Copper-Zinc Alloys (Brass CuZn)

Brass is an alloy composed of the metals copper and zinc.  The most commonly found brasses contain a zinc content from five to 45 percent.  If the zinc content is more than 45 percent, the resulting alloy is no longer usable.  The colour of brass ranges from golden red, indicating a higher percentage of copper, to light yellow, indicating a higher zinc content. 

Copper and zinc mix evenly during smelting and remain evenly distributed in solidification.  Thus, brass is an extremelly homogeneous material.  Theoretically, an unlimited number of different alloys can be created by changing the ratios of copper and zinc, but, in practice, the numbers of alloys are limited to a few dozen.  The newer European Standards list approximately 60 types of brass alloy.  Thus, the optimal physical, chemical and technological properties for any given need can be found. 

Zinc is not the only basic metal that mixes homogeneously with copper.  Aluminum, iron, manganese, nickel, silicon and tin smelted with copper also yield alloys with advantagous properties.  Brasses with additives are called special brass. Brasses that contains small amounts of lead as a third component for better machinability are referred to as chipping brass (Zehrspanungsmessinge).

 

Classification of Materials

Wrought alloys are classified into the three groups outlined in Tables 7, 8 and 9. The standards DIN CEN / TS 13388 are in accordance with DIN EN 1412:

A          Copper-zinc wrought alloys without additional other elements

B          Copper-zinc wrought alloys with lead and 

C          Copper-zinc wrought alloys with additional elements (complex or multi-material alloys).

In Group A the alloys are further classified into those with a zinc content of up to 37% and those with a zinc content of over 37%. The alloys in the under 37% category have a homogeneous alpha structure, whereas those with over 37% zinc enter into the beta phase. With a beta structure, the properties are considerably different, showing higher strengths and lower ductability at room temperature. 

The same classification is typical for Group B.  These alloys contain up to 3.5 % lead to improve the alloy's machinability.  Lead is practically insoluble in copper-zinc base alloys and thus appears in particals throughout the alloy.  This is particulary useful as a chip breaker.   

Group C contains additional elements such as aluminum, tin, nickel, iron, silicon, manganese, etc.  (see table for more details).  These additional elements shift the phase boundaries of the alloy and thus influence its structure and property.  Above all, they serve to improve strength, wear and anti-friction properties, and corrosion resistance.

Properties of Copper-Zinc (Brass) - Wrought Alloys

Physical Properties (wrought alloys)

Some important values of wrought alloys are summarized in the expandable table. 

The density of pure copper is 8,93 g/cm3 at 20°C.  This value decreases as the zinc content increases.

The modulus of elasticity decreases slightly with the zinc content to the limit of alpha region and then in the (alpha + beta) phase decreases sharply.

One of the features of copper-zinc alloys are the attractive colours.  The copper colour of the brasses change with the increase in zinc content, from golden red with CuZn5 to golden yellow with CuZn15 to greenish yellow with CuZn28 to a rich yellow tone with CuZn37.  With the appearance of beta-crystals in the second phase (α + β), the alloys display a reddish hue.

Here it is important to note that an estimation of the chemical composition based on colour is not easy because just a small change in the amount of zinc or other additional element can make a big difference in the alloy.  For example, small additions of aluminum to CuZn40Pb2 will result in a greenish-yellow colour while additions of manganese a brownish colour.  The rich variety in colours makes copper-zinc alloys and interesting material for archetecture and art.

Electrical Properties (wrought alloys)

As shown in figure 8, the electrical conductivity of alpha-brasses, decreasing with the increasing zinc content, can reach up to a value of approximately 15,5 MS/m.  CuZn5, with an electrical conductivity of just over 33 MS/m, is a material of choice for specialized applications in the electrical engineering industry.

The graph also illustrates the effect of temperature on the electrical conductivity of some alloys.

When cold-forming is performed on the material, the electrical conductivity decreases (Figure 9).

Bild 8: Elektrische Leitfähigkeit einiger Kupfer-Zink-Knetlegierungen im weichgeglühten Zustand bei Temperaturen von 20 bis 200 °C (DKI 1812) [9]
Bild 9: Einfluss des Kaltverformungsgrades auf die elektrische Leitfähigkeit einiger Kupfer-Zink-Knetlegierungen (DKI 1813) [9]

Thermal Properties (wrought alloys)

The thermal conductivity decreases with the zinc content and increases with temperature (Figure 10).

The linear coefficient of thermal expansion increases with the zinc content (Tab. 6).

The specific heat in the alpha region ranges from 0.377 to 0.390 J / g · K and is virtually independent of the copper content.  In the (alpha + beta) region, however, the specific heat increases with increased zinc concentration [1].

Magnetic Properties (wrought alloys)

Iron-free copper-zinc alloys are diamagnetic. The susceptibility of pure copper increases with the zinc content (from –0,086 × 10-6 ), and in CuZn43Pb2 is 0,19 × 10-6.  The susceptibility is dependent on temperature.  Values for the permeability of  standardized copper-zinc wrought alloys can be found in the expandable table.

Mechanical Properties at Room Temperature (wrought alloys)

The typical copper-zinc alloys are not curable (thermosetting). Thus, a higher hardness and strength can only be achieved through the cold-forming process.

The properties of copper-zinc alloys with relation to zinc content can be found in figure 11.  With increasing zinc content, up to approximately 45% zinc, the alloy's tensile strength and Brinell Hardness also increases. The breaking elongation is at its highest value when the alloy contains roughly 30% zinc. Thus, CuZn30 is ideal for cold-forming.

CuZn37, for economic reasons, is the main alloy used for cold-forming in Germany, although CuZn30 is nearly as suitable.  Certain additives improve the mechanical properties of copper-zinc alloys and in some cases an alloy is created with better wear and lubricating properties.  At cold-forming temperatures, the hardness and tensile strength of the alloy increases (Figure 12) while the breaking elongation point decreases.

The mechanical properties of copper-zinc wrought alloys, depending on the alloy's content, can be found in the expandable table.

The tensile strength of the binary copper-zinc wrought alloys as strip or sheet, depends on the composition and degree of cold forming.  A tensile strength falls between 230 and 610 N / mm2, with Brinell HB 45-180.  The Vickers hardness (HV) measurement scale requires slightly higher values than the Brinell hardness.

CuZn37 is a good material for springs (spring properties for tapes and wires). Single-phase α-copper-zinc alloys are suited to deep drawing.  The cupping test values for CuZn36, R300 (and CuZn37, R300, depending on the sheet thickness: 0.3 - 2 mm) are between between 11 to 14.3 mm.

CuZn37 ist ein guter Federwerkstoff (Federeigenschaften für Bänder und für Drähte. Einphasige α-Kupfer-Zink-Legierungen lässen sich gut tiefziehen. Die Tiefungswerte für CuZn36, R300 (und CuZn37, R300 liegen je nach Blechdicke (0,3 - 2 mm) zwischen 11 - 14,3 mm.

The fatigue strength usually determines the resistance to alternating strength.  With a decrease in copper content, the resistance to alternating strength is increased (Figure 13).  In multi-element alloys, such as CuZn37Mn3Al2PbSi, the fatigue strength is between 170 N/mm2 in a compressed state and 190 N/mm2 when in drawn temper[1]. The ratio of fatigue strength to tensile strength in copper materials is typically 0.26 to 0.33.

Mechanical Properties under higher and lower temperatures (wrought alloys)

Depending on the zinc content of the brass, heat resistance and hot elongation values at 20, 200, 300 and 400 ° C can be obtained from figure 14.

Multi-element alloys in particular have good properties at higher temperatures.  This is illustrated by the wrought alloy CuZn31Si1 in figure 15.

Copper-zinc alloy's creep strength increases (at least at lower temperatures) as the copper content decreases [11].

The properties of copper-zinc alloys in the presence of lower temperatures can be seen in figure 17 that uses the wrought alloy CuZn40Mn2Fe1 as an example.  Copper-zinc alloys do not become brittle at low temperatures.  This makes them a good material of choice for construction project made under low temperature conditions.

Properties of copper-Zinc (Brass)- casting alloys

The copper-zinc casting alloys are classified by their suitability with regard to sand (GS), casting die (GM), centrifuge (GZ), line section (GC), die casting (GP) methods.  With multi-element alloys a differentiation is made in practice between the material's solderability and mechanical properties, that is, a differentiation is made between aluminum free, soft and hard soldering alloys and high tensile alloys with aluminum content. 

Most physical properties of copper-zinc casting alloys such as density, conductivity and elongation are identical to those of the wrought alloys in both the annealed and recrystallized state.

Magnetic Properties

Copper-zinc casting alloys that are free of iron are diamagnetic.  The specific suseptability of pure copper is –0,086 × 10-6 and increases with an increased zinc content.  The suseptability is also dependent on temperature.  Values for the permeability of a standardized copper-zinc alloys are given in expandable table.

Mechanical Properties

The mechanical strength properties of copper-zinc casting alloys can be found in the exandable table.  The scale of the tensile strength reaches up to 750 N/mm2.

Die Festigkeitswerte der Kupfer-Zink-Gusslegierungen sind aus der Ausklapptabelle zu ersehen. Die Skala der Zugfestigkeitswerte reicht hier bis 750 N/mm2. Einen erheblichen Einfluss hat das Gießverfahren, wie ein Vergleich mit den Kennwerten für Sandguss in der Ausklapptabelle zeigt.

Mechanische Eigenschaften bei erhöhten Temperaturen 

Warmfestigkeits- und Warmdehnungskennwerte in Abhängigkeit vom Zinkgehalt  können Bild 14 entnommen werden.

Insbesondere „Mehrstofflegierungen“ haben bei erhöhten Temperaturen noch gute Eigenschaften. Das zeigt das Bild 18 am Beispiel der Legierung CuZn35Al1-C.

Literatur

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