1. Field of The Invention
2. Prior Art
[0010] The objects of the present inventions are achieved by provision of the following
copper alloys:
1. A free-cutting copper alloy with an excellent easy-to-cut feature which is composed
of 69 to 79 percent, by weight, of copper, 2.0 to 4.0 percent, by weight, of silicon,
0.02 to 0.4 percent, by weight, of lead and the remaining percent, by weight, of zinc.
For purpose of simplicity, this copper alloy will be hereinafter called the "first
invention alloy".
Lead forms no solid solution in the matrix but disperses in a granular form to improve
the machinability. Silicon raises the easy-to-cut property by producing a gamma phase
(in some cases, a kappa phase) in the structure of metal. That way, both are the same
in that they are effective in improving the machinability, though they are quite different
in contribution to the properties of the alloy. On the basis of that recognition,
silicon is added to the first invention alloy so as to bring about a high level of
machinability meeting the industrial requirements, while making it possible to reduce
greatly the lead content. That is, the first invention alloy is improved in machinability
through formation of a gamma phase with the addition of silicon.
The addition of less than 2.0 percent, by weight, of silicon can not form a gamma
phase sufficient enough to secure an industrially satisfactory machinability. With
the increase in the addition of silicon, the machinability improves. But with the
addition of more than 4.0 percent, by weight, of silicon, the machinability will not
go up in proportion. The problem is, however, that silicon is high in melting point
and low in specific gravity and also liable to oxidize. If silicon in a single form
is fed into the furnace in the melting step, silicon will float on the molten metal
and is oxidized into oxides of silicon or silicon oxide, hampering the production
a silicon-contained copper alloy. In producing the ingot of silicon-contained copper
alloy, therefore, silicon is usually added in the form of a Cu-Si alloy, which boosts
the production cost. In the light of the cost of making the alloy, too, it is not
desirable to add silicon in a quantity exceeding the saturation point or plateau of
machinability improvement - 4.0 percent by weight. An experiment showed that when
silicon is added in the amount of 2.0 to 4.0 percent, by weight, it is desirable to
hold the content of copper at 69 to 79 percent, by weight, in consideration of its
relation to the content of zinc in order to maintain the intrinsic properties of the
Cu-Zn alloy. For this reason, the first invention alloy is composed of 69 to 79 percent,
by weight, of copper and 2.0 to 4.0 percent, by weight, of silicon respectively. The
addition of silicon improves not only the machinability but also the flow of the molten
metal in casting, strength, wear resistance, resistance to stress corrosion cracking,
high-temperature oxidation resistance. Also, the ductility and dezincing corrosion
resistance will be improved to some extent.
The addition of lead is set at 0.02 to 0.4 percent by weight on this ground. In the
first invention alloy, a sufficient level of machinability is obtained by adding silicon
that has the aforesaid effect even if the addition of lead is reduced. Yet, lead has
to be added in the amount not smaller than 0.02 percent by weight if the alloy is
to be superior to the conventional free-cutting copper alloy in machinability, while
the addition of lead exceeding 0.4 percent would have adverse effects, resulting in
a rough surface condition, poor hot workability such as poor forging behaviour and
low cold ductility. Meanwhile, it is expected that such a small content of not higher
than 0.4 percent by weight will be able to clear the lead-related regulations however
strictly they are to be stipulated in the advanced nations including Japan in the
future. On that ground, the addition range of lead is set at 0.02 to 0.4 percent by
weight in the first and also second to eleventh invention alloys which will be described
later.
2. Another embodiment of the present invention is a free-cutting copper alloy also
with an excellent easy-to-cut feature which is composed of 69 to 79 percent, by weight,
of copper; 2.0 to 4.0 percent, by weight, of silicon; 0.02 to 0.4 percent, by weight,
of lead; one selected from among 0.02 to 0.4 percent, by weight, of bismuth, 0.02
to 0.4 percent, by weight, of tellurium, and 0.02 to 0.4 percent, by weight, of selenium;
and the remaining percent, by weight, of zinc. This second copper alloy will be hereinafter
called the "second invention alloy".
That is, the second invention alloy is composed of the first invention alloy and,
in addition, one selected element from among 0.02 to 0.4 percent, by weight, of bismuth,
0.02 to 0.4 percent, by weight, of tellurium, and 0.02 to 0.4 percent, by weight,
of selenium.
Bismuth, tellurium and selenium as well as lead do not form a solid solution with
the matrix but disperse in granular form to enhance the machinability. That makes
up for the reduction of the lead content. The addition of any one of those elements
along with silicon and lead could further improve the machinability beyond the level
hoped from the addition of silicon and lead. From this finding, the second invention
alloy is worked out in which one element selected from among bismuth, tellurium and
selenium is mixed. The addition of bismuth, tellurium or selenium as well as silicon
and lead could make the copper alloy so machinable that complicated forms could be
freely cut out at a high speed. But no improvement in machinability can be realized
from the addition of bismuth, tellurium or selenium in the amount less than 0.02 percent,
by weight. Meanwhile, those elements are expensive as compared with copper. Even if
the addition exceeds 0.4 percent by weight, the proportional improvement in machinability
is so small that the addition beyond that does not pay off economically. What is more,
if the addition is more than 0.4 percent by weight, the alloy will deteriorate in
hot workability such as forgeability and cold workability such as ductility. While
it might be feared that heavy metals like bismuth would cause a problem similar to
that of lead, a very small addition of less than 0.4 percent by weight is negligible
and would present no particular problems. From those considerations, the second invention
alloy is prepared with the addition of bismuth, tellurium or selenium kept to 0.02
to 0.4 percent by weight. In this regard, it is desired to keep the combined content
of lead and bismuth, tellurium or selenium to not higher than 0.4 percent by weight.
That is because if the combined content exceeds 0.4 percent by weight, if slightly,
then there will begin a deterioration in hot workability and cold ductility and also
there is fear that the form of chippings will change from (B) to (A) in Fig. 1. But
the addition of bismuth, tellurium or selenium, which improves the machinability of
the copper alloy though a mechanism different from that of silicon as mentioned above,
would not affect the proper contents of copper and silicon. On this ground, the contents
of copper and silicon in the second invention alloy are set at the same level as those
in the first invention alloy.
3. Another embodiment of the present invention is a free-cutting copper alloy also
with an excellent easy-to-cut feature which is composed of 70 to 80 percent, by weight,
of copper; 1.8 to 3.5 percent, by weight, of silicon; 0.02 to 0.4 percent, by weight,
of lead; at least one selected from among 0.3 to 3.5 percent, by weight, of tin, 1.0
to 3.5 percent, by weight, of aluminum, and 0.02 to 0.25 percent, by weight, of phosphorus;
and the remaining percent, by weight, of zinc. This third copper alloy will be hereinafter
called the "third invention alloy".
Tin works the same way as silicon. That is, if tin is added, a gamma phase will be
formed and the machinability of the Cu-Zn alloy will be improved. For example, the
addition of tin in the amount of 1.8 to 4.0 percent by weight would bring about a
high machinability in the Cu-Zn alloy containing 58 to 70 percent, by weight, of copper,
even if silicon is not present. Therefore, the addition of tin to the Cu-Si-Zn alloy
could facilitate the formation of a gamma phase and further improve the machinability
of the Cu-Si-Zn alloy. The gamma phase is formed with the addition of tin in the amount
of 1.0 or more percent by weight and the formation reaches the saturation point at
3.5 percent, by weight, of tin. If tin exceeds 3.5 percent by weight, the ductility
will drop instead. With the addition of tin in the amount less than 1.0 percent by
weight, on the other hand, an insufficient gamma phase will be formed. If the addition
is 0.3 or more percent by weight, then tin will be effective in uniformly dispersing
the gamma phase formed by silicon. Through that effect of dispersing the gamma phase,
too, the machinability is improved. In other words, the addition of tin in the amount
not smaller than 0.3 percent by weight improves the machinability.
Aluminum is, too, effective in facilitating the formation of the gamma phase. The
addition of aluminum together with or in place of tin could further improve the machinability
of the Cu-Si-Zn alloy. Aluminum is also effective in improving the strength, wear
resistance and high-temperature oxidation resistance as well, as the machinability
and also in keeping down the specific gravity. If the machinability is to be improved
at all, aluminum will have to be added in the amount of at least 1.0 percent by weight.
But the addition of more than 3.5 percent by weight could not produce the proportional
results. Instead, that could lower the ductility as is the case with tin.
As to phosphorus, it has no property of forming the gamma phase as tin and aluminum.
But phosphorus works to uniformly disperse and distribute the gamma phase formed as
a result of the addition of silicon alone or with tin or aluminum or both of them.
That way, the machinability improvement through the formation of gamma phase is further
enhanced. In addition to dispersing the gamma phase, phosphorus helps refine the crystal
grains in the alpha phase in the matrix, improving hot workability and also strength
and resistance to stress corrosion cracking. Furthermore, phosphorus substantially
increases the flow of molten metal in casting. To produce such results, phosphorus
will have to be added in the amount not smaller than 0.02 percent by weight. But if
the addition exceeds 0.25 percent by weight, no proportional effect can be obtained.
Instead, there would be a fall in hot forging property and extrudability.
In consideration of those observations, the third invention alloy is improved in machinability
by adding to the Cu-Si-Pb-Zn alloy (first invention alloy) at least one selected from
among 0.3 to 3.5 percent, by weight, of tin, 1.0 to 3.5 percent, by weight, of aluminum,
and 0.02 to 0.25 percent, by weight, of phosphorus.
Meanwhile, tin, aluminum and phosphorus are to improve the machinability by forming
a gamma phase or dispersing that phase, and work closely with silicon in promoting
the improvement in machinability through the gamma phase. In the third invention alloy
to which silicon is added along with tin, aluminum or phosphorus, the addition of
silicon is smaller than that in the second invention alloy to which is added bismuth,
tellurium or selenium which replaces silicon of the first invention in improving machinability.
That is, those elements bismuth, tellurium and selenium contribute to improving the
machinability, not acting on the gamma phase but dispersing in the form of grains
in the matrix. Even if the addition of silicon is less than 2.0 percent by weight,
silicon along with tin, aluminum or phosphorus will be able to enhance the machinability
to an industrially satisfactory level as long as the percentage of silicon is 1.8
or more percent by weight. But even if the addition of silicon is not larger than
4.0 percent by weight, adding of tin, aluminum or phosphorus together will silicon
will saturate the effect of silicon in improving the machinability, when the silicon
content exceeds 3.5 percent by weight. On this ground, the addition of silicon is
set at 1.8 to 3.5 percent by weight in the third invention alloy. Also, in consideration
of the addition amount of silicon and also the addition of tin, aluminum or phosphorus,
the content range of copper in this third invention alloy is slightly raised from
the level in the second invention alloy and copper is properly set at 70 to 80 percent
by weight.
4. A free-cutting copper alloy also with an excellent easy-to-cut feature which is
composed of 70 to 80 percent, by weight, of copper; 1.8 to 3.5 percent, by weight,
of silicon; 0.02 to 0.4 percent, by weight, of lead; at least one element selected
from among 0.3 to 3.5 percent, by weight, of tin, 1.0 to 3.5 percent, by weight, of
aluminum, and 0.02 to 0.25 percent, by weight, of phosphorus; one element selected
from among 0.02 to 0.4 percent, by weight, of bismuth, 0.02 to 0.4 percent, by weight,
of tellurium, and 0.02 to 0.4 percent, by weight, of selenium; and the remaining percent,
by weight, of zinc. This fourth copper alloy will be hereinafter called the "fourth
invention alloy".
The fourth invention alloy has any one selected from among 0.02 to 0.4 percent, by
weight, of bismuth, 0.02 to 0.4 percent, by weight, of tellurium, and 0.02 to 0.4
percent, by weight, of selenium in addition to the components in the third invention
alloy. The grounds for mixing those additional elements and setting those amounts
to be added are the same as given for the second invention alloy.
5. A free-cutting copper alloy with an excellent easy-to-cut feature and with a high
corrosion resistance which is composed of 69 to 79 percent, by weight, of copper;
2.0 to 4.0 percent, by weight, of silicon; 0.02 to 0.4 percent, by weight, of lead;
at least one element selected from among 0.3 to 3.5 percent, by weight, of tin, 0.02
to 0.25 percent, by weight, of phosphorus, 0.02 to 0.15 percent, by weight, of antimony,
and 0.02 to 0.15 percent, by weight, of arsenic, and the remaining percent, by weight,
of zinc. This fifth copper alloy will be hereinafter called the "fifth invention alloy".
The fifth invention alloy has, in addition to the first invention alloy, at least
one element selected from among 0.3 to 3.5 percent, by weight, of tin, 0.02 to 0.25
percent, by weight, of phosphorus, 0.02 to 0.15 percent, by weight, of antimony, and
0.02 to 0.15 percent, by weight, of arsenic.
Tin is effective in improving not only the machinability but also corrosion resistance
properties (dezincification corrosion resistance) and forgeability. In other words,
tin improves the corrosion resistance in the alpha phase matrix and, by dispersing
the gamma phase, the corrosion resistance, forgeability and stress corrosion cracking
resistance. The fifth invention alloy is thus improved in corrosion resistance by
the property of tin and in machinability mainly by adding silicon. Therefore, the
contents of silicon and copper in this alloy are set at the same as those in the first
invention alloy. To raise the corrosion resistance and forgeability, on the other
hand, tin would have to be added in the amount of at least 0.3 percent by weight.
But even if the addition of tin exceeds 3.5 percent by weight, the corrosion resistance
and forgeability will not improve in proportion to the amount added of tin. It is
no good economy.
As described above, phosphorus disperses the gamma phase uniformly and at the same
time refines the crystal grains in the alpha phase in the matrix, thereby improving
the machinability and also the corrosion resistance properties (dezincification corrosion),
forgeability, stress corrosion cracking resistance and mechanical strength. The fifth
invention alloy is thus improved in corrosion resistance and others through the action
of phosphorus and in machinability mainly by adding silicon. The addition of phosphorus
in a very small quantity, that is, 0.02 or more percent by weight could produce results.
But the addition in more than 0.25 percent by weight would not be so effective as
hoped from the quantity added. Rather, that would reduce the hot forgeability and
extrudability.
Just as phosphorus, antimony and arsenic in a very small quantity - 0.02 or more percent
by weight - are effective in improving the dezincification corrosion resistance and
other properties. But the addition exceeding 0.15 percent by weight would not produce
results in proportion to the quantity added. Rather, it would affect the hot forgeability
and extrudability as phosphorus applied in excessive amounts.
Those observations indicate that the fifth invention alloy is improved in machinability
and also corrosion resistance and other properties by adding at least one element
selected from among tin, phosphorus, antimony and arsenic (which improve corrosion
resistance) in quantities within the aforesaid limits in addition to the same quantities
of copper and silicon as in the first invention copper alloy. In the fifth invention
alloy, the additions of copper and silicon are set at 69 to 79 percent by weight and
2.0 to 4.0 percent by weight respectively - the same level as in the first invention
alloy in which any other machinability improver than silicon and a small amount of
lead is not added - because tin and phosphorus work mainly as corrosion resistance
improver like antimony and arsenic.
6. A free-cutting copper alloy also with an excellent easy-to-cut feature and with
a high corrosion resistance which is composed of 69 to 79 percent, by weight, of copper;
2.0 to 4.0 percent, by weight, of silicon; 0.02 to 0.4 percent, by weight, of lead;
at least one element selected from among 0.3 to 3.5 percent, by weight, of tin, 0.02
to 0.25 percent, by weight, of phosphorus, 0.02 to 0.15 percent, by weight, of antimony,
and 0.02 to 0.15 percent, by weight, of arsenic; one element selected from among 0.02
to 0.4 percent, by weight, of bismuth, 0.02 to 0.4 percent, by weight, of tellurium,
and 0.02 to 0.4 percent, by weight, of selenium; and the remaining percent, by weight,
of zinc. This sixth copper alloy will be hereinafter called the "sixth invention alloy".
The sixth invention alloy has any one element selected from among 0.02 to 0.4 percent,
by weight, of bismuth, 0.02 to 0.4 percent, by weight, of tellurium, and 0.02 to 0.4
percent, by weight, of selenium in addition to the components in the fifth invention
alloy. The machinability is improved by adding, in addition to silicon and lead, any
one element selected from among bismuth, tellurium and selenium as in the second invention
alloy and the corrosion resistance and other properties are raised by adding at least
one selected from among tin, phosphorus, antimony and arsenic as in the fifth invention
alloy. Therefore, the additions of copper, silicon, bismuth, tellurium and selenium
are set at the same levels as those in the second invention alloy, while the additions
of tin, phosphorus, antimony and arsenic are adjusted to those in the fifth invention
alloy.
7. A free-cutting copper alloy also with an excellent easy-to-cut feature and with
an excellent high strength feature and high corrosion resistance which is composed
of 62 to 78 percent, by weight, of copper; 2.5 to 4.5 percent, by weight, of silicon;
0.02 to 0.4 percent, by weight, of lead; at least one element selected from among
0.3 to 3.0 percent, by weight, of tin, 0.2 to 2.5 percent, by weight, of aluminum,
and 0.02 to 0.25 percent, by weight, of phosphorus; and at least one element selected
from among 0.7 to 3.5 percent, by weight, of manganese and 0.7 to 3.5 percent, by
weight, of nickel; and the remaining percent, by weight, of zinc. The seventh copper
alloy will be hereinafter called the "seventh invention alloy".
Manganese and nickel combine with silicon to form intermetallic compounds represented
by MnxSiy or NixSiy which are evenly precipitated in the matrix, thereby raising the
wear resistance and strength. Therefore, the addition of manganese and nickel or either
of the two would improve the high strength feature and wear resistance. Such effects
will be exhibited if manganese and nickel are added in the amount of not less than
0.7 percent by weight respectively. But the saturation state is reached at 3.5 percent
by weight, and even if the addition is increased beyond that, no proportional results
will be obtained. The addition of silicon is set at 2.5 to 4.5 percent by weight to
match the addition of manganese or nickel, taking into consideration the consumption
to form intermetallic compounds with those elements.
It is also noted that tin, aluminum and phosphorus help to reinforce the alpha phase
in the matrix, thereby improving the machinability. Tin and phosphorus disperse the
alpha and gamma phases, by which the strength, wear resistance and also machinability
are improved. Tin in the amount of 0.3 or more percent by weight is effective in improving
the strength and machinability. But if the addition exceeds 3.0 percent by weight,
the ductility will fall. For this reason, the addition of tin is set at 0.3 to 3.0
percent by weight to raise the high strength feature and wear resistance in the seventh
invention alloy and also to enhance the machinability. Aluminum also contributes to
improving the wear resistance and exhibits its effect of reinforcing the matrix when
added in the amount of 0.2 or more percent by weight. But if the addition exceeds
2.5 percent by weight, there will be a fall in ductility. Therefore, the addition
of aluminum is set at 0.2 to 2.5 in consideration of improvement of machinability.
Also, the addition of phosphorus disperses the gamma phase and at the same time pulverizes
the crystal grains in the alpha phase in the matrix, thereby improving the hot workability
and also the strength and wear resistance. Furthermore, it is very effective in improving
the flow of molten metal in casting. Such results will be produced when phosphorus
is added in the amount of 0.02 to 0.25 percent by weight. The content of copper is
set at 62 to 78 percent by weight in the light of the addition of silicon and the
property of manganese and nickel of combining with silicon.
8. A free-cutting copper alloy also with an excellent easy-to-cut feature and with
an excellent high-temperature oxidation resistance which comprises 69 to 79 percent,
by weight, of copper, 2.0 to 4.0 percent, by weight, of silicon, 0.02 to 0.4 percent,
by weight, of lead, 0.1 to 1.5 percent, by weight, of aluminum, and 0.02 to 0.25 percent,
by weight, of phosphorus, and the remaining percent, by weight, of zinc. The eighth
copper alloy will be hereinafter called the "eighth invention alloy".
Aluminum is an element which improves the strength, machinability, wear resistance
and also high-temperature oxidation resistance. Silicon, too, has a property of enhancing
the machinability, strength, wear resistance, resistance to stress corrosion cracking
and also high-temperature oxidation resistance. Aluminum works to raise the high-temperature
oxidation resistance when it is used together with silicon and that in not smaller
than 0.1 percent by weight. But even if the addition of aluminum increases beyond
1.5 percent by weight, no proportional results can be expected. For this reason, the
addition of aluminum is set at 0.1 to 1.5 percent by weight.
Phosphorus is added to enhance the flow of molten metal in casting. Phosphorus also
works for improvement of the aforesaid machinability, dezincification corrosion resistance
and also high-temperature oxidation resistance in addition to the flow of molten metal.
Those effects are exhibited when phosphorus is added in the amount not smaller than
0.02 percent by weight. But even if phosphorus is used in more than 0.25 percent by
weight, it will not result in a proportional increase in effect rather weakening the
alloy. For this consideration, the addition of phosphorus settles down on 0.02 to
0.25 percent by weight.
While silicon is added to improve the machinability as mentioned above, it is also
capable of improving the flow of molten metal like phosphorus. The effect of silicon
in improving the flow of molten metal is exhibited when it is added in the amount
of not smaller than 2.0 percent by weight. The range of the addition for the flow
improvement overlaps that for improvement of the machinability. These taken into consideration,
the addition of silicon is set to 2.0 to 4.0 percent by weight.
9. A free-cutting copper alloy also with excellent easy-to-cut feature coupled with
a good high-temperature oxidation resistance which is composed of 69 to 79 percent,
by weight, of copper; 2.0 to 4.0 percent, by weight, of silicon; 0.02 to 0.4 percent,
by weight, of lead; 0.1 to 1.5 percent, by weight, of aluminum; 0.02 to 0.25 percent,
by weight, of phosphorus; one element selected from among 0.02 to 0.4 percent, by
weight, of bismuth, 0.02 to 0.4 percent, by weight, of tellurium, and 0.02 to 0.4
percent, by weight, of selenium; and the remaining percent, by weight, of zinc. The
ninth copper alloy will be hereinafter called the "ninth invention alloy".
The ninth invention alloy contains one element selected from among 0.02 to 0.4 percent,
by weight, of bismuth, 0.02 to 0.4 percent, by weight, of tellurium and 0.02 to 0.4
percent, by weight, of selenium in addition to the components of the eighth invention
alloy. While a high-temperature oxidation resistance as good as in the eighth invention
alloy is secured, the machinability is further improved by adding one element selected
from among bismuth and other elements which are as effective as lead in raising the
machinability.
10. A free-cutting copper alloy also with excellent easy-to-cut feature and a good
high-temperature oxidation resistance which is composed of 69 to 79 percent, by weight,
of copper; 2.0 to 4.0 percent, by weight, of silicon; 0.02 to 0.4 percent, by weight,
of lead; 0.1 to 1.5 percent, by weight, of aluminum; 0.02 to 0.25 percent, by weight,
of phosphorus; at least one selected from among 0.02 to 0.4 percent, by weight, of
chromium and 0.02 to 0.4 percent, by weight, of titanium; and the remaining percent,
by weight, of zinc. The tenth copper alloy will be hereinafter called the "tenth invention
alloy".
Chromium and titanium are intended for improving the high-temperature oxidation resistance.
Good results can be expected especially when they are added together with aluminum
to produce a synergistic effect. Those effects are exhibited when the addition is
no less than 0.02 percent by weight, whether they are added alone or in combination.
The saturation point is 0.4 percent by weight. For consideration of such observations,
the tenth invention alloy has at least one element selected from among 0.02 to 0.4
percent by weight of chromium and 0.02 to 0.4 percent by weight of titanium in addition
to the components of the eighth invention alloy and thus further improved over the
eighth invention alloy with regard to the high-temperature oxidation resistance.
11. A free-cutting copper alloy also with excellent easy-to-cut feature and a good
high-temperature oxidation resistance which is composed of 69 to 79 percent, by weight,
of copper; 2.0 to 4.0 percent, by weight, of silicon; 0.02 to 0.4 percent, by weight,
of lead; 0.1 to 1.5 percent, by weight, of aluminum; 0.02 to 0.25 percent, by weight,
of phosphorus; at least one element selected from among 0.02 to 0.4 percent, by weight,
of chromium and 0.02 to 0.4 percent, by weight, of titanium; one element selected
from among 0.02 to 0.4 percent, by weight, of bismuth, 0.02 to 0.4 percent, by weight,
of tellurium and 0.02 to 0.4 percent, by weight, of selenium; and the remaining percent,
by weight, of zinc. The eleventh copper alloy will be hereinafter called the "eleventh
invention alloy".
The eleventh invention alloy contains any one selected from among 0.02 to 0.4 percent,
by weight, of bismuth, 0.02 to 0.4 percent, by weight, of tellurium and 0.02 to 0.4
percent, by weight, of selenium in addition to the components of the tenth invention
alloy. While as high a high-temperature oxidation resistance as in the tenth invention
alloy is secured, the eleventh invention alloy is further improved in machinability
by adding one element selected from among bismuth and other elements which are as
effective as lead in raising the machinability.
12. A free-cutting copper alloy also with further improved easy-to-cut feature obtained
by subjecting any one of the preceding respective invention alloys to a heat treatment
for 30 minutes to 5 hours at 400 to 600°C. The twelfth copper alloy will be hereinafter
called the "twelfth invention alloy".
The first to eleventh invention alloys contain machinability improving elements such
as silicon and have an excellent machinability because of the addition of such elements.
The effect of those machinability improving elements could be further enhanced by
heat treatment. For example, the first to eleventh invention alloys which are high
in copper content with gamma phase in small quantities and kappa phase in large quantities
undergo a change in phase from the kappa phase to the gamma phase in a heat treatment.
As a result, the gamma phase is finely dispersed and precipitated, and the machinability
is improved. In the manufacturing process of castings, expanded metals and hot forgings
in practice, the materials are often force-air-cooled or water cooled depending on
the forging conditions, productivity after hot working (hot extrusion, hot forging
etc.), working environment and other factors. In such cases, with the first to eleventh
invention alloys, the alloys with a low content of copper in particular are rather
low in the content of the gamma phase and contain beta phase. In a heat treatment,
the beta phase changes into gamma phase, and the gamma phase is finely dispersed and
precipitated, whereby the machinability is improved.
But a heat treatment temperature at less than 400°C is not economical and practical
in any case, because the aforesaid phase change will proceed slowly and much time
will be needed. At temperatures over 600°C, on the other hand, the kappa phase will
grow or the beta phase will appear, bringing about no improvement in machinability.
From the practical viewpoint, therefore, it is desired to perform the heat treatment
for 30 minutes to 5 hours at 400 to 600°C.