CA1046208A - Brazing of aluminium - Google Patents
Brazing of aluminiumInfo
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- CA1046208A CA1046208A CA288,543A CA288543A CA1046208A CA 1046208 A CA1046208 A CA 1046208A CA 288543 A CA288543 A CA 288543A CA 1046208 A CA1046208 A CA 1046208A
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Abstract
ABSTRACT OF THE DISCLOSURE
Brazing of aluminum is preformed by the employment of a flux comprising a mixture of non-hygroscopic potassium fluoaluminates, essentially free from unreacted potassium fluoride. The flux is preferably produced by fusing together commercial grades of potassium fluoride and aluminum fluoride.
The flux may advantageously be employed in the form of an aqueous slurry.
The advantage of the method is that the flux is totally water-insoluble and may be stored in the form of a slurry without risk of disproportionation.
Furthermore it leaves no soluble residues after brazing.
Brazing of aluminum is preformed by the employment of a flux comprising a mixture of non-hygroscopic potassium fluoaluminates, essentially free from unreacted potassium fluoride. The flux is preferably produced by fusing together commercial grades of potassium fluoride and aluminum fluoride.
The flux may advantageously be employed in the form of an aqueous slurry.
The advantage of the method is that the flux is totally water-insoluble and may be stored in the form of a slurry without risk of disproportionation.
Furthermore it leaves no soluble residues after brazing.
Description
The present inYention relates to the joining b~
brazing of ~heet, tube and other forms of aluminium. ~he term 1'brazing sheet" i8 hereinafter e~plo~ed for convenience to include tube and other form~ of aluminium suitable for brazing.
Aluminium brazing sheet having a surface cladding on one or both faces of an aluminium alloy havin~ a melting point which i~ lower by, say, 30 - 40C. than the core of the sheet is extensively used in the production of heat ex-changer~. The core may be aluminium or an alumi~ium alloy.
This surface layer forms the hard ~older b~ means o~ which compo~ent~ ~ade from sueh brazing sheet may be joined togetherO In the brazing operation the assembly o~ such components to be joined is subjected to a temperature at which the ~urface layer i~ melted without melting the core.
In order to be able to achieve that result under industrial conditions without risk there muæt be a differ~nce in the melting points of the order of 30 - 40C.
In order to produce a brazed joint a flux is employed to remove the alllmin;um oxide coating present both on the suriace of the brazin~ ~heet and the all~m;nium to which it is to be joined. Conventionally the flux employed for the puIpose has been a mixture of chloride ~alts, including alkali metal and alkaline earth metal chlorides.
~hese water soluble materials are corrosive to aluminium in $he presence of moistureO Consequently at the end of the brazing operation the brazed as~embl~ must be ~ubjected to a AJ~/4~40 2 .. . . , . :
cleaning operation to remove the water soluble flux. Even so there are usually inclusions of the flux in the metal of the joint which may result in corrosion after a relatively short interval, particularly where the brazed assembly may be subjected to humid conditions.
According to the present invention there is provided a method of producing a brazed joint between aluminium or aluminium alloy components which comprises interposing between the facing surfaces of said components a layer of an aluminium brazing alloy of lower melting point than said components and a layer of a flux comprising an intimate mixture of substan-tiallyw~ter-insbl~ble~po~assiumfluoALuman~te complexes essentially free of unreacted potassium fluoride, said flux being in finely divided form, heating said components and said flux to a temperature above the mè~lting point of said brazing alloy and said flux and below the melting point of said aluminium or aluminium alloy and then allowing said components and said layer of aluminium brazing alloy to cool to form a brazed joint between said surfaces.
It is already known to braze aluminium without the use of a flux under vacuum or inert gas conditions, but the capital cost of the equipment employed is very high. Moreover a major disadvantage of the fluxless brazing methods is that much closer tolerances must be observed for assembly than for flux brazing. Any failure to maintain very close - tolerances results in the brazed assemblies being rejected due to incompletely filled joints.
It is an object of the present invention to provide a process for producing a brazed joint between aluminium oraluminium alloy components employing a flux which has the characteristics of being non-hygroscopic prior to brazing and substantially insoluble in water after brazing, whilst exhibiting the necessary qualities of becoming reactive at a temperature below the melting point of the hard solder layer and acting as a flux or ~;~
10~6Z08 solvent for aluminium oxide and being substantially unreactive with almminium whilst in the molten condition.
In the most widely employed brazing sheet the core is clad with a hard solder layer formed of an aluminium-silicon eutectic melting at about 577 C. and therefore the flux employed should become reactive at a temperature below that value.
- 3a -104tjZ08 It has already been proposed in ~ritish Patent ~o~ 1,055,914 to produce a flux for soldering aluminium by mixing 53 - 55% Al~3 with 47 to 45% KF, within which range a known eutectic point o¢cur~, having a melting point of about 560C. In this prior proposal the materials are either mixed dry, with ~ubsequent addition of water, or the EF is added in aqueous solution. In both alternatives the re~ultant paste is dried at a temperature below 200C.
~he material produced by that method was reported as leaving a brittle, non-hygroæcopic residue at the end of the brazing o~eration. Although the method of production results in a~ i~timate admixture o~ the soluble EF with the in~oluble Al~3 there does not appear to be a complete reaction between the fluoride components and the resultant material i8 hygros-copic and i~ thus unsuitable for use in aqueouæ slurry.
Slurrying of this material in w~ter would result in æolution o~ KF and consequent possibility of disproportionation of the flux on drying and melting point variability.
The rlu2 material of British Patent No. 1,055,914 was recommended for use in inert and non-hygroæcopic vehicles, which would obviate the above-mentioned difficulties.
However, ~or many brazing operations, particularly closed heat exchanger assemblies, such as radiators for motor vehicles and evaporators for air conditioner~, the conditions preclude the use of resinous binders. In such case the relative absence o~ oxygen makes it impossible to burn off more than a very slight amount of the carbonaceous material , - .
:
~046208 in the brazing operation, BO that the flux must be en-trained in a vehicle which is substantially completel~
evaporable. Of æuch vehicles water iB by far the most ~uitable, both from aæpects of cost and operating convenience, since it leaves no residue and require~ no oxy~en ~or burn-off.
X-ray di~fraction examination of the solidi~ied residue of the eutecti¢ mixture of EF and AlF3, which occuræ
at about 45.8/o EF and 54.2/o AlF3 indicates that virtually all the fluoride contents are in the form Of ~3~'F6 and ~AlF
which are very sparingly soluble in water and are non-h~groæ¢opic. In fact the fuæed eutectic consists of these two pha~es ~nd not of KF and AlF3.
We have therefore appreciated, in accordance with the present invention, that in order to permit the flux to be employed in the form o~ an aqueous slurry it iæ necessary to co~vert the flux materials eæsentially into a mixture of inæoluble potaBSiUm fluoaluminate complexes before appli-cation to the hard solder (alloy surface coating of the brazing sheet) and that it should be essentially free of unreacted KF. ~he simplest and preferred method of obtain-ing such a mixture is to fuse together ~1~3 and KF in correct proportions, allow the mix to cool and then OEind the cooled mass to an appropriatel~ small particle size to allow it to be 3uspended in water in the form of a thin slurry. It iæ
found that grinding to -100 mesh (smaller than 150 microns) is generally satisfactory, but it is preferred to grind it - ., ~ . - . . . . . . . .
~04~208 to -150 mesh or even -200 me~h (smaller than 104 and 75 microns respe¢tively). It is however possible to prepare E~ 6 and K~lF4 separately and to mix them in the required proportion~. ~he preparation of XAlF4 has been described by Bro~et in Z. ~norg. Algem. Chemie, Vol. 239, pages 301-304, (1938).
T~picall~ the flux material i8 made up into a thin ~lurry by the addition of 2 parts of water to 1 part by weight of finely ground flux. A very small amount of surface active agent is found to assist the deposition of a uniform layer of flux on the aluminium surface (which may be the aluminium solder surface or the surface of the aluminium to which the solder is to be joined) and it i8 preferable to add a very small ~mount, such a~ ~%, of a conventional hydrox~-ethyl cellulose thickening agent to maintain the ~lux in ~uspension. ~he amount of carbonaceous material is too small to lead to the for~ation of unacceptable carbon deposits during the brazing operation.
It is preferred that the relative proportions of EF
and ~1~3 emplo~ed in the preparation of the flux shQuld be a3 ~lose to the eutectic point as possible. Whilst the melti~g point ~how~ in the published diagram (Journal American Cerami¢ Society, 42, pages 631-4, De~ember 1966) ri~ very r~pidly if the quantit~ of EF rises above that required for the eutectic, there is only a slight rise in melting point to about 574C where the AlF3 rises ab~ve the eutectic up to a total of about 60~ (50 mole~ %A1~3~.
.
..
~046208 The liquidus point of a mixt~re of potassium fluoaliminate complexes varies depending upon the composition of the mixture, expressed as relative proportions of AlF3 and KF~ reaching a minimum (about 560 C) at the aforementioned eutectic composition. For many purposes, it is expecially preferred in the practice of the invention to imploy a flux at or close to this eutectic composition; however, in a broader sense, the invention embraces the use as Muxes of intimate mixtures of potassium fluoaluminate complexes having a composition corresponding to an AlF3/KF
ratio, in parts by weight, preferably between about 60:40 and about 50:50 and~ as stated~ essentially free of unreacted potassium fluoride.
At compositions corresponding to an AlF3 content below about 60%
the mixture of potassium fluoaluminate complexes, in dry state, consists essentially of K3AlF6 and KAlF4. At higher levels of AlF3 content within the stated range, the mixtures are constituted of KAlF4 with some unreacted AlF3 (which is insoluble in water) but, again, essentially free of unreacted KF; such mixtures (KAlF4~L*h AlF3) are embraced within the term ~mixtures of potassium fluoalu~inate complexes" as used herein. Between that part of the flux composition range in which the flux is a mixture of KAlF4 ~wPth K3AlF6, and that part of the range in which the flux is a mixture of XAlF4 with AIF3, there is a unique point (i.e. a unique value of the AlF3/KF ratio, slightly below that corresponding to 60% AlF3) at which the composition consists of KAlF4 alone. Thus the stated composition range inherently includes this point at which the "mixture of complexes" is solely KAlF4;
in other words, pure KAlF4 lies at a unique intermediate point within the stated range. Accordingly, the term "mixtures of potassium fluoaluminate complexes" as used herein further inherently embraces a composition consisting essentially of KAlF4. Minor amounts of other fluorides (e.g. LiF, NaF, or CaF2i) may be incorporated in the flux.
A preferred flux for use in the process of the present invention 104Si2~)8 consists essentially of an intimate admixture of K3AlF6 and KAlF4 in such amounts that the potassium fluoride/aluminuum fluoride ratio is 40-50:60-50% and is essentially free of unreacted KF.
All percentages herein are by weight~ except where otherwise stated.
In is found that, surprisingly in relation to the above-quoted published data, satisfactory fluxing is effected throughout the quoted range, although the effectiveness of the flux decreases away from the eutectic point.
Small quantities of alkali- or alkaline-earth metal or ~inc fluo- -rides, up to a total of about 5 mole %, can be tolerated, providing that the melting point of the flux is not raised above that of the hard solder.
However, the presence of such fluorides does not appear to confer any benefits by reducing the melting point below that of the KF/AlF3 eutectic and all have the effect of raising the melting point to some extent even where the KF/AIF3 proportions have been adjusted to provide optimum melting point ~-conditions.
In preparing the flux by the fusion method from technical grade KF and AlF3 the materials in finely ground, dry state are mixed in proportions to yield KF and AlF3 within the above relative proportions. The purity of the KF is not critical and technical or commercial grade KF has been found to be satisfactory. The impurities normally associated with technical KF are therefore acceptable and, - 7a -104t~Z08 to facilitate weighing and mixing with the Al~3, the ~F
iP grou~d to at least -100 me~h and should be moi~ture-free to avoid difficulties in the fusion operation.
In the case of the aluminium fluoride, the effec-tivene~s of the flux appears to d;~;nish with decreasing purity of this component. The best technical results are obtain~d with pure distilled AlF3. On the other hand, a usable ~lux has been prepared from aluminium fluoride with a~ 3 content as low as 6~/o. ~he purity of commercially available aluminium fluoride dependY largely on the method of manufacture and the preferred commercial material is 95%
purity alum;n;um fluoride which i~ produced by the fluo-8ilicic acid process and is available at reaso~able cost.
~he principal impurity i~ this material is alumina while the othçr normally-a~sociated impurities such a~ ~ulphate~, iron-oxide, silica, etc., are believed to have little effect but should each be le~s than 0.3%. ~luminium fluorides of lower purity and produced by alternative processes have been used succesæfully but to enqure con-sistent success, that iæ, over a reasonably wide range of furnacin~ brazing conditions, the minimum purity should be 90Yo Al~3 As stated above, both KF a~d ~1~3 should be dr~
before the fu~ion reaction to avoid the possibility of the hydrolysis of AlF3.
Example Potassium fluoride and aluminium fluoride in a 10~08 thoroughly dried condition are ground to pass through a 70 me~h screen (~maller than 212 micron~ he ground m~terials are mixed in the proper proportions and mixed intimately. The mixture is then placed in a graphite crucible, which is placed in a furnace at 625-650C. ~_ the mixture fuses it i~ Qtirred with a platinum stirrer.
~he molte~ material i~ then quickly poured into a mould to ~olidify.
Flux material made as described above was ground to -200 mesh (finer than 75 microns) and wa~ slurried with water to form a thin slurry a~ deæ¢ribed above and the - material was employed in the production of a heat exchanger of the wafer and fi~ type, produced from hard solder-coated brazi~g sheet of the type already discussed. The aqueous slurry was applied to the facing surraces of the assembly at a rate of about 250 gm~/sq. metre. With this flux application sound joint~ were formed when the assembly wa~
subjected to a temperature of about 600C. in the brazing furnace.
Unlike the conventional chloride fluxes, the fused aluminium fluoride/potas~ium fluoride complex flux is non-h~groscopic and does not tend to become entrapped in the liquid filler metal. This allows the flux material to be expo~ed to the atmosphere or to be formed into an aqueou~
slurry prior to brazing without the problem~ of oxychlorido formation, and permits strong den~e corrosion-free joints to be formed.
_9
brazing of ~heet, tube and other forms of aluminium. ~he term 1'brazing sheet" i8 hereinafter e~plo~ed for convenience to include tube and other form~ of aluminium suitable for brazing.
Aluminium brazing sheet having a surface cladding on one or both faces of an aluminium alloy havin~ a melting point which i~ lower by, say, 30 - 40C. than the core of the sheet is extensively used in the production of heat ex-changer~. The core may be aluminium or an alumi~ium alloy.
This surface layer forms the hard ~older b~ means o~ which compo~ent~ ~ade from sueh brazing sheet may be joined togetherO In the brazing operation the assembly o~ such components to be joined is subjected to a temperature at which the ~urface layer i~ melted without melting the core.
In order to be able to achieve that result under industrial conditions without risk there muæt be a differ~nce in the melting points of the order of 30 - 40C.
In order to produce a brazed joint a flux is employed to remove the alllmin;um oxide coating present both on the suriace of the brazin~ ~heet and the all~m;nium to which it is to be joined. Conventionally the flux employed for the puIpose has been a mixture of chloride ~alts, including alkali metal and alkaline earth metal chlorides.
~hese water soluble materials are corrosive to aluminium in $he presence of moistureO Consequently at the end of the brazing operation the brazed as~embl~ must be ~ubjected to a AJ~/4~40 2 .. . . , . :
cleaning operation to remove the water soluble flux. Even so there are usually inclusions of the flux in the metal of the joint which may result in corrosion after a relatively short interval, particularly where the brazed assembly may be subjected to humid conditions.
According to the present invention there is provided a method of producing a brazed joint between aluminium or aluminium alloy components which comprises interposing between the facing surfaces of said components a layer of an aluminium brazing alloy of lower melting point than said components and a layer of a flux comprising an intimate mixture of substan-tiallyw~ter-insbl~ble~po~assiumfluoALuman~te complexes essentially free of unreacted potassium fluoride, said flux being in finely divided form, heating said components and said flux to a temperature above the mè~lting point of said brazing alloy and said flux and below the melting point of said aluminium or aluminium alloy and then allowing said components and said layer of aluminium brazing alloy to cool to form a brazed joint between said surfaces.
It is already known to braze aluminium without the use of a flux under vacuum or inert gas conditions, but the capital cost of the equipment employed is very high. Moreover a major disadvantage of the fluxless brazing methods is that much closer tolerances must be observed for assembly than for flux brazing. Any failure to maintain very close - tolerances results in the brazed assemblies being rejected due to incompletely filled joints.
It is an object of the present invention to provide a process for producing a brazed joint between aluminium oraluminium alloy components employing a flux which has the characteristics of being non-hygroscopic prior to brazing and substantially insoluble in water after brazing, whilst exhibiting the necessary qualities of becoming reactive at a temperature below the melting point of the hard solder layer and acting as a flux or ~;~
10~6Z08 solvent for aluminium oxide and being substantially unreactive with almminium whilst in the molten condition.
In the most widely employed brazing sheet the core is clad with a hard solder layer formed of an aluminium-silicon eutectic melting at about 577 C. and therefore the flux employed should become reactive at a temperature below that value.
- 3a -104tjZ08 It has already been proposed in ~ritish Patent ~o~ 1,055,914 to produce a flux for soldering aluminium by mixing 53 - 55% Al~3 with 47 to 45% KF, within which range a known eutectic point o¢cur~, having a melting point of about 560C. In this prior proposal the materials are either mixed dry, with ~ubsequent addition of water, or the EF is added in aqueous solution. In both alternatives the re~ultant paste is dried at a temperature below 200C.
~he material produced by that method was reported as leaving a brittle, non-hygroæcopic residue at the end of the brazing o~eration. Although the method of production results in a~ i~timate admixture o~ the soluble EF with the in~oluble Al~3 there does not appear to be a complete reaction between the fluoride components and the resultant material i8 hygros-copic and i~ thus unsuitable for use in aqueouæ slurry.
Slurrying of this material in w~ter would result in æolution o~ KF and consequent possibility of disproportionation of the flux on drying and melting point variability.
The rlu2 material of British Patent No. 1,055,914 was recommended for use in inert and non-hygroæcopic vehicles, which would obviate the above-mentioned difficulties.
However, ~or many brazing operations, particularly closed heat exchanger assemblies, such as radiators for motor vehicles and evaporators for air conditioner~, the conditions preclude the use of resinous binders. In such case the relative absence o~ oxygen makes it impossible to burn off more than a very slight amount of the carbonaceous material , - .
:
~046208 in the brazing operation, BO that the flux must be en-trained in a vehicle which is substantially completel~
evaporable. Of æuch vehicles water iB by far the most ~uitable, both from aæpects of cost and operating convenience, since it leaves no residue and require~ no oxy~en ~or burn-off.
X-ray di~fraction examination of the solidi~ied residue of the eutecti¢ mixture of EF and AlF3, which occuræ
at about 45.8/o EF and 54.2/o AlF3 indicates that virtually all the fluoride contents are in the form Of ~3~'F6 and ~AlF
which are very sparingly soluble in water and are non-h~groæ¢opic. In fact the fuæed eutectic consists of these two pha~es ~nd not of KF and AlF3.
We have therefore appreciated, in accordance with the present invention, that in order to permit the flux to be employed in the form o~ an aqueous slurry it iæ necessary to co~vert the flux materials eæsentially into a mixture of inæoluble potaBSiUm fluoaluminate complexes before appli-cation to the hard solder (alloy surface coating of the brazing sheet) and that it should be essentially free of unreacted KF. ~he simplest and preferred method of obtain-ing such a mixture is to fuse together ~1~3 and KF in correct proportions, allow the mix to cool and then OEind the cooled mass to an appropriatel~ small particle size to allow it to be 3uspended in water in the form of a thin slurry. It iæ
found that grinding to -100 mesh (smaller than 150 microns) is generally satisfactory, but it is preferred to grind it - ., ~ . - . . . . . . . .
~04~208 to -150 mesh or even -200 me~h (smaller than 104 and 75 microns respe¢tively). It is however possible to prepare E~ 6 and K~lF4 separately and to mix them in the required proportion~. ~he preparation of XAlF4 has been described by Bro~et in Z. ~norg. Algem. Chemie, Vol. 239, pages 301-304, (1938).
T~picall~ the flux material i8 made up into a thin ~lurry by the addition of 2 parts of water to 1 part by weight of finely ground flux. A very small amount of surface active agent is found to assist the deposition of a uniform layer of flux on the aluminium surface (which may be the aluminium solder surface or the surface of the aluminium to which the solder is to be joined) and it i8 preferable to add a very small ~mount, such a~ ~%, of a conventional hydrox~-ethyl cellulose thickening agent to maintain the ~lux in ~uspension. ~he amount of carbonaceous material is too small to lead to the for~ation of unacceptable carbon deposits during the brazing operation.
It is preferred that the relative proportions of EF
and ~1~3 emplo~ed in the preparation of the flux shQuld be a3 ~lose to the eutectic point as possible. Whilst the melti~g point ~how~ in the published diagram (Journal American Cerami¢ Society, 42, pages 631-4, De~ember 1966) ri~ very r~pidly if the quantit~ of EF rises above that required for the eutectic, there is only a slight rise in melting point to about 574C where the AlF3 rises ab~ve the eutectic up to a total of about 60~ (50 mole~ %A1~3~.
.
..
~046208 The liquidus point of a mixt~re of potassium fluoaliminate complexes varies depending upon the composition of the mixture, expressed as relative proportions of AlF3 and KF~ reaching a minimum (about 560 C) at the aforementioned eutectic composition. For many purposes, it is expecially preferred in the practice of the invention to imploy a flux at or close to this eutectic composition; however, in a broader sense, the invention embraces the use as Muxes of intimate mixtures of potassium fluoaluminate complexes having a composition corresponding to an AlF3/KF
ratio, in parts by weight, preferably between about 60:40 and about 50:50 and~ as stated~ essentially free of unreacted potassium fluoride.
At compositions corresponding to an AlF3 content below about 60%
the mixture of potassium fluoaluminate complexes, in dry state, consists essentially of K3AlF6 and KAlF4. At higher levels of AlF3 content within the stated range, the mixtures are constituted of KAlF4 with some unreacted AlF3 (which is insoluble in water) but, again, essentially free of unreacted KF; such mixtures (KAlF4~L*h AlF3) are embraced within the term ~mixtures of potassium fluoalu~inate complexes" as used herein. Between that part of the flux composition range in which the flux is a mixture of KAlF4 ~wPth K3AlF6, and that part of the range in which the flux is a mixture of XAlF4 with AIF3, there is a unique point (i.e. a unique value of the AlF3/KF ratio, slightly below that corresponding to 60% AlF3) at which the composition consists of KAlF4 alone. Thus the stated composition range inherently includes this point at which the "mixture of complexes" is solely KAlF4;
in other words, pure KAlF4 lies at a unique intermediate point within the stated range. Accordingly, the term "mixtures of potassium fluoaluminate complexes" as used herein further inherently embraces a composition consisting essentially of KAlF4. Minor amounts of other fluorides (e.g. LiF, NaF, or CaF2i) may be incorporated in the flux.
A preferred flux for use in the process of the present invention 104Si2~)8 consists essentially of an intimate admixture of K3AlF6 and KAlF4 in such amounts that the potassium fluoride/aluminuum fluoride ratio is 40-50:60-50% and is essentially free of unreacted KF.
All percentages herein are by weight~ except where otherwise stated.
In is found that, surprisingly in relation to the above-quoted published data, satisfactory fluxing is effected throughout the quoted range, although the effectiveness of the flux decreases away from the eutectic point.
Small quantities of alkali- or alkaline-earth metal or ~inc fluo- -rides, up to a total of about 5 mole %, can be tolerated, providing that the melting point of the flux is not raised above that of the hard solder.
However, the presence of such fluorides does not appear to confer any benefits by reducing the melting point below that of the KF/AlF3 eutectic and all have the effect of raising the melting point to some extent even where the KF/AIF3 proportions have been adjusted to provide optimum melting point ~-conditions.
In preparing the flux by the fusion method from technical grade KF and AlF3 the materials in finely ground, dry state are mixed in proportions to yield KF and AlF3 within the above relative proportions. The purity of the KF is not critical and technical or commercial grade KF has been found to be satisfactory. The impurities normally associated with technical KF are therefore acceptable and, - 7a -104t~Z08 to facilitate weighing and mixing with the Al~3, the ~F
iP grou~d to at least -100 me~h and should be moi~ture-free to avoid difficulties in the fusion operation.
In the case of the aluminium fluoride, the effec-tivene~s of the flux appears to d;~;nish with decreasing purity of this component. The best technical results are obtain~d with pure distilled AlF3. On the other hand, a usable ~lux has been prepared from aluminium fluoride with a~ 3 content as low as 6~/o. ~he purity of commercially available aluminium fluoride dependY largely on the method of manufacture and the preferred commercial material is 95%
purity alum;n;um fluoride which i~ produced by the fluo-8ilicic acid process and is available at reaso~able cost.
~he principal impurity i~ this material is alumina while the othçr normally-a~sociated impurities such a~ ~ulphate~, iron-oxide, silica, etc., are believed to have little effect but should each be le~s than 0.3%. ~luminium fluorides of lower purity and produced by alternative processes have been used succesæfully but to enqure con-sistent success, that iæ, over a reasonably wide range of furnacin~ brazing conditions, the minimum purity should be 90Yo Al~3 As stated above, both KF a~d ~1~3 should be dr~
before the fu~ion reaction to avoid the possibility of the hydrolysis of AlF3.
Example Potassium fluoride and aluminium fluoride in a 10~08 thoroughly dried condition are ground to pass through a 70 me~h screen (~maller than 212 micron~ he ground m~terials are mixed in the proper proportions and mixed intimately. The mixture is then placed in a graphite crucible, which is placed in a furnace at 625-650C. ~_ the mixture fuses it i~ Qtirred with a platinum stirrer.
~he molte~ material i~ then quickly poured into a mould to ~olidify.
Flux material made as described above was ground to -200 mesh (finer than 75 microns) and wa~ slurried with water to form a thin slurry a~ deæ¢ribed above and the - material was employed in the production of a heat exchanger of the wafer and fi~ type, produced from hard solder-coated brazi~g sheet of the type already discussed. The aqueous slurry was applied to the facing surraces of the assembly at a rate of about 250 gm~/sq. metre. With this flux application sound joint~ were formed when the assembly wa~
subjected to a temperature of about 600C. in the brazing furnace.
Unlike the conventional chloride fluxes, the fused aluminium fluoride/potas~ium fluoride complex flux is non-h~groscopic and does not tend to become entrapped in the liquid filler metal. This allows the flux material to be expo~ed to the atmosphere or to be formed into an aqueou~
slurry prior to brazing without the problem~ of oxychlorido formation, and permits strong den~e corrosion-free joints to be formed.
_9
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of producing a brazed joint between aluminium or aluminium alloy components which comprises interposing between the facing surfaces of said components a layer of an aluminium brazing alloy of lower melting point than said components and a layer of a flux comprising an intimate mixture of substantially water-insoluble potassium fluoaluminate complexes essentially free of unreacted potassium fluoride, said flux being in finely divided form, heating said components and said flux to a temperature above the melting point of said brazing alloy and said flux and below the melting point of said aluminium or aluminium alloy and then allowing said components and said layer of aluminium brazing alloy to cool to form a brazed joint between said surfaces.
2. A method according to claim 1 wherein composition of the potassium fluoaluminate complexes corresponds to a ratio of aluminium fluoride to potassium fluoride in the range of 60:40 to 50:50 parts by weight.
3. A method according to claim 1 comprising applying said flux in the form of a slurry in an evaporable liquid.
4. A method according to claim 3 wherein said liquid is water.
5. A method according to claim 4 wherein at least one of a pair of facing surfaces of said components is clad with a layer of aluminium brazing alloy.
6. A method according to claim 1 in which a layer of finely divided flux admixed with finely divided aluminium brazing alloy is interposed between a pair of facing surfaces.
7. A method according to claim 2 wherein the flux has been prepared by (a) establishing a dry mixture of divided solid aluminium fluoride and potassium fluoride in proportions providing said ratio, and (b) heating the last-mentioned mixture for effecting substantially complete fusion thereof.
8. A method according to claim 2 wherein the flux has been prepared by combining separately prepared K3AlF6 and KAIF4 to provide a mixture thereof, essentially free of unreacted potassium fluoride and having a composition corresponding to said ratio.
9. A method according to claim 1, wherein said flux has a composition corresponding to a substantially eutectic mixture of aluminium fluoride and potassium fluoride.
10. A method according to claim 1, wherein said mixture of potassium fluoaluminate complexes consists essentially of KAlF4.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB3618372A GB1438955A (en) | 1972-08-02 | 1972-08-02 | Brazing aluminium |
| ZA00735010A ZA735010B (en) | 1972-08-02 | 1973-07-23 | Improvements in or relating to brazing aluminium |
| CA177926 | 1973-08-01 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1046208A true CA1046208A (en) | 1979-01-16 |
Family
ID=27162969
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA288,543A Expired CA1046208A (en) | 1972-08-02 | 1977-10-12 | Brazing of aluminium |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1046208A (en) |
-
1977
- 1977-10-12 CA CA288,543A patent/CA1046208A/en not_active Expired
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