Copper-aluminium alloys – Kupferverband

Copper-aluminium alloys

In the technical, commercially available copper-aluminium alloys, an optimal combination of excellent corrosion resistance in a variety of aggressive media with above-average mechanical and good physical properties can be observed. This is why they occupy a special position among copper materials.

The commercially available copper-aluminium alloys contain up to 14 % Al as the main alloying addition to copper, whereby additions of 4.0 to 9.0 % exclusively of aluminium characterise the group of “two-substance alloys”, which are generally single-phase, i.e. homogeneous, in their microstructure. Alloys with about 8 to 14 % Al, which then always contain other additional elements – such as iron, nickel and manganese – to improve certain properties, form the group of multiphase, i.e. heterogeneous “multicomponent alloys”. Furthermore, a distinction is made between wrought alloys (i.e. alloys that are easy to form plastically) and cast alloys (i.e. alloys that are only easy to cast). Wrought alloys are processed from cast “formats” (e.g. rolling slabs, press billets) into semi-finished products such as sheets, strips, tubes, rods, die and open-die forgings. Castings are produced from the casting alloys by means of various mould casting processes, which are not kneaded but exclusively machined.

Physical
properties

Electrical
properties

Magnetic
properties

Mechanic
properties

Physical properties

The addition of aluminium causes a change in the colour of the copper from copper red to gold tones to a yellowish colour at about 10 % Al. The density of pure copper is 8.93 kg/dm3 at 20°C. As Fig. 3 shows, it decreases almost linearly with increasing aluminium content to about 7.5 kg/dm3 at 10 % Al. The modulus of elasticity of commercially available copper-aluminium alloys is between 105 and 130 kN/mm2 (see fold-out table). In the α-range, it decreases with increasing aluminium content and increases sharply with the occurrence of the γ2-phase . The sliding modulus is between 43 and 45 kN/mm2 (see fold-out table). The Poisson’s ratio (transverse contraction number) “ν” is 0.30-0.35.

The thermal conductivity is reduced by the aluminium content. The thermal conductivity of copper-aluminium alloys increases with temperature. The temperature coefficient of thermal conductivity increases with the aluminium content. The coefficient of thermal expansion experiences little change due to the aluminium content). The linear coefficient of thermal expansion increases with increasing temperature.

Electrical properties

The electrical conductivity and its temperature coefficient decrease with increasing aluminium content . Additives such as iron and especially manganese and nickel reduce them further. The commercially available copper-aluminium alloys achieve about 12 to 17 % of the conductivity of pure copper. The electrical conductivity of copper-aluminium alloys decreases with increasing temperature.

Magnetic properties

The magnetic properties of copper-aluminium alloys are most strongly altered by iron of the other additives. The paramagnetism, which is very weak in the binary alloy base, is considerably strengthened by iron. The critical iron content of amagnetic copper-aluminium materials (µ ” 1.001) is less than 0.15 % Fe for nickel-free alloys. For nickel-containing (about 4 % Ni) alloys, the Fe content is 0.5 %. Nickel has only a minor effect on the permeability of copper-aluminium alloys in the usual amounts. Manganese only slightly influences the permeability. The magnetic properties can be further improved by suitable heat treatment.

Mechanic properties

The mechanical properties of the standardised wrought copper-aluminium alloys are specified in the semi-finished product and cast product standards. The tensile strength of the binary copper-aluminium alloys initially increases uniformly in the as-cast state with increasing aluminium content, only to drop rapidly above about 10 % Al due to the embrittling effect of the structural constituent γ2 occurring as a decay product of the β-phase. In the commercially available heterogeneous copper-aluminium multicomponent alloys, this phase reaction is specifically influenced by the addition of further alloying elements, which explains the very high strength values of this alloy group under both static and dynamic loading. The elongation at break already reaches its maximum value at aluminium contents between 5 and 8 %. The hardness of the homogeneous alloys increases uniformly over the entire concentration range of the aluminium.

The mechanical properties of cold-formable wrought alloys are essentially dependent on the degree of deformation. In addition to the alloy constituents, grain size and deformation process, which determine hardening and texture formation and thus also the mechanical properties, are also important. In the case of cold-formable, homogeneous wrought alloys, tensile strength, yield strength and hardness increase with increasing degree of cold forming, while elongation at break and fracture constriction decrease. Depending on the composition or degree of cold forming, the tensile strength of wrought copper-aluminium alloys ranges from 340 to about 830 N/mm2.

At elevated operating temperatures, the nickel- and iron-containing wrought copper-aluminium alloys are particularly favourable because of their high initial strength. They can be used at temperatures up to 300°C (components subject to monitoring up to 250°C) due to their high temperature strength. For long-term stresses and high temperature stresses, the possible applications are determined by the creep properties (creep rupture strength). The creep properties for the material CuAl10Ni5Fe4 were determined in creep rupture tests lasting 30,000 hours. The strength values determined in this way allow the behaviour of the material to be estimated for temperatures of up to 250°C and operating times of up to 100,000 h.

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