In the following description, the vertical direction is parallel to the ascending and descending direction of an upper electrode 21, which is a movable electrode and will be described later. The resistance spot welding apparatus 1 uses resistance spot welding to join workpieces W, which are made up of two or more overlapping metal plates W1 and W2. The metal plates W1 and W2 are, for example, steel plates. The steel plates may have a surface layer made of hot-dip galvanized, alloyed hot-dip galvanized, zinc oxide, aluminum-silicon alloy, or the like. FIG. 1 illustrates an example in which workpieces W are joined, which are made up of two or more overlapping metal plates, a first metal plate W1 and a second metal plate W2.

[0009] Resistance spot welding apparatus 1 includes welding gun 10, power supply device 30, current sensor 53, voltage sensor 54, and control unit 80.

[0010] The welding gun 10 is attached, for example, to the tip of a robot arm (not shown). The gun body 11 is moved by the robot arm to a target welding point WP of the workpieces W. The welding gun 10 includes the gun body 11, a movement mechanism 20, a pair of opposing electrodes 21, 22, and a pressure device 40. In the example shown in FIG. 1 , the upper electrode 21 is a movable electrode and is attached to the upper part of the gun body 11. The lower electrode 22 is a fixed electrode and is attached to the lower part of the gun body 11 in a position opposite the upper electrode 21. The upper electrode 21 may be a fixed electrode. The lower electrode 22 may be a movable electrode.

[0011] The movement mechanism 20 raises and lowers the movable electrode. The movement mechanism 20 has a servo motor (not shown) and converts the rotational force of the servo motor into a linear movement force in the raising and lowering direction, and transmits the converted linear movement force to the movable electrode, thereby raising and lowering the movable electrode. The power supply unit 30 supplies a welding current of a target current value between the pair of electrodes 21, 22. The pressure device 40 has a cylinder (not shown) and presses the movable electrode against the workpiece W to apply pressure.

[0012] Current sensor 53 detects the current value of the welding current supplied from power supply device 30 at predetermined time intervals and transmits a signal indicating the current value to control unit 80. A toroidal coil, for example, is used as current sensor 53. Voltage sensor 54 detects the voltage value between upper electrode 21 and lower electrode 22 at predetermined time intervals and transmits a signal indicating the voltage value to control unit 80.

[0013] Control unit 80 includes a central processing unit (CPU), a microcomputer including RAM, and ROM. Control unit 80 controls the operation of resistance spot welding apparatus 1 by causing the microcomputer to execute a pre-installed program. Specifically, control unit 80 comprehensively controls the current value, current application time, pressure application force of electrodes 21, 22, current application timing, pressure application timing, etc. At least some of the functions of control unit 80 may be realized by a hardware circuit.

[0014] 2 is a flowchart showing a resistance spot welding method, and the flow shown in FIG. 2 is executed for each welding point WP of the workpieces W, for example.

[0015] In step S1, the resistance spot welding apparatus 1 performs a pre-energization process. The pre-energization process is a process of performing pre-energization to form an alloy layer. In the pre-energization process, the resistance spot welding apparatus 1 forms an alloy layer by adhering the joining interface WS of the workpieces W by pre-energization. The presence of the alloy layer around the nugget formed on the workpieces W prevents the molten workpieces W from scattering and reduces the possibility of spattering. In the pre-energization process, the resistance spot welding apparatus 1 performs pre-energization until the alloy layer reaches a predetermined size. At the end of the pre-energization process, the temperature of the workpieces W reaches, for example, a temperature range of 1200°C to 1500°C, which is the temperature range for forming an alloy layer.

[0016] In step S2, the resistance spot welding apparatus 1 performs a main current process after the pre-current process. The main current process is a process of performing main current to form a nugget. In the main current process, the resistance spot welding apparatus 1 melts the joining interface WS of the workpieces W by main current, thereby forming a nugget.

[0017] Fig. 3 is a graph showing an example of a current pattern in resistance spot welding. Fig. 4 is a graph showing another example of a current pattern in resistance spot welding. In Figs. 3 and 4, the vertical axis represents current, and the horizontal axis represents time.

[0018] In the pre-energization process, at least one of slope energization and constant current energization and pulsation energization are performed. The pre-energization process ends when the execution of at least one of slope energization and constant current energization is completed. In the main energization process, at least slope energization and constant current energization are performed. The main energization process starts with slope energization before constant current energization.

[0019] In pulsation current application, the resistance spot welding apparatus 1 repeatedly performs high-current application, in which a current higher than a predetermined threshold current is applied, and low-current application, in which a current equal to or lower than the threshold current is applied, at predetermined time intervals. Pulsation current application may start with high-current application or low-current application. Pulsation current application preferably includes high-current application two or more times and five or less times. The threshold current for pulsation current application can be determined experimentally or empirically. Thus, the current values ​​I1 and I2 for pulsation current application can be determined experimentally or empirically. The current value I1 for high-current application is preferably 8 kiloamperes or more and 15 kiloamperes or less. When high-current application is performed two or more times in pulsation current application, the current values ​​I1 for the high-current application may be the same or different. The current value I2 during low current conduction is preferably at least one-third of the current value I1 during high current conduction and not more than two-thirds of the current value I1 during high current conduction. When low current conduction is performed two or more times during pulsation conduction, the current values ​​I2 during each low current conduction may be the same or different. The current conduction time T1 during high current conduction is preferably at least 4 milliseconds and not more than 8 milliseconds. When high current conduction is performed two or more times during pulsation conduction, the current conduction times T1 during each high current conduction may be the same or different. The current conduction time T2 during low current conduction is preferably at least 4 milliseconds and not more than 8 milliseconds. When low current conduction is performed two or more times during pulsation conduction, the current conduction times T2 during each low current conduction may be the same or different. During pulsation conduction, the time interval between repeated high current conduction and low current conduction is preferably less than 10 ms. The total energization time T3 in the pulsation energization is preferably 20 milliseconds or more and 40 milliseconds or less.

[0020] In the slope conduction, the resistance spot welding apparatus 1 conducts current while increasing it over time. The execution of the slope conduction is completed when the current has increased to a preset current value I4. The current values ​​I3 to I5 in the slope conduction can be determined experimentally or empirically. The current value I3 at the start of the slope conduction performed in the pre-current conduction step may be equal to or greater than the current value I2 in the low-current conduction of the pulsation conduction performed in the pre-current conduction step, or may be less than the current value I2 in the low-current conduction. As shown in FIG. 3 , when the pre-current conduction step ends with slope conduction, the following occurs. In this case, the current value I4 at the end of the slope conduction corresponding to the end of the pre-current conduction step is smaller than the current value I1 in the high-current conduction and larger than the current value I2 in the low-current conduction of the pulsation conduction performed in the pre-current conduction step. In this case, the current value I4 at the end of the slope energization, which corresponds to the end of the pre-energization step, is smaller than the current value I5 in the constant current energization executed in the main energization step.

[0021] In constant current energization, the resistance spot welding apparatus 1 energizes at a constant current. The execution of constant current energization is completed when predetermined energization times T6 and T7 have elapsed. The current value I5 in constant current energization can be determined experimentally or empirically. The current value I5 in the constant current energization performed in the main energization process is greater than the current value IE at the end of the pre-energization process. The current value I5 in the constant current energization performed in the main energization process is greater than the current value I2 in the low current energization of the pulsation energization performed in the pre-energization process. The current value I5 in the constant current energization performed in the main energization process may be greater than or less than the current value I1 in the high current energization of the pulsation energization performed in the pre-energization process. As shown in FIG. 4, when the pre-energization process is completed by constant current energization, the following occurs. In this case, the current value I6 in the constant current energization corresponding to the end of the pre-energization process is smaller than the current value I1 in the high current energization and larger than the current value I2 in the low current energization of the pulsation energization performed in the pre-energization process. Also, in this case, the current value I6 in the constant current energization corresponding to the end of the pre-energization process is smaller than the current value I5 in the constant current energization performed in the main energization process.

[0022] 3 , the resistance spot welding apparatus 1 performs a pre-energization step in which pulsation energization is performed for 30 milliseconds followed by slope energization for 250 milliseconds. Specifically, in the pulsation energization, the resistance spot welding apparatus 1 performs a high current energization at a preset current value I1 for 6 milliseconds, followed by a low current energization at a preset current value I2 for 6 milliseconds. The resistance spot welding apparatus 1 then performs a high current energization at the preset current value I1 for 6 milliseconds, followed by a low current energization at the preset current value I2 for 6 milliseconds, and finally a high current energization at the preset current value I1 for 6 milliseconds. In other words, the pulsation energization includes three high current energizations and two low current energizations. During the ramp current conduction in the pre-energization step, the resistance spot welding apparatus 1 increases the current over time from a current value I3, which is greater than the current value I2 during the low-current conduction and less than the current value I1 during the high-current conduction. The resistance spot welding apparatus 1 then terminates the pre-energization step when the current has increased to a preset current value I4. After completing the pre-energization step, the resistance spot welding apparatus 1 then performs ramp current conduction for 34 milliseconds, followed by constant current conduction for 34 milliseconds, as the main energization step. During the ramp current conduction in the main energization step, the resistance spot welding apparatus 1 increases the current over time from the current value I4 at the end of the pre-energization step (IE), i.e., the current value I4 at the end of the ramp current conduction in the pre-energization step, to a preset current value I5. The resistance spot welding apparatus 1 then terminates the main energization step after performing constant current conduction at the preset current value I5.

[0023] In the example shown in Fig. 4, the resistance spot welding apparatus 1 performs a pre-energization process by applying pulsation current for 30 milliseconds followed by constant current energization for 200 milliseconds. In the pre-energization process shown in Fig. 4, the resistance spot welding apparatus 1 applies constant current at a preset current value I6. After applying constant current in the pre-energization process, the resistance spot welding apparatus 1 ends the pre-energization process. The energization pattern of the pulsation current and the energization pattern in the main energization process are the same as those shown in Fig. 3.

[0024] To form an alloy layer that suppresses spatter generation, the resistance spot welding apparatus 1 sets the current value IE at the end of the pre-current process to be smaller than the limiting current value. The limiting current value is the current value at which a nugget can be formed without spatter generation during the pre-current process and the main current process. The limiting current value varies depending on the material of the metal plates W1 and W2 constituting the workpieces W, the thickness of the metal plates W1 and W2 constituting the workpieces W, and the welding conditions. The limiting current value is calculated in advance of the actual production by performing the pre-current process and the main current process using test pieces made of the same material and with the same thickness as those used in the actual production under the same welding conditions as those used in the actual production. The welding conditions include, for example, the current values ​​I1 to I6, the current application times T1 to T7, the pressure applied by the electrodes 21 and 22, the current application timing, and the pressure application timing.

[0025] FIG. 5 shows the occurrence of spatter when joining two or more overlapping steel plates (W) to be joined. The thickness of the steel plates constituting the joined members (W) and the welding conditions are the same in each diagram of FIG. 5. FIG. 5 shows the relationship between the current value (IE) at the end of the pre-energization process, the ratio of the alloy layer diameter (LM) to the nugget diameter (LN), and the occurrence of spatter. The alloy layer diameter (LM) is the diameter of the alloy layer (M). The nugget diameter (LN) is the diameter of the nugget (N). Both the alloy layer diameter (LM) and the nugget diameter (LN) are dimensions along the surface direction of the joined members (W). The nugget diameter (LN) can be estimated, for example, using the amount of expansion in the direction perpendicular to the surface direction of the joined members (W) and the electrical resistance value between the pair of electrodes (21, 22).

[0026] As shown in the left diagram of FIG. 5 , when the current value IE at the end of the pre-current process is set to be equal to or less than the lower limit current value IL, the current value IE at the end of the pre-current process is smaller than when the current value IE at the end of the pre-current process is set to be greater than the lower limit current value IL. As a result, when the current value IE at the end of the pre-current process is set to be equal to or less than the lower limit current value IL, the diameter LM of the alloy layer formed at the joining interface WS of the workpieces W is smaller than when the current value IE at the end of the pre-current process is set to be greater than the lower limit current value IL. Therefore, when the current value IE at the end of the pre-current process is set to be equal to or less than the lower limit current value IL, the nugget N grows rapidly during the main current process, and the nugget N cannot be covered by the alloy layer M. As a result, spatter occurs during the main current process. In other words, the alloy layer diameter LM is correlated with the occurrence of spatter. In addition, when the metal plates W1 and W2 constituting the workpiece W are steel plates, the lower limit current value IL is a current value 1,500 amperes smaller than the limit current value IS, regardless of whether the surface of the steel plate is plated or not.

[0027] As shown in the right diagram of FIG. 5 , when the current value IE at the end of the pre-current process is set to be equal to or greater than the upper limit current value IU, the current value IE at the end of the pre-current process is larger than when the current value IE at the end of the pre-current process is set to be smaller than the upper limit current value IU. Therefore, when the current value IE at the end of the pre-current process is set to be equal to or greater than the upper limit current value IU, not only the alloy layer M but also the nugget N begins to form in the pre-current process. In this case, when the current value IE at the end of the pre-current process is set to be equal to or greater than the upper limit current value IU, the diameter difference LD between the nugget diameter LN and the alloy layer diameter LM is smaller than when the current value IE at the end of the pre-current process is set to be smaller than the upper limit current value IU. Therefore, when the current value IE at the end of the pre-current process is set to be equal to or greater than the upper limit current value IU, the nugget N is formed in the pre-current process, but the alloy layer M cannot cover the periphery of the nugget N, and spatter occurs in the pre-current process. In other words, the ratio of the nugget diameter LN to the alloy layer diameter LM correlates with the occurrence of spatter. In addition, when the metal plates W1 and W2 constituting the joined member W are steel plates, the upper limit current value IU is a current value 500 amperes smaller than the limit current value IS, regardless of whether the surface of the steel plate is plated or not.

[0028] 5, when the current value IE at the end of the pre-energization step is set to be greater than the lower limit current value IL and less than the upper limit current value IU, an alloy layer M and nugget N of the desired size can be formed, and no spattering is observed. Therefore, when the metal plates W1 and W2 constituting the workpieces W are both steel plates, the resistance spot welding device 1 sets the current value IE at the end of the pre-energization step to be greater than the lower limit current value IL and less than the upper limit current value IU.

[0029] According to the above embodiment, the resistance spot welding apparatus 1 performs a pre-energization step before the main energization step to form the nugget N, thereby forming an alloy layer M around the formation position of the nugget N to prevent the molten workpieces W from spattering. By ending the pre-energization step at a current value smaller than the limiting current value IS, the resistance spot welding apparatus 1 can create a desired molten state that suppresses the generation of spatter, even when the quality of the workpieces W varies. This makes it possible to suppress the generation of spatter regardless of the quality variation of the workpieces W.

[0030] Furthermore, according to the above embodiment, when two or more overlapping steel plates are joined by resistance spot welding to form workpieces W, the resistance spot welding apparatus 1 can set the current value IE at the end of the pre-energization step to be greater than the lower limit current value IL and less than the upper limit current value IU. This makes it possible to more reliably suppress the generation of spatter when the metal plates W1 and W2 constituting the workpieces W are steel plates. Furthermore, when the metal plates W1 and W2 constituting the workpieces W are steel plates, the current value IE at the end of the pre-energization step may be 1000 amperes less than the limit current value IS. This makes it possible to more reliably suppress the generation of spatter.

[0031] Furthermore, according to the above embodiment, the resistance spot welding apparatus 1 can perform at least one of slope current application and constant current application after pulsation current application in the pre-current application step. This allows a larger alloy layer M to be formed while maintaining the ratio between the nugget diameter LN and the alloy layer diameter LM, thereby more reliably suppressing spatter generation in the pre-current application step.

[0032] Furthermore, according to the above embodiment, the resistance spot welding apparatus 1 can start the main current application process by using a sloped current. This reduces the possibility of a sudden increase in temperature of the welding point WP of the workpieces W compared to when the main current application process is performed using only a constant current. This prevents the nugget N from growing rapidly, thereby more reliably preventing spatter from occurring during the main current application process.

[0033] Furthermore, according to the above embodiment, the resistance spot welding apparatus 1 can perform both slope current application and constant current application in the main current application step, which allows the nugget N to grow in a shorter time than when the main current application step is performed using only slope current application.

[0034] Furthermore, according to the above embodiment, the resistance spot welding apparatus 1 can perform pulsation current application by repeatedly applying a high current and a low current at time intervals of less than 10 milliseconds in the pre-current application step. By repeatedly applying a high current and a low current at short time intervals, the high current can raise the temperature of the welding point WP of the workpieces W to a temperature range where the alloy layer M is formed, while the low current can suppress the generation of spatter. In other words, the alloy layer M of the desired size can be formed in a short time while suppressing the generation of spatter.

[0035] B. Other Embodiments: (B1) The metal plates W1 and W2 constituting the workpieces W may be made of materials other than steel plates. In this case, the current value IE at the end of the pre-energization process can be set by calculating the lower limit current value IL and the upper limit current value IU relative to the limit current value IS based on the difference between welding using a test piece and actual production, as in Figure 5.

[0036] (B2) The pre-energization step may include pulsation energization two or more times. The pre-energization step may include slope energization two or more times. The pre-energization step may include constant current energization two or more times. The main energization step may include slope energization two or more times. The main energization step may include constant current energization two or more times.

[0037] 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 of 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. [Explanation of symbols]

[0038] 1...resistance spot welding device, 10...welding gun, 11...gun body, 20...moving mechanism, 21...upper electrode, 22...lower electrode, 30...power supply, 40...pressure device, 53...current sensor, 54...voltage sensor, 80...controller, I1 to I6...current value, IE...current value at the end of pre-energization process, IL...lower limit current value, IS...limit current value, IU...upper limit current value, LD...diameter difference, LM...alloy layer diameter, LN...nugget diameter, M...alloy layer, N...nugget, T1 to T7...energization time, W...workpiece, W1...first metal plate, W2...second metal plate, WP...welding point, WS...welding interface

Claims

1. 1. A resistance spot welding method comprising: a pre-current application process in which an alloy layer is formed by applying current to bond the joining interface of two or more overlapping metal plates; a main current application process for forming a nugget by melting the joining interface by current application after the pre-current application process, a current value at the end of the pre-energization step is a limiting current value calculated in advance by performing the pre-energization step and the main energization step using a test piece, and is smaller than the limiting current value at which the nugget can be formed without generating spatter.

2. 2. The resistance spot welding method according to claim 1, a current value at the end of the pre-energization step that is greater than a lower limit current value that is 1,500 amperes less than the limit current value and less than an upper limit current value that is 500 amperes less than the limit current value, when the metal plate is a steel plate.

3. 2. The resistance spot welding method according to claim 1, In the pre-energization step, pulsation energization, which repeats a high current energization in which a current higher than a predetermined threshold current is energized and a low current energization in which a current equal to or less than the threshold current is energized at predetermined time intervals; At least one of slope energization, in which an electric current is energized while increasing with the lapse of time, and constant current energization, in which an electric current is energized at a constant level, is executed; the pre-energization step is terminated when execution of at least one of the slope energization and the constant current energization is completed.

4. 2. The resistance spot welding method according to claim 1, In the main current application step, Slope current conduction, which increases the current over time, and constant current energization, wherein energization is performed at a constant current.

5. 5. The resistance spot welding method according to claim 4, the main current supplying step is started with the slope current supplying prior to the constant current supplying.

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