WO2007131993A1

WO2007131993A1 - Flux for brazing of aluminium

Info

Publication number: WO2007131993A1
Application number: PCT/EP2007/054659
Authority: WO
Inventors: Ulrich Seseke-Koyro; Andreas Becker; Mirko Wilshusen; Sandrine Dulac; Armand Jacques Marie Gabriel
Original assignee: Solvay Fluor Gmbh; Alcan Rhenalu
Priority date: 2006-05-15
Filing date: 2007-05-14
Publication date: 2007-11-22

Abstract

Flux for brazing of aluminium The invention discloses fluxes or compositions with fluxes which comprise fluoroaluminate anions and cations of potassium, cesium and of at least one kind of metals selected from the group consisting of cerium, bismuth and lanthanum. Such fluxes can be used for brazing aluminium and aluminium alloys with magnesium. They are especially suitable for brazing parts at least one of which has a thickness of less than 0.5 mm, for example brazing sheet to form a tube.

Description

Flux for brazing of aluminium

The invention relates to a flux for brazing of aluminium, especially aluminium alloys containing magnesium and a brazing process.

Assembled parts made of aluminium or aluminium alloys can be produced by assembling and brazing respective aluminium (alloy) parts. Usually, a flux based on complex fluoroaluminates is applied, e.g. a potassium fluoroaluminate or mixtures of such fluoroaluminates. Such fluxes remove aluminium oxide and possibly impurities on the surface of the parts to be brazed which otherwise would inhibit brazing.

Fluxing agents based on potassium fluoroaluminate are very well suited to braze aluminium and aluminium alloys with a low content of magnesium. Such a process is disclosed in GB 1 438 955. The preparation of such fluxes is described, inter alia, by Willenberg, US-A 4,428,920; Meshri, US-A 5,318 764, and by Kawase, US-A 4,579,605.

For alloys of aluminium with higher magnesium content, good brazing results are achieved using a fluoroaluminate flux comprising cesium. It can be observed, however, that brazing of thin sheets (e.g. with a thickness lower than 0.3 mm), an optimal brazing result cannot be achieved.

International patent application WO 2005/092563 discloses additives which are said to improve the flow of the solder and to refine the surface of the brazed parts. As suitable additives, metals of the 2. to 5. group of the periodic system of the elements like Sr, In, Sn, Sb or Bi are mentioned, or metals of the number 21 to 30, 39 to 47 and/or 57 to 79 of the periodic system. Preferred metals are zirconium, niobium, cerium, yttrium and lanthanum. Mentioning is made that - if also cesium is incorporated - also aluminium alloys with magnesium may be brazed.

To perform brazing, the flux and a solder metal or alloy (or a precursor thereof, like silicon powder or potassium fluorosilicate) is applied to the parts to be brazed. The flux can be applied in the form of a composition, e.g. as a dispersion in water or organic solvents, as a paste, optionally together with a binder, thixotropic agents and other additives. It also can be applied electrostatically as a powder. Upon heating, first the flux melts and cleans the surface, and then the solder/filler alloy melts. After cooling, the parts are brazed. Objective of the present invention is to provide a flux which is suitable for brazing aluminum alloys, especially in the form of thin sheets. This and other objectives are solved by the fluxes of the present invention.

The flux according to the present invention is based on fluoroaluminate, comprises potassium cations and additionally comprises cesium cations and at least one metal cation selected from the group consisting of bismuth, cerium and lanthanum. In a preferred embodiment, the flux consists of fluoroaluminate anions, potassium cations, cesium cations and at least one cation selected from the group consisting of bismuth, cerium and lanthan. The term "fluoroaluminate" denotes AlF₄, AIF5 and AlF₆ anions. The term

"fluoroaluminate" also includes two or three of these anions as well as possible adducts like AlF₄-AlFs (which could be given as AI2F₉) and the like.

The flux can be prepared by mechanically mixing potassium fluoroaluminate and cesium fluoroaluminate or CsF and suitable compounds of lanthanum, cerium or bismuth. Here, the fluorides of lanthanum, cerium or bismuth are assumed to be the best compounds, though the respective oxides, hydroxides, carbonates, oxifluorides or fluorohydroxides or other compounds may provide an acceptable flux. In this case, the flux also comprises other anions than fluoroaluminate. Though the fluxes of the present invention can be prepared like that, they are preferably obtainable and obtained by co precipitation because it is assumed that they are more homogenous. This method is described later.

The content of cesium cations preferably lies in the range of 0.2 to 4 % by weight, preferably 0.5 to 2.5 % by weight, especially in the range of 0.8 to 2.0 % by weight. The content of each of the cations selected from the group consisting of bismuth, cerium and lanthanum cations preferably lies in the range between 0.2 to 4 % by weight, preferably in the range of 0.5 to 2.5 % by weight, especially in the range of 0.8 to 2 % by weight, still more preferably in the range of 0.8 to 1.5 % by weight. If more than one of Ce, Bi or La cations are comprised, they may sum up to 6 % by weight, preferably up to 4 % by weight, especially to 3.5 % by weight, still more preferably to 2.5 % by weight. The balance to 100 % by weight is constituted by the potassium cations and the fluoroaluminate anions and optionally fluoride anions. Cations and anions neutralize each other so that the total electric charge of the flux is zero. It is preferred that only one kind of cations selected from the group consisting of cerium, lanthanum and bismuth is comprised in the flux. Preferred cation from the group consisting of bismuth, cerium and lanthanum is the cerium cation. It is comprised, as given above, preferably in an amount between 0.2 and 4 % by weight. Preferred ranges are given above. The most preferred range is between 0.8 and 1.5 % by weight. A preferred flux consists of potassium, cesium, cerium and fluoroaluminate. The preferred content of Cs cations lies in the range of 0.2 to 4 % by weight. Cerium cations are comprised in 0.2 to 4 % by weight, potassium and fluoroaluminate and optionally fluoride anions being the balance to 100 % by weight. The content each of Cs and Ce cations is especially preferred in the range of 0.5 to 2.5 % by weight, still more preferably in the range of 0.8 to 2.0 % by weight. Cerium cations are especially preferred comprised in the range of 0.8 to 1.5 % by weight.

The flux according to the invention can be applied for those purposes for which fluoroaluminate fluxes are generally used. For example, it can be used for brazing parts of aluminium or aluminium alloys, especially alloys of aluminium and magnesium, such as alloys with 0.5 % by weight or more of magnesium. It is very suitable for brazing parts with a thickness of less than 0.3 mm, such as brazing sheet used for tube stock for heat exchangers. The term "parts" shall also comprise a single part to be brazed itself, e.g. a sheet two sides of which are joined together by brazing to form a tube.

In principle, the flux can be prepared as disclosed in WO 2005/092563. It was mentioned above that the flux can be prepared by mechanically mixing potassium fluoroaluminate, cesium fluoroaluminate and suitable compounds, like the fluorides, of cerium, bismuth and lanthanum. If made in such a manner, the flux also comprises other anions than fluoroaluminate. It is preferred to produce the flux by a wet process, especially by co precipitation because homogeneity of the cations is better in such kinds of processes. It is especially preferred to start with fluoroaluminium acid which in turn can be prepared from alumina or hydrates of alumina and aqueous hydrofluoric acid. Then, precipitation to form a solid comprising potassium cations and fluoroaluminate anions is initiated by adding a suitable potassium salt, usually potassium hydroxide in aqueous solution.

The cerium, bismuth and lanthanum cations, respectively, can be added at any suitable point in this process. For example, they could be added in the form of oxides, hydroxides, fluorides, metallates (e.g., bismuthates or lanthanates), fluorometallates (being the cation in the metallate or being incorporated into the metallate or fluorometallate anion), carbonates, their hydrates or in the form of other salts like nitrates or chlorides to the alumina or alumina hydrate before, during or after the reaction with hydrofluoric acid. It is preferred to add them before reaction with HF so that a co precipitate is formed. If desired, the compounds could be added to the hydrofluoric acid. This may be advantageous if the compounds have a low solubility in water.

Of course, if desired, mixtures of different cerium compounds, different bismuth compounds or different lanthanum compounds can be applied. If two or more kinds of the Ce, Bi or La cations are to be incorporated, of course mixtures of such compounds can be added. Cerium oxide, bismuth oxide and lanthanum oxide are very suitable as starting compounds.

Cesium cations are added to the reaction mixture preferably in the same way like the potassium ions. For example, cesium hydroxide or CsOH lye can be added, e.g. to the fluoroaluminium acid, to the HF or even to the aluminum or aluminum hydrate.

After precipitation, the precipitate can be dried and either applied as such or in the form of a composition. It is also possible to adapt the particle size to the desired purpose. For example, the average particle size could be shifted to bigger particles by agglomeration or to smaller particle size by grinding. If desired, fractions could be sieved out.

Starting compounds have different solubility in water or acid. So, sometimes the amount of the metal cations and the ions in the starting material may differ from the amount of respective elements as found in the precipitated product. For example, while the content of cesium should be 1 % by weight in the precipitated product, because the amount of starting materials can be calculated to provide such a product, the precipitated product may comprise less than the expected amount of 1 % cesium because part of cesium remains in solution. In such a case, it is very easy to determine the amount of starting material needed to obtain a product with desired cesium content by simple tests. The flux can be applied as such in powder form, e.g. by electrostatic application to the parts.

It also can be applied in the form of a composition. For example, it could be applied as aqueous dispersion or as a dispersion in organic solvents, like Cl to C5 alcohols with one, two or three OH groups like methanol, ethanol, n-propanol, i-propanol or glycol. Other organic solvents are suitable too, like pyrrolidones, ethers or ether alcohols, like diethylenglycolmonobutylether. Such dispersions usually comprise 10 to 75 % by weight of the flux.

The compositions may also comprise binders like ethylcellulose and then can be in the form of a paste. The compositions may also contain film-forming polymers such as polyacrylates, polyvinyls, polyamines, polyenes, polyisoprenes and the like. In this case, the composition can be applied like a lacquer. The polymers vaporize during brazing.

It is also possible to use water-soluble polymers like polyvinylalcohol as a binder. If desired, the polyvinyl alcohol can be applied as bag for the flux. This allows safe dustfree handling of the flux (and possible solid additives) during transportation and application in the form of aqueous flux dispersion.

The compositions may also comprise a solder alloy, e.g. aluminium-zinc alloy or aluminium-silicon alloy, a precursor of the solder like silicon powder or copper or alkali metal fluorosilicates. Additives may be comprised like potassium fluorozincates or potassium fiuorostannates. These additives improve the corrosion resistance because they form a zinc or tin layer during brazing.

Among other improvements as described in WO 2005/092563 like surface cleaning or activation, improvement of solder flow, reduced roughness of the solidified flux after brazing, it was observed that the fluxes according to the invention make it possible to also braze aluminium parts with high magnesium content, e.g. higher than 0.3 % by weight, preferably equal to or higher than 0.5 % by weight, e.g. up to 0.8 % by weight or even more, especially if these parts have a low thickness, e.g. 0.5 mm or less, preferably 0.3 mm or less. Another aspect of the present invention is to provide a process for brazing aluminium parts, especially parts made of aluminium with magnesium content, preferably with a content of mote than 0.3 % by weight of magnesium, more preferably with a content equal to or more than 0.5 % by weight of magnesium and higher. The process for brazing of aluminium parts, especially parts of alloys of aluminium and magnesium whereby a flux is applied to one or both of the parts to be brazed is characterized by applying a flux based on fluoroaluminate, which flux comprises potassium cations and additionally comprises cesium cations and at least one metal cation selected from the group consisting of bismuth, cerium and lanthan. In a preferred embodiment, a flux is applied which consists of fluoroaluminate anions, potassium cations, cesium cations and at least one cation selected from the group consisting of bismuth, cerium and lanthan. The term "fluoroaluminate" denotes AlF₄, AIF5 and AlF₆ anions. The term "fluoroaluminate" also includes two or three of these anions as well as possible adducts like AlF₄-AlFs (which could be given as AI2F₉) and the like. The process can be performed with a flux which optionally t also comprises fluoride anions.

In a preferred embodiment, the brazing process is performed with a flux with a content of cesium cations preferably in the range of 0.2 to 4 % by weight, more preferably in the range of 0.5 to 2.5 % by weight, especially in the range of 0.8 to 2.0 % by weight. The content of the cations selected from the group consisting of bismuth, cerium and lanthanum cations preferably lies in the range between 0.2 to 4 % by weight, preferably in the range of 0.5 to 2.5 % by weight, still more preferably in the range of 0.8 to 2.0 % by weight, most preferably in the range of 0.8 to 1.5 % by weight. If more than one of Ce, Bi or La cations are comprised, they may sum up to 6 % by weight, preferably 4 % by weight, especially to 3.5 % by weight. The balance to 100 % by weight is constituted by the potassium cations and the fluoroaluminate anions and optionally fluoride anions. Cations and anions neutralize each other so that the total electric charge of the flux is zero. Further preferred embodiments, especially concerning the content of cesium, cerium, bismuth and lanthanum.

The process is preferably performed with a homogenous flux which may be obtained, as described above, by co precipitation.

The brazing process can be performed in a known manner. The flux or flux composition can be applied to one or both parts to be joined by dry application, e.g. electrostatically. Alternatively, it can be applied in a wet process, in the form of dispersion in water or an organic solvent, or as a paste. Ingredients of such compositions are described above. The composition can be sprayed onto the parts, or by painting or dipping. Modern technologies may be applied like plasma coating or high-speed coating. It is preferred to apply the flux in a range of 3 to 7 g/m². While higher or lower values are possible, the strength of the joint may be undesirably low at lower ranges, or wasteful in view of the flux at higher values.

Additives like solder metal or solder precursors can be applied separately to the parts, or they can be comprised in the flux composition. For example, it is possible to paint a composition comprising flux and solder precursor onto the parts, if desired, drying them and start brazing directly after drying or later. In principle, only known methods to apply the heat needed for brazing can be applied, e.g. torch brazing, induction brazing or laser brazing. A preferred method to perform brazing is the method known as "controlled atmosphere brazing", shortly CAB method. This kind of brazing is performed in an oven in inert atmosphere such as nitrogen. By applying this method, very good brazing joints can be achieved.

The temperature at which brazing is performed depends from the solder or solder precursor used. The solder usually melts at higher temperatures than the flux. When the parts are heated, first the flux melts and cleans the surface, and then the solder (or the solder precursor forming the solder) melts and effects joining of the parts. It is preferred to perform brazing at a temperature between 450⁰C and 620⁰C. The pressure preferably corresponds to ambient pressure (1 Bar abs.).

A preferred embodiment of the present invention provides fort he brazing of thin parts of aluminium comprising magnesium, especially in amount of more than 0.5 % by weight. Such parts are useful to form tubes, e.g. for heat exchangers.

The following examples are intended to describe the invention further without the intent to limit it. Example 1 : Preparation of potassiumfluoroaluminate-Cs-La

Reaction :

Al(OH)₃ + 4 HF + KOH/CsOH + La₂O₃ --> potassium(cesium 1.1 %) (lanthanum 1.03 %) fluoroaluminate + 4 H₂O

Carrying out of the process :

8.330 g water and 4.351 g HF (in the form of 8.668 g of an aqueous solution with an HF concentration of 50.2 %) were mixed in a container equipped with stirrer, dropping funnel an means to provide protection against evaporation loss of hydrogen fluoride. The appropriate amount OfAl(OH)₃ was added through the dropping funnel. Lanthanum oxide was added in portions to the mixture in the container. The cesium lye was added to the potassium lye, the mixture in the reactor was heated to around 80⁰C, and the combined lye of cesium hydroxide and potassium hydroxide was added thereto. The precipitated product was stirred during a post-reaction phase of 2.5 hours. The precipitate was dried at around 200⁰C and homogenized by milling.

Yield : 7.517 g. Cesium content : 1.1 % by weight. Lanthanum content : 1.03 % by weight.

A DTA (differential thermo analysis) was performed under nitrogen for the product and showed an onset at 574°C. The extrapolated peak was found to be 596.7°C. Upon solidification of a melted sample, the point of solidification was found to be 561.2°C.

An analysis of the particle size revealed an x50 value of 3.9 μm (i.e., 50 % of the particles had a particle size up to 3.9 μm).

Example 1 was repeated with varying amounts of cesium hydroxide and lanthanum oxide to prepare potassium fluoroaluminate with the following contents (made round) of cesium and lanthanum, respectively :

Example 2 : Preparation of potassiumfluoroaluminate-Cs-Bi Al(OH)₃ + 4 HF + KOH/CsOH + Bi₂O₃ --> potassium(cesium 0.74%)(bismuth 0.72%)fluoroaluminate + 4 H₂O

Carrying out of the process :

8.340 g water and 4.351 g HF (in the form of 8.668 g of an aqueous solution with an HF concentration of 50.2 %) were mixed in a container equipped with stirrer, dropping funnel an means to provide protection against evaporation loss of hydrogen fluoride. The appropriate amount OfAl(OH)₃ was added through the dropping funnel. Bismuth oxide was added in portions to the mixture in the container. The cesium lye was added to the potassium lye, the mixture in the reactor was heated to around 80⁰C, and the combined lye of cesium hydroxide and potassium hydroxide was added thereto. The precipitated product was stirred during a post-reaction phase of 2.5 hours. The precipitate was dried at around 200⁰C and homogenized by milling.

Yield : 7.580 g. Cesium content : 0.74 % by weight. Bismuth content : 0.72 % by weight.

A DTA (differential thermo analysis) was performed under nitrogen for the product and showed an onset at 576.5°C. The extrapolated peak was found to be 597.1°C.

Upon solidification of a melted sample, the point of solidification was found to be 569.5°C.

An analysis of the particle size revealed an x50 value of 3.57 μm (i.e., 50 % of the particles had a particle size up to 3.57 μm).

Example 2 was repeated with varying amounts of cesium hydroxide and lanthanum oxide to prepare potassium fluoroaluminate with the following contents (made round) of cesium and bismuth, respectively :

Example 3 : Preparation of potassiumfluoroaluminate-Cs-Ce Al(OH)₃ + 4 HF + KOH/CsOH + CeO₂ --> potassium(cesium 0.82 %) (cerium 1.01%) fluoroaluminate + 4 H₂O

Carrying out of the process :

8.340 g water and 4.351 g HF (in the form of 8.668 g of an aqueous solution with an HF concentration of 50.2 %) were mixed in a container equipped with stirrer, dropping funnel an means to provide protection against evaporation loss of hydrogen fluoride. The appropriate amount OfAl(OH)₃ was added through the dropping funnel. Cerium oxide was added in portions to the mixture in the container. The cesium lye was added to the potassium lye, the mixture in the reactor was heated to around 80⁰C, and the combined lye of cesium hydroxide and potassium hydroxide was added thereto. The precipitated product was stirred during a post-reaction phase of 2.5 hours. The precipitate was dried at around 200⁰C and homogenized by milling.

Yield : 7.560 g. Cesium content : 0.82 % by weight. Cerium content : 1.01 % by weight.

A DTA (differential thermo analysis) was performed under nitrogen for the product and showed an onset at 574.7°C. The extrapolated peak was found to be 598.4°C.

Upon solidification of a melted sample, the point of solidification was found to be 566.5°C.

An analysis of the particle size revealed an x50 value of 4.47 μm (i.e., 50 % of the particles had a particle size up to 4.47 μm).

Example 3 was repeated with varying amounts of cesium hydroxide and oxide to prepare potassium fluoroaluminate with the following contents (made round) of cesium and lanthanum, respectively :

Brazing of sheets using fluxes

General procedure :

Angle-on-coupon samples were brazed in a glass furnace where the base alloy consisted of an Al-Mg alloy with a content of 0.5 % by weight Mg. The thickness was 0.25 mm. The brazing agent was applied as slurry to form a coating with a weight of 5 g/m² and dried. The brazing was performed in a nitrogen atmosphere at a temperature of up to 605⁰C.

After cooling the brazed samples, the joints were evaluated by considering the ratio between the total length of the angle ("V length") and the length of the brazed joint. A ratio of brazed joint length to V length of 100 % is labeled "A", a ratio of 90% is labeled "B", a ratio of 75% is labeled "C", a ratio of 50% is labeled "D", and if there were no joint, it would have been labeled "E". The following experiments and respective evaluations were performed :

Table 1 : Flux composition and observed ratio of brazed joint length/ V length

Brazability improves with higher content of Cs and the other metals. Even at a content of 1 % by weight of cesium and 0.5 % by weight of the other metals, good brazing joints are achieved. From a point of costs and reliability of the joints, fluxes comprising 1 % by weight of Cs and Ce, Bi or La seem to optimal. Brazing with a potassium fluoroaluminate flux with 1 % cesium (but no cerium, bismuth or lanthanum) failed to give an acceptable joint.

SWAAT ("Sea water acetic acid test") corrosion tests have shown that, concerning corrosion, no differences between the three fluxes and a comparison example performed with 2 % by of Cs (without any of the three other metals) were noted after 200 and 400 cycles.

For brazing, synthesis, toxicity and environmental reasons, a flux comprising 1 % by weight each of Cs and Ce is preferred.

Claims

C L A I M S

1. Flux based on fluoroaluminate, comprising potassium cations and additionally comprising cesium cations and at least one kind of metal cations selected from the group consisting of cerium, bismuth, and lanthanum.

2. Flux according to claim 1 that it consists of fluoroaluminate anions, potassium cations, cesium cations and at least one kind of metal cations selected from the group consisting of cerium, bismuth and lanthanum.

3. Flux according to claim 1, characterized in that the content of cesium in the flux is in the range of 0.2 to 4 % by weight, preferably 0.5 to 2.5 % by weight, especially preferred in the range of 0.8 to 2.0 % by weight.

4. Flux according to one of the preceding claims, characterized in that the content of cerium, bismuth and/or lanthanum lies in the range of 0.2 to 4 % by weight.

5. Flux according to claim 4, characterized that the content each of cerium, bismuth or lanthanum lies in the range of 0.5 to 2.5 % by weight, preferably in the range of 0.8 to 2.0 % by weight, still more preferably in the range of 0.8 to 1.5 % by weight.

6. Flux according to claim 1, characterized that cerium is selected as the kind of cations from the group consisting of cerium, bismuth and lanthanum.

7. Flux according to claim 1, characterized in that potassium cations and fluoroaluminate anions are the balance to 100 % by weight of the flux.

8. Flux according to one of the preceding claims, characterized in that it is substantially homogenous.

9. Flux composition, comprising 10 to 75 % by weight of the flux according to one or more of claims 1 to 8, the balance to 100 % by weight being formed by one or additives like solvent, binder, solder, precursor, thickener, and film-forming substances.

10. Process for the preparation of a flux according to one of claims 1 to 9, characterized in that fluoroaluminium acid is reacted with a potassium compound and a cesium compound to form a precipitate which comprises potassium anion, cesium anions and fluoroaluminate anions, whereby additionally a cerium compound, a bismuth compound, and/or a lanthanum compound is comprised in the aluminium acid and/or is added during the precipitation reaction with the potassium compound and the cesium compound.

11. Process according to claim 10, characterized in that the potassium and cesium compounds are mainly or completely added in the form of the hydroxides, preferably in the form of an aqueous solution.

12. Process according to claim 10 or 11, characterized in that the cerium compound, the bismuth compound and/or the lanthanum compound is added in the form of the oxides or carbonates.

13. Process according to one of claims 10 to 14, characterized in that the cerium compound, bismuth compound and/or lanthanum compound are added during the preparation of fluoroaluminium acid by reaction of alumina or alumina hydrate with hydrofluoric acid.

14. Process for the preparation of a flux according to one of claims 1 to 9, characterized in that potassium fluoroaluminate, cesiumfiuoroaluminate or CsF and a suitable cerium compound, bismuth compound or lanthanum compound, preferably the fluorides, are mechanically mixed.

15. Process for brazing two or more separate aluminium parts or an aluminium part with itself, characterized in that a flux of claims 1 to 8 or a flux composition of claim 9 is applied as brazing flux.

16. Process according to claim 15, characterized in that the aluminium of the aluminium part is an alloy of aluminium and magnesium, preferably with a content of at least 0.3 % by weight, more preferably with 0.5 % by weight or more of magnesium.

17. Process according to claim 15 or 16, characterized in that the aluminium part or at least one of the two or more aluminium parts to be brazed have a thickness of less than 0.5 mm, preferably less than 0.3 mm.