JP2851203B2 - Resistance spot welding method for Al-Mg based alloy sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for resistance spot welding of an Al-Mg alloy sheet having excellent strength and formability, which is used for an automobile body sheet and the like. Can be obtained.

[0002]

2. Description of the Related Art Conventionally, Al-Mg based alloys used for automobile body sheets and the like are A5052 (Al-Mg).
2.6 wt% Mg-0.25 wt% Cr) alloy, A5182
(Al-4.5 wt% Mg-0.35 wt% Mn) alloy, A
5083 (Al-4.5 wt% Mg-0.7 wt% Mn-
0.15wt% Cr) alloys and the like.
All contents were less than 5%.

At present, there is resistance spot welding as one means for joining body sheet materials to each other, and this has been studied by many researchers, and MIL standards, WES standards, and the like have been determined.

According to these standards, spot welding conditions for a 0.7 to 1.2 mm thick plate of the above Al-Mg based material are set to 2
30-300kgf, welding current 23000-33000
A, energization time is 6 to 8 cycles. There is no regulation for single-phase rectifier welders and inverter welders.

[0005] In an aluminum alloy sheet containing less than 5 wt% of Mg, an electrode life of about 1000 points is obtained under these welding conditions. The electrode life in this case means that when continuous spot welding is performed by resistance spot welding, the electrode is worn and the diameter of the nugget at the welded portion on the thinnest plate (thinst plate) side to be joined is 4 × t ^{1. / 2} x 0.9 mm (t is the thinnest plate thickness (mm)) or less. The bonding strength of the bonding portion has a linear relationship with the nugget diameter, and it has been found that the bonding strength has no practical problem if the nugget diameter is equal to or larger than the value of the nugget diameter represented by the above equation. .

However, recently developed Mg of more than 5 wt%
When resistance spot welding is applied to an aluminum alloy having excellent formability containing, under the same welding conditions as described above, the penetration of the welded portion melts up to the surface as shown in FIG. 2, that is, the nugget (3) becomes the two plate materials (1). The surface of (2) reaches the surface to cause cracks and the like, and the electrode life becomes 800 points or less. This is because the high Mg content aluminum alloy is Mg
The melting point is reduced by the addition of a larger amount of the material, and the material melts with less energy compared to the low Mg-containing material due to the increase in the electrical resistance of the material. It is considered that alloying occurs between the electrode and the material, the electrode is consumed, and the life of the electrode is shortened.

On the other hand, the electrode life in spot welding in the case of plated steel sheets currently used in automobiles is 2,000.
It is said to be 3000 points. It is said that there is no problem in mass production if the hitting point of this degree is possible even in the case of an aluminum alloy. However, as described above, an aluminum alloy containing 5 wt% or more of Mg is suitable for a conventional low Mg-containing aluminum alloy. The electrode life when welding under the welding conditions was 800 points or less as described above, and did not reach the electrode life in the case of a plated steel sheet.

Therefore, as means for solving the above problems, Japanese Patent Application Laid-Open Nos. 61-159288 and 63
No. 278679 proposes a method of welding by inserting an insert material between a material and an electrode.

[0009]

In the automobile industry, energy saving of automobiles is progressing in order to prevent global warming. However, it is said that energy saving of about 10% can be achieved by reducing vehicle weight by 10%. Therefore, the usage of aluminum alloy is increasing. As described above, when an aluminum alloy is used for a body sheet, a conventional A5052 alloy, an A5182 alloy, an A5 alloy containing less than 5% by weight of Mg.
Since 083 alloy and the like have insufficient formability, a high Mg-containing aluminum alloy of 5 wt% or more has recently been developed as described above.

However, this material is the same as the conventional A
When resistance spot welding was performed under conditions suitable for a 5052 alloy or the like, the material was melted to the vicinity of the surface of the material and alloyed with the electrode, thereby shortening the life of the electrode and cracking. When the electrode life is shortened in this way, frequent electrode replacement and dressing (polishing of the electrode tip) are required, and it becomes difficult to adapt the aluminum alloy to a mass production line of an automobile. Further, if a crack occurs in the welded portion, strength, corrosion resistance, and the like are reduced, causing a problem in safety. In addition, the method using the insert material is difficult to apply on site from the viewpoint of installation of the device, productivity and cost.

[0011]

SUMMARY OF THE INVENTION In view of the foregoing, the present invention has been made to solve these problems as a result of various studies. The present invention has a long electrode life, a large joint strength, and an improvement in joint reliability. And a resistance spot welding method for an Al alloy plate.

That is, according to the present invention, the thinnest plate is 0.7-1.
A plurality of Al-Mg (5 to 9 wt%) alloy plates having a thickness of 2 mm are overlapped and subjected to resistance spot welding at a welding current of 18000 to 36000 A and a conduction time of 3 to 15 cycles, thereby joining these alloy plates to each other. In the method, at least the thickness of the oxide film on the surface of the spot weld joint of these alloy plates is set to 100 to 250 Å and the electrode pressing force of the welding machine is set to 350 to 450 kgf. It is effective to use a single-phase AC welding machine, a single-phase rectification welding machine or an inverter welding machine as the machine.

[0013]

According to the above-mentioned means, the diameter of the nugget at the welded portion on the thinnest plate side is 4 × t ^1/2 × 0.9 mm = 3.6 t ^1/2 mm or more, so that there is no problem in strength. Further, as shown in FIG. 1, the penetration of the welded portion did not reach the surface on the electrode contact side, but the penetration rate (the thickness of one plate (1) joined in FIG. (D) × 100% when the depth of the nugget (3) formed on the ()) side is d.
Is adjusted to 20-85%. Therefore, alloying between the electrode and the material is suppressed, and the life of the electrode is dramatically improved.

The reason why the welding conditions are adjusted so that the penetration rate is 20 to 85% is that when the penetration rate is 20% or less, the shear strength of the joint is low, and when the joining plates are peeled off from each other. This is because, as shown in FIG. 3, the nuggets (3) are sheared apart from each other at the surface of the plate material joint, and the nuggets (3) do not become button breaks remaining on the plate as shown in FIG. Is 85% or more, alloying of the electrode and the material occurs, and the electrode life is likely to be shortened. Therefore, in resistance spot welding, it is necessary to adjust the welding conditions so that the diameter of the nugget on the thinnest plate side is 3.6 t ^1/2 or more and the penetration rate is 20 to 85%.

Conventionally, a welding current of 18,000 to 36
Resistance spot welding was performed under the conditions of 000 A and an energization time of 3 to 15 cycles. This is because the welding current is 1800
If it is less than 0A, the nugget diameter does not reach 3.6t ^1/2 ,
If it exceeds 6000 A, the penetration rate exceeds 85%. If the energization time is less than 3 cycles, the heat generation is small, so that the nugget diameter does not satisfy 3.6t ^1/2 or more, and if it exceeds 15 cycles, the penetration rate also exceeds 85%.

Therefore, in the present invention, the thickness of the oxide film on the surface of the bonding material is further set to 100 to 250 Å. The reason is that if the thickness of the oxide film is less than 100 Å, the contact resistance is too small and the heat generated during welding is small, and the nugget diameter does not satisfy the condition of 3.6 t ^1/2 mm or more. This is because excessive heat generation causes the penetration rate to exceed 85% and increases internal defects. Therefore, the thickness of the oxide film is 100-250.
Need to be angstrom.

If the electrode pressure is less than 350 kgf, the contact resistance between the materials is high, so that the penetration increases, and the penetration rate exceeds 85%. If the electrode pressure exceeds 450 kgf, the contact resistance decreases, and heat is generated. This is because the size becomes small and the condition that the nugget diameter is 3.6 t ^1/2 or more is not satisfied.

The Al-Mg alloy plate to be used in the present invention is an Al-Mg alloy plate containing 5 to 9 wt% of Mg. The reason why the amount of Mg in the material is limited to 5 to 9 wt% is that
If it is less than wt%, elongation is low and formability is not sufficient, and 9 wt%
If the ratio exceeds the limit, it is difficult to produce a material, and the life of the electrode during resistance welding becomes extremely poor. In addition,
In the present Al-Mg based alloy, Cu 0.5 wt% or less, M
0.3 wt% or less for each of n, Cr, and Zr, 0.5 wt% or less for each of Fe and Si, 0.1 wt% or less for Ti, and 0.02 wt% for B
Hereinafter, even if Be is contained in an amount of 0.05 wt% or less, the effect of the present invention is not affected. According to the present invention, two or more of these Al-Mg-based alloy plates are stacked, and the thinnest plate having a thickness of 0.7 to 1.2 mm is joined by resistance spot welding. If the electrode life is 3000, comparable to plated steel plate
You get more than points.

[0019]

Embodiments of the present invention will be specifically described below.

<Example 1> As welding materials, Al-6wt% Mg-0.1wt% Cu
A -0.01 wt% Mn alloy having a thickness of 1.2 mm, a width of 30 mm and a length of 200 mm was used. The welding machine used was a single-phase AC welding machine, and the electrode used was a cap-tip type DR type of chrome copper of 16 mmφ. The thickness of the oxide film of this welding material was adjusted by pickling, and the film thickness was measured by Auger electron spectroscopy and shown in Table 1. In the conventional method, no treatment for washing the oxide film on the surface is performed. Two of these plate members were completely overlapped and continuously welded at a point interval of 30 mm at one point / 2 seconds under the conditions shown in Table 1.

The welded plate was peeled off with a peel test jig (4) shown in FIG. 5, and the diameter of the nugget (3) was measured with a caliper. The value of (major axis + minor axis) 径 2 (mm) was defined as the nugget diameter. did. The penetration rate was determined for the nugget of the weld joint at the fifth hit point based on the above equation. The initial nugget diameter in the table is a value for the nugget at the joint at the first hit point. Then, the initial nugget diameter and penetration rate, and the number of welding points at which the nugget diameter became 3.6 t ^1/2 or less were determined as the electrode life.
It was shown to.

In the table, Nos. 1 to 8 are the methods of the present invention, and Nos. 9 to
No. 16 is a comparison method, Nos. 17 to 19 are conventional methods, and No. 17 is a MIL standard (MIL-W-6858D "WELDING, RE
SISTANCE: SPOT AND SEAM ”)
No. 18 is a commercial standard, No. 19 is WES 7302-197
9 "Spot welding standard" (aluminum and aluminum alloy). (However, heat was not performed afterwards)

[0023]

[Table 1]

As is evident from Table 1, No. 1
1 to 8 each had an initial nugget diameter of 3.6 t ^1/2 =
The electrode satisfies 3.6 × 1.2 ^1/2 = 3.9 mm or more, and the electrode life exceeds 3000 points, which is much better than the conventional method. On the other hand, in the comparative method, the initial nugget diameter was less than 3.9 mm and spot welding was impossible from the beginning, or the electrode life was less than 3000 points.

Example 2 The welding material was an aluminum alloy plate made of Al-6.8 wt% Mg, having a thickness of 1.0 mm, a width of 30 mm and a length of 200 mm.
mm. The welding machine uses a single-phase rectifying welding machine, and the electrode is a chrome copper 16mmφ cap tip type R
Shape, R = 80 mm was used. Then, as in Example 1, the surface was pickled and the thickness of the oxide film was measured.

Two of these plate members were completely overlapped and continuously welded at a point interval of 30 mm at one point / 2 seconds under the conditions shown in Table 2. Then, the initial nugget diameter, penetration rate and electrode life of the welded plate were examined in the same manner as in Example 1, and the results are shown in Table 2.

In the table, Nos. 1 to 8 are the methods of the present invention, and Nos. 9 to
No. 16 is a comparison method, Nos. 17 to 19 are conventional methods, and No. 17 is a welding condition conforming to the MIL standard.
Is a commercial standard, and No. 19 is a condition of WES. (However, heat was not performed afterwards)

[0028]

[Table 2]

As is evident from Table 2, No. 1
1 to 8 each had an initial nugget diameter of 3.6 t ^1/2 =
The electrode life satisfies 3.6 × 1.2 ^1/2 = 3.6 mm or more and the electrode life exceeded 3,000 points, which was far superior to the conventional method. On the other hand, in the comparative method, spot welding was impossible because the initial nugget diameter was less than 3.6 mm, or the electrode life was less than 3000 points.

Example 3 As welding materials, an aluminum alloy plate made of Al-5.2 wt% Mg, having a plate thickness of 1.0 mm and a plate thickness of 0.7
mm plates were used. The welding machine uses an inverter welding machine and the electrode is a chrome copper 16mmφ cap tip type R.
Shape, R = 20 mm was used. Then, the surface was pickled in the same manner as in Example 1, the thickness of the oxide film was adjusted, and the film thickness was measured.

Each of the welding materials is 30 mm wide × 200 mm long.
And sandwiching the one with a thickness of 1 mm between two sheets with a thickness of 0.7 mm, 3
The sheets were completely overlapped and were continuously welded at an interval of 30 mm at one point / 2 seconds. The initial nugget diameter, penetration rate and electrode life of the welded plate were examined in the same manner as in Example 1, and the results are shown in Table 3.

In the table, Nos. 1 to 8 are the methods of the present invention, and Nos. 9 to
No. 16 is a comparison method, Nos. 17 to 19 are conventional methods, and No. 17 is a welding condition conforming to the MIL standard.
Is a commercial standard, and No. 19 is a condition of WES. (However, heat was not performed afterwards)

[0033]

[Table 3]

As is apparent from Table 3, the No. 1 of the present invention example
1 to 8 each had an initial nugget diameter of 3.6 t ^1/2 =
The electrode satisfies the condition of 3.6 × 0.7 ^1/2 = 3.0 mm or more and the electrode life exceeds 3000 points, which is far superior to the conventional method. On the other hand, in the comparative method, spot welding could not be performed because the initial nugget diameter was less than 3.6 mm, or the electrode life was less than 3000 points.

[0035]

As described above, according to the present invention, in particular, Mg
In the case of resistance spot welding of Al-Mg alloy sheet containing up to 9 wt%, a sufficient nugget diameter and the same electrode life as a plated steel sheet of 3000 points or more can be obtained. This greatly contributes to improving the resistance spot welding that has been performed.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a normal resistance spot welding joint.

FIG. 2 is a cross-sectional view showing a joint portion of spot welding with excessive penetration.

FIG. 3 is a side sectional view showing a sheared state of a joint.

FIG. 4 is a perspective view showing a state in which a button of a joint is broken.

FIG. 5 is an explanatory diagram showing a method of using a peel test jig.

[Explanation of symbols]

 1, 2 plate material 3 nugget 4 peel test jig

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-4-358094 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B23K 11/18

Claims (2)

(57) [Claims] 1. A plurality of Al—Mg (5 to 9 wt%) alloy sheets each having a thickness of 0.7 to 1.2 mm, the thinnest sheet of which is overlapped with a welding current of 18000 to 36000 A and a conduction time of 3 to 1
In a method of joining these alloy plates to each other by resistance spot welding in five cycles, at least the thickness of the oxide film on the surface of the spot weld joint of these alloy plates is reduced.
100 to 250 angstroms and an electrode pressing force of a welding machine of 350 to 450 kgf.
-Resistance spot welding method for Mg-based alloy sheet. 2. The method according to claim 1, wherein a single-phase AC welding machine, a single-phase rectification welding machine or an inverter welding machine is used as the spot welding machine.