Room temperature organization
Ordinary brass is a copper-zinc binary alloy, its zinc content varies widely, so its room temperature organization is also very different. According to the Cu-Zn binary state diagram, there are three kinds of room temperature organization of brass: brass with zinc content below 35%, the microstructure at room temperature consists of single-phase α-solid solution, known as α-brass; brass with zinc content in the range of 36% to 46%, the microstructure at room temperature consists of (α + β) two phases, known as (α + β) brass (two-phase brass); zinc content More than 46% to 50% of the brass, the microstructure at room temperature consists of only β phase, known as β brass.
Pressure processing performance
α single-phase brass (from H96 to H65) has good plasticity and can withstand hot and cold processing, but α single-phase brass is prone to medium-temperature embrittlement during hot processing such as forging, and its specific temperature range varies with the amount of Zn, generally between 200 and 700℃. Therefore, the temperature during hot working should be higher than 700℃. Single-phase α brass in the brittle zone is mainly due to the presence of Cu-Zn alloy system in the α-phase area of Cu3Zn and Cu9Zn two ordered compounds, in the low temperature heating occurs when the order of the transformation, so that the alloy becomes brittle; In addition, the alloy exists in the presence of trace amounts of lead, bismuth, harmful impurities and copper to form a low-melting-point eutectic film distributed in the grain boundaries, hot working intergranular rupture. Practice shows that the addition of trace cerium can effectively eliminate the medium temperature embrittlement.
Two phase brass (from H63 to H59), the alloy organization in addition to good plasticity of the α-phase, but also appeared by the electronic compound CuZn-based β solid solution. β-phase at high temperatures has a high degree of plasticity, and low temperatures β′-phase (ordered solid solution) nature of the hard brittle. Therefore (α + β) brass should be forged in the hot state. β brass with zinc content greater than 46% to 50% cannot be pressure machined because of its hard and brittle properties.
Mechanical properties
The mechanical properties of brass are different due to the different zinc content, and the mechanical properties of brass change with the different zinc content. For α brass, both σb and δ keep increasing with the increase of zinc content. For (α+β) brass, the room temperature strength increases until the zinc content is increased to about 45%. If the zinc content is further increased, the strength decreases sharply due to the appearance of the more brittle r-phase (solid solution based on the Cu5Zn8 compound) in the alloy organization. (The room temperature plasticity of (α+β) brasses always decreases with increasing zinc content. So the zinc content of more than 45% of the copper-zinc alloy has no practical value.
The use of ordinary brass is extremely wide such as water tank band, water supply and drainage pipes, medallions, bellows, serpentine tubes, condenser tubes, bullet casings and a variety of complex shapes of stamping products, small hardware, and so on. With the increase of zinc content from H63 to H59, they can well withstand the hot state processing, mostly used in machinery and electrical appliances, various parts, stamping parts and musical instruments.
In order to improve the corrosion resistance, strength, hardness and cutting brass, etc., in the copper – zinc alloy by adding a small amount (generally 1% to 2%, a small number of up to 3% to 4%, a very few up to 5% to 6%) tin, aluminum, manganese, iron, silicon, nickel, lead and other elements, constituting a ternary, quaternary, or even quintuple alloy, that is, for the complex brass, also known as special brass.
Zinc equivalent factor
The organization of complex brass can be deduced from the “zinc equivalent factor” of the elements added to the brass. Because in the copper-zinc alloy to add a small amount of other alloying elements, usually only to make the Cu-Zn state diagram of the α / (α + β) phase area to the left or right. So the organization of special brass is usually equivalent to that of ordinary brass with increased or decreased zinc content. For example, the organization of Cu-Zn alloy with 1% silicon is equivalent to the organization of Cu-Zn alloy with 10% zinc. Therefore, the “zinc equivalent” of silicon is 10, and the “zinc equivalent coefficient” of silicon is the largest, so that the α/(α + β) phase boundary in the Cu-Zn system is significantly shifted to the copper side, i.e., the α-phase region is strongly narrowed. The “zinc equivalence factor” of nickel is negative, i.e. the α-phase region is enlarged.
Special brass in the α-phase and β-phase is a multi-complex solid solution, its strengthening effect is larger, while the α and β-phase in the ordinary brass is a simple Cu-Zn solid solution, its strengthening effect is lower. Although the zinc equivalent is comparable, the nature of multivariate solid solution and simple binary solid solution is not the same. Therefore, a small amount of multiple strengthening is a way to improve the performance of the alloy.
