JP7662005B2 - Resistance Spot Welding Method - Google Patents

Description

This disclosure relates to a resistance spot welding method.

Generally, resistance spot welding, a type of lap resistance welding, is used to join overlapping steel sheets. In order to obtain good weld quality in resistance spot welding, it is necessary to suppress the scattering of molten metal, or so-called splashing.

As a measure against the occurrence of expulsion, Patent Document 1 proposes a method in which the current pattern is a multi-stage current pattern of three or more stages, the welding conditions such as current flow time and welding current are adjusted so that the appropriate current range (ΔI: the current range in which a nugget with a desired nugget diameter or more and a molten residual thickness of 0.05 mm or more can be stably formed) is 1.0 kA or more, preferably 2.0 kA or more, and a cooling time is provided between each stage. Patent Document 2 proposes a method in which the current pattern is a two-stage current pattern of initial current flow and main current flow, the welding conditions such as current flow time and welding current are adjusted to increase the welding current value at the limit of expulsion occurrence, and a cooling time is provided between each stage. Patent Document 3 proposes a method in which the current pattern is a three-stage current pattern of initial current flow, constant current flow, and main current flow, and the welding conditions such as current flow time and welding current are adjusted to suppress heat generation during the constant current period and to make the temperature distribution of the plate assembly uniform by heat conduction in order to increase the welding current value at the limit of expulsion occurrence.

Patent No. 3849539 JP 2021-79416 A JP 2021-79410 A

Ji et al., Journal of Mechanical Science and Technology 28 (11) (2014) 4761-4769

When welding a plating layer that contains a large amount of elements other than Al and Zn, expulsion may not be prevented even at a constant current. However, Patent Documents 1 to 3 do not include any idea of adjusting the welding conditions according to the component composition of the plating layer.

This disclosure has been made in consideration of the above circumstances, and aims to provide a resistance spot welding method that can prevent expulsion even when the plating layer contains large amounts of elements other than Al and Zn.

The inventors conducted extensive research to achieve the above objective. As a result, they discovered that expulsion can be suppressed and nugget formation can be stabilized by appropriately repeating cooling and heating depending on the component composition and plating amount of the plating layer.

Although there are some unclear points regarding the stabilization mechanism of nugget formation as described above, the inventors currently believe as follows. It is believed that expulsion that occurs during resistance spot welding is significantly affected by the heat generation pattern at the beginning of current flow. This heat generation pattern correlates with the wet spread of the plating layer between the steel sheets. Furthermore, the wet spread of the plating layer correlates with the electrical path (current flow diameter or resistance). Since the compositional components and plating amount of the plating layer affect the wet spread of the plating layer, it is believed that by appropriately repeating heating and cooling according to the compositional components and plating amount of the plating layer, the wet spread of the plating layer is stabilized, resulting in stable welding and suppression of expulsion.

The present invention was made based on the above findings. That is, the gist of the present invention is as follows.

[1] A resistance spot welding method in which a sheet set of multiple overlapping steel sheets is sandwiched between a pair of electrodes and joined by passing current through the electrodes while applying pressure,
At least one of the plurality of steel sheets is a surface-treated steel sheet having a plating layer mainly composed of Al or Zn,
The energization is divided into at least two stages, an initial energization and a main energization, and a current stop time is provided between the initial energization and the main energization,
a resistance spot welding method in which, when an initial current flow time is t1 [ms] and the current flow pause time is tc [ms], a relationship between t1 and tc among a total sheet thickness T [mm], a pressurizing force P [kN], a plating thickness D [μm], and a content (mass %) of an element other than Al and Zn contained in the plating layer, satisfies any one of the following formulas (1) to (3) depending on the content X:
Notes
When 0.01≦X＜0.20,
(0.05+0.1×X)×(1+(D×T)/P)/50 ≦ tc/(t1+tc) ≦ (1.2+0.5×X)×(1+(D×T)/P)/50 …(1)
When 0.20≦X＜0.40,
(0.03+0.2×X)×(1+(D×T)/P)/50 ≦ tc/(t1+tc) ≦(1.1+X)×(1+(D×T)/P)/50 …(2)
When 0.40≦X≦0.50,
(-0.09+0.5×X)×(1+(D×T)/P)/50 ≦ tc/(t1+tc) ≦ (0.9+1.5×X)×(1+(D×T)/P)/50 …(3)

[2] The resistance spot welding method described in [1] above, in which the surface-treated steel sheet is subjected to a hot stamping process in which the steel sheet is heated to 850 degrees or higher, and then spot-welded.

[3] The resistance spot welding method described in [1] or [2] above, in which t1 is 5 ms or more and 100 ms or less.

[4] The resistance spot welding method described in any one of [1] to [3] above, in which tc is 20 ms or less.

[5] A resistance spot welding method according to any one of [1] to [4] above, in which t1 [ms] and the main current flow time tm [ms] satisfy t1 < tm.

According to the present disclosure, it is possible to provide a resistance spot welding method that can prevent expulsion even when the plating layer contains a large amount of elements other than Al and Zn.

FIG. 1 is a diagram for explaining an overview of a resistance spot welding method. FIG. 2 is a schematic diagram of a current pattern in a resistance spot welding method using an inverter DC resistance spot welder. 1 is a conceptual diagram of a current pattern in a resistance spot welding method using an AC resistance spot welder. FIG. FIG. 1 is a diagram showing the relationship between the upper limit and lower limit values of tc/(t1+tc) and the ratio X in the case where the total sheet thickness T is 3.0 mm, the plating thickness is 20 μm, and the pressure P is 3.5 kN.

The following describes an embodiment of the present invention. Note that the present invention is not limited to the following embodiment. In addition, in this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower and upper limits.

As shown in FIG. 1, the resistance spot welding method according to the present disclosure is a method in which a plate set 3 consisting of multiple overlapping steel plates 1, 2 is sandwiched between a pair of upper and lower electrodes 4, 5, and while applying pressure, a high welding current is passed between the upper and lower electrodes for a short period of time to form and join a joint 6. A spot-like weld 6 is obtained by utilizing the resistance heat generated by passing a high-current welding current. This spot-like weld 6 is called a nugget, and is the part where the steel plates 1, 2 melt and solidify at the contact point when a current is passed through the overlapping steel plates, thereby joining the steel plates together in a spot-like manner.

At least one of the multiple steel sheets is a surface-treated steel sheet having a plating layer mainly composed of Al or Zn. Two or more or three or more of the multiple steel sheets may be surface-treated steel sheets having a plating layer mainly composed of Al or Zn. All of the multiple steel sheets may be surface-treated steel sheets having a plating layer mainly composed of Al or Zn.

The plating layer being mainly composed of Al or Zn means that the total content (mass%) of Al and Zn contained in the plating layer is 0.50 or more.

The plating layer further contains elements other than Al and Zn. The ratio X of the content (mass%) of the elements other than Al and Zn contained in the plating layer is 0.01 to 0.50. Here, the ratio X of the content (mass%) of the elements other than Al and Zn contained in the plating layer is defined by the following formula:
X=(100-[Al]-[Zn])/100
[Al]: Content (mass%) of Al contained in the plating layer
[Zn]: Zn content (mass%) in the plating layer
The element contained in the plating layer other than Al and Zn may be, for example, at least one selected from the group consisting of Fe, Si, Mg, Sn, and Ni.

In particular, the plating layer of hot stamping steel sheet contains elements with low vapor pressure such as Zn, Mg, and Sn. Therefore, in resistance spot welding in which hot stamping steel sheet is used as the joined material, there is a problem that expulsion is likely to occur even at low currents. However, according to the present resistance spot welding method, expulsion can be suppressed even in resistance spot welding in which hot stamping steel sheet is used as the joined material.

In this resistance spot welding method, the current is divided into at least two stages, an initial current and a main current, and a current rest period is provided between the initial current and the main current. As described above, it is believed that the expulsion that occurs during resistance spot welding is greatly affected by the heat generation pattern at the beginning of the current. This heat generation pattern correlates with the wet spreading of the plating layer between the steel sheets. It is believed that the occurrence of expulsion is suppressed when the wet spreading of the plating layer extends over a wide area. Therefore, the inventors have discovered that the wet spreading of the plating layer can be stabilized by adjusting the initial current time t1 [ms] and the subsequent current rest time tc [ms] according to the component composition and plating amount of the plating layer, and have arrived at this resistance spot welding method.

In the present resistance spot welding method, when an initial current flow time is t1 [ms] and the current supply pause time is tc [ms], the relationship between t1 and tc in relation to a total sheet thickness T [mm], a pressure P [kN], a plating thickness D [μm], and a proportion X of a content (mass %) of elements other than Al and Zn contained in the plating layer satisfies any one of the following formulas (1) to (3) depending on the proportion X:
When 0.01≦X＜0.20,
(0.05+0.1×X)×(1+(D×T)/P)/50 ≦ tc/(t1+tc) ≦ (1.2+0.5×X)×(1+(D×T)/P)/50 …(1)
When 0.20≦X＜0.40,
(0.03+0.2×X)×(1+(D×T)/P)/50 ≦ tc/(t1+tc) ≦(1.1+X)×(1+(D×T)/P)/50 …(2)
When 0.40≦X≦0.50,
(-0.09+0.5×X)×(1+(D×T)/P)/50 ≦ tc/(t1+tc) ≦ (0.9+1.5×X)×(1+(D×T)/P)/50 …(3)

The current rest time tc is the time without current flow after the end of the initial current flow time t1, and is the time from the end of the initial current flow to the start of main current flow. By setting the current rest time tc so as to satisfy the above formulas (1) to (3), the temperature of the plating layer can be lowered before the plating layer that has spread between the steel sheets by wetting is discharged outside the sheet assembly, even if the plating layer has a low melting point or boiling point. This ensures a sufficient current flow diameter between the steel sheets and prevents the occurrence of splashing. The welding current value may or may not be constant during the initial current flow and main current flow.

As the proportion X of elements other than Al and Zn contained in the plating layer increases, a dwell time is required to sufficiently wet and spread the plating layer between the steel sheets. Therefore, the proportion of tc in t1 and tc must be increased as the proportion X increases. For example, Figure 4 shows the relationship between the upper and lower limits of tc/(t1+tc) and the proportion X when the total plate thickness T is 3.0 mm, the plating thickness is 20 μm, and the pressure P is 3.5 kN. The upper and lower limits of tc/(t1+tc) were calculated based on the above formulas (1) to (3). As shown in Figure 4, as the proportion X of elements other than Al and Zn increases, both the upper and lower limits of tc/(t1+tc) increase.

Schematic diagrams of the current pattern in the above-mentioned resistance spot welding method are shown in Figures 2 and 3. Figure 2 shows the current pattern in an inverter DC resistance spot welding machine, and Figure 3 shows the current pattern in an AC resistance spot welding machine. The dotted line in Figure 3 indicates, for example, the root mean square value (hereinafter referred to as "RMS"), and the period when the current value is equal to or greater than the RMS is the current flow time, and the period when the current value is less than the RMS is the rest time. Figures 2(a) and 3(a) show an overview of the current flow pattern when the current flow consists of two stages, an initial current flow and a main current flow, but the current flow pattern is not limited to two stages. In one example, as shown in Figures 2(b) and 3(b), three or more stages of multi-stage current flow including two or more stages of initial current flow may be performed before the main current flow. When performing multi-stage current flow, it is sufficient that the initial current flow time t1 of the first stage initial current flow and the current flow rest time tc immediately thereafter satisfy the above formulas (1) to (3). However, as shown in Figures 2(b) and 3(b), current flow with the same initial current flow time t1 and current flow pause time tc as the first stage may be repeated in multiple stages.

t1 [ms] may be controlled to satisfy formulas (1) to (3), but is preferably 5 ms or more and 100 ms or less. If t1 is 5 ms or more, the coating layer melts and the molten coating can wet and spread between the steel sheets. If t1 is 100 ms or less, the molten coating can be prevented from being swept out of the system.

tc [ms] may be controlled to satisfy formulas (1) to (3), but is preferably 20 ms or less. If tc is 20 ms or less, the coating that has spread between the steel sheets is stable, and the molten state of the nugget can be maintained. There is no particular lower limit for tc, but in order to stabilize the coating that has spread between the steel sheets, it is preferably 5 ms or more.

tm [ms] is not particularly limited, but in order to obtain an appropriate nugget diameter, t1 < tm is preferable, and more preferably 100 ms or more. In addition, from the viewpoint of construction time constraints, tm is preferably 1000 ms or less.

Above, we have explained the relationship between the initial current flow time t1 and the rest time tc with respect to the occurrence of expulsion during welding, but below we will also discuss other factors that affect the occurrence of expulsion during resistance spot welding.

The total plate thickness T [mm] is the total plate thickness of the multiple steel plates that are stacked together to form a plate assembly. If the total plate thickness T is small, the nugget is more likely to reach the steel plate surface in the event of excessive heat generation. This is thought to make it easier for splashing to occur. The total plate thickness T [mm] can be controlled to satisfy formulas (1) to (3), but it is preferable to set it to 0.8 mm or more, for example, and 10 mm or less.

The pressure P [kN] is the pressure applied to the plate assembly by the pair of electrodes from the start of the initial current flow to the end of the main current flow. The pressure P may be constant throughout the welding, but it does not have to be constant. It is believed that if the pressure P increases, the discharge of the molten metal plating around the weld is promoted, the current flow diameter becomes insufficient, and splashing is more likely to occur. Therefore, the pressure P [kN] is controlled to satisfy formulas (1) to (3). The pressure P [kN] may be controlled to satisfy formulas (1) to (3), but it is preferable to set it to 2 kN or more, and 10 kN or less, for example. The pressure P is measured as follows. The pressure P can be measured using a pressure gauge such as a load cell.

The plating thickness D [μm] is the thickness of the plating layer per side of the surface-treated steel sheet. When multiple steel sheets each have a plating layer, the plating thickness D is the average value of the plating layer thicknesses of the multiple sheets. If the plating thickness D of the surface-treated steel sheet to be welded becomes small, the current diameter becomes insufficient. For this reason, it is considered that the heat generated at the beginning of the current flow becomes excessive, and splashing is likely to occur. As a result, the amount of the plating layer in contact with the base material in the vicinity of the weld is reduced, and the current diameter is reduced. For this reason, it is considered that splashing is likely to occur. For this reason, the plating thickness D is adjusted to satisfy formulas (1) to (3). The plating thickness D may be adjusted to satisfy formulas (1) to (3), but for example, it is preferable to set it to 10 μm or more from the viewpoint of corrosion resistance, and it is preferable to set it to 40 μm or less from the viewpoint of manufacturing costs. The plating thickness is measured as follows. That is, the plating thickness is determined by randomly selecting 10 locations from the steel plate, cutting the steel plate in the thickness direction at those locations, mirror-polishing the cross section, and measuring the plating thickness at those 10 locations using an optical microscope or a secondary electron image or a backscattered electron image (magnification 500x).

The proportion X of the content (mass%) of elements other than Al and Zn is a value showing the total content (mass%) of elements other than Al and Zn contained in the plating layer as a percentage. When multiple steel sheets each have a plating layer, the proportion Xc of the content (mass%) of elements other than Al and Zn is calculated by formula (4). In formula (4), steel sheets 1 and 2 are one and the other of two opposing steel sheets.
Xc=(T1×X1+T2×X2)/(T1+T2)…(4)
however,
T1: plating thickness of steel sheet 1, T2: plating thickness of steel sheet 2
X1: Content (mass%) of elements other than Al and Zn in steel plate 1
X2: Content (mass%) of elements other than Al and Zn in steel plate 2

For the reasons mentioned above, we came up with the above formulas (1) to (3). The coefficients in the formulas were determined experimentally to find the optimal coefficients depending on X.

The type of steel used to form the plate assembly is not particularly limited. The manufacturing method for the steel plate may be any method, such as cold rolling or hot rolling, and the structure of the steel plate may be any method. There is no problem in using hot-pressed steel plates as the steel plates of the plate assembly. There is also no particular limitation on the thickness of the steel plate, and in one example, it may be a thickness that can be used for a general automobile body. The thickness of the steel plate may be, for example, 0.5 mm or more and 4.0 mm or less.

The plate combination may consist of multiple sheets of the same type of steel, or multiple sheets of different types of steel. There is no problem if the thicknesses of the steel sheets are different, and a combination of a steel sheet with a plating layer that has a lower melting point than the steel sheet and a steel sheet without a plating layer may be used.

The plate assembly may be a plate assembly of three or more overlapping steel plates. When a plate assembly of three or more overlapping steel plates is used as the material to be joined, the welding conditions are adjusted so that the above formulas (1) to (3) are satisfied for the plate assembly between two opposing steel plates with the smallest product of the plating thickness D and the plate thickness T.

Furthermore, the current value and time during which electricity is applied may be constant throughout the welding, but they do not have to be constant. Also, there is no problem in using a control method that monitors parameters such as resistance and voltage during welding and changes the current value and time of application in response to fluctuations in these parameters.

In addition, resistance spot welding can be performed with a gap between the steel sheets or with the electrode tilted relative to the steel sheets.

The above-mentioned resistance spot welding method can be used to obtain a welded component in which multiple steel plates, including at least one surface-treated steel plate having a metal plating layer, are welded together. When welding is performed using the above-mentioned resistance spot welding method, the occurrence of expulsion at the welded portion can be suppressed, making it possible to manufacture a welded component made of surface-treated steel plates having a low-melting point plating layer.

As a welding device that can be used with this resistance spot welding method, a welding device equipped with a pair of upper and lower electrodes and capable of freely controlling the pressure and welding current during welding can be used. There are no particular limitations on the pressure mechanism (air cylinder, servo motor, etc.), type (fixed type, robot gun, etc.), and electrode shape of the welding device. Examples of the shape of the electrode tip include DR type (dome radius type), R diameter (radius type), and D type (dome type), etc., as described in JIS C 9304:1999.

Conditions other than those mentioned above can be determined by standard methods.

Examples of the present disclosure are shown below. Note that the present disclosure is not limited to the following examples. For each of the two-ply and three-ply plate combinations in which steel plate 1 and steel plate 2 shown in Table 1 were overlapped, resistance spot welding was performed under the conditions shown in Table 2 to produce joints (welded members). Note that the melting points of the base metals of the test materials in this example are in the range of 1400 to 1570°C, and the melting points of the plating are in the range of 400 to 700°C. The tensile strengths shown in Table 1 are the tensile strengths obtained by preparing JIS No. 5 tensile test pieces from each steel plate in a direction parallel to the rolling direction and conducting tensile tests in accordance with the provisions of JIS Z 2241:2011.

The welding equipment used was an inverter DC resistance spot welding machine or an AC resistance spot welding machine, and the electrodes were DR-type chromium copper electrodes. The two electrodes were the same. The current was passed in two stages, an initial current and a main current, and when an inverter DC resistance spot welding machine was used, the current value (welding current) during current was kept constant. The main current time tm was 420 ms. The pressure was the pressure at the end of current, and was kept constant throughout current and until the end of current. Resistance spot welding was performed at room temperature, with the electrodes always water-cooled.

For each joint obtained, the welded portion was cut in the thickness direction of the steel plate, the cross section was mirror-polished, and the nugget diameter was measured using an optical microscope (magnification 10x). In the case of three-ply plates, the same procedure was carried out for the plate assembly with the smallest product of plating thickness D and plate thickness T.

In addition, the occurrence of expulsion during welding was observed. Ten joints were fabricated and observed under the same conditions, and the results were judged according to the following criteria. (A) to (C) were deemed acceptable, and (D) was deemed unacceptable. Here, ΔW is the range between the welding current value at which expulsion occurs and the welding current value at which 4√t occurs, which is determined by experimentally determining the weld lobe (relationship between current value and nugget diameter) at 0.5 kA intervals for the welding current, and determining the welding current value at which 4√t occurs from the approximation curve of the data thus obtained. Note that t is the thickness of the thinnest steel plate used. Incidentally, in spot welding, the required nugget diameter is determined for each steel plate thickness t, and the calculation formula is 4√t.
(A): ΔW is 1.0 kA or more for all 10 joints. (B): 1 out of 10 joints has ΔW less than 1.0 kA. (C): 2 out of 10 joints have ΔW less than 1.0 kA. (D): 3 or more out of 10 joints have ΔW less than 1.0 kA.

As shown in Table 2, the evaluation results for ΔW above show that all of the fittings (examples of the present invention) in which the pause time tc was set to satisfy the requirements of this disclosure were rated (passed) as either (A) or (C).

The occurrence of expulsion during welding was also observed. Ten joints were fabricated and observed under the same conditions, and the results were judged according to the following criteria. The judgment results are shown in Table 2. (E) to (G) were judged as pass, and (H) was judged as fail.
(E): No shattering occurred in any of the 10 joints. (F): One of the 10 joints shattered. (G): Two of the 10 joints shattered. (H): Three or more of the 10 joints shattered.

As shown in Table 2, all of the fittings (examples of the present invention) in which the pause time tc was set to meet the requirements of this disclosure were rated as either E or G (passed).

1, 2 Steel plate 3 Plate assembly 4, 5 Electrode 6 Welded part (nugget)

Claims (4)

In a resistance spot welding method in which a sheet set of multiple overlapping steel sheets is sandwiched between a pair of electrodes and joined by passing current through the electrodes while applying pressure,
At least one of the plurality of steel sheets is a surface-treated steel sheet having a plating layer mainly composed of Al or Zn,
The energization is divided into at least two stages, an initial energization and a main energization, and a current stop time is provided between the initial energization and the main energization,
When an initial current application time is t1 [ms] and the current application rest time is tc [ms], the relationship between t1 and tc satisfies any one of the following formulas (1) to (3) in relation to a total sheet thickness T [mm], a pressure P [kN], a plating thickness D [μm], and a ratio X of a content (mass%) of elements other than Al and Zn contained in the plating layer, according to the ratio X:
a resistance spot welding method , wherein the t1 [ms] and the main current flow time tm [ms] satisfy t1 < tm .
Notes
When 0.01≦X＜0.20,
(0.05+0.1×X)×(1+(D×T)/P)/50≦tc/(t1+tc)≦(1.2+0.5×X)×(1+(D×T)/P)/50…(1)
When 0.20≦X＜0.40,
(0.03+0.2×X)×(1+(D×T)/P)/50≦tc/(t1+tc)≦(1.1+X)×(1+(D×T)/P)/50…(2)
When 0.40≦X≦0.50,
(-0.09+0.5×X)×(1+(D×T)/P)/50≦tc/(t1+tc)≦(0.9+1.5×X)×(1+(D×T)/P)/50…(3) The resistance spot welding method according to claim 1, in which the surface-treated steel sheet is subjected to hot stamping in which the sheet is heated to 850 degrees or higher, and then spot-welded. The resistance spot welding method according to claim 1 or 2, wherein t1 is 5 ms or more and 100 ms or less. The resistance spot welding method according to claim 1 or 2, in which tc is 20 ms or less.

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