Title of Invention

A PROCESS FOR SOLDERING COMPONENTS OF ALUMINIUM AND ALUMINIUM ALLOYS USING A FLUX

Abstract

A process for soldering components of aluminium and aluminium alloys using a flux, wherein the flux is applied in solid form, as an aqueous or organic suspension, as an aqueous or organic slurry or as a paste, and contains alkali fluorozincate or mixtures of alkali fluoride and zinc fluoride, and soldering is effected at a temperature in the range from 420 to 590°C, with "alkali" signifying potassium, caesium or rubidium.

Full Text

FORM 2 THE PATENTS ACT, 1970 [39 OF 1970] COMPLETE SPECIFICATION [See Section 10; Rule 13] TITLE "A PROCESS FOR SOLDERING COMPONENTS OF ALUMINIUM AND ALUMINIUM ALLOYS USING A FLUX" APPLICANT We, SOLVAY FLUOR GMBH, of Hans-Bockler-Allee 20, 30173 Hannover, Germany, The following specification particularly describes and ascertain the nature of the invention and the manner in which it is to be performed:- Solvay Fluor und Derivate GmbH 30173 Hannover New fluxing agents Description The invention relates to novel fluxes for soldering aluminium and aluminium alloys, a soldering process and soldered components. Assemblies (for example radiators for vehicle motors or heat exchangers) consisting of parts made of aluminium or aluminium alloys can be produced by soldering (brazing) these parts. Advantageously, a flux based on fluoroaluminate is used which frees the surface of the components which are to be soldered together from oxidic adhesions. Fluxes based on potassium fluoroaluminate are particularly suitable for aluminium or low-magnesium aluminium alloys. One such process is disclosed in British Patent 1 438 955. The production of corresponding fluxes is described, for example, by Willenberg, US-A 4,428,920 and Meshri, US-A 5,318,764, and by Kawase, US-A 4,579,605. Fluxes which contain fluoroaluminates of caesium are described, for example, in Suzuki, US-A 4,670,067 and Shimizu, US-A 5,171,377. Such fluxes, which may additionally contain potassium fluoroaluminate fluxes, are particularly well suited for soldering aluminium alloys having a relatively high magnesium content. US Patent 4,906,307 discloses a process for soldering components made of aluminium alloys. According to one embodiment, provision is made to use a flux which contains K2SiF6, ZnF2, NaF and AIF3. Soldered-plated components are soldered. In soldering, the procedure is such that the flux (for example in the form of a slurry) and a solder metal are applied to the components to be joined. The components are placed together in the desired position and heated. First the flux melts and cleans the surface, then the solder melts. Then the parts are allowed to cool. US-A-5,190,596 teaches that a metal which forms a eutectic with the aluminium upon soldering can be added to the flux instead of a solder metal. Suitable metals are copper, zinc and germanium, in particular silicon. The addition of certain metal fluorosilicates in certain quantities may make the solder metal superfluous (see EP-A 810 057 and German Patent Application 196 36 897.9). In the latter patent application, it is disclosed that a mixture of potassium fluoroaluminate flux and potassium fluorosilicate, in which the potassium fluorosilicate is contained in a quantity of 6 to 50% by weight, makes a solder metal unnecessary. In the British Patent Specification 1,438,955 mentioned first hereinbefore, it is explained that relatively small quantities of alkali metal zinc fluorides, up to 5 mole percent, can be tolerated in the flux. However, it is stated that their presence does not yield any advantages in relation to lowering the melting point, but rather they all have the effect of raising the melting point. Haramaki, US-A 4,645,119, discloses fluxes based on potassium fluoroaluminate which contain 3 to 30 % by weight ZnF2, optionally in the form of KZnF3. The zinc fluoride decomposes at the soldering temperature, and the metallic zinc covers the soldered parts or the entire surface of the components which are to be soldered together, and imparts improved protection against corrosion to the aluminium. It is an object of the present invention to provide a novel application process and also novel fluxes which can be used therefor. This object is achieved by the process according to the invention and the novel flux. The process according to the invention for soldering aluminium and aluminium alloys using a flux based on complex fluorides provides for the flux to contain alkali fluorozincate or mixtures of alkali fluoride and zinc fluoride as flux, and for soldering to be effected at a temperature in the range from 420 to 590°C, with "alkali" signifying potassium, caesium or rubidium. The fact that alkali fluorozincate and mixtures of alkali fluoride and zinc fluoride have a flux action at these temperatures is an unexpected finding. Surprisingly, however, alkali fluorozincates act as fluxes even when the soldering is performed at temperatures far below the melting point of the alkali fluorozincate used. The melting points of KZnF3 and K2ZnF4 are e.g. 870°C and 737°C, so a soldering operation should not take place at all at temperatures below 600°C. The following explanation is offered: in the presence of solder-forming constituents such as silicon, an Al-Si eutectic forms. Owing to electrochemical processes, this generates an alkali fluoroaluminate flux in situ; for example, it is assumed that KZnF3+AI (from the Al-Si alloy) reacts to form KAIF4 or KF and AIF3 and Zn metal. However, this is only an attempted explanation which might be able to explain the phenomena such as the soldering operation and the formation of Zn. Mixtures of alkali fluoride (or alkali fluorides) and zinc fluoride yield usable solders. The molar ratio of alkali fluoride to zinc fluoride may be in the range of about 1:1, e.g. from 1:1.05 to 1.05:1. However, there may also be a relatively great excess of one of the two constituents, in particular the zinc fluoride. Preferably, however, alkali fluorozincates are used, because they yield better solders. The term "alkali fluorozincate" within the scope of the present invention comprises compounds of the general formula (MF)x(ZnF2)y, with M = K, Rb, Cs and 0>x>4 and 0>y>4. "Alkali fluoride" comprises the fluorides of potassium, rubidium and caesium. Preferably x and y are integers, namely, independently of each other, 1, 2, 3 or 4; however, x and y may also be in a hypostoichiometric ratio to one another. Although either x, y or both are then greater than 0, they do not represent an integer. In this case, it is preferred if y is greater than x. The term "flux" within the scope of the present invention comprises those compounds which have the desired surface-cleaning action (in particular removal of oxidic layers) during soldering. The flux may consist of alkali fluorozincate; other fluxes are then not contained therein. The flux may also contain other fluxes in addition to alkali fluorozincate. For example, the flux may be a mixture of alkali fluorozincate with alkali fluoroaluminate, for example potassium fluoroaluminate and/or caesium fluoroaluminate. The alkali fluorozincate may be present as a pure compound or as a mixture of alkali fluorozincates. For example, pure potassium fluorozincate or pure caesium fluorozincate may be used. These may be compounds which are present in one or more phases. For example, pure KZnF3 or alternatively mixtures of KZnF3 and K2ZnF4 may be used. However, corresponding mixtures with different alkali metal cations may also be used. Preferred fluorozincates are potassium fluorozincate and caesium fluorozincate. These may of course also be contained as a mixture. If caesium fluorozincate is contained as the sole fluorozincate in the flux, it is present in a quantity of 5 or more percent by weight. Preferably the alkali fluorozincate is contained in the flux in a quantity of more than 30% by weight, in particular in a quantity of 50 or more percent by weight. The percentages relate to the flux used as 100% by weight. If it is not pure alkali fluorozincate fluxes which are us«d, other fluxes represent the remainder to make up to 100% by weight of the mixture, in particular fluxes based on potassium and/or caesium fluoroaluminate. Thus, fluxes may contain upto 95% by weight potassium fluoroaluminate or caesium fluroaluminate in addition to the alkali fluorozincate. The flux can frequently be used as such, without the addition of auxiliaries. For example, solder-plated aluminium sheets may be soldered with pure flux. In addition to the flux, ready-to-use compositions may if desired contain auxiliaries. The flux may also contain auxiliaries such as binders, dispersing agents, solder metal, solder-metal precursors, solder-forming materials such as metal fluorosilicates, in particular alkali fluorosilicates, or stabilisers. Fluxes consisting of pure alkali metal fluorozincate and fluxes which additionally contain potassium fluoroaluminate and/or auxiliaries, can be used very well in the process according to the invention. If binder is contained in the flux, it is expediently contained in a quantity of 10 to 90% by weight. If solder metal is contained in the flux, it is expediently contained in a quantity of 25 to 75% by weight. The flux, as described in US-Patents 5,100,048 and 5,190,596 may contain admixed solder-forming metals such as silicon, copper or germanium. These are then contained in a quantity of approx. 10 to approx. 80% by weight. It is also possible to use larger or smaller quantities than those described above. The minimum or maximum quantity which is in fact effect^e can be determined by small-scale tests (soldering tests). Metal fluorosilicate, such as alkali fluorosilicate, for example potassium hexafluorosilicate, can also be contained therein as solder-metal precursor. If it is contained, the quantity is expediently in the range from 5 to 95% by weight. Thus the flux may contain, 5 to 95% by weight alkali flurozincate and 95 to 5% by weight alkali flurosilicate or consisting thereof with "alkali" signifying potassium, caesium and rubidium. The above percentages relate to the total flux used as 100% by weight. As shown in German Application 196 36 897.9, soldering is possible without solder, if at least 6% by weight foSiFe is contained in the flux. The same applies, according to EP-A 810 057, to fluxes which contain 7 to 15% by weight metal fluorosilicates such as Cs2SiF6, CsHSiF6, or CsKSiF6. For K2SiF6 25 to 50, or even up to 75% by weight is advantageous for this. However, even if metal fluorosilicates are contained in the flux in small quantities, for example in a quantity of 1 to less than 6% by weight, the flux properties in terms of the wetting properties of the surface to be soldered, but also the melting point of the flux are positively affected. If it is planned to use the flux in the form of a slurry, it may also contain dispersing agents which stabilise the suspension. The flux can be applied in known manner to the components of aluminium or aluminium alloys which are to be joined. Dry application based on electrostatic spraying technology is possible owing to the good fluidisation properties of the fluxes. Alternatively, the flux can be applied to the materials to be joined in the form of aqueous or organic suspensions or as a paste. Aqueous or organic slurries expediently contain 15 to 75% by weight of the flux. It is also possible to use suspensions of the flux in organic liquids, expediently the substances usually used as organic solvents, such as alcohols, in particular methanol, ethanol, propanol or isopropanol and also polyols. Other organic liquids ("carriers") are ethers, e.g. diethyleneglycol monobutylether, ketones such as acetone, esters of alcohols, diols or polyols. An example of a binder for the use as a paste is ethylcellulose. By means of film-forming agents, usually polymers, which are soluble in organic solvents, e.g. acetone, fluxes with optionally solder or solder-precursor can be applied to the workpiece, and yield a securely-adhering film once the solvent has been evaporated. Suitable polymers are e.g. (meth)acrylates. The film-forming agent then evaporates upon soldering. When being used, the solder metal, if required, may be contained in the flux (as an admixed powder), it may be already applied to the components to be soldered as a plating, or may be applied in addition to the flux. The soldering temperature is dependent on the solder used or the solder-forming metal or material. Below a solder -metal liquidus temperature of 450°C by definition the term "soft soldering" (= "soldering") is used, and the term "hard soldering" (= "brazing") above this temperature. There are low-melting solders, such as zinc- aluminium solders, which can be used for soldering from temperatures as low as 390GC, or pure zinc solder, which can be used from temperatures as low as 420°C. Other solders can be soldered at higher temperature. AI-Si-[Cu] solders can be used from [530 °C] or 575 °C upwards. Generally, a soldering temperature of up to 600°C is sufficient. Preferably, soldering is carried out at 390°C to 600°C, in particular at 420 to 590°C. In this case, ambient pressure prevails. Soldering, e.g. in a vacuum, with evaporation of the flux, as described in JP-A-03/099 795, does not fall within the scope of the present invention. Torch soldering or furnace-soldering, in particular in an inert atmosphere (e.g. N2 atmosphere), may be carried out. Soldering may be effected in a controlled atmosphere or in a non oxidizing flame. Known fluxes can be used for the process according to the invention. Japanese Application 71/293 699, for example, discloses fluxes consisting of potassium fluorozincate in a specific molar ratio. US-A-4,645,119 discloses a flux based on potassium fluoroaluminate, which also contains potassium fluorozincate. The potassium fluorozincate is used as an additive to improve corrosion resistance, not as a flux. European Patent Application EP-A-O 659 519 discloses a flux for aluminium soldering which contains potassium fluoride, zinc fluoride and aluminium fluoride within specific ranges. Potassium fluorozincates may possibly be contained herein. Novel fluxes are described below which can be used in the process according to the invention and are likewise a subject of the invention. One subject of the invention is a flux, usable for soldering aluminium and aluminium alloys, which contains alkali metal fluorozincate and solder metal or in particular a solder-metal precursor and optionally alkali metal fluoroaluminate and optionally auxiliaries, or consists thereof, with "alkali" signifying potassium, caesium and rubidium. The preferred alkali metal fluorozincate is potassium fluorozincate and/or caesium fluorozincate; the preferred solder-metal precursor is silicon, copper, zinc or germanium, or a metal fluorosilicate, preferably an alkali metal fluorosilicate, in particular potassium fluorosilicate and/or caesium fluorosilicate. If desired, conventional auxiliaries such as binders, supports or stabilisers may be contained therein. From as low as 2% by weight alkali fluorozincate, positive effects on the soldering behaviour can be detected. The auxiliaries, for example binders, may be contained in a quantity of 10 to 90% by weight, relative to the total weight of the flux. According to one embodiment, the flux preferably contains 5 to 95% by weight alkali fluorozincate (as sole constituent acting as a flux) and 5 to 95% by weight solder or solder-metal precursor, or consists thereof, with "alkali" signifying potassium, caesium or rubidium. If alkali fluoroaluminate is contained in the flux in addition to the alkali fluorozincate and solder metal or solder-metal precursor, the quantities are preferably 5 to 90% by weight alkali fluorozincate, 5 to 90% by weight solder or solder-metal precursor and 5 to 90% by weight alkali fluoroaluminate. The flux may consist of these components, or auxiliaries may be contained in a quantity of 10 to 90% by weight, relative to the total weight of the flux. According to a particularly preferred embodiment, the flux contains alkali fluorozincate, alkali fluoroaluminate and at least one solder-metal precursor. The preferred alkali fluorozincate is potassium fluorozincate and caesium fluorozincate, the preferred solder metal is silicon, germanium, zinc or copper or alkali metal fluorosilicate, preferably potassium fluorosilicate or caesium fluorosilicate. The flux may consist of the above constituents. The alkali metal fluorozincate is preferably contained in the flux in a quantity of 2 to 20% by weight, the alkali metal fluoroaluminate in a quantity of 20 to 80% by weight and the solder-metal precursor in a quantity of 10 to 50%. If desired, conventional auxiliaries such as binders, supports or stabilisers (for the suspension) may be contained therein, in this case preferably in a quantity of 30 to 70% by weight, relative to the total weight of the flux. One further subject of the invention is a flux which can be used for soldering aluminium and aluminium alloys and contains more than 5% by weight, preferably more than 5 mole percent, but less than 100% by weight caesium fluorozincate and potassium fluoroaluminate or caesium fluoroaluminate as remainder to make up to 100% by weight. This flux preferably contains more than 30% by weight, in particular 50 or more percent by weight, caesium fluorozincate. The advantage of this flux which, if desired, may contain the conventional auxiliaries such as binders, supports or stabilisers, is that it is also possible to solder magnesium-containing aluminium alloys very well. This is ascribed to the caesium cation. Alternatively, therefore, mixtures of potassium fluorozincate and caesium fluoroaluminate or caesium fluorozincate can also be used very effectively. Using the flux according to the invention or the flux according to the invention [sic], assemblies consisting of soldered parts made of aluminium or aluminium alloys can be produced. The required alkali fluorozincates can be produced in various ways. For example, alkali fluoride, e.g. caesium fluoride or potassium fluoride, can be melted with zinc fluoride in the desired ratio. Alternatively, it is possible to operate in aqueous solution. Thus alkali fluorides and zinc fluoride can be caused to react from aqueous solution, forming alkali zinc fluoride, and the resulting alkali zinc fluoride can be isolated if desired. To this end, a zinc fluoride solution, which if desired has been produced fresh from zinc oxide and aqueous HF, is reacted with a potassium fluoride solution which if desired has been obtained freshly from potassium hydroxide and aqueous HF. It is worked up in that the resulting solids are separated off from the aqueous supernatant solution and are then dried. Another procedure provides for reacting a solution of alkali bifluorides (i.e. adducts of HF and alkali fluoride) with zinc oxide. Thus the alkali fluoride and/or the zinc fluoride can be produced by metathesis of other alkali or zinc salts by means of HF or alkali or ammonium bifluoride in the solution. Information about phase diagrams, based on thermal and X-ray analyses, are described by 0. Schmidt-Dumont and Horst Bornefeld in Z. anorg. altgem. Chem. 287 (1956), pages 120 to 137. Information on Cs4Zn3F10 is described by D. Babel in Z. Naturforsch. 20a (1965), pages 165 and 166. A novel method for the production of fluorometallates is described by M. K. Chaudhuri, S. K. Ghosh and Z. Hiese in J. Chem. Soc. Dalton Trans. (1984), pages 1763 to 1964. Other than as assumed in the prior art, alkali fluorozincates are suitable as fluxes in aluminium soldering or soldering of aluminium alloys such as Mg-AI alloys at temperatures of 600°C and less. It is not necessary to operate in a vacuum with flux vapour. The residue is non-corrosive, and can be painted over. The range of known fluxes is enlarged in a manner which was impossible to predict. The following examples are intended to explain the invention further, without limiting its scope. Examples Example 1: Preparation of potassium fluorozincate (JF 009400) Zinc oxide is reacted with aqueous HF to form a zinc fluoride solution (Solution 1). Solution 1 is added, with stirring, to a previously prepared aqueous KF HF solution (Solution 2 consisting of 22.3 g KF and 16 g HF). Stirring is continued for one hour, and the resulting solids are filtered off. The solids are dried at 110°C in an air-recirculating cabinet. Yield: 95.4 % (of theory) Analysis: XRD confirms pure KZnF3; identification with reference spectrum. DTA up to 650°C no recognisable phase conversion Example 2: Preparation of caesium fluorozincate (JF 009403) 30 g CsOH is reacted with aqueous HF to form CsFHF in solution. 16.3 g zinc oxide is added in portions to this solution, with stirring. It is worked up as in Example 1. Yield: « 52.8 % (of theory) Analysis: 33.9 % Cs, 37.9 % Zn XRD, no reference spectrum present. DTA: several onsets, in particular at 368.5°C, 558.8 "C and 664.6°C. Example 3: Preparation of caesium fluorozincate (JF 009404) 60 g CsOH is reacted with aqueous HF to form CsF HF in solution. 16 g zinc oxide is added to this solution in portions, with stirring. It is worked up as in Example 1. Yield: 52.8 % (of theory) Analysis: 49.0 % Cs, 27.2 % Zn XRD, no reference spectrum present. DTA: small onset 499 "C, main peak at 583 °C onset. Example 4: Preparation of caesium fluorozincate (JF 009415) As in Example 3, but is subsequently stirred for 2.5 hours at approximately 90°C. It is worked up as in Example 1. Yield: 67.3 % (of theory) Analysis: 58% Cs, 26.1 % Zn XRD, no reference spectrum present. Example 5: Preparation of caesium fluorozincate (JF 009418) 45 g CsOH is reacted with aqueous HF to form CsFHF in solution. 16.3 g zinc oxide was added to this solution in portions, with stirring, and is subsequently stirred for 2 hours at approximately 80°C. It is worked up as in Example 1. Yield: 73.5 % (of theory) Analysis: 85.5% Cs, 36.2 % Zn XRD, no reference spectrum present. DTA: Onsets at 502.4 °C, 556.3 °C and 586.4 °C. Example 6; Preparation of sodium fluorozincate (JF 009419) 16 g NaOH is reacted with aqueous HF to form NaF HF in solution. 32.6 g zinc oxide is added in portions to this solution, with stirring. It is worked up as in Example 1. Yield: 95.0% (of theory) Analysis: XRD, identification with reference spectrum 2011 82. DTA: Onset at 648.4 °C. Example 7: Preparation of rubidium fluorozincate (JF 009420) 20.5 g RbOH is reacted with aqueous HF to form Rb HF in solution. 16.3 g zinc oxide is added in portions to this solution, with stirring. It is worked up as in Example 1. Yield: 93.8 % (of theory) Analysis: XRD, reference spectrum 20 10 16. DTA: Maxima at 638.6 °C and 683.9 °C. Soldering tests General soldering conditions: In order to obtain a homogenous distribution of the flux on the surface, a defined quantity of flux with one to two drops of isopropanol is triturated on the surface of an aluminium or AIMg section [25 x 25 mm], with or without solder or solder plating. Then this section was provided with an angled aluminium piece [approx. 45°, length 40 mm, height 5 mm] and was left until the isopropanol had evaporated. This section was then placed in the preheated [approx. 400 °C for ZnAI soldering, approx. 520 °C for AlSi(Cu) soldering] soldering furnace which was flooded with a controlled atmosphere [nitrogen dewpoint -40°C] (so-called CAB brazing) and heated to soldering temperature [soldering of the angled piece with the section, at up to 600 °C, depending on solder] [so-called CAB brazing processes]. Nocolok® is potassium fluoroaluminate. WE CLAM: 1. A process for soldering components of aluminium and aluminium alloys using a flux, wherein the flux is applied in solid form, as an aqueous or organic suspension, as an aqueous or organic slurry or as a paste, and contains alkali fluorozincate or mixtures of alkali fluoride and zinc fluoride, and soldering is effected at a temperature in the range from 420 to 590°C, with "alkali" signifying potassium, caesium or rubidium. 2. A process as claimed in claiml, wherein potassium fluorozincate and/or caesium fluorozincate is contained in the flux. 3. A process as claimed in claim 1, wherein if caesium fluorozincate is contained in the flux, it is present in a quantity of more than 5 mole percent. 4. A process as claimed in claim 1, wherein potassium fluorozincate and/or caesium fluorozincate is contained in a quantity of more than 30% by weight in the flux. 5. A process as claimed in claim 4, wherein potassium fluorozincate and/or caesium fluorozincate is contained in the quantity of 50 or more percent by weight in the flux. 6. A process as claimed in claim 1, wherein the flux consists of alkali fluorozincate or of alkali fluorozincate and an auxiliary. 7. A process as claimed in claim 1 wherein potassium fluoroaluminate or caesium fluoroaluminate is contained in a quantity of up to 95% by weight in addition to the alkali fluorozincate. 8. A process as claimed in claim 1 or 6, wherein the flux contains auxiliaries such as binders, solder metal, solder-metal precursors or stabilisers for suspensions. 9. A process as claimed in claim 8, wherein the flux contains alkali fluorosolicate, with "alkali" signifying potassium, caesium or rubidium. 10. A process as claimed in claim 9 for the solder-free soldering of aluminium and aluminium alloys, wherein the flux contains the alkali fluorosilicate, in particular potassium fluorosilicate, in a quantity of at least 5 to 95% by weight as a solder-metal precursor. 11. A process as claimed in claim 1, wherein the flux is used in the form of an aqueous or alcoholic slurry. 12. A process as claimed in claim 1 wherein soldering is effected in a controlled atmosphere or in a non-oxidising flame. 13. A process as claimed in claim 1 wherein Mg-containing alloys of aluminium are soldered. 14. A flux used for soldering aluminium and aluminium alloys, containing or consisting of alkali metal fluorozincate, solder-metal precursor and optionally alkali metal fluoroaluminate, with "alkali" signifying potassium, caesium or rubidium 15. A flux used for soldering aluminium and aluminium alloys, containing more than 5 mole percent, preferably more than 30% by weight, but less than 100% by weight, caesium fluorozincate. 16. A flux containing or consisting of 5 to 95% alkali fluorozincate and 5 to 95% by weight solder metal, solder-metal precursor, such as silicon, copper, zinc and/or germanium or alkali fluorosilicate, with "alkali" signifying potassium, caesium or rubidium. 17. A flux as claimed in claim 16, wherein it contains or consists of 5 to 90% alkali fluorozincate, 5 to 90% solder-metal precursor and 5 to 90% by weight potassium fluoroaluminate. 18. A flux containing 5 to 95% by weight alkali fluorozincate and 95 to 5% by weight alkali fluorosilicate, or consisting thereof with "alkali" signifying potassium, caesium or rubidium.

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