Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND 0~ THE INVENTION
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Copper base alloys are widel~ used in industry and are
characterized by high formability~ good conductivity and pleasing
appearance. A high percentage of all copper base alloys are
utilized in the form of strip or sheet. The method of producing
strip or sheet to final gauge usually involves alternate steps
of deformation and annealing. It is often found in certain alloys
that annealing after deformation, particularly at thinner gauges,
produces undesirable blistering. These blisters are gas filled
defects which become apparent when the alloy is heated. As the
temperature is raised, gas pressure inside the defect i creases,
thus expanding and de~orming the surrounding metal whlch has a
low yield strength because of the elevated temperature. This
problem is particularly common in CDA Alloy 638 which contains
2.5 to 3.1% aluminum, 1.5 to 2.1% sillcon, .25 to .55% cohalt,
balance essentially copper. Unless otherwise noted, all percentages
in this application are weight percentages.
SUMMARY OF THE INVENTION
The present invention compri~es a process for the production
of copper strip which results in a blister free product. The
process is a comparatively simple one which can be applied using
standard equipment commonly a~ailable in a commercial copper
alloy production facility. The process of the present invention
includes a hot rolling step followed by a diffusion annealing
step performed under carefully controlled conditions. The
diffusion anneal step reduces the hydrogen content of the alloy
without permittlng blister formation. Following the diffusion
anneal the alloy is cold worked according to a particular
schedule. This cold working operation welds shut the i~ternal
defects so that blistering will not occur during subsequent
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- annealing operations. The present invention is broadly appli-
cable to a wide range of copper alloys but is particularly
useful in connection with the production of CDA Alloy 638.
It is an object of the present invention to provide
~ a production method for producing high quality copper alloy
- strip.
It is a further object of the present invention to
provide a processing technique which minimizes blister forma-
tion in copper alloys.
It is an object of the present invention to provide ,
; a method for producing blister free copper alloy material using
as a starting material a copper alloy which has been hot worked
at least 50% to a thickness of from .200 to .750" including the i;
steps of:
A. annealing the copper alloy material at a
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temperature of from 40 to 70% of the absolute
melting temperature of the alloy for a time of
from 1 to 2~ hours; and
B. cold working the material at least 60%.
Further objects will become apparent when the follow-
ing description of the preferred embodiments and claims are
considered.
DESCRIPTION OF THE P
The present invention provides a process for producing ~
blister free copper alloy sheet or strip through the use of a ,-
process which includes the steps of casting, hot working, ' :
diffusion annealing, cold rolling and optionally a further
annealing step. The following description will provide detailed `
parameters for each of the steps in the process of the present
invention.
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The casting of the alloy may be performed using
any process which will produce a sound ingot. It is preferred,
however, to use a process in which a minimum surface area of
molten metal is exposed -to the atmosphere during casting.
or this reason it is preferred to use D.C. casting.
Regardless of precautions taken, a certain amount
of hydrogen will be present within the metal if the casting
operation is performed in a normal atmosphere. Hydrogen
pickup can occur from moisture or dirt in the charge mater~
ials, moisture and impurities in the flux or melt cover,
moisture in the air and moisture or dirt in the mold. As
an approximation molten copper alloys can hold four times
as much hydrogen as solidified copper alloys at
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similar temperatures. Thus, it is common that solidified copper
alloys contain more hydrogen than would be present under
equilibrium conditions.
The ingot is then hot worked, usually by rolling, using an
appropriate hot working temperature. In the case of CDA Alloy 638
which contains 2.5 to 3.5% aluminum, 1.5 to 2.1% silicon, .25 to
.55% cobalt, balance essentially copper, an appropriate hot working
temperature is from 80oo to 920C, preferably 850 to 900C. In
general, the hot working temperature will be from .7 to .95 Tm -
where Tm is the absolute melting point of the alloy. During the
initial stages of hot working internal cracking occurs and it is
to these internal cracks which dissolved hydrogen may diffuse and
subsequently cause blisters. Hydrogen is present in the metal
itself in dissociated or atomic form. Hydrogen in internal
defects will ccmbine to form molecular hydrogen, H2. Molecular
hydrogen is essentially insoluble in copper alloys and will not
diffuse through copper alloys. It is desirable to ho~ work more
than 50% since partial healing or bonding of these internal
;- cracks occurs. As lncreased deformation occurs, some of the
~; 20 defects heal as ~heir surfaces bond together. It is preferred
that the hot working reduction be from 75 to 95% since material
made with reductions of this order of magnitude has fewer internal
defects than material made with lower reduction. Complete healing
Or internal cracks is not possible because of the presence of
hydrogen within the defect which interferes with the complete
bonding of the internal crack surfaces. The final gauge after
hot working must be from 0.200 ~o 0.750" and is preferably from
0.300 to 0.550". The importance Or this requirement will be made
clear in a subsequent paragraph.
me hot worked strip is then annealed under conditions which
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will permit the diffusion of hydrogen from within the strip to
the surface of the strip and then to the surrounding environment.
; The temperature and metal thickness required are interrelated
such that the metal will not yield under the action of the
internal gas pressure, but rather will permit the hydrogen which
~s trapped in the defects to dissociate and diffuse out of the
metal. It is ~ost surprising that at the temperatures employed
the molecular hydrogen within the defects can dissociate to
permit its diffuslon out of the void through the metal and to the
surrounding environment. This is particularly unusual since at
the temperatures involved, hydrogen in the surrounding atmosphere
will not dissociate and thus cannot enter the metal. The annealing
temperature should fall within the range of .4 to .7 Tm where Tm
is the absolute melting point of the alloy. In the case of CDA
Alloy 638 the temperature range is approximately 450 to 650C.
Naturally, the time of the treatment must be selected so as to
permit the diffusion of the hydrogen out of the metal. The time
limitation is affected by the thickness of the strip which controls
the average diffusion distance for the hydrogen. It is ~urther
limited by the temperature of the treatment. In general, periods
froim 1 to 24 hours are appropriate. Increasing the strip
thickness requires longer diffusion times for the same temperature,
and for strlps of the same thickness longer times are required at
lower temperatures. It is important for the temperature range
contemplated that the strip be no thinner than 0.200" since thin
strips have less ability to resist the expansion of defects from
increased lnternal hydrogen pressure than do thick strips. It is
also important that the length of the diffusion anneal treatment
not be any longer than necessary since undesirable changes to the
metallurgical microstructure and properties of the alloy may occur.
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These undesirable changes include changes in the amount and
distribution of second phases, depletion of solute ~ements and/or
undesirable increases in grain size.
The efficacy of this di~fusion annealing treatment ls
lndependent of the furnace atmosphere employed since the atomic
hydrogen will recombine at the free surface of the metal and since
the molecular hydrogen in the atmosphere cannot dlffuse into the
alloy. Thus, either reducing~inert,or oxidizing environments are
allowable. It is preferred to use conventional reducing atmos-
pheres in order to minimize surface oxidation during this annealingstep.
After the diffusion annealing step the strip is cold rolled
at least 60% and preferably at least 75%. This cold rolling
operation serves to weld together the internal defects.
Reductions of less than 60% do not provide adequate bonding of
internal defect surfaces. However, if the strip is to be annealed
subsequent to this cold rolling step reductions as low as 40% may
be satisfactory. Such optional annealing may be carrled out at
temperatures o~ from .4 to .9 Tm for times of from 5 seconds to
24 hours. Opt~onally, bonding may also be obtained if the
rolling operation is performed at temperatures above room l -
temperature.
Following the cold rolling operation the strip may optionally
be annealed so as to obtain the desired mechanical properties
such as strength and ductllity. This an~ealing operation is
desirable in that lt will help to remove any vestige of the prior
internal defects. Following the optional annealing step, further
operations may be performed. If for example it ls desired to
have a ~inal product having mechanical properties which correspond
to those which result from 10% cold work, it would be necessary
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to anneal the material following the first cold rollin~ step and
then cold roll to 10% since the first cold rolling step must
lncorporate a higher amount of deformation.
Although the preceding discussion has been in terms of the
production of copper strip or sheet it will be appreciated that
the process of the present inventlon is equally applicable to
other material forms such as rod and wire. The process of the
present invention is applicable to all copper alloys in which
blistering occurs as a result of entrapped hydrogen.
o mis invention may be embodied in other forms~or carried
out in other ways without departing from the spirit or essential
characterlstics thereof. The present embodiment is therefore to
be considered as in all respects illustrative and not restrictive,
the scope of the invention being indicated by the appended claims
and all changes which come within the meaning and range of
equivalency are intended to be embraced therein.
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