Copper alloy castings are used in applications that require superior
corrosion resistance, high thermal or electrical conductivity, good
bearing-surface qualities or other special properties. Casting makes
it possible to produce parts whose shape cannot be easily obtained by
fabricating methods such as forming or machining.
Composition of copper casting alloys may differ from those of their
wrought counterparts for various reasons. Generally, casting permits
greater latitude in the use of alloying elements, because the effects
of composition on hot or cold working properties are not important.
Castability should not be confused with "fluidity", which is the ability
of a molten alloy to fill a mold cavity completely in every detail.
Castability, on the other hand, is the ease with which an alloy response
to ordinary foundry practice, without requiring special techniques for
gating, risering, melting, sand conditioning or any of the other factors
involved in making good castings.
High fluidity often ensures good castability, but it is not solely
responsible for that quality in a casting alloy.
Foundry alloys generally are classed as "high shrinkage" or "low shrinkage".
To the former class belong the manganese bronzes, aluminum bronzes, silicon
bronzes, silicon brasses and some nickel silvers. They are more fluid than
the low-shrinkage red brasses, more easily poured, and give high-grade
castings in sand, permanent mold, plaster, die and centrifugal casting
processes. With high-shrinkage alloys, careful design is necessary to:
- promote directional solidification,
- avoid abrupt changes in cross section,
- avoid notches (by using generous fillets), and
- properly place gates and risers,
all of which help avoid internal shrinks and draws.
Turbulent pouring must be avoided to prevent formation of dross. Liberal
use of risers or exothermic compound ensures adequate molten metal to
feed all sections of the casting.
All copper alloys can be successfully cast in sand. Sand casting is the
most economical casting method and allows the greatest flexibility in
casting size and shape.
Permanent mold casting is best suited for tin, silicon, aluminum and
manganese bronzes, and for yellow brasses.
Die-casting is well suited for yellow brasses, but increasing amounts
of permanent mold alloys are also being die cast. Size is a definite
limitation for both methods, although large slabs weighing as much as
4500 kg have been cast in permanent molds. Brass die-castings generally
weigh less than 0.2 kg. The limitation of size is due to reduced die
life with larger castings.
Virtually all copper alloys can be cast successfully by the centrifugal
casting process. Casting of virtually any size from less than 100 g to
more than 22 000 kg have been made.
Most copper-base casting alloys containing tin, lead or zinc have only
moderate tensile and yield strengths, low to medium hardness, and high
elongation. When higher tensile or yield strength is required, the
aluminum bronzes, manganese bronzes, silicon brasses, silicon bronzes
and some nickel silvers are used instead. Most of the higher strength
alloys have better-than-average resistance to corrosion and wear.
Properties of the castings themselves are almost always lower and depend
on section size and process variables. Tensile strengths for cast test
bars of aluminum bronzes and manganese bronzes range from 450 to 900 MPA,
depending on composition; aluminum bronzes attain maximum tensile strength
only after heat treatment.
Although manganese and aluminum bronzes often are used for the same
applications, the manganese bronzes are handled more easily in the
foundry. As-cast tensile strengths as high as 800 MPa and elongations
of 15 to 20% can be obtained readily in sand castings, and slightly
higher values in centrifugal castings. Stresses may be relieved at
175 to 200oC.
Lead may be added to the lower-strength manganese bronzes to increase
machinability, but at the expense of decreased tensile strength and
elongation. Lead content should not exceed 0,1% in high-strength manganese
bronzes. Although manganese bronzes range in hardness from 125 to 250 HB,
they are readily machined with proper tools.
Tin is added to the low-strength manganese bronzes to enhance resistance
to dezinfication, but should be limited to 0,1% in high-strength manganese
bronzes unless great sacrifices in strength and ductility can be accepted.
Manganese bronzes are specified for marine propellers and fittings,
pinions, ball-bearing races, worm wheels, gear-shift forks and
architectural work. Manganese bronzes also are used for rolling-mill
screwdown nuts and slippers, bridge trunnions, gears and bearings,
all of which require high strength and hardness.
Most aluminum bronzes contain from 0,75 to 4% Fe to refine grain
structure and increase strength. Alloys containing from 8 to 9,5% Al
cannot be heat-treated unless other elements (such as nickel or manganese)
in amounts over 2% are added as well. They have higher tensile strength
and greater ductility and toughness than any of the ordinary tin bronzes.
Applications include valve nuts, cam bearings, impellers, hangers in
pickling baths, agitators, crane gears and connecting rods.
The heat treatable aluminum bronzes contain from 9,5 to 11,5% Al, in
addition to iron, with or without nickel or manganese. These alloys
resemble heat-treated steels in structure and in response to quenching
and tempering; castings are quenched in water or oil from temperatures
between 760 and 925oC and tempered at 425 to 650oC depending
on exact composition and required properties.
Aluminum bronzes resist corrosion in many substances, including pickling
solutions. When corrosion occurs, it often proceeds by dealuminification,
a form of dealloying in which aluminum is lost preferentially. Duplex
alpha-plus-beta aluminum bronzes are more susceptible to dealloying
than the all-alpha aluminum bronzes.
Aluminum bronzes have a high fatigue limit, considerably greater than
that of manganese bronzes or any other cast copper alloy. Unlike those
of Cu-Zn and Cu-Sn-Pb-Zn alloys, the mechanical properties of aluminum
and manganese bronzes do not decrease much with increases in casting
cross section. This is because of a narrow freezing range, which
results in a denser structure when castings are properly designed
and properly fed.
Whereas manganese bronzes become hot short above 230oC, aluminum
bronzes can be used at temperatures as high as 400oC for short periods of
time without appreciable loss in strength.
Unlike manganese bronzes, many aluminum bronzes increase in yield strength
and hardness, but decrease in tensile strength and elongation, on slow
cooling in the mold. While some manganese bronzes precipitate a
relatively soft phase during slow cooling, aluminum bronzes
precipitate a hard constituent rather rapidly within the narrow
temperature range 565 to 480oC. Hence, large castings, or smaller
castings that are cooled slowly, will have properties different from
small castings cooled relatively rapidly. The same phenomenon occurs
on heat treating the hardenable aluminum bronzes. Cooling slowly through
the critical temperature range after quenching, or tempering at
temperatures within this range, will decrease elongation. Addition of
2 to 5% Ni greatly diminishes this effect.
Nickel brasses, silicon brasses and silicon bronzes, although generally
higher in strength than red metal alloys, are used more for their corrosion