Method of brazing an aluminum material

A method of brazing an aluminum material. At least a portion of the material is brought into contact with a coating solution containing cesium and fluorine ions, whereby a chemically coated flux layer composed of cesium fluoroaluminate or a mixture thereof with aluminum fluoride is formed on the surface of the material. Then, the material is heated and joined to another material with a brazing alloy.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
This invention relates to a method of brazing an aluminum material, i.e. 
aluminum or an alloy thereof, by forming a chemically coated flux layer on 
its surface and heating it. 
2. Description of the Prior Art: 
Brazing of an aluminum material is usually carried out by employing an 
Al-Si eutectic alloy having a melting point which is somewhat lower than 
that of the aluminum material. In order to enable the brazing alloy to 
combine satisfactorily with the aluminum material, it is necessary to 
remove any contaminant, such as an oxide film, from the surface of the 
aluminum material. A flux is applied to the surface of the material to 
remove any such contaminant therefrom. 
The inventors of this invention found potassium pentafluoroaluminate 
(K.sub.2 AlF.sub.5) effective as a flux for brazing an aluminum material. 
A brazing method employing this flux forms the subject matter of Japanese 
Patent Application No. 191311/1983 corresponding to U.S. Application No. 
659,423. It is characterized by forming a flux layer of K.sub.2 AlF.sub.5 
on the surface of the aluminum material by chemical conversion coating. 
This layer begins to melt at a temperature of about 560.degree. C. and 
removes an oxide film from the surface of the aluminum material without 
undergoing any chemical reaction with aluminum. 
It has, however, been considered desirable to develop a flux having a lower 
melting point to enable a lower brazing temperature to reduce the amount 
of heat required and facilitate the brazing operation. Moreover, K.sub.2 
AlF.sub.5 is not suitable as a flux for brazing an aluminum material 
containing magnesium, since its chemical reaction with magnesium renders 
brazing difficult. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of this invention to provide a method of 
brazing an aluminum material employing a flux having a lower melting point 
than that of K.sub.2 AlF.sub.5, and having a lower reactivity with 
magnesium. 
This object is attained by a method which comprises bringing at least a 
portion (desired for brazing) of an aluminum material into contact with a 
coating solution containing cesium and fluorine ions to form a flux layer 
composed of cesium fluoroaluminate or a mixture of cesium fluoroaluminate 
and aluminum fluoride on the surface of that materialby chemical 
conversion coating, and heating that portion of the aluminum material to 
join it to another material with a brazing alloy at a temperature of lower 
than the melting point of the aluminum material and higher than that of 
the brazing alloy. 
The chemically conversion-coated layer is formed by the reaction of the 
ions in the solution with the aluminum in the aluminum material. It has a 
very small thickness and yet is a very effective brazing flux. This flux 
has a melting point which is lower than that of conventional flux. 
Therefore, it makes it possible to use a brazing alloy having a lower 
melting point and facilitates the brazing operation. It is also useful for 
brazing an aluminum material containing magnesium. The residue which is 
left after brazing does not cause any corrosion of the aluminum material, 
since it is hard to dissolve in water.

DETAILED DESCRIPTION OF THE INVENTION 
According to this invention, an aluminum material is brought into contact 
with a coating solution containing cesium (Cs) and fluorine (F) ions, 
whereby a flux layer is chemically coated on the aluminum material. 
The term "aluminum material" as herein used means aluminum or an aluminum 
alloy. The aluminum alloy is an alloy composed of aluminum and at least 
one other metal, such as silicon (Si), copper (Cu), manganese (Mn), zinc 
(Zn), titanium (Ti), chromium (Cr), zirconium (Zr) or magnesium (Mg). 
Specific examples include those alloys which are designated as A3003, 
A3004 and A7072 in Japanese Industrial Standard (JIS). The term also means 
aluminum or an aluminum alloy having a surface covered by another alloy 
having a melting point which is 10.degree. C. to 100.degree. C. lower than 
the base metal or alloy, for example, a eutectic alloy of aluminum and 
silicon having a silicon content of from 7 to 12% by weight. Specific 
examples include a brazing sheet comprising a sheet of A3003 alloy and a 
layer of A4343 alloy clad thereon, and designated as, for example, BA12PC. 
The coating solution can be prepared by a variety of methods. One of them 
is to dissolve cesium fluoride (CsF) and hydrogen fluoride (HF) in water. 
Another example is to dissolve a carbonate or hydroxide of cesium in water 
and add hydrogen fluoride. Still another example is to dissolve an acidic 
fluoride of cesium in water. The method is not restricted to the above. 
The aluminum material is dipped in the coating solution, or otherwise 
brought into contact therewith. The cesium and fluorine ions in the 
solution react with aluminum in the material and form therewith a layer 
comprising at least one cesium fluoroaluminate complex or a mixture of at 
least one cesium fluoroaluminate complex and aluminum fluoride 
(hereinafter referred to as fluxing compound) bonded strongly with the 
surface of the aluminum material. The cesium fluoroaluminate complex 
includes a series of substances, such as Cs.sub.3 AlF.sub.6, CsAlF.sub.4 
and Cs.sub.2 AlF.sub.5. H.sub.2 O, which comprises cesium (Cs), aluminum 
(Al) and fluorine (F). 
The coating solution must include cesium and fluorine ions. The solution 
preferably contains 0.01 to 1.0 mol of cesium ions and 0.02 to 2.0 mols of 
fluorine ions per liter and has a pH of 2 to 6 in order to form 
efficiently a chemically coated layer of a fluxing compound having a high 
flux effect. If the solution contains smaller quantities of cesium and 
fluorine ions, the chemical reaction proceeds so slowly that an 
undesirably long time is required for forming a sufficiently large amount 
of a fluxing compound for obtaining an effective flux. No increase of 
cesium and fluorine ions over the ranges hereinabove stated is useful, 
since it does not bring about any corresponding increase in the amount of 
the fluxing compound which is formed. If the solution has a pH above 6, it 
reacts with aluminum only at an undesirably low rate. The use of a 
solution having a pH below 2 should also be avoided, since it heavily 
etches the aluminum material and forms an undesirably rough surface 
thereon. In order to adjust the pH of the solution, it is preferable to 
add hydrogen fluoride, as it is one of the substances used to prepare the 
solution. 
The aluminum material may be dipped in the solution as hereinabove stated. 
It is also effective to coat or spray the solution on at least that 
portion of the aluminum material which is to be brazed to another 
material. In this case, a relatively large quantity of the solution should 
be used to supply a sufficiently large quantity of cesium and fluorine 
ions. 
If the aluminum material is brought into contact with the solution by 
dipping or otherwise, the solution, which contains a mixture of cesium and 
fluorine ions, destroys an oxide film on the surface of the aluminum 
material, and the cesium and fluorine ions react with the aluminum ions in 
the material to form a chemically-coated layer of a fluxing compound on 
the surface of the material. The formation of the layer depends on the 
temperature of the solution. A satisfactory chemical reaction takes place, 
even if the solution is at room temperature. It is, however, more 
effective to use a solution having a temperature of from 40.degree. C. to 
70.degree. C. in order to remove the oxide film from the aluminum material 
completely and rapidly, so that the resulting fluxing compound may form a 
layer bonded still more strongly to the surface of the aluminum material. 
The solution may be applied to the aluminum material before it is formed 
into a particular shape or assembled into a particular object, or 
thereafter. The aluminum material can be degreased with an organic 
solvent, such as trichloroethylene, or otherwise cleaned before it is 
brought into contact with the solution. 
The length of time for which the aluminum material should be maintained in 
contact with the solution depends on various factors, including the cesium 
and fluorine ion concentrations of the solution, its pH and its 
temperature. It is, however, usually in the range of, say, 0.5 to five 
minutes. 
The cesium fluoroaluminate resulting from the chemical reaction as 
hereinabove described is a complex CsF-AlF.sub.3. It appears to have 
complicated structures, as is obvious from FIG. 1, which is a phase 
diagram of the system CsF-AlF.sub.3 [Zeitschrift fuer Anorganische und 
Allgeneine Chemie, 81,357 (1913)]. The phase diagram of any complexes 
containing more than 25 mol % of AlF.sub.3 is not clearly known as shown 
in FIG. 1. 
The inventors of this invention tried to identify this complex salt by 
X-ray diffraction, but could not identify it, since none of the 
chemically-coated layers showed a diffraction pattern coinciding with that 
of any substance having a known crystal structure, such as Cs.sub.3 
AlF.sub.6 or CsAlF.sub.4. CsF-AlF.sub.3 complexes have an extremely 
complicated phase equilibrium pattern, i.e. the complexes can take various 
crystal forms. 
The fluxing compound melts, or begins to melt, at a temperature of 
450.degree. C. to 480.degree. C. when its molar ratio of AlF.sub.3 /CsF is 
in the range of 67/33 to 26/74. The melted fluxing compound removes the 
oxide film from the surface of the aluminum material to enable a brazing 
alloy to flow satisfactorily thereon. It does not exert any adverse effect 
on aluminum. 
The fluxing compound can also be formed if the aluminum material in the 
solution is employed as an anode, and if an electric current is supplied 
thereto. The cathode used under these circumstances is a material having a 
surface area equal to that of the anode, and which is not ionized in the 
solution. Carbon is a typical example. 
The formation of the fluxing compound can also be carried out by employing 
an alternating current. If an AC voltage is applied to two pieces of 
aluminum material, a fluxing compound is formed on the material having a 
positive voltage as compared with the other piece, while no coating is 
formed on the material of negative voltage as compared with the other one. 
The formation of the fluxing compound takes place at a lower rate when 
either a DC or AC voltage is applied, than when no voltage is applied. 
Thus, the use of an electric current is effective for promoting the 
formation of a chemically coated layer composed of a desired quantity of a 
fluxing compound. 
Some unreacted cesium and fluorine ions remain on the surface of the 
aluminum material subjected to the chemical conversion coating. Those ions 
can be washed away in water, but do not cause any problem later, even if 
they are not removed. 
The aluminum material can, then, be dried so that water may be removed from 
its surface. This step is effective for causing the remaining ions to 
react with aluminum to form an additional fluxing compound, even when it 
is not washed as hereinabove stated. The material can be allowed to dry in 
the open air, though this method requires a relatively long time. 
Therefore, it is effective to blow air having a temperature ranging from 
ordinary temperature to 100.degree. C. against the material. It is still 
more effective to employ air having a temperature of 100.degree. C. to 
200.degree. C., as it removes water from the fluxing compound and heats it 
to form a coated layer united more strongly with the surface of the 
aluminum material. The complete drying of the material is also desirable 
from a standpoint of avoiding (during brazing) generation of water vapor 
which will raise the dewpoint of furnace atmosphere. It is also 
advantageous from the standpoint of avoiding the generation of harmful 
hydrogen fluoride vapor. 
The chemically-coated layer on the aluminum material preferably contains 
approximately 0.1 to 5 g of the fluxing compound per square meter. This 
quantity renders it effective as a brazing flux. 
The chemically-coated layer may be formed on the aluminum material in its 
original shape, for example, in the form of a wire, plate or block. 
Alternatively, it can be formed after the material has been formed into a 
part having a desired shape, such as a cooling water tube or fin for an 
automobile radiator, or after any such part has been combined with another 
part to form a temporarily assembled object which is ready for brazing. 
If the flux layer is formed on the aluminum material in its original shape, 
it is, then, worked into a desired shape and combined with another 
material to prepare a temporarily assembled object. The material with 
which it can be combined is an aluminum material which may, or may not, 
have a chemically-coated flux layer formed thereon, or to which a flux has 
been applied in an ordinary way. The flux layer is so strongly united with 
the material that it does not substantially peel away when the material is 
worked. A flux layer containing 0.1 to 3 g of a fluxing compound per 
square meter is, among others, capable of withstanding considerable hard 
work. If the layer contains over 5 g of the fluxing compound per square 
meter, care should be taken to avoid any peeling thereof when bending the 
material greatly. 
The temporarily assembled object has two or more portions at which its 
constituent parts should be brazed together. It is necessary to apply a 
brazing alloy to each of those portions before subjecting the object to 
the brazing operation. In this connection, it is effective and desirable 
to employ as at least one of its constituent parts a material on which a 
brazing alloy is clad before they are assembled. Alternatively, it is, of 
course, possible to apply a brazing alloy in the form of a rod, sheet, 
wire or powder to each of such portions. 
A fluxing compound may be formed on the brazing alloy, too, as hereinbefore 
described, before it is applied to the parts to be brazed. The brazing 
alloy preferably has a melting point which is about 10.degree. C. to 
100.degree. C. higher than that of the flux. It is usual to employ a 
eutectic alloy of aluminum and silicon having a silicon content of from 7 
to 12% by weight (e.g., A4343 or A4047 according to the designation of 
JIS). It is, however, possible to use also an alloy having a lower melting 
point, such as an Al-Si-Cu alloy (A4145) which begins to melt at a 
temperature of about 521.degree. C., or an Al-Si-Cu-Zn alloy which begins 
to melt at about 516.degree. C. 
After the brazing alloy has been applied, each of those portions at which 
the parts are to be brazed is heated by a torch or a heating furnace. The 
furnace preferably contains a non-oxidizing atmosphere, such as nitrogen, 
though a furnace containing atmospheric air can also be used. If the 
material is heated, the fluxing compound first melts to exhibit a flux 
effect. It reacts with Al.sub.2 O.sub.3 on the surface of the aluminum 
material and removes it therefrom. This effect manifests itself, even if 
the aluminum material may contain magnesium. The fluxing compound does not 
react with aluminum. 
It is not clear why the fluxing compound is effective as a flux for brazing 
an aluminum material containing magnesium, too. It is, however, likely 
that as it has a lower melting point than any conventional fluoride flux, 
it may prevent the vaporization of magnesium from the aluminum material 
before the brazing alloy begins to flow, or that it may dissolve the 
magnesium fluoride (MgF.sub.2) which prevents the flow of the brazing 
alloy. 
If the temperature is further raised, the brazing alloy melts and flows 
satisfactorily on the surface of the aluminum material to penetrate all of 
those portions at which the parts are to be brazed. 
The invention will now be described more specifically with reference to a 
plurality of examples. 
EXAMPLE 1 
A plurality of brazing sheets 1 (designated by JIS as BA12PC) and an equal 
number of sheets of aluminum material 2 containing 0.8 to 1.2% by weight 
of magnesium (designated by JIS as A6061) were chemically treated under 
the conditions shown at Run Numbers 1 to 6 in TABLE 1. Each of the brazing 
sheets 1 had a pair of surfaces clad with an aluminum alloy containing 7% 
by weight of silicon. It had a width of 2 cm, a length of 3 cm and a 
thickness of 1.6 mm. Each of the sheets 2 was 3 cm square and had a 
thickness of 1.0 mm. Each brazing sheet 1 and one of the sheets 2 were put 
together to form a temporarily assembled object having a brazing joint 3 
as shown in FIG. 2. The chemical analysis of the flux layer formed for Run 
No. 1 indicated that it had an Al/Cs atomic ratio of 33/67. The results of 
its X-ray diffraction are shown in FIG. 3 by way of example, and the 
results of its thermal analysis in FIG. 4. 
The object was held at 610.degree. C. for two minutes in a furnace having a 
nitrogen atmosphere, and allowed to cool outside the furnace. A fillet was 
formed at the joint 3 by a brazing alloy. Two shapes of fillets were 
formed. One of them was very satisfactory with a uniform width as shown in 
FIG. 5. The other is shown in FIG. 6, and was also satisfactory, though 
its width was not uniform. The right column in TABLE 1 shows the results 
of evaluation of the fillet formed under the different conditions. 
Run No. C1 is a comparative example in which a conventional potassium 
fluoroaluminate flux having an AlF.sub.3 /KF molar ratio of 45/55 and in 
the form of a powder having a particle size of 200 mesh was applied in a 
quantity of 3 g/m.sup.2 to the surface of an aluminum material. No 
satisfactory brazing was possible, as the brazing alloy failed to flow 
properly. 
TABLE 1 
______________________________________ 
Conditions of chemical coating treatment 
Composition of 
Run solution Tempera- 
Time 
No. (mol/liter) pH ture (.degree.C.) 
(min.) 
Evaluation 
______________________________________ 
1 CsHF.sub.2 : 
1.0 3.0 20 1 Satisfactory 
2 CsHF.sub.2 : 
0.01 4.0 60 2 Substantially 
satisfactory 
3 Cs(OH): 0.2 3.5 20 1 Satisfactory 
HF: 0.25 
4 CsF: 0.1 3.2 20 1 Satisfactory 
HF: 0.1 
5 CsF: 0.2 6.0 60 2 Substantially 
HF: 0.01 satisfactory 
6 CsF: 0.2 2.0 20 0.5 Substantially 
HF: 0.8 satisfactory 
Cl KF--AlF.sub.3 flux Unsatisfactory 
______________________________________ 
EXAMPLE 2 
A plurality of 3 cm square, 1.0 mm thick sheets 5 of an aluminum alloy 
designated by JIS as A3004 were chemically treated under the conditions 
shown in TABLE 2 at Run Nos. 7 to 9 according to this invention, and at C2 
to C4 for comparative purposes. Each two of the sheets 5 were put together 
into a T-shaped object having therebetween a degreased brazing alloy 4 in 
the form of a strip having a width of 0.5 cm, a length of 3 cm and a 
thickness of 0.2 mm, as shown in FIG. 7. The brazing alloy had been 
degreased by trichloroethylene. Each object was held at the temperature 
shown in TABLE 2 for two minutes in a furnace filled with nitrogen, 
whereby the two sheets were brazed together. The procedure of EXAMPLE 1 
was repeated for evaluating the resulting fillet. The results are shown in 
the right column of TABLE 2. 
TABLE 2 
__________________________________________________________________________ 
Conditions of chemical coating treatment 
Run Composition of so- 
Temp. 
Time 
Brazing 
No. 
Brazing alloy (wt. %) 
lution (mol/liter) 
pH (.degree.C.) 
(min.) 
temp. (.degree.C.) 
Evaluation 
__________________________________________________________________________ 
7 Al--10Si CsHF.sub.2 : 
0.1 3.5 
20 1 610 Satisfactory 
C2 Al--10si KHF.sub.2 : 
0.1 3.0 
20 1 610 Unsatisfactory 
8 Al--10Si--4Cu--10Zn 
CsHF.sub.2 : 
1.0 3.5 
20 1 580 Satisfactory 
C3 Al--10Si--4Cu--10Zn 
KHF.sub.2 : 
1.0 3.0 
20 1 580 Unsatisfactory 
9 Al--75Zn CsF: 1.0 3.0 
20 1 500 Satisfactory 
HF: 1.0 
C4 Al--75Zn KF: 1.0 3.0 
20 1 500 Unsatisfactory 
HF: 1.0 
__________________________________________________________________________