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 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.
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.
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.
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.
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.
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