Method for monitoring resistance spot welding and system therefor

Abstract

To provide a method for monitoring a bonding condition of an iron substrate and an aluminum substrate on the basis of in-situ information.SOLUTION: According to the present invention, a method performs energization between electrodes that sandwich an iron substrate and an aluminum substrate, and monitors resistance spot welding for welding the ion substrate and the aluminum substrate at a joint part containing an intermetallic compound formed by melting the aluminum substrate by heat transfer from the ion substrate. A condition of the joint part can be accurately determined by paying attention to energization time or an energization amount from a starting point at which a temporal change in an electric resistance value between the electrodes becomes slow. The starting point is specified, for example, by an intersection point between an approximation straight line that indicates the temporal change in the electric resistance value in a section preceding the starting point and an approximation straight line that indicates a temporal change in an electric resistance value in a section that delays to the starting point.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for monitoring the bond condition (good or bad) of a resistance spot welded iron base and aluminum base. [Background technology]

[0002] At manufacturing sites for car bodies, aircraft bodies, housings, structures, etc., multiple components (workpieces to be joined) are often resistance spot welded (simply called "spot welding").

[0003] Spot welding is now being performed not only between the same materials (e.g., steel plates) but also between dissimilar materials (e.g., steel plate and aluminum alloy plate). As the materials to be joined become more diverse, proposals have been made to stably ensure the quality of spot welding (joint strength, etc.), for example, as described in the following patent documents. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2009-226467 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Document 1, when spot welding a steel plate and an aluminum alloy plate, the point at which the electrical resistance between the electrodes drops by a predetermined amount (melting resistance judgment reference value) from the initial current value is detected as the melting start point of the joint part of the aluminum alloy plate, and the current value applied at the melting start point is reduced. This reduces the amount of heat generated in the joint part, suppresses the amount of intermetallic compounds produced, and stabilizes the joint strength (

[0016] ,

[0027] ,

[0028] ,

[0030] ,

[0031] , Fig. 5, etc.).

[0006] However, to detect the melting start point as in Patent Document 1, it is necessary to find a threshold value (melting resistance judgment reference value: ΔR aluminum melting) by repeating many experiments for each combination of plate materials to be spot-welded. In the first place, the electric resistance value between the electrodes is easily changed by various disturbances that occur at the production site, such as the gap between the materials to be joined (steel plate and aluminum alloy plate), the contact state between the surfaces to be joined, the perpendicularity (surface perpendicularity deviation) between the axial center of the material to be joined and the spot welding electrode, and the intrusion or biting of foreign matter (press oil, etc.) into the surfaces to be joined. For this reason, it is difficult to set an appropriate electric resistance value itself as the threshold value. For this reason, it is difficult to accurately grasp the melting start point on the aluminum alloy plate side by simply comparing the set threshold value with the electric resistance value.

[0007] Incidentally, when one looks at Figure 4 of Patent Document 1, the electrical resistance between the electrodes suddenly and monotonically decreases after the melting start point. Patent Document 1 also considers that suppressing the growth and amount of intermetallic compounds formed between the steel sheet and the aluminum alloy sheet is effective in improving the joining strength (

[0010] ,

[0016] ). However, such a description in Patent Document 1 is different from the knowledge obtained by the present inventor through many experiments.

[0008] The present invention has been made in consideration of the above circumstances, and has an object to provide a new method etc. capable of monitoring the joining condition between spot-welded dissimilar materials. [Means for solving the problem]

[0009] Through intensive research, the inventors have newly discovered that when substantial melting begins near the joining surface of an aluminum substrate that is spot-welded to an iron substrate, the change over time (slope) of the electrical resistance between the electrodes becomes gentle. By developing this result, the present invention, which will be described below, has been completed.

[0010] <Method for monitoring resistance spot welding> The present invention is a method for monitoring resistance spot welding in which an iron base and an aluminum base are welded at a joint containing an intermetallic compound formed by passing an electric current between electrodes holding the iron base and the aluminum base and melting the aluminum base by heat transfer from the iron base, the method comprising: an origin identifying step of identifying an origin at which a change in electrical resistance value between the electrodes over time becomes gradual; and a determination step of determining a state of the joint based on the time or amount of electric current passing from the origin.

[0011] According to the present invention, the state of the joint between the iron base and the aluminum base (also simply referred to as the "joint state") can be determined based on the current flow history (in-situ information) between the electrodes obtained at the spot welding site (in-situ). Therefore, the quality (good or bad) of the welded object (product) between the iron base and the aluminum base can be monitored on-site, and the occurrence or outflow of defective products can be prevented upstream.

[0012] The reason why the bonding state between the iron and aluminum substrates can be grasped by the present invention is believed to be as follows. When a current is passed between the iron and aluminum substrates sandwiched between the electrodes, the iron substrate, which has a higher electrical resistivity, becomes hotter than the aluminum substrate and starts to melt. The contact resistance decreases due to the expansion of the contact area of ​​the iron substrate and the expansion of the melting caused by the melting, and the electrical resistance between the electrodes drops sharply.

[0013] Thereafter, when the aluminum substrate (near the joining surface) begins to melt (change phase from solid to liquid) due to heat transfer from the iron substrate, the change in electrical resistance between the electrodes also becomes gradual due to the change in resistance and contact state accompanying the melting of the aluminum substrate.

[0014] Conversely, it can be considered that the boundary (i.e., starting point) where the time change (slope) of the electrical resistance between the electrodes suddenly decreases and then becomes gradual coincides approximately with the start of melting of the aluminum substrate (near the joined surfaces). Depending on the amount of heat input from that boundary to the aluminum substrate, the melting of the aluminum substrate and the melting reaction (production of intermetallic compounds) progress, and the state (size, structure, texture, etc.) of the joint between the iron substrate and the aluminum substrate are determined.

[0015] Therefore, even in spot welding, which is repeated according to a certain chart (timing of energization, duration, amount of energization, pressure, etc.), if the energization duration and amount of energization are monitored from the point (starting point) at which the change in the electrical resistance between the electrodes becomes gradual, it becomes possible to indirectly and accurately determine the joining condition (presence or absence of joining defects, etc.) even at production sites, etc.

[0016] 《Monitoring device》 The present invention can be understood as not only a monitoring method but also a monitoring device. For example, the present invention may be a monitoring device for resistance spot welding in which an iron base and an aluminum base are welded at a joint containing an intermetallic compound formed by passing a current between electrodes holding the iron base and the aluminum base and melting the aluminum base by heat transfer from the iron base, the monitoring device for resistance spot welding comprising: an origin identifying means for identifying an origin at which a time change in electrical resistance value between the electrodes becomes gradual; and a determining means for determining a joint state between the iron base and the aluminum base based on a current passing time or current amount from the origin.

[0017] <Resistance spot welding system> The present invention can also be understood as a resistance spot welding system. For example, the present invention may be a resistance spot welding system that welds an iron base and an aluminum base at a joint containing an intermetallic compound formed by passing a current between electrodes that sandwich the iron base and the aluminum base and melting the aluminum base by heat transfer from the iron base, the resistance spot welding system including an origin identifying means that identifies an origin at which a time change in electrical resistance value between the electrodes becomes gradual, and a determining means that determines a joint state between the iron base and the aluminum base based on a current passing time or current amount from the origin.

[0018] The resistance spot welding system may be, for example, one in which the above-mentioned monitoring device is attached to a resistance spot welding machine, or one in which the monitoring device is incorporated into a control device for the resistance spot welding machine. The resistance spot welding system may not only determine the joint state between the iron base and the aluminum base, but may also reflect (feed back) starting point information (the time and date from the start of current flow to the starting point, etc.), changes in electrical resistance over time (history, rate of change (slope) itself, rate of change before and after the starting point, etc.) in the welding conditions as needed or at an appropriate time.

[0019] "others" (1) In this specification, unless otherwise specified, "melting of the aluminum substrate" means that the aluminum substrate does not melt by its own Joule heat alone, but melts due to the addition of heat transferred from the iron substrate.

[0020] The "amount of current flow" in this specification is an index that reflects the amount of heat (Q) input to the parts to be joined (particularly the aluminum substrate), and is, for example, an integral value calculated from the current value (I) between the electrodes, the voltage value (V), the electrical resistance value (R), the current flow time (t), etc.

[0021] (2) The size of the joint (nugget), whether circular or not, is referred to as the "nugget diameter" as appropriate. The nugget diameter is determined as the diameter of a circle that approximately envelops the outer edge of the nugget.

[0022] (3) The elements of a method, i.e., "steps" and elements of a product, i.e., "means," can be interpreted as interchangeable. Steps or means executed by a computer are understood as elements of a program (including recording media, data structures, etc.) that is loaded into a computer.

[0023] (4) Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the various numerical values ​​or numerical ranges described in this specification may be used as a new lower limit or upper limit value to create a new range such as "a to b". Unless otherwise specified, "x to ykA" in this specification means xkA to ykA. The same applies to other unit systems (such as μm). [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic diagram showing an overview of spot welding. [Diagram 2] 4 is an example of a time chart relating to spot welding. [Diagram 3] 1 is an example of temperature distribution obtained by simulating spot welding. [Figure 4] 1 is a graph showing the change over time in electrical resistance between electrodes. [Diagram 5] 1 is a graph showing the relationship between the main current flow time and the joint strength or the nugget diameter. [Figure 6] 1 is a bar graph showing the relationship between the amount of heat input in the main current supply and the number of piles in the joint (nugget). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] One or more components selected from the present specification may be added to the above-mentioned components of the present invention. The contents described in this specification may be applicable to devices, systems, etc. as well as methods. Which embodiment is best depends on the target, required performance, etc.

[0026] Iron and aluminum substrates (1) The iron substrate and the aluminum substrate can be of any composition or shape. The iron substrate can be an iron alloy (especially steel) or pure iron. The aluminum substrate can be an aluminum alloy or pure aluminum. The iron substrate and the aluminum substrate can be of the same shape or different shapes. Below, we will explain these substrates by taking typical examples such as a steel plate and an aluminum alloy plate (referred to as "Al alloy plate").

[0027] The steel plate and the Al alloy plate may each be one sheet, or at least one of them may be multiple sheets. The multiple sheets may be the same or different in thickness, composition, surface treatment state (plating or not, etc.).

[0028] (2) The steel sheet is, for example, a cold-rolled steel sheet, a hot-rolled steel sheet, a high-strength steel sheet, a hot-stamped steel sheet, etc. At least one steel sheet may be surface-treated on one or both sides. Examples of the surface-treated layer of the steel sheet include a zinc-plated layer, an Al-Si-based plating layer, a zinc oxide layer, a Zn-Fe alloy layer, etc. Representative examples of zinc-plated steel sheets include hot-dip galvanized steel sheets, electrogalvanized steel sheets, and alloyed hot-dip galvanized steel sheets. The alloyed hot-dip galvanized steel sheet may be an aluminum-zinc alloy-plated steel sheet (so-called Galvalume Steel Sheet (registered trademark)).

[0029] In addition, even if a surface treatment layer made of a metal with a lower melting point than the Al base material (such as an Al alloy) that constitutes the aluminum base is present on the joining surface side and begins to melt before (priority to) the aluminum base, it is possible to monitor the joining condition using the present invention.

[0030] (3) The aluminum substrate (Al alloy plate) is made of, for example, an Al alloy of 2000 series to 8000 series, particularly 5000 series or 6000 series. For the 5000 series, for example, an Al alloy equivalent to A5052, A5083, A5005, etc., specified in JIS is used. For the 6000 series, for example, an Al alloy equivalent to A6022, A6016, A6N01, A6061, A6082, etc., specified in JIS is used. Note that the Al alloy referred to in this specification also includes the A1000 series.

[0031] (4) The thickness of the iron substrate (steel plate, etc.) is, for example, 0.4 to 2.5 mm, or 0.6 to 1.8 mm. The thickness of the aluminum substrate (Al alloy plate, etc.) is, for example, 0.8 to 3 mm, or 1 to 2 mm. The thicknesses of the materials to be joined may be the same or different.

[0032] 《Joint part》 Spot welding creates a joint (joint layer) containing intermetallic compounds between the iron and aluminum substrates. The shape (structure, etc.) of the joint can change depending on the amount of current (amount of heat input) or the time that current is passed from the start of melting (starting point) of the aluminum substrate.

[0033] When the current flow time or current flow amount from the starting point is within an appropriate range, a welded product having a desired joint can be obtained. The joint may have a first layer made of a first intermetallic compound formed on the iron substrate side and a second layer made of a second intermetallic compound formed on the aluminum substrate side, as described in detail in JP 2023-66438 A, for example. Such first and second layers are described below. The contents of JP 2023-66438 A are also incorporated into this specification as appropriate.

[0034] (1) First layer The first layer is on the iron substrate side of the joint and has a first intermetallic compound and a first pile. The first intermetallic compound may be, for example, Al 5 Fe 2 The primary projections are mainly composed of. The primary projections are integrally connected to the iron substrate and protrude like piles into the layered first intermetallic compound. In other words, the primary projections extend from the iron substrate in a comb-like shape, and the first intermetallic compound fills the spaces between the primary projections and covers the bonding interface of the iron substrate. The primary projections may have the same composition as the iron substrate, or may be an iron alloy containing more Al or plating components (e.g., Zn) than the iron substrate.

[0035] Since a plurality (a large number) of first piles are uniformly distributed in the first intermetallic compound, the first layer is firmly bonded to the iron substrate, and the propagation and development of cracks that may occur in the first layer can be suppressed. Note that such piles may shrink or even disappear if the amount of heat input (current application time, current application amount) from the starting point becomes excessive.

[0036] (2)Second layer The second layer is on the aluminum substrate side of the joint and consists of a second intermetallic compound (mainly Al 3 The second layer may be in the form of a layer having a substantially uniform thickness or in the form of comb teeth. The comb-teeth-shaped second layer has, for example, in addition to a base layer made of the second intermetallic compound, second projections extending in a columnar manner toward the aluminum substrate. The second projections enhance the anchor effect to firmly bond the second layer to the aluminum substrate, and can also inhibit the propagation and development of cracks that may occur in the second layer.

[0037] The thickness of the first layer is, for example, 1 to 10 μm, 2 to 7 μm, or even 3 to 5 μm. The combined thickness of the first layer and the second layer is, for example, 2 to 15 μm, 3 to 10 μm, or even 4 to 8 μm.

[0038] 《Manufacturing method》 The spot welding of an iron substrate and an aluminum substrate is carried out, for example, through the following heating and cooling steps.

[0039] (1) Heating process (main energization process) The heating process is performed by passing electricity between the electrodes that hold the iron and aluminum substrates. At this time, the iron substrate heats up before the aluminum substrate, and the aluminum substrate begins to melt at the joint (near the joint interface with the iron substrate) due to the heat transfer from the iron substrate in addition to its own resistance heating. The iron substrate may be heated in advance by spot welding steel plates together.

[0040] The maximum current value between the electrodes in the heating step is, for example, 20 kA or less. More specifically, the current value is 10 to 18 kA (even less than 18 kA), or 11 to 16 kA.

[0041] The current flow time is, for example, 200 to 600 ms or 300 to 500 ms. During the current flow time, the current value may be constant or may vary. For example, when upslope current flow is performed in which the current value is monotonically increased (time change rate ≧0), efficient spot welding can be achieved while suppressing the generation of spatters and dust.

[0042] (2) Cooling process In the cooling step, the joined parts after the heating step are cooled through the electrodes. The cooling step may be performed by cutting off the current (non-current passing), or may be performed while passing a small current value (which may be constant or may decrease monotonically, etc.).

[0043] (3) Pre-energization process Before the heating step (main current application step), a pre-current application step (pre-current application step) may be performed to familiarize the contact state between the iron substrate and the aluminum substrate. In the pre-current application step, a current value (e.g., 3 to less than 10 kA, 5 to 9 kA) smaller than that in the heating step may be applied. After the pre-current application step (before the main current application step), the current to the electrodes may be cut off (non-current application) or reduced to lower the temperature near the bonding interface. This reduces the contact resistance of the bonded parts, and allows the transition to the main current application step while suppressing the generation of spatters and dust. The period is, for example, 10 to 300 ms, 50 to 150 ms.

[0044] Overwatch The state of the joint between the iron base and the aluminum base (the form of the joint) depends greatly on the amount of heat input to the joint (particularly the aluminum base) from the time when the aluminum base substantially starts to melt (react) (the start of melting). However, it is not easy to directly grasp the start of melting or the amount of heat input. Therefore, in the present invention, the starting point, current flow time, and current flow amount, which are indirect indicators of these, are obtained from the current flow history (in-situ information) between the electrodes obtained at the spot welding site (on the spot: in-situ), and the joint state is monitored (the quality of the welded work is judged) based on these. The specific method is as follows.

[0045] (1) Identifying the starting point (steps / means) The time when the aluminum substrate and the iron substrate substantially start to melt is indicated, for example, by the time (starting point) when the change in the electrical resistance between the electrodes starts to slow down. Such a starting point is determined, for example, from the intersection of a first line indicating the change in the electrical resistance in a first region before the starting point and a second line indicating the change in the electrical resistance in a second region after the starting point. The current flow time and current flow amount can be calculated from the time (point) of the intersection.

[0046] The first line and the second line are obtained, for example, from data (curve, plot, coordinates, etc.) showing the time change of the electrical resistance value as an approximation line (regression line) of the time change. The approximation line is obtained, for example, as a linear function obtained by regression analysis using the least squares method or the like based on a plurality of points (for example, 50 to 200 points, 100 to 150 points) arbitrarily or continuously extracted from a range in which the time change is stable. The starting point of the first line is a point (for example, a point 10 to 20 points later) where the resistance change stabilizes from the maximum point that appears at the beginning of the current flow, and for the second current flow, a point (for example, a point 10 to 20 points later) where the resistance change stabilizes from the point where the slope changes after the first line is used.

[0047] (2) Judgment (step / means) The shape (structure, size, etc.) of the joint (nugget) formed between the iron and aluminum substrates largely depends on the amount of heat input from the start of melting of the aluminum substrate. The amount of heat input is indexed by the amount of current or the time of current flow from the starting point.

[0048] Since the change over time in the electrical resistance between the electrodes after the starting point is gradual, if a current is applied at a substantially constant value, the amount of heat input from the start of melting is indexed by the time of current application from the starting point. On the other hand, if current is applied while changing the current value (for example, upslope current application, downslope current application, setting of non-current application sections, etc.), the amount of heat input from the start of melting is indexed by the amount of current applied obtained by integrating the current value (squared) after the starting point.

[0049] By comparing the current flow time or current flow amount from the starting point with a preset threshold value, it is possible to judge the bonding state (bonding quality) of the iron base and the aluminum base. Note that the threshold value in this specification also includes a numerical range. The comparison with the threshold value includes not only a simple comparison of magnitude, but also whether or not the value falls within a predetermined range.

[0050] When the energization time or amount of energization is outside a predetermined range (including below or equal to a threshold value), a warning or the like may be issued. If the production site responds to the warning promptly, the occurrence or outflow of defectively joined products can be prevented.

[0051] Incidentally, the monitoring (quality control) of the welded products according to the present invention may be performed for each welded product, for each fixed number of products, or at fixed time intervals. A monitoring device may be provided for each welding device (robot, etc.), or may be provided for a selected welding device. The results (current flow time and current flow amount from the starting point) obtained from multiple monitoring devices may be integrated to monitor (determine) the overall joining condition. EXAMPLES

[0052] Various specimens (welds) were produced by resistance spot welding two steel plates (iron base) and an Al alloy plate (aluminum base) stacked in order, and the relationship between their current flow history and the joining state was clarified. The present invention will be described in more detail with reference to such specific examples.

[0053] An outline of the spot welding performed in this example is shown in Fig. 1. A plate assembly in which a first steel plate, a second steel plate, and an Al alloy plate were laminated in this order was used as the joined material. The spot welding was performed by passing electricity through a pair of electrodes that applied pressure to the surface of the plate assembly (the lower surface of the first steel plate and the upper surface of the Al alloy plate). For convenience of explanation, unless otherwise specified, the direction of the arrows shown in Fig. 1 is the up-down direction or the left-right direction.

[0054] <Sample Preparation> (1) Material to be joined The first steel plate was a non-plated cold-rolled steel plate (440 MPa class / plate thickness: 1.4 mm), the second steel plate was a galvannealed steel plate (270 MPa class / plate thickness: 0.8 mm), and the Al alloy plate was a drawn material equivalent to JIS A6022 / plate thickness: 1.2 mm.

[0055] The zinc-plated steel sheets had a zinc-plated layer (metal layer) with a thickness of about 8 μm. The melting point of the zinc-plated layer itself was about 420°C, and the melting point of the Al alloy sheet was about 650°C. Each sheet material was used for spot welding as it was, without surface polishing, etc. Each sheet material was also cut into strips (30 mm x 100 mm) for use.

[0056] (2) Electrode The first electrode on the first steel plate side and the second electrode on the Al alloy plate side used the same DR type (JIS C9304) commercially available tip (OBARA Corporation). The tip was forcibly cooled by supplying forced circulating cooling water (flow rate: 2.7 L / min) to the inside (inner cylindrical part) of the tip. The electrode was made of chromium copper (Cr: 1 mass%, Cu: balance) and had an electrical conductivity of 80% IACS.

[0057] As shown in the enlarged view of Figure 1, the electrode size was as follows: tip diameter (nominal diameter D2): φ16 mm, thickness of the bottom tip: 12 mm, radius of curvature of the tip shoulder: 8 mm, radius of curvature of the tip surface: R1: 40 mm, and tip diameter (D1) was 12 mm.

[0058] (3) Welding conditions Spot welding was performed using a servo pressure spot welding machine (ARO PA235KVAMF). The current pattern (time chart) is shown in Figure 2. The electrode pressure (F) for the plate assembly was kept constant at 4kN. Current was applied by controlling the DC current as follows:

[0059] First current value (I 1 The first current application process (pre-current application process) was carried out with a current supply voltage of 8kA and a current application time of 50ms. After this current application, the plate assembly was cooled through both electrodes with a non-current application period of 100ms (first cooling process). This reduced the electrical resistance between the plates (particularly between the second steel plate and the Al alloy plate) to a specified range, stabilizing the subsequent second current application process.

[0060] After the first cooling step, the second current step (heating step / main current step) was performed by up-slope current that linearly and monotonically increased the current value. 2 ) was set to an initial value of 10kA and increased at a rate of 1kA / 100ms. The current application time in the second current application process (referred to as "main current application time") was 100ms, 200ms, 300ms, 400ms, 500ms or 600ms unless otherwise specified. After the second current application process, as in the first cooling process, the plate assembly was cooled through both electrodes in a non-current state (100ms) (second cooling process). In this way, multiple samples with different main current application times (main current amounts) were produced.

[0061] <Numerical analysis of temperature distribution> The temperature distribution in the welded parts that occurs during the second current application process was numerically analyzed using the resistance welding simulation software SORPAS. An example of the analytical model used and the analytical results (temperature distribution) are shown in Figure 3.

[0062] As can be seen from FIG. 3, first, a first liquid phase is generated in the joining portion of the first steel plate and the second steel plate in the second current application process, and solidifies in the second cooling process to become a first nugget that joins the first steel plate and the second steel plate. In parallel with this, a second liquid phase is generated in the joining portion of the second steel plate and the Al alloy plate in the second current application process, and solidifies in the second cooling process to become a second nugget (joint) that joins the Al alloy plate and the second steel plate. Note that the above-mentioned second current value (I 2 ), the above-mentioned simulation confirmed that the Al alloy plate does not melt directly due to its own resistance heat alone, but melts due to conductive heat from the second steel plate.

[0063] "test" Each sample was subjected to a tensile shear test (JIS Z 3136) to measure the load at break. This measurement was repeated three times to obtain the tensile shear strength (average value).

[0064] "observation" The fracture surface of the joint (second nugget) between the Al alloy plate and the second steel plate was observed with an inverted metallurgical microscope (Olympus GX53) and a scanning electron microscope (SEM: Hitachi High-Tech S-3600N). In addition, the components of the structure shown in the SEM image were analyzed with an energy dispersive X-ray spectrometer (EDX) attached to the SEM.

[0065] By the way, in the case of a two-layer joint, the first layer on the second steel plate side is Al. 5 Fe 2 phase (first intermetallic compound), and the second layer on the Al alloy plate side is Al 3 The Fe phase (second intermetallic compound) was predominant.

[0066] The first layer is Al5 Fe 2 In the second layer, the first piles, which were elongated and extended integrally from the second steel plate, were distributed almost uniformly. 5 Fe 2 Al formed on the 3 Relatively thick columnar secondary projections that extended integrally from the Fe phase and grew so as to bite into the Al alloy plate were distributed approximately uniformly.

[0067] The thickness of the intermetallic compound layer in the two-layer joint was about 1 to 6 μm in total. 5 Fe 2 The thickness of the second layer (Al 3 The thickness of the Fe phase was about 0.3-2 μm.

[0068] The pile distribution (protrusion distribution) was also observed according to the description in JP 2023-66438 A. In the case of the joint of the two-layer structure, there were 5 to 50 piles per 50 μm (reference length: L) along the surface direction of the first layer.

[0069] <<Origin Identification Step>> Spot welding was performed according to the current pattern in Figure 2, and the change in electrical resistance between the electrodes over time was measured. An example is shown in Figure 4. Figure 4 also shows the temperature distribution (simulation results) of the parts to be joined during the second current flow (main current flow) process.

[0070] As can be seen from Figure 4, there existed a range (first section) in which the electrical resistance between the electrodes suddenly decreased, and a range (second section) in which the time change in the electrical resistance gradually decreased during the second current application step. It was also found that the time change in the electrical resistance in the first section could be approximately indexed by the first approximation line L1, and the time change in the electrical resistance in the second section could be approximately indexed by the second approximation line L2.

[0071] From the simulation results at the time (tp) of the intersection (P) of both lines, it was confirmed that the Al alloy plate actually starts to melt at that time. Therefore, it was found that the time (tp) should be set as the starting point for determining the start of melting of the Al alloy plate.

[0072] The first approximate straight line L1 and the second approximate straight line L2 are regression lines obtained by the least squares method based on 80 to 200 points extracted at intervals of 20 to 50 ms from approximately the center of the first and second sections. For example, if the starting point is assumed to be approximately 200 ms from the start of the second current application process, the first approximate straight line can be obtained based on points extracted from a region preceding the starting point (for example, approximately 100 ms from the start of the second current application process). Similarly, the second approximate straight line can be obtained based on points extracted from a region lagging behind the starting point (for example, approximately 300 ms from the start of the second current application process).

[0073] Decision Step (1) Bonding strength and nugget diameter The joint strength (tensile shear strength) and the first and second nugget diameters measured from the fracture surface of each sample (test piece) with different current application times are shown in Figure 5. The first nugget diameter is the diameter of the first intermetallic compound (Al 5 Fe 2 ) and the second nugget diameter is the diameter (d1) of the envelope circle of the second intermetallic compound (Al 3 The diameter (d2) of the envelope circle of the welded joint (Fe) was taken as d1. Overall, d2>d1. Figure 5 also shows the temperature distribution (simulation results) of the welded parts during the second current application (main current application) process.

[0074] As is clear from Fig. 5, in the range beyond the starting point, the joint strength and nugget diameter increased approximately in proportion to the current flow time. Incidentally, it was also found that the joint strength has a stronger correlation with the first nugget diameter than with the second nugget diameter.

[0075] On the other hand, in the range before the starting point, even if a nugget was formed between the first steel plate and the second steel plate, a substantial nugget was not formed between the Al alloy plate and the second steel plate, and the joining strength was significantly reduced. This is considered to be because, in such a range, the Al alloy plate is not substantially melted, and a substantial nugget is not substantially formed between the second steel plate and the Al alloy plate.

[0076] When the current application time exceeded the starting point, a large nugget was formed due to the progress of melting of the aluminum alloy plate, but the joint strength decreased. This was probably because the pile extending from the second steel plate to the first nugget (first layer) decreased and even disappeared.

[0077] (2) Number of piles Main power supply time (T 2 ) and the amount of current at that time (I 2 ) and the input heat (I 2 2 ×T 2 The piles extending from the second steel plate to the first nugget were counted based on SEM images of the fracture surfaces of each sample (test piece) with different diameters (×R). The relationship between the number of piles and the amount of heat input is shown in Figure 6. Figure 6 also shows the fracture surfaces (SEM images) of each sample belonging to the area with few piles and low strength and low heat input, the area with many piles and high strength, and the area with few piles and high heat input where strength is reduced.

[0078] As can be seen from Figure 6, the number of piles increased in proportion to the amount of heat input (time or amount of current flow from the starting point). However, piles were hardly observed in the samples before the starting point, and piles decreased in the samples with excessive current flow time after the starting point. These were consistent with the joint strength shown in Figure 5.

[0079] From the above, it was clarified that the starting point at which the change in electrical resistance between the electrodes becomes gradual essentially indicates the time when the aluminum base begins to melt due to spot welding, and the current flow time and current flow amount after the starting point essentially indicate the state (size, structure, etc.) of the joint between the iron base and the aluminum base. Therefore, it was confirmed that the joining state can be judged and the quality of the weld can be monitored by identifying the starting point from the current flow history (in-situ information) related to spot welding and comparing the current flow time and current flow amount after the starting point with threshold values, etc.

[0080] Incidentally, since the time when the aluminum substrate starts to melt is likely to vary due to various factors, simply monitoring the current flow time and current flow amount from the start of the current flow time does not allow the bonding state of the iron substrate and the aluminum substrate to be accurately grasped as in the present invention.

Claims

1. 1. A method for monitoring resistance spot welding in which an iron base and an aluminum base are welded at a joint including an intermetallic compound formed by melting the aluminum base by heat transfer from the iron base by passing a current between electrodes that sandwich the iron base and the aluminum base, the method comprising the steps of: a starting point identifying step of identifying a starting point at which a time change in the electrical resistance value between the electrodes becomes gradual; a determination step of determining a state of the joint based on a current flow time or a current flow amount from the starting point; A method for monitoring resistance spot welding comprising:

2. 2. The method for monitoring resistance spot welding according to claim 1, wherein the starting point identifying step identifies the starting point from an intersection of a first straight line indicating a time change in the electrical resistance value in a first section before the starting point and a second straight line indicating a time change in the electrical resistance value in a second section after the starting point.

3. 3. The method for monitoring resistance spot welding according to claim 1, wherein the determining step issues a warning when the current supply time or the current supply amount is outside a predetermined range.

4. 1. A resistance spot welding system for welding an iron base and an aluminum base at a joint including an intermetallic compound formed by melting the aluminum base by heat transfer from the iron base by passing a current between electrodes that sandwich the iron base and the aluminum base, comprising: a starting point identifying means for identifying a starting point at which a time change in the electrical resistance value between the electrodes becomes gradual; a determination means for determining a state of the joint based on a current flow time or current flow amount from the starting point; A resistance spot welding system comprising:

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