Copper-zinc alloys (brass)
Brass is an alloy of the metals copper and zinc. The common compounds contain a zinc content of five to 45 percent. Beyond that, no usable alloys are produced. The colour spectrum ranges from golden red with a high copper content to light yellow with a high zinc content.
Copper and zinc mix optimally in the melt and remain evenly distributed in each other even during solidification. Brass is therefore a very homogeneous material. Although theoretically an infinite number of alloys between copper and zinc can be produced, in practice the number of brass grades is limited to a few dozen. The new Euronorms list about 60 grades. This makes it possible to produce largely all the desired physical, chemical and technological properties.
But not only the two basic metals are excellently soluble in each other. Numerous other elements such as aluminium, iron, manganese, nickel, silicon and tin can be added to the melt and thus new alloys with advantageous properties can be obtained. Brasses with such specific additives are called special brasses. Those types of brass that contain small amounts of lead as a third component for better machinability are also called free-cutting or machining brasses.
However, the uses are quite different. Brass products are often found, for example, in sanitary installations such as pipes, fittings or fixtures. Here it is the high corrosion resistance and stability of the material that is valued. This is also the reason why ship propellers are made of brass. But brass is also used as valves, bearings or pipes in power plants and mechanical engineering. Other areas of application are control, measurement and regulation technology, vehicle construction, precision mechanics or electrical engineering and electronics. In the latter two, the copper alloy is used for the production of terminal contacts, plug connections, waveguides or antennas, among other things, due to its good conductivity.
Classification of materials
The wrought alloys listed in tables 7, 8 and 9 of the standard DIN CEN/TS 13388 are divided into three groups according to DIN EN 1412:
- A: Wrought copper-zinc alloys without further alloying elements.
- B: Wrought copper-zinc alloys with lead and
- C: Wrought copper-zinc alloys with further alloying elements (multi-material alloys).
In group A, a distinction is made between alloys with a zinc content of up to 37 % and alloys with more than 37 % zinc. Alloys with less than 37 % zinc have a homogeneous α-structure. In alloys with more than 37 % zinc, β also appears as a second phase, which significantly changes the properties. The same classification is also common for group B. Up to 3.5 % lead is added to these alloys to improve machinability. Lead is insoluble in copper-zinc alloys. The lead inclusions act as chip breakers in the structure. In group C, the alloys contain additions of aluminium, tin, nickel, iron, silicon, manganese, etc. These additives more or less shift the phase boundaries of the copper-zinc system; they influence the structure and the properties. Above all, they serve to improve the strength as well as the sliding and wear properties and the corrosion resistance.
The pure (binary) copper-zinc alloys can be divided into three main groups based on their microstructure.
The first group of alloys up to approx. 37 % zinc has a uniform microstructure (α-phase) and crystallises in a face-centred cubic lattice.
The second group, characterised by approx. 37 to 46 % zinc, contains with the β-phase, which solidifies in a body-centred cubic lattice, an additional structural component of lower plasticity, whose share in the total structure increases with the zinc content (α+β-structure).
The third group of materials with approx. 46 to 50 % zinc again consists of a uniform structure (β-phase).
With even higher zinc contents, the γ-phase appears as a further microstructure component, whose extreme brittleness makes such alloys technically unusable.
The properties of the binary copper-zinc alloys change relatively evenly depending on the zinc content in the region of the α-mixed crystals. With higher zinc contents, on the other hand, one usually observes abrupt changes in properties with the appearance of β-mixed crystals in the structure. Apart from the zinc content, the properties are influenced by the content of other alloying elements. Suitable alloy variants are thus available for certain requirements.
Free-cutting brass (MS58)
For the so-called free-cutting brass, the term “MS58” is still widely used in practice. Actually, it is no longer permissible to still speak of MS58 today, as this quality no longer exists according to the standard. In DIN 17660, Aug. 1954 edition, the following composition of the then MS 58 is given:
- Cu 57-59.5; Pb 1-3; Zn rest; Fe 0.5, Sn 0.3, Al 0.1, Mn 0.2, Ni 0.5, Sb 0.02, other 0.2 max.
Although the materials to be used today fit into this analysis, a division into three alloy groups and initially five alloys was made in 1967, which was reduced in 1974 to the three alloys that still exist today. The differences between the materials CuZn39Pb2, CuZn39Pb3 and CuZn40Pb2 are derived from their specific applications. Each of these materials has been optimised with regard to specific processing methods:
- CuZn39Pb2 (CW612N according to EN, 2.0380 according to former DIN): good hot forming properties, limited cold forming properties (bending, riveting, flanging); good punchable drilling and milling quality;
- CuZn39Pb3 (CW614N according to EN, 2.0401 according to former DIN): good hot working properties; main alloy for machining on automatic machines;
- CuZn40Pb2 (CW617N acc. to EN, 2.0402 acc. to former DIN): good hot working properties, limited cold working properties; alloy for all machining processes and precisely drawn extrusions.
For drinking water applications, minor differences in the technological properties do not play a significant role. Moreover, regulations limit the number of materials that can be used, even within the above-mentioned group. In case of dispute, the designation MS58 can cause considerable problems. The so-called MS58 is a common material that is practically always available from stock. Therefore, customers are often advised to use this material, although a more precisely specified quality according to the applicable standards or another brass material that is less in demand would sometimes be more suitable.
Properties of wrought copper-zinc (brass) alloys
Physical properties (wrought alloys)
The density of pure copper at 20 °C is 8.93 g/cm3. This value decreases with increasing zinc content. The modulus of elasticity decreases slightly with the zinc content up to the limit of the α-region, and strongly in the (α+β)-region. A characteristic of copper-zinc alloys is their attractive colour. The copper colour changes with increasing zinc content through golden red at CuZn5, golden yellow at CuZn15 and greenish yellow at CuZn28 to a rich yellow hue at CuZn37. With the appearance of β-crystals in the two-phase (α+β)-copper-zinc alloys, the hue changes to reddish. However, it should be mentioned here that when estimating the composition on the basis of the colour, the addition of small amounts of other alloying elements can change it greatly. For example, small additions of aluminium to CuZn40Pb2 result in a greenish-yellow colour and of manganese in a brownish colour. This makes the copper-zinc alloys interesting for architecture and art.
Electrical properties (wrought alloys)
The electrical conductivity of α-brass drops with increasing zinc content to a value of about 15.5 MS/m, CuZn5 with a conductivity of still over 33 MS/m is a sought-after material for special applications in the field of electrical engineering. As the degree of cold forming increases, the electrical conductivity decreases.
Thermal properties (wrought alloys)
The thermal conductivity decreases with the zinc content and increases with the temperature. The linear coefficient of thermal expansion increases with the zinc content. The specific heat is almost independent of the copper content in the α-area with 0.377 to 0.390 J/g × K. The specific heat increases with the zinc content. In the (α+β)-region it increases with increasing zinc concentration.
Magnetic properties (wrought alloys)
Iron-free copper-zinc alloys are diamagnetic. The specific susceptibility of pure copper of -0.086 × 10-6 increases with the zinc content, down to -0.19 × 10-6 for CuZn43Pb2. It is temperature-dependent.
Mechanical properties at room temperature (wrought alloys)
The usual copper-zinc alloys cannot be age-hardened. Therefore, apart from alloy hardening, high hardness and strength characteristics can only be achieved by cold forming. With increasing zinc content – up to about 45 % Zn – tensile strength and Brinell hardness increase. The elongation at break reaches a maximum value at about 30 % Zn. CuZn30 is best for cold forming. CuZn37, the main alloy for cold forming in Germany for economic reasons, is, however, only slightly inferior to CuZn30 in cold forming capability. Certain alloying additions improve the mechanical properties of the copper-zinc alloys, and for some alloys also the wear and sliding properties. Tensile strength and hardness increase with the degree of cold forming, while elongation at break decreases. The tensile strength of wrought binary copper-zinc alloys as strip or sheet is between 230 and over 610 N/mm2, depending on the composition and the degree of cold forming, which determines the material condition, the Brinell hardness HB is between 45 and 180; the Vickers hardness HV is slightly higher than the Brinell hardness due to the measuring method.
CuZn37 is a good spring material (spring properties for strips and for wires. Single-phase α-copper-zinc alloys can be deep-drawn well. The deep drawing values for CuZn36, R300 (and CuZn37, R300 are between 11 – 14.3 mm depending on the sheet thickness (0.3 – 2 mm). The fatigue strength is usually determined as alternating strength. The fatigue strength increases with decreasing copper content. In the case of “multi-material alloys”, the fatigue strength of CuZn37Mn3Al2PbSi, for example, is between 170 N/mm2 in the pressed and 190 N/mm2 in the drawn state . The ratio of fatigue strength to tensile strength is between 0.26 and 0.33, which is within the usual range for copper materials.
Mechanical properties at elevated and low temperatures (wrought alloys)
Especially “multi-material alloys” still have good properties at elevated temperatures. The creep strength of copper-zinc alloys increases – at least at low temperatures – with decreasing copper content. Copper-zinc alloys do not become brittle at low temperatures. This enables their use as construction materials in the low-temperature range.
Properties of copper-zinc (brass) casting alloys
The copper-zinc casting alloys are classified according to their suitability as sand (GS), gravity die (GM), centrifugal (GZ), continuous casting (GC) and pressure die casting (GP). In the case of multi-component alloys for mould casting, a further distinction is made in practice between the alloys according to their brazing behaviour and mechanical properties; accordingly, a distinction is made between aluminium-free alloys suitable for soft and hard brazing and high-strength alloys containing aluminium.
Most of the physical properties such as density, conductivity and expansion of cast copper-zinc alloys are comparable to those of wrought alloys in the soft-annealed and recrystallised state.
Magnetic properties: Iron-free copper-zinc casting alloys are diamagnetic. The specific susceptibility of pure copper of -0.086 × 10-6 increases with the zinc content. It is temperature dependent.
Mechanical properties: The scale of tensile strength values here ranges up to 750 N/mm2. The casting process has a considerable influence, as a comparison with the characteristic values for sand casting shows.
Mechanical properties at elevated temperatures: Especially “multi-material alloys” still have good properties at elevated temperatures.
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