JP2005111483A - Electrode for spot welding - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for spot welding excellent in continuous spotting properties when a hard-to-weld stock is used. <P>SOLUTION: In the electrode for spot welding, the metallic structure after heating at 500°C for 30 min is the fibrous one along a line parallel to an electrode axial center. Alternatively, the electrode for spot welding comprises, by mass, 0.3 to 1.2% Cr, and the balance Cu with inevitable impurities, and having a metallic structure comprising Cr based precipitates whose major axis elongates along a line parallel to the electrode axial center, wherein, the ones having an aspect ratio of ≥5 are present by ≥1 piece/100 μm<SP>2</SP>. Further, Zr, Si and Sn can be incorporated therein. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrode for a spot welder.

In general, many spot welding machines for industrial use use a copper alloy, for example, chrome copper for the electrode tip. The reason why chromium copper is used for the electrode is that the wear resistance and the like are taken into consideration, but the number of spots that can be spot-welded with one electrode tip is about 5000 when using a cold-rolled steel sheet.
However, in actuality, in order to maintain the quality of such spot welding, the current situation is that the electrode tips are replaced or dressed before the limit hitting point is reached. If spot welding is repeatedly performed with the same electrode tip, the tip portion of the electrode tip is gradually worn due to melting damage or the like, and the quality of the spot welding is deteriorated.
In addition, robot type spot welders used in automobile manufacturers' body assembling processes, for example, perform spot welding of about 200 to 400 spots per hour, so it is necessary to frequently replace the electrode tips. However, this replacement operation is automatically performed by a robot or the like from the viewpoint of labor saving.
However, since the line must be stopped during the electrode tip exchange, this is a bottleneck in the automatic assembly process. If two spot welders are installed at the same location and one of the two welders is used in turn, the line can be stopped due to electrode replacement, but the equipment cost is simply doubled. is there.

As prior arts, there are JP-A-2-263956, JP-A-5-77061, JP-A-7-314153, and the like that disclose techniques related to electrodes for spot welding. The first disclosure relates to a method for producing a Cr-Cu electrode, the next disclosure defines the alumina distribution of the alumina-dispersed copper electrode, and the last disclosure defines the composition of the electrode. The latter two disclosures describe that good characteristics are exhibited when welding Al plates, but the melting temperature of Al plates is about 600 ° C, while Sn-based plated steel and Al-based plated steel The melting temperature of the steel sheet during welding is 1500 ° C. or higher, and the plated steel sheet is much higher in terms of the heat load of the electrodes. Therefore, it has been extremely difficult to weld difficult-to-weld materials such as Sn-based and Al-based plated steel sheets from the prior art.
JP-A-2-263956 JP-A-5-77061 Japanese Unexamined Patent Publication No. 7-314153

In general, when the surface-treated steel sheet is used, the number of hit points by one electrode tip is often lower than that of a cold-rolled steel sheet, and when the surface-treated steel sheet is frequently used, the above-mentioned chip replacement frequency becomes higher. In particular, the Sn-Zn plated steel sheet, which has begun to be used as a material for automobile fuel tanks in recent years, has Sn as the main component of its plating layer, and because it reacts very easily with the copper of the electrode tip, it has a low number of dots that can be welded with the electrode tip. There is a problem. Further, a hot-dip Al-plated steel sheet used as a material for an automobile exhaust system or an automobile fuel tank has the same problem because the main component of the plating layer is Al and easily reacts with copper of the electrode tip. When these surface-treated steel sheets are used, the number of hit points by the electrode tip is about 200 to 500 points, which is reduced to 1/10 or less compared to the above cold-rolled steel plate. The hassle is getting bigger.
Then, this invention makes it a subject to provide the electrode for spots excellent in the continuous spot property when using a hardly soluble raw material.

The present invention has been made to solve the above problems. The present inventors have studied in detail the factors on the steel plate side, the electrode side, and the welding conditions that affect the life of the electrode tip in spot welding, and as a result of optimizing the factor on the electrode side, the life of the electrode tip Succeeded in improving. That is, the present invention has been made with the knowledge that the influence of the metal structure of the electrode, particularly the precipitation form of the Cr-based precipitate, and the copper crystal structure on the life of the electrode tip is great. By using the electrode tip according to the present invention, it is possible to obtain a stable life of the electrode tip even when a difficult-to-weld material such as the Sn—Zn-plated steel plate or the hot-dip Al-plated steel plate is used.
In the present invention, the precipitation state of Cr inside the electrode and the metal structure of the electrode are regulated to suppress the reaction between Sn or Al and the electrode tip, but the clear reason is not clear at present. Absent. As factors on the electrode side that govern the reactivity between the electrode and the steel plate, deformation resistance of the electrode tip, electrical conductivity of the electrode, thermal conductivity, etc. can be considered. Generally, the deformation resistance (hardness) and electrical conductivity of the electrode Are in a trade-off relationship. However, in spot welding, the flow of electricity and heat flows between the electrode tip, the water-cooled end, and the electricity supply end, that is, in a direction parallel to the axis of the electrode. For this reason, if the precipitates or the metal structure is distributed on a line parallel to the axis, it is considered that it is difficult to prevent the flow of electricity and heat.

The present invention completed based on such knowledge is as follows.
(1) An electrode for spot welding, wherein the metal structure after heating the electrode at 500 ° C. for 30 minutes is fibrous along a line parallel to the electrode axis.
(2) The spot welding electrode according to (1), wherein Cr: 0.3 to 1.2% is contained in mass%, and the balance is Cu and inevitable impurities.
(3) The electrode for spot welding according to (1), characterized in that Al: 0.05 to 2% by mass and the balance is made of Cu and inevitable impurities.
(4) The spot welding according to (2), further comprising one or more of Zr: 0.01 to 0.4%, Si: 0.05 to 0.8%, Sn: 0.05 to 0.8% by mass% Electrode.
(5) Cr-based precipitates containing Cr: 0.3 to 1.2% by mass, the balance being Cu and inevitable impurities, and extending in the metal structure along a line parallel to the electrode axis. An electrode for spot welding, comprising: a precipitate having a major axis of 7 μm or more and an aspect ratio of 5 or more of 1 piece / 100 μm square or more.
(6) Containing Cr: 0.3-1.2% by mass, the balance consisting of Cu and inevitable impurities, and the metal structure after heating the electrode at 500 ° C. for 30 minutes is along a line parallel to the electrode axis An electrode for spot welding, comprising a Cr-based precipitate having an elongated major axis, wherein one of the precipitates has a major axis of 7 μm or more and an aspect ratio of 5 or more per 100 μm square.
(7) (5) to (6) characterized by further containing one or more of Zr: 0.01 to 0.4%, Si: 0.05 to 0.8%, Sn: 0.05 to 0.8% by mass% The electrode for spot welding as described.
(8) The electrode for spot welding as described in (1) to (7), wherein the hardness is 140 to 180 as Vickers hardness.

  As described above, the present invention can provide an electrode tip for spot welding that has a longer life than the conventional one when using a hardly weldable material such as a Sn-Zn plated steel plate or an Al plated steel plate. According to the present invention, it is possible to reduce the line stoppage in the automobile assembly process, which can contribute to more efficient automobile production in the future.

Next, the reason why the present invention is limited as described above will be described in detail.
In this specification, “%” defining the steel composition and the plating composition is “mass%”.
First, the reasons for limitation regarding the components of the electrode will be described.
Cr: The most widely used electrode tip is a component system called chromium copper, and contains 0.3 to 1.2% of Cr in Cu. This Cr forms precipitates by heat treatment and raises the hardness of the electrode copper. Similarly, in the present invention, it is desirable to form a Cr precipitate, and 0.3% or more is desirable in order to precipitate Cr.
On the other hand, the solubility of Cr is about 1% at 1000 ° C., and even if 1.2% or more is added, it does not form a solution by heat treatment and makes precipitation difficult, so 1.2% is the upper limit.

Zr, Si, Sn: These elements coexist with Cr, affect the precipitation of Cr, and affect the hardness and conductivity. In order to obtain these effects, 0.01% as Zr, 0.05% as Si, and 0.05% or more as Sn are desirable, and adding more than necessary increases costs and decreases manufacturability, so the upper limit is 0.4% as Zr, Si and Sn are set to 0.8%.
Al: Al is added as an oxide in the form of so-called alumina-dispersed copper. Alumina-dispersed copper is said to be suitable for a surface-treated steel sheet because it has little decrease in hardness at high temperatures. At this time, since the hardness can be increased by adding 0.05% or more of alumina as Al, the lower limit is made this amount. When adding more than necessary, the conductivity decreases, so when adding, the upper limit is desirably 2%.

  Next, the structure of the electrode will be described. As described above, it is desirable that the structure be a structure along a line parallel to the axis from the viewpoint of increasing heat and electric conductivity in a direction parallel to the axis. When the metal structure is a fibrous structure or a chromium-copper material, such an effect can be obtained by distributing Cr-based precipitates along a line parallel to the axis. It is preferable that the Cr-based precipitate is stretched along a line parallel to the axial center and has a precipitation form having an aspect ratio of 5 or more and a major axis of 7 μm or more. The existence of one or more such precipitates in a 100 μm square improves the life of the electrode tip. These metal structures can be determined by dividing the electrode into two on the plane including the axis and observing the cross-sectional structure. Observation is possible with an optical microscope, and a suitable solution for etching is a 10% ammonium persulfate aqueous solution mixed with a small amount of aqueous ammonia. Since ammonia in ammonia water is volatile, it is difficult to define the exact amount of ammonia water, but the structure does not change with the amount of ammonia and can be observed. However, it is necessary to shorten the etching time in order to observe the precipitate, and to increase the etching time in order to observe the metal structure.

  The aspect ratio of the Cr-based precipitate is 5 or more. There is no particular upper limit, but it is often 20 or less. The major axis is 7 μm or more. Although there is no upper limit, it is often 30 μm or less. The form of these precipitates is determined from a cross-sectional optical microscope. Actually, the size of the true precipitate may vary depending on where the precipitate becomes the polished surface, but it is difficult to measure accurately, and therefore, the cross-sectional structure that can be observed relatively easily is determined. The density of the precipitates is one or more of the above precipitates in a 100 μm square. In the measurement, it is desirable to calculate the average value by observing about three visual fields at random and obtaining the number of each. Although there is no upper limit, it is often 20 or less when the upper limit of Cr content is 1.2%.

It is desirable that the hardness of the electrode tip is in the range of 140 to 180 in terms of hardness.
The load for measuring the hardness is 5 kgf. If the electrode hardness is high, the electrode will be less deformed, and this will be advantageous for the electrode life. On the other hand, if the hardness is high, the electrical conductivity will be low and the electrode will generate more heat, which will be disadvantageous for the electrode life. It becomes. These balanced hardness ranges are the above ranges.
The electrode hardness is determined by the components and the thermal history during production, and is particularly affected by solution treatment and aging treatment conditions.
Since these conditions affect the structure and precipitate form of the electrode, the hardness can be controlled within the above range by manufacturing the structure and the form of the precipitate.

  Next, a method for manufacturing an electrode having such a structure will be described. The electrode is produced by adjusting the components and forming a cast copper alloy into a rod shape by a process such as hot pressing, followed by solution treatment at about 950 ° C. After that, it is manufactured through quenching, cold working and aging at about 475 ° C. Among these processes, the metal structure and the form of precipitates are affected by the solution treatment, cold working, and aging treatment processes. When the aging temperature is low, a fibrous structure tends to be formed. In addition, as the degree of cold working increases, naturally a fibrous cold-worked structure tends to remain. When the solution treatment temperature is low, the aspect ratio of Cr-based precipitates tends to increase. The electrode of the present invention can be obtained by adjusting these process conditions. However, the present invention does not specifically define the production method, and the electrode of the present invention can be produced by methods other than these.

  EXAMPLES Next, an Example demonstrates this invention in detail. Using a high-frequency melting furnace, the chromium copper alloy whose Cr content was adjusted to 0.85% was melted and uniformly stirred, and then cast into a graphite mold. 120mm diameter and 250mm length with the composition shown in Table 1 An ingot was produced. A 92mm diameter, 200mm length billet for extrusion is formed from the ingot, extruded into a bar with a diameter of 20-32mm at a temperature of 900 ° C, solution treatment at a temperature of 850-950 ° C, and water cooling After quenching, cold drawing was performed to a diameter of 16 mm. Further, aging treatment was performed at 400 to 500 ° C.

The solution treatment time was 30 minutes and the aging treatment time was 4 hours.
The drawn rod material was formed into a spot welding electrode having a tip portion having a tip diameter of 6 mm (40R) and a cooling hole, and set in a spot welder. As a welded material, Sn-8% Zn-plated steel sheet (Product name: Eco-Coat-T, Material: Extremely low carbon steel, Plate thickness: 0.8mm, Plating adhesion amount: 40g / m ^{2 on} one side, Post-treatment coating amount: One side 100 mg / ^{m 2,} oiling), melting
An Al-plated steel sheet (trade name: Al sheet, material: extra-low carbon steel, plate thickness: 0.8 mm, plating adhesion amount: 30 g / m ^{2 on} one side, post-treatment coating amount: 100 mg / m ^{2 on} one side, oiling) was used. Welding conditions were a preload of 200 kgf, a welding current of 9 to 9.5 kA, a continuous welding speed of 3 s / time, and the interval between the upper and lower electrodes before welding was 30 mm.
The electrode life was determined to be NG when the button diameter was cut by 4√t (t: mm) when the button diameter was measured by peeling the welded part every 25 points using the peel method.
Further, the structure was observed after the electrode was further annealed at 500 ° C. for 1 hour. For observation, etching was performed with a solution obtained by adding a small amount of aqueous ammonia to a 10% ammonium persulfate aqueous solution, and the photograph was taken with an optical microscope at a magnification of 400 times.

  Table 1 shows the relationship between the manufacturing conditions of the electrode tip, the structure, and the number of consecutive dots when using a Sn-Zn plated steel sheet (trade name: Ecocoat-T). In each structure, a fibrous structure or a stretched Cr-based precipitate along a line parallel to the axis of the electrode was observed. From this table, it can be seen that there is a significant correlation between the electrode structure and the number of consecutive dots. The hardness of these electrodes was 140 to 170 in terms of Vickers hardness, but no significant correlation was found between the hardness and the number of continuous hit points. Using the No. 1 and 3 electrodes, the hot-dip Al-plated steel sheet was tested in the same manner. As a result, 1500 points were obtained when the No. 1 electrode was used, and 400 points were obtained when the No. 3 electrode was used. Even when a hot-dip Al-plated steel sheet was used, a remarkable effect was recognized. An example of the electrode structures of Nos. 2 and 3 is shown in FIGS. FIG. 1 shows the observation of Cr precipitates with slightly weak etching, and FIG. 2 shows the metal structure with strong etching. In FIG. 1, the one having a dark color contrast is a Cr-based precipitate. Note that the black trace in the center of FIG. 2 is an indentation obtained by measuring the hardness, and is not related to the structure.

On the other hand, the same applies to electrodes made of alumina-dispersed copper (1% as Al), or Cr-Zr (Cr: 0.7%, Zr: 0.2%), Cr-Si (Cr: 0.7%, Si: 0.1%) electrodes. The test was conducted. The basic manufacturing conditions were No. 1 in Table 1. However, since alumina-dispersed copper is not a precipitation hardening type, solution treatment and quenching treatment are not performed.
Using the electrode manufactured in this manner, a spot welding test was performed under the above conditions using a Sn-Zn plated steel sheet, 900 points for alumina dispersed copper, 850 points for Cr-Zr system, 800 points for Cr-Si system. A continuous hitting point was obtained.
When the structure of the electrode at this time was observed by the method described above, all of them exhibited a fibrous structure.

It is a figure which shows the Cr-type deposit of the cross-sectional organization of the electrode of this invention example and a comparative example. It is a figure which shows the metal organization of the cross-section organization of the electrode of this invention example and a comparative example.

Claims (8)

  An electrode for spot welding, wherein the metal structure after heating the electrode at 500 ° C. for 30 minutes is fibrous along a line parallel to the electrode axis.   2. The spot welding electrode according to claim 1, comprising Cr: 0.3 to 1.2% by mass%, the balance being Cu and inevitable impurities.   2. The spot welding electrode according to claim 1, wherein Al: 0.05 to 2% by mass and the balance is made of Cu and inevitable impurities.   The electrode for spot welding according to claim 2, further comprising one or more of Zr: 0.01 to 0.4%, Si: 0.05 to 0.8%, Sn: 0.05 to 0.8% in terms of mass%.   Cr: 0.3 to 1.2% by mass%, the balance is made of Cu and inevitable impurities, and the metal structure contains Cr-based precipitates whose major axis extends along a line parallel to the electrode axis, An electrode for spot welding characterized in that one of the precipitates having a major axis of 7 μm or more and an aspect ratio of 5 or more is 1 piece / 100 μm 2 or more.   Containing Cr: 0.3-1.2% by mass%, the balance consists of Cu and inevitable impurities, and the metal structure after heating the electrode at 500 ° C for 30 minutes extends along the line parallel to the electrode axis 1. An electrode for spot welding, comprising a Cr-based precipitate, wherein the precipitate has a major axis of 7 μm or more and an aspect ratio of 5 or more of 1 piece / 100 μm 2 or more.   The spot according to claim 5 or 6, further comprising one or more of Zr: 0.01 to 0.4%, Si: 0.05 to 0.8%, Sn: 0.05 to 0.8% in mass%. Welding electrode.   8. The spot welding electrode according to claim 7, wherein the hardness is 140 to 180 as Vickers hardness.

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