Selection and Application of Copper Alloy Castings

요약:

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.

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:

  1. promote directional solidification,
  2. avoid abrupt changes in cross section,
  3. avoid notches (by using generous fillets), and
  4. 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.

Mechanical Properties

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 resistance.

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