Naturally Hard Copper Alloys

Alloys like brasses (CuZn), tin bronzes (CuSN), and German silver (CuNiZn), for which the required hardness is achieved by cold working are defined as naturally hard alloys. Included in this group are also the silver bronzes (CuAg) with 2 – 6 wt% of Ag.

5.1.4.1 Copper-Zinc Alloys (Brasses)

Copper-zinc alloys are widely used as contact carrier materials in switching devices for electrical power engineering because of their high electrical conductivity, the higher mechanical strength combined with good formability compared to pure copper Table 1 (Tab. 5.7) and Table 2 (Tab. 5.8), and at the same time their reasonable economic costs. Especially suitable are the brasses with up to 37 wt% Zn content which are according to the phase diagram all made up from the " -phase of the CuZn system Figure 1(Fig. 5.5). It is important to note the strong dependence of the electrical conductivity and mechanical strength on the zinc content Figure 2(Fig. 5.6).

Table 5.7: Physical Properties of Selected Copper-Zinc Alloys

Material/ Designation EN UNS	Composition [wt%]	Density [g/cm3]	Electrical Conductivity [MS/m] [% IACS]		Electrical Resistivity [μΩ·cm]	Thermal Conductivity [W/(m·K)]	Coeff. of Linear Thermal Expansion [10-6/K]	Modulus of Elasticity [GPa]	Softening Temperature (approx. 10% loss in strength) [°C]	Melting Temp Range [°C]
CuZn5 CW500L C21000	Cu 94 - 96 Zn Rest	8.87	33	57	3.8	243	18.0	127		1055 - 1065
CuZn10 CW501L C22000	Cu 89 - 91 Zn Rest	8.79	25	43	4.0	184	18.2	125		1030 - 1045
CuZn15 CW502L C23000	Cu 84 - 86 Zn Rest	8.75	21	36	4.8	159	18.5	122	ca. 250	1005 - 1025
CuZn20 CW503L C24000	Cu 79 - 81 Zn Rest	8.67	19	33	5.3	142	18.8	120	ca. 240	980 - 1000
CuZn30 CW505L C26000	Cu 69 - 71 Zn Rest	8.53	16	28	6.3	124	19.8	114	ca. 230	910 - 940
CuZn37 CW508L C27200	Cu 62 - 64 Zn Rest	8.45	15.5	27	6.5	121	20.2	110	ca. 220	900 - 920
CuZn23Al3Co CW703R C68800	Cu 73.5 Al 3.4 Co 0.4 Zn Rest	8.23	9.8	17	10.2	78	18.2	116	ca. 280	950 - 1000

Table 5.8: Mechanical Properties of Selected Copper-Zinc Alloys

Material	Hardness Condition	Tensile Strength Rm [MPa]	0,2% YieldStrength Rp02 [MPa]	Elongation A50 [%]	Vickers Hardness HV	Bend Radius1) perpendicular to rolling direction	Bend Radius1) parallel to rolling direction	Spring Bending Limit σFB [MPa]	Spring Fatigue Limit σBW [MPa]
CuZn5	R 230 R 270 R 340	230 - 280 270 -350 340 - 440	≤ 130 ≥ 200 ≥ 280	36 12 4	45 - 90 70 - 120 110 - 160	0 x t 0 x t	0 x t 0 x t	250	130
CuZn10	R 240 R 280 R 350	240 - 290 280 - 360 350 - 450	≤ 140 ≥ 200 ≥ 290	36 13 4	50 - 100 80 - 130 110 - 160	0 x t 0 x t	0 x t 0 x t	260	140
CuZn15	R 300 R 350 R 410 R 480 R 550	300 - 370 350 - 420 410 - 490 480 - 560 ≥ 550	≤ 150 ≥ 270 ≥ 360 ≥ 420 ≥ 480	16 8 3 1	85 - 120 100 - 150 125 - 155 150 - 180 ≥ 170	0 x t 0 x t 0 x t 1 x t	0 x t 0 x t 1 x t 3 x t	300	160
CuZn20	R 270 R 320 R 400 R 480	270 - 320 320 - 400 400 - 480 480 - 570	≤ 150 ≥ 200 ≥ 320 ≥ 440	38 20 5 3	55 - 105 95 - 155 120 - 180 ≥ 150	0 x t 0 x t 0 x t	0 x t 0 x t 0 x t	320	180
CuZn30	R 270 R 350 R 410 R 480	270 - 350 350 - 430 410 - 490 480 - 580	≤ 160 ≥ 200 ≥ 430 ≥ 430	40 21 9 4	95 - 125 120 - 155 150 - 180 170 - 200	0 x t 0 x t 0 x t 1 x t	0 x t 1 x t 2 x t 3 x t	330	180
CuZn37	R 300 R 350 R 410 R 480 R 550	300 - 370 350 - 440 410 - 490 480 - 560 550 - 640	≤ 180 ≥ 200 ≥ 260 ≥ 430 ≥ 500	38 19 8 3	55 - 105 95 - 155 120 - 190 ≥ 150 ≥ 170	0 x t 0 x t 0 x t 0.5 x t 1 x t	0 x t 0 x t 0 x t 1 x t 3 x t	350	190
CuZn23Al3Co	R 660 R 740 R 820	660 - 750 740 - 830 ≥ 820	≥ 580 ≥ 660 ≥ 780	10 3 2	190 - 220 210 - 240 ≥ 235	0 x t 1 x t	0 x t 2 x t	≥ 400	230

¹⁾ t: Strip thickness max. 0.5 mm

The main disadvantages of these alloys are with increasing zinc content the also increasing tendency towards tension crack corrosion and the poorer stress relaxation properties compared to other copper alloys.

One of the special brass alloys used as a contact carrier material is CuSn23Al3Co. This material exhibits significantly higher mechanical strength than the standard brass alloys. Even so this material is a naturally hardening alloy, a suitable heat treatment allows to further increase its strength.

Figure 1 Fig. 5.5: Strain hardening of Ag 99.95 by cold working

Figure 2 Fig. 5.6: Mechanical properties of brass depending on the copper content (after cold working of 0 and 50%)

Figure 3 Fig. 5.7: Strain hardening of CuZn36 by cold forming

Figure 4 Fig. 5.8: Softening of CuZn36 after 3 hrs annealing after 25% cold working

Figure 1: Phase diagram of copper-zinc for the range of 0-60 wt% zinc

Figure 2: Mechanical properties of brass depending on the copper content (after cold working of 0 and 50%)

Figure 3: Strain hardening of CuZn36 by cold forming

Figure 4: Softening of CuZn36 after 3 hrs annealing after 25% cold working

5.1.4.2 Copper-Tin Alloys (Tin Bronze)

Because of their good elastic spring properties and formability the copper-tin alloys CuSn6 and CuSn8 are standard materials for spring contact elements in electrome-chanical components such as connectors, switches, and relays Table 3 (Tab. 5.9) and (Tab.5.10). Besides these other alloys such as CuSn4 and CuSn5 and the multi-metal tin bronze CuSn3Zn9 have significant usage – mainly in North America. Figure 5.10 Figure 5 shows the copper rich side of the phase diagram for the CuSn system. The mechanical property values achieved by cold forming are superior to these of the brass alloys Figure 6(Fig. 5.11). They increase significantly with increasing Sn content. The work hardening and softening behavior are shown for the example of CuSn8 in Figure 7 Figures 5.12 and Figure 8 5.13. The stress relaxation properties for CuSn alloys are good for up to 100°C, deteriorate however quickly for temperatures above 150°C.

Fig. 5.9: Softening of CuZn36 after 3 hrs annealing after 50% cold working (im Text nicht zu finden!)

Softening of CuZn36 after 3 hrs annealing after 50% cold working

Phase diagram of the Cu-Sn system for the range of 0 – 30 wt% Sn

Table 5.9: Physical Properties of Copper-Tin Alloys

Material Designation EN UNS	Composition [wt%]	Density [g/cm3]	Electrical Conductivity [MS/m] [% IACS]		Electrical Resistivity [μΩ·cm]	Thermal Conductivity [W/(m·K)]	Coeff. of Linear Thermal Expansion [10-6/K]	Modulus of Elasticity [GPa]	Softening Temperature (approx. 10% loss in strength) [°C]	Melting Temp Range [°C]
CuSn4 CW450K C51100	Sn 3.5 - 4.5 P 0.01 - 0.4 Cu Rest	8.85	12.0	20	8.3	118	18.0	120	ca. 260	960 - 1060

Table 5.10: Mechanical Properties of Copper-Tin Alloys

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Figure 6 Fig. 5.11: Mechanical properties of tin bronze depending on the tin content (cold working 0 and 50%)

Figure 7 Fig. 5.12: Strain hardening of CuSn8 by cold working

Figure 8 Fig. 5.13: Softening of CuSn8 after 3 hrs annealing after 50% cold working

Figure 6: Mechanical properties of tin bronze depending on the tin content (cold working 0 and 50%)

Figure 7: Strain hardening of CuSn8 by cold working

Figure 8: Softening of CuSn8 after 3 hrs annealing after 50% cold working

5.1.4.3 Copper-Nickel-Zinc Alloys (German Silver)

Despite its lower electrical conductivity, the good spring properties, high corrosion resistance, and the good workability make copper-nickel-zinc alloys a frequently used spring contact carrier in switches and relays. As illustrated in the phase diagram the most commonly used materials are in the " -range which means that they are single-phase alloys Figure 9 (Fig. 5.14). The formability and strength properties of german silver are comparable to those of the copper-tin alloys. The work hardening and softening behavior is illustrated on the example of CuNi12Zn24 in Figure 10 Figures 5.15 and Figure 11 5.16.

The relaxation behavior of Cu-Ni-Zn alloys is superior to the one for the tin bronzes. Additional advantages are the very good weldability, brazing properties, and the high corrosion resistance of these copper-nickel-zinc alloys.

Fig. 5.14: Copper rich region of the ternary copper-nickel-zinc phase diagram with indication of the more commonly available german silver materials

Copper rich region of the ternary copper-nickel-zinc phase diagram with indication of the more commonly available german silver materials

Table 5.11: Physical Properties of Copper-Nickel-Zinc Alloys

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Table 5.12: Mechanical Properties of Copper-Nickel-Zinc Alloys

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Figure 10 Fig. 5.15: Strain hardening of CuNi12Zn24 by cold working

Figure 11 Fig. 5.16: Softening of CuNi12Zn24 after 3 hrs annealing after 50% cold working

Figure 10: Strain hardening of CuNi12Zn24 by cold working

Figure 11: Softening of CuNi12Zn24 after 3 hrs annealing after 50% cold working

5.1.4.4 Copper-Silver-(Cadmium) Alloys (Silver Bronze)

Besides the low-allowed CuAg0.1 other copper materials with higher silver contents (2-6 wt%) are also used as contacts carrier materials. Some of them contain additionally 1.5 wt% Cd. The phase diagram Figure 12 (Fig. 5.17) shows that in principle the CuAg alloys can be precipitation hardened, but the possible increase in mechanical strength is rather small.

Copper-silver alloys have good spring properties and compared to other spring materials have a high electrical conductivity (Tables 5.13 and 5.14). The mechanical strength values in the strongly worked condition are comparable to those of the copper-tin alloys. Work hardening and softening behavior are shown for the example of CuAg2 (Figs. 13 – 15)(Figs. 5.18 – 5.20). For the relaxation behavior the silver bronzes are superior to German silver and tin bronze.

Because of their good spring properties combined with high electrical conductivity silver bronzes are suitable for the use contact springs in relays under higher current loads. Taking advantage of their high temperature stability they are also used as current carrying contacts in high voltage switchgear and as electrode material for resistance welding.

Fig. 5.17: Phase diagram of copper-silver for the range of 0 – 40 wt% silver

Phase diagram of copper-silver for the range of 0 – 40 wt% silver

Table 5.13: Physical Properties of Selected Copper-Silver-(Cadmium) Alloys

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Table 5.14: Mechanical Properties of Selected Copper-Silver-(Cadmium) Alloys

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Figure 13 Fig. 5.18: Strain hardening of CuAg2 by cold working

Figure 14 Fig. 5.19: Softening of CuAg2 after 1 hr annealing after 40% cold working

Figure 15 Fig. 5.20: Softening of CuAg2 after 1 hr annealing after 80% cold working

Strain hardening of CuAg2 by cold working

Softening of CuAg2 after 1 hr annealing after 40% cold working

Softening of CuAg2 after 1 hr annealing after 80% cold working

References

References