JP3242726B2 - Aluminum alloy material for resistance spot welding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum alloy material for resistance spot welding suitable for use by performing resistance spot welding as a structural material of a transport machine such as an automobile.
In particular, the present invention relates to an aluminum alloy material for resistance spot welding that exhibits excellent effects of preventing internal defects in a resistance spot weld, improving joint strength, and extending the life of an electrode.

[0002]

2. Description of the Related Art In recent years, aluminum (Al) or an aluminum alloy has excellent corrosion resistance and aesthetic appearance (design), and is light in weight. It's being used.

In recent years, Al or Al alloys have been used in the field of automobiles and the like by taking advantage of the above-mentioned properties of Al alloys, and the opportunities for applying resistance spot welding to Al or Al alloys have increased. ing.

However, Al or an Al alloy material has a thermal conductivity and an electrical conductivity that are about twice that of iron, and a high current and a short welding time are required for resistance spot welding using local heat generation. However, the welding range is very limited, and the welding operation requires great care.

[0005] In addition, Al or Al alloy material has a large volumetric shrinkage ratio at the time of solidification as compared with iron, and resistance spot welding solidifies at a faster cooling rate than at the time of arc welding or the like. Weld cracks easily occur inside the weld metal and in the heat-affected zone. Such weld cracks are not only rejected based on the acceptance / rejection criteria based on the appearance test of the inspection method of the JISZ3140 spot welded part, but are also large in parts where aesthetics such as automobile panel materials are actually required. It becomes a defect. In addition, it also affects deterioration of corrosion resistance due to penetration of the electrode material (Cu-based) into the inside of the crack.

Further, aluminum or aluminum alloy has a larger difference in the amount of dissolved hydrogen in liquid and solid than iron, and as a result, blowholes are easily generated.

As described above, cracks and blowholes are caused by the above-described quality deterioration peculiar to resistance spot welding,
As a result, variations in strength occur, which results in a shortened life of the electrode, which causes strength deterioration. That is, the electrode material falls off in the surface crack, and the abrasion of the electrode tip is promoted. In addition, the occurrence of cracks and blowholes makes the power distribution during welding non-uniform, and the life of the electrodes is shortened due to local heat generation. Therefore, in order to prevent the above welding defects, the welding conditions such as the pressing force and the welding current have been improved,
Studies have been made on removal of the oxide film and control of main additive elements.

[0008]

However, in removing the oxide film and controlling the main additive elements, it cannot be said that sufficient measures have been taken to prevent welding defects. In terms of welding work, although defects can be reduced by high pressure and two-stage pressing conditions, applicable welding conditions are very narrow, and the influence on other factors such as electrode deterioration is a problem. Become.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and prevents the occurrence of cracks and blowholes in a weld joint, reduces the variation in joint strength peculiar to resistance spot welding, and improves heat generation characteristics. Aluminum alloy for resistance spot welding, which can reduce the current and increase the strength, prevent the deterioration of the electrode, extend the life of the electrode, improve the quality of the resistance spot welding and save energy The purpose is to provide materials.

[0010]

According to the present invention SUMMARY OF] resistance spot welding aluminum alloy material, Mg; 0.2 to 7 wt%, Si; 1.5 wt% or less, Cu; 0.6 to 1.
It is characterized by containing 0% by mass and Zr; 0.04 to 0.3% by mass, with the balance being Al and unavoidable impurities.

[0011]

[0012]

[Action] present inventors, the above object quality proof intensive studies in order to achieve results, in particular with likely to occur welding defects such as weld cracks and blowholes, automotive timber such as the spot welding is utilized many Al-Mg type used and A
the l-Mg-Si-based alloy, to alloy obtained by adding Cu which becomes a cause of weld cracking of those for improving the strength of the material and moldability, by adding Zr, that these disadvantages can be eliminated It has been found to be very effective. This Zr has already been used as an additive in the field of arc welding, but the present invention has revealed, for the first time, the effect of eliminating the above-mentioned disadvantages peculiar to resistance spot welding.

[0013] That is, Mg; 0.2 to 7 mass%, Si;
1.5 mass% or less, C u; containing 1.0 mass% or less,
The aluminum alloy plate and the balance of Al and unavoidable impurities, by adding Zr 0.04 to 0.3 mass%, it is possible to achieve the object of the present invention.

[0014] In order to improve the occurrence of weld cracks and blowholes in spot welds, or to ensure weldability such as strength, the addition of Zr provides resistance spots separately from the effects known in arc welding. Effective for welding-specific mechanisms.

First, as is known in arc welding,
It is necessary to make the crystal grain size of the weld metal part fine. In arc welding, from the aspect of component dilution, it is more effective to add Zr to a wire than to add Zr to a material.
General. Therefore, in arc welding, cracks in the weld metal part are reduced.

However, in the case of resistance spot welding as in the present invention, it is necessary to add Zr to the base metal, so that cracks in the weld metal can be reduced, and defects of the base material such as the shadow of the base material can be reduced. Can also be prevented.

In order to obtain such an effect, it is necessary to regulate the amount of Zr added to 0.04 to 0.3% by weight.
If the Zr content is less than 0.04% by weight, the effect of the addition cannot be obtained. Conversely, if the Zr content exceeds 0.3% by weight, a large crystallized substance is generated, leading to deterioration in workability of the material itself. Further, by adding Zr in this range, the following effects unique to resistance spot welding are exhibited.

First, the range of variation in strength when continuous spot welding is performed is reduced, and the life of the electrode is extended. The mechanism is as follows: (1) Arc welding rapidly cools at about 1 × 10 ³ ° C / sec, but resistance spot welding rapidly cools at about 1.5 × 10 ³ ° C / sec or more. Further, solidification cracks are more likely to occur. Zr has the effect of improving the fluidity of the molten metal,
Even if cracks occur once during the solidification period, the ability to fill the Zr-added material increases, and the occurrence of cracks is reduced.
Zr also affects the shape or distribution of the melt at the grain boundaries, and has the effect of increasing the interface contact angle at the grain boundaries. Also in this case, Zr has an effect of preventing grain boundary cracking. (2) In the resistance spot welding, a blow hole is formed in a welding condition region in which characteristics required for welding such as weldability strength and nugget diameter 5√t (t is a plate thickness) are satisfied, that is, a region in which required characteristics are secured. The occurrence of is remarkable. It is thought that the cause of the blowhole is hydrogen in the metal and the oxide film, but Zr has a property of easily forming a hydride compound with hydrogen. Zr and H ₂
Has a higher reaction energy than the reaction energy of H ₂ with another metal, and forms relatively stable zirconium hydride. Therefore, the weld metal containing Zr is H ₂ is absorbed, blowholes is reduced. (3) The Zr-added material reduces cracks and blowholes by the mechanism shown in (1) and (2). The variation in joint strength is mainly attributable to the fact that the fracture path is dominated by cracks and blowholes. By reducing these cracks and blowholes as described above, the range of variation in joint strength can be reduced. Is also possible.

On the other hand, the addition of Zr realizes an improvement in joint strength and an improvement in workability, as well as a reduction in the range of strength variation and a longer life of the electrode. First, the strength is as follows: (4) In the case of resistance spot welding, segregation is remarkable in the weld metal despite the fact that the cooling rate is faster than that of arc welding. In particular, components such as Mg are apt to segregate, causing a reduction in material strength and grain boundary cracking. However, by adding Zr, by making the crystal grains in the weld metal fine, segregation is suppressed and a larger amount of solid solution in the matrix that governs strength and hardness is secured. As a result, the strength and hardness are prevented from deteriorating, and the strength is increased. (5) In the case of resistance spot welding, a corona bond portion (crimped portion) that has a significant effect on welding strength is stabilized by adding Zr. This effect is remarkably confirmed when the Zr content is 0.08% by weight or more, and the tensile strength at one point is about 50%.
The strength increases by about kgf. (6) When welding a middle plate and a thick plate, cracks and blowholes are reduced in the weld metal even when the nugget diameter is 5√t (t is the plate thickness), which is the average nugget diameter normally specified in JIS. Therefore, the shear area is wide and the strength is secured. (7) When welding a thin plate, a relatively small heat input is possible because the strength in the weld metal is high.

Next, the following operational effects are obtained as workability improvements. (8) By adding Zr, the resistance value of the plate itself increases, and welding can be performed at a lower current condition than in the past. (9) Since the temperature of the electrode is lowered by lowering the current, the pickup of aluminum on the copper surface of the electrode is also reduced, and the life of the electrode can be extended. Further, the thermal conductivity of the aluminum plate also decreases due to the increase in the resistance value of the aluminum plate. As a result, the penetration of the nugget in the electrode direction becomes shallower, so that the temperature of the electrode tip can be lowered, the pickup is reduced, and the electrode has a longer life.

Next, the reason for adding other components and the reason for limiting the composition of the aluminum alloy material according to the present invention will be described. Mg Mg is an element that contributes to the strength and workability of the base material and the weld. However, if the Mg content is less than 0.3% by weight, the effect cannot be obtained. If the Mg content exceeds 7% by weight, the strength is increased, but the rolling processability is deteriorated, and cracks are likely to occur during rolling. In the actual production process becomes difficult. Further, when the Mg content is 7% by weight or more, a precipitate is easily generated at a crystal grain boundary in the welded portion,
It causes welding cracks. Therefore, the Mg content is 0.3
To 7% by weight. Si Si is an element that contributes to the improvement of strength. However, as the Si content increases, coarse crystallization occurs, and the precipitation of impurity elements at the crystal grain boundaries of the weld metal part causes cracks, resulting in workability. Is greatly reduced. Therefore, the Si content is 1.5
% By weight or less. Cu Cu, like Mg, is an element that contributes to improvement in strength and workability. In particular, when Cu is made to coexist with Mg, the effect of adding Cu is further increased, and particularly, the effect of reducing softening during baking of paint is large. However, when the Cu content exceeds 1.0% by weight, eutectic melting is liable to occur, although it is effective in improving the strength, and it becomes difficult to produce the material in an actual production process. Therefore,
The Cu content is 1.0% by weight or less. Inevitable impurities In the present invention, Cr, F
e, V, Ti, Be and the like.

[0022]

Next, examples of the present invention will be described in comparison with comparative examples.

A 500 mm thick ingot of an aluminum alloy having the chemical components and crystal grain sizes shown in Table 2
, And then hot-rolled at a temperature of 500 to 280 ° C. until the sheet thickness becomes 3.5 mm. Subsequently, cold rolling was performed to a sheet thickness of 1.5 mm, and thereafter, rapid cooling was performed at an average cooling rate of 400 ° C./min as intermediate annealing. Next, cold rolling (final cold working rate: 30%) was performed to a thickness of 1 mm.

This 1 mm thick aluminum alloy plate is
After rapidly heating to 0 ° C. and holding at that temperature for 30 seconds,
A final heat treatment of rapid cooling at an average cooling rate of 00 ° C./min was performed to obtain a test material.

The obtained test materials were subjected to resistance spot welding, and crack resistance, blow hole resistance, variation in strength, and continuous spotting property were examined. The welding conditions are shown in Table 1 below, and the evaluation results are shown in Table 3 below.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

However, the evaluation method is as follows. (1) Investigation of cracks in weld metal section Nugget sections were taken with a projector, and the size of the cracks was observed on a photograph.

◎: No cracks ○: Cracks of 0.5 mm or less Δ: Cracks of 0.5 to 1.0 mm ×: Cracks of 1.0 mm or more (2) Crack investigation of heat-affected zone Nugget cross section was observed with a microscope. .

；: No cracks ×: Cracks (3) Existence of blow holes Nugget sections were taken by a projection method and observed on photographs. Then, the degree of cracking was evaluated based on the porosity. The porosity is 10% for the vacancy on the center line of the nugget part of the welded section.
Holes of 0.5 mm or more were measured in the photograph at × 2.

；; 0 ；; less than 2.5% Δ; 2.5 to 1.0% ×; 10% or more (4) Variation in joint strength Evaluation was made based on the standard deviation of strength by total pulling at 100 continuous dots.

◎: 80 N or less ○: 80 to 130 N Δ: 130 to 180 N ×: 180 N or more (5) Continuous hitting point Evaluated by the number of hit points obtained by dividing the minimum value of JISA grade.

◎: 3000 points or more ；: 3000 to 1500 points Δ; 1500 to 1000 points ×; less than 1000 points (6) Overall evaluation The overall evaluation was made based on the following criteria.

◎: All of the highest evaluations ；: Either of 4 or 5 was ◎, and all of them were ○ or more ；: All were 以上 or more ；; All were △ or more ；; All x △

As is apparent from Table 3, No. No. 1 to No. 3 and No. 3 No. 6 to No. Eleven example materials are:
Since it is a trial material within the range that satisfies all of the specified conditions of the present invention, it has excellent effects on crack resistance, blow hole removal, strength variation, and continuous hitting property.
In addition, No. In the reference examples 4 and 5, the same excellent effect was obtained.
The fruit was obtained. In this case, the Mg-rich No. 1 which is considered to be prone to cracking. Even with the aluminum materials of 5 and 6, no welding defects and variations in strength are observed due to the addition of Zr. In addition, the Cu-rich No. 1 is also susceptible to cracking. The materials of Examples 7, 8, and 10 have a high Mg content of N.
o. Although the aluminum material of No. 7 shows a slight decrease in weldability, it shows excellent results as a whole. The effect is further improved by adding Si, which is unlikely to cause cracking. Therefore, the so-called Al-Mg-based and Al-Mg-S
In the i-based alloy, the effect of Zr addition is extremely excellent.

On the other hand, No. Twelve comparative examples had low Mg materials and tended to have few weld cracks, but had blowholes which are considered to be caused by hydrogen in metals and the like, and deterioration in strength variation was confirmed. Also, in Comparative Example No. 1
In No. 3, Zr is excessively added, and its weldability is improved as compared with the others. However, a large crystallized substance is generated, and the dispersion of strength and the continuous spotting property are inferior. No. 1 in which Mg is relatively rich. In Comparative Example No. 14, Mg was segregated at the crystal grain boundaries, and cracking and continuous hitting were inferior due to strength deterioration. Furthermore, the Mg-rich No. 1 In Comparative Example No. 15, although Zr was added for the time being, the amount of the added Zr was small, so that the welding cracks and the strength varied greatly, and the blow holes also exceeded the porosity of 50%. In addition, S which is hard to cause welding cracks
No. i added i. Even in Comparative Examples 16 to 19, No. 16 in which Cu was added richly. No. 16 has the worst characteristics. 17, 18, and 19 are Si or a large amount of Z
Although the characteristics are improved as a whole by adding r, it is still not at a satisfactory level.

[0038]

As described above, since the aluminum alloy material according to the present invention contains Zr of a predetermined composition, the defect (crack, crack,
Blowholes), variations in joint strength, and continuous hitting life can be improved, and resistance spot weldability can be improved while making use of the characteristics of an aluminum alloy having excellent corrosion resistance and light weight. For this reason, the present invention makes a great contribution to the weight reduction of transport vehicles such as automobiles.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Seiji Sasabe 100-1 Urakawachi, Miyama-ji, Fujisawa-shi, Kanagawa Prefecture Inside the Kobe Steel Fujisawa Works (72) Inventor Kikuo Toyose 15 Kinuigaoka, Moka-shi, Tochigi Prefecture Kobe Steel, Ltd. Moka Works (56) References JP-A-1-162594 (JP, A) JP-A-4-32534 (JP, A) JP-A-5-105981 (JP, A) -47127 (JP, B1) JP-B 61-50141 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 21/00-21/18

Claims (1)

(57) [Claims] 1. Mg: 0.2 to 7% by mass; Si;
5% by mass or less, Cu; 0.6 to 1.0% by mass and Z
r: an aluminum alloy material for resistance spot welding, comprising 0.04 to 0.3% by mass, with the balance being Al and unavoidable impurities.