Copper-nickel-zinc alloys

The properties and processing of copper-nickel-zinc alloys are significantly influenced by the main alloying elements copper, nickel and zinc as well as by other additional elements such as lead, manganese and tin.

As a base metal, copper is decisive for toughness. It facilitates cold forming. Nickel improves tarnish resistance, especially at elevated temperatures, and corrosion resistance. It increases the modulus of elasticity and electrical resistance. With increasing nickel content, the melting range is shifted to higher temperatures. Zinc contributes to the strengthening of the alloys and improves the hot workability, but reduces the corrosion resistance to aggressive media. Increasing zinc content reduces the melting range. Lead makes the wrought alloys easier to machine, but reduces the toughness and increases the sensitivity to hot cracking during annealing. The hot formability of the oc alloys is strongly impaired by lead, so that they are usually only formed cold. In contrast, the good hot formability of the (oc + [š)~alloys is not significantly affected by lead. In the casting alloys to which lead is added up to 9 %, it improves the castability, especially for the production of pressure-tight castings.

Probably all copper-nickel-zinc alloys contain some manganese (max. 0.5 to 0.7 %) as a permissible admixture; it reduces annealing brittleness, has a deoxidising and desulphurising effect. Tin is an essential component in cast alloys because it lowers the melting point and makes the melt thin. It increases strength and hardness, but reduces elongation. Wrought alloys do not contain tin because it has an embrittling effect and worsens the hot formability.

Applications

Because of their good strength and spring properties, their colour, their low conductivity for heat and electricity as well as their easy galvanisability, copper-nickel-zinc alloys have found many specific fields of application. The use of these alloys is determined by the composition and thus by both the properties and the processing possibilities. For electrical engineering and electronics, these materials are particularly interesting because of their strength and toughness, adequate electrical conductivity, considerable modulus of elasticity compared to other copper alloys, better tarnish resistance and corrosion resistance. This combination of properties is particularly advantageous for electrical contacts, springs, etc. CuNi18Zn20 is mainly used for springs and diaphragms in low-current technology, for electrical resistance wires, pressure gauge spring tubes, etc. Drawn and stamped parts are used for lamp bases, switch covers, housings and similar electrical engineering fittings made of sheet metal or strip. Contact bimetallic strips are used to manufacture contact parts and spring elements that are subject to mechanical and electrical stress. The prerequisites for this are not only the corrosion resistance and the good spring properties of the material, but also its adequate solderability. The bimetallic strips have inserts of gold or silver or palladium alloys. For some applications, however, the high zinc content of the nickel silver is a disadvantage, as it can transfer to the contact material via the gas phase during soft annealing under inert gas. In such cases, copper-nickel-tin alloys can be used as an alternative.

Properties

Physical properties

Thermal and electrical properties

Strength properties

Physical properties

The copper-nickel-zinc alloys standardised in Germany consist of 47 to 64 % Cu, 10 to 25 % Ni and 15 to 42 % Zn, depending on the intended use. Some alloys also have other elements added to improve certain properties or workability. Elements of this kind are e.g. lead, manganese or tin. Copper-nickel-zinc alloys get their silver-like colour from the interaction of the alloy components nickel and zinc. Alloys with higher copper contents are yellowish. With increasing zinc content, they take on a greenish hue. The colour of an alloy with about 20 % Ni comes closest to that of silver. Even higher nickel contents gradually lead to the colour of pure nickel. The melting ranges of copper-nickel-zinc alloys increase with the nickel and copper contents. For a rough calculation, the following rule of thumb was used in the past: Melting range in °C = 10 x (wt. % Ni) + 5 x (wt. Cu) + 600. The modulus of elasticity of the standardised lead-free wrought alloys is approximately between 125 and 140 kN/mm².

Thermal and electrical properties

The thermal and electrical conductivity is low compared to other copper materials. Because of their low electrical conductivity, which is approximately between 3 and 5 m/Ω * mm², they can be used as resistance materials. In comparison, the copper-zinc alloys (brasses), for example, have an electrical conductivity of about 13 m/Ω * mm² and above. The thermal conductivity of the copper-nickel-zinc alloys is also low at 21 to 33 W/m * K; the copper-zinc alloys (brasses) nevertheless have values above 113 W/m * K. Copper-nickel-zinc alloys are non-magnetic; this is important for some areas of application.

Strength properties

The relatively high strength for copper alloys can be greatly increased by cold forming due to the large hardening capacity of copper-nickel-zinc alloys. Depending on the composition, the tensile strength is between 340 and over 610 N/mm2; it can reach over 830 N/mm2 for round spring wires made of CuNi18Zn20 according to DIN EN 12166. The strong hardening by cold forming is expressed in the large difference in the characteristic values for tensile strength, 0.2 proof stress and hardness in the soft and hard state. The Brinell hardness is approximately between 85 and 190 HB, the Vickers hardness of strips and strips for leaf springs according to DIN EN 1654 made of CuNi18Zn20 is more than 230 H.V. At elevated temperatures, the tensile strength does not drop significantly up to about 300 °C. Depending on the expected service life, the tensile strength of CuNi18Zn20 may be higher than that of CuNi18Zn20. Depending on the expected service life, creep behaviour may already have to be taken into account at these temperatures. Like all copper materials, the copper-nickel-zinc alloys do not show any signs of embrittlement at low temperatures; they are therefore well suited for use for low-temperature purposes. For the calculation, strength values at room temperature are assumed. This practically means taking into account a safety factor that increases with falling temperature.

Both in the form of strip and wire, CuNi18Zn20 in particular is an excellent spring material. Spring strips made of CuNi18Zn20 are standardised in DIN EN 1654. While the Vickers hardness and the smallest bending radius are specified as acceptance values in Tab. 1 of this standard, the spring bending limit is specified instead of the hardness in Tab. 2 of the standard for tempered strips. The spring bending limit is a characteristic value for the spring force. The spring bending limit is increased by an annealing treatment after finish rolling in the temperature range of 200 to 300 °C (“annealing effect”). This heat treatment ensures that the spring force hardly changes in continuous operation even at increased temperatures and also causes a strong reduction of any residual stresses in the strip. Spring wires made of CuNi18Zn20 are standardised in DIN EN 12166. Tab. 1 of this standard contains, among other things, characteristic values for the tensile strength of strain-hardened CuNi18Zn20 in the tempered state as a function of the wire diameter. The standard specifies mandrel diameters for the winding test according to DIN ISO 7802 for work-hardened alloys. Cupping values for the alloys CuNi12Zn24 and CuNi18Zn20, which have particularly good deep drawing properties, are given in DIN EN 1652.

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