Porosity-free electrical contact material, pressure cast method and apparatus

A 100% dense, porosity free copper-chromium contact has been prepared in which deleterious porosity has been eliminated. This copper-chromium contact has been produced by pressurizing liquid copper to infiltrate an evacuated chromium based, lightly sintered, highly porous preform. The electrical contact has one of either a homogeneous Cr distribution and a graded Cr distribution. The apparatus used to effect the molten metal infiltration has two independent, physically separated chambers--a first cold chamber and a second hot chamber. The first chamber is under no applied pressure except inside a gating system used to transfer molten Cu into the porous preform in the first chamber. The new contact has about 15-30% Cr material and a high erosion resistant contact surface. The graded Cr distribution has a Cr rich layer with about 25-50% by weight Cr, an intermediate Cr layer with about 15-20% by weight Cr, a low Cr layer with about 5-15% Cr and a Cr poor layer with about 1-5% Cr above a copper substrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electrical contacts for use in power interruption 
and control devices and more particularly concerns an improved contact 
with a 100% dense, porosity-free microstructure having enhanced electrical 
interruption performance. A copper-chromium contact has been prepared in 
which deleterious porosity has been eliminated. This copper-chromium 
contact has been produced by pressurizing liquid copper to infiltrate an 
evacuated chromium based, lightly sintered, highly porous preform. 
BACKGROUND INFORMATION 
It is well known to use a basic contact in a device such as a vacuum 
interrupter. Typically the contact is attached to a copper electrode by 
brazing. One family of the more common contact alloys is based on the 
copper-chromium system. Sometimes with the addition of other elements. 
Two current methods of fabricating contacts are powder metallurgy 
processing (P/M) and capillary pressure infiltration. These two techniques 
are well-established but both have inherent problems. 
One problem that has arisen during the recent past with regard to contacts 
produced by these two methods is related to contact surface erosion and/or 
welding resulting in a reduction in life of the contacts and hence in the 
interrupter/contactor life. The prior P/M based contacts typically exhibit 
a few percent porosity or have an actual sintered density of 96-98% of the 
theoretical density. Capillary pressure driven contact infiltration 
techniques suffer from solidification shrinkage porosity, which is due to 
shrinkage of molten Cu upon solidification. The presence of the 
.gtoreq.2-4% porosity in contacts adversely affects the performance of the 
contacts in the vacuum interruption tubes. This 2-4% porosity is thought 
to provide are anchoring sites on the contact surface, which in contact 
with the resulting stationary arc can lead to rapid, local erosion, and 
hence, reduced contact life. Furthermore, because the porosity may contain 
some residual gases, when an arc is initiated at the porosity sites, these 
residual gases will leave the contact and cause an increase in partial 
vacuum pressure in the vacuum tube thereby reducing the performance of the 
vacuum interrupter. 
Another problem of the P/M method was the inability to tailor the Cu--Cr 
composition and microstructure. The ability to vary Cr particulate size 
and composition as a function of location was nearly impossible. By the 
same token, the Cr composition of the inner bulk regions of the contact 
was difficult to dilute to improve thermal conductivity and to reduce the 
cost of raw materials via savings on expensive Cr consumption per contact. 
Finally, the P/M technique is a very expensive technique in terms of 
contact manufacturing. 
SUMMARY OF THE INVENTION 
Applicants have made a novel and improved contact, which eliminates the 
observed porosity in contacts made by both conventional powder 
metallurgical and infiltration based processing using pressure 
infiltration. Applicants have invented a copper-chromium contact in which 
such deleterious porosity has been eliminated. The 100% dense, porosity 
free contact is produced by pressurizing liquid copper to infiltrate an 
evacuated chromium based, lightly sintered, highly porous preform. 
An improved contact is made from a blended Cr and Cu powder mixture. A 
contact made from the above powders is about 15-30% by weight Cr material. 
A 50% by weight Cr/50% by weight Cu of mesh size of about -325/-325, gives 
a preform volume void fraction of about 0.61 and a preform volume Cr 
fraction of about 0.22. A 25% by weight Cr/25% by weight Cr/50% by weight 
Cu of mesh size of about -325/100 to 200/-325 gives a preform volume void 
fraction of about 0.61 and a preform volume Cr fraction of about 0.23. The 
above cited preforms are then infiltrated with molten Cu to produce 
contacts which have a 100% dense, porosity free microstructure. One 
embodiment of the improved contact comprises a Cr rich layer having about 
25-30% by weight Cr, an intermediate Cr layer having about 15-25% Cr, a 
low Cr layer having about 5-15% Cr and a Cr poor layer having about 1-5% 
Cr. This embodiment has a Cr rich layer of about 0.5 to 10 mils thickness, 
an intermediate Cr layer of about 0.5 to 10 mils thickness, a low Cr layer 
of about 0.5 to 10 mils thickness, and a poor Cr layer of about 0.5 to 10 
mils thickness. A copper conductive layer of about 250-375 mils is below 
the Cr poor layer. The improved contact has a Cu/Cr interface cohesive 
strength and the mating surface resists melting and erosion. 
The method of making the improved electrical contact comprises the steps of 
selection and blending of Cr and Cu mixtures to form a blended powder, 
light sintering of said blended powder to produce rigid porous preforms 
and pressure infiltration and solidification of molten Cu in the porous 
Cu/Cr preforms to obtain a 100% dense, porosity free microstructure. The 
Cu and Cr powder selection, blending and sintering steps further comprise 
selecting a mixture of Cu and Cr powders. The powder is selected from the 
group consisting of 50% by weight Cr/50% by weight Cu of powder mesh size. 
(-325/-325) and a 25% by weight Cr/25% by weight Cr/50% by weight Cu of 
powder mesh size (-325/100-200/-325). The Cr and Cu mixtures are then 
blended to form a blended powder. The step of blending comprises blending 
for 35 to 50 minutes and pouring into a container. The blended powder is 
deoxidized and lightly sintered to form a rigid preform. The step of 
sintering further comprises treating with hydrogen to precoat/presinter at 
about 900.degree. C. to 1100.degree. C. for the Cu/Cr blended powder. The 
step of sintering takes about 20 to 40 minutes. 
The steps of Cu pressure infiltration and solidification of said preforms 
further comprises placing the preform in a heated preform container, 
placing the preform container in a first chamber of a pressure chamber, 
placing a crucible containing deoxidized Cu in a second chamber, 
evacuating said pressure chambers, heating the Cu in the container to a 
temperature of about 1150.degree. C. to 1200.degree., and heating the 
preform container to about 950.degree. C. to 1000.degree. C. Both molten 
Cu and the preform are kept at their respective temperatures for about 15 
to 25 minutes. The molten Cu is then pressurized with N.sub.2 gas to about 
800 to 1100 Psi and transferred through a gating system to infiltrate said 
preform. The pressure infiltration and solidification of the preform 
produces a 100% dense, porosity free microstructure. 
The apparatus for pressure casting a preform comprises a first chamber of a 
pressure chamber containing a sealed preform container in which a preform 
is placed, means for heating the preform container and the preform, a 
second chamber containing a crucible, heating means to keep a metal in the 
crucible molten, a gating system connecting the crucible in the second 
chamber to the preform container in the first chamber, means for 
evacuating the first and second chambers of the pressure chamber, and 
means for pressurizing the molten metal from the crucible into the preform 
container through said gating system. The first chamber and second chamber 
are physically separated chambers. The pressure in the second chamber 
containing the crucible ranges from 800 Psi to 1,100 Psi. The first 
chamber containing the preform is not pressurized during transfer. The 
metal in the crucible is heated to about 1150.degree. C. to 1200.degree. 
C. and the preform is heated to about 950.degree. C. to 1000.degree. C. 
The crucible and preform are held at their respective temperatures for 
about 15 to 25 minutes. The molten Cu is pressurized and transferred 
through the gating system to infiltrate the preheated preform. The 
pressure casting is a continuous operation. 
The new porosity free contact has the following advantages over a powder 
metallurgical (P/M) product: 
(a) The new contact is a 100% dense, porosity free material due to pressure 
application during infiltration and solidification. 
(b) The new contact has lower oxide content resulting from the use of 
molten metal infiltration of Cu. 
(c) The new contact has enhanced Cu/Cr interface cohesion strength and 
improved erosion behavior. 
(d) The new contact has a lower gas content (e.g., H.sub.2, O.sub.2 and 
N.sub.2), 
(e) The new contact has a more uniform distribution and if desired can have 
a tailored Cr distribution. 
(f) The new contact has lower production costs than the powder 
metallurgical products of the prior art. 
The new porosity free contact has the following advantages over a vacuum 
infiltrated product: 
(a) minimal or zero shrinkage porosity with the new contact; 
(b) a faster rate of infiltration of the preform; 
(c) an enhanced Cr distribution, where there is less dissolution of Cr in 
the molten Cu; and 
(d) tailored compositional gradients, having a selectively engineered Cr 
distribution.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An improved contact is made from a blended Cr and Cu powder mixture. A 
contact made from the above powders is about 15-30% by weight Cr material. 
A 50% by weight Cr/50% by weight Cu of mesh size of about -325/-325 gives 
a preform volume void fraction of about 0.61 and a preform volume Cr 
fraction of about 0.22. A 25% by weight Cr/25% by weight Cr/50% by weight 
Cu of mesh size of about -325/100 to 200/-325 gives a preform volume void 
fraction of about 0.61 and a preform volume Cr fraction of about 0.23. The 
above cited preforms are infiltrated with molten Cu to form contacts which 
have a 100% dense, porosity free microstructure. One embodiment of the 
improved contact comprises a Cr rich layer having about 25-30% by weight 
Cr, an intermediate Cr layer having about 15-25% Cr, a low Cr layer having 
about 5-15% Cr and a Cr poor layer having about 1-5% Cr. The Cr rich layer 
is about 0.5 to 10 mils thick, the intermediate Cr layer is about 0.5 to 
10 mils thick, the low Cr layer is about 0.5 to 10 mils thick, and the 
poor Cr layer is about 0.5 to 10 mils thick. A copper conductive layer of 
about 250-375 mils is below the Cr poor layer. The improved contact has a 
Cu/Cr interface cohesive strength and the matting surface resists melting 
and erosion. 
A method to fabricate the new Cu--Cr electrical contacts comprises the 
following fabrication steps: 
1. selection and blending of Cr/Cu powder mixture; 
2. light sintering of a blended powder to produce a rigid, porous preform; 
and 
3. Cu pressure infiltration and solidification in the porous Cu/Cr preform. 
1. Cu and Cr Powder Selection and Blending 
High purity Cr and Cu powders are used to make porous Cr/Cu based preforms. 
The powders are blended in a V-shaped blender for about 35 to 50 minutes, 
preferably 45 minutes and are gently and freely poured into graphite 
containers for subsequent light sintering. Blended mixtures of free (-325) 
mesh Cu and fine (-325) mesh Cr are used. This provides a contact with a 
homogeneous Cr distribution. In order to produce a contact containing a 
lower Cr fraction (e.g., 0.20 or 0.25 Cr), Cu powders are added to the Cr 
powder and they were blended together in a V-shaped blender. For example, 
when -325 mesh Cr and -325 mesh Cu powders were blended with a 1:1 mixing 
weight ratio, the final void fractional volume was 0.61. In this case, 
because of the presence of the Cu powders, the Cr volume fraction of the 
preform was 0.22. When an equal amount of -325 mesh Cu powder was added to 
a blend containing (-325) mesh Cr+(100-200) mesh Cr, the volume fractions 
of void and Cr were 0.58 and 0.23, respectively. This provides a contact 
with a graded Cr distribution. The various combinations of blended powder 
discussed are summarized hereinbelow. 
______________________________________ 
Powder Blend 
Composition 
Powder Mesh Preform Vol. 
Preform Vol. 
(weight %) Size Void Fraction 
Cr. Fraction 
______________________________________ 
50 Cr/50 Cu 
-325/-325 0.61 0.22 
25 Cr/25 Cr/50 Cu 
-325/100-200/-325 
0.58 0.23 
______________________________________ 
The void fractions are based on calculations given as follows: 
______________________________________ 
Vv: void volume fraction 
Vm: metal volume fraction 
VCu: Cu volume fraction 
VCr: Cr volume fraction 
mCu: weight of Cu powder 
Vv = 1 - Vm = 1 - (VCu - VCr) 
mCr: weight of Cr powder 
Vv = 1 - {(mCu/.rho.Cu) + (mCr/.rho.Cr)}/Vt! 
.rho.Cu: 
density of Cu 
(8.92 g/cc) 
.rho.Cr: 
density of Cr 
(7.2 g/cc) 
Vt: total volume of 
preform based on 
shape (e.g., Vt: .pi. r.sup.2 h 
for cylinder where 
.pi.: 3.14, r: radius of 
preform and h: height 
of preform) 
______________________________________ 
2. Light Sintering 
The blended powder mixtures were carefully poured into cylindrical graphite 
molds. The molds were then placed in a hydrogen furnace. The powders were 
deoxidized and lightly sintered and came out of the furnace as rigid, 
handleable preforms. The temperatures for each 30 minutes sinter were 
about 900.degree. C. to 1100.degree. C., preferably about 1000.degree. C. 
for Cu/Cr powder mixtures. All preforms had shrunk slightly due to the 
light sintering but were rigid enough to handle for pressure casting. The 
metallography of the preforms showed good macro-and microstructures with 
uniform powder distribution. 
3. Pressure Infiltraation and Soldification of Preforms 
The process of making infiltrated Cu/Cr contacts is outlined as follows and 
is shown in FIG. 1. The porous Cr/Cu based preforms were infiltrated with 
molten Cu in a closed chamber using pressurized nitrogen gas. First, a 
preform 1 was placed in a sealed steel container 3. The container 3 was 
then placed in the cold first chamber 5 of a pressure chamber. Pieces of 
oxygen free solid Cu are placed in a graphite crucible 9 in a second 
chamber 7 of the pressure chamber. The preform container is connected with 
the graphite crucible via a steel gating system 13. The chambers 5 and 7 
were sealed, evacuated and the furnace heating systems 15 and 21 were 
turned on. The solid Cu melted and the temperature of the molten Cu 17 was 
allowed to reach about 1150.degree. to 1180.degree. C. The temperature of 
the steel preform container was independently controlled and kept at about 
1150.degree. C. to 1200.degree. C. Keeping the temperature of the preform 
below the melting temperature of Cu is critical for two reasons: (1) Cu 
powders in the preform can remain solid, and (2) the degree of sintering 
in the preform (which will cause shrinkage in the preform) can be 
minimized. The molten Cu 17 and the preform 3 were held at the above 
temperatures for about 15 to 25 minutes. The second chamber 7 was then 
pressurized 19 from about 800 to 1100 Psi, preferably 1000 Psi, and the 
pressurized molten Cu is transferred through the gating system 13 into the 
preform 1. 
The molten metal infiltration approach explained hereinabove differs from 
the existing prior art in the following ways: 
1. This chamber has two independent, physically separated sections--a hot 
section and a cold section; 
2. The cold section is under no applied pressure except inside the gating 
pipe. This is different than the prior art's approach where the preform 
containing molds or containers were under a pressurized atmosphere along 
with the molten metal. The merit of the new method is that this design 
reduces pressurization gas consumption, and therefore it can be more 
economical enabling the pressure levels to be increased to 10,000 Psi or 
higher if required. Currently, the pressure levels of the prior art are 
limited to pressures in the 1300-2500 Psi range. 
3. FIG. 2 shows the microstructure of the 100% dense, porosity free Cu/Cr 
contact produced by the pressure casting method of the present invention. 
4. Pressure east Cu/Cr materials of the present invention can be tailored 
to have functionally different regions in the contact. A functionally 
graded contact would have, for example, a Cr rich layer 23 with about 
25-50% Cr which gives a high erosion resistance contact outer surface. 
Underneath is an intermediate Cr layer 25 with about 15-25% Cr; next a low 
Cr layer 27 with about 5-15% Cr and a Cr poor layer 29 of about 1-5% Cr. 
This is shown in FIG. 3. The Cr rich layer is about 0.5 to 10 mils thick, 
the intermediate Cr layer is about 0.5 to 10 mils thick and the Cr poor 
layer is about 0.5 to 10 mils thick. After the graded preform is pressure 
infiltrated with copper, a copper substrate of about 250 to 375 mils thick 
is layered on the Cr poor layer. In this way, the composition of the 
inner-bulk region of the contact can be diluted to improve thermal 
conductivity and to reduce the cost of raw materials via savings on 
expensive Cr consumption per contact. The melting temperature of the 
surface can be increased thereby and the erosion rate of the contact can 
be decreased. 
While specific embodiments of the invention have been described in detail, 
it will be appreciated by those skilled in the art that various 
modifications and alternatives to those details could be developed in 
light of the overall teachings of the disclosure. Accordingly, the 
particular arrangements disclosed are meant to be illustrative only and 
not limiting to the scope of the invention, which is to be given the full 
breadth of the appended claims.