JPH09196881A - Computing method for interface voltage of resistance welding and its interface resistance, and monitoring method for welding quality - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the accuracy of monitoring the welding quality by computing a voltage or resistance occurring on the interface of an electrode and a welded material from voltage between the electrodes to sandwich the welded material or the resistance between the electrodes computed from the voltage between the electrodes and welding current. SOLUTION: In the case where the resistance of an electrode it self is denoted by electrode resistance Rec; resistance inside a welded material by plate resistance Rreal; resistance occuring on the interface of the electrode and the welded material by resistance between a plate and the electrode Rde; and resistance occurring on the mutual interfaces of the welded materials by resistance between the plate and the plate Rdc; a measured resistance Rmeas measured from the electrode or an electrode holder is expressed as follows; Rmeas=2Re1+2Rreal+2Rde+Rdc. The resistance Rei can be measured by supplying the current in a state where the welded material is not sandwiched. The resistance Rde can not finally ignore influence on account of the wear of the surface of the electrode with the increase of the number of times of welding. The resistance Rde is computed to precisely judge the quality of welding.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention particularly relates to a method for calculating an interface voltage or an interface resistance of resistance welding used for spot welding and a method for monitoring welding quality.

[0002]

2. Description of the Related Art Resistance welding, particularly spot welding, has been used for various products using steel sheets, but in recent years, the welding failure tends to increase. That is, in the past, since a mild steel plate was generally a material to be welded, there was little energization failure, and the welding quality could be kept relatively stable if welding conditions were controlled to be constant. However, galvanized steel sheets and high-strength steel sheets have begun to be used in large quantities in place of mild steel sheets, and the occurrence of poor welding has increased. From such a background, the appearance of an apparatus capable of accurately controlling welding quality, rather than simply monitoring welding conditions, has been awaited.

To address this problem, various welding quality monitoring devices have been developed so far for the purpose of determining the quality of the welding result after the completion of welding. For example, in the one developed up to now, (1) the inter-chip resistance is determined from the welding current and the welding voltage, and the quality of the welding result is judged from the change pattern thereof. One example thereof is JP-A-56-158286. (2) The inter-chip voltage is compared with the time change of the preset reference voltage, and the quality is judged by whether the difference is within the allowable value or not. No. 59-14312, further, an effective component that effectively contributes to heat generation in the welded portion is extracted from the inter-chip voltage, and the quality of the welding result is determined from the time integral value of the effective component. As an example,
No. 9-40550 and Japanese Patent Application Laid-Open No. 59-61580, (3) The heat generation temperature is detected, and the quality of the welding result is determined from the temperature change pattern. No. 216246, and (4) ultrasonic waves are transmitted between the materials to be welded, and the quality of the welding result is determined from the amount of the transmission. One example is disclosed in Japanese Unexamined Patent Publication No. 52-94841. What was disclosed,
(5) One using displacement of the electrode tip during welding, which is disclosed as an example thereof in JP-B-60-40955, (6) Welding current is detected, and the upper and lower limit values thereof are monitored to obtain a welding result. There are things that try to keep constant. (7)
A computer is used to calculate the nugget diameter using a heat conduction model. As an example, Sano: Study on numerical analysis method of current passage and temperature distribution in spot welding, Master's thesis of Osaka University Graduate School of Welding (Showa 54) , Xiu: A study on speeding up of quality monitoring for numerical calculation aided resistance spot welding, and one disclosed in the master's thesis of welding department of Osaka University (Heisei 3). In order to directly control the welding machine, (8) calculate the base metal temperature distribution from the heat conduction model, estimate the nugget diameter from the temperature distribution, and correct the temperature distribution using the electrode movement amount during welding. There is Japanese Patent Publication No. 7-16791.

[0004]

[Problems to be Solved by the Invention] (1) to (6)
In the conventional method up to, it is indispensable to perform a preliminary experiment at the welding site for each welding material and obtain the relationship between the welding quality and the discrimination standard in advance. It was necessary to consider the change in interface resistance at the contact interface between the electrode tip and the material to be welded due to wear and deformation of the electrode tip. However, it is difficult to estimate even the change in the interfacial resistance in a preliminary experiment at the welding site, and conventionally it was only possible to roughly determine the quality of the welded portion, and the applicable range was limited. Therefore, using a conventional welding quality monitoring device together with the resistance welding machine,
Even if trying to secure the welding quality, it is not versatile and uncertain, and it is the fact that welding is performed under excessive welding conditions at the welding site in order to avoid welding defects. Excessive welding current is used at the welding point, resulting in excessive power loss, electrode wear, and dust. (7) has a possibility of solving the above-mentioned problems, and the biggest drawback is that it takes time to solve the heat conduction equation. For this reason, a method of calculating the nugget diameter at a high speed has been devised, and a device for monitoring all welding points at a welding site after the end of welding has been put to practical use. (8) is a method of correcting the calculation error of (7), but since the movement amount of the electrode is used, a movement amount detection device is required, resulting in high cost.
Further, it may not be applicable when the welding position is the end of the material to be welded.

The first invention is a further development of the method (7) to solve the above problems, and has general versatility in the voltage or resistance generated at the interface during welding, which is the main factor of the calculation error. A second aspect of the present invention provides a welding quality monitoring method using the method of the first aspect of the present invention in consideration of wear and deformation of electrode tips, which are difficult to apply by the conventional welding quality monitoring method. It is an object.

[0006]

In order to achieve the above object, the present invention provides a method for calculating an interfacial voltage or an interfacial resistance in resistance welding according to the first aspect of the invention, in which a voltage between electrodes sandwiching a material to be welded,
Alternatively, the voltage or resistance generated at the interface between the electrode and the welded material is calculated using the resistance between the electrodes calculated from the voltage between the electrodes and the welding current and at least the voltage or resistance determined by the specific resistance heat generation of the welded material. It is characterized by doing. In the welding quality monitoring method of the second invention, it is determined by the voltage between the electrodes sandwiching the material to be welded, or the resistance between the electrodes calculated from the voltage between the electrodes and the welding current, and at least the specific resistance heat generation of the material to be welded. It is characterized in that the interface voltage or interface resistance between the electrode and the material to be welded is calculated using voltage or resistance, and the welding quality is determined using the calculated voltage or resistance.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION With the above construction, the method of calculating the interfacial voltage or the interfacial resistance of resistance welding according to the first aspect of the present invention is such that the interface between the electrode tip and the surface of the material to be welded when the electrode tip is worn or deformed during welding. The change in voltage or resistance can be calculated, and the voltage or resistance at the interface, which has a great influence on the monitoring of welding quality, can be incorporated into the method for monitoring welding quality. A welding quality monitoring method according to a second aspect of the present invention is a welding quality monitoring method that uses a heat conduction model and incorporates the method for calculating the interface voltage or the interface resistance of the resistance welding according to the first aspect of the present invention. Has the effect of enabling the monitoring of.

Embodiments of the present invention will be described below with reference to the drawings. First, the calculation method of the interface voltage or the interface resistance of the first invention will be described with reference to FIGS. 1, 2, 3, and 4.
This will be described with reference to FIG. FIG. 1 shows the resistance generated between the electrode and the material to be welded and their interface. The resistance of the electrode itself is the electrode resistance Rel, and the resistance inside the material to be welded is the plate resistance Rr.
If eal, the resistance generated at the interface between the electrode and the material to be welded is the plate-to-electrode resistance Rde, and the resistance generated at the interface between the materials to be welded is the plate-to-plate resistance Rdc, it is measured from the electrode or electrode holder. The measured resistance Rmeas is represented by the sum of the above-mentioned resistances and is as shown in the formula (I).

Rmeas = 2Rel + 2Rreal + 2
Rde + Rdc ... (I). The formula (I) shows the case where the upper and lower electrodes are welded by welding two plates, but the measured resistance Rmeas can be represented by the sum of the resistances of the respective portions even when a large number or different upper and lower electrodes are used. Here, the heat generation of the welded portion, that is, the resistance component related to the formation of the molten portion is the plate resistance Rre
al, and the electrode resistance Rel, the plate-to-electrode resistance Rde, and the plate-to-plate resistance Rdc do not directly contribute to the heat generation of the welded portion. When the resistance between the electrodes is used to monitor the welding quality, an error may occur in the determination of the welding quality depending on the size of the resistance component that does not represent heat generation in the welded portion and the time when it affects.

Plate-to-electrode resistance Rde and plate-to-plate resistance Rd
c is the surface resistance related to the uneven shape on the electrode surface or the plate surface. FIG. 2 is a schematic diagram for explaining unevenness on the surface. As shown in the figure, the surface irregularities can be represented by a fine high-frequency component called roughness and a low-frequency component called waveness. Among these, the high frequency component disappears in the initial stage of the passing electrode due to the collapse due to the electrode pressurization and the decrease in the yield stress due to the temperature rise due to the energization. That is, the surface resistance due to the high frequency component disappears immediately after the start of energization, and the time ratio of the measured resistance Rmeas becomes extremely small, so that it can be ignored. Even when the welding quality is monitored using the inter-electrode resistance, it does not cause an error in determining the welding quality. On the other hand, the low-frequency component causes concentrated resistance due to the narrowing of the current-carrying path at the interface, and the resistance component accounts for a large proportion of the measured resistance Rmeas and has a long influence time. Applying the surface irregularities described above to the plate-to-plate interface and the plate-to-electrode interface, the plate-to-plate interface becomes a fresh interface with each welding, so the surface irregularity becomes a high-frequency component. . That is, the plate-to-plate resistance Rdc has almost no effect on the formation of the nugget. At the plate-electrode interface, the electrode surface wears as the number of weldings increases, so that the uneven shape of the surface contains low-frequency components, and if the electrode wears further, a good contact state cannot be obtained. In particular, when a galvanized steel plate is welded, an alloy layer is formed on the surface of the electrode, so that the progress of electrode wear becomes large. That is, the influence of the plate-to-electrode resistance Rde due to the contact state between the plate surface and the electrode interface cannot be ignored, and when the welding quality is monitored using the electrode-to-electrode resistance, an error may occur in quality determination. Therefore, in order to determine the welding quality with higher accuracy, it is necessary to calculate the plate-to-electrode resistance Rde according to the state of the interface between the plate surface and the electrode surface and remove it from the measured resistance Rmeas.

The electrode resistance Rel can be measured by energizing the material to be welded without sandwiching it. That is, the electrode resistance Rel can be treated as a known value.

FIG. 3 shows an example of measurement results of resistance change between chips during welding using a measuring line. The upper part of the figure shows the measurement result using a new electrode, and the lower part shows the measurement result using a worn electrode after 1000 times of welding. Measured from the measurement resistance Rmeas from the electrode holder and the plate-electrode interface part, respectively. The resistance Rr is displayed. In particular, the resistance Rr indicates the plate resistance Rreal obtained by removing the plate-to-electrode resistance Rde by measuring from the plate-electrode interface portion.
When a new electrode is used, the contact between the plate surface and the electrode surface is good, so the measured resistance Rreal from the electrode holder
And the resistance Rr measured from the plate-to-electrode interface portion are almost the same. That is, the measured resistance Rmeas indicates the heat generation state of the welded portion, and the welding quality can be monitored using the measured resistance Rmeas. When the consumable electrode is used, the resistance Rr measured from the plate-electrode interface is considerably smaller than the measured resistance Rmeas. This is because the contact state between the consumed electrode surface and the plate surface is not good, and the influence of the plate-to-electrode resistance Rde described above is increased. That is, the measured resistance Rmeas at the time of electrode wear is largely influenced by the resistance component that does not represent the heat generation state of the welded portion, and if the measured resistance Rmeas is used for monitoring the welding quality, there is a risk of causing a quality determination error. In order to monitor the welding quality with higher accuracy, it is necessary to use the resistance Rr measured from the plate-electrode interface.
Although it is possible to actually measure the resistance Rr using the measurement line as shown in this embodiment, it is difficult to attach the measurement line to the plate-electrode interface at the welding line in the actual manufacturing site.

FIG. 4 is a flow chart for carrying out the interface resistance calculation method for calculating the voltage or resistance generated at the interface between the electrode and the material to be welded according to the first invention. The thickness of the material to be welded, the number of layers, the physical constants of the plate material and plate material, the shape and type of the electrode, and the physical constants of the electrode material are input in advance and based on the given values. Set the boundary conditions for the heat conduction model. This heat conduction model is a mathematical model that consists of the geometrical shape and physical constants of the weld, and is used for numerical analysis from the voltage and current of the weld. Then, from the welding current flowing through the weld and the voltage applied to the weld, a numerical analysis using a heat conduction model is performed to calculate the current-carrying diameter of the weld, the current distribution, the potential distribution, and the current density distribution. If the heat generation calculation and the heat conduction calculation are performed from the current density and the specific resistance, the temperature distribution of the weld can be estimated. Further, the heat generation state during the welding process can be sequentially estimated by performing the preset time step for each very short time from the start to the end of welding. Here, the combined resistance in the plate, that is, the plate resistance Rreal can be estimated from the specific resistance of each part of the welded portion calculated for each time. Then, by using the estimated plate resistance Rreal and the measured resistance Rmeas, the plate-to-electrode resistance Rde can be estimated at each time step. FIG. 5 is an explanatory diagram of a method of estimating the plate-to-electrode resistance Rde using the measured resistance Rmeas and the plate resistance Rreal estimated from the heat conduction model.

At an arbitrary time t0 during welding, the plate resistance Rreal estimated by the heat conduction model is Rc. At the start of energization, the sheet resistance Rreal is estimated by the initial temperature of the material to be welded given by the initial boundary condition setting. At time t1 which is the next step, Rm ′ is measured as the measured resistance Rmeas, but since the measured value includes the surface resistance generated at the interface between the plate and the electrode, it should be used for numerical analysis. The resistance Rsim obtained by subtracting is the plate resistance Rc at time t0.
And the measured resistance Rm ′ at time t1. Therefore, a constant internal division point is obtained from the difference between the measured resistance Rm ′ and the plate resistance Rc, and t is calculated as the calculated resistance Rsim used for the numerical analysis.
Rs ′ in 1 is used. Here, the difference between the measured resistance Rm ′ and the calculated resistance Rs ′ is estimated as the resistance between the plate and the electrode at time t1. Numerical analysis using a heat conduction model is performed from the calculated resistance Rs ′ and the welding current at time t1, and the estimated combined resistance of the welded portion becomes the plate resistance Rc ′. At time t2, the calculated resistance Rs ″ is estimated in the same manner as described above using the plate resistance Rc ′ estimated at time t1 and the measured resistance Rm ″ at time t2. By repeating this process at each set time step until the end of welding, the measured resistance R
The plate-to-electrode resistance Rde can be calculated from the difference between meas and the calculated resistance Rsim. As described above, according to the present invention, it is possible to calculate the interface resistance between the electrode and the material to be welded, which does not represent the heat generation state of the welded portion. Further, even in the conventional welding quality monitoring method using the electrode resistance, the influence of the interface resistance can be eliminated by the method of the present invention, so that more accurate welding quality determination and monitoring can be performed. It can be carried out. In particular, it is effective when the electrodes are consumed or the galvanized steel sheet is welded.

Although the case where the resistance between the electrodes is used has been described in the present embodiment, the same effect of the invention can be obtained by the same method even when the voltage between the electrodes is used.

Next, the welding quality monitoring method of the second invention will be described with reference to FIGS. 6, 7 and 8. FIG. 6 is a flow chart for implementing the welding quality monitoring method of the second invention. However, according to the method of calculating the interface voltage or interface resistance of the first invention, the resistance or voltage of the plate-electrode interface is calculated by performing a numerical analysis using a heat conduction model from the voltage or resistance between the electrodes and the welding current. Then, the temperature distribution of the welded portion can be estimated. Therefore, as shown in FIG. 6, it is possible to estimate the fusion zone by calculating the range above the fusion temperature of the material to be welded from the temperature distribution of the weld zone estimated by numerical analysis at each time step. is there. Then, from the estimation result of the fusion zone, it is possible to estimate the size of the fusion zone at the time when the formation of the fusion zone or the welding end, that is, the nugget, the fusion zone diameter at the plate-to-plate interface, that is, the nugget diameter, etc., and monitor the welding quality. Is what you can do. Furthermore, by calculating the voltage or resistance generated at the plate-electrode interface and subtracting it from the measured inter-electrode voltage or resistance, the welding quality can be monitored with higher accuracy than if the measured inter-electrode resistance or voltage was used as is. It can be performed. 7 and 8 show experimental results using the method for monitoring welding quality according to the present invention. FIG. 7 is an experimental result in which the measured voltage between electrodes is directly applied to the numerical analysis by the heat conduction model without using the calculation method of the interface voltage or the interface resistance of the first invention, but FIG. 8 is the welding result of the second invention. The experimental result by the quality monitoring method is shown. Welding conditions are galvanized steel sheets, two sheets with a plate thickness of 2.0 mm, welding current 10.0 kA
Constant, pressure 400 kgf, welding time 22 cycle
Welding was continuously performed at (60 Hz), and welding was performed until the measured value of the nugget diameter by the fracture test was 0 mm. In this embodiment, the measured resistance Rm is referred to with reference to the actually measured plate resistance.
75% of the difference of the plate resistance Rc estimated as
In addition to c, the calculated resistance is Rsim. The nugget diameter was gradually reduced due to the consumption of the electrode due to the increase in the number of weldings, and the nugget diameter was not generated after about 2,300 weldings. When the measured inter-electrode voltage shown in FIG. 7 is used as it is, a numerical analysis including the resistance between the plate and the electrode due to electrode consumption is performed, so that a significant nugget diameter estimation error occurs after about 2000 weldings. Will occur. On the other hand, when the welding quality monitoring method of the second invention shown in FIG. 8 is used, the resistance between the plate and the electrode due to the consumption of the electrode is calculated, and the numerical analysis is performed using the heat generation resistance of the welded portion. Therefore, the nugget diameter can be accurately estimated. As described above, according to the present embodiment, the voltage or resistance generated at the interface between the material to be welded and the electrode is calculated, and the heat resistance of the weld is calculated from the voltage or resistance measured between the electrodes by using this. Since the numerical analysis is performed using the conduction model, highly accurate welding quality monitoring can be realized regardless of the contact state between the electrode and the workpiece. Needless to say, each invention is not limited to this embodiment, and various modifications can be made without departing from the spirit of each invention.

[0017]

EFFECT OF THE INVENTION The method of calculating the interface voltage or interface resistance of resistance welding of the first aspect of the invention is that the voltage or resistance between the measured electrodes is generated at the interface between the material to be welded and the electrode by numerical analysis using a heat conduction model. Since the voltage or resistance to be measured can be calculated, the voltage or resistance at the interface, which has a great influence on the monitoring of the welding quality, can be incorporated into the method for monitoring the welding quality, which enables more accurate monitoring of the welding quality. It is effective.

The welding quality monitoring method according to the second aspect of the present invention is more effective by incorporating the method for calculating the interfacial voltage or the interfacial resistance of the resistance welding according to the first aspect of the invention into the estimation calculation method for the fusion zone by the numerical analysis using the heat conduction model. This has the effect of realizing accurate welding quality monitoring.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of resistance or voltage generated at an electrode and a material to be welded and its interface.

FIG. 2 is a schematic diagram of the surface condition of the electrode or the material to be welded.

FIG. 3 is a graph showing the results of an experiment in which the interelectrode resistance and the plate resistance are actually measured.

FIG. 4 is a flowchart for performing calculation of interface voltage or interface resistance.

FIG. 5 is a conceptual diagram showing a method of calculating a calculated resistance from a measured resistance and an estimated plate resistance.

FIG. 6 is a flowchart for implementing a welding quality monitoring method using the interface voltage or interface resistance calculation method.

FIG. 7 is a graph showing the results of a nugget diameter estimation experiment performed without using the method of calculating the interface voltage or interface resistance.

FIG. 8 is a graph showing an experimental result of nugget diameter estimation when a method of calculating interface voltage or interface resistance is used.

[Explanation of symbols]

 Rmeas Resistance between electrodes Rel Resistance of electrode part Rde Resistance generated at interface between welding target and electrode Rdc Resistance generated at interface between welding target Rreal Real resistance of welding target Rr Resistance between welding target and electrode Resistance measured from the interface Vmeas Voltage between electrodes Vel Voltage of electrode part Vde Voltage generated at interface between welded material and electrode Vdc Voltage generated at interface between welded materials Vreal Voltage due to heat generation of resistance of welded material

Claims (2)

[Claims] 1. A voltage between electrodes sandwiching a material to be welded, or a resistance between electrodes calculated from a voltage between electrodes and a welding current, and at least a voltage or resistance determined by heat generation of the resistance of the material to be welded, A method for calculating the interface voltage or resistance of resistance welding, which comprises calculating the voltage or resistance generated at the interface between the electrode and the material to be welded. 2. The voltage between electrodes sandwiching the material to be welded, or the resistance between the electrodes calculated from the voltage between the electrodes and the welding current, and at least the voltage or resistance determined by the specific resistance heat generation of the material to be welded, A method for monitoring welding quality, characterized in that a voltage or resistance generated at an interface between an electrode and a material to be welded is calculated, and welding quality is judged by using the calculated voltage or resistance.