Resistance spot welding process and resistance spot welding device

Abstract

A workpiece (W) prepared by laminating two or more metal sheets (P1, P2, P3) is welded using a resistance spot welding apparatus (1). A first current application control is performed on the electrode pair (21, 22) holding the workpiece (W) such that a first current (I1) flows between the electrode pair (21, 22). Subsequently, a second current application control is performed on the electrode pair (21, 22) such that a current flowing between the electrode pair (21, 22) decreases from the first current (I1) to a second current (I2) that is smaller than the first current (I1). Subsequently, a third current application control is performed on the electrode pair (21, 22) such that a current flowing between the electrode pair (21, 22) increases from the second current (I2) to a third current (I3) which is greater than the second current (I2).Subsequently, a fourth current application control is carried out on the electrode pair (21, 22) such that a fourth current (I4) flows between the electrode pair (21, 22).

Description

PRIOR ARTThe present disclosure relates to a resistance spot welding method and a resistance spot welding apparatus.In order to make a weld lens effective in resistance spot welding while preventing the formation of a spatter, a method is already known for supplying a constant current before applying a main current. Japanese Unexamined Patent Application Publication No. 2021-079410 discloses an execution of energization control by which a constant current is supplied for a prescribed period after initial energization to gradually increase a welding current, followed by supplying a larger constant current.SUMMARYIn resistance spot welding, when a workpiece prepared by laminating two or more metal sheets is welded, inconsistencies in heat generation between the metal sheets may result in deterioration in welding quality. Such inconsistencies in heat generation may be caused by a difference in resistance between the metal sheets. The difference in resistance may be caused, for example, by differences in tensile strength and sheet thickness between the metal sheets.If inconsistencies in heat generation occur between the metal sheets, an uneven weld spot is formed on the workpiece, leading to possible insufficient weld penetration in some portions of the metal sheets.In view of the foregoing, according to an aspect of the present disclosure, it is preferable to provide a method that can achieve high-quality welding when resistance spot welding is performed on a workpiece prepared by laminating two or more metal sheets.According to an aspect of the present disclosure, there is provided a resistance spot welding method for welding a workpiece prepared by laminating two or more metal sheets using a resistance spot welding apparatus. The resistance spot welding device includes a pair of electrodesThe resistance spot welding method includes performing a first current application control on the electrode pair such that a first current flows between the electrode pair in a state where the workpiece is held between the electrode pair at both ends of the workpiece in a lamination direction of the two or more metal sheets. The resistance spot welding method may further include performing a second current application control on the electrode pair such that a current flowing between the electrode pair decreases from the first current to a second current smaller than the first current. The second energization control may be performed subsequent to the first energization control.The resistance spot welding method may further include performing a third current application control on the electrode pair such that a current flowing between the electrode pair increases from the second current to a third current larger than the second current. The third energization control may be performed subsequent to the second energization control.The resistance spot welding method may further include performing a fourth current application control on the electrode pair such that a fourth current flows between the electrode pair. The fourth energization control may be performed subsequent to the third energization control. The fourth current may be a constant current. The first current may be a constant current that is less than the fourth current. The third current may be greater than the fourth current.According to this resistance spot welding method, warping of the metal sheets can be facilitated by the first and second energization controls. The warping helps to create a state in which a current application path is concentrated in a narrow zone. Moreover, the discharge of the current to the surfaces of the metal sheets can be facilitated by temporarily supplying a large current by the third current application control, thereby making it possible to achieve melting of a wide range of the metal sheets in the lamination direction. This melt prevents inconsistencies in the properties of the materials in the direction of lamination. Thus, inconsistencies in the melt between the metal sheets can be prevented when the welding of the workpiece is finished in the fourth energization control.Thus, according to this resistance spot welding method, it is possible to achieve high quality welding of the workpiece prepared by laminating two or more metal sheets. Moreover, excellent welding can be achieved by restricting the fourth current to a relatively small current in the fourth energization control. Therefore, according to this resistance spot welding method, high quality welding can be achieved while preventing spatter.According to an aspect of the present disclosure, the two or more metal sheets may include at least two metal sheets having resistances, tensile strengths, or sheet thicknesses different from each other. When the metal sheets include metal sheets having resistances different from each other due to such a difference in resistance, the heat generation and melting between the metal sheets may be inconsistent.Metal sheets having different strengths have resistances different from each other. Metal sheets having different sheet thicknesses have resistances different from each other. Thus, in a case where two or more metal sheets include metal sheets having different tensile strengths or sheet thicknesses from each other, it may become possible that the melt between the metal sheets is inconsistent.According to an aspect of the present disclosure, the above-mentioned first, second, third, and fourth energization controls may prevent inconsistencies in the melt between the metal sheets caused by the difference in resistance, tensile strength, or sheet thickness. This improves the welding quality.According to an aspect of the present disclosure, the workpiece may include a workpiece in which two metal sheets among the two or more metal sheets located at the two ends of the workpiece in the lamination direction have resistances, tensile strengths, or sheet thicknesses different from each other. The above resistance spot welding method is effective in improving the quality of welding such a workpiece.According to an aspect of the present disclosure, the workpiece may include a workpiece prepared by laminating the two or more metal sheets such that the resistance, the tensile strength, or the sheet thickness of the metal sheets in the lamination direction increases or decreases. According to an aspect of the present disclosure, the workpiece may include a workpiece in which, of the two or more metal sheets, a tensile strength of a first metal sheet located at a first end of the workpiece in the lamination direction is different from a tensile strength of a second metal sheet located at the center of the workpiece in the lamination direction; a tensile strength of a third metal sheet located at a second end of the workpiece opposite the first end in the lamination direction is less than a sum of the tensile strength of the first metal sheet and the tensile strength of the second metal sheet; and the tensile strength of the second metal sheet located at the center of the workpiece in the lamination direction is a tensile strength of a metal sheet held between the first metal sheet and the third metal sheet located at both ends of the workpiece among the two or more metal sheets, or a sum of tensile strengths of one or more metal sheets held between the first metal sheet and the third metal sheet among the two or more metal sheets. The above resistance spot welding method can improve the quality of welding to the workpiece satisfying these conditions.According to an aspect of the present disclosure, a ratio H 1 / H 2 of a total sheet thickness H 1, which is a sum of thicknesses of the two or more metal sheets of the workpiece in the lamination direction, to a thickness H 2, which is a thickness of a metal sheet having a smaller thickness of two metal sheets located at the two ends of the workpiece in the lamination direction, may be 3.5 or more. The above resistance spot welding method can improve the quality of welding to the workpiece satisfying these conditions.According to an aspect of the present disclosure, the workpiece may include a workpiece having a strength difference of 445 MPa or more at both ends. According to an aspect of the present disclosure, the workpiece may include a workpiece having a strength difference of 255 MPa or more and a strength ratio of 4.29 or more at both ends. Here, the difference in strength at the both ends is a difference in tensile strengths of the two metal sheets located at the both ends of the workpiece in the lamination direction. The strength ratio is a value obtained by dividing a sum of the tensile strengths of the two or more metal sheets of the workpiece by a tensile strength of a metal sheet having a lower tensile strength among the two metal sheets located at the both ends of the workpiece in the lamination direction. The above resistance spot welding method can improve the quality of welding to the workpiece satisfying these conditions.According to an aspect of the present disclosure, the two or more metal sheets may include a high strength steel sheet. The resistance spot welding method of the present disclosure can improve the quality of welding to the workpiece including the high-strength steel sheet.According to an aspect of the present disclosure, a resistance spot welding device for welding a workpiece prepared by laminating two or more metal sheets may be provided. The resistance spot welding device may include a pair of electrodes and a controller. The pair of electrodes may be arranged to hold the workpiece at both ends of the workpiece in the laminating direction of the two or more metal sheets.The controller may be configured to control the application of current between the pair of electrodes. The controller may be configured to perform a first current application control on the pair of electrodes such that a first current flows between the pair of electrodes holding the workpiece.The controller may be configured to perform a second current application control on the electrode pair such that a current flowing between the electrode pair decreases from the first current to a second current smaller than the first current. The second energization control may be performed subsequent to the first energization control.The controller may be configured to perform a third current application control on the electrode pair such that a current flowing between the electrode pair increases from the second current to a third current larger than the second current. The third energization control may be performed subsequent to the second energization control.The controller may be configured to perform a fourth current application control on the electrode pair such that a fourth current flows between the electrode pair. The fourth energization control may be performed subsequent to the third energization control. The fourth current may be a constant current. The first current may be a constant current that is less than the fourth current. The third current may be greater than the fourth current.This resistance spot welding apparatus can improve the quality of welding as well as the aforementioned resistance spot welding method.BRIEF DESCRIPTION OF THE DRAWINGSHereinafter, exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings, in which: FIG. 1 is a schematic illustration of a resistance spot welding apparatus; FIG. 2 is a schematic diagram showing an electrical configuration of the resistance spot welding apparatus; FIG. 3 is a graph showing a typical current waveform; FIG. 4 is a flow chart showing a welding process including current controls; FIG. 5 is a graph showing a specific example of the current waveform of a specific workpiece; FIG. 6A is a schematic cross-sectional illustration of a workpiece describing a first phase of initial energization; FIG. 6B is a schematic cross-sectional illustration of the workpiece describing a second phase of initial energization; FIG. 7A is a schematic cross-sectional view of a workpiece describing a melt caused by a large current application; FIG. 7B is a cross-sectional view of a workpiece describing growth of a weld bead upon main flow application; FIG. 7C is a cross-sectional view of a workpiece describing growth of a welding bead in main flow application; FIG. 8A is a diagram describing a modified example of a current waveform; FIG. 8B is a diagram describing a modified example of the current waveform; FIG. 8C is a diagram describing a modified example of the current waveform; FIG. 9A is a diagram describing a modified example of the current waveform; FIG. 9B is a diagram describing a modified example of the current waveform; and FIG. 9C is a diagram describing a modified example of the current waveform.DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTSA resistance spot welding device 1 shown in FIG. 1 is configured to weld a workpiece W prepared by laminating two or more metal sheets by resistance spot welding.The workpiece W may include two or more steel sheets than the metal sheets. The workpiece W may include at least two steel sheets having different resistances. The workpiece W may include at least two steel sheets having different tensile strengths. The workpiece W may include at least two steel sheets having different sheet thicknesses. The higher the tensile strength of the steel sheet, the greater the resistance of the steel sheet. The greater the sheet thickness of the steel sheet, the greater the resistance of the steel sheet. The "resistor" in the present disclosure is an electric resistor.An example of the workpiece W shown in FIG. 1 is a workpiece including three steel sheets stacked on each other. The workpiece W includes a first steel sheet P 1, a second steel sheet P 2, and a third steel sheet P 3.The first steel sheet P 1 may be, for example, but not limited to, hot dip galvanized steel sheet. The first steel sheet P 1 may be, for example, a high-strength steel sheet having a tensile strength of 440 MPa (megapascal) or more. The first steel sheet P 1 may be, for example, an SCGA 440 steel sheet having a thickness of 1.4 mm.The second steel sheet P 2 may be, for example, but not limited to, hot dip galvanized steel sheet. The second steel sheet P 2 may be, for example, a high-strength steel sheet having a tensile strength of 1180 MPa (megapascal) or more. The second steel sheet P 1 may be, for example, an SCGA 11180 steel sheet having a thickness of 1.4 mm. The high-strength steel sheet having such an extremely high tensile strength is also referred to as a material of very high tensile stress.The third steel sheet P 3 may be, for example, but not limited to, a cold-rolled steel sheet. The third steel sheet P 3 may be, for example, a high-strength steel sheet having a tensile strength of 1470 MPa (megapascal) or more. The third steel sheet P 3 may be, for example, an SCGA 1470 steel sheet having a thickness of 2 mm.In the example of the workpiece W shown in FIG. 1, the first steel sheet P 1, the second steel sheet P 2, and the third steel sheet P 3 are layered in this order. Hereinafter, the direction in which two or more metal sheets included in the workpiece W are laminated is referred to as a laminating direction. The lamination direction corresponds to the normal directions of the first steel sheet P 1, the second steel sheet P 2, and the third steel sheet P 3, and the normal direction of the surface of the workpiece. The lamination direction corresponds to the thickness directions of the first steel sheet P 1, the second steel sheet P 2, and the third steel sheet P 3, and the thickness direction of the workpiece.The resistance spot welding apparatus 1 includes a resistance welding device 20.The resistance welding device 20 includes a first electrode 21 and a second electrode 22. The second electrode W is disposed over the workpiece 22 such that the workpiece W is sandwiched between the first electrode 21 and the second electrode 22. The first electrode 21 is movable in the up-and-down direction relative to the second electrode 22.The first electrode 21 and the second electrode 22 each contact the workpiece W during welding. The first electrode 21 contacts the third steel sheet P 3 which is the metal sheet located as the lowermost layer of the workpiece W. The second electrode 22 contacts the first steel sheet P 1 which is the metal sheet located as the uppermost layer of the workpiece W. The first electrode 21 and the second electrode 22 hold both sides of the workpiece W in the laminating direction to apply a pressure. In this state, a welding current is supplied between the first electrode 21 and the second electrode 22 and flows through the workpiece W. The workpiece W is welded by resistance heating formed by the welding current.As shown in FIG. 2, the resistance spot welding apparatus 1 includes a welding power source 30, a current sensor 40, and a controller 50 as elements of an electric system for the resistance welding device 20.The welding power source 30 is configured to supply the welding current between the first electrode 21 and the second electrode 22. The current sensor 40 is disposed on a rail between the welding power source 30 and the first electrode 21 or on a rail between the welding power source 30 and the second electrode 22. The current sensor 40 is configured to detect a current I supplied between the first electrode 21 and the second electrode 22, and input the detected signal to the controller 50. The current I is the above-mentioned welding current.The controller 50 controls energization between the first electrode 21 and the second electrode 22 by controlling the welding power source 30. specifically, the controller 50 is configured as a energization controller to control the current I flowing between the first electrode 21 and the second electrode 22 such that the current I changes in accordance with the current pattern shown in FIG. 3.More specifically, the controller 50 is configured to perform feedback control of the current I flowing between the first electrode 21 and the second electrode 22 based on the current I detected by the current sensor 40. Hereinafter, the first electrode 21 and the second electrode 22 are collectively referred to as a pair of electrodes 21, 22.The controller 50 initiates the control process shown in FIG. 4 when a welding initiation command for welding the workpiece W is input via a manipulator not shown. In the control process, the controller 50 controls the current I between the electrode pair 21, 22 such that the melt current changes in accordance with the current waveform shown in FIG. 3. This control helps to achieve good resistance spot welding on the workpiece W.Specifically, the controller 50 performs first energization control (S 110) on the electrode pair 21, 22 for a period C 1 that is a period from a welding initiation point T 0 for welding the workpiece W to a first point T 1 at which a certain period of time has elapsed since the welding initiation point T 0. In the period C 1, the first current application control is performed such that a first current I 1 flows between the electrode pair 21, 22 holding the workpiece W. The first current I 1 is a constant current.Subsequent to the first energization control (S 110), the controller 50 performs a second energization control (S 120) on the electrode pair 21, 22 for a period C 2 that is a period from the first point T 1 to a second point T 2. In the period C 2, the second current application control is performed such that the current I flowing between the electrode pair 21, 22 decreases from the first current I 1 to a second current I 2 that is smaller than the first current I 1. The second current I 2 is greater than zero.Subsequent to the second energization control (S 120), the controller 50 performs third energization control on the electrode pair 21, 22 for a period C 3 that is a period from the second point T 2 to a third point T 3 and a period C 4 that is a period from the third point T 3 to a fourth point T 4 (S 130).In the period C 3, the second current application control is performed such that the current I flowing between the electrode pair 21, 22 decreases from the second current I 2 to a third current I 3 larger than the second current I 2. In the period C 4, the third current application control is performed such that the current I flowing between the electrode pair 21, 22 maintains the third current I 3.Subsequent to the third energization control (S 130), the controller 50 performs fourth energization control (S 140) on the electrode pair 21, 22 for a period C 5 that is a period from the fourth point T 4 to a fifth point T 5. In the period C 5, the fourth current application control is performed such that a fourth current I 4 smaller than the third current I 3 flows between the electrode pair 21, 22.In the period C 5, the fourth current application control is performed such that a constant current flows between the electrode pair 21, 22 as the fourth current I 4. As mentioned above, also in the period C 1 ranging from the welding initiation point T 0 to the first point T 1, the current between the electrode pair 21, 22 is controlled such that a constant current flows between the electrode pair 21, 22 as the first current I 1. The first current I 1 is smaller than the fourth current I 4 for a reason explained later.In the periods C 3 and C 4 from the second point T 2 to the fourth point T 4, the current between the electrode pair 21, 22 is controlled, for a reason explained later, such that a current larger than the fourth current I 4 flows between the electrode pair 21, 22 as the third current I 3.The controller 50 continuously performs these first, second, third, and fourth energization control as a consecutive energization control from the point T 0 and stops energization between the electrode pair 21, 22 at the fifth point T 5. The resistance spot welding to the workpiece W is completed at the fifth point T 5.Now, an object of the above energization control in accordance with the current waveforms will be explained. The energization from the welding initiation point T 0 to the first point T 1 corresponds to a first stage of initial energization in the present embodiment, and is performed to heat the workpiece W by the resistance heating inside the workpiece W such that the workpiece W does not melt. The first current I 1 and the length of the period C 1 ranging from the point T 0 to the point T 1 are determined such that the workpiece W does not melt.FIG. 5 shows a specific example of the current waveform applicable when the workpiece W is a workpiece (hereinafter referred to as a focused workpiece) including an SCGA440 steel sheet having a thickness of 1.4 mm as the first steel sheet P 1, an SCGA1180 steel sheet having a thickness of 1.4 mm as the second steel sheet P 2, and an SPC1470 steel sheet having a thickness of 2 mm as the third steel sheet P 3. The pressure applied between the pair of electrodes 21, 21 is 5.51 kN (kilonewtons).The horizontal axis in FIG. 5 represents hours. The vertical axis in FIG. 5 represents the current I. The graph in FIG. 5 shows ratios of time periods between the points T 0, T 1, T 2, T 3, T 4, and T 5 and ratios between the currents I 1, I 2, I 3, and I 4 in detail, provided that the time and the current I are zero at the origin.FIG. 6A conceptually shows that the inside of the workpiece W is heated in the first phase of the initial energization. In FIG. 6A, the broken line conceptually shows a portion of the workpiece W that is heated in a case where the resistance of the second steel sheet P 2 is higher than the resistance of the first steel sheet P 1 and the resistance of the third steel sheet P 3 is higher than the resistance of the second steel sheet P 2. The portion shown by the broken line corresponds to the resistance center of the workpiece W. This example of the combination of the first steel sheet P 1, the second steel sheet P 2, and the third steel sheet P 3 includes the combination of the first steel sheet P 1, the second steel sheet P 2, and the third steel sheet P 3 in the above focused workpiece.The first phase of the initial energization is performed not only for heating a resistance center but also for forming a warp in the metal sheets included in the workpiece W to limit a energization path in the workpiece W to a narrow zone.The warp of the metal sheets described herein is a warp formed such that the metal sheets adjacent to each other in the lamination direction are more separated from each other in the lamination direction as the distance from the center of the current application path between the first electrode 21 and the second electrode 22 increases.FIG. 6A shows that the warping of the first steel sheet P 1 causes the first steel sheet P 1 and the second steel sheet P 2 to separate from each other at zones away from the center of the energization path. FIG. 6A similarly shows that the warping of the third steel sheet P 3 causes the second steel sheet P 2 and the third steel sheet P 3 to separate from each other in zones away from the center of the energization path.Such separations of the metal sheets limit the current application path, which causes the current to flow concentratedly inside the workpiece W. Hereinafter, a concentrated current flow between the electrode pair 21, 22 at a desirable current density through a limited region in the workpiece W is referred to as a current concentration.The energization from the first point T 1 to the second point T 2 corresponds to the second phase of the initial energization performed to fully heat a zone in the workpiece W held between the first electrode 21 and the second electrode 22 with a gradual heat input to the workpiece W.The broken lines in FIG. 6B conceptually show that, in the second phase of the initial energization, the heat propagates from the resistance center and the region in the workpiece W held between the first electrode 21 and the second electrode 22 is completely heated.This heating serves to stably form the warp of the metal sheets. In other words, by providing a step to gradually decrease the current I as seen in the current control from the first point T 1 to the second point T 2, the warp of the metal plates is stably formed and the current concentration is stably obtained.In the present embodiment, the controller 50 controls a current flowing between the electrode pair 21, 22 based on the detected signal supplied from the current sensor 40. However, when the warp of the metal sheets cannot be stably formed, the current density changes at the time when the current flows in the lamination direction of the workpiece W, which then changes the manner of heat generation. Accordingly, in order to achieve a stable current concentration, it is important to perform the current control to include a downward slope between the first point T 1 and the second point T 2 (S 120).The energization from the second point T 2 to the fourth point T 4 serves to facilitate the discharge of the current to a surface of a metal sheet having a relatively low resistance by supplying a large current and achieve the heat generation and melting over the entirety of the workpiece W from its upper surface to its lower surface between the electrode pair 21, 22.When a steel sheet having a relatively low resistance, such as the first steel sheet P 1 of the focused workpiece, is present on the surface of the workpiece W and a large current is not supplied, there is a possibility that the Joule heat generated on this low-resistance steel sheet is insufficient, and thus welding of the first steel sheet P 1 ends as insufficient. If a large current is supplied, it is possible to prevent deterioration of welding quality due to such insufficient heat generation on the steel sheet with a relatively low resistance.FIG. 7A shows that due to the supply of a large current in the third energization control, a wide range of melt is formed between the electrode pair 21, 22, causing material fusion between adjacent steel sheets of the first steel sheet P 1, the second steel sheet P 2, and the third steel sheet P 3. The material fusion weakens the resistance differences between the metal sheets.The energization from the fourth point T 4 to the fifth point T 5 corresponds to the main energization and is performed to finish the welding of the workpiece W. The fourth current I 4 and the time period of the period C 5 from the point T 4 to the point T 5 (i.e., the main current application time) are determined so that an appropriate weld lens G is formed in the workpiece W.The main current application serves to spread the melt in the workpiece W from the resistance center to the periphery and to form a suitable weld tip G in the workpiece W. Since the resistance difference is alleviated by the material fusion caused by the supply of a large current in the preceding step, the melt in the workpiece W distributes more uniformly than in a case where the third current application control is omitted. Thereby, a suitable weld tip G is formed in the workpiece W.FIGS. 7B and 7C show that the weld tip G grows gradually as a result of the main current application. As shown in FIG. 7C, spreading of the weld joint G over the entirety of the metal sheets (the first steel sheet P 1, the second steel sheet P 2, and the third steel sheet P 3) in the workpiece W results in excellent welding quality.More specifically, in the present embodiment, efficient heat generation and melting of the metal sheets can be achieved, and an appropriate weld metal G can be formed with a relatively small current because the current concentration is achieved.Further, in the present embodiment, since the current concentration in the process up to the second phase of the initial energization can be stably obtained, current tolerance in the main energization can be increased. In other words, a relatively wide range current as the fourth current I 4 may be allowed (tolerated) to achieve a suitable weld. In FIG. 5, a thick arrow on the fourth current I 4 means that the tolerance of the fourth current I 4 is large.Moreover, in the present embodiment, spatter with a small current in the main current application is prevented, and excellent welding can be achieved because the third current application control helps achieve a wide range of material fusion in the workpiece W in the lamination direction.The resistance spot welding device 1 and the resistance spot welding method of the present embodiment have been explained above. This welding method works productively when the workpiece W prepared by laminating two or more metal sheets each having different resistance is welded. Examples of the metal sheets having different resistances include metal sheets having different tensile strengths and metal sheets having different sheet thicknesses.When there is a difference in resistance between the metal sheets in the workpiece W, the resistance heating is small in a portion having a low resistance compared to other portions. In the present embodiment, the dissipation of the current to the metal sheets having relatively low resistances can be facilitated by supplying a large current. Further, the difference in resistance can be mitigated by causing the material fusion between the metal sheets disposed adjacently to each other. Accordingly, it is possible to weld two or more metal sheets having different resistances excellently.The resistance spot welding method provided in accordance with the current pattern including the above-mentioned large current portion L operates particularly effectively when a workpiece W including three metal sheets as exemplified by a combination of the first steel sheet P 1, the second steel sheet P 2, and the third steel sheet P 3 satisfies at least one of a first condition, a second condition, a third condition, or a fourth condition, each of which is expressed in the following inequality.First condition: X1<X3second condition: X1>X2 and (X1+X2)>X3Third condition: X1<X2 and (X1+X2)>X3Fourth condition: X1≥X2>X3Here, the premise is that, in the workpiece W, the first metal sheet is the metal sheet having the tensile strength X 1, the second metal sheet is the metal sheet having the tensile strength X 2, and the third metal sheet is the metal sheet having the tensile strength X 3.The first metal sheet and the third metal sheet are located at both ends of the workpiece W in the lamination direction. The second metal sheet is disposed between the first metal sheet and the second metal sheet. The first metal sheet corresponds to the first steel sheet P 1 illustrated in FIG. 1. The second metal sheet corresponds to the second steel sheet P 2. The third metal sheet corresponds to the third steel sheet P 3. The first metal sheet, the second metal sheet, and the third metal sheet may correspond to the third steel sheet P 3, the second steel sheet P 2, and the first steel sheet P 1, respectively. In other words, in order to understand the first condition, the second condition, the third condition, and the fourth condition, the first metal sheet, the second metal sheet, and the third metal sheet may be understood to be laminated from top to bottom or from bottom to top. In other words, the first metal sheet and the tensile strength X 1 may be read as the third metal sheet and the tensile strength X 3; and the third metal sheet and the tensile strength X 3 may be read as the first metal sheet and the tensile strength X 1.The first condition is that the tensile strength X 1 of the first metal sheet located at a first end of the workpiece W is smaller than the tensile strength X 3 of the third metal sheet located at a second end of the workpiece W opposite the first end. In other words, the first condition is that the two metal sheets located at both ends of the workpiece W have different tensile strengths X 1 and X 3.The second condition is that the tensile strength X 1 of the first metal sheet located at the first end of the workpiece W in the lamination direction is larger than the tensile strength X 2 of the second metal sheet located at the center of the workpiece W; and that the tensile strength X 3 of the third metal sheet located at the second end of the workpiece W in the lamination direction is smaller than the sum of the tensile strength X 1 of the first metal sheet and the tensile strength X 2 of the second metal sheet. The second end of the workpiece W in the lamination direction is the end opposite the first end of the workpiece W in the lamination direction.The third condition is that the tensile strength X 1 of the first metal sheet located at the first end of the workpiece W is smaller than the tensile strength X 2 of the second metal sheet located at the center of the workpiece W; and that the tensile strength X 3 of the third metal sheet located at the second end of the workpiece W is smaller than the sum of the tensile strength X 1 of the first metal sheet and the tensile strength X 2 of the second metal sheet.In summary, the second condition and the third condition can be expressed as follows.In other words, the second condition and the third condition are that the tensile strength X 1 of the first metal sheet located at the first end of the workpiece W is different from the tensile strength X 2 of the second metal sheet located at the center of the workpiece W; and that the tensile strength X 3 of the third metal sheet located at the second end of the workpiece W is less than the sum of the tensile strength X 1 of the first metal sheet and the tensile strength X 2 of the second metal sheet.The fourth condition is that two or more metal sheets are laminated in the lamination direction so that the tensile strength increases or decreases. The lamination direction described herein includes the direction from the first metal sheet to the third metal sheet and the direction from the third metal sheet to the first metal sheet.The tensile strengths X 1, X 2, and X 3 of the first condition, the second condition, the third condition, and the fourth condition may be read as resistances X 1, X 2, and X 3. The tensile strengths X 1, X 2, and X 3 may be read as sheet thicknesses X 1, X 2, and X 3. The greater the tensile strength, the greater the resistance. The greater the sheet thickness, the greater the tensile strength.When the first condition is satisfied, the resistance center of the workpiece W is deviated toward the third metal sheet in the lamination direction of the workpiece W due to the third metal sheet having a large resistance. When the fourth condition is satisfied, the resistance center of the workpiece W is deviated toward the first metal sheet in the lamination direction of the workpiece W due to the first metal sheet having a large resistance.Accordingly, in a method of easily growing the weld tip G from the resistance center satisfying one of these conditions, provided by applying the conventional welding method to the workpiece W, insufficient welding may occur in the metal sheets disposed at the ends of the workpiece W.According to the present embodiment, as shown in FIG. 7A, it is possible to achieve the melt of the first metal sheet having a relatively small resistance by supplying a large current and to properly achieve the welding of the workpiece W. Thus, it is also possible to prevent spatter.The above-mentioned first condition, second condition, third condition, and fourth condition are applicable in a case where the workpiece W is prepared by laminating four or more metal sheets. In a case where the above-mentioned conditions are satisfied, when "the second metal sheet" is replaced with "a set of layered metal sheets" and "the tensile strength X 2 of the second metal sheet" is replaced with a sum of the tensile strengths of the metal sheets included in "the set of layered metal sheets", the resistance spot welding method of the present embodiment works effectively to weld the workpiece W excellently. In the case where the workpiece W is prepared by laminating four or more metal sheets, "the second metal sheet located at the center of the workpiece W" is one or more metal sheets held between two metal sheets located at both ends of the workpiece W in the second condition and the third condition. In this case, "the tensile strength X 2 of the second metal sheet located at the center of the workpiece W" is the sum of the tensile strengths of one or more metal sheets held between two metal sheets located at both ends of the workpiece W.In welding tests in various environments, it has been found that the resistance spot welding method of the present embodiment contributes to appropriate welding of the workpiece W even when the workpiece W satisfies one of the following fifth condition or sixth condition.Fifth condition: The difference ΔX of strengths at both ends is 445 MPa or more.Sixth condition: The difference ΔX of strengths at both ends is 255 MPa or more, and a ratio R of strengths between both ends is 4.29 or more.The difference in strengths at both ends is a difference in tensile strengths of the metal sheets located at both ends of the workpiece in the lamination direction. In other words, the difference ΔX in strengths at both ends is a difference in tensile strengths between two metal sheets of the workpiece W contacting the pair of electrodes 21, 22. Using the above tensile strengths X1, X2and X3, the difference ΔXof the strengths at both ends can be expressed in the following formula: ΔX=|X1-X3|.The ratio R of strengths corresponds to the ratio R=R 1 / R 2, where R 1 is the sum of the tensile strengths of the metal sheets of the workpiece W in the lamination direction, and R 2 is the tensile strength of the metal sheet having a lower tensile strength among the two metal sheets located at both ends of the workpiece W including two or more metal sheets in the lamination direction. Using the above tensile strengths X1, X2and X3, R1may be expressed in the following formula: R1=(X1+X2+X3). R2may be expressed in the following formula: R2=min {X1, X3} using the MIN function. Here, the ratio R of strengths is expressed in the following formula:As the difference ΔX in strengths at both ends increases, the melt of the metal sheet having a relatively low tensile strength does not proceed due to the low resistance. As the ratio R of strengths increases, the weld joint G is directed in one direction. Thus, the resistance spot welding method of the present embodiment is very useful on the workpiece W satisfying the above-mentioned fifth condition and sixth condition.The resistance spot welding method of the present embodiment generally also works effectively in welding the workpiece W whose sheet thickness satisfies the following seventh condition.Seventh condition: Ratio H of the sheet thickness is 3.5 or more.The ratio H of the sheet thickness described herein is the ratio H 1 / H 2, where H 1 is the total sheet thickness that is the sum of the thicknesses of the metal sheets included in the workpiece W in the lamination direction, and H 2 is the thickness of the metal sheet having the smaller thickness among the two metal sheets located at both ends of the workpiece W including the metal sheets in the lamination direction.[Modified Examples]The current waveform should not be limited to the example shown in FIG. 3, and may be modified from the current waveform of any one of FIGS. 8A, 8B, 8C, 9A, 9B, or 9C. As is understood from comparisons between FIGS. 3 and 8A, 8B, 8C, 9A, 9B, and 9C, the initial energization including a downward slope and a large current before the main energization particularly contributes to excellent welding.In the current waveform shown in FIG. 8A, the current waveform of the large current portion L 1 is a triangular shape having an upward slope and a sharp peak, unlike the trapezoid shown in FIG. 3. In the current waveform shown in FIG. 8B, the current waveform of the large current portion L 2 is a rectangular shape. In the current waveform shown in FIG. 8C, the current waveform of the large current portion L 3 is a triangular shape having a downward slope starting at the tip.In the current waveform shown in FIG. 9A, the current waveform of the large current portion L 4 is a trapezoid having an upward slope and a downward slope. In the current waveform shown in FIG. 9B, the current waveform of the large current portion L 5 is a triangular shape having an upward slope, a downward slope, and a sharp peak. In the current waveform shown in FIG. 9C, the current waveform of the large current portion L 6 is a trapezoid starting at the tip.[Other Embodiments]The present disclosure should not be limited to the above embodiments and may be embodied in various other ways. For example, the above resistance spot welding device 1 and the resistance spot welding method may be used in welding the workpiece W prepared by laminating two metal sheets.Functions of one element in the above-mentioned embodiment may be distributed to two or more elements. Functions of two or more elements may be integrated into one element. A part of the configuration in the above-mentioned embodiment may be omitted. At least a part of the configuration of the aforementioned embodiment may be added to or replaced with another configuration of the embodiment. Any or all forms encompassed by the technical idea characterized by the languages used in the claims are embodiments of the present disclosure.[Technical Ideas Disclosed in Present Description]It should be understood that the present disclosure discloses the following ideas.[Item 1]Item 1 is a resistance spot welding method for welding a workpiece prepared by laminating two or more metal sheets using a resistance spot welding device including a pair of electrodes.The method comprises:performing a first current application control on the electrode pair such that a first current flows between the electrode pair in a state where the workpiece is held between the electrode pair at both ends of the workpiece in a lamination direction of the two or more metal sheets;performing a second current application control on the electrode pair subsequent to the first current application control such that a current flowing between the electrode pair decreases from the first current to a second current smaller than the first current;performing a third current application control on the electrode pair subsequent to the second current application control such that a current flowing between the electrode pair increases from the second current to a third current larger than the second current; andperforming a fourth current application control on the electrode pair subsequent to the third current application control such that a fourth current flows between the electrode pair.The fourth current is a constant current.The first current is a constant current less than the fourth current.The third current is greater than the fourth current.[Item 2]Item 2 is the resistance spot welding method according to Item 1. the two or more metal sheets include at least two metal sheets having resistances, tensile strengths, or sheet thicknesses different from each other.[Item 3]Item 3 is the resistance spot welding method according to Item 1 or Item 2, The workpiece includes a workpiece in which two metal sheets among the two or more metal sheets located at the two ends of the workpiece in the lamination direction have resistances, tensile strengths, or sheet thicknesses different from each other.[Item 4]Item 4 is the resistance spot welding method according to any one of Items 1 to 3.The workpiece includes at least the following (i) and / or (ii):(i) a workpiece prepared by laminating the two or more metal sheets such that the resistance, tensile strength or sheet thickness of the two or more metal sheets increases or decreases in the laminating direction; or(ii) a workpiece in which, of the two or more metal sheets, a tensile strength of a first metal sheet located at a first end of the workpiece in the lamination direction is different from a tensile strength of a second metal sheet located at a center of the workpiece in the lamination direction; a tensile strength of a third metal sheet located at a second end of the workpiece opposite the first end in the lamination direction is less than a sum of the tensile strength of the first metal sheet and the tensile strength of the second metal sheet; and the tensile strength of the second metal sheet located at the center of the workpiece in the lamination direction is a tensile strength of a metal sheet held between the first metal sheet and the third metal sheet located at the both ends of the workpiece among the two or more metal sheets in the lamination direction, or a sum of tensile strengths of one or more metal sheets held between the first metal sheet and the third metal sheet among the two or more metal sheets.[Item 5]Item 5 is the resistance spot welding method according to any one of Items 1 to 4.A ratio H 1 / H 2 of a total sheet thickness H 1, which is a sum of thicknesses of the two or more metal sheets of the workpiece in the lamination direction, to a thickness H 2, which is a thickness of a metal sheet having a thickness smaller than two metal sheets of the two or more metal sheets located at both ends of the workpiece in the lamination direction, is 3.5 or more.[Item 6]Item 6 is the resistance spot welding method according to any one of Items 1 to 5.The workpiece comprises at least one workpiecea workpiece having a strength difference of 445 MPa or more at both ends and / ora workpiece having a strength difference of 255 MPa or more at both ends and a strength ratio of 4.29 or more.The difference in strength at the both ends is a difference in tensile strength of two metal sheets located at the both ends of the workpiece in the lamination direction. The strength ratio is a value obtained by dividing a sum of the tensile strengths of the two or more metal sheets of the workpiece by a tensile strength of a metal sheet having a lower tensile strength among the two metal sheets located at the both ends of the workpiece in the lamination direction.[Item 7]Item 7 is the resistance spot welding method according to any one of Items 1 to 6.The two or more metal sheets include a high strength steel sheet.[Item 8]Item 8 is a resistance spot welding device for welding a workpiece prepared by laminating two or more metal sheets.The device comprises:a pair of electrodes arranged to hold the workpiece in the laminating direction of the two or more metal sheets at both ends of the workpiece; anda controller configured to control current application between the pair of electrodes.The controller is configured:performing first current application control on the electrode pair such that a first current flows between the electrode pair holding the workpiece;performing second current application control on the electrode pair subsequent to the first current application control such that a current flowing between the electrode pair decreases from the first current to a second current smaller than the first current;performing third current application control on the electrode pair subsequent to the second current application control such that a current flowing between the electrode pair increases from the second current to a third current larger than the second current; andperforming fourth current application control on the electrode pair subsequent to the third current application control such that a fourth current flows between the electrode pair. The fourth current is a constant current.The first current is a constant current less than the fourth current.The third current is greater than the fourth current.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedJP 2021-079410

[0002]

Claims

A resistance spot welding method for welding a workpiece (W) prepared by laminating two or more metal sheets (P1, P2, P3) using a resistance spot welding apparatus (1) including a pair of electrodes (21, 22), the method comprising: performing a first current application control (S110) on the pair of electrodes (21, 22) such that a first current (I1) flows between the pair of electrodes (21, 22) in a state in which the workpiece (W) is held between the pair of electrodes (21, 22) at both ends of the workpiece (W) in a laminating direction of the two or more metal sheets (P1, P2, P3); performing a second current application control (S 120) on the electrode pair (21, 22) subsequent to the first current application control such that a current flowing between the electrode pair (21, 22) decreases from the first current (I 1) to a second current (I 2) smaller than the first current (I 1); performing a third current application control (S 130) on the electrode pair (21, 22) subsequent to the second current application control such that a current flowing between the electrode pair (21, 22) increases from the second current (I 2) to a third current (I 3) larger than the second current (I 2); and performing a fourth current application control (S 140) on the electrode pair (21, 22) subsequent to the third current application control such that a fourth current (I 4) flows between the electrode pair (21, 22), wherein the fourth current (I 4) is a constant current, wherein the first current (I 1) is a constant current smaller than the fourth current (I 4), and wherein the third current (I 3) is larger than the fourth current (I 4).The resistance spot welding method according to claim 1, wherein the two or more metal sheets (P1, P2, P3) comprise at least two metal sheets having resistances, tensile strengths or sheet thicknesses different from each other.The resistance spot welding method according to claim 1 or 2, wherein the workpiece (W) includes a workpiece in which two metal sheets (P1, P3) among the two or more metal sheets (P1, P2, P3) located at the both ends of the workpiece in the lamination direction have resistances, tensile strengths, or sheet thicknesses different from each other.The resistance spot welding method according to any one of claims 1 to 3, wherein the workpiece comprises at least one of (i) and / or (ii): (i) a workpiece prepared by laminating the two or more metal sheets (P1, P2, P3) such that the resistance, the tensile strength, or the sheet thickness of the two or more metal sheets increases or decreases in the laminating direction, or (ii) a workpiece in which, of the two or more metal sheets (P1, P2, P3), a tensile strength of a first metal sheet (P1) located at a first end of the workpiece in the laminating direction is different from a tensile strength of a second metal sheet (P2) located at a center of the workpiece in the laminating direction; a tensile strength of a third metal sheet (P 3) disposed at a second end of the workpiece opposite to the first end in the lamination direction is smaller than a sum of the tensile strength of the first metal sheet (P 1) and the tensile strength of the second metal sheet (P 2); and the tensile strength of the second metal sheet (P 2) located at the center of the workpiece in the lamination direction is a tensile strength of a metal sheet held between the first metal sheet (P 1) and the third metal sheet (P 3) located at the both ends of the workpiece among the two or more metal sheets (P 1, P 2, P 3), or a sum of tensile strengths of one or more metal sheets (P 2) held between the first metal sheet (P 1) and the third metal sheet (P 3) among the two or more metal sheets (P 1, P 2, P 3).The resistance spot welding method according to any one of claims 1 to 4, wherein a ratio H1 / H2 of a total sheet thickness H1 which is a sum of thicknesses of the two or more metal sheets (P1, P2, P3) of the workpiece (W) in the lamination direction to a thickness H2 which is a thickness of a metal sheet having a thickness of a thickness of two metal sheets (P1, P3) of the two or more metal sheets (P1, P2, P3) located at both ends of the workpiece in the lamination direction is 3.5 or more.The resistance spot welding method according to any one of claims 1 to 5, wherein the workpiece (W) includes at least one workpiece having a difference in strength of 445 MPa or more at the both ends and / or one workpiece having a difference in strength of 255 MPa or more at the both ends and a ratio of strength of 4.29 or more, and wherein the difference in strength at the both ends is a difference in tensile strengths of two metal sheets (P1, P3) located at the both ends of the workpiece (W) in the lamination direction, and the ratio of strength is a value obtained by dividing a sum of the tensile strengths of the two or more metal sheets (P1, P2, P 3) of the workpiece (W) is obtained by a tensile strength of a metal sheet having a lower tensile strength from the two metal sheets (P 1, P 3) located at the both ends of the workpiece in the lamination direction.The resistance spot welding method according to any one of claims 1 to 6, wherein the two or more metal sheets (P1, P2, P3) comprise a high strength steel sheet.A resistance spot welding apparatus (1) for welding a workpiece (W) prepared by laminating two or more metal sheets (P1, P2, P3), the apparatus comprising: a pair of electrodes (21, 22) arranged to hold the workpiece (W) at both ends of the workpiece in the laminating direction of the two or more metal sheets (P1, P2, P3); and a controller (50) configured to control energization between the pair of electrodes (21, 22), wherein the controller (50) is configured to: perform a first energization control on the pair of electrodes (21, 22) such that a first current (I1) flows between the pair of electrodes (21, 22) holding the workpiece (W); a second current application control to the electrode pair (21, 22) subsequent to the first current application control such that a current flowing between the electrode pair (21, 22) decreases from the first current (I1) to a second current (I2) smaller than the first current (I1); a third current application control to the electrode pair (21, 22) subsequent to the second current application control such that a current flowing between the electrode pair (21, 22) increases from the second current (I2) to a third current (I3) larger than the second current (I2); and performing a fourth current application control on the electrode pair (21, 22) subsequent to the third current application control such that a fourth current (I4) flows between the electrode pair (21, 22), wherein the fourth current (I4) is a constant current, wherein the first current (I1) is a constant current less than the fourth current (I4), and wherein the third current (I3) is greater than the fourth current (I4).

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