WO2003015979A1

WO2003015979A1 - Aluminium product having excellent brazing characteristics

Info

Publication number: WO2003015979A1
Application number: PCT/EP2002/009112
Authority: WO
Inventors: Ed Morley; Morten Syslak; Jan Halvor Nordlien
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 2001-08-15
Filing date: 2002-08-09
Publication date: 2003-02-27
Anticipated expiration: 2004-02-15
Also published as: NO20013961D0

Abstract

An aluminium product having excellent brazing characteristics and having at least one substantially flat surface, said flat surface at least partially having a composition, capable of providing joints in a brazing process, wherein the flat surface at least partially is coated with a composition comprising a flux powder and a synthetic resin based, as its main constituent , on methacrylate homopolymer or a methacrylate copolymer and in that the weight ratio of the flux powders to the synthetic resin is in the range from 9:1 to 1:2.

Description

Aluminium product having excellent brazing characteristics

The invention relates to an aluminium product having excellent brazing characteristics and having at least one flat surface, said flat surface at least partially having a composition, capable of providing joints in a brazing process.

Such an aluminium product is lαiown from GB-A-2.334.531 and from WO 01/38040 Al.

In WO 01/38040 there is described the use of K2SiF6 and mixtures of this with KZnF3 and/or up to 50 % potassiumfluoaluminates which applied upon the surface of an aluminium product provided a coating which upon heating to an elevated temperature provided a flux and a brazing alloy by reaction with aluminium. These so-called reactive flux coatings can provide good joint formation in controlled atmosphere brazing.

Some brazed joints cannot be developed in a reliable manner using two facing faces coated with such reactive flux without the presence of a braze cladding layer. This was partly due to the fact that the gap filling was not always guaranteed because of an inadequate amount of brazing compositions. As a result thereof air gaps may still be present in the brazed joint, reducing its strength and making it susceptible to corrosion. The use of braze clad alumin- ium product, especially in products having a flat surface is generally known and when used it is not necessary to use a reactive flux as described above but it would be sufficient to utilise a flux e.g. a flux based on up to 100% potassiumfluoroaluminates. Such flux must be applied immediately before the brazing process starts and in any case after the final shaping of the product has taken place, as such flux caoting is hard and brittle and will easily be removed from the product because of lack of adherence to the surface of the product.

It is therefor an object of the invention to provide an aluminium product as defined above, which can be provided with a surface composition which in a normal standard brazing process can be used, and which makes it possible to apply the flux immediately in the production process itself of the aluminium product. This object is achieved in that the flat surface is at least partially coated with a composition comprising a flux powder and a synthetic resin based, as its main constituent, on methacrylate homopolymer or a methacrylate copolymer, and in that the weight ratio of the flux powder to the synthetic resin is in the range from 9: 1 to 1 :2.

By using such a composition it became possible to apply the flux as a regular coating which has sufficient flexibility and adherence to the surface to withstand further handling of the product and deformation steps in order to obtain the final product.

Otherwise it has been found that it is important that the ratio between the amount of flux powder present in the coating composition compared to the amount of binder lies within defined limits. If the amount of binder becomes too high this has a negative influence on the brazing quality obtained by means of that coating composition. If the amount of binder becomes too low, the adhesion to the aluminium surface might become too low, so that the coating might easily be removed during handling of the coated product.

In view of these requirements it is preferred to use a composition in which the weight ratio of the flux powder to the synthetic resin is in the range from 9:1 to 3:2, more preferably in the range of 8: 1 to 3 : 1.

In the latter range the balance between brazing characteristics and the adhesion proved to be optimal for a wide range of applications, while at the same time the amount of binder was of such a level that with sufficient adhesion the decomposition of the binder before or during the brazing became very easy and didn't result in any hazardeous components.

In case the size of the flux powder allows so, it is preferred to use a weight ratio of the flux powder to the synthetic resin in the range from 6:1 to 5:1. This is especially true if the flux powder has a particle size of less than 5 um. Because of this small particle size it becomes feasible to use this range and it guarantees a good balance between adhesion and brazing characteristics. Otherwise and especially where the particle size of the flux powder is larger, it is preferred to use a weight ratio of the flux particles to binder in the range from 5:1 to 3:1; This higher amount of binder is required in view of the application of the caoting.

Flux powders are generally known in the art. In brazing aluminium components flux powders are used on the one hand to remove the oxide layer which is inherently present on any aluminium surface, and otherwise to wet the aluminium surface so as to allow the brazing agent to adhere to the surface. A brazing agent according to the invention is an aluminium alloy or composition having a lower melting point than the aluminium or aluminium alloy used in the components) to be brazed, or a metal or a composition which will react with the aluminium or aluminium alloy of the component(s) to be brazed so as to form an alloy or composition having such a lower melting point.

According to the invention, different types of flux powder can be used. For the purpose of this invention flux powders are divided into two categories. The first category will be named "standard flux powder". Such a standard flux owder requires the presence of a brazing agent, as defined above in order to perform the brazing operation. Typical examples of such brazing agents are Zn, Si or aluminium alloys thereof. Such a brazing agent might be present as a surface layer (i.e. a clad layer) or it might be incorporated in the caoting composition, i.e. The coating composition consists of flux powder, a brazing agent or agents and a binder,

The second category of flux powder will be named "reactive flux powder". It consists of a chemical composition or a mixture of chemical compositions which upon heating will perform the double function of generating a fluxing composition which will remove the oxyde layer and wet the aluminium surface and provide a brazing agent, commonly a composition which together with the aluminium of the component will generate an aluminium alloy with lower melting point. Standard flux powders which e.g. can be used according to the invention are the following:

Potassium fluoroaluminates such as potassium tetrafluoroaluminate (KAF ), Potassium pentafluoroaluminate (K₂A1F₅, K₂A1F₅ -H₂O) and potassium hexafluoroaluminate (K₃A1F₆). Examples of such fluxes have been described in GB-A 1,438,955, US-A 4,428,920, US-A-3,951,328, US-A 5,318,764 and US-A 4,579,605.

Oxyfluoroaluminum such as Al₂F4O and A1FO

Hydroxyfluoroaluminium such as AlF₂(OH), AlF₂(OH>H₂O and AlF(OH)₂

Fluoroborates such as potassium tetrafluoroborate (KBF₄) and sodium tetrafluoroborate (NaBF₄). Examples of such fluxes haven been described in GB-A 899,171, GB-A 1,007,039 and US-A-4,235,649.

Fluorozincates such as potassium trifluorozincate (KZnF₃), potassium tetrafluorozincate (K₂ZnF ) caesium trifluorozincate (CsZnF₃) and caesium tetrafluorozincate (Cs₂ZnF₄). Examples of such fluxes haven been described in DE-A-19,913,111 and WO-A- 99,48641.

Caesiumfluoroaluminate complexes such as caesium fluoride (CsF), caesium potassium fluoroaluminate complexes (Cs_aKbAl_cF_d), caesium hexafluoroaluminate (Cs₃AlF₆), caesium tetrafluoroaluminate (CsAlF₄, CsAlF₄-H₂O) and caesium pentafluoroaluminate (Cs₂AlF₅, Cs₂AlF₅-H₂O). Examples of such fluxes haven been described in US-A-4,670,067, US-A- 5, 171 ,377 (both caesium fluoroaluminates and caesium fluorides), US-A-5,806,752

(Casium fluoride), US-A- 5,771,962 (Casium fluoride) and US-A- 4,655,385(both caesium fluoroaluminates and caesium fluorides).

Otherwise so-called superfluid fluxes can be used as well.

Furthermore it is possible to use combinations of standard flux powders and reactive flux powders in the same coating composition.

Alkali metal fluorosilicates such as caesium hexafluorosilicate (Cs₂SiF₆), potassium hexafluorosilicate (K₂SiF₆), lithium hexafluorosilicate (Li₂SiF₆), rubidium hexafluorosilicate (Rb₂SiFβ), sodium hexafluorosilicate (Na₂SiFδ) and ammonium hexafluorosilicate ((NH₄)₂SiF₆). Examples of such fluxes haven been described in US-A-5,785,770, DE-A- 19,636,897, US-A-5,985,233, US-A-6,019,856, US-A- 5,980,650 and WO-A-98,10887.

Alkali bimetal fluorosilicates such as potassium caesium hexafluorosilicate (KCsSiF₆), lithium caesium hexafluorosilicate (LiCsSiF₆), rubidium caesium hexafluorosilicate (RbCsSiFδ), rubidium potassium hexafluorosilicate (RbKSiFβ) and ammonium caesium hexafluorosilicate (NH4CsSiF₆).

Alkali metal bifluorosilicates (also referred to as alkali metal hydrofluorosilicates) such as caesium hydrofluorosilicate (CsHSiFβ), potassium hydrofluorosilicate (KHSiFβ), lithium hydrofluorosilicate (LiHSiFβ) and ammonium hydrofluorosilicate (NFMHSiFβ). As for the synthetic resins to be used in the present invention, those mainly comprising a homopolymer of a methacrylate or a copolymer of two or more methacrylates are preferred. Given as specific examples of such a methacrylic acid ester are methyl methacrylate, ethyl methacrylate, propyl methacrylate, 2-methylpromyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, 2-etylhexyl methacrylate, octyl methacrylateisodecyl methacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, diathylaminoethyl methacrylate, 2-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, t-butylaminoethyl methacrylate, glycidyl methacrylate, metaterahydrofurfuryl methacrylate, and the like.

The coating can be applied by different techniques, such as by means of a roll coating technique, whereupon the applied coating is heated and dried to evaporate the organic solvent in the mixed flux composition. Roll-transfer printing can also be used as an applica- tion method but roll coating is preferred since it offers superior control of the quality of the deposited layer.

The coating thickness is dependent upon the flux powder used, and when applicable also the way the brazing agent is present.

In case the coating consists of a binder and a standard flux powder, the coating thickness is applied in quantities of 3-20 g/m² If the coating quantity is below 3 g/m² the problem arises that local insufficiency of the flux powder will occur resulting in an inadequate brazing quality. More than 20 g/m² is not needed and it is only a waist of expensive flux powder.

For this combination of binder and standard flux powder it is preferred to use a coating quantity of 4-15 g/m², most preferably of 4-8 g/m².

As there is no brazing agent present in the coating of this situation, the brazing agent can either be added by applying the coating to a surface which has been clad with a brazing agent, or the coating has been applied to an unclad aluminium surface which will be brazed to a surface which has been clad with a brazing agent.

In case the coating consists of a binder and a reactive flux powder, the coating is applied in quantities of 12-30 g/m².

In this situation the coating quantity needs to be somewhat larger as the brazing agent must be generated by the coating composition. The upper and the lower limits are to some extend defined by the ration between the flux powder and the binder. But within this range sufficient brazing agent will be generated while at the same time the fluxing function will be performed adequately.

For this combination of binder and reactive flux powder it is preferred to use a coating of 15-25 g/m², most preferably 18-22 g/m².

In case the coating consists of a binder , a standard flux powder and a brazing agent the coating is applied in quantities of 8-78 g/m².

In this case the brazing agent might be the critical factor as it si necessary to have a minimum presence. If the amount of brazing agent becomes too low, local insufficiency may easily occur resulting in a poor brazing joint. If otherwise the amount of brazing agent becomes too high this might result into local corrosion problems. For this combination of binder, flux powder and brazing agent it is preferred to use coating quantities of 8-25 g/m², most preferably 10-15 g/m².

It must be remarked that in the latter combination the flux powder might consist either of a reactive flux powder, a standard flux powder or mixtures thereof.

Preferably the synthetic resin has a a composition which volatilizes in the range of 200-450°C, preferably below 400°C.

In this way a coating is obtained the binder of which will evaporate at the temperatures normally used in the brazing process, so that the binder cannot negatively influence the characteristics of the obtained brazing joint.

In a most preferred embodiment the synthetic resin comprises at least 10 % by weight of a resin commercially available as Polaroid B 48 of Rohm & Haas.

It has been found that this resin fulfils all requirements for a satisfying the coating, especially with respect to ease of use while applying the coating. Moreover no hazardeous or dangereous fumes are generated during the brazing process, and no negative influences have been found with respect to the environment.

The invention also relates to an aluminium product having excellent brazing characteristics and having at least one flat surface, wherein the flat surface is either provided with a coating in accordance with the one described above.

It is also an objection of this invention to provide a product provided with a coating having a reduced amount of flux powder which is sufficient to provide the flux and brazing composition for the braze joint to be made.

This object has been obtained in that the coating is applied in the form of a pattern.

In this way the coating can be applied in such an overall quantity that the average coating thickness is equal to the amount actually needed to provide the required brazing. Otherwise it becomes possible to apply the caoting only to these areas where the brazing is actually done, thereby reducing the amount of flux and brazing composition to be used.

In accordance with the invention such pattern type coating can easily be applied by means of the conventional coating methods, which allow the application of pattern by using adapted coating application implements.

Preferably the pattern is composed of stripes and/or dots.

By using such a partial coating with a regular pattern it becomes possible to use standard coating techniques as are common in the art.

The invention also relates to a process of making a shaped product consisting of aluminium or an aluminium alloy comprising the steps of - producing an intermediate object by extrusion or rolling and having at least one flat surface providing at least one flat surface with a coating according to the invention as described above, reshaping of the intermediate object into the shaped product, and - heating the shaped product to a temperature sufficient to provide at least one brazed connection between defined parts of the shaped products.

Examples of such products are described PCT/EP01/04981.

The process is different dependent upon the type of coating used. In case the coating consists of a reactive flux powder and binder or a flux powder, a binder and a brazing agent, it can be performed as such. In case the coating consists of a standard flux powder and a binder at least a portion of the intermediate object has to be provided with a clad of a brazing agent.

A first example is so-called wrapped tube, which has been obtained by producing a flat shaped object by e.g. rolling, and subsequently bending this flat object so as to obtain a substantially cylindrical shape with overlapping side portions. At least one part of the overlapping side portions has been coated according to the invention while the object is in its flat shape, so that the coating can be applied in a very simple way by means of conventional coating systems, such as e.g. described in PCT/EP99/09162. In case the wrapped tube is intended to be used as a header tube or manifold in a heat exchanger it is possible to provide the side portions of the flat shaped object with recesses or cut-outs, which after shaping into a wrapped tube form holes for the end portions of heat exchanging tubes. Also for this purpose the outer wall portion may be coated according to the invention before bending. When subsequently the obtained tube and if present the inserted heat exchanging tubes are placed in a brazing oven, or the like, no additional flux application is required and a brazed connection between the overlapping side portions and between the tubes is directly obtained because of the presence of the braze flux containing coating.

In another embodiment a wrapped tube is obtainable by means of the process according to the invention. This type of tube may be especially useful in high pressure applications such as brake lines or hydraulic applications. The intermediate object is a flat sheet which at least at one side is provided with a braze flux retaining coating. Subsequently the flat sheet is wrapped to a cylindrical tube whereby a number of layers are formed so that the wall of the tube is thicker than the thickness of the flat sheet. Preferably two or more layers are formed, dependent upon the application of the final product. In a last step the wrapped tube is heated to a temperature above the required braze temperature, which is dependent upon the flux used whereby as a result of the presence of the flux retaining coating between the layers of the wrapped tube a firm connection between the successive layers can be obtained.

In another embodiment folded multiple port flat tubes have been made by means of a coating according to the invention. The intermediate object for such tubes is a flat sheet of rectangular shape. After coating at least one side of the sheet according to the invention the sheet is folded into the desired shape and subsequently heated in order to obtain the required brazed connections.

Tubes obtained by these production methods can used either as heat exchanging tubes or as manifolds. It is obvious that the invention is not restricted to the disclosed embodiments, but that within the scope of the claims it is possible to apply modifications.

Claims

1. An aluminium product having excellent brazing characteristics and having at least one substantially flat surface, said flat surface at least partially having a composition, capable of providing joints in a brazing process, characterized in that the flat surface at least partially is coated with a composition comprising a flux powder and a synthetic resin based, as its main constituent, on methacrylate homopolymer or a methacrylate copolymer and in that the weight ratio of the flux powders to the synthetic resin is in the range from 9: 1 to 1 :2.

A product according to claim 1, characterized in that the weight ratio of the flux powders to the synthetic resin is in the range from 9: 1 to 3:2.

3. A product according to claim 2, characterized in that the weight ratio of the flux powders to the synthetic resin is in the range from 8 : 1 to 3 : 1.

4. A product according to claim 3, characterized in that the weight ratio of the flux powders to the synthetic resin is in the range from 6: 1 to 5: 1.

5. A product according to claim 3, characterized in that the weight ratio of the flux powders to the synthetic resin is in the range from 5:1 to 3:1.

6. A product according to anyone of the claims 1 - 5, characterized in that in case of a coating consisting of binder and a standard flux powder, the coating is applied in quantities of 3-20 g/m² .

7. A product according to claim 6, characterized in that the coating is applied in quantities of 4-15 g/m², preferably 4-8 g/m² .

8. A product according to claim 6 or 7, characterized in that the coating is applied to a material clad with a brazing composition.

9. A product according to anyone of the claims 1 - 5, characterized in that in case of a flux coating consisting of binder and a reactive flux powder, the coating is applied in quantities of 12-30 g/m² .

10. A product according to claim 9, characterized in that the coating is applied in quantities of 15-25 g/m², preferably 18-22 g/m² .

11. A product according to anyone of the claims 1 - 5, characterized in that in case of a flux coating consisting of binder, a flux powder and a brazing agent, the coating is applied in quantities of 8-78 g/m² .

12. A product according to claim 11, characterized in that the coating is applied in quantities of 8-25 g/m², preferably 10-15 g m² .

13. A product according to claim 1 - 5, characterized in that the coating consists of a binder, a reactive flux powder, a standard flux powder and a brazing agent in particu- late form.

14. A product according to anyone of the claims 1 - 7 or 11 - 12, characterized in that the flux contains a fluoroaluminate of one or more alkaline metals, or other fluorcom- plexes or mixtures thereof.

15. A product according to claim 14, characterized in that the alkaline metal is selected from the group of potassium, caesium, rubidium, or mixtures thereof.

16. A product according to anyone of the claims 1 - 15, characterized in that the synthetic resin has a composition which volatilizes in the range of 200-450° C, preferably below 400°C.

17. A product according to claim 16, characterized in that the synthetic resin comprises at least 10 % by weight of a resin commercially available as Paraloid B 48 of Rohm & Haas.

18. An aluminium product according ot anyone of the claims 1 - 5 or 9 - 10, characterized in that the flux coating is a fluorcomplex with potassiumfluorosilicates applied on top of an unclad aluminium surface.

19. An aluminium product having excellent brazing characteristics and having at least one flat surface, wherein the flat surface is provided with a coating in accordance with anyone of the preceding claims, characterized in that the coating is applied in the form of a pattern.

20. A product according to claim 19, characterized in that the pattern is composed of stripes and/or dots.

21. A process of making a shaped product consisting of aluminium or an aluminium alloy comprising the steps of : - producing an intermediate object by extrusion or rolling and having at least one substantially flat surface providing at least one flat surface with a coating according to anyone of the claims 1 - 5 or 8 - 20, reshaping of the intermediate object into the shaped product, and - heating the shaped product to a temperature sufficient to provide at least one brazed connection between defined parts of the shaped products.

22. A process of making a shaped product consisting of aluminium or an aluminium alloy comprising the steps of : - producing an intermediate object by extrusion or rolling and having at least one substantially flat surface providing at least one flat surface with a coating according to anyone of the claims 6 - 7, or anyone of the claims 14 - 17 or 19 - 20 as far as these are dependent upon claims 6 - 7, - providing at least a portion of the intermediate obj ect with a clad of a brazing composition, reshaping of the intermediate object into the shaped product, and heating the shaped product to a temperature sufficient to provide at least one brazed connection between a coated part and a clad part of the shaped product.

23. A process according to anyone of the claims 21 - 22, characterized in that the reshap- ing at least involves a bending operation of at least one part of the flat surface of the intermediate product

24. A process according to any one of the claims 21 - 23 characterized in that the product is a folded tube.

25. A process according to any one of the claims 21 - 23, characterized in that the product is a welded tube.

26. A process according to any one of the claims 21 - 23, characterized in that the product is a wrapped tube.

27. A process according to anyone of the claims 21 - 22, characterized in that the product is a manifold tube.