Brazing composition for aluminum material, aluminum material for brazing, and method of brazing aluminum material

A brazing composition for aluminum materials capable of functioning as a flux and a brazing material with a single component and providing a solid brazed joint exhibiting little local fusion. The brazing composition for aluminum materials which has the function of removing the oxide film on the surface of the aluminum material to be joined at a temperature lower than the melting point of the aluminum material and forming a eutectic aluminum alloy braze which fuses at such a low temperature by reaction with the aluminum materials, wherein the brazing composition comprises a first powder comprising at least one or more M--Si--F compounds comprising, in addition to at least Si and F, hydrogen or an alkaline metal or hydrates thereof or comprises this first powder as an component.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a brazing composition for aluminum
 materials, an aluminum material for brazing, and a method of brazing an
 aluminum material. More particularly, the present invention relates to
 brazing of aluminum materials comprising aluminum or aluminum alloys
 corresponding to brazing using an anticorrosive flux.
 2. Description of the Background Art
 Brazed products prepared by assembling aluminum materials or parts and
 brazing them have been widely used for heat exchangers such as automotive
 heat exchangers because of their lightweight and high conductivity. In the
 manufacture of such brazed products, various brazing methods are employed
 today, which are roughly divided into a flux brazing method and a vacuum
 brazing method. Among the brazing using a flux, brazing using an
 anticorrosive flux is in the mainstream, in which a compound containing
 fluorine (F), for example, fluorine compounds such as KF, AlF.sub.3,
 KAlF.sub.4, K.sub.2 AlF.sub.5, K.sub.3 AlF.sub.6, CsF, RbF, LiF, NaF, and
 CaF.sub.2, a mixture of these compounds, or fused and solidified products
 of these compounds is used as the main component.
 In this brazing method, the flux is applied to a brazing material made of
 Al--Si alloys and heated to remove an oxide film on the surface of the
 aluminum materials to be joined. At the same time, the brazing material
 and the aluminum materials are provided with wettability to join the
 aluminum materials. As the brazing materials used in the above brazing
 using an anticorrosive flux, a brazing sheet comprising aluminum or
 aluminum alloys as a core material with brazing alloys such as Al--Si
 alloys clad on both sides or one side of the core material is used. Such a
 brazing sheet is manufactured by joining the core material and the brazing
 alloys by hot rolling and spreading to a predetermined thickness by cold
 rolling. This makes the manufacturing process complicated and increases
 the number of the manufacturing steps, thereby increasing the material
 costs. Moreover, since the brazing material contains hard Si, the molding
 cost also increases. Furthermore, in the manufacture of aluminum-made heat
 exchangers using such a brazing sheet, a step of applying an anticorrosive
 flux before heating for brazing further increases the cost.
 In order to solve these problems of the prior art, a method which comprises
 applying a brazing composition comprising a mixture of metal Si powder and
 an anticorrosive flux to aluminum materials, assembling the aluminum
 material and the other material to be joined, and heating these aluminum
 materials to braze them without using a brazing sheet has been proposed
 (U.S. Pat. No. 5,100,048, U.S. Pat. No. 5,190,596, Publication of
 Translation of International Patent Application No. 504485/1994). This
 method uses a mixture of metal Si powder or other metal powders which form
 a eutectic alloy with aluminum, and a flux powder which removes an oxide
 film as the brazing composition. The mixture is applied to the aluminum
 materials to be joined and heated while the metal Si powder or other metal
 powders which form a eutectic alloy with aluminum in the mixture is in
 contact with the aluminum surfaces to diffuse Si and the like in the
 aluminum material and create a state similar to a eutectic structure of
 eutectic alloys such as Al--Si alloys with the aluminum components to join
 the aluminum materials.
 However, in this method, the metal powder which forms a eutectic alloy with
 aluminum, such as a metal Si powder used as a component of the mixture, is
 relatively expensive. Moreover, the powder grinding operation for
 preparing the metal powder to a predetermined particle size is difficult
 because of its hardness, thereby increasing the manufacturing cost.
 If a small amount of the metal powder (Si) having a large particle size is
 included in the mixture, such a metal powder fuses the aluminum material
 that is a base metal during the steps from heating to cooling for brazing
 to cause a large amount of local fusion. This brings about problems in the
 resulting brazed products. For example, in the case of heat exchangers,
 the necessary characteristics such as pressure resistance and corrosion
 resistance may be impaired. In the case of brazing parts which require a
 large amount of a brazing composition, if the mixture of the brazing
 composition is applied thickly, the metal (Si) does not react with the
 aluminum components in the aluminum material and remains as a black
 residue.
 SUMMARY OF THE INVENTION
 In view of the above situation of the prior art, the present inventors have
 conducted extensive studies of brazing using an anticorrosive flux. As a
 result, the present inventors have discovered that a specific compound
 comprising, in addition to at least F and Si, hydrogen or an alkaline
 metal or hydrates thereof can function as a flux which removes an oxide
 film which is present on the surface of the aluminum materials and hinders
 the joining of the aluminum materials by the reaction with the aluminum
 material at a temperature lower than the melting point of the aluminum
 material to be joined, and can function as a brazing material which forms
 a eutectic aluminum alloy braze which fuses at such a low temperature and
 integrally joins the aluminum materials by the subsequent cooling. The
 present invention has been completed on the basis of this finding.
 Accordingly, an object of the present invention is to provide a brazing
 composition for aluminum materials wherein a single component can function
 as a flux and a brazing material (differing from a conventional brazing
 composition comprising a flux and a brazing material in combination) to
 provide a solid brazed joint exhibiting little local fusion, an aluminum
 material for brazing using the brazing composition, and a method of
 manufacturing the same, and a method of brazing aluminum materials using
 the brazing composition.
 In order to achieve the above object, the present invention provides a
 brazing composition which has the function of removing the oxide film on
 the surface of the aluminum material to be joined at a temperature lower
 than the melting point of the aluminum material and forming a eutectic
 aluminum alloy braze which fuses at such a low temperature by the reaction
 with the aluminum materials, wherein the brazing composition comprises a
 first powder which comprises at least one or more M-Si-F compounds
 comprising, in addition to at least Si and F, hydrogen or an alkaline
 metal or hydrates thereof, or comprises this first powder as a major
 component and other components. Specifically, according to the brazing
 composition of the present invention, the compound comprising, in addition
 to at least F (fluorine) and Si (silicon), hydrogen or an alkaline metal
 or hydrates thereof reacts with the aluminum materials (the surface of the
 aluminum material) to be joined at a temperature lower than the melting
 point of the aluminum materials to decompose into two or more compounds
 having the function of effectively removing the oxide film on the surface
 of the aluminum material which hinders joining of the aluminum materials
 and forming a eutectic aluminum alloy braze as a brazing material to form
 an integrally brazed joint by subsequent cooling. According to the brazing
 composition of the present invention, because a layer of the flux which
 removes the oxide film and a layer which forms a eutectic aluminum alloy
 braze and forms the brazed joint as a brazing material are formed on the
 surface of the aluminum material at a temperature slightly lower than the
 brazing temperature, local fusion on the brazed area of the aluminum
 material after brazing can be prevented.
 Since the brazing composition of the present invention functions as both
 the flux and brazing material, there is no need to use either the mixture
 prepared by selecting the brazing material and flux and mixing these
 components or a brazing sheet, as in a conventional method. Moreover, the
 surface of the aluminum material where the aluminum material and the
 brazing composition has reacted exhibits little local fusion.
 Furthermore, the powder of the above specific M--Si--F compounds or the
 hydrates thereof which constitute the brazing composition of the present
 invention can be easily ground in comparison with a conventionally used
 hard Si powder. Because of this, local fusion due to the large particle
 size of the powders can be effectively prevented. According to the brazing
 composition of the present invention, if a powder of a large particle size
 is used, since local fusion is effectively controlled by the properties of
 the compound which provides such a powder, a solid brazed joint exhibiting
 little local fusion can be advantageously obtained.
 According to a preferred embodiment of the brazing composition for aluminum
 materials of the present invention, the first powder of the brazing
 composition comprises at least one or more M--Si--F compounds or hydrates
 thereof so that the weight ratio of M:Si:F: is 5-50%:5-50%:20-80% (wherein
 M represents hydrogen or an alkaline metal, provided that the sum of M,
 Si, and F is 100%). In the present invention, hexafluorosilicic acid or
 alkaline metal salts thereof, in particular, potassium hexafluorosilicate
 or sodium hexafluorosilicate is advantageously used as the M--Si--F
 compound.
 According to another preferred embodiment of the present invention, the
 brazing composition of the present invention comprises 50 wt % or more of
 at least one or more of hexafluorosilicic acid, an alkaline metal salt
 thereof, or hydrates of these compounds as the first powder comprising one
 or more M--Si--F compounds or hydrates thereof and, in the remaining
 proportion, second powders comprising at least one or more powders
 selected from the group consisting of metal powders consisting of Al, Si,
 Cu, Zn, Ge, Sr, or Bi, alloy powders of at least one of these metals and
 aluminum, oxides or fluorides of these metals, alkaline metal salts of
 silicic acid or hydrates thereof, hexafluorosilicate (excluding alkaline
 metal salt) or hydrates thereof, a brazing flux comprising 20-45% of K,
 10-25% of Al, and 45-70% of F in an elemental proportion, and a fluoride
 flux. In the combination of the first and second powders, potassium
 hexafluorosilicate or sodium hexafluorosilicate is advantageously used as
 the alkaline metal salt of hexafluorosilicic acid.
 When applying the brazing composition of the present invention to the
 aluminum materials to be joined, the brazing composition is dispersed in a
 volatile solvent or water in slurry form and used as a brazing coating.
 Particularly, it is preferable that a resin which disperses and disappears
 at 550.degree. C. or lower and does not hinder brazing properties be added
 to such a brazing coating. The addition of such a resin improves
 properties such as uniformity of the coated surface and adhesion of the
 coating.
 The present invention further provides a method of manufacturing aluminum
 materials for brazing which comprises coating the above brazing coating on
 the surface of the aluminum materials to be joined by a roll transfer
 method to form a layer of the brazing composition. The present invention
 also provides an aluminum material for brazing prepared by the above
 manufacturing method and the like, specifically, an aluminum material for
 brazing which comprises a coating layer of the brazing composition of the
 present invention applied to the surface of the aluminum material to be
 joined. This coating layer advantageously comprises 1-50 g/m.sup.2 of Si
 in the first powder comprising one or more M--Si--F compounds or hydrates
 thereof in the brazing composition. The present invention provides a
 method of brazing aluminum materials which comprises applying the brazing
 composition of the present invention to two aluminum materials at least in
 the joined areas, heating the aluminum materials to remove the oxide film
 on the joined surfaces of the aluminum materials with the brazing
 composition, and forming a eutectic aluminum alloy braze by the reaction
 of the brazing composition and the aluminum materials to braze the
 aluminum materials into an integrated product. According to the method of
 brazing aluminum materials of the present invention, since a specific
 M--Si--F compound which constitutes the brazing composition functions as
 both the flux and brazing material, aluminum materials can be brazed using
 such a single component. Therefore, there is no need to separately select
 the components for the brazing material and flux and mix these components
 in the preparation of the brazing composition, thereby effectively
 reducing the cost for the materials and advantageously simplifying the
 manufacturing process.
 DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENT
 According to the brazing composition for aluminum materials of the present
 invention, a first powder comprising at least one or more M--Si--F
 compounds which comprise, in addition to at least Si (silicon) and F
 (fluorine), hydrogen or an alkaline metal or hydrate thereof is used as
 the constituent component or one of the components. In brazing the
 materials made of aluminum or aluminum alloys, the first powder forms a
 flux layer which removes the oxide film and a layer which functions as a
 brazing material for forming a brazed joint by forming a eutectic aluminum
 alloy braze on the surface of the aluminum material at a temperature lower
 than the melting point of the aluminum material to be joined. Because of
 this, the first powder removes the oxide film on the surface of the
 aluminum material as a flux and forms a eutectic aluminum alloy braze
 which fuses at such a low temperature to advantageously achieve the object
 of the present invention.
 As the first powder comprising at least one or more specific M--Si--F
 compounds or hydrates thereof, a compound prepared so that the weight
 ratio of M (hydrogen or an alkaline metal):Si (silicon):F (fluorine) in
 the compound is 5-50%:5-50%:20-80% (wherein M+Si+F=100%) is advantageously
 used. Although a composition in which the weight ratios of M and F are
 less than 5% and more than 80%, respectively causes no problem in brazing,
 use of a large amount of F which removes the oxide film increase the
 manufacturing cost. Moreover, plate crystals of the flux excessively
 adsorb to the surface of the aluminum material and remain as a residue,
 thereby impairing the appearance as well as uniformity of the surface
 treatment after the flux brazing. If the weight ratios of M and F are more
 than 50% and less than 20%, respectively, the amount of F for removing the
 oxide film is insufficient, thereby resulting in inferior joining.
 Moreover, because of the insufficient fusion of Si which can form a
 eutectic with the surface of the aluminum materials, residues of Si which
 do not fuse remain in the non-joint areas. If the weight ratios of Si is
 less than 5%, the use of a large amount of F which removes the oxide film
 increases the manufacturing cost. Moreover, plate crystals of the flux are
 excessively adsorbed to the surface of the aluminum materials and remain
 as a residue, thereby impairing the appearance as well as uniformity of
 the surface treatment after the flux brazing. If the weight ratio of Si is
 more than 50%, because of insufficient fusion of the Si which can form a
 eutectic with the surface of the aluminum material, residues of Si which
 do not fuse remain in the non-joint areas.
 In the present invention, as examples of the above M--Si--F compounds,
 hexafluorosilicic acid (H.sub.2 SiF.sub.6) or alkaline metal salts thereof
 can be used. Specific examples include Li.sub.2 SiF.sub.6, Na.sub.2
 SiF.sub.6, K.sub.2 SiF.sub.6, Cs.sub.2 SiF.sub.6, Rb.sub.2 SiF.sub.6, and
 H.sub.2 SiF.sub.6. Of these, potassium hexafluorosilicate (K.sub.2
 SiF.sub.6) and sodium hexafluorosilicate (Na.sub.2 SiF.sub.6) are
 advantageously used. The above hexafluorosilicic acid or alkaline metal
 salts thereof can be used as a hydrate such as H.sub.2 SiF.sub.2.2H.sub.2
 O and Li.sub.2 SiF.sub.6.2H.sub.2 O.
 The brazing composition of the present invention comprises only the first
 powder which comprises at least one or more M--Si--F compounds or hydrates
 thereof or comprises 50 wt % or more of the first powder as the major
 component and other components. As the above other components
 (subcomponent: second powder) which are used in combination with the first
 powder, various known components are used in order to improve the
 characteristics of the brazed part and the like. This subcomponent (second
 powder) comprises at least one or more compounds selected from the group
 consisting of metal powders such as Al, Si, Cu, Zn, Ge, Sr, or Bi, alloy
 powders of at least one of these metals and aluminum, oxides or fluorides
 of these metals, an alkaline metal salt of silicic acid or hydrates
 thereof, hexafluorosilicate (excluding the alkaline metal salt) or
 hydrates thereof, brazing flux comprising 20-45% of K, 10-25% of Al, and
 45-70% of F in an elemental proportion, and fluoride flux. The second
 powder is used in the remaining proportion (less than 50 wt %) in addition
 to 50 wt % or more of the first powder in the brazing composition. The
 addition of the second powder as the subcomponent improves joining
 properties, provides corrosion resistance, sacrificial anode effect,
 effective brazing at a low temperature, and the like.
 Specifically, for example, Si (metal), Al--Si alloy, SiO.sub.2, other Si
 alloys, or a compound containing Si mainly ensures and adjusts the brazing
 material necessary for the fillet which is formed on the brazed joint. Zn,
 Al--Zn alloys, ZnF.sub.2.4H.sub.2 O, other Zn alloys or a compound
 containing Zn adjusts the potential difference between the joined members
 to ensure the sacrificial anode effect. Cu, Al--Cu alloys, Cu.sub.2 O,
 CuF.sub.2.2H.sub.2 O, other Cu alloys, or a compound containing Cu mainly
 improves the hardness of the joined members. Ge, Al--Ge alloys, GeO.sub.2,
 other Ge alloys, or a compound containing Ge mainly lowers the reaction
 temperature with the aluminum materials to control the brazing
 temperature. Sr, SrF, Al--Sr alloys, Bi, Al--Bi alloys, Bi.sub.2 O.sub.3,
 other Bi or Sr alloys, or a compound containing Bi or Sr mainly improves
 the fluidity of the braze to promote brazing properties. Al, Al.sub.2
 O.sub.3, AlF.sub.3, other Al alloys, or a compound containing Al controls
 the degree of reaction and fusion with the aluminum base metal, adjusts
 the melting point of the flux, and the like. Hexafluorosilicates such as
 BaSiF.sub.6, CaSiF.sub.6, SrSiF.sub.6, ZnSiF.sub.6 or hydrates thereof,
 KF, AlF.sub.3, KAlF.sub.4, K.sub.2 AlF.sub.5, K.sub.3 AlF.sub.6, CsF, RbF,
 LiF, NaF, CaF.sub.2, a mixture of two or more of these compounds, or a
 composition obtained by fusing and solidifying these compounds adjusts the
 melting point of the flux for removing the oxide film or the melting point
 of the flux while maintaining the amount of brazing fusion. Alkaline metal
 salts of silicic acid such as Na.sub.2 SiO.sub.3, Li.sub.2 SiO.sub.3, and
 K.sub.2 SiO.sub.3 or hydrates thereof contribute to the formation of the
 brazing material. As described above, in the brazing composition of the
 present invention, various metals or compounds are optionally blended as
 the subcomponent (second powder) in an appropriate amount in addition to
 the first powder as the essential component.
 In the present invention, substances which improve the object of the
 present invention other than the substances described above, for example,
 a substance which diffuses or fuses in the aluminum materials by reacting
 with the aluminum material, or provides the brazed product with desired
 characteristics by reacting with the flux upon heating for brazing can be
 used in the brazing composition in combination with the first powder.
 As the aluminum material to which the brazing composition of the present
 invention is applied, various materials such as a plate or extruded
 materials comprising aluminum or aluminum alloys, and molded products or
 assembled products thereof are used. A layer of the brazing composition is
 formed to a predetermined thickness on various aluminum materials such as
 aluminum raw materials, processed products, and assembled products
 according to techniques described below. The brazing composition of the
 present invention is applied to the aluminum materials made of aluminum or
 aluminum alloys having a desired shape according to a conventional
 technique. The brazing composition is generally prepared in slurry form by
 dispersing the composition in a volatile solvent or water to be used as a
 brazing coating having a function of a brazing flux and a function of
 forming a eutectic aluminum alloy braze which fuses at a low temperature
 as described above and applied to the objective aluminum materials. Use of
 such a coating improves the uniformity of the coating on the surface of
 the aluminum material, ensures multiplication and simplicity of the
 coating method advantageously, and improves properties such as adhesion of
 the coating. In the present invention, in order to ensure these effects, a
 brazing coating comprising the brazing composition prepared in slurry form
 in a volatile solvent or water and a resin which decomposes and disappears
 at 550.degree. C. or lower and does not hinder brazing properties is
 advantageously used. As water which is used in the preparation of this
 brazing coating, since impurities contained in water hinder brazing
 properties, use of pure water is preferable. As the above volatile
 solvent, conventionally known solvents are appropriately selected
 according to the desired thickness of the coating, degree of surface
 roughness (uniformity), and the like for each constituent member of the
 brazed product such as heat exchangers. Of these, organic solvents such as
 propanol, 2-propanol, butanol, toluene, xylene, and ethylbenzene are
 advantageously used.
 The brazing composition is prepared in slurry form in such a volatile
 solvent or water as the brazing coating so that the solid weight ratio is
 50% or more. As the resin which is added to the brazing coating, resins
 which completely volatilize or disappear at a brazing temperature or less,
 in particular, at the fusing temperature or less of the brazing material
 are used in order to prevent the brazing properties from being hindered.
 Resins which can decompose and disappear at 550.degree. C. or lower and do
 not hinder the brazing properties are generally used. As examples of such
 resins, homopolymers or copolymers prepared by polymerizing one monomer or
 two or more monomers such as methyl methacrylate, ethyl methacrylate,
 n-butyl methacrylate, 2-ethylhexyl methacrylate, and propyl methacrylate,
 copolymers of these monomers and other vinyl monomers, and the like can be
 given. These resins are appropriately selected according to the desired
 characteristics such as coating thickness and surface uniformity (degree
 of surface roughness) in combination with the types of the dispersion
 medium (solvent or water). The weight ratio of the resin used in the
 coating is preferably 50% or less, since use of the resin in a too large
 an amount increases the cost and the like.
 According to the present invention, the objective aluminum material for
 brazing is manufactured by applying the above brazing coating to at least
 the joined surface of the aluminum material made of aluminum or aluminum
 alloys using a conventional method to form the coating layer of the
 brazing composition. In the manufacture of the aluminum material for
 brazing, conventional methods such as spray coating, immersion coating,
 and roll transfer coating (roll coating) are appropriately adopted for
 continuously applying the brazing coating to the surface of the aluminum
 materials. Since a spray gun sometimes clogs during the spray coating and
 stability of the coating sometimes becomes inferior during immersion in
 the immersion method, the roll transfer method which exhibits superior
 coating stability and superior processability is adopted as a preferable
 coating method in the present invention. In roll transfer coating, coating
 conditions such as materials for the surface of the roller and the forward
 or reverse rotation of a coating roller and an application roller are
 appropriately determined according to the desired coating thickness,
 surface uniformity (degree of surface roughness), and the like. The roll
 transfer conditions suitable for the object are appropriately selected.
 The coating layer comprising the brazing composition of the present
 invention is formed to a predetermined thickness on the joined surface of
 the aluminum materials for the brazing thus obtained. Such a coating layer
 is generally formed to a thickness so that the Si in the first powder
 comprising one or more M--Si--F compounds or hydrates thereof in the
 brazing composition is contained in a ratio of 1-50 g/m.sup.2. If the Si
 content (Si-reduced value) in the coating layer is too small, although the
 aluminum materials can be joined, fused brazing materials cannot form an
 effective fillet, whereby the brazed product exhibits insufficient
 performance, or the hardness of the brazed joint becomes inferior. If the
 Si content of the coating layer is too large, fusion and corrosion caused
 by the fillet formed at the joint of the base metal are significant,
 thereby impairing brazing properties, corrosion resistance, and the like .
 According to the aluminum material for brazing of the present invention,
 the thickness of the coating layer comprising the brazing composition
 formed on the surface of the aluminum material is appropriately determined
 in the range of the above Si-reduced values (1-50 g/m.sup.2) corresponding
 to the functions of each constituent material of the objective brazed
 product, namely, the desired amount of the fillet at the joint. As the
 coating layer having a larger Si-reduced value (thicker coating) is
 formed, the fillet formed at the joint becomes bigger. For example, in the
 case of a fin for a heat exchanger, the coating layer having a Si-reduced
 value in the range from 1-20 g/m.sup.2 is preferable. In the case of a
 tube used as a refrigerant circulator of a heat exchanger, a coating layer
 in the range from 2-30 g/m.sup.2 is preferable. In the case of a tank
 formed at an entrance of a heat exchanger which connects the refrigerant
 circulators, a coating layer in the range from 5-50 g/m.sup.2 is
 preferable.
 The present invention exhibits superior characteristics when brazing
 aluminum materials using the brazing composition or the aluminum material
 thus obtained in comparison to conventional brazing using an anticorrosive
 flux. Specifically, the present invention comprises applying the brazing
 composition to at least the joined surfaces of the aluminum materials and
 heating the composition and the aluminum materials to remove the oxide
 film present on the surface of the joined areas of the aluminum materials
 and the brazing composition, and forming a eutectic aluminum alloy braze
 by the reaction of the brazing composition and the aluminum material to
 braze these aluminum materials to an integral joined product (brazed
 product), whereby a solid brazed joint can be easily formed. In this
 brazing method, the brazing composition is applied to at least one of the
 joined surfaces of two aluminum materials as the coating layer and the
 like. The aluminum materials are assembled (attached) to the objective
 joint form in the presence of the brazing composition. The aluminum
 materials are heated at a temperature lower than the melting point of
 these aluminum materials, generally at 580-620.degree. C., to react the
 brazing composition and the surfaces of the aluminum materials. By this
 reaction, the oxide film which is present on the surfaces of the aluminum
 materials and hinders the joining is removed and a eutectic aluminum alloy
 braze which fuses at such a temperature is formed. The aluminum materials
 are then cooled to obtain the brazed product of two aluminum materials.

EXAMPLES
 The present invention will now be described in more detail by way of
 examples below, which should not be construed as limiting the present
 invention. It should be clearly understood that numerous modifications,
 amendments, and variations of the present invention other than the
 following Examples and as specifically described herein are possible on
 the basis of the knowledge of a person who is skilled in the art.
 Example 1
 Various aluminum materials for brazing shown in Table 1 were prepared.
 Specifically, various brazing coatings in slurry form were prepared by
 mixing brazing compositions of the Examples shown in Table 1 with the same
 weight of purified water. Coating layers comprising various brazing
 compositions having a Si-reduced coating weight shown in Table 1 were
 applied to one side of the surface of a first aluminum sheet (material:
 A3003, thermally refined O-type, thickness: 1.0 mm, width: 25 mm, length:
 60 mm) using a bar coater to obtain first aluminum sheets as the aluminum
 materials for brazing.
 In the preparation of the brazing compositions, powders having an average
 particle diameter of about 30 .mu.m were used in Nos. 1-6 and Nos. 11-16.
 In No. 7 and No. 8, powders having an average particle diameter of about
 20 .mu.m were used. In No. 9 and No. 10, powders having an average
 particle diameter of about 60 .mu.m were used. The mixing ratio of
 KAlF.sub.4 :K.sub.3 AlF.sub.6 was 1:1.
 TABLE 1
 Si-
 reduced
 Brazing composition Content (wt %) coating
 Content weight
 No. Component (wt %) M Si F (g/m.sup.2)
 Example
 1 Na.sub.2 SiF.sub.6 100 24 15 61 2
 2 Na.sub.2 SiF.sub.6 100 24 15 61 45
 3 K.sub.2 SiF.sub.6 100 35 13 52 2
 4 K.sub.2 SiF.sub.6 100 35 13 52 45
 5 K.sub.2 SiF.sub.6 /Na.sub.2 SiF.sub.6 50/50 30 14 56
 2
 6 K.sub.2 SiF.sub.6 /Na.sub.2 SiF.sub.6 50/50 30 14 56
 45
 Comparative
 Example
 7 Si/(KAlF.sub.4 +
 K.sub.3 AlF.sub.6) 33/67 30 33 37 2
 8 Si/(KAlF.sub.1 +
 K.sub.3 AlF.sub.6) 33/67 30 33 37 45
 9 ZnSiF.sub.6.4 H.sub.2 O 100 -- 19 81 2
 10 ZnSiF.sub.6.4 H.sub.2 O 100 -- 19 81 45
 11 Na.sub.2 SiF.sub.6 100 24 15 61 0.5
 12 Na.sub.2 SiF.sub.6 100 24 15 61 55
 13 K.sub.2 SiF.sub.6 100 35 13 52 0.5
 14 K.sub.2 SiF.sub.6 100 35 13 52 55
 15 K.sub.2 SiF.sub.6 /Na.sub.2 SiF.sub.6 50/50 30 14 56
 0.5
 16 K.sub.2 SiF.sub.6 /Na.sub.2 SiF.sub.6 50/50 30 14 56
 55
 The first aluminum sheet was placed as a horizontal plate so that the
 coating layer of the brazing composition is the upper side. A second
 aluminum sheet (material: A3003, thermally refined O-type, thickness: 1.0
 mm, width: 25 mm, length: 55 mm) was assembled as a vertical plate on the
 first aluminum sheets into a T shape and secured with a jig. This
 assembled material consisting of the first and second aluminum sheets was
 put into a brazing oven maintained at an oxygen concentration of 100 ppm
 or below and a dew point of -30.degree. C. or below in a nitrogen
 atmosphere and heated at 600.degree. C. for 3 minutes for brazing. After
 cooling to 500.degree. C. or below in the oven, the assembled material
 secured with a jig was taken out to obtain various samples in which the
 first and second aluminum sheets were brazed into a T-shape.
 The size of the fillet and appearance of the surfaces of the samples thus
 obtained were evaluated by naked eye observation. The rate of the fillet
 formed at the joint (joining rate), the corrosion depth of the joint, and
 the maximum fusion depth of the non-joint areas were measured. The results
 are shown in Table 2.
 The joining rate (%) used herein is determined by (length of the fillet
 formed at the joint/length where the horizontal plate and the vertical
 plate joined (length of joint)). The corrosion depth of the fillet at the
 joint and the maximum fusion depth of the non-joint areas were determined
 by measuring the area where the deepest fusion of the base metal due to
 the corrosion of the brazing material was observed by using enlarged
 sectional microphotography.
 TABLE 2
 Evaluation results
 Maximum
 Corrosion Joining fusion depth
 depth at Size of rate at non-joint Appear-
 No. joint fillet (%) area (.mu.m) ance
 Example
 1 Shallow Small 100 15 Good
 2 Shallow Large 100 45 Good
 3 Shallow Small 100 16 Good
 4 Shallow Large 100 42 Good
 5 Shallow Small 100 14 Good
 6 Shallow Large 100 41 Good
 Comparative
 Example
 7 Shallow Small 100 52 Good
 8 Deep Large 100 104 Great
 quantity of
 residue (S)
 9 0 Great
 quantity of
 residue
 10 0 Great
 quantity of
 residue
 11 None None 100 11 Good
 12 Excessive Excessive 100 62 Great
 quantity of
 residue (F)
 13 None None 100 12 Good
 14 Excessive Excessive 100 69 Great
 quantity of
 residue (F)
 15 None None 100 16 Good
 16 Excessive Excessive 100 58 Great
 quantity of
 residue (F)
 Note:
 (S): Black (brown) residue due to Si
 (F): Excessive crystalline residue of flux powder
 As is clear from the results shown in Table 2, in Nos. 1-6, are the
 Examples of the present invention, the fillet was effectively formed,
 showing effective brazing of the aluminum materials with a small corrosion
 depth in the joint area. In addition, the maximum fusion depth of the
 non-joint area was also small (less than 50 .mu.m), and corrosion due to
 the residue on the surface was scarcely observed. On the other hand, in
 No. 7 which is the Comparative Example, the non-joint area exhibited a
 large value for the maximum fusion depth. In No. 8, the non-joint area
 exhibited a large value for the maximum fusion depth and brown residue was
 observed on the surface. In No. 9 and No. 10 in which the brazing
 composition comprises, in addition to F and Si, metal components other
 than hydrogen or an alkaline metal, the aluminum materials could not be
 brazed. In Nos. 11, 13, and 15, the fillet was not formed though the
 aluminum materials were joined. In Nos. 12, 14, 16, though brazing with a
 joining rate of 100% was achieved, the non-joint area exhibited a large
 value for the maximum fusion depth and a great quantity of flux residue
 was observed on the surface.
 Example 2
 Using various brazing compositions comprising mixed powders shown in Tables
 3 and 4, brazing properties of the aluminum materials were evaluated. Each
 brazing composition was applied so that the Si-reduced coating weight in
 K.sub.2 SiF.sub.6, which is the constituent component, was about 10
 g/m.sup.2. The brazing operations and evaluation of the brazed joint were
 conducted in the same manner as in Example 1. The results are shown in
 Tables 5 and 6.
 TABLE 3
 Brazing composition
 No. Component Content (wt %)
 17 K.sub.2 SiF.sub.6 /Si 70/30
 18 K.sub.2 SiF.sub.6 /Cu 70/30
 19 K.sub.2 SiF.sub.6 /Zn 70/30
 20 K.sub.2 SiF.sub.6 /Ge 70/30
 21 K.sub.2 SiF.sub.6 /Sr 70/30
 22 K.sub.2 SiF.sub.6 /Bi 70/30
 23 K.sub.2 SiF.sub.6 /AlF.sub.3 90/10
 24 K.sub.2 SiF.sub.6 /LiF 90/10
 25 K.sub.2 SiF.sub.6 /CsF 90/10
 26 K.sub.2 SiF.sub.6 /KF 90/10
 27 K.sub.2 SiF.sub.6 /NaF 90/10
 28 K.sub.2 SiF.sub.6 /(KAlF.sub.4 + K.sub.3 AlF.sub.6) 70/30
 29 K.sub.2 SiF.sub.6 /Cu/Zn 70/15/15
 30 K.sub.2 SiF.sub.6 /SiO.sub.2 70/30
 31 K.sub.2 SiF.sub.6 /ZnO.sub.2 70/30
 32 K.sub.2 SiF.sub.6 /SiO.sub.2 /ZnF.sub.2 .multidot. 4H.sub.2 O
 70/15/15
 33 K.sub.2 SiF.sub.6 /Si/SiO.sub.2 70/15/15
 34 K.sub.2 SiF.sub.6 /Si/(KAlF.sub.4 + K.sub.3 AlF.sub.6)
 70/15/15
 TABLE 3
 Brazing composition
 No. Component Content (wt %)
 17 K.sub.2 SiF.sub.6 /Si 70/30
 18 K.sub.2 SiF.sub.6 /Cu 70/30
 19 K.sub.2 SiF.sub.6 /Zn 70/30
 20 K.sub.2 SiF.sub.6 /Ge 70/30
 21 K.sub.2 SiF.sub.6 /Sr 70/30
 22 K.sub.2 SiF.sub.6 /Bi 70/30
 23 K.sub.2 SiF.sub.6 /AlF.sub.3 90/10
 24 K.sub.2 SiF.sub.6 /LiF 90/10
 25 K.sub.2 SiF.sub.6 /CsF 90/10
 26 K.sub.2 SiF.sub.6 /KF 90/10
 27 K.sub.2 SiF.sub.6 /NaF 90/10
 28 K.sub.2 SiF.sub.6 /(KAlF.sub.4 + K.sub.3 AlF.sub.6) 70/30
 29 K.sub.2 SiF.sub.6 /Cu/Zn 70/15/15
 30 K.sub.2 SiF.sub.6 /SiO.sub.2 70/30
 31 K.sub.2 SiF.sub.6 /ZnO.sub.2 70/30
 32 K.sub.2 SiF.sub.6 /SiO.sub.2 /ZnF.sub.2 .multidot. 4H.sub.2 O
 70/15/15
 33 K.sub.2 SiF.sub.6 /Si/SiO.sub.2 70/15/15
 34 K.sub.2 SiF.sub.6 /Si/(KAlF.sub.4 + K.sub.3 AlF.sub.6)
 70/15/15
 TABLE 5
 Joining Corrosion Maximum fusion depth at
 No. rate (%) depth at joint non-joint area (.mu.m) Appearance
 17 100 Shallow 27 Good
 18 100 Shallow 19 Good
 19 100 Shallow 34 Good
 20 100 Shallow 35 Good
 21 100 Shallow 32 Good
 22 100 Shallow 25 Good
 23 100 Shallow 34 Good
 24 100 Shallow 28 Good
 25 100 Shallow 24 Good
 26 100 Shallow 30 Good
 27 100 Shallow 20 Good
 28 100 Shallow 27 Good
 29 100 Shallow 20 Good
 30 100 Shallow 27 Good
 31 100 Shallow 20 Good
 32 100 Shallow 27 Good
 33 100 Shallow 20 Good
 34 100 Shallow 27 Good
 TABLE 6
 Joining Corrosion Maximum fusion depth at
 No. rate (%) depth at joint non-joint area (.mu.m) Appearance
 35 100 Shallow 90 Good
 36 75 Excessive 150 Good
 corrosion
 37 68 Shallow 76 Good
 38 45 Excessive 71 Good
 corrosion
 39 53 Excessive 69 Good
 corrosion
 40 47 Excessive 75 Good
 corrosion
 41 100 Shallow 33 Great
 quantity of
 residue
 42 100 Shallow 29 Great
 quantity of
 residue
 43 100 Shallow 29 Great
 quantity of
 residue
 44 100 Shallow 20 Great
 quantity of
 residue
 45 100 Shallow 22 Great
 quantity of
 residue
 46 100 Shallow 19 Great
 quantity of
 residue
 47 100 Excessive 40 Good
 corrosion
 48 95 Shallow 22 Good
 49 65 Shallow 28 Great
 quantity of
 residue
 50 88 Shallow 30 Great
 quantity of
 residue
 51 100 Shallow 90 Great
 quantity of
 residue
 As is clear from the results shown in Tables 3-6, in Nos. 17-34, in which
 the brazing composition according to the present invention comprises the
 first powder comprising at least one or more M--Si--F compounds or
 hydrates thereof, excellent brazed products were obtained. In Nos. 35-51,
 in which the brazing compositions comprising a small amount of the first
 powder were used, the joining rate was low, the joint and the non-joint
 area exhibited large values for the corrosion depth and the maximum fusion
 depth, or a great quantity of residue was observed on the surface.
 Example 3
 Potassium hexafluorosilicate (K.sub.2 SiF.sub.6) was used as the M--Si--F
 compound to prepare a powder coating No. 1. Various solvents shown in
 Table 7 were added to this powder and an acrylic resin comprising n-butyl
 methacrylate as the major component was added as the resin to prepare
 coatings Nos. 2-5 in slurry forms. As the K.sub.2 SiF.sub.6 powder, a
 powder having an average particle diameter of about 30 .mu.m was used. The
 solid concentration in the coatings of Nos. 2-5 was adjusted to 60 wt %
 and the resin concentration was adjusted to 15 wt % of the solid
 component. The coatings thus prepared were applied to aluminum sheets
 similar to those used in Example 1 using the application methods shown in
 Table 7 so that the Si-reduced coating weight was about 10 g/m.sup.2. The
 brazing properties of the aluminum sheets were evaluated in the same
 manner as in Example 1. The results are shown in Table 8. As is clear from
 the results shown in Table 8, brazed products exhibiting superior
 properties were obtained when using the coatings Nos. 1-5.
 TABLE 7
 Brazing Blend of Application to
 Coating No. composition Solvent resin Aluminum sheet
 1 K.sub.2 SiF.sub.6 None Powder coating
 2 K.sub.2 SiF.sub.6 Water Blended Brush coating
 3 K.sub.2 SiF.sub.6 2-propanol Blended Brush coating
 4 K.sub.2 SiF.sub.6 Xylene Blended Brush coating
 5 K.sub.2 SiF.sub.6 Xylene Blended Roll transfer
 coating
 TABLE 7
 Brazing Blend of Application to
 Coating No. composition Solvent resin Aluminum sheet
 1 K.sub.2 SiF.sub.6 None Powder coating
 2 K.sub.2 SiF.sub.6 Water Blended Brush coating
 3 K.sub.2 SiF.sub.6 2-propanol Blended Brush coating
 4 K.sub.2 SiF.sub.6 Xylene Blended Brush coating
 5 K.sub.2 SiF.sub.6 Xylene Blended Roll transfer
 coating
 As is clear from the above explanations, since the brazing composition of
 the present invention can remove the oxide film from the surfaces of the
 aluminum materials to be joined at a temperature lower than the melting
 point of the aluminum materials and can form a eutectic aluminum alloy
 braze which fuses at such a low temperature by reaction with the aluminum
 materials, there is no need to use either a brazing sheet or a mixture of
 a brazing material and flux as in a conventional method. If a powder
 having a large particle size is used, since local fusion is effectively
 controlled, a solid brazed joint exhibiting little local fusion can be
 obtained. Because of this, the cost for the materials as well as the
 manufacturing cost can be reduced due to the simple manufacturing process.
 An aluminum material for brazing is easily prepared by using the brazing
 coating prepared from the brazing composition of the present invention. A
 solid brazed product exhibiting little local fusion can be obtained by
 applying the brazing composition to the joined surfaces of the aluminum
 materials of the present invention and the like and heating these aluminum
 materials to braze them.
 Obviously, numerous modifications and variations of the present invention
 are possible in light of the above teachings. It is therefore to be
 understood that, within the scope of the appended claims, the invention
 may be practiced other than as specifically described herein.