Flux for Brazing
This application claims priority to European application No. 14158706.3,
the whole content of this application being incorporated herein by reference for
all purposes.
The invention concerns a flux for brazing, a process for brazing metal parts
employing said flux, a flux composition containing said flux, aluminum parts
coated with said flux or said flux composition, a process for brazing and a brazed
metal object obtainable by said brazing process.
It is well known in the art that brazing of aluminum parts can be performed
utilizing fluxes based on alkali metal fluoroaluminates. Fluxes of this type are
generally considered to be noncorrosive. If contacted for extended times with
water or aqueous liquids, aluminum parts brazed with potassium fluoroaluminate
based fluxes show signs of corrosion. This is disclosed by Bo Yang et al. in
Journal of ASTM International, Vol. 3, Issue 10 (2006). This corrosion seems to
be caused by fluoride ions which are leached from brazing residues if the brazed
parts are in contact with water or aqueous systems. This is especially
disadvantageous when the brazed metal parts, which often bear contaminations
from flux residue after brazing causing the corrosion and/or fluoride leaching,
are used for coolers for stationary or mobile refrigeration equipment, such as air
conditioning systems, or stationary heat exchangers or in cooling water for
engines, for example engines for powering vehicles, as these are commonly
continuously in contact with water or aqueous systems. Two effects have major
disadvantages in such equipment: Corrosion of the metal parts, and clogging due
to the reaction of the flux residue (most likely leached fluoride) with components
of the cooling liquid, causing solid substances. Some improvement of the
corrosion effect was achieved by addition of Li compounds to a flux system in
WO2010060869.
JP patent application S61-099569 discloses a brazing flux comprising, on
one hand, 73.6 to 99.7 % KA1F4 and 0.2 to 18.4 % KF, and on the other hand,
0.1 to 8 % of at least one additive selected from the group consisting of LiF, NaF
and CaF2. According to the examples explaining that invention, the content of
KF is from 9.8 to 1 1 . All percentages are given as % by weight.
CN patent application 101434014 discloses a flux for joining aluminum
and steel. The flux contains an increased amount of K3A1F6 relative to the
eutectic amount which is said to be KA1F4 + 28% K3A1F6.
EP patent application 0131444 discloses a flux for brazing aluminum parts
which provides, after brazing, provides a corrosion-inhibiting metal film. This
metal film is formed by the reaction between ZnF2 or SnF2 which are present in
the flux, and aluminum from the parts to be brazed.
It has now been found that the presence of at least one additive salt
comprising a cation selected from the group consisting of earth alkali metals and
an anion selected from the group consisting of F , C0 3
2-, O2- , nitrate, phosphate,
borate, metaborate and oxalate in a brazing flux based on a fundamental flux
comprising from 80 mol% to 100 mol% KA1F enhances and thus improves the
corrosion resistance of the brazed aluminum parts against water and aqueous
systems compared to fluxes which do not contain such additives. The flux
according to the invention provides for efficient fluxing and brazing while
corrosion of the brazed parts is significantly reduced or even essentially
prevented. Without wishing to be bound by any theory, it is believed that the flux
composition according to the invention prevents or reduces the formation of
solid, generally fluoride containing, flux residues, on the brazed part, thereby
improving resistance to corrosion. The flux residues further display an enhanced
"hydrophilicity", which is indicated by an improved wettability of the flux
residue; "hydrophilicity" in this context does not indicate solubility. The
improved wettability of the flux residue results, for example, in enhanced drying
of aqueous systems and/or water on the brazed part. Thereby, the contact time of
such an aqueous system and/or water with the brazed part, and thus the corrosion
of the brazed part, is reduced.
Thus, the flux of the invention comprises a) a brazing flux which is
denoted as "fundamental flux", b) at least one additive salt, and optionally c) a
brazing additive.
Thus, neither ZnF2 nor SnF2 must be present in the flux.
Accordingly, the present invention concerns a flux comprising a
fundamental flux, wherein the fundamental flux comprises from 80 mol% to 100
mol% KA1F , the content of K3A1F6 is equal to or lower than 2 mol % including
0 mol%, and wherein the flux further comprises from 0.1 to 20 weight% of at
least one additive salt, wherein the at least one additive salt comprises at least
one anion selected from the group consisting of F , C0 3
2- , O2-, nitrate,
phosphate, borate, metaborate and oxalate, and at least one cation selected from
the group consisting of earth alkali metal cations.
The invention further concerns a flux composition comprising the said flux and
at least one fluxing additive selected from the group consisting of binders,
thickeners, surfactants, thixotropic agents and solvents.
Aluminum parts for brazing which are coated at least partially with said flux
and/or a said flux composition are also claimed. The invention further concerns a
process for brazing of metal parts made from aluminum or aluminum alloys with
metal parts made from aluminum, aluminum alloys, copper, titanium or steel,
which comprises the steps of
a) coating metal parts at least partially with the aforementioned flux and/or the
aforementioned flux composition;
b) optionally drying the at least partially coated metal parts;
c) assembling the at least partially coated metal parts;
d) heating the assembled, at least partially coated metal parts to a temperature
sufficiently high to braze the at least partially coated metal parts;
e) brazing the at least partially coated metal parts;
f) optionally cooling the brazed parts.
The invention also concerns a brazed metal object, obtainable by the
aforementioned process.
The flux according to the present invention comprises a fundamental flux
and at least one additive salt.
The term "fundamental flux" denotes the constituents of the flux of the
invention but excluding the additive salt. The distinction between "fundamental
flux" and "flux" according to the invention is made because the constitution of
the "fundamental flux" is best expressed in % mol in respect of the main
constituent or main constituents while the constitution of the "flux" is best
expressed in % by weight wherein the sum of "fundamental flux" and the at least
one additional salt, or, if more than one additive salt is present, of all additive
salts is set to 100% by weight.
In other words, the flux of the invention comprises from 80 mol% to 100
mol% KA1F4, from 0.1 to 20 weight% of at least one additive salt, wherein the at
least one additive salt comprises at least one anion selected from the group
consisting of F , CO3
2- , O2-, nitrate, phosphate, borate, metaborate and oxalate,
and at least one cation selected from the group consisting of earth alkali metal
cations; and optionally, if the content of KA1F4 is from 80 mol% to less than 100
mol , at least one or more flux component which is selected from the group
consisting of K2AIF5, K3A1F6, potassium fluorozincates, cesium
fluoroaluminates, potassium fluorostannates and cesium fluorostannates, and
hydrates of all the foregoing, where applicable.
In the following, the fundamental flux will be explained in detail.
According to the present invention, the fundamental flux generally
comprises from 80 mol % to 100 mol % KA1F4. Thus, KA1F4 is the main
constituent of the fundamental flux. Often, the KA1F4 content is equal to or
higher than 80 mol . Preferably, the KA1F4 content is equal to or higher than 90
mol . Even more preferably, the KA1F4 content is equal to or higher than 95
mol . Most preferably, the KA1F4 content is equal to or higher than 98 mol .
Often, the KA1F4 content is equal to or lower than 100 mol . Preferably, the
KAIF4 content is equal to or lower than 98 mol . Even more preferably, the
KAIF4 content is equal to or lower than 95 mol . In a very preferred aspect of
the invention, the fundamental flux comprises exclusively or essentially
exclusively KA1F4. Mol % refers to the molar percentage of KA1F4 with respect
to the constitution of the fundamental flux.
Thus, according to one embodiment, the fundamental flux comprises 100
mol % of KAIF4, and thus, consists of KA1F4. In this embodiment, no additional
fundamental flux component is present.
According to another embodiment of the present invention, the
fundamental flux further comprises at least one additional fundamental flux
component, wherein the at least one or more component is selected from the
group consisting of K2AIF5, K3A1F6, potassium fluorozincates, cesium
fluoroaluminates, potassium fluorostannates and cesium fluorostannates, and
hydrates of all the foregoing, where applicable. Preferred additional fundamental
flux components are K2AIF5, K2AIF5 H2O, potassium fluorozincates, potassium
fluorostannates, cesium fluorostannates and cesium fluoroaluminates. A most
preferred additional fundamental flux component is K2AIF5, its hydrate or
combinations thereof. K3A1F6 may also be contained; generally the content of
K3A1F6 is equal to or lower than 2 mol , and more preferably, it is equal to or
lower than 1mol , and especially preferably, it is equal to or lower than 0.5
mol , including 0 mol . Especially preferably, the fundamental flux does not
contain added K3A1F6. The at least one additional fundamental flux component
generally is present in a molar content of from greater than 0 to 20 mol with
the limitation given above in view of K3A1F6. The molar content in the
fundamental flux of the at least one additional fundamental flux component adds
up with the molar content of the KA1F4 to 100 mol of the fundamental flux.
Often, the content of the at least one additional fundamental flux component is
equal to or higher than 0 mol . Preferably, the content of the at least one
additional fundamental flux component is equal to or higher than 2 mol . Even
more preferably, the content of the at least one additional fundamental flux
component is equal to or higher than 5 mol . Often, the content of the at least
one additional fundamental flux component is equal to or lower than 20 mol .
Preferably, the content of the at least one additional fundamental flux component
is equal to or lower than 10 mol . Even more preferably, the content of the at
least one additional fundamental flux component is equal to or lower than 5
mol . In a very preferred aspect of the invention, any other additional
fundamental flux component except KA1F4 is absent or essentially
absent.Commercially available fundamental fluxes which can be used as such or
as
fundamental flux component include, for example, Nocolok ® Flux (a
mixture of KA1F4 and K2AIF 5), Nocolok ® Flux Drystatic (a mixture of KA1F4
and K2AIF 5 with a specific range of particle size distribution), Nocolok ® Cs Flux
(a mixture of KA1F4, K2AIF 5 and cesium fluoroaluminate) and Nocolok ® Zn
Flux (potassium fluorozincate).In the present invention, weight% refers to the
total weight of the flux, if not otherwise defined.
Chlorides, NaF and free KF are considered as undesired impurities. The
content of chlorides, e.g. LiCl, NaCl and KC1, in the flux of the invention is
equal to or greater than 0 to equal to or lower than 0.1 % by weight. The content
of NaF and KF in the flux is equal to or greater than 0 to lower than 0.2 % by
weight. Preferably, the flux is essentially free of chlorides, NaF and KF. The
term "essentially free" denotes that chlorides, NaF and KF are contained at most
in undesired traces, preferably, that the total content of chlorides is equal to or
lower than 0.05 % by weight and the total content of the sum of NaF and KF is
preferably equal to or lower than 0.1 % by weight. The term "KF" denotes free
KF which is not present in complex form, e.g. in the form of KA1F4. The content
of these impurities is given relative to the flux set as 100 % by weight.
Most preferably, the fundamental flux does not contain added chlorides,
added NaF and added KF.
Preferably, the flux of the invention, if at all, contains ZnF2 and SnF2 only
as undesired traces, e.g. in amounts from equal to or greater than 0 to equal to or
lower than 0.05% by weight. Preferably, the flux does not contain ZnF2 or SnF2.
According to the present invention, the flux comprises at least one additive
salt. The at least one additive salt comprises at least one anion selected from the
group consisting of F , C0 3
2-,0 2- , nitrate, phosphate, borate, metaborate and
oxalate, and at least one cation selected from the group consisting of earth alkali
metal cations. For example, the at least one additive salt is selected from the
group consisting of CaF2, BeF2, MgF2, BaF2 SrF2, CaO, BeO, MgO, BaO, SrO,
CaC0 3, BeC0 3,MgC0 3, BaC0 3, and SrC0 3. More preferably, the at least one
additive salt is selected from the group consisting of CaF2, BeF2, MgF2, BaF2
and SrF2. The at least one additive salt most preferably is CaF2 or MgF2 or a
mixture thereof. The at least one additive salt is generally present in the flux in a
weight percentage of from 0.1 to 20 weight%. Often, the at least one additive salt
is present in the flux in a weight percentage of equal to or more than 0.5
weight%. Preferably, the at least one additive salt is present in the flux in a
weight percentage of equal to or more than 1 weight%. Most preferably, the at
least one additive salt is present in the flux in a weight percentage of equal to or
more than 1.5 weight . Often, the at least one additive salt is present in the flux
in a weight percentage of equal to or less than 18 weight . Preferably, the at
least one additive salt is present in the flux in a weight percentage of equal to or
less than 15 weight . Most preferably, the at least one additive salt is present in
the flux in a weight percentage of equal to or more than 10 weight . In a very
preferred embodiment of the invention, if the content of KA1F4 in the
fundamental flux is equal to or higher than 80 mol % (mol % of fundamental
flux), the at least one additive salt is present in the flux in an amount of equal to
or higher than 2.5 weight % (weight %relating to the total weight of the flux).
In one aspect of the present invention, the flux may further comprise at
least one brazing additive. If appropriate the at least one brazing additive is
preferably selected from the group consisting of Si, LiOH, LiF, Li
fluoroaluminates, lithium potassium fluoroaluminates, solder metals and solder
metal precursors. "Solder" is also referring to "filler" or "filler metal". Especially
preferred brazing additives are selected from the group consisting of solder
metals and Si. Exemplary solder metals are Al-Si alloys, whereas examples of
solder metal precursors are Si, copper or germanium. Commercially available
fundamental fluxes which already comprise brazing additives as described above
include Nocolok Sil Flux. The weight percentages or fundamental flux, at least
one salt additive and optionally at least one brazing additive add up to 100
weight .
In one embodiment, no brazing additive is present.
In another embodiment, a brazing additive is present Generally, the at least
one brazing additive is then present in the flux in a weight percentage of from
more than 0 to 30 weight . Often, the at least one brazing additive is present in
the flux in a weight percentage of equal to or more than 0.5 weight .
Preferably, the at least one brazing additive is present in the flux in a weight
percentage of equal to or more than 1 weight . Most preferably, the at least one
brazing additive is present in the flux in a weight percentage of equal to or more
than 1.5 weight . Often, the at least one brazing additive is present in the flux
in a weight percentage of equal to or less than 30 weight . Preferably, the at
least one brazing additive is present in the flux in a weight percentage of equal to
or less than 15 weight . Most preferably, the at least one brazing additive is
present in the flux in a weight percentage of equal to or more than 10 weight .
In a preferred embodiment of the invention, when Li is present in the flux as
brazing additive, the molar content of KA1F4 in the fundamental flux is equal to
or higher than 95 mol , wherein mol % relates to the molar composition of the
fundamental flux. The brazing additive is preferably selected from the group
consisting of Si and solder metal, especially solder metal consisting of Al-Si
alloys.
In a very preferred embodiment, the flux of the invention consists of
KA1F , at least one additive salt comprising a cation selected from the group
consisting of earth alkali metals and an anion selected from the group consisting
of F , O2- , and C0 3
2- , and the flux optionally further contains K2AIF 5,
K2A1F5-H20 , cesium fluoroaluminate, Si and/or a solder metal, especially a Si-
Al alloy.
In an especially preferred embodiment, the flux of the invention consists of
KA1F , at least one additive salt selected from the group consisting of CaF2,
BeF2, BaF2 SrF2, and the flux optionally further contains at least one compound
selected from the group consisting of K2AlFs, K2AlFs-H20 , cesium
fluoroaluminate, Si and solder metal, especially a Si-Al alloy.If desired, fluxes
with specific particle sizes can be selected for specific methods of application.
For example, the particles, including any brazing additives, may have the particle
size distribution as disclosed in US-A 6,733,598, which is incorporated by
reference into the present patent application and are especially suitable for
application according to the dry method, e.g. by electrostatic power, as is further
detailed in US-A 6,733,598. The particles of the flux may be of a coarser nature
than the finer particles disclosed in said US 6,733,598. Such coarser fluxes are
very suitable for the application in the form of a flux composition including the
flux dispersed a solvent; they can, for example, be applied by painting, printing
or spraying onto the parts. The flux may also have a distinct particle size
distribution as disclosed in US application US2013/0037172, which is
incorporated by reference into the present patent application which results in
specific advantageous sedimentation behavior and viscosity in wet
applications. The flux can be applied as such as dry powder, for example,
electrostatically as described in US-A 6,733,598 or by applying a low
temperature plasma, as described in WO 2006/100054, which is incorporated by
reference into the present patent application. In this plasma process, finely
divided flux powder is partially molten by a low temperature plasma beam and
sprayed onto the surface of the aluminum parts to be joined. The flux formed
from the flux and the brazing additives mentioned above can be applied to the
brazing process according to the principles mentioned above.
As to the potential content of chlorides, NaF and KF in the flux which are
considered as undesired impurities, it is referred to the remarks made above in
respect to the fundamental flux. The content of chlorides, e.g. LiCl, NaCl and
KC1, in the flux of the invention is equal to or greater than 0 to equal to or lower
than 0.1 % by weight. The content of NaF and KF in the flux of the invention is
equal to or greater than 0 to equal to or lower than 0.1 % by weight. Preferably,
the flux of the invention is essentially free of chlorides, NaF and KF. The term
"essentially free" denotes that chlorides, NaF and KF are contained at most in
undesired traces, preferably, that the total content of chlorides is equal to or
lower than 0.05 % by weight and the total content of the sum of NaF and KF is
preferably equal to or lower than 0.1 % by weight. The content of these
impurities is given relative to the flux of the invention set as 100 % by weight.
Most preferably, the flux of the invention does not contain added chlorides,
added NaF and added KF.
The invention also concerns a flux composition comprising the flux of the
invention. The flux composition of the present invention which is suitable for the
wet fluxing method contains the flux, optionally including one or more of the
brazing additives as described above, and at least one of the fluxing additives
selected from the group consisting of solvents, binders, thickeners, suspension
stabilizers, antifoaming agents, surfactants and thixotropic agents.
In one preferred embodiment, the flux composition comprising the flux
contains the flux suspended in a solvent, especially in water, water-free organic
liquids or aqueous organic liquids. Preferred liquids are those that have a boiling
point at ambient pressure ( 1 bar abs) of equal to or lower than 350°C. The term
"suspended in water" does not exclude that a part of the flux composition is
dissolved in the liquid; this may be the case especially when water or aqueous
organic liquids are contained. Liquids that are preferred are deionized water,
mono-, di- or tribasic aliphatic alcohols, especially those with 1 to 4 carbon
atoms, e.g. methanol, ethanol, isopropanol, or ethylene glycol, or glycol alkyl
ethers, wherein alkyl preferably denotes linear CI to C4 alkyl or branched
aliphatic C3 to C4 alkyl, including methyl, ethyl, iso-propyl, n-propyl, n-butyl,
iso-butyl, sec-butyl, and tert-butyl. Non-limiting examples are glycol monoalkyl
ethers, e.g. 2-methoxyethanol or diethylene glycol, or glycol dialkyl ethers, for
example, dimethyl glycol (dimethoxyethane). Mixtures comprising two or more
of the liquids are also suited very well. Isopropanol or mixtures containing
isopropanol are especially suitable.
The composition comprising the flux suspended in a liquid may also
contain further fluxing additives, for example, thickener, surfactants or
thixotropic agents.
In an especially preferred embodiment, the flux is present in the form of a
flux composition wherein the flux is suspended in a liquid which also contains a
binder. Binders improve, for example, the adhesion of the flux mixture after their
application on the parts to be brazed. Thus, the wet flux method using a flux
composition comprising flux, binder and water, organic liquid or aqueous
organic liquid is a preferred embodiment of the brazing process of the present
invention.
Suitable binders can be selected for example from the group consisting of
organic polymers. Such polymers are physically drying (i.e., they form a solid
coating after the liquid is removed), or they are chemically drying (they may
form a solid coating e.g. under the influence of chemicals, e.g. oxygen or light
which causes a cross linking of the molecules), or both. Suitable polymers
include polyolefines, e.g. butyl rubbers, polyurethanes, resins, phthalates,
polyacrylates, polymethacrylates, vinyl resins, epoxy resins, nitrocellulose,
polyvinyl acetates or polyvinyl alcohols. Flux compositions containing water as
liquid and water-soluble polymers, for example, polyurethane, or polyvinyl
alcohol as binder are especially suitable because they have the advantage that,
during the brazing process, water is evaporated instead of possibly flammable
organic liquids.
The compositions may include other additives which improve the
properties of the composition, for example, suspension stabilizers, surfactants,
especially nonionic surfactants, e.g. Antarox BL 225, a mixture of linear C8
to CIO ethoxylated and propoxylated alcohols, thickeners, e.g. methyl butyl
ether, thixotropic agents, e.g. gelatine or pectines, or a wax as described
in EP-A 1808264.
The content of the flux (including fundamental flux, the at least one
additive salt and, if present, other additives, e.g. solder metal, solder precursor)
in the total composition (including liquid or liquids, thixotropic agents,
surfactants and binders, if present) generally is equal to or greater than 0.75 %by
weight. Preferably, it is equal to or greater than 1 %by weight. More preferably,
the flux content in the composition is equal to or greater than 5 %by weight,
very preferably, equal to or greater than 10 %by weight of the total flux
composition. Generally, the flux content in the composition is equal to or lower
than 70 %by weight. Preferably, it is equal to or lower than 50 %by weight.
The binder, if present, is generally contained in an amount of equal to or
greater than 0.1 %by weight, preferably equal to or greater than 1 %by weight
of the total flux composition. The binder, if present, is generally contained in an
amount equal to or lower than 30 %by weight, preferably equal to or lower
than 25 %by weight of the total composition.
The thixotropic agent, if present, is generally contained in an amount of
equal to or greater than 1 %by weight of the total flux composition. Generally, if
present, it is contained in an amount equal to or lower than 20 %by weight,
preferably equal to or lower than 10 %by weight of the total flux composition.
The thickener, if present, is generally contained in an amount of equal to or
greater than 1%by weight, preferably equal to or greater than 5 %by weight of
the total flux composition. Generally, the thickener, if present, is contained in an
amount equal to or lower than 15 %by weight, preferably equal to or lower
than 10 %by weight of the total composition.
Highly suitable flux compositions for wet applications contain 10 to 70 %
by weight of the flux (including the at least one additive salt and optional solder
metal or solder precursor), 1 to 25 %by weight binder, 0 to 15 %by weight of a
thickener, 0 to 10 % by weight of a thixotropic agent, and 0 to 5 % by weight of
other additives, e.g. a surfactant or a suspension stabilizer. Preferably, the
reminder to 100 % by weight is water, an organic solvent or an aqueous organic
solvent.
In one specific embodiment, the flux composition comprising the flux is
free of any water or water-free or aqueous organic liquid, but contains the flux
(including the at least one additive salt, and optionally one or more of the solder
metal or precursor) as described above, and a water-soluble organic polymer as a
binder which is present in the form of a water soluble package for the flux. For
example, polyvinyl alcohol is very suitable as water-soluble package for the flux
as described in US patent application publication 2006/0231 162. Such packages
can be handled without dust formation, and after addition of water, they form a
suspension in water including a flux and the water soluble polymer as binder.
Another aspect of the present invention is the provision of a process for
brazing of metal parts made from aluminum or aluminum alloys with metal parts
made from aluminum, aluminum alloys, copper, titanium or steel comprising a
step wherein the aforementioned flux or the aforementioned flux composition
comprising the flux is applied to a part of the surface (including those parts of
the surface which will be joined during brazing) or the entire surface of the parts
to be brazed, so that the metal parts are at least partially coated. Optionally, the at
least partially coated metal parts can be dried. After fluxing, these parts are
assembled, heated to a temperature sufficiently high to braze the at least partially
coated metal parts and brazed, or, alternatively, the parts to be brazed are
assembled, then fluxed, heated to a temperature sufficiently high to braze the at
least partially coated metal parts and then brazed. Optionally, the brazed parts
can be subjected to a heat treatment after brazing. The flux or flux composition
comprising the flux is described in detail above.
For the claimed process for brazing of metal parts, the flux can be applied
according to the dry fluxing method described above and further referenced to in
US-A 6,733,598. The wet flux compositions can alternatively be applied to the
metal parts according to methods known in the art. For example, they can be
sprayed onto the surface thus forming coated metal parts; alternatively, they can
be applied by immersing the metal parts to be coated into the flux composition;
or by painting or printing the flux composition onto the metal parts to be brazed
thus forming coated parts. It has to be kept in mind that the term "metal"
includes aluminum alloys. Generally, the at least partially coated metal parts
made from aluminum or aluminum alloys may be brazed with metal parts made
from aluminum, aluminum alloys, copper, titanium or steel. The liquid-free flux
composition containing flux, water-soluble binder and optionally further
additives in the form of a package can be put into water before use to form an
aqueous flux composition containing suspended flux mixture and dissolved
binder.
Generally, the metal parts at least partially coated with the wet flux
composition are dried (this is of course not necessary in parts coated according to
the dry method unless applies fluoroaluminate hydrates and wants to remove
crystal water before starting the brazing process). Drying can be performed
independently from brazing; the dried, at least partially flux-coated metal parts
can then be stored until they are brazed. Alternatively, drying can be performed
directly in the brazing apparatus or in a separate drying apparatus just before the
brazing operation.
Generally, for brazing, the at least partially coated metal parts to be joined
by brazing are assembled (before or after drying if coated according to a wet
process) and heated to a "holding temperature" of from about 540°C to about
650°C.The holding temperature often is equal to or more than 540°C. More
preferably, the holding temperature is equal to or more than 550°C. Even more
preferably, the holding temperature is equal to or more than 560°C. Often, the
holding temperature is equal to or less than 650°C. More preferably, the holding
temperature is equal to or less than 620°C. Even more preferably, the holding
temperature is equal to or less than 610°C. In order to reach the holding
temperature, a temperature gradient is used which is suitable for the brazing
system, for example taking into account the metal parts to be brazed, the flux or
flux composition, and brazing equipment which is employed. For example, a
heating rate of 30°/min may be suitable. The time during which the holding
temperature is applied, also referred to as holding time, often is equal to or more
than 30 seconds. More preferably, the holding time is equal to or more than 60
seconds. Even more preferably, the holding time is equal to or more than 90
seconds. Often the holding time is equal to or less than 10 minutes. More
preferably, the holding time is equal to or less than 5 minutes. Most preferably,
the holding time is equal to or less than 3 minutes. The heating can be done in
an inert gas atmosphere, e.g. helium, nitrogen, argon and/or xenon atmosphere,
or mixtures of those gases; this is also referred to as "CAB" conditions
(controlled atmosphere brazing).
In one embodiment of the invention, the brazing process also comprises a
step of cooling the brazed parts. This cooling may be performed actively, for
example by applying a stream of inert gas to the brazed parts, or inactively by
letting the temperature drop to ambient temperature after the heating is stopped.
Preferably, the step of cooling the brazed parts is performed in an inert gas
atmosphere, e.g. helium, nitrogen, argon and/or xenon atmosphere, or mixtures
of those gases.
It was found that brazing products which were brazed with the flux of the
invention which comprises the at least one additive salt generally are very
resistant to corrosion due to the very low solubility, specifically low fluoride
leaching, of the flux residue after brazing. This also has the favorable effect that
solids that form by interaction of aqueous systems that are employed e.g. in
coolers / heat exchangers with the brazing residue, are avoided, which prevents
clogging of the cooler / heat exchanger systems.
A further aspect of the present invention concerns aluminum parts or
aluminum alloy parts, coated at least partially with the additive salt containing
flux, or the flux composition comprising the flux, of the present invention. These
parts preferably are parts used to produce heat exchangers, e.g. tubes and fins.
Another aspect of the present invention concerns a brazed metal object,
which is obtainable by the following process for brazing of metal parts made
from aluminum or aluminum alloys with metal parts made from aluminum,
aluminum alloys, copper, titanium or steel, which comprises the steps of
a) coating metal parts at least partially with a flux according to the foregoing
description and/or a flux composition according to the foregoing description;
b) optionally drying the at least partially coated metal parts;
c) assembling the at least partially coated metal parts;
d) heating the assembled, at least partially coated metal parts to a temperature
sufficiently high to braze the at least partially coated metal parts;
e) brazing the at least partially coated metal parts;
f) optionally cooling the brazed parts.
The process details by which the brazed metal object is obtainable are
identical to those described above for the "process for brazing of metal parts
made from aluminum or aluminum alloys with metal parts made from aluminum,
aluminum alloys, copper, titanium or steel", as described above.
In one embodiment of the present invention, the brazed metal object,
obtainable as previously described by the process for brazing of metal parts made
from aluminum or aluminum alloys with metal parts made from aluminum,
aluminum alloys, copper, titanium or steel, forms part of a cooler for stationary
or mobile refrigeration equipment, such as air conditioning equipment, or of a
stationary heat exchanger. Generally, the brazed metal object may be present in
any section of the cooler of the stationary or mobile refrigeration equipment, or
of the stationary heat exchanger. Often, the brazed metal parts obtainable by the
previously described process are cooler parts which are in contact with cooling
agents. Such cooling agents can be either aqueous, for example water-alcohol
mixtures, wherein preferably ethyleneglycol is used, or non-aqueous, for
example halogenated hydrocarbons or oils. The brazed metal objects are
especially preferred for use in motor vehicles, as part of the internal combustion
engine cooler, or as part of the air conditioning appliance in motor vehicle. The
term "motor vehicle" includes cars, motor cycles and buses, but also ships,
airplanes and other transport vehicles. Another advantageous application for the
said brazed metal parts is air conditioning equipment which cools rooms and/or
spaces, for human or animal occupancy, or storage. This air conditioning
equipment may be either mobile or stationary.
Should the disclosure of any patents, patent applications, and publications
which are incorporated herein by reference conflict with the description of the
present application to the extent that it may render a term unclear, the present
description shall take precedence.
The examples which follow are intended to illustrate the present invention
without, however, limiting the scope thereof.
Example 1
Nocolok Flux® with 3% of BaF2 was homogenized thoroughly in a ball mill. An
amount of 5 gram/m2 of this powder blend was evenly distributed on a cladded
(4343) Aluminum 3003 coupon of 2.5 by 2.5 cm in size. The cladded Aluminum
part was assembled with an aluminum angle. The assembled parts were placed
in a lab brazing furnace (volume 2.5 L), with a controlled atmosphere of nitrogen
gas (11 L/min). The brazing furnace containing the assembled parts was heated
with a rate of 30°C/min. Upon reaching the holding temperature of 605°C, the
furnace was held at 605°C for 2 minutes. The heating was stopped and the setup
cooled under nitrogen atmosphere. Overall brazing time was 22 minutes, cooling
down was 4 minutes.
The brazed coupon - down to ambient temperature- was then soaked 10 days at
ambient temperature in a closed beaker containing 40 ml DI-water. (DI-water:
deionized water).
Fluoride content in mg/L of soaking water:
Nocolok® +3% BaF2: 6,5
Nolocok® w/o additive: 7,5
Example 2
KA1F4 with 3%, respectively 1.5%, of BaF2 was homogenized thoroughly in a
ball mill. An amount of 5 gram/m2 of this powder blend was evenly distributed
on a cladded (4343) Aluminum 3003 coupon of 2.5 by 2.5 cm in size. The
cladded Aluminum part was assembled with an aluminum angle. The assembled
parts were placed in a lab brazing furnace (volume 2.5 L), with a controlled
atmosphere of nitrogen gas (11 L/min). The brazing furnace containing the
assembled parts was heated with a rate of 30°C/min. Upon reaching the holding
temperature of 605°C, the furnace was held at 605°C for 2 minutes. The heating
was stopped and the setup cooled under nitrogen atmosphere. Overall brazing
time was 22 minutes, cooling down was 4 minutes.
The brazed coupon - down to ambient temperature- was then soaked 10 days at
ambient temperature in a closed beaker containing 40 ml DI-water. (DI-water:
deionized water).
Fluoride content in mg/L of soaking water:
KAIF 4 +3% BaF2: 4,5
KA1F4 +1.5% BaF2: 4,0
KA1F4 w/o additive: 5,5

C L A I M S
1. A flux comprising a fundamental flux, wherein the fundamental flux
comprises from 80 mol to 100 mol KA1F4, the content of K3A1F6 in the flux
is equal to or lower than 2 mol % including 0 mol , the content of free KF in the
flux is lower than 0.2 % by weight including 0 % by weight, and wherein the
flux further comprises from 0.1 to 20 weight% relative to the total weight of the
flux of at least one additive salt, wherein the at least one additive salt comprises
at least one anion selected from the group consisting of F , C0 3
2-, O2-, nitrate,
phosphate, borate, metaborate and oxalate, and at least one cation selected from
the group consisting of earth alkali metal cations.
2. The flux according to claim 1 wherein the at least one additive salt is
selected from the group consisting of CaF2, BeF2, MgF2, BaF2 SrF2, CaO, BeO,
MgO, BaO, SrO, CaC0 3, BeC0 3,MgC0 3, BaC0 3, and SrC0 3.
3. The flux according to claim 2 wherein the at least one additive salt is
selected from the group consisting of CaF2, BeF2, MgF2, BaF2 and SrF2.
4. The flux according to any of claim 1 to 3, wherein the fundamental flux
comprises one or more components selected from the group consisting of
K AIF5, K3A1F6, potassium fluorozincates, cesium fluoroaluminates, potassium
fluorostannates and cesium fluorostannates, and hydrates wherein the one or
more components are comprised in the fundamental flux from 0 to 20 mol ,
adding up with the content of KA1F to 100 mol .
5. The flux according to any of the claims 1 to 4, wherein the flux further
comprises at least one brazing additive selected from the group consisting of Si,
LiOH, LiF, Li fluoroaluminates, lithium potassium fluoroaluminates, solder
metals and solder metal precursors.
6. The flux according to any of the claims 1 to 5, wherein the at least one
earth alkali metal additive salt is present in an amount of equal to or more than 1
weight%.
7. The flux according to any of the claims 1 to 6, wherein the at least one
additive salt is present in an amount of from equal to or less than 15 weight%.
8. The flux according to any of the claims 1 to 7, wherein the fundamental
flux comprises a molar content of equal to or higher than 98 mol KA1F4.
9. A flux composition comprising a flux according to any one of the
claims 1 to 8, and at least one fluxing additive selected from the group consisting
of solvents, binders, thickeners, suspension stabilizers, antifoaming agents,
surfactants and thixotropic agents.
10. Aluminum parts or aluminum alloy parts for brazing, coated at least
partially with a flux according to any one of the claims 1 to 8 and/or a flux
composition according to claim 9.
11. A process for brazing of metal parts made from aluminum or
aluminum alloys with metal parts made from aluminum, aluminum alloys,
copper, titanium or steel, which comprises the steps of
a) coating metal parts at least partially with the flux according to any one of the
claims 1 to 8 and/or a flux composition according to claim 9;
b) optionally drying the at least partially coated metal parts;
c) assembling the at least partially coated metal parts;
d) heating the assembled, at least partially coated metal parts to a temperature
sufficiently high to braze the at least partially coated metal parts;
e) brazing the at least partially coated metal parts;
f) optionally cooling the brazed parts.
12. The process according to claim 11, wherein step c) and/or d) are
performed at a temperature equal to or higher than 540°C, and at a temperature
equal to or lower than 650°C.
13. The process according to any one of the claims 11 or 12, wherein step
c) and/or d) are performed in the presence of a protective gas containing equal to
or more than 75% by volume of at least one gas selected from the group
consisting of helium, nitrogen, argon and xenon.
14. A brazed metal object, obtainable according to the process of anyone
of claims 1 1 to 13.
15. The brazed metal object according to claim 14, which forms part of a
cooler for stationary or mobile refrigeration equipment, such as air conditioning
systems, or of a stationary heat exchanger.