Copper-Zinc Alloys: The Brasses

ทฤษฎี:

The copper alloys may be endowed with a wide range of properties by varying their composition and the mechanical and heat treatment to which they are subjected. For this reason they probably rank next to steel in importance to the engineer.

The important alloys of copper and zinc from an industrial point of view are the brasses comprised within certain limits of zinc content. The addition of zinc to copper results in the formation of a series of solid solutions which, in accordance with usual practice, are referred to in order of diminishing copper content as the á, â,a, etc., constituents.

The copper alloys may be endowed with a wide range of properties by varying their composition and the mechanical and heat treatment to which they are subjected. For this reason they probably rank next to steel in importance to the engineer.

The important alloys of copper and zinc from an industrial point of view are the brasses comprised within certain limits of zinc content. That portion of the constitutional diagram which refers to these alloys is given in the Figure 1.

Figure 1. Constitutional Diagram of the Copper-Zinc Alloys

The addition of zinc to copper results in the formation of a series of solid solutions which, in accordance with usual practice, are referred to in order of diminishing copper content as the a, b, g, etc., constituents. The diagram may be summarized as follows:

Percentage composition Constituent just below the freezing point Constituent after slow cooling to 400°C
Copper Zinc
100 to 67.5 0 to 32.5 a a
67.5 to 63 32.5 to 37 a + b a
63 to 61 37 to 39 b a
61 to 55.5 39 to 45.5 b a + b`
55.5 to 50 45.5 to 50 b b`
50 to 43.5 50 to 56.5 b b` + g
43.5 to 41 56.5 to 59 b + g b` + g

Further changes in composition of the a and b` phases below 400°C are only observed after prolonged annealing.

There is a certain connection between the properties and the microstructure which may be expressed in general terms.

The tensile strength increases with increase in zinc content, rises somewhat abruptly with the appearance of b, and reaches a maximum at a composition corresponding roughly to equal parts of a and b. It falls off rapidly at the appearance of the g constituent.

Elongation rises to a maximum and begins to fall again before the composition reaches the limit of the a solution. It falls considerably as the amount of b increases, and is very small in the presence of g.

The a constituent shows the greatest resistance to shock. This is diminished by the presence of b, and the alloy becomes extremely brittle when g is present.

Hardness is greatly increased by the presence of b and still further when g appears.

Alloys containing a phase only are specially suitable for cold working, and may be hot- or cold rolled. Those containing a and b will suffer very little deformation without rupture in the cold rolling and may only be hot rolled. The b constituent may also be forged, rolled or hot extruded, but alloys containing g should invariably be avoided for any mechanical treatment.

Designation system of brasses

The brasses of industrial importance are often designated by their copper and zinc content.

C 23000 - Red Brass (85 Cu, 15 Zn)

This alloy is used for ornaments and for cheap jewellery which is to be gilded: it withstands cold-work, cupping, etc. On account of the range of solidification, the cast material has a dendritic structure.

If cooled very slowly or annealed, diffusion takes place, yielding polyhedral grains of uniform composition. The process of diffusion is assisted by mechanical deformation of the grains by hot- or cold work followed by annealing. The changes which occur in rolling and annealing are similar to those described for 70:30 brass.

C 26000 - Cartridge Brass (70 Cu: 30 Zn)

This alloy, which is used widely for tubes, sheets and wires, also shows a dendritic structure of the a solid solution when chill fast. The b constituent does not begin to appear in the cast structure until the zinc exceeds 32% except in the presence of an additional element like aluminum or tin.

After annealing, the alloy consists of homogeneous solid solution, and it is specially suitable for cold-working. To withstand this treatment, especially drawing, it is necessary that the brass should be perfectly sound and free from impurities.

Since high grade 70:30 brass is usually made from the purest copper and zinc available without admixture of any but the cleanest scrap, these impurities are chiefly inclusions of dross (oxides or silicates) or charcoal. Such inclusions, if present, frequently lead to failure of the material during manufacture or in use. They become entrapped in the solidifying metal, either by splashing or by rapid solidification in moulds of small cross section.

It is a frequent procedure in casting brass to draw it into rod to employ very long moulds of very small cross section, in order to minimize subsequent mechanical treatment. Ingots made in such moulds are most liable to contain inclusions and to show piping to a great depth, resulting in central unsoundness over a considerable length of the ingot. To ensure soundness it is necessary to cast in a mould such that the cross section is large enough to give relatively slow cooling. The mould and stream of molten metal should be so arranged as to avoid splashing; the dimensions of the mould and speed of pouring should be such as to result in the ingot solidifying from bottom upwards.

The effect of cold-work on the microstructure is to break down the crystal grains by plastic deformation, and so crush them into confused debris. Annealing after cold-work results in recrystalization and subsequent crystal growth.

C 28000 - Muntz Metal (60 Cu: 40 Zn)

The molten metal begins to freeze at about 905°C, and dendrites of the b solution are formed. With sufficiently slow cooling through the range of solidification the alloy consists of homogeneous b constituent when just solid, but, on cooling, this solution retains less copper and at 770°C the a constituent separates from the homogeneous b and increases in amount as the temperature falls. The structure on reaching atmospheric temperature is therefore a mixture of a and b, the relative proportions of which may be controlled to some extent by the rate of cooling.

For example, a thin section of 60:40 brass quenched from 800°C consists of homogeneous b. With a larger section it is impossible to suppress completely the separation of a, but a specimen rapidly cooled from this temperature always contains more b than a specimen more slowly cooled. These microstructural characteristics are accompanied by changes in mechanical properties which can be deduced from the known hardness and brittleness of the b constituent and the softness and ductility of the a constituent.

Hot-rolled 60:40 brass, the rolling of which has been stopped above 700°C, shows a uniform structure in longitudinal and transverse directions. After the separation, the a and b constituents are each elongated in the direction of rolling, giving the normal structure of rolled 60:40 brass. The lower temperature of finishing, the smaller will be the grain size. If, however, rolling is continued much below 600°C, recrystalization does not keep pace with the deformation and the metal is cold-worked.

Brazing solder (50 Cu: 50 Zn)

This alloy, if cooled sufficiently slowly through the range of solidification, consists of homogeneous b solution, which, however, may decompose on cooling if the copper content is less than 50%. At atmospheric temperature the b solution will retain a maximum of just 50% of zinc if no impurities are present, but any content of zinc over 50% causes the separation of the g constituent, which increases in amount as the temperature falls. Its presence renders the alloy extremely hard and brittle.

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