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The mechanical properties of metals to a large extent depend on their crystallo-graphic structure and temperature (Figure 1.11). Among the metals used in electrical construction, a special attention is paid to copper. Its mechanical properties signifi-cantly depend on thermal and plastic processing and on the contents of admixtures.

TABLE 1.2

Important Electrical, Thermal, and Chemical Properties of Copper

Properties Values Remarks

Electrical conductivity of purest electrolytic copper at 20°C

59.77 × 10⁶ S/m See Figure 1.7 for the influence of admixtures Electrical conductivity of standard, annealed

copper at 20°C, after international standard and Polish Standard PN/E-4

58 × 10⁶ S/m Adopted as reference 100%

for evaluation of conductivity of other metals

Temperature coefficient of resistivity at temperature 0–150°C, after international standard and Polish Standard PN/E-4

0.00393 1/K For purest Cu, 0.0043 1/K

Electron work functions 4.07–2.61 eV

Thermal conductivity λ at 20°C 385–394 W/(m K) Coefficient of thermal linear expansion at

temperature 20–100°C

16.5 × 10⁻⁶ 1/K Standard copper as per PN/E-4, 17 × 10⁻⁶ 1/K Specific heat at temperature 20°C 385 J/(kg K)

Melting point temperature 1083 ± 0.1°C

Casting temperature 1150–1200°C

Recrystallization temperature 200–300°C

Annealing temperature 500–700°C

In dry and humid air, in water, in hydrochloric or sulfuric acid of concentration less than 80%

Not oxidized and not active In dry air at temperature 100°C Creates protective oxide layer In dry air at temperature ≥200°C Oxidizes (color coating)

In oxygen Oxidizes at room temperature

With sulfur at heating Creates Cu₂S not protecting against corrosion With nitrogen Creates compounds Cu₃N, CuN₃, Cu₆N₂ obtained

with indirect methods In hydrochloric and sulfuric acid, in ammonia

at heating, in nitrogen acid

Dilutes

Source: Adapted from Handbook of Electrical Materials. (in Russian) Vol. 2, Moscow: Gosenergoizdat, 1960; Wesolowski K.: Materials Science. Vols. I, II, III. (in Polish) Warsaw: WNT, 1966.

In the case of copper, the relation of the extension force P versus elongation Δl does not show an evident border of plasticity P_pl, in contrast to the analogical graph for steel (Figure 1.12). Up to the limit of P_s, the elongation is exclusively elastic, and above that limit—it is elastic and plastic. The point P_H (limit of proportionality) is where the curve begins deviation from the straight line. The force P_r represents the limit of strain endurance to lengthening. Modulus of elasticity (Young’s modulus) is the ratio of stress, below the limit of elasticity, to the relative elongation (strain), that is, the slope of the linear portion of the stress–strain curve.

Plastic processing of copper at cold temperatures (squeeze), appearing during draw-out, causes its hardening with a significant increase of endurance to split (up to 400–500 N/mm²), and a small elongation at split (1–2%). Such a wire is strongly elastic at bending and is not suitable for winding.

120

100

%

80

60

400 100 Cu

4 1 3 2 5

Brass Cu

Al Steel

200 300 400 500 °C

FIGURE 1.11 Dependence of mechanical strength of metals on temperature (per Babikov):

1—aluminum, 2—brass, 3—hard copper or at short-duration heating, 4—electrolytic copper or at long-lasting heating, 5—steel. (Adapted from Kozłowski L.: Elements of Atomic Physics and Solid. Cracow: AGH, 1972.)

P

0 Δl

P

P_prP_el P_pl P_prP_el

P_pl

P_len P_len

0 Δl

(a) (b)

FIGURE 1.12 Tensile forces versus elongation of a sample for (a) copper and (b) steel.

Annealing, which consists of heating copper to a temperature of several hundred degrees Celsius (above the recrystallization temperature) and then rapid cooling, causes significant softening of copper. Along with the softening, a remarkable reduc-tion of endurance to split takes place (up to about 200 N/mm²) as well as an increase of plasticity. It also lessens the resistivity, by 2–3%.

The recrystallization temperature is the temperature at which the removal of internal stresses of plastic deformation of crystals appears, by increasing the size of some crystals at the cost of others. The recrystallization temperature varies in the range between 280°C and 400°C, for different sorts of copper and for different deformations, and is accompanied by a sudden fall of strain strength and hardness and at the same time an increase of elongation. In the case of pure copper, the initial temperature of recrystallization is about 180°C [1.23]. For most metals, the higher the deformation, the lower the recrystallization temperature. Any impurities and ingredients in copper increase its recrystallization temperature.

In turbine generators of older construction, rotor windings were made of soft electrolytic copper with the modulus of elasticity of 10⁵ N/mm², limit of elastic-ity of 42 N/mm², and CTE of 17 × 10⁻⁶ K⁻¹. At such small strengths of copper, any faster starting or stopping of large turbogenerators was accompanied by a per-manent deformation of conductors in the slots, caused by reciprocal interaction of thermal expansion and the friction of the conductors on slots. It resulted in damages of winding, insulation, or clampings. Application of copper along with the addition of 0.07–0.1% of silver, with cold press treatment, increased its limit of elasticity to 150 N/mm², which reduced the risk of such damages. Also, for windings of large transformers, sometimes a copper with silver additions is used, which, contrary to normal copper, does not lose its increased elasticity obtained by plastic treatment during exploitation. Due to similar reasons, for the rotor windings, sometimes a conducting aluminum alloy is used, for example, Cond-Al (Latek [1.35]) with a elasticity limit of 11 deca-newton (daN)/mm² and a thermal coefficient of expan-sion of 13.1 × 10⁻⁶ 1/K. In Table 1.3, selected important mechanical properties of copper are collected.

For commutator bars of electric machines, sometimes an abrasion-resistant, suf-ficiently hard copper is used, with a hardness of at least 75 daN/mm².

Because components of electric circuits and material for construction elements (clamping plates, consoles) of electric machines and transformers are under haz-ard from strong magnetic leakage fields, various bronzes are used occasionally.

These bronzes contain, apart from copper, tin, beryllium, chromium, magnesium, zinc, cadmium, silicon, and other metals. At properly selected composition, these alloys can reach endurance to split in the order of 80–100 daN/mm², and even more.

However, they have a higher resistivity than copper. For instance, cadmium bronzes with contents of 0.9% Cd have after broaching the conductivity of 83–90% of the conductivity of copper and tensile strength up to 73 daN/mm². They are used for manufacturing slipper conductors and corresponding contact elements (commuta-tors) due to their high abrasion resistance. Beryllium bronzes (2.25% Be) reach a strength of 110 daN/mm² at a conductivity of 30% [1.23].

Aluminum, often used as a material replacing copper, is about 3.5 times lighter than copper, and its resistivity is 1.65 times higher than the resistivity of copper.

Aluminum conductors of the same length and resistance as that of copper will be about 2 times lighter than the copper conductor. However, its diameter must be 1.28 times bigger than the diameter of the copper conductor of the same resistance. It is additionally difficult in the case of replacing copper conductors by aluminum con-ductors, when the space is limited.

In Table 1.4, selected important technical properties of aluminum are presented.

Pure aluminum has a remarkably (2–3 times) lower mechanical strength than copper, but its alloys with magnesium (Mg), silicon (Si), iron (Fe), and so on have much better mechanical properties. For instance, the alloy called aldrey (0.3–0.5%

Mg; 0.4–0.7 Si, and 0.2–0.3% Fe) has the mass density and conductivity almost the same as aluminum, and the tensile strength (35 daN/mm²) close to the strength of copper. This material is used for the conductors of overhead power lines.

In the air, aluminum is always covered by a thin layer of oxide (about 0.001 mm) which protects the metal against further corrosion, but also makes it difficult to join aluminum conductors with each other. Welding aluminum elements or aluminum with copper requires a special technology. On a humid contact of Al–Cu appears an TABLE 1.3

Basic Mechanical Properties of Copper at Temperature 20°C

Properties Unit Casting

Plastic Processing Dependence on Increase of Annealing Temperature

Soft Hard

Limit of tensile strength daN/mm² 18–22 20–28 25–50 Decreases Limit of proportionality daN/mm² — 2.2–3 14–20

Limit of plasticity at residual elongation 0.2%

daN/mm² 6–7 23–38

Relative elongation before disruption

% 18 18–50 0.5–5 Grows

Relative narrowing % 23 To 75 To 55

Modulus of elasticity

Static daN/mm² — 11,700 12,200–13,200

Dynamic daN/mm² — 7400 11,200

Brinell hardness daN/mm² 40 35–37.5 65–120 Decreases

Limit of compressive strength

daN/mm² 157 — —

Settlement at compression % 65

Specific striking viscosity daN/mm² 5.3 15.6 Limit of shear strength daN/mm² — 19 43 Limit of fatigue strength

at torsion

daN/mm² — 2.8 4.3

Limit of fatigue strength at bending

daN/mm² — — 11

Mass density at temperature 20°C

g/cm³ 8–8.9 8.87–8.89 8.85

Source: After Handbook of Electrical Materials. (in Russian) Vol. 2, Moscow: Gosenergoizdat, 1960.

electric cell with a current flow from aluminum to copper, which causes a consider-able corrosion of the aluminum conductor.