ALUMINIUM ALLOYS AND THEIR USE

This application claims priority to European Application No. 17206208.5 the whole content of this application being incorporated herein by reference for all purposes.

The present invention concerns compositions comprising a flux, a metal additive, and a specific binder, aluminum or aluminum alloy parts at least partially coated with the flux composition manufactured by the process, and a brazing method.

Inorganic fluorides and fluoride complexes are well known to be suitable as a flux for brazing parts of aluminum and aluminum alloys to parts of aluminum and aluminum alloys, and even to parts of copper, steel and titanium. Especially useful are potassium fluoroaluminates, optionally comprising cesium and lithium fluoroaluminates. See US-A-4 670,067 and EP-A-2671670. Cesium containing mixtures are especially suitable for fluxing parts of aluminum alloyed with Mg, lithium containing mixtures improve the resistance of brazed parts against corrosion. Other suitable fluxes are zinc fluoride, potassium

fluorozincates, potassium fluorostannates, potassium fluorosilicate and other such inorganic compounds. The fluxes may be applied to the parts to be brazed as a wet composition which comprises the flux, a solvent and a binder and often also other additives. While the flux removes undesired oxide layers on the parts to be brazed, solder metals, solder alloys or solder alloy precursors provide a reliable joint between the parts.

One object of the present invention is to provide a composition useful as flux especially for brazing of parts of aluminum or of aluminum alloyed with Mg to parts of aluminum, aluminum alloy, especially Al-Mg alloy, steel, copper and titanium which provides joints protected against corrosion. It is further an object of the present invention to provide parts at least partially coated with the composition, and a brazing method using the composition of the present invention. These and other objects are provided by the invention as outlined in the description and claims.

One aspect of the invention relates to a flux composition. The flux composition for brazing of aluminum and aluminum alloys comprises a fluoride salt based flux; at least one metal additive selected from the group consisting of solder metal, solder alloy, Si, Ge and Cu; a binder; and a solvent wherein the solvent comprises at least one diester of formula (I) R ¹ 0-C(0)-(CH ₂ ) _X -C(0)0R ² (I)

wherein R ¹ denotes Cl to C5 alkyl, R ² denotes Cl to C5 alkyl, and x is an integer 1 to 6, or a mixture of diesters of formula (I).

Suitable as diesters (dibasic esters) are the diesters of malonic acid (x = 1), succinic acid (x = 2), glutaric acid (x = 3), adipic acid (x = 4), pimelic acid (x =

5) and subaric acid (x = 6). The composition may also contain minor amounts of diesters of dibasic acids with a C7 or C8 chain or diesters unsaturated dibasic acids such as maleic acid and fumaric acid.The term“minor amounts” preferably denotes a total content of these C7, C8 or unsaturated diesters of equal to or lower than 5 % by weight, relative to the total weight of the composition.

Preferably, in diesters of formula (I), R ¹ and R ² are the same or different and denote methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl or t-butyl.

Preferably, x is 2, 3 or 4, including mixtures of these selected diesters. Preferred diesters are dimethyl succinate, dimethyl adipate, dimethy glutarate or mixtures of the foregoing. Solvent systems containing such diesters and which can preferably be used according to the present invention include Santosol®

(available from Cytec), Dowanol® products or Rhodiasolv products, such as RHODIASOLV® RPDE (available from Solvay).

The flux composition preferably comprises two or more diesters of formula

(I). Still more preferably, the flux composition comprises two or more diesters selected from the group consisting of diesters wherein x is 2, 3 or 4.

Especially preferably, the solvent comprises a mixture of two or more diesters of formula (I) selected from the group consisting of diesters wherein x is 2, 3 or 4, and R ¹ and R ² are the same or different and denote methyl or ethyl.

If desired, the flux composition may contain minor amounts of water and/or organic liquids other than diesters. The term“minor amounts” preferably denotes a range of from greater than 0 to equal to or less than 20 % by weight, preferably equal to or less than 5 % by weight, relative to the total weight of the solvent in the composition. In other words, in the flux composition according to the invention, the diester of formula (I) constitutes equal to or more than 80 %- wt of the total solvent content set to 100 %-wt, still more preferably, equal to or more than 95 %-wt of the total solvent content set to 100 %-wt, and most preferred, the diesters mentioned above are the only solvent.

If minor amounts of one or more of other organic liquids are present, they are preferably selected from the group consisting of mono-, di- or tribasic aliphatic alcohols, especially those with 1 to 4 carbon atoms, e.g. methanol, ethanol, isopropanol, alkylene glycols, e.g. ethylene glycol, or glycol alkyl ethers, wherein alkyl preferably denotes linear C 1 to C4 alkyl or branched aliphatic C3 to C4 alkyl, including methyl, ethyl, iso-propyl, n-propyl, n-butyl, iso-butyl, sec-butyl, and tert-butyl. Non-limiting examples are glycol monoalkyl ethers, e.g. 2-methoxyethanol or diethylene glycol, or glycol dialkyl ethers, for example, dimethyl glycol (dimethoxy ethane). Mixtures comprising two or more of the liquids are also suitable.

Suitable fluxes are known in the art of brazing. Alkali metal fluxes of fluoroaluminates, fluorozincates, fluorosilicates and fluorostannates and combinations thereof with zinc fluoride, stannum fluoride are especially suitable. Preferably, the term“alkali metal” denotes here potassium and lithium.

The term“solder metal” or“solder ahoy” according to the present invention intends to denote a metal or alloy used to create a permanent bond between metal workpieces, and is often used for brazing processes at

temperatures of equal to or higher than 450°C.

Preferably, the fluoride salt based flux in the composition of the invention is selected from the group consisting of potassium fluoroaluminate, lithium fluoroaluminate, cesium fluoroaluminate, potassium fluorozincate, potassium fluorostannate, potassium fluorosilicate, zinc fluoride, stannum fluoride, and mixtures of two or more thereof.

In the flux composition of the invention, especially preferably, the flux is selected from the group consisting of potassium fluoroaluminate zinc fluoride or a combination of zinc fluoride and potassium fluoride, and potassium

fluorozincate, and most preferably, the flux is potassium fluorozincate.

If fluxes other than potassium fluorozincate are contained, the flux composition may also contain elemental zinc, preferably in powder form.

The term“potassium fluoroaluminate” denotes KAlF ₄ , K ₂ AIF ₅ ,

K ₂ AIF ₅ H ₂ O, and K ₃ AlF ₆ .

Often, the composition comprises a potassium fluoroaluminate selected from the group consisting of KAlF ₄ , K ₂ AIF ₅ , K ₂ AIF ₅ H ₂ O, and mixtures thereof. Optionally, one or more of the other inorganic fluorides and fluoride complexes given above may additionally be comprised.

Generally, the content of K ₃ AIF ₆ in the fluoroaluminate mixture is equal to or lower than 5% by weight; preferably, the content of K ₃ AlF ₆ is equal to or lower than 2% by weight; and especially preferably, the content of K ₃ AlF ₆ is equal to or lower than 1% by weight, including 0 %.

The composition is especially useful as flux for brazing of parts of aluminum or aluminum alloy to other parts of aluminum, of aluminum alloys, of steel, of copper or of titanium.

In one embodiment, the potassium fluoroaluminate is essentially present in the form of KAlF ₄ . In another embodiment, the potassium fluoroaluminate in the fluoroaluminate mixture is essentially in the form of K ₂ AIF ₅ , K ₂ AIF ₅ H ₂ O, or mixtures thereof. In still another embodiment which is an especially preferred one, the potassium fluoroaluminate is present in the form of mixtures comprising KAIF ₄ and at least one of K ₂ AIF ₅ and K ₂ AIF ₅ H ₂ O. In a very preferred embodiment, the potassium fluoroaluminate is present in the form of mixtures comprising KAlF ₄ and K ₂ AIF ₅ .

In this embodiment, the flux preferably comprises KAIF ₄ and at least one of K ₂ AIF ₅ and K ₂ AIF ₅ H ₂ O wherein the sum of KAIF ₄ and at least one of K ₂ AIF ₅ and K ₂ AIF ₅ H ₂ O is set to 100 wt-%, the content of KAIF ₄ is preferably equal to or greater than 70 wt-%; the content of KAIF ₄ is preferably equal to or lower than 90 wt-%. In a highly preferred embodiment, the potassium fluoroaluminate is present in the form of mixtures comprising KAIF ₄ and K ₂ AIF ₅ in a weight ratio of KAlF ₄ :K ₂ AlF ₅ = 75:25 to 85:15.

For brazing of aluminum alloys containing Mg, a cesium fluoroaluminate may be present. Here, the content of CsAlF ₄ , CS ₂ AIF ₅ or CS ₃ AIF ₆ is preferably equal to or lower than 3%, more preferably equal to or lower than 2% by weight of the total fluoroaluminate content set as 100% by weight.

In another embodiment, which is the preferred one, the flux comprises or consists of potassium fluorozincate.

The particle size is of the flux often in a range of from 0.1 pm to 100 pm. preferably, most of the particles have a particle size in a range of from 0.2 pm to 60 pm. Other solid constituents are preferably in the same particle size range or close to it. Preferred ranges for solder metal, solder alloy and/or solder alloy are given further below. In a particularly preferred aspect, the flux has a particle size distribution of X50 from 1 to and including 7 pm, preferably X50 from 1 to and including 5 pm. It is advantageous if the particle size distribution of the flux has a X90 value of equal to or lower than 25 pm, preferably equal to or lower than 23 pm. The respective X values as reported in the present invention are determined by laser diffraction. They can be measured, for example, in a Sympatec Helos particle size analyzer, e.g. model Helos H2068. The analysis refers to secondary particles.

The flux composition contains a binder. Preferably, binders are selected from the group consisting of organic polymers. Such polymers are physically drying (i.e., they form a solid coating after the liquid is removed), or they are chemically drying (they may form a solid coating e.g. under the influence of chemicals, e.g. oxygen or light which causes a cross linking of the molecules), or both. Suitable polymers include polyolefines, e.g. butyl rubbers, polyurethanes, resins, phthalates, polyacrylates, polymethacrylates, vinyl resins, epoxy resins, and nitrocellulo se .

More preferably, the binder is a polyacrylate ester.

Besides flux, binder and solvent, the composition of the invention may further comprise additives.

For example, the composition may comprise additives which improve the brazing. For example, they may contain a solder metal, e.g. in the form of powder or small particles. This may be unnecessary if solder-plated parts are brazed. The composition may additionally or instead comprise elemental Si, Ge or Cu which function as solder metal precursor as described in US-A 5,190,596. It is assumed that the precursors form Al-Si solder, Ge-Al solder or Cu-Al solder during brazing.

The composition may comprise Li compounds as additive as disclosed in WO2011/098120 and W02010/060869. Suitable sources of Li ions as additives are, for example, LiF, Li ₃ AlF ₆ , LiOH, Li oxalate or Li ₂ C0 ₃ . Flux mixtures containing Li ions often display reduced corrosion of their brazing residue.

The composition may also comprise alkaline earth metal compounds such as CaF ₂ , CaC0 ₃ , MgF ₂ , MgC0 ₃ , SrF ₂ , SrC0 ₃ , BaF ₂ , BaC0 ₃ , and mixtures of two or more of said second components which also provide reduced corrosion as disclosed in WO 2015/13595.

The composition may comprise compounds, especially salts, of strontium, indium, tin, antimony, bismuth, zirconium, niobium, cerium, yttrium, titanium or lanthanum, e.g. the respective halides, nitrates, carbonates or oxides or any mixture thereof. Such additives can be added during manufacture of the flux mixture or after its manufacture. Often, the respective fluorides, such as TiF ₄ , ZrF ₄ , CeF ₃ , CeF ₄ , YF ₃ or alkaline fluorometallates, e.g. K ₂ ZrF ₆ or K ₂ TiF ₆ , are added after manufacture as disclosed in WO 2005/092563 and WO

2007/131993. As mentioned above, the preferred flux is potassium fluorozincate. In this embodiment, and especially in embodiments wherein other fluxes than potassium fluorozincate are present, the composition may additionally contain elemental zinc, preferably in powder form. Here, the content of zinc is preferably equal to or greater than 0, more preferably, equal to or greater than 2 % by weight, and the content of zinc is preferably equal to or lower than 10 % by weight, more preferably, equal to or lower than 8 % by weight; all percentages relative to the total weight of the composition.

The composition may also comprise additives which improve handling of the compositions. For example, the composition may comprise at least one additive selected from the group consisting of surfactants, thixotropic agents, defoaming agents, thickeners, biocides, coloring agents, antioxidants, corrosion inhibitors, and suspension stabilizers.

As antifoaming agent, e.g. silicon oil, glycerine, or preferably propylene glycol may be present. As a biocide, for example, biocides based on 5-chloro-2- methyl-4-isothiazolin-3-one and/or 2-methyl-4-isothiazolin-3-one is suitable. As antioxidant (e.g., BHT, butyl hydroxytoluene), may be applied. As corrosion inhibitor, benzotriazol may be used. A suitable suspension stabilizer is for example methyl cellulose. Suitable thixotropic agents are for example gelatin and pectin. Suitable thickeners are, for example, waxes, hardened oil, e.g.

hardened castor oil, fatty acid amides and polyamides, as described in US-A 8,075,706, and ethers, e.g. methyl butyl ether. Preferred thickeners are ethylene bis-stearamide waxes. Such waxes are available, for example, from Munzing Chemie GmbH under the tradename Ceretan ®, e.g. Ceretan7006, 7008 and 7008. These are micronized waxes with low particle sizes (in the micrometer range). Preferred compositions comprise a thickener, especially a wax thickener, preferably an ethylene bis stearamide wax.

Surfactants (also known as wetting agent), if present, provide uniform coating, and compensate to some extent for surfaces not cleaned prior to application.

In the following, preferred ranges are given for the constituents for the flux. The percentages refer to the total weight of the composition (including solder metal, solder alloy or solder precursor, flux, solvent and any other ingredients) set as 100 % by weight. To be noted that all constituents add up to 100 % by weight. Preferably, the flux is contained in the composition in an amount of equal to or higher than 1 % by weight, more preferably equal to or higher than 5 % by weight. Preferably, it is contained in an amount of equal to or lower than 60 % by weight, more preferably equal to or lower than 50 % by weight. A preferred range is thus from equal to or higher than 1 % by weight to of equal to or lower than 60 % by weight, more preferably in a range of from equal to or higher than 5 % by weight to of equal to or lower than 50 % by weight. If two or more fluxes are present, then the figures given here relate to the sum of fluxes.

Solder metal, solder alloy metal (such as Al-Si alloys) or solder alloy precursor (such as Si, Ge or Cu) are preferably contained in the composition in an amount of equal to or higher than 5 % by weight, more preferably equal to or higher than 5 % by weight. Preferably, solder metal, solder alloy metal or solder alloy precursor is contained in an amount of equal to or lower than 60 % by weight, more preferably equal to or lower than 50 % by weight. A preferred range is thus from equal to or higher than 5 % by weight to of equal to or lower than 60 % by weight, more preferably in a range of from equal to or higher than 10 % by weight to of equal to or lower than 50 % by weight. If a combination of two or all three of the group consisting of solder metal, solder alloy metal and solder alloy precursor is contained, then the sum of both or all three are preferably in the amounts and the ranges given here. A preferred solder material is AlSil2.

The solder metal, solder alloy and/or solder alloy precursor preferably has a particle size distribution of X50 from 16 to and including 25 pm, preferably X50 from 17 to and including 23 pm, and X90<45pm, preferably X90 from 39 to and including 55 pm, more preferably X90 from 40 to and including 51 pm.

As mentioned above, in one embodiment, the composition may also contain Li compounds, alkaline earth metal compounds organic compounds or salts of strontium, indium, tin, antimony, bismuth, zirconium, niobium, cerium, yttrium, titanium or lanthanum, e.g. the respective halides, nitrates, carbonates or oxides or any mixture thereof. Such kind of additive may be present in a range of from equal to or greater than 0, preferably, equal to or greater than 1 % by weight. Preferably, such kind of additive is present in an amount of equal to or lower than 10 % by weight. If two or more of such additives (Li compounds, alkaline earth metal compounds organic compounds or salts of strontium etc.) are present, then the figures given here relate to their sum. In another embodiment, the composition is free of such additives (Li compounds, alkaline earth metal compounds organic compounds or salts of strontium etc.).

The binder is preferably present in an amount of equal to or higher than 2 % by weight, more preferably, equal to or higher than 5 % by weight.

Preferably, the binder is contained in an amount of equal to or lower than 40 % by weight, more preferably equal to or lower than 25 % by weight. A preferred range is thus from equal to or higher than 2 % by weight to of equal to or lower than 40% by weight, more preferably in a range of from equal to or higher than 5 % by weight to of equal to or lower than 25 % by weight. If two or more binders are present, then the figures given here relate to the sum of binders.

The diester is preferably present in an amount of equal to or higher than 5 % by weight, more preferably, equal to or higher than 10 % by weight, still more preferably, equal to or higher than 15 % by weight. Preferably, the diester is contained in an amount of equal to or lower than 60 % by weight, more preferably equal to or lower than 50 % by weight, still more preferably in an amount of equal to or lower than 40 % by weight. A preferred range is thus from equal to or higher than 5 % by weight to of equal to or lower than 60 % by weight, more preferably in a range of from equal to or higher than 10 % by weight to of equal to or lower than 50 % by weight, still more preferably in a range of from equal to or higher than 15 % by weight to of equal to or lower than 40 % by weight. Often, as stated above, the diester solvent is composed of two or three or even more dibasic esters, and the figures given here relate to the sum of dibasic ester compounds.

The composition may, as mentioned above, contain additionally water and/or organic liquids other than the diester or diesters. Preferably, their amount is 0. Preferred percentages of diester solvent in case water and/or other organic liquids are present are given above. Preferably, the upper and lower limits and ranges for the diester content given above apply also for the total solvent content, i.e. the sum of diester and additionally contained water and/or other organic solvent.

The composition may further contain other additives, e.g. surfactants, thixotropic agents, defoaming agents, thickeners, biocides, coloring agents, antioxidants, corrosion inhibitors, and/or suspension stabilizers. If present, the amount of additive is in a range of from equal to or greater than 0 to equal to or lower than 10 % by weight, preferably equal to or lower than 5 % by weight. If two or more than 2 additives given here are contained, the figures relate to the sum of them.

A further aspect of the present invention concerns aluminum parts or aluminum alloy parts, coated with the composition according to the present invention. These parts preferably are parts used to produce heat exchangers, e.g. tubes and fins, useful for coolers or air conditioning systems of cars or lorries, or for producing“HVAC” apparatus. HVAC means heating, ventilation, air conditioning - the technology of indoor environmental comfort.

Another aspect of the present invention concerns assembled parts of aluminum or aluminum alloys brazed using a composition according to the present invention. These parts are, preferably, parts used in transferring heat from one medium to another medium. Preferably, these parts are used in heat exchangers or cooling systems of cars and lorries and in HVAC apparatus.

Generally, the parts coated with the wet composition are dried (this is of course not necessary in parts coated according to the dry method unless applies fluoroaluminate hydrates and wants to remove crystal water before starting the brazing process). Drying can be performed independently from brazing; the dried coated parts can then be stored until they are brazed. Alternatively, drying can be performed directly in the brazing apparatus or in a separate drying apparatus just before the brazing operation.

The weight per area of the flux is preferably equal to or higher than 3 g/m2. More preferably, it is equal to or higher than 5 g/m2. Preferably, it is equal to or lower than 40 g/m2.

A process for brazing of parts made from aluminum or aluminum alloys wherein the wet composition comprising is coated on at least one of the parts to be brazed, and the parts are heated to a temperature sufficiently high to braze the parts, is also an aspect of the present invention.

The brazing is performed in a manner well known to the expert. The solder metal or solder metal precursor is contained in the composition, and thus, parts to be brazed need not be plated with solder metal. At least one of the parts to be brazed is coated at least partially with the composition; the parts are then assembled and heated to fluidize first the flux mixture and then the solder metal. If the coated parts have not yet been dried as described above, solvent and other constituents with respectively low boiling point or decomposition point vaporize during the process of heating the parts to the brazing temperature. For brazing, the coated parts to be joined by brazing are assembled (before or after drying if coated according to a wet process) and heated to about 570°C or higher, preferably up to about 600 °C or even higher, e.g. up to

6 lO°C, especially if potassium fluoroaluminate fluxes optionally containing cesium fluoroaluminate or lithium fluoroaluminate as additive. Brazing can be performed in an inert gas atmosphere, e.g. in a nitrogen or argon atmosphere. It is also possible to braze in open air (torch brazing). A preferred method for brazing of parts of aluminum, aluminum alloy, steel, copper and titanium with parts of aluminum, aluminum alloy comprises steps wherein at least one of the parts to be joined is coated at least partially with a mixture of the invention, a composition of the invention, or both, the parts to be joined are assembled, and heated to a temperature of equal to or higher than 570°C.

Preferably, a flux composition is applied comprising the fluoride salt based flux, solder metal, solder alloy or solder metal precursor as described above, the binder and the solvent wherein the solvent comprises or consists of the diester of formula (I) or a respective mixture of diesters. Preferably, a solder alloy, especially based on Al and Si, is applied.

The advantage of the composition is that in one step, flux for oxide removal, solder metal, solder metal alloy or solder alloy precursor are applied in one step to the parts to be brazed and zinc as corrosion protecting agent is preferably included or formed from potassium fluorozincate. Another important advantage is the low flammability of the composition.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The following examples are intended to explain the invention in further detail without wishing to limit. Example 1: Manufacture of a composition

Aluminum-silicon alloy powder (D ₅₀ = 18 pm), KZnF ₃ (available from Solvay Fluor GmbH under the tradename NOCOLOK® Zn Flux, particle size distribution 50 % (X ₅₀ ) 1.5 - 6 pm), acrylic resin (available from Rohm & Haas under the tradename Paraloid® B67), dibasic ester mixture composed of dimethyl glutarate (main compound), dimethyl succinate, and dimethyl adipate (available from Solvay under the trade name Rhodiasolv® RPDE), a micronized ethylene-bis-stearamide-wax (available from Munzing Chemie GmbH under the trade name Ceretan® MA 7006), and a thixotropic agent were mixed such that the composition contained the following amounts:

Aluminum-silicon alloy powder 36.15 % by weight

KZnR 25.42 % by weight

acrylic resin 13.30 % by weight

dibasic ester mixture 23.13 % by weight

wax 0.4 % by weight

thixotropic agent 1.6 % by weight

The composition was in the form of a suspension.

Example 2: Al brazing using the composition The composition of example 1 is brushed onto uncladded aluminum 3003 coupons of 2.5 by 2.5 cm. The flux load is approximately 30 gram/m ² . Two angles are assembled and brazed under typical controlled atmosphere aluminum brazing conditions (in N ₂ inert gas). The organic ingredients evaporate while raising the temperature, and at approximately 300 °C, binder, solvent, thixotropic agent and wax are burnt off without any residue. The temperature is further raised to 600 °C to form a brazed joint of the angles. After cooling, the formed joints are inspected and found to be satisfactory.