EP3898071A1

EP3898071A1 - Brazing flux, brazing flux composition and process for manufacturing

Info

Publication number: EP3898071A1
Application number: EP19845813.5A
Authority: EP
Inventors: Hans-Walter Swidersky; UIf UNVERRICHT
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 2018-12-20
Filing date: 2019-12-20
Publication date: 2021-10-27
Also published as: CN113242778A; WO2020126090A1

Abstract

The present invention concerns novel fluxes and compositions comprising such fluxes, wherein the fluxes have a specific particle size characteristic, in particular fluxes which comprise particles with a particle size which is equal to or larger than 100 pm in an amount of equal to or less 0.01 wt%. The invention further concerns a process for the manufacture of such fluxes with specific particle size characteristics. Other objects of the present invention are flux compositions comprising aforesaid fluxes, aluminum or aluminum alloy parts at least partially coated with the flux or flux composition, and a brazing process and brazed metal object obtainable by said brazing method.

Description

BRAZING FLUX BRAZING FLUX COMPOSITION AND PROCESS

FOR MANUFACTURING

This application claims priority to EP No. 18214758.7 filed on 20/12/2018, the whole content of this application being incorporated herein by reference for all purposes.

The present invention concerns novel fluxes and compositions comprising such fluxes, wherein the fluxes have a specific particle size characteristic. The invention further concerns a process for the manufacture of such fluxes with specific particle size characteristics. Other objects of the present invention are flux compositions comprising aforesaid fluxes, aluminum or aluminum alloy parts at least partially coated with the flux or flux composition, and a brazing process and brazed metal object obtainable by said brazing method.

Fluxes are used to remove oxide layers on aluminum or aluminum alloy part surfaces which are to be joined by brazing or welding. The fluxes are applied to the aluminum or aluminum alloy part surfaces in various ways, for example by dry application, plasma application, electrostatic application, in aqueous suspensions and suspensions comprising organic suspension agents.

The suspensions can comprise further additives such as binders. Most of the application techniques depend on fluxes with a stable particle size distribution to ensure uniform and reliable application in the brazing process. W001/30531, for example, describes a brazing flux with a specific particle size distribution which is particularly advantageous for dry application.

It has now been found that fluxes which comprise equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm have certain advantages. For example, flux layers which are treated

mechanically by applying pressure to the flux layer applied to the surface to ensure uniform thickness, which comprise more than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm can indent the surface of the aluminum or aluminum alloy part, causing defects. Fluxes which comprise more than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm which are applied by air transportation or sprayed in suspensions can block transportation lines or application nozzles, or may dissolve unevenly in plasma or gas. It is known in the art that fluxes can be separated into fractions of particular particle size by sieving, as described in WOO 1/30531. This can have the disadvantage of limitations in throughput in large production volume and/or clogging of sieves. Fluxes can also be milled to handle, manipulate or control particles of larger size, but this often impacts the overall particle size distribution and thus application behaviour, which may not be advantageous in order to be able to keep established application procedures. It was now found, surprisingly, that fluxes of particular particle size distributions can be obtained by a process which comprises the steps of a) providing a brazing flux of a first particle size distribution, b) submitting the brazing flux to a step of air classification to obtain at least one fraction of a brazing flux of a second particle size distribution. The term“fraction” denotes the portion of brazing flux of a second particle size distribution which was obtained from the brazing flux of a first particle size as a partial fraction of said brazing flux of a first particle size.

The invention concerns thus, in a first aspect, a process for the manufacture of a brazing flux, which comprises the steps of a) providing a brazing flux of a first particle size distribution, b) submitting the brazing flux to a step of air classification to obtain at least one fraction of a brazing flux of a second particle size distribution. The invention concerns further a brazing flux, which comprises equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm, and more preferably equal to or larger than 71 pm. Another object of the present invention is a flux composition comprising the brazing flux which comprises equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm, and more preferably equal to or larger than 71 pm and at least one fluxing additive or at least one brazing additive. The invention also concerns parts of aluminum, aluminum alloy, steel, copper and titanium, coated at least partially with at least one aforesaid flux, and/or the aforesaid flux composition, a method for brazing of parts of aluminum, aluminum alloy, in particular aluminum alloy comprising magnesium, steel, copper and titanium with parts of aluminum or aluminum alloy wherein at least one of the parts to be joined is coated at least partially with at least one aforesaid flux and/or the aforesaid flux composition, the parts to be joined are assembled, and heated to a temperature of equal to or higher than 570°C. Brazed metal objects obtained by the aforesaid process are also object of the present invention. In the present invention, designations in singular are in intended to include the plural;“a binder” is intended to denote also“more than one binder” or“a plurality of binders”.

In the context of the present invention, the term“comprising” is intended to include the meaning of“consisting of’. All aspects and embodiments of the present invention are combinable.

Where ranges are given, the end points of the range are included.

In a first aspect, the invention concerns a process for the manufacture of a brazing flux, which comprises the steps of a) providing a brazing flux of a first particle size distribution, b) submitting the brazing flux to a step of air classification to obtain at least one fraction of a brazing flux of a second particle size distribution. Generally, brazing fluxes are known to the person skilled in the art, in particular non-corrosive brazing fluxes. Preferably, brazing fluxes according to the invention comprise at least one compound of the group consisting of potassium fluoroaluminates, such as KA1F , K₂A1F₅, K₂A1F₅ H₂0, caesium fluoroaluminates such as CSA1F₄, CS₂AIF₅, CS₃AIF₆, and caesium potassium fluoroaluminates such as KCS2AI3F12, alkalifluorozincates such as potassium fluorozincate or caesium fluorozincate, potassium fluorostannate, such as K₂SnF₆, and caesium fluorostannate. Each of the foregoing can be amorphous and/or be partially or fully in the form of one or more XRD distinguishable phases. Generally, brazing fluxes and their manufacture is known; for example, potassium fluoroaluminates can be manufactured from HA1F obtained from HF and Al(OH)₃ or AI2O3, and KOH. This is described for example in

US4,428,920, US4,579,605 and US5, 968, 288. US 3,951,328, US6,221,129 or US3,971,501 describe a flux based on KA1F₄ and K₃A1F_6. US4,689,092 describes a flux based on potassium fluoroaluminate and caesium

fluoroaluminate. In one aspect, preferred fluxes comprise KA1F and at least one of K2AIF5 and K2AIF5 H2O, and the weight ratio between KA1F₄ (including any hydrate if present) and K₂A1F₅ (including any hydrate K₂A1F₅ H₂0 if present) is from 1 :99 to 99: 1. Often, it is in the range of 1 : 10 to 10: 1. In another preferred aspect, the flux essentially consists of KA1F and at least one of K₂A1F₅ and K2AIF5 H2O. “Essentially” denotes preferably that their sum constitutes equal to or more than 95 % by weight, more preferably, equal to or more than 98 % by weight of the flux. A flux comprising 10 to 40 % by weight of K₂A1F₅,

K₂A1F₅ H₂0 or any mixtures thereof, the balance to 100 % by weight being essentially KA1F₄ is very suitable. Generally, the content in K₃ A1F₆ in a flux comprising potassium fluoroaluminate is low, preferably equal to or less than 5% by weight, more preferably equal to or less than 3% by weight, even more preferably equal to or less than 1% by weight. Most preferably, K₃A1F₆ is absent in the flux comprising potassium fluoroaluminate, which is equal to 0 weight % of K₃A1F₆ in the flux comprising potassium fluoroaluminate. In another aspect, the flux comprises or consists of caesium fluoroaluminate, in the form

of CSA1F₄, CS₂A1F₅, CS₃A1F₆, KCS₂A1₃F_I2, their hydrates and any mixture of two, three or more thereof. CSA1F and CS₂A1F₅, or their hydrates, and mixtures thereof are preferred. CSA1F₄ is most preferred. Often, the flux comprising caesium fluoroaluminate, preferably CSA1F , further comprises K₂A1F₅ and optionally KA1F _. Fluxes containing potassium fluoroaluminate and caesium cations, e.g. in the form of caesium fluoroaluminate, as described in US4670067 and US4689062 are also very suitable. Those caesium-containing fluxes are especially suitable to solder, weld or, in particular, braze aluminum-magnesium alloys. The weight ratio of KA1F₄ and K₂A1F₅ is preferably as described above. In fluxes containing potassium fluoroaluminate and caesium cations, the Cs content is, calculated as content in CsF, between 2 and 74 mol-%. The sum of KA1F₄, K₂A1F₅ and the caesium fluoroaluminate compound or compounds, including any hydrate, in such fluxes is preferably equal to or greater than 95 % by weight, more preferably equal to or more than 98 % by weight. The content of K₃A1F₆ is preferably equal to or less than 2 % by weight, and most preferably equal to or less than 1 % by weight including 0 % by weight.

According to the process wherein at least one fraction of a brazing flux of a second particle size distribution is obtained, step a) in which a brazing flux of a first particle size distribution is provided, can comprise manufacturing steps, such as precipitation and/or drying of a flux, as described in processes known to the person skilled in the art, as in the above-cited publications, or one or more steps wherein commercial fluxes are provided. Such commercial fluxes can be, for example, a flux consisting of KA1F and K₂A1F₅, preferably in a weight ratio of about 4: 1, which is known as Nocolok^®; KZnF₃ known as Nocolok^® Zn Flux; potassium hexafluorosilicate such as Nocolok^® CB Flux; KA1F and K₂A1F₅ and Li additive also known as Nocolok^® Li Flux; Nocolok^® Flux Drystatic and Nocolok^® Cs Flux (SM). In step b) the brazing flux of a first particle size distribution is submitted to a step of air classification to obtain at least one fraction of a brazing flux of a second particle size distribution. Air classification apparatuses are known and include, without limitation, zigzag classifiers, single- channel classifiers, multichannel classifiers, elbow-jet air classifiers, deflector- wheel classifiers and vane classifiers. The choice of classifier and classification conditions depends on the particle size distribution of the brazing flux provided in step a) and the intended separation to be obtained in step b). Classification performance is controlled and optimized by adjusting air volume and throughput, and other factors such as, for example the classifying wheel's rotational speed when a deflector- wheel classifier is used. Particularly preferential are air classifiers of the brand Noll SEPARANO^® E, such as Type 9420 or 9360. The process can comprise one or more steps b) to obtain one or more fractions of desired particle size particle size distributions. If more than one step b) is comprised in the process, the steps can be consecutive or interspersed with other steps, for example milling or sieving steps. The term“air classification” is to be distinguished from classical sieving methods. Sieving methods involve the segregation of various sizes of particles using cloth or screens through which fractions of particles pass, while larger particles are retained. As described above, the sieving technique has various disadvantages, such as limitations in throughput in large production volume and/or clogging of sieves. Air

classification, in contrast, basically is a mechanical separation process in which particles are separated according to their ratio of inertia and/or gravity to flow resistance in a gas stream; it can be described as a classification process that exploits the principle of separation by gravity or centrifugal force.

A preferred process according to the present invention is a process wherein the brazing flux provided to step a) comprises particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm, and more preferably equal to or larger than 71 pm in an amount of equal to or more than 0.05 wt%, more preferably equal to or more than 0.5 wt%, and more preferably equal to or more than 1 wt% and wherein the at least one fraction of a brazing flux of a second particle size distribution comprises equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm, and more preferably equal to or larger than 71 pm. In one aspect of the process according to the present invention, the brazing flux provided to step a) comprises particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm, and more preferably equal to or larger than 71 pm in an amount of equal to or more than 5 wt%. In one preferred aspect, the brazing flux provided to step a) comprises particles with a particle size which is equal to or larger than 100 mih, preferably equal to or larger than 80 pm, and more preferably equal to or larger than 71 pm in an amount of between 1 and 10 wt%, and more preferably of between 1 and 5 wt%. In another aspect, the brazing flux provided to step a) comprises particles with a particle size which is equal to or larger than 80 pm in an amount of equal to or less than 10 wt%, preferably equal to or less than 8 wt%, and more preferably equal to or less than 6 wt%. The amount of particles with a particle size which is equal to or larger than 100, 80 or 71 pm is determined by a suitable method, such as, for example, an air jet sieve or a wet screening method. For the wet sieving method, a weighed sample is suspended in a suitable solvent, such as an aliphatic alcohol, water, ketone, or a

hydrocarbon, optionally treated with ultrasound and screened over a sieve suitable to separate the particles with a particle size which is equal to or larger than 100, 80 or 71 pm. The sieve residue is dried and weighed to determine the amount of particles with a particle size which is equal to or larger than 100, 80 or 71 pm. According to one preferred aspect of the present invention, the amount of particles the particles with a particle size which is equal to or larger than 100, 80 or 71 pm in the brazing flux provided to step a) and/or at least one fraction of a brazing flux of a second particle size distribution, and/or brazing flux which comprises equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm is determined by air jet sieving or the wet sieving method as described above.

In one aspect of the present invention, the brazing flux provided to step a) comprises at least one compound of the group consisting of potassium

fluoroaluminates, such as KA1F , K₂A1F₅, K₂A1F₅ H₂0, caesium

fluoroaluminates such as CSA1F₄, CS₂AIF₅, CS₃AIF₆, potassium caesium fluoroaluminates KCS₂AI₃F₁₂, alkalifluorozincates such as potassium

fluorozincate or caesium fluorozincate, potassium fluorostannate, such as K₂SnF₆, and caesium fluorostannate. Each of the foregoing can be amorphous and/or be partially or fully in the form of one or more XRD distinguishable phases Preferably, the brazing flux provided to step a) comprises at least one compound of the group consisting of KA1F , K₂A1F₅, K₂A1F₅ H₂0, CSA1F , CS₂AIF₅, CS₃AIF₆, KCS₂AI₃F₁₂, KZnF₃, wherein each of the foregoing can be amorphous and/or be partially or fully in the form of one or more XRD distinguishable phases. Particularly preferred fluxes provided to step a) are described above. Potassium fluorozincates such as KZnF₃ or K₂ZnF₆ areknown for example from WO2009153312 and WOOl/74715. The term“at least one compound” intends to denote the presence of each of the compounds

individually, or in a mixture with at least one other compound.

According to one aspect of the present invention, the flux provided to step a) is a flux as described in WO 01/30531, in particular the preferred fluxes therein identified by the values given in Table A and Table B of WO 01/30531.

It should be noted that larger particles are often not registered with required definition in such an R2 laser diffraction screen, which is often used for measurements of fluxes such as the fluxes described in WO 01/30531. Small amounts of particles of a particle size of 100, 80 or 71 pm in an amount of equal to or more than 0.05 wt%, which are still significant for various characteristics of the flux, are more suitably determined by the method above of wet sieving.

Another object of the present invention is a brazing flux which comprises equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm and more preferably equal to or larger than 71 pm. The flux can comprise equal to or less than 0.01 wt%, preferably equal to or less than 0.005 wt% or more preferably equal to or less than 0.0002 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm and more preferably equal to or larger than 71 pm. In a most preferred aspect, the wt% of fraction of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm and more preferably equal to or larger than 71 pm is below the measurement limit of the scale, which is set to 0.1 mg, which equals, if the sample submitted to wet sieving is lOOg, less than 0.0001 wt%. Such fluxes are considered to be essentially free of particles with a particle size which is equal to or larger than 100, 80 or 71 pm. The term“equal to or less than 0.01 wt%” includes as lower limit the detection threshold for the selected particle size measurement method, which preferably is the wet sieving or air jet method. Thus, if the measurement method detects no (“zero”) particles equal to or larger than 100, 80 or 71 pm respectively, this denotes the lower limit for the value wt% being 0.

Preferably, the brazing flux comprises at least one compound of the group consisting of KA1F₄, K₂A1F₅, K₂A1F₅ H₂0, CSA1F₄, CS₂A1F₅, CS₃A1F₆,

KCS₂A1₃F_I2, alkalifluorozincates such as potassium fluorozincate, e.g. KZnF₃ or K₂ZnF₆, caesium fluorozincate, potassium fluorostannate, such as K₂SnF₆, and caesium fluorostannate, wherein each of the foregoing can be amorphous and/or be partially or fully in the form of one or more XRD distinguishable phases. More preferred fluxes are detailed above. Especially preferred fluxes comprise at least 80 to 100 wt% of at least one compound of the group consisting of KAIF₄, K₂AIF₅ and K₂AIF₅ H₂O. In another preferred aspect, such a flux further comprises one or more caesium potassium fluoroaluminates.

The invention concerns further a flux composition comprising a brazing flux which comprises equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm and more preferably equal to or larger than 71 pm as described above and at least one fluxing additive and/or at least one brazing additive. Brazing additives intend to denote additives that improve or modify the joint between the brazed parts, e.g. improve the brazing of Al-Mg alloys or improve generally the surface properties of the joint, for example, form an anticorrosive coating or brazing residue. Fluxing additives intend to denote additives which modify or improve the way of applying the fluxing of the parts to be joined. Fluxing additives according to the present invention often are selected from the group consisting of solvents, binders, surfactants, thixotropic agents, defoaming agents, antioxidants, thickeners, and suspension stabilizers. A suitable antioxidant is, for example, BHT (butylated hydroxytoluene). A suitable corrosion inhibitor is, for example, benzotriazol. A suitable defoaming agent is, for example, silicon oil or glycerin. A suitable suspension stabilizer is for example sodium carboxy methylcellulose. A suitable thixotropic agent is, for example, gelatin or pectin. Suitable binders can be selected for example 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, cellulose derivatives, polyvinyl acetates or polyvinyl alcohols. Further suitable binders are glycols such as, for example, ethylenglycol or glycols derived from ethylenglycol. Flux

compositions containing water as liquid and water-soluble polymers, for example, polyurethane, or polyvinyl alcohol as binder are especially suitable because they have the advantage that, during the brazing process, water is evaporated instead of possibly flammable organic liquids. A thickener can be used for adjusting the viscosity and to keep the flux longer in suspension; this improves, e.g., shelf life and provides better performance in spray application. 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. Surfactants (also known as wetting agent), if present, can provide uniform coating, and compensate to some extent for surfaces not cleaned prior to application. Preferred surfactants are nonionic surfactants, e.g. Antarox^® BL 225, a mixture of linear C8 to CIO ethoxylated and propoxylated alcohols.

The content of the brazing flux of the invention (including, if present, other brazing additives, e.g. filler metal, filler precursor, or metal salts, improving the brazing or surfaces properties) in the total flux composition (including fluxing additives such as liquid or liquids, thixotropic agents, surfactants and binders, if present) generally is equal to or greater than 0.75 % by weight. Preferably, it is equal to or greater than 1 % by weight. More preferably, the content of the brazing flux, optionally including brazing additives, in the flux composition is equal to or greater than 5 % by weight, very preferably, equal to or greater than 10 % by weight of the total flux composition.

Generally, the content of the brazing flux of the invention (including, if present, other additives, e.g. filler metal, filler precursor, or metal salts, improving the brazing or surfaces properties) in the total flux composition (including fluxing additives such as liquid or liquids, thixotropic agents, surfactants and binders, if present) in the flux composition is equal to or lower than 70 % by weight. Preferably, it is equal to or lower than 50 % by weight.

The binder, if present, is generally comprised in an amount of equal to or greater than 0.1 % by weight, preferably equal to or greater than 1 % by weight of the total flux composition. The binder, if present, is generally contained in an amount equal to or lower than 30 % by weight, preferably equal to or lower than 25 % by weight of the total flux composition.

The thixotropic agent, if present, is generally comprised in an amount of equal to or greater than 1 % by weight of the total flux composition. Generally, if present, it is contained in an amount equal to or lower than 20 % by weight, preferably equal to or lower than 10 % by weight.

The thickener, if present, is generally comprised in an amount of equal to or greater than 1 % by weight, preferably equal to or greater than 5 % by weight of the total flux composition. Generally, the thickener, if present, is comprised in an amount equal to or lower than 15 % by weight, preferably equal to or lower than 10 % by weight of the total composition. Highly suitable flux compositions for wet applications comprise 10 to 70 % by weight of the brazing flux and optionally brazing additives (including filler metal, filler precursor, modifying and anticorrosive agents, e.g. metal salts, improving the brazing or surfaces properties), 1 to 25 % by weight binder, 0 to 15 % by weight of a thickener, 0 to 10 % by weight of a thixotropic agent, and 0 to 5 % by weight of other additives, e.g. a surfactant or a suspension stabilizer. Preferably, the remainder to 100 % by weight is water, an organic solvent or an aqueous organic solvent. The flux composition can be free of any water or water-free or aqueous organic liquid, but comprises the brazing flux and optionally one or more brazing additive (such as the filler metal or precursor, modifying or anticorrosive agents which improve the brazing process or the properties of the brazed product or other additives, e.g. those described above) and a water-soluble organic polymer as a binder which is present in the form of a water soluble package for the flux composition. For example, polyvinyl alcohol is very suitable as water-soluble package for the flux composition as described in US patent application publication 2006/0231162. Such packages can be handled without dust formation, and after addition of water, they form a suspension in water including a flux and the water soluble polymer as binder.

Suitable brazing additives are known. For example, the flux composition can comprise ZnF₂ which is disclosed, for example, in W099/48641. SnF₂ is also a suitable additive. Such additives can improve the formed joint because a protective zinc coating or tin coating is formed during brazing.

The flux composition can comprise elemental Si, Ge or Cu as additive which serve as solder metal precursor as described in US-A 5,190,596.

The flux composition can comprise alkali hexafluoro silicates, in particular

K₂SiF₆ or Cs₂SiF₆ or mixtures thereof. Such additives are disclosed, for example, in WO00/73014 and render a part or even all of solder metal unnecessary because elemental Si is formed during brazing which serves as solder alloy precursor.

The flux composition can comprise Li compounds as additive as disclosed in WO2011/098120 and W02010/060869. Suitable sources of Li ions as additives are, for example, LiF, Li₃AlF₆, LiOH, Li oxalate or Li₂C0_3. Flux composition containing Li ions often display reduced corrosion of their brazing residue. Particularly preferred flux compositions comprising Li compounds are fluxes which are based on K₂A1F₅, precursors of K₂A1F₅, or hydrates of the foregoing as disclosed in WO2011/098120 and W02010/060869. Precursors of K₂AIF₅ include KZnF₃, K₂SiF₆, CS₂A1F₆, their hydrates and mixtures of the foregoing. The flux according to the present invention can comprise a lithiumfluoroaluminate, in particular Li₃ A1F_6. In a one aspect, the flux essentially consists of the lithiumfluoroaluminate.

The flux composition can also comprise alkaline earth metal compounds such as CaF₂, CaC0₃, MgF₂, MgC0₃, SrF₂, SrC0₃, BaF₂, BaC0₃, and mixtures of two or more of said second components which also provide reduced corrosion as disclosed in WO 201513595.

The flux composition can comprise compounds, especially salts, of 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 , ZrF₄, CeF₃, CeF , YF₃ or alkaline fluorometallates, e.g. K₂ZrF₆ or K₂TiF₆, are added after manufacture as disclosed in WO 2005/092563 and WO 2007131993.

The flux composition can comprise one or more brazing additive selected from the group consisting of ZnF₂, SnF₂, K₂SiF₆, Cs₂SiF₆, Li compounds, alkaline earth metal compounds, compounds of indium, antimony, bismuth, zirconium, niobium, cerium, yttrium, titanium, and lanthanum; solder metal; Si, Ge and Cu.

Brazing additives can be preferably comprised in an amount of more than 0 % by weight up to 30% by weight, relative to the total weight of flux

composition and additive or additives.

The invention further concerns parts of aluminum, aluminum alloy, steel, copper and titanium, coated at least partially with a brazing flux comprising equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm and more preferably equal to or larger than 71pm according to the invention, and/or a flux composition according to the invention. The regions of the parts that are coated with flux and/or flux composition generally in a load of 3 to 30 g/m² (amount flux in g per surface in m²). The flux and/or flux composition is applied to the aluminum, aluminum alloy, steel, copper and/or titanium parts according to techniques known to the skilled person, such as dry application, plasma application, dispersion application, paint application, paste application, and can comprise steps such as drying and rolling. Another object of the present invention concerns a method for brazing of parts of aluminum, aluminum alloy, in particular aluminum alloy comprising magnesium, steel, copper and titanium with parts of aluminum or aluminum alloy wherein at least one of the parts to be joined is coated at least partially with a brazing flux comprising equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm and more preferably equal to or larger than 71pm according to the invention, and/or a flux composition according to the invention, the parts to be joined are assembled, and heated to a temperature of equal to or higher than 570°C. The brazed metal object obtainable by such a process is a further object of the present invention.

Brazing fluxes comprising equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm and more preferably equal to or larger than 71pm can have a number of advantages. For example, flux layers which are treated mechanically by applying pressure to the flux layer applied to the surface to ensure uniform thickness, which comprise more than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm and more preferably equal to or larger than 71pm can indent the surface of the aluminum or aluminum alloy part, causing defects. Fluxes which comprise more than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm, preferably equal to or larger than 80 pm and more preferably equal to or larger than 71pm which are applied by air transportation or sprayed in suspensions can block transportation lines or application nozzles, or may dissolve unevenly in plasma or gas. The present process for the manufacture of a brazing flux was found to have the disadvantage of overcoming limitations in throughput in large production volume and/or clogging of sieves in the separation of fractions of larger particle size. Milling often impacts the overall particle size distribution and thus application behaviour, which in some cases is not desirable in order to be able to keep established application procedures.

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 further explain the invention without intending to limit the scope. General remark: Percentages are given in % by weight unless otherwise indicated.

Example 1 :

A batch of NOCOLOK^® Flux Drystatic was submitted to air classification in a Noll SeparaNo E9000 Type 9360air classifier. The amount of particles with a particle size of 71 pm and more were determined with the air jet sieve method before the air classification to be 2.5 wt%. After the air classification step, the amount of particles with a particle size of 71 pm and more were determined with the air jet sieve method after the air classification to be 0.00 wt% (no particle fraction of this size recovered in the air jet sieve method).

A glycol flux paste was manufactured comprising the product of example 1. The paste was applied through a nozzle to aluminum alloy parts to be brazed. The paste comprising the product of example 1 showed significantly less clogging of the nozzle compared to NOCOLOK^® Flux Drystatic.

Claims

1. Process for the manufacture of a brazing flux, which comprises the steps of a) providing a brazing flux of a first particle size distribution, b) submitting the brazing flux to a step of air classification to obtain at least one fraction of a brazing flux of a second particle size distribution.

2. Process according to claim 1, wherein the brazing flux provided to step a) comprises particles of a particle size which is equal to or larger than 100 pm in an amount of equal to or more than 0.05 wt%.

3. The process according to claim 1 or 2, wherein the brazing flux comprises at least one compound of the group consisting of KA1F , K₂A1F₅,

K₂AIF₅ H2O, CSA1F₄, CS₂AIF₅, CS₃AIF₆, KCS₂AI₃F₁₂, amorphous potassium fluoroaluminates, amorphous caesium fluoroaluminates, amorphous caesium potassium fluoroaluminates, potassium fluorozincate, caesium fluorozincate, potassium fluorostannate, and caesium fluorostannate.

4. The process according to claim 3, wherein the brazing flux comprises at least one compound of the group consisting of KA1F , K₂A1F₅, K₂A1F₅ H₂0, CSA1F , CS₂AIF₅, CS₃AIF₆, KCS₂AI₃F₁₂, KZnF₃, K₂ZnF₆, amorphous potassium fluoroaluminates, amorphous caesium fluoroaluminates and amorphous caesium potassium fluoroaluminates.

5. A brazing flux which comprises equal to or less than 0.01 wt% of particles with a particle size which is equal to or larger than 100 pm.

6. A brazing flux according to claim 5, wherein the brazing flux comprises at least one compound of the group consisting of KA1F , K₂A1F₅, K₂AIF₅ H₂O, CSA1F , CS₂AIF₅, CS₃AIF₆, KCS₂AI₃F₁₂, amorphous potassium fluoroaluminates, amorphous caesium fluoroaluminates, amorphous caesium potassium fluoroaluminates, potassium fluorozincate, caesium fluorozincate, potassium fluorostannate, and caesium fluorostannate.

7. The brazing flux according to anyone of claim 5 to 7, which comprises or consists of from 80 to 100 wt% of at least one compound of the group consisting of KA1F₄, K₂AIF₅ and K₂AIF₅ FLO.

8. The brazing flux according to claim 7, further comprising one or more caesium potassium fluoroaluminates.

9. The process according to any one of claims 1 to 4, or the brazing flux according to any one of claims 5 to 8, wherein the particle size is determined by air jet sieving or wet sieving.

10. A flux composition comprising the flux of any one of claims 5 to 9 and at least one fluxing additive and/or at least one brazing additive.

11. The flux composition according to claim 10, wherein the at least one fluxing additive is selected from the group consisting of solvents, binders, surfactants, thixotropic agents, defoaming agents, corrosion inhibitors, antioxidants, thickeners, and suspension stabilizers.

12. The flux composition according to claim 10 or 11, wherein the at least one brazing additive is selected from the group consisting of ZnF₂, SnF₂, K₂SiF₆, Cs₂SiF₆, Li compounds, alkaline earth metal compounds, compounds of indium, antimony, bismuth, zirconium, niobium, cerium, yttrium, titanium, and lanthanum, solder metal, Si, Ge and Cu.

13. Parts of aluminum, aluminum alloy, steel, copper and titanium, coated at least partially with a flux according to any one of claims 5 to 8, and/or a flux composition according to any one of claims 10 to 12.

14. A method for brazing of parts of aluminum, aluminum alloy, in particular aluminum alloy comprising magnesium, steel, copper and titanium with parts of aluminum or aluminum alloy wherein at least one of the parts to be joined is coated at least partially with a flux of any one of claims 5 to 9 and/or a flux composition of any one of claims 10 to 12, the parts to be joined are assembled, and heated to a temperature of equal to or higher than 570°C.

15. A brazed metal object obtainable by the process of claim 14.