Process for cladding precious metals to precipitation hardenable materials

A process for cladding precious metals to precipitation hardenable materials. In accordance with one aspect of the present invention, are the steps of (i) placing a precious metal layer of a first selected thickness atop a selected beryllium-copper alloy base metal strip of a second selected thickness to approximate a desired final product thickness, (ii) cold rolling the layer and strip to reduce their respective thicknesses by generally more than 50%, (iii) heating the layer and strip to a first selected temperature generally within a range of 1000.degree.-1300.degree. F., (iv) maintaining the first temperature for a first selected time to promote metallic bonding of the layer to the strip while softening the base metal, (v) pickling the layer and strip to remove surface oxides, (vi) cold rolling the layer and strip to a thickness generally 11% greater than that of the desired final product thickness, (vii) heating the layer and strip to a second selected temperature generally within a range of 1250.degree.-1400.degree. F., (viii) maintaining the second temperature for a second selected time to effect dissolution of the beryllium into the copper and growth of metallic grains in the alloy to a desired size, with minimal diffusion of the base metal into the precious metal, (ix) pickling bonded strip and layer to remove surface oxides, (x) cold rolling the layer and strip generally to the desired final product thickness, (xi) heating the layer and strip to a third selected temperature generally within a range of 500.degree.-800.degree. F. and (xii) maintaining the third temperature for a third selected time to resurrect strength, ductility and conductivity of the base metal.

FIELD OF THE INVENTION 
The present invention relates to processes for joining precious metals to 
precipitation hardenable materials and more particularly to a process for 
cladding a precious metal to a precipitation hardenable base metal which 
minimizes use of precious metal while sustaining strength, ductility and 
conductivity of the base metal. 
BACKGROUND OF THE INVENTION 
When cladding a precious metal to a precipitation hardenable base metal, 
thermally induced diffusion (or interdiffusion) between the metals is 
common. The result is diminished purity of the precious metal. Because 
interdiffusion is a function of temperature, it becomes significant when 
cladding with base metals of relatively high annealing temperatures, e.g., 
beryllium-copper alloys. 
To compensate for interdiffusion, substantial amounts of precious metal 
have been used in excess of that required for the finished product, i.e., 
electronic connectors. For this reason, beryllium-copper alloys have been 
found relatively costly for use as a base metal. 
Conventional copper alloys with relatively low annealing temperatures such 
as phosphor bronze are processed, e.g., by cladding, with minimal 
interdiffusion. While relatively economical for use as a base metal, 
phosphor bronze lacks the strength, ductility and conductivity of 
beryllium-copper alloys. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, there is provided 
an improved process for cladding precious metals to beryllium-copper 
alloys. The process comprises the steps of (i) placing a precious metal 
layer of a first selected thickness atop a selected beryllium-copper alloy 
base metal strip of a second selected thickness to approximate a desired 
final product thickness, (ii) cold rolling the layer and strip to reduce 
their respective thicknesses by generally more than 50%, (iii) heating the 
layer and strip to a first selected temperature generally within a range 
of 1000.degree.-1300.degree. F., (iv) maintaining the first temperature 
for a first selected time to promote metallic bonding of the layer to the 
strip while softening the base metal, (v) pickling the layer and strip to 
remove surface oxides, (vi) cold rolling the layer and strip to a 
thickness generally 11% greater than that of the desired final product 
thickness, (vii) heating the layer and strip to a second selected 
temperature generally within a range of 1250.degree.-1400.degree. F., 
(viii) maintaining the second temperature for a second selected time to 
effect dissolution of the beryllium into the copper and growth of metallic 
grains in the alloy to a desired size, with minimal diffusion of the base 
metal into the precious metal, (ix) pickling the strip and layer to remove 
surface oxides, (x) cold rolling the layer and strip generally to the 
desired final product thickness, (xi) heating the layer and strip to a 
third selected temperature generally within a range of 
500.degree.-800.degree. F. and (xii) maintaining the third temperature for 
a third selected time to resurrect strength, ductility and conductivity of 
the base metal. 
Although the present invention is described in connection with a copper 
alloy which includes beryllium, it may be adapted for cladding precious 
metals to other precipitation hardenable materials such as alloys of 
nickel, titanium and iron. 
Accordingly, it is an object of the present invention to clad a precious 
metal to a beryllium-copper alloy base metal with minimal consumption of 
precious metal while preserving strength, ductility and conductivity of 
the base metal. 
Another object of the present invention is to provide for the simple, 
efficient and economical manufacture of precious metal clad 
beryllium-copper alloy strip products. 
Still another object of the present invention is to clad precious metals to 
beryllium-copper alloys at relatively low process temperatures to reduce 
interdiffusion. 
Still other objects and advantages of the present invention will become 
apparent from the following description of the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Precipitation hardenable materials of high strength, ductility and 
conductivity such as beryllium-copper alloys have been found desirable for 
use as base metals in electronic connectors, e.g., for automobiles and the 
like. Because conventional methods of electroplating (or cladding) 
phosphor bronze with precious metals are performed with relatively little 
interdiffusion, beryllium-copper alloys have been generally less 
economical for commercial use. 
It has been found, however, that by using annealing temperatures notably 
lower and for longer times than those of conventional processes, 
interdiffusion between precious metals and beryllium-copper alloys is 
reduced exponentially. Less precious metal is then required to produce the 
finished product, lowering costs. 
In accordance with one aspect of the present invention, there is provided 
an improved process for cladding precious metals to beryllium-copper alloy 
base metals. The process comprises the steps of (i) placing a precious 
metal layer of a first selected thickness atop a selected beryllium-copper 
alloy base metal strip of a second selected thickness to approximate a 
desired final product thickness, (ii) roll bonding the layer to the strip 
to reduce their respective initial thicknesses by generally more than 50%, 
(iii) heating the layer and strip to a first selected temperature 
generally within a range of 1000.degree.-1300.degree. F., (iv) maintaining 
the first temperature for a first selected time to promote metallic 
bonding of the layer to the strip while softening the base metal, (v) 
pickling the layer and strip to remove surface oxides, (vi) roll bonding 
the layer to the strip to a thickness generally 11% greater than that of 
the desired final product thickness, (vii) heating the layer and strip to 
a second selected temperature generally within a range of 
1250.degree.-1400.degree. F., (viii) maintaining the second temperature 
for a second selected time to effect dissolution of the beryllium into the 
copper and growth of metallic grains in the alloy to a desired size, with 
minimal diffusion of the base metal into the precious metal, (ix) pickling 
the strip and layer to remove surface oxides, (x) roll bonding the layer 
to the strip generally to the desired final product thickness, (xi) 
heating the layer and strip to a third selected temperature generally 
within a range of 500.degree.-800.degree. F. and (xii) maintaining the 
third temperature for a third selected time to resurrect strength, 
ductility and conductivity of the base metal. 
Referring now to FIGS. 1 and 2, there is shown generally a precious metal 
clad beryllium-copper alloy strip product 10. To achieve a product having, 
for example, a base metal 0.010 inch thick 1/4 HM in beryllium-copper 
alloy with 0.00003 inch thick nominal precious metal thickness, a stripe 
of precious metal 20 is layered initially to a first selected thickness, 
e.g., about 0.000197 inch, upon one edge of a beryllium-copper alloy base 
metal strip 40 of a second selected thickness, e.g., approximately 0.062 
inch. The thicknesses are selected depending on device, function and 
specifications, as will be appreciated by those skilled in the art. 
Precious metal stripe 20 is preferably a layered composite which comprises 
a layer 21 of gold joined, e.g., by cladding, to layer 22 of palladium or 
palladium alloy. Examples include a palladium-silver alloy containing 
about 60% palladium and about 40% silver and a palladium-nickel alloy of 
about 80% palladium and about 20% nickel. 
Next, a first bonding step is performed. During this step, the precious 
metal layer is roll bonded to the base metal strip, e.g., preferably by 
cold rolling, reducing their respective initial thicknesses by generally 
more than 50%. In the present example, the base metal strip is reduced in 
thickness from approximately 0.062 inch to 0.0285 inch. The precious metal 
thickness is reduced proportionately. 
As best seen in FIG. 2, a diffusion barrier 30 such as a layer of nickel is 
placed between base metal strip 40 and precious metal stripe 20 to further 
reduce interdiffusion between the bonded layers. In an alternative 
embodiment, the diffusion barrier is omitted. 
In accordance with one aspect of the present invention, cold rolling is 
performed using a rolling mill. The mill has rotating rolls which are 
separated by a selected distance from one another, the distance 
corresponding to the desired thickness of the strip product. Operating 
temperatures are generally below that which would soften the materials to 
be cold rolled. For instance, with beryllium-copper alloys, the maximum 
process temperature is around 200.degree. F. Preferably, a lubricating oil 
is used during cold rolling to maintain a uniform surface finish and 
extract heat. 
During a first heating phase, the bonded precious metal layer and 
beryllium-copper alloy base metal are heated to a first temperature 
generally within a range of 1000.degree.-1300.degree. F. A temperature of 
1150.degree. F. has been found suitable. This temperature is maintained 
for a first selected time, e.g., about 3-5 minutes, sufficient to soften 
the base metal and promote metallic bonding of the precious metal layer to 
the base metal strip. 
Suitable first temperatures are as much as about 250.degree. F. below those 
of conventional processes for cladding beryllium-copper alloys, for 
example, 1450.degree.-1550.degree. F. By lowering the temperature, 
interdiffusion between the precious metal and beryllium-copper alloy is 
decreased, thereby reducing precious metal requirements. Because less 
precious metal is needed, the present invention advantageously provides 
cost effective production of precious metal clad beryllium-copper alloy 
strip products. 
As an intermediate step, the layer and strip are pickled, preferably in a 
bath of either sulfuric or nitric acid, to remove oxides from base metal 
surfaces which typically develop during high temperature annealing 
operations. The pickling process preferably involves one or more 
pre-pickling stages followed by a bright pickle stage. During the 
pre-pickle stage(s), the layer and strip are immersed in a first hot acid 
solution, e.g., 20% to 30% sulfuric acid at a temperature generally within 
a range of 150.degree.-180.degree. F. Alternatively, a solution of caustic 
soda is used. 
The bright pickle stage then utilizes a second hot acid solution, e.g., 
about 15% to 20% nitric acid at a temperature ranging generally from room 
temperature to about 100.degree. F. Alternatively, the acid solution 
comprises around 20% to 30% sulfuric acid at a temperature ranging 
generally from room temperature to about 150.degree. F. After each stage, 
the layer and strip are immersed in rinsing and neutralizing solutions, as 
is known by those skilled in the art. 
During the next step of the present invention, the bonded precious metal 
layer and beryllium-copper alloy base metal strip are again roll bonded to 
one another, e.g., preferably by cold rolling, but this time to a combined 
thickness generally 11% greater than that of the final product, e.g., 
about 0.011 inch. 
A second heating phase is then performed. During this phase, the bonded 
layer and base metal are exposed to a second selected temperature 
generally within a range of 1250.degree.-1400.degree. F. Like the first 
heating phase, the second temperature is substantially lower than those 
temperatures used during conventional processing of beryllium-copper 
alloys, such as 1450.degree. F. The objective, again, is to reduce 
precious metal requirements by decreasing the amount of interdiffusion 
between the precious metal and beryllium-copper alloy. 
This second temperature, e.g., about 1350.degree. F., is maintained for a 
second selected time sufficient to cause dissolution of beryllium into 
copper (within the beryllium-copper alloy) and growth of the alloy's 
metallic grains to a size desirable for selected mechanical properties, 
i.e., strength, ductility and conductivity. It will be understood, 
however, by those skilled in the art that the duration of heating is not 
so long as to cause significant diffusion of the base metal into the 
precious metal. A second selected time of generally 1-3 minutes has been 
found suitable. 
The bonded precious metal layer and base metal are again pickled in acid to 
remove metal oxides from the metal surface which may develop during 
heating, and roll bonded, e.g., by cold rolling, this time to a final 
product thickness, such as 0.010032 inch. 
Finally, the bonded layer and base metal strip are heated to a third 
selected temperature generally within a range of 500.degree.-800.degree. 
F. The third temperature, e.g., approximately 700.degree. F., is 
maintained for a third selected time, up to about 4 hours, sufficient to 
develop the desired mechanical properties in the strip product, such as 
strength (hardness), ductility and conductivity. 
A variety of tests have been conducted using the present process. 
Representative examples are set forth below. 
EXAMPLE 1 
______________________________________ 
Base Metal Start Thickness 
0.060 inch 
Beryllium Content 1.90 weight % 
Precious Metal Start Thickness 
0.000360 inch total 
(0.000040 inch Au) 
(0.000320 inch Pd--Ag alloy) 
Roll Bonding 0.060 to 0.028 inch 
Annealing 1300.degree. F. for 2 min. 
Pickling sulfuric acid bath 
Cold Rolling 0.0280 to 0.0112 inch 
Annealing 1300.degree. F. for 4 min. 
Pickling sulfuric acid bath 
Cold Rolling 0.0112 to 0.0100 inch 
Annealing 700.degree. F. for 10 min. 
Base Metal Final Thickness 
0.0100 inch 
Ultimate Tensile Strength 
130 ksi 
0.2% Offset Yield Strength 
108 ksi 
% Elongation in 2" 
14 
Electrical Conductivity 
21.2% IACS 
% Stress Remaining after 
90 
1000 hours @ 125.degree. C. 
Precious Metal Final Thickness 
0.000056 inch total 
(0.000006 inch Au) 
(0.000050 inch Pd--Ag alloy 
Initial Contact Resistance 
4.3 milliohms 
Contact Resistance - 1000 hours 
Measured at 150.degree. C. 
3.7 milliohms 
Measured at 200.degree. C. 
3.7 milliohms 
______________________________________ 
EXAMPLE 2 
______________________________________ 
Base Metal Start Thickness 
0.054 inch 
Beryllium Content 1.76 weight % 
Precious Metal Start Thickness 
0.000204 inch total 
(0.000021 inch Au) 
(0.000183 inch Pd--Ag alloy) 
Roll Bonding 0.054 to 0.018 inch 
Annealing 1300.degree. F. for 2 min. 
Pickling sulfuric acid bath 
Cold Rolling 0.0180 to 0.0112 inch 
Annealing 1350.degree. F. for 3.3 min. 
Pickling sulfuric acid bath 
Cold Rolling 0.0112 to 0.0100 inch 
Annealing 725.degree. F. for 4 min. 
Base Metal Final Thickness 
0.0100 inch 
Ultimate Tensile Strength 
118 ksi 
0.2% Offset Yield Strength 
97.3 ksi 
% Elongation in 2" 
17 
Electrical Conductivity 
21.5% IACS 
% Stress Remaining after 
87 
1000 hours @ 125.degree. C. 
Precious Metal Final Thickness 
0.000190 inch total 
(0.000035 inch Au) 
(0.000155 inch Pd--Ag alloy) 
Initial Contact Resistance 
1.5 milliohms 
Contact Resistance - 1000 hours 
Measured at 150.degree. C. 
3.0 milliohms 
Measured at 200.degree. C. 
3.0 milliohms 
______________________________________ 
Again, by using annealing temperatures notably lower than those of 
conventional processes, the coefficient of diffusion (or interdiffusion) 
between the precious metal layer and the base metal is reduced 
exponentially. This results in a substantial reduction in the precious 
metal needed to produce the finished strip product, lowering costs. In 
this manner, the present process advantageously produces strip products of 
precious metal clad beryllium-copper alloy of desired physical and 
mechanical properties, e.g., those within standard published temper 
designations, but at a significantly lower cost than conventional methods. 
Although the embodiments illustrated herein have been described for use 
with a beryllium-copper alloy, an equivalent process could be practiced on 
other precipitation hardenable materials such as alloys of nickel, 
titanium or iron, giving consideration to the purpose for which the 
present invention is intended. Similarly, precious metals and their alloys 
other than those of gold or silver may be clad to beryllium-copper alloys 
or other suitable precipitation hardenable materials. 
Various modifications and alterations to the present invention may be 
appreciated based on a review of this disclosure. These changes and 
additions are intended to be within the spirit and scope of the invention 
as defined by the following claims.