DE202010017865U1 - Flux for forming a non-soluble solder residue - Google Patents

Abstract

A modified flux for aluminum brazing comprising a basic flux comprising K2AlF5 or a precursor thereof which forms K2AlF5 during brazing, and a Li salt in an amount corresponding to 80% to 120% of the stoichiometric amount required is included to convert all of the K2AlF5 to K2LiAlF6 during brazing.

Description

The invention relates to a flux for brazing aluminum, which forms a brazing residue with very low solubility in water, a method for brazing and brazed aluminum parts, which are obtained when using the flux.

It is well known in the art that brazing of aluminum parts can be carried out using fluxes based on alkali metal fluoroaluminates. Fluxes of this type are generally considered to be corrosion resistant. See for example that U.S. Patent 3,971,501 , which applies a flux based on KAlF ₄ and K ₃ AlF ₆ , or U.S. Patent 4,689,092 which applies a flux based on potassium fluoroaluminate and cesium fluoroaluminate. The U.S. Patent 6,949,300 discloses the kinetic spraying of a brazing composition onto metal substrates which comprises corrosion inhibitors, brazing filler and / or anti-corrosion fluxes.

The EP-A-0 091231 discloses a flux comprising LiF in an amount of from 2% to 7% by weight. The flux has a lowered melting point and is said to be suitable for brazing aluminum alloys containing magnesium. In two examples, articles were brazed with a flux containing K ₃ AlF ₆ , KAlF ₄ and Li ₃ AlF ₆ . The brazed articles were subjected to a brine test and showed no corrosion after a treatment of 1000 hours.

In the GB-A 2 224 751 For example, a method of treating an aluminum workpiece will be described. A treatment of the workpiece with a carbon oxide, for. During brazing, is provided. In this case, the workpiece is black. The formation of a black coating is improved when LiF is present in the flux.

The JP 07-009123 discloses a flux having a melting point of 560 ° C or lower, which is suitable for brazing aluminum alloys containing magnesium. In two examples fluxes with 3 and 10 wt.% LiF were used.

The U.S. Patent 5,802,752 discloses a method of flame brazing (torch brazing) outdoors. A flux is used which contains a potassium fluoroaluminate flux and from 1 to 30% by weight, most preferably from 6 to 11% by weight, of cesium fluoride, lithium fluoride, or both. The flux is well suited for brazing aluminum alloys containing magnesium.

The EP-A-0 347 106 discloses a method of treating aluminum workpieces by heating in an oxidizing atmosphere to improve their corrosion resistance. In some of the examples, among other inorganic additives, LiCl, Li ₃ AlF ₆ or LiF are added in an amount of 4.8% by weight. The effect is enhanced when additives such as alkali and alkaline earth metal salts are added to the flux.

The EP-A-0 541 259 discloses a flux comprising KF-AlF ₃ and, in an amount of 0.75 to 16.5 wt%, LiF. The flux is obtained by melting suitable amounts of KF, LiF and AlF ₃ in a crucible. The brazed parts, when immersed in water, impart a relatively low conductivity to the water.

When aluminum parts brazed with potassium fluoroaluminate fluxes are contacted with water or aqueous fluids, they show signs of corrosion. This is done by Bo Yang et al. in Journal of ASTM International, Vol. 3, Issue 10 (2006) , disclosed. The corrosion can be detected by the appearance of turbidity in the water or in the liquid and seems to cause, for example, the formation of aluminum hydroxide.

This corrosion appears to be caused by fluoride ions which are washed out of braze residues when the brazed parts are in contact with water for extended periods of time, e.g. For at least a day or longer.

It is known that aluminum is always coated with alumina, which prevents a good braze joint; The purpose of adding a flux is to clean the surface of the aluminum parts to be brazed and to noticeably remove the oxide layer. Fluxes based on potassium fluoroaluminate are often used. Often these fluxes consist of KAlF ₄ or a mixture of KAlF ₄ and K ₂ AlF ₅ . The content of K ₃ AlF ₆ in the flux should be low. During the brazing process is often K ₃ AlF _{6 is} formed, and these fluxes can be considered as a precursor for K ₃ AlF ₆ . It is assumed that K ₂ AlF ₅ disproportionates to KAlF ₄ and K ₃ AlF ₆ according to the following equation: 2K ₂ AlF ₅ → KAlF ₄ and K ₃ AlF ₆ (1)

Other fluxes, e.g. As potassium fluorozincate, form potassium fluoroaluminates in situ: 2Al + 3KZnF ₃ → 3Zn + KAlF ₄ + K ₂ AlF ₅ (2)

The formed K ₂ AlF ₅ disproportionates according to equation (1) to give KAlF ₄ and K ₃ AlF ₆ . Thus, such fluxes are precursors of K ₂ AlF ₅ and K ₃ AlF ₆ , and the overall equation is: 4Al + 6KZnF ₃ → 6Zn + 3KAlF ₄ + K ₃ AlF ₆ (2a)

K ₂ SiF ₆ forms K ₂ AlF ₅ , and see (3b), then also K ₃ AlF ₆ : 4Al + 3K ₂ SiF ₆ → 3Si + 2KAlF ₄ + 2K ₂ AlF ₅ (3) 4Al + 3K ₂ SiF ₆ → 3Si + 3KAlF ₄ + K ₃ AlF ₆ (3b)

The formed K ₂ AlF ₅ disproportionates as described above to give KAlF ₄ and K ₃ AlF ₆ . Also potassium fluorostannates are precursors of K ₂ AlF ₅ and K ₃ AlF ₆ .

Upon prolonged contact with water, K ₃ AlF ₆ KF was found to liberate, causing corrosion on the surface of the brazed aluminum parts.

The aim of the present invention is to provide an improved flux which provides brazed aluminum parts having improved anti-corrosion properties, especially after contact with water. Another objective is to provide a brazing process using the new flux. Yet another objective is to provide brazed aluminum parts with improved corrosion protection, especially when in contact with water.

It has been found that when lithium salts are added in specific amounts to fluxes for aluminum brazing, K ₂ LiAlF _{6 is formed} in place of K ₃ AlF ₆ in the braze residue. K ₂ LiAlF ₆ is much less soluble than K ₃ AlF ₆ when in contact with stagnant water. Such contact with stagnant water occurs, for example, when brazed parts are stored outdoors.

In the context of the present invention, the term "comprising" is intended to include the meaning of "consisting of."

In one aspect, the invention relates to a modified flux for aluminum brazing, which provides low solubility residuals in water. The modified flux of the invention is suitable for aluminum brazing and contains a basic flux comprising K ₂ AlF ₅ or a precursor thereof and a Li salt in an amount corresponding to 80% to 120% of the stoichiometric amount is required to convert all of the K ₂ AlF ₅ to K ₂ LiAlF ₆ during brazing.

The term "basic" does not refer to the meaning "pH over 7" but refers to the meaning "essential". It is preferred that the molar ratio of the Li salt and K ₂ AlF ₅ present in the flux or formed from the precursor is 0.8: 1 to 1.2: 1 for Li salts of the LiA type, 0.4: 1 to 0.6: 1 for Li salts of the type Li ₂ B and 0.25: 1 to 0.4: 1 for Li salts of the type Li ₃ C. The preferred ranges are 0.9: 1 to 1.1: 1 for Li-type LiA salts, 0.5: 1 to 0.55: 1 for Li- ₂ B Li salts, and 0.3: 1 to 0.36: 1 for Li salts of the Li ₃ C type. The term "Li salt of the LiA type" refers to Li salts having a monovalent anion A ^- , e.g. As LiF, LiCl or Li acetate. The term "Li salt of the type Li ₂ B" refers to Li salts having a divalent anion B ^2- , e.g. B. Li ₂ SO ₄ , Li ₂ CO ₃ , or lithium oxalate. The term "Li salts of the type Li ₃ C" denotes Li salts with trivalent anion C ^3- . By contrast, the relationship is different for the Li ₃ AlF ₆ compound. This is because Li ₃ AlF ₆ also forms K ₂ LiAlF ₆ according to the following equation (4): 4K ₂ AlF ₅ + Li ₃ AlF ₆ → 2KAlF ₄ and 3K ₂ LiAlF ₆ (4)

Accordingly, the molar ratio between Li ₃ AlF ₆ and K ₂ AlF ₅ present in the flux or formed during brazing is 0.2: 1 to 0.3: 1, and preferably 0.22: 1 to 0.28: 1st

Particularly preferred molar ratios between Li salt and K ₂ AlF ₅ present or formed are 1: 1 to 1.1: 1 for Li salts of the LiA type, especially for LiF, 0.5: 1 to 0.55 : 1 for Li salts of type Li ₂ B, 0.33: 1 to 0.36: 1 for Li salts of Li ₃ C type which are not Li ₃ AlF ₆ , and 0.25: 1 to 0.275: 1 for Li ₃ AlF ₆ . Preferably, the content of K ₃ AlF ₆ in the basic flux is preferably 2 wt% or lower, more preferably 1 wt% or lower, inclusive, 0 wt%. This content is calculated for the flux on a dry weight basis. Consequently, the content of K ₃ AlF ₆ in the modified flux is of comparable size; in fact, this is slightly lower because the Li salt is trapped in the modified flux. Often, the content of K ₃ AlF ₆ in the modified flux is preferably 1.99 wt% or lower and even 1.82 wt% or lower, depending on the amount of added Li salt.

The modified flux according to one embodiment contains a mixture of potassium salt or salts (fluoroaluminate or fluoroaluminates, fluorozincates or fluorosilicates) and lithium salt or salts. Such a flux can be prepared by the dry process by mixing the respective salts. In another embodiment, the lithium content is homogeneously distributed in the potassium salt. Such a flux can be prepared in a wet process by co-precipitation. This will be explained later.

In one embodiment, the basic flux K ₂ comprises AlF ₅ . This embodiment is preferred. In another embodiment, the basic flux comprises a precursor of K ₂ AlF ₅ .

In the following, basic fluxes comprising K ₂ AlF _{5 are} described in detail.

• • Basischen Flussmitteln, umfassend oder bestehend aus K₂AlF₅.
• • Basischen Flussmitteln, umfassend oder bestehend aus KAlF₄ und K₂AlF₅.
• • Basischen Flussmitteln, umfassend oder bestehend aus K₂AlF₅, Cäsiumfluoraluminat und gegebenenfalls KAlF₄.

Preferred basic fluxes of this embodiment are selected from the group consisting of:

• • Basic fluxes comprising or consisting of K ₂ AlF ₅ .
• • Basic fluxes comprising or consisting of KAlF ₄ and K ₂ AlF ₅ .
• • Basic fluxes comprising or consisting of K ₂ AlF ₅ , cesium fluoroaluminate and optionally KAlF ₄ .

The potassium fluoroaluminates may be partially or fully in the form of their hydrates; z. For example, K ₂ AlF _{5 may be} partially or completely present in the form of K ₂ AlF ₅ .H ₂ O.

It is known that K ₂ AlF _{5 is} in a form which may be rehydrated and is in a form which is irreversibly dehydrated. Any of the forms or a mixture thereof in any desired ratio may be present in the fluxes. Details concerning their manufacture and use are in the US Pat. No. 5,980,650 specified. For example, a precipitated K ₂ AlF ₅ raw product is dried in a dryer at 570 ° C for 0.5 second residence time. The resulting product contains irreversibly dehydrated K ₂ AlF ₅ .

The term "consisting of" means that the basic flux does not contain significant amounts of other components, e.g. On other fluxes; Preferably, the flux does not contain more than 2% by weight of other ingredients, e.g. B. other fluxes. The content of K ₃ AlF ₆ in all fluxes of this embodiment is preferably 2% by weight or lower, more preferably 1% by weight or lower inclusive 0% by weight. This content is calculated for the modified flux on a dry weight basis.

Comprise or basic flux, the K ₂ AlF ₅ are made therefrom may be of K ₂ AlF ₅ and / or its hydrate, K ₂ AlF ₅ · H ₂ O, included. The total content of K ₂ AlF ₅ is preferably 95% by weight or higher. The preferred content of K ₃ AlF ₆ , if present, is as stated above and is preferably 2% by weight or less.

Basic fluxes comprising or consisting of KAlF ₄ and K ₂ AlF ₅ will now be described in detail.

In these basic fluxes KAlF ₄ and K ₂ AlF ₅ and, if present, their hydrates are the main constituents. Often, the potassium fluoroaluminate consists essentially of a mixture of KAlF ₄ and K ₂ AlF ₅ or their hydrates; "Substantially" means preferably that their sum is 95% by weight or more, more preferably 98% by weight or more of the basic flux; in particular, at most 2 Wt% of K ₃ AlF ₆ , preferably 2 wt% or less, and most preferably 1 wt% or less, including 0%. The weight ratio between KAlF ₄ (including any hydrate, if any) and K ₂ AlF ₅ (including any hydrate, if any) is very flexible. It can be 1:99 to 99: 1. Often it ranges from 1:10 to 10: 1. A basic flux comprising 10 to 40% by weight of K ₂ AlF ₅ , K ₂ AlF ₅ .H ₂ O, or any mixtures thereof, the remainder to 100% by weight being essentially KAlF ₄ , is very suitable ,

The following describes basic fluxes which comprise or consist of K ₂ AlF ₅ , cesium fluoroaluminate and optionally KAlF ₄ . Basic fluxes containing potassium fluoroaluminate and cesium cations, e.g. In the form of cesium fluoroaluminate, as in US Patent 4670067 and US 4689062 described are also very suitable. These cesium-containing basic fluxes are particularly suitable for brazing aluminum-magnesium alloys. The weight ratio of KAlF ₄ and K ₂ AlF ₅ is preferably as described above. The Cs content, calculated as content in CsF, is between 2 and 74 mol%. The cesium is preferably in the form of CsAlF ₄ , Cs ₂ AlF ₅ , Cs ₃ AlF ₆ , their hydrates and any mixture of two, three or more thereof. The sum of KAlF ₄ , K ₂ AlF ₅ and the cesium fluoroaluminate compound or compounds, including any hydrate, is preferably 95% by weight or more, more preferably 98% by weight or more. The content of K ₃ AlF ₆ is preferably 2 wt% or lower, and most preferably 1 wt% or lower, including 0 wt%.

The flux of the present invention contains one of the above-mentioned basic fluxes and a proper amount of a lithium salt. The mixture of the basic flux and the lithium salt is referred to as a "modified flux". As mentioned above, the lithium content in this modified flux depends on the content of K ₂ AlF _5, including its hydrate, if present; 0.8 to 1.2 molar equivalents of the lithium salt with a monobasic anion are present in the modified flux per mole of K ₂ AlF ₅ . This is explained with regard to LiF as the lithium salt. As an example, the calculations are for a basic flux consisting of 80 wt% KAlF ₄ and 20 wt% K ₂ AlF ₅ . Such flux is available from Solvay Fluor GmbH, Hannover, Germany, under the trade name Nocolok ^®. 100 g of this flux contains 20 g of K ₂ AlF ₅ , which corresponds to 0.1 mol, since K ₂ AlF _{5 has} a molecular weight of 200 g. Thus, the modified flux of the invention contains 0.08 to 0.12 moles of LiF. This corresponds approximately to the addition of 2.08 g to 3.12 g of LiF. Accordingly, the modified flux comprises 2.08 g: (100 + 2.08 g) = 2.04 wt% to 3.12 g: (100 + 3.12 g) = 3.0% wt% LiF.

Table 1 below sets forth the minimum and maximum levels of LiF in a modified flux with varying content of K ₂ AlF ₅ (in one example K ₂ AlF ₅ .H ₂ O). The modified flux including KAlF ₄ , K ₂ AlF ₅ .H ₂ O and LiF is set as 100% by weight. Table 1: Modified fluxes containing LiF Content of K ₂ AlF ₅ in basic flux [g / 100 g basic flux] Minimum added amount of LiF [in g per 100 g basic flux] Maximum added amount of LiF [in g per 100 g basic flux] Minimal content of LiF in modified flux [% by weight] Maximum content of LiF in modified flux [% by weight] 5 0.52 0.78 0.52 0.77 10 1.04 1.56 1.02 1.53 15 1.56 2.34 1.54 2.3 20 2.08 3.12 2.04 3.02 25 2.6 3.9 2.53 3.75 30 3.12 4.68 3.03 4.47 35 3.64 5.46 3.51 5.18 40 4.16 6.24 3.99 5.87

In a preferred embodiment, the ratio of the LiA type compound, preferably LiF, and K ₂ AlF ₅ in the modified flux is 0.9: 1 to 1.1: 1.

Table 2 lists a list of modified fluxes of this preferred embodiment. Table 2: Preferred Modified Fluxes Containing LiF Content of K ₂ AlF ₅ in basic flux [g / 100 g basic flux] Minimum added amount of LiF [in g per 100 g basic flux] Maximum added amount of LiF [in g per 100 g basic flux] Minimal content of LiF in modified flux [% by weight] Maximum content of LiF in modified flux [% by weight] 5 0.59 0.72 0.59 0.71 10 1.17 1.43 1.16 1.4 15 1.76 2.15 1.73 2.1 20 2.34 2.86 2.29 2.78 25 2.93 3.58 2.85 3.46 30 3.51 4.29 3.39 4.11 35 4.1 5.01 3.94 4.77 40 4.68 5.72 4.47 5.41

It is particularly preferred if the molar ratio of the LiA type compound, preferably LiF and K ₂ AlF _5, in the modified flux is 1: 1 to 1.1: 1.

Table 3 below describes modified fluxes with the basic flux addition of Li ₃ AlF ₆ . The amount was calculated according to the equation (4). Table 3: Modified fluxes comprising added Li ₃ AlF ₆ Content of K ₂ AlF ₅ in basic flux [g / 100 g basic flux] Minimum amount of Li ₃ AlF _{6 to be added} [in g per 100 g basic flux] Maximum amount of Li ₃ AlF _{6 to be added} [in g per 100 g of basic flux] Minimal content of Li ₃ AlF ₆ in modified flux [% by weight] Maximum content of Li ₃ AlF ₆ in modified flux [% by weight] 5 0.81 1.22 0.8 1.21 10 1.62 2.43 1.6 2.37 15 2.43 3.65 2.37 3.52 20 3.24 4.86 3.14 4.63 25 4.05 6.08 3.89 5.73 30 4.86 7.29 4.63 6.79 35 5.67 8.51 5.37 7.84 40 6.48 9.72 6.09 8.86

According to a further embodiment, the basic flux and thus the modified flux comprises a precursor of K ₂ AlF ₅ . Preferred precursors are potassium fluorozincate and K ₂ SiF ₆ . KZnF ₃ is a basic flux which is a precursor of K ₂ AlF ₅ and thus K ₃ AlF ₆ . It forms K ₂ AlF ₅ according to equation (2), as indicated above. The addition of Li salts prevents the formation of K ₃ AlF ₆ from this compound, as shown for the LiF addition: 2Al + 3KZnF ₃ + LiF → 3Zn + KAlF ₄ + K ₂ LiAlF ₆ (5)

Since the molecular weight of KZnF _{3 is} 161.4 and 0.8 to 1.2 moles of monobasic Li salt are used per mole of K ₂ AlF ₅ formed, about 4.3 g to 6.5 g LiF (or the respective Amounts of other Li salts with monobasic anions) per 100 g of KZnF _{3 added to} the basic flux. Thus, the modified flux comprises about 4 to 6.1% by weight of LiF or any other Li salt with monobasic anions.

The amount of Li ₃ AlF ₆ can be calculated from equation (6): 8Al + 12KZnF ₃ + Li ₃ AlF ₆ → 12Zn + 6KAlF ₄ + 3K ₂ LiAlF ₆ (6)

About 6.5 g to 10 g of Li ₃ AlF _{6 are} added per 100 g of KZnF ₃ . Accordingly, very little Li salt is needed to reduce the corrosive influence of cooling water or stagnant water on articles brazed with the potassium fluorozincate flux. The modified flux comprises 6.1 to 9.1 wt% Li ₃ AlF ₆ .

These calculations assume that the flux consists of the Li salt and KZnF ₃ . The expert can easily recalculate the amounts of Li salt added to the basic flux if the basic flux contains additives. For example, if the basic flux consists of 70 wt% KZnF ₃ and 30 wt% silicon, the amount of LiF to be added would be 4 x 0.7 g = 2.8 g to 6.5 x 0.7 g = 4.6 g; consequently, the modified flux contains from 2.7 g to 4.4% by weight of LiF. The amount of Li ₃ AlF ₆ would be 4.6 g to 7 g, and the modified flux contains 4.4 to 6.5 g of Li ₃ AlF ₆ . However, it was found that the anti-corrosion effect of Li salts for basic fluxes of the potassium fluoroaluminate type described above was better.

Accordingly, modified fluxes include K ₂ AlF ₅ and 10.5 g to 15.5 g LiF or 16.2 g to 24.3 g Li ₃ AlF ₆ per 100 g K ₂ AlF ₅ . Particularly preferred modified fluxes include 11.7 g to 14.3 g LiF or 18.2 g to 22.3 g Li ₃ AlF ₆ per 100 g K ₂ AlF ₅ .

Alternatively, flux KZnF ₃ and 4.3 g to 6.5 g LiF or 6.5 g to 10 g Li ₃ AlF ₆ per 100 g KZnF ₃ . Fluxes often include KZnF ₃ 4.8g to 6g LiF or 7.3g to 9.2g Li ₃ AlF ₆ per 100g KZnF ₃ .

The basic fluxes, which can be modified by the addition of Li salts, as well as their preparation are known and often commercially available. Basic fluxes based on potassium fluoroaluminate are described, for example, in US Pat U.S. Patent 3,951,328 . U.S. Patent 4,579,605 and U.S. Patent 6,221,129 described. Basic flux precursors, especially potassium hexafluorosilicate, may also be used.

Alkalimetal fluorozincate basic fluxes are disclosed, for example, in U.S. Pat U.S. Patents 6,432,221 and 6,743,409 disclosed.

The fluxes optionally contain braze metal precursors, especially Si. The particle size of Si is preferably 30 μm or smaller.

If desired, the various basic flux, lithium salt and any additive powders, if used, may be blended and / or ground to obtain a more homogeneous modified or smaller particle size modified flux.

The preferred flux substantially or consists of potassium fluoroaluminate, especially K ₂ AlF ₅ or a mixture of KAlF ₄ and K ₂ AlF ₅ , or of potassium fluorozincate and one of LiF and Li ₃ AlF ₆ in the amounts based on K ₂ AlF ₅ , which are contained in the flux or are formed during brazing as briefly outlined above.

Li ₃ AlF ₆ , LiF and other Li salts, e.g. LiOH or Li ₂ CO ₃ , are commercially available. While inorganic basic Li compounds, for example LiOH or Li ₂ CO ₃ , are very suitable in the wet process, as in US Pat U.S. Patent 4,428,920 described, other Li compounds, eg. As LiF, optionally together with the above-mentioned basic Li compounds. Fluoroaluminic acid can be produced from aluminum hydroxide and HF in the stoichiometric amounts concerned. Li ₃ AlF ₆ is available from Solvay Fluor GmbH, Hannover, Germany.

The preparation of potassium fluoroaluminate fluxes with varying levels of KAlF ₄ and K ₂ AlF ₅ is described in U.S. Pat U.S. Patent 4,579,605 described. Aluminum hydroxide, hydrofluoric acid and a potassium compound, e.g. B. KOH dissolved in water are reacted. The patent discloses that by applying the Starting materials in specific molar ratios and concentrations and maintaining specific reaction temperatures, the content of KAlF ₄ and K ₂ AlF ₅ in the resulting flux mixture can be previously set.

The Li cations can be introduced into the modified flux in two main ways: the wet process and the dry process. According to the wet process, the modified fluxes are prepared in processes which include at least one precipitation step. For example, potassium lithium fluoroaluminates can be prepared by reacting aluminum hydroxide with HF to produce fluoroaluminic acid, which in turn is then reacted with potassium hydroxide in the presence of Li salts, for example LiF, Li ₃ AlF ₆ , LiOH, Li-oxalate or Li ₂ CO ₃ (or even Li metal - but there is a risk due to the formation of H ₂ ) is reacted so that precipitates potassium fluoroaluminate containing Li cations. The advantage is a fairly homogeneous distribution of Li content in the precipitate formed.

Alternatively, the modified flux can be prepared in a dry process by mechanically mixing the basic flux and the Li salt in the desired proportions. Also, organic and inorganic Li compounds generally appear suitable here. Preferably, fluorine-containing Li compounds, if desired in the form of mixtures of two or more such compounds, are used as the source of Li cations. It is possible to use compounds containing only Li cations. For example, compounds or mixtures of compounds containing Li cations and other alkali metal cations, preferably K and / or Cs cations, may be employed. Frequently, LiF or lithium fluoroaluminate is used as the source of Li cations.

The modified flux, whether obtained by the wet process or the dry process, is in principle applicable in the same way as the basic flux for brazing. It can be as such, for. B. be used as a dry flux electrostatically or by plasma spraying. It can also be applied in a wet fluxing or wet fluxing process. Details are given below when the aspect of the present invention relating to a brazing method is explained in detail below.

As mentioned above, the modified flux improves the anti-corrosion properties of parts brazed therewith. It is recognized in the art that especially fluoroaluminate fluxes are basically non-corrosive to aluminum or aluminum alloys. Nevertheless, under certain circumstances - prolonged contact with water, especially with stagnant water, or with aqueous fluids such as cooling fluid (cooling water) - corrosion appears to occur. This can be recognized by a white precipitate (believed to be aluminum hydroxide) found in the water or aqueous humor.

Thus, the modified fluxes are preferably used for improving the durability of aluminum parts which after brazing are subjected to an additional step where they are in contact with water or aqueous compositions, especially with stagnant water such as rainwater or cooling water, for an extended period of time. This often leads to a washing out of fluoride. The term "longer periods" refers to a contact period that lasts at least one day, preferably two days. The term "longer period" has no specific upper limit. This can last for over a week or more. In the case of water containing, for example, cooling liquid, the contact between the liquid and aluminum may take years, e.g. 1 year or less, 2 years or less and even 5 years or less.

In the present invention, the term "caused by fluoride ions" is used. The reason is that the largest corrosive influence is attributed to the fluoride ion. It is possible that other species originating from the flux residue dissolution may have corrosive properties. Thus, the term "caused by fluoride ions" does not exclude the possibility that corrosion is caused by other species present in the water or aqueous solution or by other chemical mechanisms.

The term "water" excludes water from natural sources, e.g. As rainwater, water that is formed as dew, and water that forms after a snow melt, a. It includes artificial water sources, eg. As tap water, a. The term "water" is also meant to include aqueous compositions. The term "aqueous compositions" in its broadest sense includes any composition containing water and at least one additional ingredient, e.g. As an inorganic or organic salt, and often liquid ingredients, for example, an organic liquid, for. B. one monobasic or dibasic alcohol, and comes into contact with brazed aluminum parts. It includes, for example, cooling fluid which typically contains, in addition to water, additional antifreeze compounds, especially glycol, and additives, for example anticorrosion agents or colorants, such as those used in stationary refrigerator water radiators or stationary heat exchangers or automotive cooling water. Preferably, the term "water" refers to rainwater, water formed by snow or dew; and the term "aqueous compositions" preferably designates cooling water in internal combustion engines.

In one embodiment of this aspect of the invention, the aluminum parts that are made more resistant to corrosion by using Li ⁺ -modified flux are heat-treated with oxygen or oxygen dissolved in air or inert gases, e.g. B. in mixtures containing oxygen and argon and / or nitrogen, post-brazed after brazing. It has been found that fluoride which has been washed out of flux residues after prolonged periods of contact of the aluminum parts with water has less corrosive effect on the brazed parts compared to brazed parts without heat treatment in air or the oxygen containing gases.

In this embodiment, the brazed parts are subjected to a heat treatment in an oxygen-containing atmosphere. Preferably, the temperature during the heat treatment is 400 ° C or higher. It is preferably equal to or lower than 530 ° C. If desired, the temperature may be higher. A preferred oxygen-containing atmosphere is air.

The duration of the heat treatment is preferably equal to or longer than 10 seconds, more preferably equal to or longer than 30 seconds. It is preferably 1 hour or shorter, especially 15 minutes or shorter.

An oxidizing heat treatment for improving the corrosion resistance is already known from EP-A-0 034706 known. However, it is not clear from the description of said relevant EP application against which type of corrosion or corrosion caused by which corrosive agent the protected aluminum parts could be protected. A reference to the examples indicates that protection against corrosion caused by salt water should occur. The said EP application does not deal with problems caused by fluoride ions which have been washed out of the flux after contact with water for a prolonged period of time, for example for one day or more.

According to another embodiment, no additional treatment of the parts in oxygen or in an oxygen-containing atmosphere is performed. Preferably, the use of the modified flux of the invention is the sole anticorrosive treatment of the brazed parts.

In a preferred embodiment, the terms "water" and "aqueous composition" in the context of the present invention do not include salt water, especially salt water according to the AST-GM43 SWAT test.

The modified flux may be used for brazing in the same manner as known to those skilled in the art of basic fluxes. The above-mentioned modified flux may, for example, be used per se as a dry powder, optionally together with additives, e.g. B. by electrostatic application. For example, the above-mentioned modified fluxes may be applied to the aluminum parts in a wet process, optionally together with additives. In the following, suitable additives will be described in more detail.

There are two main categories of additives: braze additives that improve the joint between the brazed parts, e.g. B. improve the brazing of Al-Mg alloys or generally improve the surface properties of the joint, and Fluxierungsadditive that modify or improve the Fluxierungsweg the parts to be joined. Useful additives will now be explained in some detail.

In the following paragraphs, brazing additives that improve or modify brazing are discussed with respect to potassium fluoroaluminate, which is the preferred example of a basic flux.

In one embodiment, the modified flux further contains at least one magnesium compatibilizer selected from the group consisting of cesium fluoroaluminates, cesium fluorozincates and potassium fluorozincates. Such a flux is also suitable for brazing aluminum alloys containing equal to or greater than 0.5% by weight of magnesium. The content of the magnesium compatibilizer is preferably equal to or higher than 0.5% by weight of the flux, that is, the sum of potassium fluoroaluminate, LiF or lithium fluoroaluminate, and the magnesium compatibilizer. It is preferably equal to or lower than 20% by weight of the flux.

The flux may be additionally modified by metal salts of metals of the main group or subgroups of the Periodic Table of the Elements, for example by halides, nitrates, carbonates or oxides of zirconium, niobium, lanthanum, yttrium, cerium, titanium, strontium, indium, tin, antimony or Bismuth, as described in US Patent Application Publication 2007-0277908. These additives may preferably be contained in an amount equal to or less than 3% by weight of the total dry weight of the flux.

The flux can also braze (fill) metal, z. As Al-Si alloys, or brazing metal precursors, for example, silicon, copper or germanium, as in the U.S. Patent 5,1000,486 described include. The braze metal precursors, if present in the flux, are preferably present in an amount of from 2 to 30% by weight of the total flux.

Another flux suitable for aluminum brazing contains potassium fluorozincate, the compound containing the Li cation, and optionally Si. Again, if included, the Si is preferably present in an amount of from 2 to 30 percent by weight of the total flux.

If desired, fluxes with specific particle sizes can be chosen for specific application methods. For example, the particles including any brazing additives may have the particle size distribution as shown in U.S. Pat US Pat. No. 6,733,598 disclosed, and are particularly suitable for the application according to the dry method, for. B. by electrostatic energy.

The particles of the flux may be of a coarser nature than the finer particles found in the US 6,733,598 be revealed. Such coarser fluxes are very suitable for use in the form of a flux composition which includes the flux dispersed in a solvent; they can be applied to the parts, for example, by brushing, printing or spraying.

The flux, including optionally modifying metal salts or magnesium compatibilizers, for example those described above, can be used as such with or without additives, for example as a dry powder, electrostatically or by the application of a low temperature plasma as described in U.S. Pat WO 2006/100054 described, be applied. In this plasma process, finely divided flux powder is partially melted by a low temperature plasma jet and sprayed onto the surface of the aluminum parts to be joined.

The modified flux with or without one or more of the above-mentioned brazing additives may also be applied according to the wet process in the form of a flux composition. Here, the flux composition comprises the modified flux or flux optionally containing one or more of the braze additives and / or fluxing additives.

A wet-spray flux composition containing the flux described above is another embodiment of the present invention. This flux composition (and thus also the brazing method according to the present invention in which the flux composition can be applied) will now be explained in detail.

The flux composition of the present invention suitable for the wet fluxing process contains the modified flux optionally including one or more of the brazing additives and at least one of the fluxing additives selected from the group consisting of solvents, binders, thickeners, suspension stabilizers, Defoaming agents, surfactants and thixotropic agents.

In a preferred embodiment, the flux composition contains the flux suspended in the solvent, especially in water, anhydrous organic liquids or aqueous organic liquids. Preferred liquids are those which have a boiling point at ambient pressure (1 bar abs.) Equal to or lower than 350 ° C. The term "suspended in water" does not exclude that part of the flux composition is dissolved in the liquid; This can be special then be the case if water or aqueous organic liquids are contained. Liquids which are preferred are deionized water, mono-, di- or tribasic aliphatic alcohols, especially those having from 1 to 4 carbon atoms, e.g. For example, methanol, ethanol, isopropanol or ethylene glycol, or glycol alkyl ethers, wherein alkyl is preferably linear or branched aliphatic C1 to C4 alkyl. Non-limiting examples are glycol monoalkyl ethers, e.g. For example, 2-methoxyethanol or diethylene glycol, or glycol dialkyl ethers, for example dimethyl glycol (dimethoxyethane). Mixtures comprising two or more of the liquids are also very suitable. Isopropanol or mixtures containing isopropanol are particularly suitable.

The composition comprising the flux suspended in a liquid may also contain other fluxing additives, for example thickeners, surfactants or thixotropic agents.

In a particularly preferred embodiment, the flux is in the form of a flux composition in which the flux is suspended in a liquid which also contains a binder. Binders, for example, improve the adhesion of the flux mixture after it has been applied to the parts to be brazed. Thus, the wet flux method using a flux composition comprising flux, binder and water, organic liquid or aqueous organic liquid is a preferred embodiment of the brazing method of the present invention.

Suitable binders may be selected, for example, from the group consisting of organic polymers. Such polymers are physically drying (ie, they form a solid coating after the liquid has been removed), or are chemically drying (they can form a solid coating, eg, under the influence of chemicals, eg, oxygen or Light, which causes crosslinking of the molecules), or both. Suitable polymers include polyolefins, e.g. For example, butyl rubbers, polyurethanes, resins, phthalates, polyacrylates, polymethacrylates, vinyl resins, epoxy resins, nitrocellulose, polyvinyl acetates or polyvinyl alcohols. Flux compositions containing water as a liquid and water-soluble polymers, for example polyurethane, or polyvinyl alcohol as a binder are particularly suitable because they have the advantage of evaporating water during the brazing operation in place of potentially flammable organic liquids.

The compositions may include other additives which enhance the properties of the composition, for example, suspension stabilizers, surfactants, especially nonionic surfactants, e.g. Antarox BL 225, a mixture of linear C 8 to C 10 ethoxylated and propoxylated alcohols, thickeners, e.g. Methylbutyl ether, thixotropic agents, e.g. Gelatin or pectins, or a wax, as in EP-A 1808264 described.

The content of the modified flux (including the basic flux, Li-containing additive and, if present, other additives, e.g., filler metal, filler precursor, additives, e.g., metal salts, to improve the braze or surface properties) in U.S. Patent No. 5,376,874 Total composition (including liquid or liquids, thixotropy generating agents, surfactants and binders, if present) is generally equal to or greater than 0.75 weight percent. It is preferably equal to or higher than 1% by weight. More preferably, the flux content in the composition is equal to or greater than 5 weight percent, more preferably equal to or greater than 10 weight percent, based on the total flux composition.

Generally, the level of modified flux in the composition is equal to or less than 70% by weight. It is preferably equal to or lower than 50% by weight.

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

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

The thickening agent, if present, is generally contained in an amount equal to or higher than 1% by weight, preferably equal to or higher than 5% by weight of the total flux composition. Generally, the thickening agent, if present, is contained 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 contain from 10 to 70 weight percent of the flux (including filler metal, filler precursor, modifying and anti-corrosion agents, e.g., metal salts, to improve braze and surface properties), 1 to 25 weight percent binder , 0 to 15% by weight of a thickening agent, 0 to 10% by weight of a thixotropic agent and 0 to 5% by weight of other additives, e.g. B. a surfactant or a suspension stabilizer. Preferably, the balance is 100% by weight of water, an organic solvent or an aqueous organic solvent.

In a specific embodiment, the flux composition is free of any water or anhydrous or aqueous organic liquid, but contains the flux (and optionally one or more of the filler metal or precursor, modifying or anti-corrosive agent, the brazing process, or the properties of the brazed product or other additives, for example those described above) as described above, and a water-soluble organic polymer as a binder which is in the form of a water-soluble packing for the flux. For example, polyvinyl alcohol is very suitable as a water-soluble packing for the flux, as described in US Patent Application Publication 2006/0231162. Such packages can be handled without dusting, and after the addition of water they form a suspension in water which includes a flux and the water-soluble polymer as a binder.

The content of Li salt with respect to K ₂ AlF ₅ present or formed during brazing is not affected by the additives, with the exception of K ₂ SiF ₆ , which forms K ₂ AlF ₅ during brazing and Thus, when calculating the amount of the required Li salt must be considered.

When the solvent is water or water and an organic liquid, e.g. B. contains an alcohol, it is preferred to apply Li salts with low solubility in water, for. B. compounds having a solubility of 0.5 g or less in water at 20 ° C; LiF and Li ₃ AlF ₆ are also very suitable here.

Another aspect of the present invention is to provide a method of brazing aluminum parts, which comprises the step of spreading the modified flux or the composition containing the modified flux onto a part of the surface (including those parts of the surface which during brazing) or the entire surface of the brazing parts is applied. After fluxing, these parts are assembled and brazed, or alternatively, the parts to be brazed are assembled, then floated and then brazed. In an alternative, the brazed parts are subjected to oxidative heat treatment after brazing; in another alternative they are not subjected to an oxidative heat treatment.

The flux may be applied according to the dry fluxing method described above. Alternatively, the wet blast compositions may be applied to the aluminum parts according to methods known in the art. For example, they may be sprayed onto the surface to form coated aluminum parts, alternatively they may be immersed in the flux composition by immersing the aluminum parts to be coated; or by brushing or printing the flux composition on the aluminum parts to be brazed, thus forming coated parts. It should be noted that the term "aluminum" includes aluminum alloys, especially magnesium-containing alloys. The liquid-free flux composition containing fluxes, water-soluble binder and optionally other additives in the form of a package may be added to water prior to use to form an aqueous flux composition containing suspended flux mixture and dissolved binder.

Generally, the parts coated with the wet flux composition are dried (of course this is not necessary for parts which are coated according to the dry process, unless fluoroaluminate hydrates are applied and it is desired to remove water of crystallization prior to starting the brazing operation). The drying can be done independently of brazing, the dried flux coated parts can then be stored until they are brazed. Alternatively, the Drying takes place directly in the brazing device or in a separate drying device immediately before the brazing operation.

A method of brazing aluminum or aluminum alloy parts in which the modified flux according to the invention or a flux composition comprising the modified flux and additives is coated on at least one of the parts to be brazed, and the parts are heated to a temperature is sufficiently high for brazing the parts.

For brazing, the coated parts to be joined by brazing are assembled (before or after drying when coated by a wet process) and heated to about 560 ° C to about 610 ° C. This can be done in an inert gas atmosphere, for. B. in a nitrogen or argon atmosphere. It is also possible to braze it outdoors or in the open air (burner brazing).

It has been found that aluminum parts brazed to the flux of the invention containing lithium fluoroaluminate are generally very resistant to corrosion.

Another aspect of the present invention relates to aluminum parts or aluminum alloy parts coated with the modified flux of the present invention. These parts are preferably parts which are suitable for the production of heat exchangers, for. As of tubes and fins or fins, in coolers or air conditioning systems of cars or trucks or for the production of "HVAC" equipment can be used. HVAC means heating, ventilation, air conditioning - the technology of indoor climate comfort.

Another aspect of the present invention relates to composite parts of aluminum or aluminum alloys that are brazed using a flux or flux composition according to the present invention. These parts are preferably parts that are used in the transfer of heat from one medium to another medium. Preferably, these parts are used in heat exchangers or cooling systems of cars and trucks and in HVAC equipment.

In the international patent application PCT / EP2009 / 065566 , previously filed by the Applicant and not yet published, certain compositions of a Li-modified flux have already been described. In one embodiment of the invention are modified fluxes, compositions or coated parts, brazing processes using the flux or flux compositions and brazed parts, as in said PCT / EP2009 / 065566 be refused, if required by law. For example, the following blends are disclosed therein: in Table 1, a modified flux consisting of 3 wt.%, 3.7 wt.%, And 4 wt.% Li ₃ AlF ₆ , the remainder being 100 wt. % Nocolok ^® is a modified flux, consisting of about 80 wt .-% KAlF ₄ and about 20 wt .-% K ₂ AlF _5; a modified flux consisting of 66 wt% KAlF ₄ , 28 wt% K ₂ AlF ₅ , and 6 wt% K ₃ AlF ₆ ; a modified flux consisting of 76 wt% KAlF ₄ , 19 wt% K ₂ AlF ₅ and 5 wt% Li ₃ AlF ₆ ; a modified flux consisting of 66 wt.% KAlF ₄ , 28 wt.% K ₂ AlF ₅ and 6 wt.% Li ₃ AlF ₆ ; a modified flux consisting of 67% by weight KAlF ₄ , 28% by weight K ₂ AlF ₅ and 5% by weight Li ₃ AlF ₆ ; a modified flux consisting of 77% by weight KAlF ₄ , 19% by weight K ₂ AlF ₅ and 4% by weight Li ₃ AlF ₆ ; a modified flux, comprising potassium fluoroaluminate, cesium fluoroaluminate with a ratio of K: Cs of 98: 2, available as Nocolok ^® Cs from Solvay Fluor GmbH, and 5 wt .-% Li ₃ AlF _6; a modified flux, containing 5 wt .-% Li ₃ AlF _6, wherein the balance to 100 wt .-% Nocolok ^®; a flux consisting of 75 parts of KZnF ₃ , 25 parts of silicon powder and 5 parts of Li ₃ AlF ₆ ; a flux containing 38% by weight K ₂ AlF ₅ .H ₂ O, 57% by weight KAlF ₄ and 5% by weight Li ₃ AlF ₆ ; a flux contains 36.8% by weight K ₂ AlF ₅ .H ₂ O, 55.2% by weight KAlF ₄ and 8% by weight Li ₃ AlF ₆ ; and brazing techniques for some of these fluxes.

It will be apparent to those skilled in the art from the above description that the described invention provides a modified flux which can be used for aluminum brazing. Besides the advantage that it makes the brazed parts less corrosive in contact with water or aqueous compositions, the preformed modified flux has the further advantage that it can be easily applied. Even if made by mechanically mixing the ingredients, they are quite homogeneous over the stored volume. Nevertheless, if desired, especially in wet application processes, the constituents of the basic flux, optionally any additives, and the Li salt may be used separately. If desired, the basic flux and any other additives, if present, are predispersed in the solvent. The Li salt is then added to this composition, preferably just prior to application of the dispersion to the parts to be brazed. The advantage of this embodiment is that of each Use of a flux composition, be it a flux in dry form or already predispersed in water, an organic solvent or a mixture of water and an organic solvent, and yet the advantages of Li salts with respect to that they render the brazed aluminum parts corrosion resistant in contact with water or aqueous compositions. Suitable organic solvents are already mentioned above; Ethanol, isopropanol and butanol are preferred solvents.

Accordingly, the invention also provides a method of brazing aluminum parts in which a flux comprising K ₂ AlF ₅ or containing a precursor of K ₂ AlF ₅ , the K ₂ AlF ₅ during the brazing process for aluminum brazing and a lithium salt having a low solubility in water in water or an aqueous solution is dispersed separately to form a dispersion containing the flux and the lithium salt, after which at least one aluminum part is at least partially coated with the dispersion and at least two composite parts be brazed. The flux is preferably selected from fluxes comprising or consisting of KZnF ₃ , K ₂ AlF ₅ , K ₂ AlF ₅ .H ₂ O, mixtures thereof and K ₂ AlF ₅ and / or K ₂ AlF ₅ .H ₂ O mixed with KAlF ₄ , and more preferably from fluxes comprising or consisting of K ₂ AlF ₅ , K ₂ AlF ₅ .H ₂ O, mixtures thereof and K ₂ AlF ₅ and / or K ₂ AlF ₅ .H ₂ O mixed with KAlF ₄ .

The flux and the lithium salt may be dispersed in dry form in water, the organic solvent or the aqueous organic dispersion. Additives commonly used in flux dispersions may be added. Preferred additives are selected from fluxing additives selected from the group consisting of binders, thickeners, suspension stabilizers, antifoams, surfactants, and thixotropic agents. Preferred solvents, binders, thickeners and thixotropic agents are mentioned above.

In order to provide the dispersion used in this invention, water, the organic solvent or the aqueous organic liquid as a carrier can be put in a container with mixing means such as a stirrer, dry flux and dry Li salt can be added in any order simultaneously or sequentially and dispersed. Alternatively, the flux may be added to the carrier as a composition comprising additives, e.g. B. as a composition comprising the flux and at least one additive selected from the group consisting of binders, thickeners, suspension stabilizers, antifoaming agents, surface active agents and thixotropic agents, which is then added to the carrier. If desired, the flux composition itself may form a dispersion in a solvent. The Li salt may be added in dry form or as a dispersion in a solvent. According to yet another alternative, the flux is provided in the form of a dispersion and the Li salt is added to this dispersion. The dispersion is applied to the parts in any desired manner, e.g. B. by brushing on the parts, by dipping the parts or by spraying applied to the parts.

The amount of added lithium salt is such that the content of Li ⁺ in the dispersion based on the dry weight of the dispersion is 0.1 wt% or higher and 4.6 wt% or lower.

Also, in this invention, because of their solubility in water, LiF and Li ₃ AlF _{6 are} the preferred Li salts, especially when aqueous flux compositions or flux compositions dispersed in a mixture of water and organic solvent as a carrier are applied.

Based on the dry weight of the dispersion, the content of 0.1% by weight of Li ⁺ corresponds to a content of about 1% by weight (exactly 0.77% by weight) of Li ₃ AlF ₆ or 0.37% by weight. % LiF. Preferably, the content of Li ⁺ in the modified flux is 0.13 wt% or higher.

The content of Li ⁺ can be very high. Generally, the content of Li ⁺ in the modified flux is 4.6% by weight or lower. This corresponds to a content of about 36% by weight of Li ₃ AlF ₆ or 17.5% by weight of LiF in the prepared dispersion. The remainder of 64% by weight on a dry basis is formed by the basic flux. Preferably, the content of Li ⁺ 1.3 wt .-% or lower. This corresponds to a content of about 10% by weight of Li ₃ AlF ₆ or about 5% by weight, on a dry basis, of LiF in the flux. More preferably, the content of Li ^{+ is} lower than 1.3 wt%. Most preferably, the content of Li ⁺ in the dispersion on a dry weight basis is 1.16 wt% or lower. This corresponds to a content of about 9 wt .-% Li ₃ AlF ₆ or about 4.3 g LiF.

Preferably, the content of Li salt in the dispersion is such that the molar ratio of the Li salt and K ₂ AlF ₅ present in the flux or formed from the precursor is 0.8: 1 to 1.2: 1 for Li salts of the type LiA, 0.4: 1 to 0.6: 1 for Li salts of the type Li ₂ B, and 0.25: 1 to 0.4: 1 for Li salts of the type Li ₃ C is. The preferred ranges are 0.9: 1 to 1.1: 1 for Li-type LiA salts, 0.5: 1 to 0.55: 1 for Li- ₂ B type Li salts, and 0.3: 1 to 0.36: 1 for Li salts of the type Li ₃ C. The term "Li salt of the type LiA" denotes Li salts with a monovalent anion A ^- , e.g. As LiF, LiCl or Li acetate. The term "Li salt of the Li ₂ B type" refers to Li salts having a divalent anion B ^2- , e.g. B. Li ₂ SO ₄ , Li ₂ CO ₃ or lithium oxalate. The term "Li salts of the type Li ₃ C" denotes Li salts with trivalent anion C ^3- . By contrast, the relationship is different for the Li ₃ AlF ₆ compound. The reason is given above and can be explained by the reaction equation which is 4K ₂ AlF ₅ + Li ₃ AlF ₆ → 2KAlF ₄ and 3K ₂ LiAlF ₆ .

Accordingly, in the dispersion, the preferred molar ratio between Li ₃ AlF ₆ and K ₂ AlF ₅ present in the flux or formed during brazing is 0.2: 1 to 0.3: 1, and preferably is 0 , 22: 1 to 0.28: 1.

Particularly preferred molar ratios between the Li salt and K ₂ AlF ₅ present or formed are 1: 1 to 1.1: 1 for LiAl type LiA salts, 0.5: 1 to 0.55: 1 for lithium salts of the type Li ₂ B, 0.33: 1 to 0.36: 1 for lithium salts of the type Li ₃ C, and 0.25: 1 to 0.275: 1 for Li ₃ AlF ₆ . Preferably, the content of K ₃ AlF ₆ in the basic flux is preferably 2% by weight or lower, more preferably 1% by weight or lower inclusive 0% by weight. This content is calculated for the dispersion on a dry weight basis. Consequently, the content of K ₃ AlF ₆ in the modified flux is of comparable size, in fact it is somewhat lower because the Li salt is trapped in the modified flux. Often, the content of K ₃ AlF ₆ in the modified flux is preferably 1.99 wt% or lower and even 1.82 wt% or lower, depending on the amount of Li salt added.

Tables 1, 2 and 3 will help those skilled in the art understand this invention and how much of the Li salt should be added according to the columns indicating the amount of Li salt to be added. It should be noted, however, that for this invention the ranges of Tables 1, 2 and 3 refer to preferred embodiments.

Preferably, 10.5 g to 15.5 g of LiF or 16.2 g to 24.3 g of Li ₃ AlF _{6 are added} per 100 g of K ₂ AlF ₅ , and more preferably 11.7 g to 14.3 g of LiF or 18 , 2 g to 22.3 g of Li ₃ AlF ₆ per 100 g of K ₂ AlF ₅ .

When the flux contains KZnF ₃ , it is preferable to add 4.3 g to 6.5 g of LiF or 6.5 g to 10 g of Li ₃ AlF ₆ per 100 g of KZnF ₃ , and particularly preferably 4.8 g to 6 g of LiF or 7.3 g to 9.2 g of Li ₃ AlF ₆ per 100 g of KZnF ₃ .

Thus, those skilled in the art could, if they wish, apply more or less of the Li salt than indicated by the above ranges. However, if the ratio is lower, the anticorrosive effect may be less than desired; if the amount is higher, the Li salt could be wasted.

For example, to perform the brazing process according to this invention, the carrier, e.g. For example, water, an organic solvent, for example, isopropanol, or a mixture of both in a mixer with a stirrer. The flux, for example a flux consisting of 80% by weight KAlF ₄ and 20% by weight K ₂ AlF ₅ , is added to the carrier. Optionally, the resulting composition may be stirred. Thereafter, or alternatively prior to the addition of the flux, a Li salt, e.g. For example, LiF is added in an amount of 2.5 g per 100 g of the flux or Li ₃ AlF ₆ in an amount of 4 g per 100 g of the flux. The resulting composition is stirred, and after a desired degree of homogenization of the resulting dispersion has been achieved, the dispersion can be applied for fluxing, e.g. B. by spraying on the parts to be brazed. The parts are then in a conventional manner, for. B. brazed in a CAB process (brazing under controlled atmosphere). Brazing metal needed to form an alloy with the aluminum parts may be present as a plating on the parts.

Alternatively, a flux composition containing a binder, e.g. Example, a polyacrylate binder or a polyurethane binder, and the flux, consisting of 80 wt .-% KAlF ₄ and 20 wt .-% K ₂ AlF ₅ , dispersed in water, can be applied. The amount of the flux in the composition is, for example, 30% by weight. The dispersion is added to a mixer containing a stirrer. LiF or Li ₃ AlF ₆ is added to the stirred dispersion in an amount of, for example, 2.5 g of LiF or 4 g of Li ₃ AlF ₆ per 100 g of the flux contained in the dispersion.

In yet another alternative, a composition comprising a flux consisting of 80 wt% KAlF ₄ and 20 wt% K ₂ AlF ₅ , and a binder, e.g. As a polyacrylate or a polyurethane, optionally with stirring, a carrier, for. As water, added in a mixing device. Thereafter or before, LiF or Li ₃ AlF _{6 is added to} the stirred dispersion in an amount of, for example, 2.5 g of LiF or 4 g of Li ₃ AlF ₆ per 100 g of the flux contained in the composition. The resulting dispersion is stirred until the desired degree of homogeneity is achieved. The brazing of aluminum parts is then carried out as described above.

Another aspect of the present invention relates to brazed aluminum parts made by brazing aluminum parts using the modified flux of the present invention. The brazed parts have a significant content of Li cations in the braze residue and show low corrosivity when they come into contact with water or cooling water for extended periods of time.

The following examples are intended to further illustrate the invention without intending to be limiting.

Examples

General procedure of the dry process: The basic flux is mixed with the Li ⁺ -containing compound and any other desired additives.

General procedure of the wet process: The flux is precipitated by its precipitation in water from starting materials containing potassium, lithium, aluminum and fluorine and optionally other compounds, for. Cesium compounds.

Example 1: Potassium fluorozincate as basic flux and its use

1.1. Preparation of the flux

KZnF ₃ (available as Nocolok ^® Zn flux from Solvay Fluor GmbH, Hannover, Germany) is mixed with Li ₃ AlF. ₆ For each 100 g of flux, 9 g of Li ₃ AlF _{6 are} added to obtain a modified flux containing 8.3 g of the Li salt.

1.2. Use of the flux for brazing

The modified flux of Example 1.1 is mixed with water as solvent and water-soluble polyurethane as binder so that the content of the modified flux in the dispersion is about 25% by weight, the resulting dispersion is coated on aluminum tubes coated with a brazing metal or are plated, sprayed. The tubes are then dried and flux coated tubes are obtained. The tubes are then fitted with aluminum fins and brazed in a known manner by heating to 600 ° C, preferably under inert gas in an oven.

1.3. Separate addition of the basic flux and the lithium salt

KZnF ₃ (available as Nocolok ^® Zn flux from Solvay Fluor GmbH, Hannover, Germany) is added with stirring to water in a tank equipped with a stirrer. With stirring, polyurethane binder and LiF are added to the water containing the basic flux. 5.5 g of LiF are added per 100 g of the flux to obtain a composition containing 5.2 g of the Li salt.

1.4. Use of the flux for brazing

The composition of Example 1.3 is sprayed on aluminum tubes clad with a braze metal. The tubes are then dried, and flux coated tubes are obtained. The tubes are then fitted with aluminum fins and brazed in a known manner by heating to 600 ° C, preferably under inert gas in an oven.

1.5 corrosion test

During brazing, a Zn coating is deposited on the tubes. This Zn coating has a comparatively low resistance to corrosion.

Example 2: Potassium fluoroaluminate / Si flux as basic flux

2.1. Preparation of the modified flux

Potassium fluoroaluminate containing Si powder as Hartlötmetallvorstufe (available as Nocolok ^® Sil with a content of 33 wt .-% Si from Solvay Fluor GmbH, Hannover, Germany), in which the weight ratio of KAlF ₄ and K ₂ AlF ₅ is 4: 1 is mixed with 4 g of Li ₃ AlF ₆ per 100 g of potassium fluoroaluminate flux to give a modified flux containing 3.8% by weight of Li ₃ AlF ₆ .

2.2. Application of the modified flux

The composition of Example 2.1 is dispersed in a dispersion of polyurethane in water (content of polyurethane about 2% by weight of the sum of water and polyurethane) and is sprayed on aluminum tubes clad with a brazing metal. The tubes are then dried and tubes coated with the modified flux and binder are obtained. The tubes are then fitted with aluminum fins and brazed in a known manner by heating to 600 ° C, preferably under inert gas in an oven.

Example 3: Cesium-containing flux as basic flux

3.1. Preparation of flux composition

A potassium fluoroaluminate flux containing cesium fluoroaluminate, available from Solvay Fluor GmbH, Hannover, Germany, ^® under Nocolok Cs Flux, with an atomic ratio of K: Cs = 98: 2, the ratio by weight of KAlF ₄ and K ₂ AlF ₅ 4: Is 1, is mixed with 4.6 g of Li ₃ AlF ₆ to obtain a flux containing 4.4 g of the Li salt. Water and a polyurethane binder are added so that the content of the modified flux in the dispersion is about 25% by weight.

3.2. Brazing with the flux composition

Parts of an aluminum alloy containing 0.5% by weight of magnesium are dipped in the dispersion of Example 3.1 and brazed at a temperature of about 600 ° C.

Example 4: Flux based on potassium fluoroaluminate for dry fluxing

A potassium fluoroaluminate flux for dry flux with a particle size distribution within the curves of 11 or as in Table B of the US Pat. No. 6,733,598 is specified is used; the flux is under the trade name Nocolok ^® Dryflux Solvay Fluor GmbH, Germany, are available.

4.1. Flux for solderless brazing of aluminum

The potassium fluoroaluminate dry flux is mixed with Si powder and Li ₃ AlF ₆ so that the content of Si in the entire flux is about 30% by weight; 4.4 g Li ₃ AlF _{6 are} added per 100 g of the potassium fluoroaluminate. The modified flux is applied by an electrostatic spray system to aluminum tubes which are coated after coating in a known manner.

4.2. Solder for solderless brazing of aluminum parts with higher Mg content

The flux of Example 4.1 is mixed with cesium tetrafluoroaluminate such that in the resulting flux mixture the atomic ratio of K: Cs is about 98: 2. The resulting flux is then applied to unplated aluminum alloy tubes containing about 0.3% by weight of magnesium. The brazing of the coated tubes is then done in a known manner by assembling the parts and heating to about 600 ° C.

Example 5: High K ₂ AlF ₅ · H ₂ O flux

5.1. Preparation of the basic flux

A potassium fluoroaluminate flux is used as in Example 19 of U.S. Patent 4,579,605 described prepared. Hydrofluoric acid containing 40% by weight of HF, potassium hydroxide containing 25% by weight of KOH and Al (OH) ₃ were reacted in a molar ratio of the raw material of Al: F: K = 1: 4: 1.5. The Al (OH) ₃ is added to the hydrofluoric acid and dissolved therein. Then the potassium hydroxide is added. The reaction mixture is kept at 60.degree. The resulting basic flux composition contains 40% by weight of K ₂ AlF ₅ .H ₂ O and 60% by weight of KAlF ₄ .

5.2. Modified flux comprising 7% by weight of Li ₃ AlF ₆

250 g of the basic flux of Example 5.1 and about 19 g of Li ₃ AlF ₆ are thoroughly mixed. The resulting flux contains 37% by weight K ₂ AlF ₅ .H ₂ O, 56% by weight KAlF ₄ and 7% by weight Li ₃ AlF ₆ .

Example 6: High dehydrogenated K ₂ AlF ₅ flux

6.1. Preparation of dehydrated K ₂ AlF ₅

A composition comprising 98.5% by weight K ₂ AlF ₅ .H ₂ O and 1.5% by weight KAlF ₄ is prepared as in Example 7 of the U.S. Patent 4,579,605 described by dissolving Al (OH) ₃ in hydrofluoric acid containing 20% by weight of HF and reacting the resulting fluoroaluminic acid with potassium hydroxide solution containing 25% by weight of KOH (molar ratio of Al: F: K = 1: 4) : 1) prepared at 30 ° C. The resulting crude product is dried in a dryer at 570 ° C, residence time 0.5 seconds. The resulting product is irreversibly dehydrated K ₂ AlF ₅ containing minimal amounts of KAlF ₄ .

6.2 Preparation of the basic flux

100 g Nocolok ^® flux (available from Solvay Fluor GmbH) comprising about 20 wt .-% K ₂ AlF _5, with the remainder to 100% KAlF ₄ is mixed with 19 g of the dehydrated K ₂ AlF ₅ of Example 6.1 , The resulting basic flux contains about 32.5 wt% K ₂ AlF ₅ and 67.5 wt% KAlF ₄ .

6.3. Preparation of a modified flux comprising K ₂ AlF ₅

100 g of the basic flux of Example 6.2 and 6.4 g of Li ₃ AlF ₆ are thoroughly mixed. The resulting flux contains 6% by weight Li ₃ AlF ₆ , about 30.5% by weight K ₂ AlF ₅ and 63.5% by weight KAlF ₄ .

Example 7: Brazing with flux with high K ₂ AlF ₅ content

7.1. Brazing with the flux of Example 5.2

Heat exchanger sections with dimensions of 10 cm x 10 cm, consisting typically of tubes and fins, are assembled. The flux of Example 5.2 is applied to the sections by immersion in a slurry of dry powdered modified flux and isopropanol (approximately 25% by weight of the modified flux in the dispersion). The specimens are weighed before and after flux loading (after drying), and so, as the surface is known, the flux loading is calculated. The mean value of the flux charge amounts to about 6 g / m ² .

The specimens are brazed using a standard CAB braze under a nitrogen atmosphere in a technical furnace. The resulting brazed assembly has improved resistance to corrosion.

Example 8 Li flux containing low melting basic flux

The applied basic flux is Nocolok ^® LM (where LM stands for low-melting). This flux is available from Solvay Fluor GmbH, Hannover, Germany. The basic flux contained about 40% by weight of K ₂ AlF ₅ (calculated on the basis of the LOH of water of crystallization of K ₂ AlF ₅ H ₂ O)

Modified flux: 11 parts of the basic flux are mechanically mixed with 1 part of Li ₃ AlF ₆ . Angle-on-coupon specimens (2.5 x 2.5 cm ² ) are brazed to flux charges of 8 g / m ² using the modified flux.

The brazed specimens are placed in 20 ml deionized water (soak tests).

After 15 days of immersion (the recipient was opened almost daily to ensure oxygen exchange) it was found that the aqueous phase of the modified flux brazed assembly remains clear. Sample specimens that were brazed with standard Nocolok ^® LM, show a haze of precipitated material, which is an indication of corrosion.

Example 9: Preparation of a modified flux dispersion in situ

In stirred water is added a binder and LiF. The stirred dispersion is then added with a flux consisting of KAlF ₄ and K ₂ AlF ₅ in which the weight ratio of KAlF ₄ and K ₂ AlF _{5 is} 4: 1. This flux is as Nocolok ^® Flux from Solvay Fluor GmbH, Hannover, Germany, are available. The flux is added in an amount of 100 g per 2.5 g of LiF.

The resulting dispersion is sprayed onto aluminum parts. The parts are then assembled, placed in a brazing furnace, heated to about 610 ° C and thereby brazed.

Example 10: Preparation of a modified flux dispersion in situ with a high LiF content

Example 9 is repeated. This time, 30 grams of the flux is added to 2.5 grams of the LiF. The concentration of LiF in the dispersion is correspondingly much higher than in Example 10. The parts are brazed but some unreacted LiF remains in the braze residue.

Example 11: Impregnation Tests

Parts brazed using a modified flux consisting of KAlF ₄ (77.6 wt%), K ₂ AlF ₅ (19.4 wt%) and Li ₃ AlF ₆ (3 wt%) are contacted with water for 10 days. Obviously, part of the braze flux residue is washed out and forms a precipitate. The Analysis of liquid and which precipitated out of the sample specimens that were mixed with the Li-modified flux and with Nocolok ^®, a flux, consisting of about 80 wt .-% KAlF ₄ and about 20 wt .-% K ₂ AlF _5, brazed , shows that the amount of leached F ^- is 5 times higher or even more for the sample which was brazed with Nocolok ^®, the amount of K ⁺ significantly higher (in the range of 100% or even more), and the Al ³⁺ content is about 6 times higher or even more. When the flux is applied in a wet process (by immersing the aluminum pieces to be joined in a suspension of the flux in isopropanol), the effect is even more noticeable than when the fluxes are applied as a dry powder. It is particularly noteworthy that for the modified flux no Zn ²⁺ is found, while in the case of Nocolok ^®, Zn cations can be identified in the lower ppm range.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

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Cited non-patent literature

• Bo Yang et al. in Journal of ASTM International, Vol. 3, Issue 10 (2006) [0009]

Claims (18)

A modified flux for aluminum brazing which comprises a basic flux comprising K ₂ AlF ₅ or a precursor thereof which forms K ₂ AlF ₅ during brazing, and a Li salt in an amount of 80% to 120% of Amount, which is stoichiometrically required to convert the entire K ₂ AlF ₅ to K ₂ LiAlF ₆ during brazing, contains. A modified flux according to claim 1, wherein the basic flux is selected from the group consisting of K ₂ AlF ₅ , mixtures of KAlF ₄ and K ₂ AlF ₅ , fluxes comprising KAlF ₄ and K ₂ AlF ₅ comprising cesium cations, K ₂ SiF ₆ and mixtures thereof. The modified flux according to claim 1, wherein the Li salt is selected from the group consisting of LiF and Li ₃ AlF ₆ . The modified flux of claim 1 containing 2% by weight or less of K ₃ AlF ₆ . The modified flux of claim 1, wherein the molar ratio of the Li salt and K ₂ AlF ₅ present in the flux or formed from the precursor during brazing is 0.8: 1 to 1.2: 1 for Li. Salts of the type LiA, 0.4: 1 to 0.6: 1 for Li salts of the type Li ₂ B and 0.25: 1 to 0.4: 1 for Li salts of the type Li ₃ C, wherein A denotes a monobasic anion, B denotes a dibasic anion, and C denotes a tribasic anion with the exception of the anion AlF ₆ ^-3 . A modified flux according to claim 1 comprising Li ₃ AlF ₆ as Li salt, wherein the molar ratio of Li ₃ AlF ₆ and K ₂ AlF ₅ present in the flux or formed from the precursor during brazing is 0.2 : 1 to 0.3: 1. A modified flux according to claim 1 comprising K ₂ AlF ₅ and comprising 10.5 g to 15.5 g LiF or 16.2 g to 24.3 g Li ₃ AlF ₆ per 100 g K ₂ AlF ₅ . A modified flux according to claim 7 comprising 11.7 g to 14.3 g LiF or 18.2 g to 22.3 g Li ₃ AlF ₆ per 100 g K ₂ AlF ₅ . A flux composition comprising a modified flux according to any one of claims 1 to 8 and at least one additive selected from the group consisting of silicon, solvents, binders, thickeners, suspension stabilizers, antifoaming agents, surface active agents and thixotropic agents. Aluminum parts for brazing, coated at least in part with a flux according to any one of claims 1 to 8 or a flux composition according to claim 9. Brazed aluminum or aluminum alloy parts wherein a modified flux according to any one of claims 1 to 8 or a flux composition according to claim 9 has been coated on at least one of the parts to be brazed and the parts heated to a temperature sufficiently high to braze the parts , Brazed aluminum parts obtained by brazing them using a flux according to any one of claims 1 to 8 or a flux composition according to claim 9. Brazed aluminum parts obtainable by a method of brazing aluminum parts in which a flux comprising K ₂ AlF ₅ or a precursor of K ₂ AlF ₅ contains the K ₂ AlF ₅ during the brazing process for aluminum brazing and dispersing a lithium salt of low solubility in water in a carrier selected from water and mixtures of water and an organic solvent which separately form a dispersion containing the flux and the lithium salt, at least a portion at least in part is coated with the dispersion and brazed. Brazed aluminum parts according to claim 13, wherein the lithium salt is selected from the group consisting of LiF and Li ₃ AlF ₆ . Brazed aluminum parts according to claim 14, wherein the flux is selected from fluxes comprising K ₂ AlF ₅ , K ₂ AlF ₅ .H ₂ O, mixtures thereof and K ₂ AlF ₅ and / or K ₂ AlF ₅ .H ₂ O, mixed with KAlF ₄ , include or consist of. Brazed aluminum parts according to claim 13, wherein the flux is applied in the form of a flux composition further comprising the modified flux and an additive. Brazed aluminum parts according to claim 16, wherein said additive is a fluxing additive selected from the group consisting of solvents, binders, thickeners, suspension stabilizers, antifoaming agents, surface active agents and thixotropic agents. Brazed aluminum parts according to claim 13, wherein the amount of added lithium salt is such that the content of Li ⁺ in the dispersion, based on the dry weight, 0.1 wt.% Or higher and 4.6 wt. % or lower.