JP2021011633A - Aluminum alloy clad material - Google Patents

Abstract

To perform brazing stably without a flux.SOLUTION: An aluminum alloy clad material includes: a sacrificial material on one surface of a core material; and a brazing material disposed on the other surface of the core material, the brazing material containing Si: 6.0-14.0%, Mg: 0.05-1.5%, Bi: 0.05-0.25%, Sr: 0.0001-0.1%, and the balance consisting of Al and inevitable impurities, and satisfying a relationship of (Bi+Mg)×Sr≤0.1. Mg-Bi-based compounds contained in the brazing material with a diameter of 0.1 μm or more and less than 5.0 μm are more than 20 in number per 10,000 μm2 visual field in a surface layer plane direction before brazing, and the Mg-Bi-based compounds with a diameter of 5.0 μm or more are less than 2 in number per 10,000 μm2 visual field, and the core material contains Mn: 1.0-1.7%, Si: 0.2-1.0%, Fe: 0.1-0.5%, Cu: 0.08-1.0%, Mg: 0.1-0.7%, and the balance consisting of Al and inevitable impurities. The sacrificial material contains Zn: 0.5-6.0%, with its Mg content limited to 0.1% or less, and a Mg concentration on the sacrificial material surface after brazing being 0.15% or less.SELECTED DRAWING: Figure 1

Description

The present invention relates to an aluminum alloy clad material for flux-free brazing that is joined by flux-free.

Aluminum automobile heat exchangers such as capacitors and evaporators are becoming smaller and lighter, and aluminum materials are becoming thinner and stronger. In the manufacture of aluminum heat exchangers, brazing is used to join joints, but in the brazing method that uses fluoride-based flux, which is currently the mainstream, the flux reacts with Mg in the material to inactivate it. Since defects are likely to occur, the use of Mg-added high-strength members is restricted. Therefore, a brazing method for joining Mg-added aluminum alloys without using flux is desired.

In flux-free brazing using Al-Si-Mg brazing material, Mg in the brazing material that has been melted and activated can be bonded by reducing and decomposing the Al oxide film (Al ₂ O ₃ ) on the surface of the joint. Become. For closed surface joints, joints that combine brazing sheets that have brazing material due to the decomposition action of the oxide film by Mg, and joints that combine brazing sheets and members to be joined (bare material) that do not have brazing material. A good joint state can be obtained (see Patent Document 1).

However, the tube and fin joints, which are typical joint shapes of general heat exchangers such as capacitors and evaporators, are easily affected by the atmosphere, and the MgO film is likely to grow on the surface of the Mg-added brazing material. Since the MgO film is a stable oxide film that is not easily decomposed, bonding is significantly hindered.
Therefore, in order to apply the flux-free technology to a general heat exchanger, a flux-free brazing brazing sheet that can obtain a stable joint state with a joint having an open portion is strongly desired.

As a method for stabilizing the bonding state of flux-free brazing, for example, an Al-Si-Mg-Bi-based brazing material shown in Patent Document 2 is used, and the distribution state of Bi particles and Mg-Bi compound particles in the brazing material is controlled. Technology has been proposed. According to this technique, by dispersing a simple substance Bi or Bi-Mg compound having a circular equivalent diameter of 5.0 to 50 μm in a brazing material, these compounds are exposed on the surface of the brazing material at the time of material production, and the exposed portion It is said that the flux-free brazing property is improved in a short brazing heat addition time by suppressing the formation of the oxide film.

Patent Publication No. 4547032 Japanese Unexamined Patent Publication No. 2014-50861

However, it is difficult to say that the flux-free brazing method proposed in the past has obtained stable bondability enough to replace the brazing method using the currently mainstream fluoride-based flux, and general heat is used. Further technical improvements are needed for widespread application to exchangers.

The present invention has been made against the background of the above circumstances, and an object of the present invention is to provide an aluminum alloy clad material for flux-free brazing capable of stably performing brazing by flux-free.

As a result of diligent studies in view of the above problems, the present inventors uniformly concentrated Bi on the surface at the time of brazing in order to further improve the brazing property of the Bi-added Al-Si-Mg-based brazing material. Is the most important, and although a coarse Mg-Bi compound of 5 μm or more is effective in suppressing the formation of an oxide film during material production, it is difficult to dissolve during brazing heat, but rather a fine 0.1 μm or more. By dispersing a Bi-Mg compound of less than 5.0 μm at a predetermined density or higher, the Mg-Bi compound is surely dissolved during brazing heat, and a metal Bi is generated, and the generated Bi is uniformly concentrated on the surface. It was found that good brazing property can be obtained by changing the brazing property.

In addition, the relationship between brazing behavior and brazing property in flux-free brazing is investigated, and in flux-free brazing, active molten brazing is generated in a short time while suppressing oxidation to form fillets. It is clarified that a high-Si brazing material is preferable because a brazing material having a low liquidus temperature and a short solid-liquid coexisting region is preferable, and it is generated at the time of casting, which is a problem with a high-Si brazing material. The method of suppressing coarse primary crystal Si was also studied repeatedly.

Furthermore, when research was conducted to further improve the stability of flux-free brazing, the brazing stability was improved by using a slight decompression in combination, which was equal to or better than the brazing method using fluoride-based flux. It was suggested that brazing stability could be obtained. However, in that case, a new problem has arisen in which Zn having a high vapor pressure evaporates and the corrosion resistance deteriorates, although it is not as high as vacuum brazing. In the Al brazing sheet, the Al sacrificial material to which Zn is added is clad, and the sacrificial anode effect protects the core material. Therefore, when Zn evaporation occurs, the corrosion resistance of the brazing sheet deteriorates.

Therefore, as a result of diligent studies from the viewpoint of obtaining high corrosion resistance even when Zn evaporates, the present inventors have made Zn evaporation by suppressing the Mg concentration in brazing on the surface of the member containing Zn to a predetermined value or less. Furthermore, it was found that the corrosion resistance does not easily deteriorate even when Zn evaporation occurs by optimizing the components of the sacrificial material, and the dispersion state of the Mg-Bi compound is appropriately controlled as described above. By combining with the Al-Si-Mg-Bi brazing material, a flux-free aluminum alloy clad material for brazing has been invented, which can obtain a stable bonding state in a joint having an open portion and has excellent corrosion resistance.

That is, in the first form of the aluminum alloy clad material, the sacrificial material is arranged on one side of the core material, and Si: 6.0 to 14.0%, Mg, by mass%, is placed on the other side of the core material. : 0.05 to 1.5%, Bi: 0.05 to 0.25%, Sr: 0.0001 to 0.1%, the balance is composed of Al and unavoidable impurities, and (Bi + Mg) × Sr An Al-Si-Mg-Bi brazing material satisfying the relationship of ≤0.1 is arranged, and
The Mg-Bi compound contained in the Al-Si-Mg-Bi brazing material has a diameter equivalent to a circle of 0.1 μm or more and less than 5.0 μm in the observation in the surface layer direction before brazing. 10000 ² was present more than 30 per field, and those with a diameter of more than 5.0μm is less than two per 10000 ² field,
Further, the core material is Mn: 1.0 to 1.7%, Si: 0.2 to 1.0%, Fe: 0.1 to 0.5%, Cu: 0.08 to 1 in mass%. It contains 0.0%, Mg: 0.1 to 0.7%, and the balance consists of Al and unavoidable impurities.
The sacrificial material contains Zn: 0.5 to 6.0% in mass%, the Mg content is regulated to 0.1% or less, and the Mg concentration on the surface of the sacrificial material after brazing is 0.15. It is characterized by being less than%.

The invention of another form of the aluminum alloy clad material is characterized in that, in the invention of the above-mentioned form, the core material further contains Mg: 0.1 to 0.7%.

The invention of another form of the aluminum alloy clad material is characterized in that, in the invention of the said form, the core material further contains Ti: 0.05 to 0.3%.

In the invention of the other form of the aluminum alloy clad material, in the invention of the above-mentioned form, the natural potentials of the lowest portion of the sacrificial material after brazing and the central portion of the core material are lower in the lowest portion of the sacrificial material. The natural potential difference is in the range of 120 to 280 mV, and the potential difference between the outermost surface and the lowest portion of the sacrificial material is within 50 mV.

In the invention of the other form of the aluminum alloy clad material, in the invention of the said form, the sacrificial material is mass%, Si: 0.2 to 0.8%, Cr: 0.05 to 0.5%, Ti: It is characterized by containing one kind or two or more kinds of 0.05 to 0.3%.

The contents specified in the present invention will be described below together with their actions.
All of the components described below are shown in% by mass.

<Blazed material>
Si: 6.0-14.0%
Si is added to form molten brazing during brazing and to form fillets at the joints. In flux-free brazing in the open part, it is important to generate active molten brazing in a short time and form fillets while suppressing oxidation, so the liquidus temperature is low and the solid-liquid coexistence region. A brazing material with a short temperature is preferable. If the content is less than the lower limit, the molten wax formation time becomes long and the molten wax becomes insufficient. On the other hand, if it exceeds the upper limit, the molten wax formation time becomes long and the material becomes hard and brittle, which makes it difficult to manufacture the material. Therefore, the Si content is set within the above range.
For the same reason, it is desirable that the Si content is 9.0% at the lower limit and 13.0% at the upper limit.

Mg: 0.05-1.5%
Mg is added to reduce and decompose the Al oxide film (Al ₂ O ₃ ). If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, MgO that reacts with oxygen in the brazing atmosphere to inhibit bonding is generated, and the material becomes hard and brittle. Material manufacturing becomes difficult. Therefore, the Mg content is set within the above range.
For the same reason, it is desirable that the Mg content is 0.1% at the lower limit and 1.2% at the upper limit, and more preferably 0.2% at the lower limit and 1.0% at the upper limit. desirable.

Bi: 0.05-0.25%
Bi is added to the surface of the material in the process of raising the temperature of the brazing, to suppress oxidation during brazing and to reduce the surface tension of the molten brazing to improve the bondability at the open portion. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, not only the effect is saturated, but also Bi oxide is easily formed on the surface of the material and bonding is hindered. Therefore, the Bi content is set within the above range.
For the same reason, it is desirable that the Bi content is 0.08% at the lower limit and 0.23% at the upper limit.

Sr: 0.0001 to 0.1%
Sr is added to suppress the formation of coarse primary crystal Si generated in a brazing material having a high Si content. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, it is oxidized on the surface of the molten metal during casting to increase dross, or to form a coarse compound, resulting in a decrease in castability. Therefore, the Sr content is set within the above range.
For the same reason, it is desirable that the Sr content is 0.0005% at the lower limit and 0.06% at the upper limit.

As an unavoidable impurity of the brazing material, Fe: may be contained in the range of 0.3% or less.

(Bi + Mg) × Sr ≦ 0.1
Since the Al-Si-Mg-Bi brazing material for flux-free brazing contains active Mg and Bi, if it coexists with Sr in a predetermined amount or more, a coarse Bi-Mg-Sr compound is contained in the molten metal during casting. Is generated and castability is reduced. This compound is more likely to be produced as the total amount of Bi and Mg increases and the Sr content increases. (Bi + Mg) × Sr indicates the critical condition for the formation of this coarse Bi-Mg-Sr compound, and by setting (Bi + Mg) × Sr ≦ 0.1, an Al-Si-Mg-Bi-based brazing material can be obtained. Even if Sr is added, a coarse Bi-Mg-Sr compound is not produced, and the effect of suppressing the production of primary crystal Si, which is the original purpose of adding Sr, can be obtained. Therefore, it is defined in the above range.
For the same reason, it is desirable to set (Bi + Mg) × Sr ≦ 0.08.

Mg-Bi-based compounds: Circle-equivalent diameters less than 0.1 to 5.0 μm are 10,000 μm. More than 20 fine Mg-Bi-based compounds are dispersed per ² visual fields, resulting in a brazing temperature rise process. When this compound is dissolved, Bi is easily uniformly concentrated on the surface of the material, and oxidation of the material is suppressed. Even if a compound having a size of less than 0.1 μm is dissolved, the above effect cannot be obtained because the amount of dissolution is small. A compound having a thickness of 5.0 μm or more is difficult to melt in the process of raising the temperature with brazing, and the compound remains as it is, so that the above effect cannot be obtained. Further, when the number of the above compounds is 20 or less per 10000 μm ² field of view, there are few dissolution points and it is difficult for Bi to be uniformly concentrated on the material surface. For the same reason, it is desirable that the number is 30 or more, and more preferably 40 or more.

The number of Mg-Bi compounds on the surface of the brazing material is as follows: The surface of the brazing material of the produced material is mirror-treated with 0.1 μm abrasive grains, and FE-EPMA (field emission electron probe microanalyzer) is used. In addition to performing automatic particle analysis, in order to measure fine compounds of 1 μm or less, a thin film is prepared by mechanical polishing and electrolytic polishing from the surface of the cut-out brazing material layer, and observed with a TEM (transmission electron microscope). Then, it is obtained by counting the number of Mg-Bi compound particles of 0.1 to 5.0 μm in the observation field of 10000 μm ² (100 μm square) in the surface direction.

Further, as a means for finely and densely distributing the Mg-Bi compound, casting is performed at a high cooling rate from a place where the molten metal temperature is high (suppressing coarse crystallization of the Mg-Bi compound and casting Mg and Bi). (Promote solid dissolution at the time and disperse in a desired state by subsequent heat treatment), and take a large total reduction amount above a certain level during hot rolling (fineness and increase in number density by promoting crushing of crystals) , Prolonging the rolling time in the high temperature range (promoting dynamic precipitation during hot rolling), lowering the hot rolling finish temperature and increasing the subsequent cooling rate (suppressing coarse precipitation due to slow cooling), etc. Can be adjusted by properly combining.

Mg-Bi-based compounds: Circle-equivalent diameter of 5.0 μm or more is 10000 μm Less than ² per 2 visual fields Coarse Mg-Bi-based compounds are difficult to melt during the brazing temperature rise process, and Bi is uniform on the material surface. It is difficult to concentrate the compound, and the amount of a fine Mg-Bi compound having a diameter of less than 5.0 μm is reduced due to the formation of a coarse compound. Therefore, it is necessary to make the value lower than the predetermined value.
The number of Mg-Bi compounds on the surface of the brazing material can be determined by the above-mentioned fully automatic particle analysis by FE-EPMA. Further, as a means for suppressing the formation of a coarse Mg-Bi-based compound, it can be adjusted by appropriately controlling the above-mentioned casting conditions and hot spreading conditions.
For example, during casting, casting at a high cooling rate from a place where the molten metal temperature is high (suppressing coarse crystallization of Mg-Bi compound), and during hot spreading, a large total reduction amount of a certain level or more should be taken (crystallized product). It can be adjusted by appropriately combining (miniaturization by promoting crushing), lowering the hot-rolled finish temperature and increasing the subsequent cooling rate (suppressing coarse precipitation due to slow cooling).

Sacrificial material Zn: 0.5 to 6.0%
Zn is added in order to make the natural potential of the material lower than that of other members, exert a sacrificial corrosion protection effect, and improve the pitting corrosion resistance of the clad material. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, the potential becomes too low and the corrosion and consumption rate of the sacrificial material becomes faster, and the pitting corrosion resistance of the clad material becomes worse due to the early disappearance of the sacrificial material. descend.
For the same reason, it is desirable that the lower limit is 0.7% and the upper limit is 5.7%.

Mg content 0.1% or less Mg has a high vapor pressure and further reduces and decomposes the Al oxide film. Therefore, the larger the Mg content, the more easily the oxide film on the material surface is destroyed during brazing. If Zn exists under the destroyed oxide film, Zn evaporates from the gaps in the destroyed oxide film. Therefore, in an environment where Zn evaporation can occur, it is possible to suppress the evaporation of Mg and the evaporation of Zn due to the destruction of the oxide film by setting the amount of Mg added to the Zn-containing layer to a predetermined value or less.
For the same reason, it is desirable to set it to 0.05% or less.

The Mg concentration on the surface of the sacrificial material after brazing is 0.15% or less Mg has a high vapor pressure and further reduces and decomposes the Al oxide film, so the more Mg, the more the oxide film on the material surface is destroyed during brazing. Cheap. If Zn exists under the destroyed oxide film, Zn evaporates from the gaps in the destroyed oxide film. Therefore, as the amount of Mg present on the surface of the sacrificial material increases during brazing, Zn evaporates constantly, and as a result, the amount of Zn evaporation increases. In the case of a clad material, even if the amount of Mg added to the sacrificial material is small, Mg diffuses from other layers such as the core material, so it is necessary to design the material in consideration of diffusion during brazing. .. The higher the Mg concentration on the surface of the sacrificial material after brazing, the higher the Mg concentration on the surface of the sacrificial material during brazing, and therefore the amount of Zn evaporation increases. Therefore, the amount of Mg on the surface of the sacrificial material after brazing is a measure of the ease of Zn evaporation, and by setting the Mg concentration on the surface of the sacrificial material after brazing to a predetermined value or less, the evaporation of Mg and the destruction of the oxide film It is possible to suppress the evaporation of Zn due to the above.
For the same reason, it is desirable to set it to 0.1% or less.

Si: 0.2-0.8%
Si is precipitated as an intermetallic compound such as simple Si, Al-Fe-Si, Al-Mn-Si, and Al-Mn-Si-Fe to disperse the origin of corrosion, thereby improving the pore corrosion resistance of the clad material. Therefore, it is added as desired. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, the corrosion rate becomes high, and the pitting corrosion resistance of the clad material is lowered due to the early disappearance of the sacrificial material.
For the same reason, it is desirable that the lower limit is 0.3% and the upper limit is 0.7%.
Further, when Si is not positively contained, Si may be contained as an unavoidable impurity in an amount of 0.05% or less.

Cr: 0.05-0.5%
Cr is precipitated as an Al—Cr-based intermetallic compound to disperse the starting point of corrosion and to form a shaded portion of solid solution Cr to form a layered corrosion form, thereby improving the pitting corrosion resistance of the clad material. Therefore, it is added as desired. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, a huge intermetallic compound is formed during casting and the rollability is lowered.
For the same reason, it is desirable that the lower limit is 0.1% and the upper limit is 0.4%.
Further, when Cr is not positively contained, Cr may be contained as an unavoidable impurity in an amount of less than 0.05%.

Ti: 0.05-0.3%
Ti precipitates as an Al-Ti intermetallic compound to disperse the origin of corrosion, and by forming a shaded portion of solid solution Ti to layer the corrosion form, the pitting corrosion resistance of the clad material is improved. Therefore, it is added as desired. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, a huge intermetallic compound is formed during casting and the rollability is lowered.
For the same reason, it is desirable that the lower limit is 0.07% and the upper limit is 0.25%.
Further, when Ti is not positively contained, Ti may be contained as an unavoidable impurity in an amount of less than 0.05%.

Heartwood Mn: 1.0 to 1.7%
Mn is precipitated as an intermetallic compound such as Al-Mn, Al-Mn-Si, Al-Mn-Fe, and Al-Mn-Si-Fe, and is added to improve the material strength. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, a huge intermetallic compound (crystallized product) is generated during casting, and the rollability is lowered.
For the same reason, it is desirable that the lower limit is 1.1%, the upper limit is 1.6%, and the lower limit is 1.2%.

Si: 0.2 to 1.0%
Si is added to improve the material strength by solid solution and to precipitate as Mg ₂ Si, Al-Mn-Si, and Al-Mn-Si-Fe intermetallic compounds. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, the melting point of the material is lowered.
For the same reason, it is desirable that the lower limit is 0.6% and the upper limit is 0.9%.

Fe: 0.1 to 0.5%
Fe is precipitated as an intermetallic compound such as Al-Mn-Fe and Al-Mn-Si-Fe and is added to improve the material strength. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, a huge intermetallic compound (crystallized product) is generated during casting, and the rollability is lowered.
For the same reason, it is desirable that the lower limit is 0.12% and the upper limit is 0.4%.

Cu: 0.08 to 1.0%
Cu is added to dissolve in solid solution and improve the material strength. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, the corrosion resistance is lowered.
For the same reason, the lower limit is preferably 0.10%, the upper limit is 0.6%, and the lower limit is more preferably 0.15%.

Mg: 0.1 to 0.7%
Mg is desired because it improves the material strength by precipitating a compound with Si and the like, and diffuses to the surface of the brazing material to reduce and decompose the oxide film (Al ₂ O ₃ ) and improve the bondability. Is added by. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, the material becomes too hard and it becomes difficult to manufacture the material.
For the same reason, it is desirable that the lower limit is 0.2% and the upper limit is 0.65%.
When Mg is not positively contained, Mg may be contained as an unavoidable impurity in an amount of 0.05% or less.

Ti: 0.05-0.3%
Ti precipitates as an Al-Ti intermetallic compound to disperse the origin of corrosion, and by forming a shaded portion of solid solution Ti to layer the corrosion form, the pitting corrosion resistance of the clad material is improved. It is added as desired to allow it. If the content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, a huge intermetallic compound is formed during casting and the rollability is lowered.
For the same reason, it is desirable that the lower limit is 0.07% and the upper limit is 0.25%.
Further, when Ti is not positively contained, Ti may be contained as an unavoidable impurity in an amount of less than 0.05%.

The natural potential difference between the lowest part of the sacrificial material after brazing and the central part of the heartwood is in the range of 70 to 280 mV (the lowest part of the sacrificial material is lower).
The natural potential difference between the lowest part of the sacrificial material and the central part of the heartwood is a measure of how much the heartwood is sacrificed and protected by the sacrificial material. If it is less than the lower limit, the core material cannot be completely protected due to insufficient potential difference, and corrosion proceeds toward the core material. If the upper limit is exceeded, overcorrosion will occur and alkaline corrosion will occur. Therefore, it is desirable to set the natural potential difference.
For the same reason, it is more desirable to set the lower limit to 120 mV and the upper limit to 250 mV.

The potential difference between the outermost surface and the lowest part of the sacrificial material is within 50 mV. In the normal potential distribution, the surface of the sacrificial material is the lowest in the sacrificial material and becomes noble toward the heartwood. In this case, since the outermost surface of the sacrificial material and the lowest portion coincide with each other, the potential difference between the outermost surface of the sacrificial material and the lowest portion becomes zero. Therefore, the potential difference between the outermost surface and the lowest portion of the sacrificial material means the magnitude of the reverse potential gradient in the sacrificial material. Good corrosion resistance can be obtained by setting this potential difference within 50 mV.
In order to reduce the potential difference between the outermost surface and the lowest portion of the sacrificial material, it is necessary to suppress Zn evaporation or use a sacrificial material component whose potential does not change significantly even if Zn evaporates. As described above, the suppression of Zn evaporation is achieved by reducing the amount of Mg added to the sacrificial material to a predetermined value or less and reducing the Mg concentration on the surface of the sacrificial material after brazing.
On the other hand, in order to obtain a sacrificial material whose potential does not change significantly even if Zn evaporates, the components constituting the sacrificial material are important. That is, other elements are usually added to the sacrificial material together with Zn which makes the potential base, but when the other elements are excluded, the potential becomes base due to the addition of Zn, but after adding Zn to some extent, the potential becomes high. Deterioration becomes slow (the effect of the amount of Zn on the potential is saturated). Therefore, by using an Al—Zn alloy as the sacrificial material and setting the Zn concentration on the surface of the sacrificial material to a predetermined value or more, the potential difference between the outermost surface and the lowest portion of the sacrificial material can be reduced even when Zn evaporation occurs. ..
Furthermore, even if Si, Cr, Ti, etc., which are less likely to affect the potential leveling by Zn among other elements, are added, the adverse effect on the above effect is small.

When the potential difference between the outermost surface and the lowest part of the sacrificial material is more than 50 mV, the surface of the sacrificial material is more noble than the lowest part, and the surface of the sacrificial material remains undissolved and corrosion progresses, so that the sacrificial material does not function effectively. Corrosion resistance of the clad material decreases due to the early arrival of corrosion on the core material. Therefore, it is desirable to set the potential difference between the outermost surface of the sacrificial material and the lowest portion within 50 mV.
For the same reason, it is desirable that the potential difference is within 40 mV, and more preferably within 25 mV.

The aluminum alloy of the core material, brazing material, and sacrificial material of the present invention also contains unavoidable impurities. For example, the sacrificial material may contain Fe and Mn as unavoidable impurities.

That is, according to the present invention, stable brazing is possible in flux-free brazing, and after brazing, it is possible to obtain the effects of having high strength and excellent corrosion resistance.

It is a figure which shows the brazing sheet for flux-free brazing in one Embodiment of this invention. It is a perspective view which shows the heat exchanger for an automobile made of aluminum in one Embodiment of this invention. It is a figure which shows the brazing evaluation model in the Example of this invention. It is a figure which shows the relationship of the potential difference of the sacrificial material outermost surface, the lowest part and the core material central part in the Example of this invention.

An embodiment of the present invention will be described below.
The aluminum alloy is melted by adjusting to the composition of the present invention. The melting can be carried out by a semi-continuous casting method.
In the present embodiment, since fine Mg-Bi compound is dispersed before brazing, coarse crystallization of Mg-Bi compound is suppressed by casting at a high cooling rate from a place where the molten metal temperature is high when casting the brazing material. While doing so, Mg and Bi are solid-dissolved in the ingot.
Specifically, the solid solubility of Mg and Bi can be increased by setting the molten metal temperature to 700 ° C. or higher.
The obtained aluminum alloy ingot is homogenized under predetermined conditions. If the homogenization treatment temperature is low, coarse Mg-Bi compounds are precipitated, and it is difficult to obtain the distribution state of the Mg-Bi compound of the present invention before brazing. Therefore, the treatment is performed at a treatment temperature of 400 ° C. or higher for 1 to 10 hours. Is desirable.

Next, the brazing material is assembled with a core material and a sacrificial material and clad-rolled hot. At this time, in the present invention, the rolling time at a predetermined temperature during hot rolling, the equivalent strain from the start to the end of hot rolling, and heat. The rolling finish temperature and the cooling rate after hot rolling are controlled to adjust the Mg-Bi compound to a predetermined size and number density.

First, by satisfying the rolling time in a predetermined temperature range during hot rolling, precipitation of a predetermined size Mg-Bi compound defined in the present invention is promoted in an environment where dynamic strain is applied. Specifically, the precipitation of fine Mg-Bi compounds is promoted by setting the rolling time between a material temperature of 400 to 500 ° C. during hot rolling to 10 min or more.

Further, by controlling the equivalent strain from the start to the end of hot spreading, it is possible to crush and refine the coarse Mg-Bi crystallized product generated at the time of casting and increase the number density. Specifically, by adjusting the slab thickness and the finish thickness so that the equivalent strain ε represented by the equation (1) becomes ε> 5.0, the Mg-Bi crystallized product becomes sufficiently fine and the number density becomes high. To increase.

ε = (2 / √3) ln (t0 / t) ・ ・ ・ Equation (1)
t0: Hot spreading start thickness (slab thickness)
t: Hot-rolled finish thickness

Further, if the finishing temperature of hot spreading is high and the temperature is maintained at a high temperature without dynamic strain, or if the cooling rate after hot spreading is slowed down, the grain boundaries and the like are coarser than those intended by the present invention. Since the Mg-Bi compound is precipitated, the hot-rolled finishing temperature is lowered to a predetermined temperature and a cooling rate of a certain level or higher is secured to suppress the precipitation of the coarse Mg-Bi compound. Specifically, the hot-rolled finishing temperature is set to 250 to 350 ° C., and the cooling rate from the finishing temperature to 200 ° C. is controlled to be faster than −20 ° C./hr to suppress the precipitation of coarse Mg-Bi compounds.

After that, after cold rolling or the like, as shown in FIG. 1, the aluminum alloy clad material 1 of the present invention in which the brazing material 3 is arranged on one side of the core material 2 and the sacrificial material 4 is arranged on the other surface of the core material 2 can get.
In cold rolling, for example, cold rolling can be performed at a total rolling reduction of 75% or more, intermediate annealing can be performed at a temperature of 200 to 450 ° C., and then final rolling can be performed at a rolling ratio of 40%. In cold rolling, the Mg-Bi compound is not easily crushed and does not deviate from the size and number density intended by the present invention, so that the conditions are not particularly limited. Further, the intermediate annealing may not be performed, or the H2n tempered product completed by the final annealing may be used.

The aluminum alloy clad material 1 made of the brazing sheet obtained in the above step is used as a component of the heat exchanger as an assembly combined with another component 10 (fins, tubes, side plates, etc. shown in FIG. 1). , Used for brazing.
The assembly is placed in a heating furnace in a non-oxidizing atmosphere under normal pressure. The non-oxidizing gas can be composed of a nitrogen gas, an inert gas such as argon, a reducing gas such as hydrogen or ammonia, or a mixed gas thereof. The pressure in the atmosphere inside the brazing furnace is basically normal pressure, but for example, in order to improve the gas replacement efficiency inside the product, a medium-low vacuum of about 100 kPa to 0.1 Pa should be set in the temperature range before melting the brazing material. Alternatively, the positive pressure may be about 5 to 100 Pa higher than the atmospheric pressure in order to suppress the mixing of outside air (atmosphere) into the furnace.

The heating furnace does not need to have a closed space, and may be a tunnel type having an inlet and an outlet for brazing material. Even in such a heating furnace, the non-oxidizing property is maintained by continuously blowing the inert gas into the furnace. As the non-oxidizing atmosphere, the oxygen concentration is preferably 50 ppm or less in terms of volume ratio.

Under the above atmosphere, for example, heating is performed at a heating rate of 10 to 200 ° C./min, and brazing joining is performed under heat treatment conditions where the ultimate temperature of the assembled body is 559 to 630 ° C.
Under brazing conditions, the faster the rate of temperature rise, the shorter the brazing time, so the growth of the oxide film on the surface of the material is suppressed and the brazing property is improved. Brazing is possible if the ultimate temperature is at least the solidus temperature of the brazing material or higher, but the fluid brazing material increases as the temperature approaches the liquidus temperature, and a good joint state can be obtained with a joint having an open portion. It will be easier. However, if the temperature is too high, brazing erosion is likely to proceed, and the structural dimensional accuracy of the assembled body after brazing is lowered, which is not preferable.

FIG. 2 shows an aluminum heat exchanger 5 in which fins 6 are formed by using the aluminum alloy clad material 1 and a tube 7 made of an aluminum alloy is used as a brazing target material. The fins 6 and the tube 7 are incorporated with the reinforcing material 8 and the header plate 9, and the aluminum heat exchanger 5 for automobiles and the like is obtained by flux-free brazing.

Various brazing sheets having the compositions shown in Tables 1, 2 and 4 and 5 (the balance is Al and unavoidable impurities) are hot under the casting conditions and homogenization conditions (brazing material) shown in Table 7, and the hot rolling conditions. A rolled plate was produced. In addition, "-" in the component indicates that the content is 0 or the amount of unavoidable impurities.
Then, a cold rolled plate having a thickness equivalent to H14 and having a thickness of 0.30 mm was produced by cold rolling including intermediate annealing. The clad ratio of each layer was 10% for the sacrificial material and 8% for the brazing material.
Further, as a member to be brazed, corrugated fins made of A3003 alloy and H14 aluminum bare material (0.06 mm thickness) were prepared.

A tube having a width of 25 mm is manufactured using the aluminum clad material, and the tube and the corrugated fin are combined so that the tube brazing material and the corrugated fin are in contact with each other. As a brazing evaluation model, a 15-stage tube and a core having a length of 300 mm are used. did. The core was heated to 600 ° C. in a brazing furnace in a nitrogen atmosphere (vacuum degree 100 kPa, oxygen content 30 ppm) and then cooled as it was, and the brazed state was evaluated.
At that time, the amount of heat input when the temperature rises from room temperature to 550 ° C. (integrated value of the product of Zn diffusion coefficient and time during brazing heat treatment) is 6 × ^10-11 m ² , and the amount of heat input until the completion of brazing is It was set to 4 × 10 ^-10 m ^2, and cooled at a rate of 100 ° C./min from the brazing temperature to room temperature.

The potential after brazing and the element concentration on the surface of the material are affected by the element diffusion state after brazing. The element diffusion state is determined by the amount of heat input if the material specifications (additional components and amount before brazing) are determined, so the amount of heat input is specified. The amount of heat input is a parameter indicating the ease of diffusion of the element, and is indicated here by the integrated value of the product of the diffusion coefficient of Zn and the time. The diffusion coefficient is calculated by the following formula.
Diffusion coefficient = frequency factor x EXP (-activation energy / (gas constant x temperature shown in absolute temperature))
Frequency factor: 1.77 × ^10-5 (m ² / s)
Activation energy: 118 (kJ / mol)
The amount of heat input until the completion of brazing is calculated by the amount of heat input of the entire brazing process from reaching the brazing temperature to cooling to room temperature.
Further, the brazing conditions including the amount of heat input are not limited to the above conditions in the present invention, and the above conditions can be used as measurement conditions for evaluation of the clad material before brazing.

The brazing conditions are not limited to the above.
The following evaluations were performed for each test material in the examples, and the evaluation results are shown in Tables 3 and 6.

Brazing property ○ Bonding rate The bonding rate was calculated by the following formula, and the superiority or inferiority between each sample was evaluated.
Fin bonding ratio = (total brazing length of fin and tube / total contact length of fin and tube) x 100
In terms of joining ratio, 90% or more was evaluated as ◯, and less than 90% was evaluated as x.

○ Fillet length The sample cut out from the core is embedded in resin, mirror-polished, and the fillet length at the joint portion 13 joint between the fin 11 and the tube 12 is measured using an optical microscope as shown in FIG. did. The number of joints to be measured was 20, and the superiority or inferiority was evaluated by using the average as the fillet length.
In terms of fillet length, 800 μm or more was evaluated as ⊚, 700 μm or more and less than 800 μm was evaluated as XX, 600 μm or more and less than 700 μm was evaluated as XX, 500 μm or more and less than 600 μm was evaluated as ◯, and less than 500 μm was evaluated as ×.

○ Coarse primary crystal Si grains The prepared brazing sheet is embedded with resin, the cross section parallel to the rolling direction is mirror-polished, the structure is revealed with Mr. Barker's solution, and then the coarse primary crystal in the brazing material layer is observed with an optical microscope. The formation state of Si was evaluated. The observation was performed at 10 locations with a field of view of 300 μm.
The case where the number of coarse Si grains having a diameter equivalent to a circle and 30 μm or more was less than 2 was evaluated as XX, the range of 2 to 9 was evaluated as ◯, and the case where 10 or more were observed was evaluated as x.

Strength after brazing Brazing sheets were installed in a furnace in a drop format, and heat treatment equivalent to brazing was performed under the above brazing conditions. Then, a sample was cut out and a tensile test was carried out at room temperature by a normal method conforming to JIS to evaluate the tensile strength.
Regarding the strength after brazing, 190 MPa or more was evaluated as ⊚, 180 MPa or more and less than 190 MPa was evaluated as ◯, 145 MPa or more and less than 180 MPa was evaluated as ◯, and less than 145 MPa was evaluated as x.

A corrosion-resistant brazing sheet was installed in a furnace in a drop format, and a heat treatment equivalent to brazing was performed under the above brazing conditions. Then, the sample was cut into a size of 30 mm × 80 mm, masked except for the sacrificial material surface, and then subjected to a corrosion test for 60 days. In the corrosion test, a 1% NaCl aqueous solution adjusted to pH 3 was used as a corrosive solution, and the test was performed with 30 minutes of spraying and 90 minutes of wetting as one cycle (corrosion test cycle, temperature, humidity, etc. are the same as SWAAT).
In the sample after the corrosion test, the corrosion product was removed by a mixed solution of chromic phosphate, and the corrosion depth was measured by observing the cross section of the maximum corroded part.
In terms of corrosion resistance, the corrosion depth is ◎ when the corrosion depth is within 20 μm, ○○ when it is inside the sacrificial material layer, ○ ○ when it exceeds the sacrificial material layer and is within half of the plate thickness, and SWAAT among the test materials that penetrated in 60 days. In the corrosion test for 40 days, it did not penetrate, and those that penetrated were evaluated as Δ, and those that penetrated within 40 days were evaluated as ×.

Mg concentration on the surface of the sacrificial material after brazing The heat treatment equivalent to brazing is performed under the above brazing conditions, and the sample after brazing is embedded in resin and mirror-polished, and the Mg concentration on the surface of the sacrificial material is analyzed by EPMA in the cross-sectional direction. Was measured. Of the measured EPMA data, the average Mg concentration in the range of 5 μm from the surface of the sacrificial material was defined as the Mg concentration on the surface of the sacrificial material. Since the Mg concentration may be detected high on the surface of the sacrificial material due to the formation of MgO, the average Mg concentration was calculated after excluding the data in which 1.0% or more was detected.

Other evaluation items ・ Potential difference between the lowest part of the sacrificial material and the central part of the core material A sample for polarization measurement was cut out from the material subjected to the heat treatment equivalent to brazing under the above brazing conditions. After masking the surface other than the measurement surface, it is immersed in a 5% NaOH solution heated to 50 ° C. for 10 seconds, then immersed in a 30% HNO3 solution for 60 seconds, washed with tap water and ion-exchanged water, and then not dried. The natural potential (reference electrode is silver / silver chloride electrode) was measured for 120 min in a 5% NaCl aqueous solution at room temperature adjusted to pH 3 as it was under the condition of being released to the atmosphere. For the natural potential, an average value of 100 to 120 min in which the value settled down was obtained.
The central portion of the core material was previously etched with NaOH or the like to expose the central portion of the core material, and then the above measurement was performed to obtain the natural potential.

-Potential difference between the outermost surface and the lowest part of the sacrificial material A sample for polarization measurement was cut out from the material subjected to the heat treatment equivalent to brazing under the above brazing conditions. After masking the surface other than the measurement surface, it is immersed in a 5% NaOH solution heated to 50 ° C. for 10 seconds, then immersed in a 30% HNO ₃ solution for 60 seconds, washed with tap water and ion-exchanged water, and then dried. The natural potential (reference electrode is silver / silver chloride electrode) was measured for 120 min in a 5% NaCl aqueous solution at room temperature adjusted to pH 3 as it was, under the condition of being released to the atmosphere. For the natural potential, an average value of 100 to 120 min in which the value settled down was obtained.
For the lowest part of the sacrificial material, the position every 3 μm from the surface of the sacrificial material is exposed in advance by etching with NaOH or the like, and then the above measurement is performed to obtain the natural potential, and the potential distribution in the cross-sectional direction is obtained. After obtaining it, the position where the lowest natural potential was obtained was set as the lowest part of the sacrificial material.
The relationship between the potential difference between the outermost surface, the lowest portion, and the central portion of the core material of the sacrificial material is shown in the reference schematic diagram of FIG.

Although the present invention has been described above based on the above-described embodiment, the scope of the present invention is not limited to the contents of the above-mentioned description, and as long as it does not deviate from the scope of the present invention, it is appropriate for the above-described embodiment. Can be changed.

1 Aluminum alloy clad material 2 Core material 3 Wax material 4 Sacrificial material 5 Aluminum heat exchanger 6 Fins 7 Tubes 10 Other components 11 Fins 12 Tubes 13 Joints

Claims (5)

A sacrificial material is placed on one side of the core material, and Si: 6.0 to 14.0%, Mg: 0.05 to 1.5%, Bi: 0.05 in mass% on the other side of the core material. It contains ~ 0.25%, Sr: 0.0001 ~ 0.1%, the balance is composed of Al and unavoidable impurities, and the relationship of (Bi + Mg) × Sr ≦ 0.1 is established in mass% of the component content. The filling Al-Si-Mg-Bi brazing material is arranged,
The Mg-Bi compound contained in the Al-Si-Mg-Bi brazing material has a diameter equivalent to a circle of 0.1 μm or more and less than 5.0 μm in the observation in the surface layer direction before brazing. 10000 ² was present more than 20 per field, and those with a diameter of more than 5.0μm is less than two per 10000 ² field,
Further, the core material is Mn: 1.0 to 1.7%, Si: 0.2 to 1.0%, Fe: 0.1 to 0.5%, Cu: 0.08 to 1 in mass%. Contains 0.0%, the balance consists of Al and unavoidable impurities,
The sacrificial material contains Zn: 0.5 to 6.0% in mass%, the Mg content is regulated to 0.1% or less, and the Mg concentration on the surface of the sacrificial material after brazing is 0.15. % Or less, an aluminum alloy clad material. The aluminum alloy clad material according to claim 1, wherein the core material further contains Mg: 0.1 to 0.7%. The aluminum alloy clad material according to claim 1 or 2, wherein the core material further contains Ti: 0.05 to 0.3%. The natural potentials of the lowest part of the sacrificial material and the central part of the core material after brazing are lower in the lowest part of the sacrificial material, the natural potential difference is in the range of 70 to 280 mV, and the most of the sacrificial material. The aluminum alloy clad material according to any one of claims 1 to 3, wherein the potential difference between the surface and the lowest portion is within 50 mV. The sacrificial material is one or more of Si: 0.2 to 0.8%, Cr: 0.05 to 0.5%, Ti: 0.05 to 0.3% in mass%. The aluminum alloy clad material according to any one of claims 1 to 4, wherein the aluminum alloy clad material is contained.

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