BONDING OF METAL WORKPIECES

COLLAGE DE PIECES EN METAL

English Abstract

A method of bonding adjacent generally planar surfaces of two
metal workpieces involves treating at least the surface of that
workpiece formed of the harder metal, or the surface of at least
one of the workpieces if equally hard, by gouging out of the
surface an array of outwardly pointing integral burrs, bringing
the surfaces into adjacency with a pressure fluent layer of
settable bonding material therebetween; applying sufficient
pressure across the surface to cause substantial deformation of
the metal of at least one of the workpieces in contact with the
metal of the other workpiece at the points where the burrs of
one workpiece surface contact the other workpiece such as to
provide metal-to-metal contact within the bonding material
layer, and setting the layer of bonding material.

Claims

CLAIMS:
1. A method of bonding adjacent generally planar surfaces
of two metal workpieces comprises treating at least the
surface of that workpiece formed of the harder metal, or the
surface of at least one of the workpieces if equally hard, by
gouging out of the surface an array of outwardly pointing
integral burrs, bringing the surfaces into adjacency with a
pressure fluent layer of settable bonding material
therebetween; applying sufficient pressure across the surface
to cause substantial deformation of the metal of at least one
of the workpieces in contact with the metal of the other
workpiece at the points where the burrs of one workpiece
surface contact the other workpiece such as to provide metal-
to-metal contact within the bonding material layer, and
setting the layer of bonding material.
2. A method of bonding according to claim 1, wherein at
least one of the surfaces to be bonded is highly passivated.
3. A method of bonding according to claim 2, wherein one
workpiece is of aluminum, and the other of aluminum,
stainless steel, a nichrome based alloy or titanium.
4. A method of bonding according to claim 1, wherein the
metal of one workpiece is much harder than that of the other
workpiece, and the pressure applied is sufficient to cause
the burrs of the harder metal workpiece to penetrate the
surface of the softer metal workpiece.
5. A bipolar separator plate for a fuel cell, comprising
metal sheets bonded by the method of claim 1.
6

Description

CA 02272115 1999-OS-17
This invention relates to the bonding of metals to provide a
secure high conductivity bond, particularly metals or pairs
of dissimilar metals difficult to bond by conventional means,
such as aluminium, titanium, stainless steel and other alloys
with highly passivated surfaces.
Welding and soldering in such cases is technically difficult,
and the heat and possible workpiece distortion involved may
not be acceptable. Adhesive bonding typically results in a
bond which is non-conductive, and whose mechanical integrity
may be suspect. Explosive bonding is expensive and
technically difficult.
It is an object of the invention to provide a technique for
cold bonding surfaces of metals, particularly highly
passivated metals, which can provide a satisfactory bond with
good and stable electrical and thermal conductivity, such as
will permit the bonded metals to be used in applications
requiring good electrical and thermal conductivity such as
bipolar separator plates in fuel cells.
According to the invention, a method of bonding adjacent
generally planar surfaces of two metal workpieces comprises
treating at least the surface of that workpiece formed of the
harder metal, or at least one of the workpieces if equally
hard, by gouging out of the surface an array of outwardly
pointing integral burrs, bringing the surfaces into adjacency
with a layer of pressure fluent bonding material
therebetween, and applying sufficient pressure across the
surface to cause substantial deformation of the metal of at
least one of the workpieces in contact with the metal of the
other workpiece at the points where the burrs of one
workpiece surface contact the other workpiece such as to
provide metal-to-metal contact within and through the
adhesive layer of bonding material, and setting the layer of
bonding material. Preferably at least one of the surfaces to
be bonded is highly passivated.
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CA 02272115 1999-OS-17
In the context of this specification, "generally planar"
means that the surfaces are locally essentially flat relative
to the burrs and the gouges from which the burrs are raised,
but does not require that the surfaces as a whole be flat
provided that they can be brought accurately parallel during
the bonding process.
The bonding material may be an adhesive such as an epoxy
adhesive, and may be a liquid or a gel or solid that will
flow under the bonding pressure applied. The setting of the
material may be either due to a curing process, or a
thermoplastic or fusible material may be used.
The burrs are preferably tapered to an outwardly directed
point. In a most preferred embodiment, the burrs are formed
in a workpiece of material such as stainless steel, a
corrosion resistant nickel-chromium based alloy such as
"Inconel~~ (trademark), or titanium, and the other workpiece
is of a much softer metal such as aluminium. In all cases,
the pressure applied is such that the burrs on one surface
will interact with the other surface to work one metal
against the other to provide direct metal-to-metal contact at
the burr sites within the covering provided by the layer of
bonding material. In the most preferred embodiment, the
burrs on the surface of the harder metal penetrate the
surface of the softer metal to help bond the two workpieces
together mechanically, thermally and electrically as well as
adhesively. Where burrs are present on both surfaces, the
burrs themselves may work against each other to promote
further metal-to-metal contact.
The invention is illustrated schematically by the
accompanying drawing in which Figure 1 illustrates two
workpieces bonded in accordance with the invention.
Referring to Figure 1, this shows how, under pressure, burrs
2 formed on a harder metal 4 such as stainless steel will
penetrate a softer metal 6 such as aluminum, while burrs 8
2

CA 02272115 1999-OS-17
formed on the latter will be deformed and flattened into
localized protuberances 8 whose metal is worked against the
surface of the stainless steel to promote metal to metal
contact. The pressure applied will attenuate the adhesive
layer 10, thus improving its thermal conductivity, while the
burrs and gouges will promote bonding, through increased
surface area and surface irregularity provided by the burrs.
The interface between the metals also extends over an
interface zone of finite thickness, providing stability of
the bond under stress and temperature cycling.
In order to test the electrical efficiency of the bonding,
four samples were prepared by bonding respectively to two
sheets of aluminum, a sheet of aluminum and a sheet of
Inconel (trademark) 625 alloy, a sheet of aluminum, a sheet
of 316L stainless steel, and a sheet of aluminum and a sheet
of titanium, using epoxy adhesive as the bonding layer and
the application of sufficient pressure during curing of the
adhesive to result in deformation of the aluminum burrs and
penetration into the aluminum of the burrs of the harder
metals as shown in Figure 1. The burrs formed on the aluminum
plates were curved tongues planed from gouges about 2mm long
and projected about 1mm above the surface. The tongues were
about 0.75mm wide and about 0.5mm thick at their base. The
burrs were formed in rows with the gouges facing alternate
directions in alternate rows about 3mm apart so that there
was no overlap between the gouges, and with burrs in a row at
a pitch of about 1.5mm. The burrs formed from the other metal
sheets projected about 0.75mm from 1.25mm gouges, and tapered
to an outwardly directed point from base dimensions of about
0.2mm width and 0.15mm thickness. The burrs were formed in
rows with the gouges facing in alternate directions in
alternate rows, the rows spaced about 2.5mm apart with burrs
in a row at a pitch of about 0.75mm. The aluminum sheets
used were 1.5mm thick, the titanium and Inconel alloy sheets
were 0.5mm thick, and the stainless steel sheet was 2.75mm
thick. The composite sheets were respectively 3.65mm,
2.29mm, 4mm and 2.81mm thick. Bonding pressures were 35
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CA 02272115 1999-OS-17
kilopascals for the first and fourth samples, and 35,000
kilopascals for the second and third samples.
The bonded samples were cut into 2.5cm squares and their
resistivity was measured under various applied pressures
within a press between copper alloy contact plates larger
than the sample plates, both the contact plates and the outer
surfaces of the sample plates being polished. Direct current
was applied between the contact plates and the voltage drop
across the samples plates measured, no corrections being made
for contact resistance or bulk resistivity of the samples.
For the two samples (aluminum-stainless steel and aluminum-
Inconel) bonded at high pressure, the resistivities were
stable at 0.023 ohms/cm2 and 0.051 ohms/cm2, over the range of
test pressures applied, over the range of currents tested,
and over the duration of the tests. Such samples would be
suitable for use as bipolar separator plates in fuel cells.
In the case of the samples bonded at low pressure, the
resistivity decreased over the duration of the tests (15-45
minutes), the decrease being greater at higher currents,
suggesting that the low pressure applied had resulted in
insufficient working to overcome fully the passivation of the
metal surfaces, although this was further overcome by the
passage of current through the sample to further break down
the passivation and reduce resistivity.
Only the fourth sample showed any change in resistivity with
pressure, but this Was probably due to a lack of flatness of
the sample which resulted in contact with the contact plates
improving as pressure increased.
Where stable electrical properties of the bond are important,
it will therefore be appreciated that sufficient pressure
should be used to provide suf f icient working at the points of
contact of the burrs with the other workpiece to provide the
necessary stability; this can be readily determined
empirically. In the case of metals of substantially different
hardness, the burring process should be such as to provide on
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CA 02272115 1999-OS-17
the harder metal burrs in the form of substantially
perpendicularly outwardly directed spikes Without a
substantial degree of retroversion, and the bonding pressure
should be sufficient to drive these spikes into the softer
metal. The burrs, if provided, on the softer metal, or on
workpieces of equal hardness, are less critical in form since
they will be crushed and flattened under pressure so that
their material is worked against that of the other workpiece . .
The function of the bonding material is not only to assist in
bonding the workpieces together, but also to seal in the
metal-to-metal contacts formed between the workpieces, and
thus prevent repassivation. Accordingly, the material used
should be capable of maintaining a hermetic seal between the
workpieces.
The method as described can also be extended to systems in
which at least one of the workpieces is not of metal,
provided that it is sufficiently ductile to permit burr
formation if burr formation is required. With workpieces of
substantial thickness, the burrs may of course be
substantially larger than those exemplified above.
5

Representative Drawing