JP2021028085A - Resistance spot weld joints for aluminum material - Google Patents

Abstract

To provide resistance spot weld joints for an aluminum material that is provided with a nugget form capable of reducing variations in CTS strength.SOLUTION: In resistance spot weld joints for aluminum materials where a plurality of aluminum materials are overlapped to be welded by resistance spot welding, a weld nugget 25 formed by resistance spot welding is configured such that in a cross-section of the overlapped surface of the aluminum materials, a plurality of blow holes BH are present in an aggregated state in the center in the overlapped surface of the weld nugget 25. Also, in a cross-section in the overlapping direction of the aluminum materials passing through the nugget center On, the largest blow hole in size of the plurality of blow holes BH is formed at a position passing through the nugget center line extending in the overlapping direction of the weld nugget 25.SELECTED DRAWING: Figure 4

Description

The present invention relates to a resistance spot welded joint of an aluminum material.

Aluminum material has low electrical resistance and high thermal conductivity. Therefore, in order to perform resistance spot welding, it is necessary to energize about 3 times as much current as in the case of steel material and to increase the pressing force of the electrode for spot welding by about 1.5 times. Further, the aluminum material is more likely to generate blow holes than the steel material, and the blow holes are particularly remarkable in the 6000 series and 5000 series aluminum alloys.

Generally, the joint strength of a spot welded joint is evaluated by tensile shear strength (TSS: Tensile Shear Strength), cross tension strength (CTS: Cross Tension Strength), etc., and in the design of a structure to which resistance spot welding is applied, It is required that the values of TSS and CTS are stable within a certain range without any variation.
However, when the material to be welded is an aluminum material, blow holes are likely to occur as described above. Therefore, as the number of spots in resistance spot welding increases, the energized state changes due to a change in the shape of the electrode tip (wear of the electrode). Then, the variation of CTS becomes remarkable.
As an evaluation method of such a spot welded joint, there is a method of forming a drill hole penetrating the nugget in the thickness direction of the joint, evaluating TSS and CTS, and setting welding conditions according to the evaluation result. (For example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2018-161659

The method for evaluating a spot welded joint in Patent Document 1 is to estimate the strength of a spot welded joint by forming a through hole having a certain area in the nugget. However, in the aluminum spot welded joint, there is a problem that even if a through hole having substantially the same area simulating a blow hole is formed, the strength of the CTS varies widely and is not stable.
Therefore, an object of the present invention is to provide a resistance spot welded joint made of an aluminum material having a nugget form capable of reducing the strength variation of CTS.

The present invention has the following configuration.
A resistance spot welded joint made of aluminum by superimposing multiple aluminum materials and performing resistance spot welding.
The molten nugget formed by the resistance spot welding
In the cross section of the overlapping surface of the aluminum materials, a plurality of blow holes are aggregated and existed in the central portion of the molten nugget in the overlapping surface.
In the cross section of the aluminum material passing through the center of the nugget in the stacking direction, the blow hole having the largest size among the plurality of blow holes is formed at a position passing through the center line of the nugget extending in the stacking direction of the molten nugget. Aluminum resistance spot welded joint.

According to the present invention, the strength variation of CTS with respect to the molten nugget formed by resistance spot welding of an aluminum material can be reduced, and a high-strength spot welded joint of an aluminum material can be obtained.

It is a schematic block diagram of the spot welder which performs resistance spot welding of an aluminum material. It is a schematic cross-sectional view which shows the state that the 1st aluminum plate and the 2nd aluminum plate were superposed and spot welded, and the molten nugget was formed. It is schematic cross-sectional view of the molten nugget when cut at the superposition surface of aluminum materials shown by the PP line of FIG. It is a schematic cross-sectional plan view of the molten nugget when cut at the superposition surface of aluminum materials shown by the PP line of FIG. It is a timing chart which shows an example of the waveform of a welding current. (A) and (B) are process explanatory views schematically showing the state of the nugget from the first stage main energization to the second stage pulsation energization. It is a graph which shows the peeling strength by the CTS test at each sampling dot which sampled every predetermined dot by repeating spot welding. It is a graph which shows the test result shown in FIG. 7 as the distribution of the peel strength with respect to the nugget diameter. It is a graph which shows the peeling strength by the CTS test at each sampling hitting point which performed spot welding repeatedly and sampled every predetermined number of times. It is a graph which shows the test result shown in FIG. 9 as the distribution of the peel strength with respect to the nugget diameter. It is a CT image which cut the molten nugget on the superposition surface of the test piece of Example 1. It is a CT image which cut the molten nugget on the superposition surface of the test piece of Comparative Example 1. It is a CT image which cut the molten nugget on the superposition surface of the test piece of the comparative example 2. It is a CT image which cut the molten nugget on the superposition surface of the test piece of Comparative Example 3. It is a CT image which cut the molten nugget on the superposition surface of the test piece of Example 2. It is sectional drawing in the plate thickness direction of the test piece of Example 2 shown in FIG. It is a CT image on the cut surface of the plate surface of the test piece of Comparative Example 4. It is sectional drawing in the plate thickness direction of the test piece of Comparative Example 4 shown in FIG. It is a CT image on the cut surface of the plate surface of the test piece of Example 3. It is sectional drawing in the plate thickness direction of the test piece of Example 3 shown in FIG. It is a CT image which cut the molten nugget on the superposition surface of the test piece of the comparative example 5. FIG. 2 is a cross-sectional view of the test piece of Comparative Example 5 shown in FIG. 21 in the plate thickness direction.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
<Spot welder>
FIG. 1 is a schematic configuration diagram of a spot welder for resistance spot welding of an aluminum material.
The spot welder 11 supplies welding power from the power supply unit 18 to the pair of electrodes 13 and 15, the welding transformer unit 17 connected to the pair of electrodes 13 and 15, the power supply unit 18, and the welding transformer unit 17. A control unit 19 and an electrode driving unit 20 for relatively moving a pair of electrodes 13 and 15 in the axial direction are provided. The control unit 19 integrally controls the current value, energization time, electrode pressing, energization timing, pressurization timing, and the like.

The spot welder 11 superimposes and sandwiches at least two plate materials of the first aluminum plate 21 and the second aluminum plate 23 between the pair of electrodes 13 and 15. Then, the first aluminum plate 21 and the second aluminum plate 23 are pressurized in the plate thickness direction by driving the electrodes 13 and 15 by the electrode driving unit 20. In this pressurized state, the welding transformer unit 17 energizes between the electrodes 13 and 15 based on a command from the control unit 19. As a result, the molten nugget 25 is formed on the overlapping surface of the first aluminum plate 21 and the second aluminum plate 23 sandwiched between the electrodes 13 and 15, and the first aluminum plate 21 and the second aluminum plate 23 are integrated. A resistance spot welded joint 27 made of aluminum is obtained.

In the above example, two aluminum plates are joined to obtain a resistance spot welded joint 27 made of an aluminum material, but the present invention is not limited to joining two aluminum plates, but three or more aluminum plates are joined. It is also preferably used when

In the following description, the stacking direction of the first aluminum plate 21 and the second aluminum plate 23 is also referred to as a plate thickness direction and a thickness direction of the molten nugget 25 (depth direction of the penetration depth). Regarding the molten nugget 25, the direction orthogonal to the stacking direction and extending radially from the center of the nugget is defined as the nugget radial direction, and the maximum diameter in the direction orthogonal to the thickness direction of the molten nugget 25 is defined as the nugget diameter. The thickness direction of the molten nugget 25 is the same as that of the first aluminum plate 21 and the second aluminum plate 23.

<1st aluminum material and 2nd aluminum material>
The first aluminum material and the second aluminum material can be aluminum or an aluminum alloy of any grade, but in particular, 5000 series, 6000 series, and 7000 series aluminum alloys in which blow holes are likely to be formed are preferably used. Be done. In addition, 2000 series, 3000 series, 4000 series, 8000 series aluminum alloys, 1000 series pure aluminum and the like can also be adopted.

As the form of the aluminum material, a cast material can be applied in addition to the aluminum plate, the extruded material, and the forged material. As the casting material, in order to reduce the blow holes existing in the base metal as much as possible, a casting material whose casting quality is improved by precision casting or overflow is preferably used.

In general, it is difficult to completely eliminate blow holes due to spot welding in spot welded joints made of aluminum. In particular, in alloy materials containing a large amount of Mg and Zn, which are elements with low vapor pressure, such as 5000 series, 6000 series, and 7000 series aluminum alloys, the occurrence of blow holes becomes remarkable, and it becomes difficult to suppress blow holes. ..

Therefore, as a result of diligently examining the change in welding strength due to the blow hole, the inventor of the present application has found that the presence form of the blow hole, not the area of the blow hole, exerts a great effect in suppressing the variation in CTS. I found.

<Agglutination of blow holes>
FIG. 2 is a schematic cross-sectional view showing a state in which the first aluminum plate 21 and the second aluminum plate 23 are overlapped and spot welded to form a molten nugget 25. Here, the overlapping direction of the first aluminum plate 21 and the second aluminum plate 23 is the Z direction, and the plate surface directions orthogonal to the Z direction are the X direction and the Y direction. The X direction and the Y direction are orthogonal to each other.

The molten nugget 25 is formed in a region including the overlapping surface of the first aluminum plate 21 and the second aluminum plate 23 having a thickness t and having a nugget width W in the direction along the plate surface. Further, indentations 29 due to pressurization of the electrodes 13 and 15 shown in FIG. 1 are formed on the outer plate surfaces of the first aluminum plate 21 and the second aluminum plate 23.

3 and 4 are schematic cross-sectional views of the molten nugget when cut at the superposition surface of the aluminum materials shown by the PP line of FIG.
The molten nugget 25 has a large number of blowholes BH of various sizes. Further, the aluminum material has high thermal conductivity, and the molten nugget 25 is formed in a short time after energization. Therefore, as shown in FIG. 3, the blow holes BH tend to be dispersedly arranged in the molten nugget 25.

In the CTS test, in order to pull the aluminum materials that have been spot-welded with resistance in the direction of peeling each other, if the blow holes BH are dispersedly arranged in the molten nugget 25 as shown in FIG. Even if the area (volume) of the blow hole BH is small, the blow hole BH near the outer peripheral edge of the molten nugget 25 becomes the starting point, and peeling can easily occur even with a low load.

Therefore, as shown in FIG. 4, the blow hole BH is aggregated in the vicinity of the nugget center On in a plan view of the molten nugget 25 on the superposition surface of the aluminum material. According to this form, even if the area (volume) of one blow hole BH is large, there is almost no blow hole BH near the outer peripheral edge of the molten nugget 25. Therefore, the presence of the blow hole BH, which can be the starting point of peeling, can be reduced, and a stable CTS can be obtained.

As for the aggregated state of the blow hole BH, in the cross section of the molten nugget 25 on the above-mentioned overlapping surface, 70% or less, preferably 75% or less, more preferably 80% or less of the nugget width W from the nugget center On of the molten nugget 25. It is preferable that all the blow holes BH are formed in the inner region M having a diameter Wc within the range. Further, the blow holes BH existing in the inner region M are preferably one, or at most three, blow holes larger than the other blow holes, rather than the state in which minute blow holes are dispersed. .. The blow hole BH is preferably formed at a position passing through the nugget center line extending in the stacking direction of the aluminum material of the molten nugget 25. That is, when there is another blow hole other than the maximum size blow hole, the other blow hole is 1/20 or less, preferably 1/30 or less, more than the maximum size blow hole. It is preferably a minute blow hole having a size of 1/50.

<Spot welding method>
The molten nugget in which the blow hole is formed as shown in FIG. 4 can be obtained, for example, by continuously energizing the welding current for a predetermined time and then energizing the pulse current a plurality of times. According to this energization method, the blow hole can be aggregated in the central portion of the molten nugget by alternately arranging the solidification portion and the shell, which will be described in detail later, in the molten nugget.

(Welding conditions)
Next, a method of forming a molten nugget by the above-mentioned energization method will be described.
The control unit 19 shown in FIG. 1 energizes between the pair of electrodes 13 and 15 from the welding transformer unit 17 at a predetermined timing. FIG. 5 is a timing chart showing an example of the waveform of the welding current.
The waveform of the welding current shown in FIG. 5 can be used as _{a main energization step of energization time T m} by the first stage continuous energization 31 and a pulsation energization step of repeatedly energizing a current of a pulse (short pulse) 32 having a short energization time. Good. The pulsation energization repeats energization suspension (pause time T _c ) and energization of the pulse 32 (energization time _Taps ) a plurality of times during the total energization time T _p. The energization waveforms of the first-stage continuous energization 31 and the second-stage pulse 32 may be rectangular, and may be other waveforms such as a triangular wave or a sine wave, or a waveform controlled by downslope or upslope. May be good. In the example shown in FIG. 5, the continuous energization 31 has a constant current, and the pulse 32 has a waveform in which a rectangular pulse is downslope-controlled. When the energization waveform of the pulsation energization is a waveform other than a rectangle such as a down slope or an up slope, the maximum current value in each pulse wave is taken as the current value of the pulsation energization. Pulsations current I _PS energization is not limited to equal to the current value I _m of the continuous current 31 may be larger than the current value I _m.

Current _{I ps} of the current value _{I m} and the second and subsequent stages of the pulse 32 of the continuous current 31 of the first stage are both set in the range of 15～60KA. Energization by current I _m of the continuous current 31, generally the final nugget size is determined. Therefore, it is sufficient to determine the optimum current value I _m in accordance with the welding purposes.

Current value _{I m} of the continuous energizing 31 is preferably 30～40KA, energization time Tm is 100 to 300 ms, preferably 150～250Ms, more preferably 180～220Ms.

In the example shown in FIG. 5, the current value of the pause time Tc of the power suspension is 0A at which the power supply between the electrodes 13 and 15 is stopped, but it does not necessarily have to be 0A, and the first aluminum plate 21 is higher than that at the time of power supply. As long as the amount of heat input to the second aluminum plate 23 can be reduced, the current value may be higher than 0A. The rest time Tc is 10 to 20 ms, preferably 10 to 15 ms, and more preferably 10 to 12 ms.

Current _{I ps} pulse 32 is preferably 30～40KA, energization time _{T ps} is 10～30Ms, preferably 15～25Ms, more preferably 18～22Ms. The number of times of repeated energization of the pulse 32 (number of pulses N) is 3 times or more, preferably 4 times or more, and more preferably 7 times or more.

<Procedure of resistance spot welding and its effect>
6 (A) and 6 (B) are process explanatory views schematically showing the state of the nugget from the main energization of the first stage to the pulsation energization of the second stage.
_{As shown in FIG. 6A, when the current value Im} is energized to the first aluminum plate 21 and the second aluminum plate 23 sandwiched between the pair of electrodes 13 and 15, the plate surfaces are brought together. The molten nugget 25 is formed around the mating surface of the above.

Next, as shown in FIG. 6B, when pulsation energization by a plurality of short pulses is performed, a plurality of shells 26 having an annular cross section (hereinafter referred to as shells) are formed inside the molten nugget 25. Will be done. When the molten nugget 25 is cut in the plate thickness direction and the cross section is observed, a striped pattern of the shell 26 can be observed concentrically from the center of the molten nugget 25 in the molten nugget 25.

By repeating the pulse energization (energization and cooling) after the main energization a plurality of times, the solidified portion, which is a columnar crystal structure, and the shell 26 are alternately formed toward the center of the nugget. When the molten nugget 25 after energizing the pulsation is observed in a cross section in the plate thickness direction, as schematically shown in FIG. 6B, a striped pattern in which the shell 26 is formed into multiple rings and is concentrically formed is observed. Observed.

By forming a plurality of shells 26 toward the center of the nugget in the molten nugget 25 by the above-mentioned procedure of resistance spot welding, the molten body surrounded by the shell 26 is gradually reduced toward the center. Therefore, even if blow holes are generated in the nugget by resistance spot welding, the generated blow holes are aggregated in the center of the nugget.

As described above, if the blow hole exists in the vicinity of the joint portion or the base material of the aluminum material (the outer peripheral portion of the nugget), it becomes a starting point of fracture or the like, so that the welding quality tends to deteriorate. However, even if the blow hole exists in the central part of the nugget where stress concentration is unlikely to occur, it does not have a great influence on the quality of the welded portion such as the joint strength.
According to this resistance spot welding method, the blow hole is aggregated in the central portion of the molten nugget by applying pulsation energization, and the quality of the welded portion can be prevented from deteriorating. Therefore, even if the aluminum material contains Mg and Zn, which are elements with low vapor pressure such as 5000 series, 6000 series, and 7000 series, and blow holes are easily formed, it is possible to suppress the variation in CTS strength due to the blow holes.

<Other spot welding methods>
In addition to the above methods, as a method of reducing the dispersion of blow holes and stabilizing the CTS, a method of replacing the electrode, dressing, and polishing while the number of hit points is small, and a method of applying pressure to the electrode according to the welding current. There is a way to control.
For example, in a 6000 series aluminum alloy, by exchanging the electrodes at 60 dots, preferably 50 dots in spot welding, it is possible to suppress the disorder of the profile of the electrode surface and always perform stable spot welding. As a result, the formation of the molten nugget is stable, and the strength variation of CTS can be suppressed.

<Experimental example 1>
A CTS test was performed on a test piece (resistive spot welded joint) in which a pair of aluminum plates were spot welded in a cross shape. Table 1 shows the material / treatment, single plate shape, and joint shape of the test piece used.

The welding conditions for spot welding are shown in Tables 2 and 3. During the preload period, a pair of aluminum plates are sandwiched by electrodes at a pressing force of 5 kN for 100 ms. As the energization conditions, in the case of a nugget diameter of 4.0√t (t is the thickness of the aluminum plate), 200ms is energized at 25kA, and in the case of a nugget diameter of 6.0√t, 200ms is energized at 38kA. Then, after energization (main energization), welding is completed with a holding period of 200 ms. From the preload period to the holding period, the pressing force by the electrodes is kept constant.

The CTS test was carried out using a tensile tester manufactured by Instron (model 5900A 5581) at a tensile speed of 10 mm / min. When the test piece was bolted to the jig for fixing the test piece, it was fastened with a torque wrench with a tightening torque of 80 Nm.

FIG. 7 is a graph showing the peel strength by the CTS test at each sampling dot sampled at predetermined times by repeating spot welding. This graph is the result when the nugget size is set to 4.0√t.

The average value of the peel strength was 2.6 kN. As for the fracture form of the joint surface up to 120 dots, both button fracture and shear fracture occurred, but when the shear fracture exceeded 120 dots, only shear fracture occurred. It is considered that this is because the tip of the electrode is worn or deformed as the number of spot welding spots increases, and the weldability deteriorates.

The maximum value of the peel strength was 3.4 kN and the minimum value was 1.8 kN, both of which were button breaks. That is, regardless of button breakage and shear breakage, the peel strength varied up to about 1.9 times.

FIG. 8 is a graph showing the test results shown in FIG. 7 as a distribution of peel strength with respect to the nugget diameter. From the results shown in FIGS. 7 and 8, it can be seen that the peel strength does not necessarily depend on the nugget diameter and the fractured form of the molten nugget.

FIG. 9 is a graph showing the peel strength by the CTS test at each sampling dot sampled at predetermined times by repeating spot welding. This graph is the result when the nugget size is set to 6.0√t. FIG. 10 is a graph showing the test results shown in FIG. 9 as a distribution of peel strength with respect to the nugget diameter.

In the cases of FIGS. 9 and 10, the average value of the peel strength was 3.9 kN, the maximum value of the peel strength was 4.66 kN, and the minimum value was 2.23 kN, and most of them were button breaks. In this case, the peel strength varied up to about 2.1 times.

<Blow hole form>
Next, the result of investigating the formation position of the blow hole in the molten nugget and its distribution form will be described from the cross-sectional image of the spot welded portion of the test piece described above. Table 4 shows the details of the X-ray CT inspection apparatus and the imaging conditions of CT images.

(Center position of nugget and blow hole on the cross section of the overlapping surface of aluminum materials)
Here, among the test pieces in which the nugget size shown in FIGS. 9 and 10 is set to 6.0√t, the test piece at the 24th RBI is Example 1, and the test piece at the 12th RBI is Comparative Example 1. The test piece with the first RBI is referred to as Comparative Example 2, and the test piece with the 20th RBI is referred to as Comparative Example 3.

FIG. 11 is a CT image obtained by cutting a molten nugget on the overlapping surface of the test piece of Example 1. FIG. 12 is a CT image in which the molten nugget is cut on the overlapping surface of the test pieces of Comparative Example 1, FIG. 13 is a CT image in which the molten nugget is cut on the overlapping surface of the test pieces of Comparative Example 2, and FIG. It is a CT image in which a molten nugget was cut on the overlapping surface of the test pieces of. The cutting positions of FIGS. 11 to 14 are the positions of the PP line shown in FIG. An indentation circle Ec, which is the outer edge of the indentation 29 shown in FIG. 2, is recognized around the blow hole BH of each CT image. The indentation circle Ec is formed concentrically with the molten nugget 25.

In Example 1 shown in FIG. 11, a single aggregated blow hole BH is formed at the center of the indentation circle Ec. That is, the blow hole BHa is formed through the center of the molten nugget.

On the other hand, in Comparative Example 1 shown in FIG. 12, a medium-sized blow hole BHb smaller than the blow hole BHa of FIG. 11 is formed at the center of the indentation circle Ec, and a small size smaller than the blow hole BHb is formed around the blow hole BHb. Blow holes BHc are formed in large numbers.

In Comparative Example 2 shown in FIG. 13, a plurality of medium-sized blow holes BHd are formed near the center of the indentation circle Ec, and a large number of small-sized blow holes BHe are further dispersed around the blow holes BHd. ing.

In Comparative Example 3 shown in FIG. 14, a large number of small-sized blow holes BHf are dispersed and formed near the center of the indentation circle Ec.

The evaluation results of Examples 1 and Comparative Examples 1 to 3 are summarized in Table 5. The peel strength of Example 1 was 4.05 kN, which was higher than the peel strengths of Comparative Examples 1, 2 and 3 of 3.49 kN, 3.50 kN and 3.36 kN. Moreover, in each case, the button was broken.

(Center position of nugget and blow hole in cross section of aluminum material in the stacking direction)
Of the test pieces in which the nugget size shown in FIGS. 7 and 8 is set to 4.0√t, the 84th test piece is referred to as Example 2, and the 81st test piece is referred to as Comparative Example 4.
FIG. 15 is a CT image obtained by cutting a molten nugget on the overlapping surface of the test piece of Example 2, and FIG. 16 is a cross-sectional view of the test piece of Example 2 shown in FIG. 15 in the plate thickness direction.

In FIGS. 15 and 16, the center of the molten nugget is shown by a dotted line, and the center of the blow hole is shown by a solid line. The blow hole of Example 2 has a deviation ΔL from the center of the nugget of 0.32 mm, and is formed at a position passing through the center line of the nugget of the molten nugget. The result of the CTS test was that the peel strength was 3.4 kN and the button was broken.

FIG. 17 is a CT image obtained by cutting a molten nugget on the overlapping surface of the test piece of Comparative Example 4, and FIG. 18 is a cross-sectional view of the test piece of Comparative Example 4 shown in FIG. 17 in the plate thickness direction.
The blow hole of Comparative Example 4 has a deviation ΔL from the center of the nugget of 1.04 mm, which is larger than that of Example 2 and is formed at a position not intersecting the nugget center line of the molten nugget. The result of the CTS test was that the peel strength was 1.8 kN and the button was broken. The blow hole volume of Example 2 is 0.57 mm ³ , and the blow hole volume of Comparative Example 4 is 0.59 mm ³ , and there is no significant difference between the two blow hole volumes.

Further, among the test pieces in which the nugget size shown in FIGS. 9 and 10 is set to 6.0√t, the test piece at the 96th RBI is referred to as Example 3 and the test piece at the 76th RBI is referred to as Comparative Example 5. ..
FIG. 19 is a CT image obtained by cutting a molten nugget on the overlapping surface of the test piece of Example 3, and FIG. 20 is a cross-sectional view of the test piece of Example 3 shown in FIG. 19 in the plate thickness direction.
The blow hole of Example 3 has a deviation ΔL from the center of the get of 0.58 mm.
It is formed at a position that passes through the nugget centerline of the molten nugget. The result of the CTS test was that the peel strength was 4.66 kN and the button was broken.

FIG. 21 is a CT image obtained by cutting a molten nugget on the overlapping surface of the test piece of Comparative Example 5, and FIG. 22 is a cross-sectional view of the test piece of Comparative Example 5 shown in FIG. 21 in the plate thickness direction.
The blow hole of Comparative Example 5 has a deviation ΔL from the center of the nugget of 0.92 mm, and is formed at a position not intersecting the center line of the nugget of the molten nugget. The result of the CTS test was that the peel strength was 2.88 kN and the button was broken. The blow hole volume of Example 3 is 5.44 mm ³ , and the blow hole volume of Comparative Example 4 is 5.44 mm ³ , and there is no big difference between the two blow hole volumes.

From the results of Examples 2 and 3 and Comparative Examples 3 and 4, when the blow hole having the largest size is formed at a position passing through the nugget center line of the molten nugget, the peel strength becomes high and does not intersect the nugget center. It can be seen that the peel strength is reduced in some cases.

The evaluation results of Examples 1 to 3 and Comparative Examples 1 to 5 above are summarized in Table 5.
In each Example and each Comparative Example, the button was broken, the peel strength of Examples 1 to 3 was 3.40 to 4.66 kN, and the peel strength of Comparative Examples 1 to 5 was 1.80 to 3.50 kN. It became. As for the peel strength, the average value of Examples 1 to 3 was higher than the average value of Comparative Examples 1 to 5.

The present invention is not limited to the above-described embodiment, and can be modified or applied by those skilled in the art based on the combination of the configurations of the embodiments with each other, the description of the specification, and well-known techniques. It is the planned invention and is included in the scope of seeking protection.

As described above, the following matters are disclosed in this specification.
(1) A resistance spot welded joint made of aluminum by superimposing a plurality of aluminum materials and performing resistance spot welding.
The molten nugget formed by the resistance spot welding
In the cross section of the overlapping surface of the aluminum materials, a plurality of blow holes are aggregated and existed in the central portion of the molten nugget in the overlapping surface.
In the cross section of the aluminum material passing through the center of the nugget in the stacking direction, the blow hole having the largest size among the plurality of blow holes is formed at a position passing through the center line of the nugget extending in the stacking direction of the molten nugget. Aluminum resistance spot welded joint.
According to this aluminum resistance spot welded joint, blow holes are aggregated and arranged in the center of the molten nugget. Therefore, as compared with the case where the blow hole is present at the outer edge of the molten nugget, the peel strength (CTS strength) between the aluminum materials can be improved, and the CTS strength variation can be reduced even if the button is broken.

(2) The aluminum material according to (1), wherein the blow hole other than the maximum size blow hole is a minute blow hole having a size of 1/20 or less of the maximum size blow hole. Resistance spot welded joints.
According to this resistance spot welded joint made of aluminum material, a blow hole larger than a minute blow hole is formed in the central portion of the molten nugget, so that the strength variation of CTS can be further reduced.

(3) The aluminum according to (1) or (2), wherein the plurality of blow holes are arranged in a radial region within 70% of the nugget width from the center of the nugget in the cross section of the overlapping surface of the molten nugget. Material resistance spot welded joints.
According to this resistance spot welded joint made of aluminum material, there is no blow hole at the outer edge of the nugget that causes the shear breakage of the molten nugget and reduces the peel strength. Therefore, when the aluminum material is peeled off, the button breaks stably and the joint is high. Strength is obtained.

11 Spot welder 13, 15 Electrode 17 Welding transformer 18 Power supply 19 Control 20 Electrode drive 21 1st aluminum plate 23 2nd aluminum plate 25 Molten nugget 26 Shell
27 Resistive spot welded joint 29 Indentation BH, BHa, BHb, BHc, BHd, BHe, BHf blow hole

Claims (3)

A resistance spot welded joint made of aluminum by superimposing multiple aluminum materials and performing resistance spot welding.
The molten nugget formed by the resistance spot welding
In the cross section of the overlapping surface of the aluminum materials, a plurality of blow holes are aggregated and existed in the central portion of the molten nugget in the overlapping surface.
In the cross section of the aluminum material passing through the center of the nugget in the stacking direction, the blow hole having the largest size among the plurality of blow holes is formed at a position passing through the center line of the nugget extending in the stacking direction of the molten nugget. Aluminum resistance spot welded joint. The resistance spot of the aluminum material according to claim 1, wherein the blow hole other than the maximum size blow hole is a minute blow hole having a size of 1/20 or less of the maximum size blow hole. Welded joint. The resistance spot welding of the aluminum material according to claim 1 or 2, wherein the plurality of blow holes are arranged in a radial region within 70% of the nugget width from the center of the nugget in the cross section of the overlapping surface of the molten nugget. Fittings.