U.S. patent application number 14/976553 was filed with the patent office on 2016-04-21 for process for producing a cu-cr material by powder metallurgy.
The applicant listed for this patent is PLANSEE POWERTECH AG. Invention is credited to Claudia Kowanda, Frank Mueller.
Application Number | 20160107237 14/976553 |
Document ID | / |
Family ID | 43646247 |
Filed Date | 2016-04-21 |
United States Patent
Application |
20160107237 |
Kind Code |
A1 |
Kowanda; Claudia ; et
al. |
April 21, 2016 |
PROCESS FOR PRODUCING A CU-CR MATERIAL BY POWDER METALLURGY
Abstract
A process for producing a Cu--Cr material by powder metallurgy
for a switching contact, in particular for vacuum switches,
includes the steps of pressing a Cu--Cr powder mixture formed from
Cu powder and Cr powder and sintering the pressed Cu--Cr powder
mixture to form the material of the Cu--Cr switching contact. The
sintering or a subsequent thermal treatment process is carried out
with an alternating temperature profile, in which the Cu--Cr powder
mixture or the Cu--Cr material is heated above an upper temperature
limit value and cooled again below a lower temperature limit value
at least twice in alternation. All of the steps are carried out at
temperatures at which no molten phase forms.
Inventors: |
Kowanda; Claudia;
(Oberaegeri, CH) ; Mueller; Frank; (Adligenswil,
CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PLANSEE POWERTECH AG |
SEON |
|
CH |
|
|
Family ID: |
43646247 |
Appl. No.: |
14/976553 |
Filed: |
December 21, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13813996 |
Feb 4, 2013 |
|
|
|
PCT/AT2011/000319 |
Aug 1, 2011 |
|
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14976553 |
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Current U.S.
Class: |
419/23 ; 419/29;
419/32 |
Current CPC
Class: |
B22F 2301/20 20130101;
B22F 3/1017 20130101; B22F 3/1017 20130101; B22F 1/0003 20130101;
B22F 3/02 20130101; C22C 1/0425 20130101; B22F 2301/10 20130101;
B22F 1/0014 20130101; B22F 2304/10 20130101; C22C 1/045 20130101;
H01H 1/0206 20130101; B22F 2998/10 20130101; B22F 2003/248
20130101; C22C 27/06 20130101; B22F 9/04 20130101; B22F 3/24
20130101; B22F 3/16 20130101; B22F 2998/10 20130101; C22C 9/00
20130101; H01H 11/048 20130101 |
International
Class: |
B22F 3/16 20060101
B22F003/16; C22C 27/06 20060101 C22C027/06; C22C 9/00 20060101
C22C009/00; B22F 9/04 20060101 B22F009/04; B22F 1/00 20060101
B22F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 3, 2010 |
AT |
GM 484/2010 |
Claims
1. A process for producing a Cu--Cr material by powder metallurgy
for a switching contact or a vacuum switch contact, the process
comprising the following steps: pressing a Cu--Cr powder mixture
formed from Cu powder and Cr powder; sintering the pressed Cu--Cr
powder mixture to form the material of the Cu--Cr switching
contact; and carrying out at least one of a sintering or subsequent
thermal treatment process with an alternating temperature profile
by heating the Cu--Cr powder mixture or the Cu--Cr material above
an upper temperature limit value and cooling the Cu--Cr powder
mixture or the Cu--Cr material again below a lower temperature
limit value at least twice in alternation, and carrying out all of
the steps at temperatures at which no molten phase forms.
2. The process according to claim 1, which further comprises
setting the upper temperature limit value in a range between
1065.degree. C. and 1025.degree. C. and setting the lower
temperature limit value at least 50.degree. C. below the upper
temperature limit value or at least 100.degree. C. below the upper
temperature limit value.
3. The process according to claim 1, which further comprises
additionally performing a step of mixing Cu powder and Cr powder to
form the Cu--Cr powder mixture.
4. The process according to claim 1, which further comprises
providing Cu particles in the Cu--Cr powder mixture having a
particle size distribution with a maximum particle diameter of
.ltoreq.80 .mu.m or .ltoreq.50 .mu.m.
5. The process according to claim 1, which further comprises
providing Cr particles in the Cu--Cr powder mixture having a
particle size distribution with a maximum particle diameter of
.ltoreq.200 .mu.m or .ltoreq.160 .mu.m.
6. The process according to claim 1, which further comprises
providing Cr particles in the Cu--Cr particle mixture having a
particle size distribution with a minimum particle diameter of
.gtoreq.20 .mu.m or .gtoreq.32 .mu.m.
7. The process according to claim 1, which further comprises
providing the Cu--Cr powder mixture with a Cu content of between
30% by weight and 80% by weight and a Cr content of between 70% by
weight and 20% by weight.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/813,996, filed Feb. 4, 2013, which was a
.sctn.371 national stage application of international application
No. PCT/AT2011/000319, filed Aug. 1, 2011, which designated the
United States; this application also claims the priority, under 35
U.S.C. .sctn.119, of Austrian application No. GM 484/2010, filed
Aug. 3, 2010. The prior applications are herewith incorporated by
reference in their entirety.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to a process for producing a
Cu--Cr material by powder metallurgy for a switching contact, in
particular for vacuum switches, and to a Cu--Cr switching contact
produced by powder metallurgy, in particular for vacuum switches.
It is concerned with producing a high-performance Cu--Cr
material.
[0003] It is known to use Cu--Cr materials for switching contacts,
in particular in the application area of the vacuum switching
principle. The vacuum switching principle has already become
established worldwide as a leading switching principle in the area
of medium voltage, i.e. in the range from about 7.2 kV to 40 kV,
and there is also an evident trend toward use at higher voltages.
Such switching contacts are used for example both for
medium-voltage vacuum circuit breakers and for vacuum
contactors.
[0004] Among the requirements for the switching contacts are a high
switching capacity that remains as constant as possible throughout
the lifetime of the contact, a high dielectric strength and minimal
erosion. It is endeavored to achieve a high erosion resistance, a
good electrical and thermal conductivity, a minimal tendency for
welding to occur during the switching operation as well as a high
dielectric strength and adequate mechanical resistance of the
switching contact.
[0005] DE 10 2006 021 772 A1 describes a process for producing
copper-chromium contacts for vacuum switches. Copper-chromium
contacts for vacuum switches are thereby produced by using a
casting or spraying process with subsequent rapid quenching to
create a thin copper-chromium sheet as a starting material for the
contacts. Concentration profiles are thereby established in a
direction perpendicular to the direction of the strip. A phase
diagram of the Cu--Cr system is also presented and described.
[0006] As can be seen from the phase diagram, in the solid phase
there is virtually no miscibility between Cu and Cr. Only in a
small region below the eutectic that is found at a temperature of
about 1075.degree. C. is there a region in which there is a slight
solubility of Cr in solid solution in Cu. The maximum solubility of
Cr in Cu in solid solution in thermodynamic equilibrium, with about
0.7 at. %, is at 1075.degree. C. At lower temperatures, the
solubility of Cr in Cu falls and, at 400 .degree. C., there is only
0.03 at. % Cr in Cu in solid solution in thermodynamic equilibrium.
A more detailed phase diagram of the Cu--Cr system is presented for
example on page 524 of the manual by M. Hansen and K. Anderko
"Constitution of Binary Alloys", McGraw-Rill Book Company, Inc.
(1958).
[0007] It follows from the phase diagram that, in the case of
Cu--Cr materials with a typical content of 30-80% by weight Cu and
70-20% by weight Cr, at temperatures below the eutectic there are
Cr grains in a Cu matrix. On account of the slight solubility of Cr
in Cu in this region, there may be a small proportion of Cr in
solid solution in the Cu matrix. Hereafter, the term Cu matrix is
used even when there is a small proportion of Cr in solid solution
in the Cu.
[0008] For producing Cu--Cr materials for switching contacts for
vacuum switching technology, purely powder-metallurgical processes,
sintering-impregnating processes and also melt-metallurgical
processes are known.
[0009] On account of the complex phase diagram of the Cu--Cr
system, the direct production of homogeneous melt materials is not
possible. For this reason, materials known as remelt materials are
often used for high-grade Cu--Cr materials for switching contacts
for vacuum switches, it being possible for example for remelting
using a laser or an arc to be employed.
[0010] Compared with melt-metallurgical production, purely
powder-metallurgical production of Cu--Cr materials proves to be
much more cost-effective for switching contacts for vacuum switches
(hereafter also referred to as vacuum switching contacts). However,
it has been found that the Cu--Cr materials produced by powder
metallurgy have so far not yet had the desired properties to a
satisfactory extent.
BRIEF SUMMARY OF THE INVENTION
[0011] It is the object of the present invention to provide a
process for producing a Cu--Cr material by powder metallurgy for a
switching contact and to provide a Cu--Cr switching contact
produced by powder metallurgy that not only provide a high erosion
resistance, a good electrical and thermal conductivity, a minimal
tendency for welding to occur during the switching operation as
well as a high dielectric strength and adequate mechanical
resistance of the switching contact but also make cost-effective
production possible.
[0012] The object is achieved by a process for producing a Cu--Cr
material by powder metallurgy for a switching contact as described
below. Advantageous developments are specified in the dependent
claims.
[0013] The process for producing a Cu--Cr material by powder
metallurgy for a switching contact, in particular for vacuum
switches, has the following steps: pressing a Cu--Cr powder mixture
formed from Cu powder and Cr powder, sintering the pressed Cu--Cr
powder mixture to form the material of the Cu--Cr switching
contact. The sintering and/or a subsequent thermal treatment
process is carried out with an alternating temperature profile, in
which the Cu--Cr powder mixture or the Cu--Cr material is heated
above an upper temperature limit value and cooled again below a
lower temperature limit value at least twice in alternation. All of
the steps are carried out at temperatures at which no molten phase
forms. The entire process for producing the Cu--Cr material is
consequently carried out purely powder-metallurgically at
temperatures that lie below the temperature of the eutectic
(1075.degree. C.) of the Cu--Cr system, so that no molten phase
forms. The term "purely powder-metallurgically" refers here to a
process in which there is no formation of a molten phase. Either
the sintering or subsequent thermal treatment process (or both)
is/are carried out with an alternating temperature profile. An
alternating temperature profile is understood here as meaning that
a temperature increase and a temperature decrease take place in
alternation, a temperature increase and a temperature decrease each
taking place at least twice. With preference, the temperature
increase and the temperature decrease take place at least three
times. The alternating temperature profile may in this case already
be executed for example during the sintering of the pressed Cu--Cr
preform. However, it is also possible for example to expose the
already (conventionally) sintered Cu--Cr material to the
alternating temperature profile in a subsequent thermal treatment
process. The upper temperature limit value may in this case be
preferably chosen such that there is the greatest possible
solubility of Cr in Cu in solid solution. The lower temperature
limit value may preferably be chosen such that there is a much
lower solubility of Cr in Cu in solid solution than at the upper
temperature limit value.
[0014] The production of the Cu--Cr material may in this case be
performed for example by the finished switching contact already
being provided in its final form, or for example such that the
switching contact is only given its final form by a suitable
finishing operation.
[0015] The purely powder-metallurgical production allows the Cu--Cr
material to be provided in a particularly cost-effective way. The
alternating temperature profile (cyclic annealing) achieves the
effect that many Cr grains with grain sizes with a cross section of
between 0.1 .mu.m.sup.2 and 50 .mu.m.sup.2 (measured in the
micrograph) are formed in a Cu matrix. The Cu--Cr material formed
consequently has a grain size distribution of the Cr grains
measured in the micrograph that has a first maximum in the region
of grain sizes with a cross section of between 0.1 .mu.m.sup.2 and
50 .mu.m.sup.2. The determination of the grain size distribution is
performed in this case microscopically in a micrograph by measuring
the surface areas of the respective Cr grains. Microscopically is
understood here as meaning by light microscopy and electron
microscopy.
[0016] A Cu--Cr material for a switching contact that is produced
in a very cost-effective way and thereby at the same time achieves
a high erosion resistance, a good electrical and thermal
conductivity, a minimal tendency for welding to occur during the
switching operation as well as a high dielectric strength and
adequate mechanical resistance of the switching contact is achieved
in this way. By realizing the alternating temperature profile, the
described advantageous grain size distribution is achieved without
any problem even when relatively coarse Cr powder (for example with
particle diameters of between 20 .mu.m and 200 .mu.m) is used as
the starting material.
[0017] In the case of a purely powder-metallurgical production
process without executing the alternating temperature profile, in
which for example Cu powder and Cr powder with maximum particle
diameters of up to approximately 200 .mu.m are used, the resultant
Cu--Cr material has a microstructure in which there are in the
micrograph relatively large Cr grains with a grain diameter in the
range between 100 .mu.m and 150 .mu.m along with some smaller Cr
grains in a Cu matrix. This then typically yields a unimodal grain
size distribution with a maximum for example at grain sizes in the
range between 100 .mu.m.sup.2 and 25000 .mu.m.sup.2. This implies
that the particle sizes of the Cr powder as the starting material
are substantially retained in the resultant Cu--Cr material unless
the alternating temperature profile is executed.
[0018] On the other hand, use of much finer-grained Cr powder as
the starting material would lead to further problems. The
production process would be made much more difficult. Fine-grained
Cr powders have a much higher oxygen content than coarse-grained
powder. As a result, the binding of the Cr phase into the Cu matrix
is made more difficult, which causes a higher porosity. It has also
been found that the degree of impurities due to oxides in fine Cr
powder fractions is higher than in coarse-grained powders. A
further difficulty in the processing of fine powders is that of
handling, in terms of avoiding the uptake of oxygen during the
production process, and that of ensuring sufficient safety in the
workplace. Furthermore, to achieve a satisfactory density and a low
porosity of the material, a higher pressing pressure is required,
or a cold working of the sintered material would be necessary. With
the specified process steps, by contrast, the desired properties of
the Cu--Cr material can be achieved in a cost-effective way using
conventional production plants.
[0019] With the process for producing the Cu--Cr material, a low
porosity, a high density, an extremely low degree of impurities,
finely and homogeneously isotropically distributed Cr grains in a
Cu matrix and a constant homogeneous chemical composition of the
Cu--Cr material are achieved. The resultant Cu--Cr material is
outstandingly suitable for switching contacts for use in vacuum
switching technology, both as a circuit breaker in the high-voltage
and medium-voltage area and as a vacuum contact in the low-voltage
area.
[0020] According to a refinement, the upper temperature limit value
lies in the range between 1065.degree. C. and 1025.degree. C. and
the lower temperature limit value lies at least 50.degree. C. below
the upper temperature limit value. The lower temperature limit
value preferably lies at least 100.degree. C. below the upper
temperature limit value. In this case, the upper temperature limit
value lies in a temperature range just below the temperature of the
eutectic (1075.degree. C.), that is to say a range in which up to
approximately 0.7 at. % Cr can be dissolved in the Cu matrix in
solid solution. This corresponds to the range in which there is the
maximum solubility of Cr in Cu in solid solution. On the other
hand, the upper temperature limit value lies far enough below the
temperature of the eutectic that the formation of a molten phase is
reliably prevented even when there are slight temperature
fluctuations. The lower temperature limit value lies well below the
upper temperature limit value, that is to say in a range in which
(in thermal equilibrium) a much smaller amount of Cr can be
dissolved in the Cu matrix in solid solution. Consequently, when
there is heating above the upper temperature limit value, Cr is
enriched in the material of the Cu matrix (up to a maximum of about
0.7 at. %). When there is cooling below the lower temperature limit
value (which corresponds to a vertical movement in the phase
diagram), the amount of Cr dissolved in solid solution exceeds the
solubility corresponding to this lower temperature value, which is
much less than 0.7 at. %. Consequently, Cr is precipitated from the
Cu matrix and Cr grains with small grain sizes form. If there is
repeated execution of the alternating temperature profile, the
number of Cr grains with small grain sizes that are formed
initially increases.
[0021] According to a refinement, the process also has the
following step: mixing Cu powder and Cr powder to form a Cu--Cr
powder mixture. In this case, the Cu--Cr powder mixture can be
provided in a simple way by using customary Cr powder and Cu
powder.
[0022] According to a refinement, the Cu particles in the Cu--Cr
powder mixture have a particle size distribution with a maximum
particle diameter of 80 .mu.m, preferably 50 .mu.m. In this case, a
reliable formation of the Cu matrix is made possible in the
sintering process and the Cu--Cr material can be reliably provided
with a low porosity and high density. The maximum particle diameter
is in this case determined by means of a screen analysis. In this
case, a screen with a corresponding mesh width (for example 80
.mu.m or 50 .mu.m) is used, and only particles that fall through
the screen are used.
[0023] According to a refinement, the Cr particles in the Cu--Cr
powder mixture have a particle size distribution with a maximum
particle diameter of .ltoreq.200 .mu.m, preferably .ltoreq.160
.mu.m. The maximum particle diameter is in turn determined by a
screen analysis with a corresponding mesh width of the screen. In
this case, the value for the maximum particle diameter is small
enough to achieve the result that there are no over-large Cr grains
in the Cu--Cr material. On the other hand, the individual particles
can also be formed large enough that there is no overt risk of
impurities due to oxides occurring, and a high density and a low
degree of porosity can be achieved in conventional production
plants.
[0024] According to a refinement, the Cr particles in the Cu--Cr
powder mixture have a particle size distribution with a minimum
particle diameter of .gtoreq.20 .mu.m, preferably .gtoreq.32 .mu.m.
The minimum particle diameter is in this case likewise determined
by a screen analysis (with a mesh width of for example 20 .mu.m or
32 .mu.m), but in this case only the particles that do not fall
through the screen are used. In this case, the minimum particle
diameter is large enough that there is no overt risk of impurities
due to oxides occurring, and a high density and a low degree of
porosity can be achieved in conventional production plants.
[0025] According to a refinement, the Cu--Cr powder mixture has a
Cu content of between 30% by weight and 80% by weight and a Cr
content of between 70% by weight and 20% by weight. In this case it
is achieved that not only a high erosion resistance and a low
welding tendency but also good electrical and thermal conductivity
and a sufficient mechanical strength can be provided. If the Cr
content exceeds 70% by weight, this leads to a notable impairment
of the thermal and electrical conductivity. If the Cr content is
less than 20% by weight, a satisfactory errosion resistance and
welding tendency cannot be achieved.
[0026] The object is also achieved by a Cu--Cr switching contact
produced by powder metallurgy as described below. Advantageous
developments are specified in the dependent claims. The Cu--Cr
switching contact may be designed for vacuum switches.
[0027] The Cu--Cr switching contact produced by powder metallurgy
has a Cu content of between 30% by weight and 80% by weight and a
Cr content of between 70% by weight and 20% by weight. The Cu--Cr
switching contact has Cr grains in a Cu matrix. A grain size
distribution of the Cr grains, measured in the micrograph, has a
first maximum in the range of grain sizes with a cross-sectional
area of between 0.1 .mu.m.sup.2 and 50 .mu.m.sup.2 . The switching
contact is produced by a powder-metallurgical process from Cu
powder and Cr powder without the formation of a molten phase. It
consequently concerns a Cu--Cr switching contact produced purely by
powder metallurgy.
[0028] A Cu matrix is understood here as meaning a material which
primarily consists of Cu, but may also have a small proportion of
Cr in solid solution. There may furthermore also be traces of
impurities. Cr grains are formed in the Cu matrix. The grain size
distribution of the Cr grains is in this case determined as
follows: a micrograph of the Cu--Cr material of the switching
contact is prepared and microscopically analyzed. In the
micrograph, the Cr grains are identified and the cross-sectional
areas of the Cr grains are measured. The evaluation is performed in
this case over a sufficiently large surface area or various surface
areas that form a sufficiently large overall surface area, so that
a representative, statistical finding is made possible. The
evaluation may be carried out for example manually or else with the
aid of suitable software. In a graphic depiction with the measured
cross-sectional area on the x axis and the associated number of
determined Cr grains with the respective cross-sectional area per
unit area (for example per mm.sup.2) on the y axis (preferably in
each case in a logarithmic representation), the grain size
distribution is evident. The grain size distribution has a maximum
in a range of grain sizes with a measured cross-sectional area of
between 0.1 .mu.m.sup.2 and 50 .mu.m.sup.2.
[0029] With the Cu--Cr switching contact produced by powder
metallurgy, the advantages described above with reference to the
process for producing a Cu--Cr material by powder metallurgy for a
switching contact are achieved. The purely powder-metallurgical
production makes particularly cost-effective production possible.
On account of the grain size distribution with the maximum in the
range of grain sizes with a cross-sectional area of between 0.1
.mu.m.sup.2 and 50 .mu.m.sup.2, the Cu--Cr switching contact has a
great number of fine Cr grains. The fine Cr grains are in this case
homogeneously distributed to the greatest extent. In this way, a
very good erosion resistance is achieved. The Cu--Cr switching
contact is obtainable by a purely powder-metallurgical process, in
which sintering or a subsequent thermal treatment process is
carried out with an alternating temperature profile, in which a
Cu--Cr powder mixture or the material of the Cu--Cr switching
contact is heated above an upper temperature limit value and cooled
again below a lower temperature limit value at least twice in
alternation and in which all of the steps are carried out at
temperatures at which no molten phase forms. The production in a
purely powder-metallurgical process is evident from the Cu--Cr
switching contact.
[0030] According to a refinement, the grain size distribution of
the Cr grains has a second maximum in the range of grain sizes with
a cross-sectional area of between 100 .mu.m.sup.2 and 10000
.mu.m.sup.2. There is consequently a bimodal Cr phase distribution
that has two maximums, a first maximum for grain sizes with a
measured cross-sectional area of between 0.1 .mu.m.sup.2 and 50
.mu.m.sup.2 and a second maximum for grain sizes with a measured
cross-sectional area of between 100 .mu.m.sup.2 and 10000
.mu.m.sup.2. This grain size distribution results from the purely
powder-metallurgical production process using coarse Cr powder, for
example with particle diameters of between 20 .mu.m and 200
.mu.m.
[0031] According to a refinement, the number of Cr grains
corresponding to the first maximum is greater than the number of Cr
grains corresponding to the second maximum, i.e. there are more
grains that have a grain size corresponding to the first maximum
than grains that have a grain size corresponding to the second
maximum. In this case, there are many fine Cr grains with
cross-sectional areas of between 0.1 .mu.m.sup.2 and 50 .mu.m.sup.2
in relation to the total number of Cr grains. A particularly
advantageous erosion resistance is achieved. If the number of Cr
grains corresponding to the first maximum is greater than the
number of Cr grains corresponding to the second maximum by a factor
of >5, there is a particularly advantageous proportion of fine
Cr grains with a small cross-sectional area.
[0032] According to a refinement, the Cu--Cr switching contact has
a relative density of >90%. In this case, a good electrical and
thermal conductivity and a high mechanical strength are reliably
provided. Such a high relative density can be reliably achieved in
conventional production plants if relatively coarse Cr powder and
Cu powder are used. Relative density is understood here as meaning
the ratio between the density achieved and the theoretically
achievable density for the composition. The combination of this
high density and the high proportion of fine Cr grains in the Cu
matrix can be achieved by the combination of using coarse Cr powder
(with particle diameters of between 20 .mu.m and 200 .mu.m) and
using an alternating temperature profile in which heating above an
upper temperature limit value and cooling again below a lower
temperature limit value are performed at least twice in
alternation.
[0033] Further advantages and developments emerge from the
following description of an embodiment with reference to the
figures.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0034] FIG. 1 shows a grain size distribution of the Cr grains in
the case of a Cu--Cr material produced by powder metallurgy in the
starting state (solid line) and after executing an alternating
temperature profile (dashed line).
[0035] FIG. 2 shows a light-microscope micrograph of a Cu--Cr
material produced by powder metallurgy.
[0036] FIG. 3 shows an light-microscope micrograph of a Cu--Cr
material produced by powder metallurgy after executing an
alternating temperature profile.
[0037] FIG. 4 schematically shows the process steps of a process
for producing a Cu--Cr material by powder metallurgy for a
switching contact.
DESCRIPTION OF THE INVENTION
[0038] A process for producing a Cu--Cr material by powder
metallurgy for a switching contact for vacuum switches according to
a first embodiment is described below with reference to FIGS. 1 to
4.
[0039] In a first step --S1-- Cu powder with a maximum particle
diameter of preferably at most 50 .mu.m is mixed with Cr powder
with a maximum particle diameter of at most 200 .mu.m (preferably
at most 160 .mu.m) and a minimum particle diameter of at least 20
.mu.m (preferably at least 32 .mu.m) to form a Cu--Cr powder
mixture. For example, a first Cu--Cr powder mixture with a Cr
content of 25% by weight and a Cu content of 75% by weight and a
second Cu--Cr powder mixture with a Cr content of 43% by weight and
a Cu content of 57% by weight were created as examples.
[0040] In a second step --S2-- the Cu--Cr powder mixture is
pressed. With preference, the Cu--Cr powder mixture is compacted by
cold pressing with a pressing pressure in a range between 400 MPa
and 850 MPa. In a subsequent step --S3-- the preform formed in this
way is sintered in a sintering process at temperatures in a
temperature range well below the temperature of the eutectic
(therefore well below 1075.degree. C.). Consequently, a molten
phase does not form in the Cu--Cr powder mixture or in the pressed
preform in any of the steps --S1-- to --S3--. The sintering process
may, for example, be carried out at temperatures in a temperature
range between 850.degree. C. and 1070.degree. C. The temperatures
must in this case be high enough that the sintering process
proceeds to a sufficient extent and with sufficient speed, and low
enough that no molten phase forms even in the event of unavoidable
temperature gradients.
[0041] A light-microscope micrograph of a Cu--Cr material produced
by powder metallurgy after step --S3-- is presented in FIG. 2 by
way of example. In FIG. 2 it can be seen that Cr grains with
different grain sizes are bound in a Cu matrix. A closer analysis
of the grain distribution in the case of the examples mentioned
showed that the grain sizes of the Cr grains corresponded
substantially to the particle sizes of the Cr powder of the
starting material.
[0042] An evaluation of the grain size distribution of the Cr
grains in the Cu--Cr material produced in such a way is represented
in FIG. 1 by a solid line. A micrograph of the Cu--Cr material was
prepared and the size of the Cr grains was microscopically examined
and measured. In this case, 10 different regions of the Cu--Cr
material were analyzed, in order to obtain a statistically
meaningful distribution. In FIG. 1, the measured cross-sectional
area of the Cr grains in .mu.m.sup.2 is plotted on the horizontal
axis in a logarithmic scale. The corresponding number of grains
normalized to a unit area of 1 mm.sup.2 is shown on the vertical
axis, likewise in a logarithmic representation. As can be seen in
FIG. 1, the Cu--Cr material has in this stage of the process a
monomodal grain size distribution with grain sizes in a range
between approximately 10 .mu.m.sup.2 and 25000 .mu.m.sup.2. The
grain size distributional has in this case a maximum that is for
grain sizes in a range greater than 100 .mu.m.sup.2.
[0043] The Cu--Cr material is subsequently subjected to a thermal
treatment process with an alternating temperature profile, as
described below. The Cu--Cr material is thereby alternately heated
to a temperature above an upper temperature limit value and cooled
to a temperature below a lower temperature limit value. In this
case, the alternating heating and cooling are performed at least
twice. It is also ensured in these process steps that no molten
phase forms, i.e. the Cu--Cr material is kept at temperatures below
the temperature of the eutectic (1075.degree. C.) of the Cu--Cr
system. This is described in further detail below.
[0044] In a step --S4-- the Cu--Cr material is heated to a
temperature above the upper temperature limit value. The upper
temperature limit value in this case preferably lies relatively
close below the temperature of the eutectic of the Cu--Cr system,
so that the Cu--Cr material is brought to a temperature just below
the temperature of the eutectic, but is far enough from the
temperature of the eutectic that formation of a liquid phase is
reliably prevented. The upper temperature limit value consequently
preferably lies in a range between 1025.degree. C. and 1065.degree.
C.
[0045] Subsequently, in a step --S5-- the Cu--Cr material is cooled
to a temperature below a lower temperature limit value. The lower
temperature limit value in this case preferably lies in a range
that is at least 50.degree. C. below the upper temperature limit
value, more preferably in a range over 100.degree. C. below the
upper temperature limit value. The lower temperature limit value in
this case preferably lies at most 250.degree. C. below the upper
temperature limit value, more preferably at most 180.degree. C.
below the upper temperature limit value. The lower temperature
limit value should be chosen such that at this value there is a
much lower solubility of Cr in solid solution in Cu than at the
upper temperature limit value. The reason for this choice will be
explained in more detail. For example, the Cu--Cr material may be
cooled to temperatures in the range of about 850.degree. C. It is
recommendable not to choose the lower temperature limit value too
low, in order to ensure an adequate degree of diffusion processes
in the Cu--Cr material. The Cu--Cr material is kept at the upper
temperature limit value and the lower temperature limit value for
some time in each case.
[0046] Subsequently, step --S4-- is repeated, i.e. the Cu--Cr
material is raised again to a temperature above the upper
temperature limit value. After that, step --S5-- is repeated, i.e.
the Cu--Cr material is cooled again to a temperature below the
lower temperature limit value. Steps --S4-- and --S5-- are repeated
altogether n times, but in total at least twice, preferably at
least three times. It has been found that, if steps --S4-- and
--S5-- are executed from 2 to 6 times (2.ltoreq.n.ltoreq.6), an
improvement in the Cu--Cr material is achieved and no further
improvement can be expected from a greater number of repetitions.
The Cu--Cr material is therefore subjected to a cyclic annealing.
At least steps --S4-- and --S5-- are carried out in a
protective-atmosphere furnace under a reducing atmosphere and/or in
a vacuum furnace, in order to avoid undesired oxidation. The
production process is subsequently ended.
[0047] FIG. 3 shows a light-microscope micrograph of a Cu--Cr
material produced by powder metallurgy after executing the
alternating temperature profile described.
[0048] In FIG. 3 it can be seen that, after carrying out the cyclic
annealing, the proportion of Cr grains with a small cross-sectional
area has increased significantly in comparison with the state
before the cyclic annealing (cf. FIG. 2). A closer analysis of the
grain size of the Cr grains shows that a bimodal grain size
distribution that has two maximums has been established.
[0049] In FIG. 1, the determined grain size distribution after
executing the alternating temperature profile is represented as a
dashed line. The grain size distribution was determined in the same
way as already described above with reference to the solid line of
FIG. 1. It is evident that, after the cyclic annealing, there is a
bimodal grain size distribution instead of the previous monomodal
grain size distribution (solid line). The grain size distribution
has a first maximum in a range of grain sizes with a
cross-sectional area of between 0.1 .mu.m.sup.2 and 50 .mu.m.sup.2.
Furthermore, the grain size distribution has a second maximum in
the range of grain sizes with a cross-sectional area of between 100
.mu.m.sup.2 and 10000 .mu.m.sup.2. The number of Cr grains
corresponding to the first maximum is greater than the number of Cr
grains corresponding to the second maximum. The number of Cr grains
corresponding to the first maximum is greater than the number of Cr
grains corresponding to the second maximum by a factor of >5.
There is furthermore a very homogeneous distribution of the Cr
grains in the Cu matrix. The proportion of Cr grains with a
cross-sectional area of <10 .mu.m.sup.2, measured in the
micrograph, is consequently very high. Consequently, the thermal
treatment with the alternating temperature profile has the effect
of achieving a shift to a high proportion of very small finely
distributed Cr grain precipitates in the Cu matrix.
[0050] With the starting materials described, having a relatively
coarse particle size of the Cr powder, very dense Cu--Cr materials
with low porosity that also have a very low degree of impurities
can be produced in a purely powder-metallurgical process by
conventional production plants. The purely powder-metallurgical
production is evident from the Cu--Cr material. On account of the
very finely distributed Cr grains, the Cu--Cr material produced
purely by powder metallurgy has a high erosion resistance, a high
dielectric strength and a sufficient mechanical strength of the
switching contact.
[0051] The formation of the finely distributed Cr grains in the Cu
matrix can be explained as follows with regard to the phase diagram
that is represented for example in DE 10 2006 021 772 A1, mentioned
at the beginning: at temperatures above the upper temperature limit
value in a region just below the temperature of the eutectic, up to
approximately 0.7 at. % Cr in solid solution can be dissolved in
the material of the Cu matrix (in thermodynamic equilibrium). When
there is cooling of the Cu--Cr matrix to a temperature below the
lower temperature limit value, the material is brought to a
temperature at which only a much smaller proportion of Cr in solid
solution can be dissolved in the material of the Cu matrix in
thermodynamic equilibrium. Consequently, during the cooling Cr is
precipitated from the material of the Cu matrix and this
precipitation takes place in the form of small grains. With renewed
heating, taking the temperature above the upper temperature limit
value, Cr in solid solution enters the material of the Cu matrix
again. With renewed lowering of the temperature below the lower
temperature limit value, Cr is precipitated again on account of the
lower solubility in solid solution, which leads to fine Cr grains.
In this way, the described bimodal grain size distribution of the
Cr grains forms.
[0052] It has been found that, for a satisfactory formation of fine
Cr grains, the temperature should go above the upper temperature
limit value and below the lower temperature limit value at least
twice. However, as from a certain number of repetitions of the
cyclic annealing, no improvement in the structure can be observed
any longer. The change in temperature between the high temperature
level and the low temperature level in the cyclic annealing should
be chosen to be sufficiently slow that Cr is reliably precipitated
from the Cu matrix during the cooling, but on the other hand not
too slow, in order that larger Cr grains do not occur again due to
grain coarsening.
[0053] Experiments with Cu--Cr powder mixtures with other ratios of
Cr and Cu were also carried out and likewise led to comparable
results. Experiments with a Cr content of 70% by weight and a Cu
content of 30% by weight also led to a comparable result with
respect to the fine Cr precipitates.
[0054] Although it has been described that the treatment with the
alternating temperature profile is not performed on the Cu--Cr
material until after step --S3-- of sintering, it is also possible
for example already to carry out the sintering process itself with
an alternating temperature profile. In this case, the pressed
Cu--Cr preform is already subjected repeatedly to steps --S4-- and
--S5-- during the sintering operation. In this case, the separate
step --S3-- is omitted and the sintering is performed during steps
--S4-- and --S5--.
* * * * *