Precipitation Hardening Copper Alloys

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Besides the naturally hard copper materials precipitation hardening copper alloys play also an important role as carrier materials for electrical contacts. By means of a suitable heat treatment finely dispersed precipitations of a second phase can be achieved which increase the mechanical strength of these copper alloys significantly.

Copper-Beryllium Alloys (Beryllium Bronze)

The cause for precipitation hardening of CuBe materials is the rapidly diminishing solubility of beryllium in copper as temperature decrease. As the phase diagram for CuBe shows, 2.4 wt% of Be are soluble in Cu at 780°C Figure 1. In this temperature range annealed CuBe alloys are homogeneous(solution annealing). The homogeneous state can be frozen through rapid cooling to room temperature (quenching). Through a subsequent annealing at 325°C the desired precipitation hardening is achieved which results in a significant increase in mechanical strength and electrical conductivity of CuBe Table 1. The final strength and hardness values depend on the annealing temperature and time as well as on the initial degree of cold working Table 2 and (Figs. 43 – 75).


As precipitation hardening alloys CuBe materials, mainly CuBe2 and CuBe1.7 have gained broad usage as current carrying contact springs because of their outstanding mechanical properties. Besides these CuCo2Be and CuNi2Be, which have medium mechanical strength and a relatively high electrical conductivity, are also used as contact carrier materials. After stamping and forming into desired contact configurations these CuBe materials are then precipitation hardened. CuBe alloys are available as semi-finished materials in a variety of cold work conditions. They can also be supplied and used in the already precipitation hardened condition without significant strength losses. In this case the hardening was already performed at the alloy producer.

Since Beryllium is rated as a carcinogen by the European regulation EU-67/548, it has been tried to reach the application properties of the well established CuBe1.7 and CuBe2 alloys with a lower Be content. The development efforts for alternate precipitation hardening materials without toxic and declaration requiring additive materials, for example CuNiCoSi, are aimed at the replacement of CuBe.

Figure 1 Phase diagram of copper- beryllium with temperature ranges for brazing and annealing treatments

Figure 2 Precipitation hardening of CuBe2 at 325°C after different cold working

Figure 3 Precipitation hardening of CuBe2 (soft) at 325°C

Figure 4 Precipitation hardening of CuBe2 (half hard) at different annealing temperatures

Figure 1: Phase diagram of copper- beryllium with temperature ranges for brazing and annealing treatments
Figure 2: Precipitation hardening of CuBe2 at 325°C after different cold working
Figure 3: Precipitation hardening of CuBe2 (soft) at 325°C
Figure 4: Precipitation hardening of CuBe2 (half hard) at different annealing temperatures


Table 1: Physical Properties of Selected Copper-Beryllium Alloys
Material
Designation
EN UNS
Composition
[wt%]
Density
[g/cm3]
Electrical
Conductivity
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]
[MS/m] [% IACS]
CuBe1.7
CW100C
C17000
Be 1.6 - 1.8
Co 0.3
Ni 0.3
Cu Rest
8.4 8 - 9a
12 - 13b
11c
14 - 16
21 - 22
19
11 - 12.5a
7.7 - 8.3b
9.1c
110 17 125a
135b
ca. 380 890 - 1000
CuBe2
CW101C
C17200
Be 1.8 - 2.1
Co 0.3
Ni 0.3
Cu Rest
8.3 8 - 9a
12 - 13b
11c
14 - 16
21 - 22
19
11 - 12.5a
7.7 - 8.3b
9.1c
110 17 125a
135b
ca. 380 870 - 980
CuCo2Be
CW104C
C17500
Co 2.0 - 2.8
Be 0.4 - 0.7
Ni 0.3
Cu Rest
8.8 11 - 14a
25 - 27b
27 - 34c
19 - 24
43 - 47
47 - 59
7.1 - 9.1a
3.7 - 4.0b
2.9c
210 18 131a
138b
ca. 450 1030 - 1070
CuNi2Be
CW110C
C17510
Ni 1.4 - 2.2
Be 0.2 - 0.6
Co 0.3
Cu Rest
8.8 11 - 14a
25 - 27b
27 - 34c
19 - 24
43 - 47
47 - 59
7.1 - 9.1a
3.7 - 4.0b
2.9c
230 18 131a
138b
ca. 480 1060 - 1100

asolution annealed, and cold rolled
bsolution annealed, cold rolled, and precipitation hardened
csolution annealed, cold rolled, and precipitation hardened at mill (mill hardened)


Table 2: Mechanical Properties of Selected Copper-Beryllium Alloys
Material Hardness
Condition
Tensile Strength Rm
[MPa]
0,2% Yield Strength
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]
CuBe1,7 R 390a
R 680a
R 1030b
R 1240b
R 680c
R 1100c
380 -520
680 - 820
1030 - 1240
1240 - 1380
680 - 750
1100 - 1200
≥ 180
≥ 600
≥ 900
≥ 1070
≥ 480
≥ 930
35
2
3
1
18
3
80 - 135
210 - 250
330 - 380
360 - 420
220 - 350
330 - 370
0 x t
1 x t
1 x t

1 x t
6 x t
0 x t
3 x t
1.5 x t

1 x t
10 x t


700
1000
390
790


260
280

260
CuBe2 R 410a
R 690a
R 1140b
R 1310b
R 690c
R 1200c
410 -540
690 - 820
1140 - 1310
1310 - 1480
690 - 760
1200 - 1320
≥ 190
≥ 650
≥ 1000
≥ 1150
≥ 480
≥ 1030
35
2
3
1
18
3
90 - 140
215 - 260
350 - 400
380 - 450
220 - 250
360 - 410
0 x t
1 x t


1 x t
5 x t
0 x t
3 x t


1.5 x t
10 x t


800
1040
400
900


270
300

280
CuCo2Be
CuNi2Be
R 250a
R 550a
R 650b
R 850b
R 520c
250 - 380
550 - 700
650 - 820
850 - 1000
520 - 620
≥ 140
≥ 450
≥ 520
≥ 750
≥ 340
20
2
10
1
5
60 - 90
160 - 200
195 - 230
240 - 290
150 - 180
0 x t
3 x t
1 x t
3 x t
1 x t
0 x t

1 x t
3.5 x t
1 x t


360
650
300


220
250
210

1) t: Strip thickness max. 0.5 mm
asolution annealed, and cold rolled
bsolution annealed, cold rolled, and precipitation hardened
csolution annealed, cold rolled, and precipitation hardened at mill (mill hardened)

Other Precipitation Hardening Copper Alloys

Copper-Chromium Alloys

As the phase diagram shows, copper-chromium has a similar hardening profile compared to CuBe Figure 5. In the hardened stage CuCr has limitations to work hardening. Compared to copper it has a better temperature stability with good electrical conductivity. Hardness and electrical conductivity as a function of cold working and precipitation hardening conditions are illustrated in (Figs. 6 – 9), Table 3 and Table 4.

Copper-chromium materials are especially suitable for use as electrodes for resistance welding. During brazing the loss in hardness is limited if low melting brazing alloys and reasonably short heating times are used.

Figure 5 Copper corner of the copper-chromium phase diagram for up to 0.8 wt% chromium

Figure 6 Softening of precipitation-hardened and subsequently cold worked CuCr1 after 4hrs annealing

Figure 7 Electrical conductivity of precipitation hardened CuCr 0.6 as a function of annealing conditions

Figure 8 Hardness of precipitation hardened CuCr 0.6 as a function of annealing conditions

Figure 9 Electrical conductivity and hardness of precipitation hardened CuCr 0.6 after cold working

Figure 5: Copper corner of the copper-chromium phase diagram for up to 0.8 wt% chromium
Figure 6: Softening of precipitation-hardened and subsequently cold worked CuCr1 after 4hrs annealing
Figure 7: Electrical conductivity of precipitation hardened CuCr 0.6 as a function of annealing conditions
Figure 8: Hardness of precipitation hardened CuCr 0.6 as a function of annealing conditions
Figure 9: Electrical conductivity and hardness of precipitation hardened CuCr 0.6 after cold working


Table 3: Physical Properties of Other Precipitation Hardening Copper Alloys
Material
Designation
EN UNS
Composition
[wt%]
Density
[g/cm3]
Electrical
Conductivity
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]
[MS/m] [% IACS]
CuCr Cr 0.3 - 1.2
Cu Rest
8.89 26a
48b
45a
83b
3.8a
2.1b
170a
315b
17 112 ca. 450 980 - 1080
CuZr Zr 0.1 - 0.3
Cu Rest
8.9 35a
52b
60a
90b
2.9a
1.9b
340a 16 135 ca. 500 1020 - 1080
CuCr1Zr
CW106C
C18150
Cr 0.5 - 1.2
Zr 0.03 - 0.3
Cu Rest
8.92 20a
43b
34a
74b
5.0a
2.3b
170a
310 - 330b
16 110a
130b
ca. 500 1070 - 1080

asolution annealed, and cold rolled
bsolution annealed, cold rolled, and precipitation hardened


Table 4: Mechanical Properties of Other Precipitation Hardening Copper Alloys

Material

Hardness

Condi- tion

Tensile

Strength Rm

[MPa]

0,2% Yield

Strength Rp02

[MPa]

Elongation

A50

[%]

Vickers

Hardness

HV

Spring Bending

Limit FFB [MPa]

CuCr

R 230a

R 400a R 450b R 550b

> 230

> 400

> 450

> 550

> 80

> 295

> 325

> 440

30

10

10

8

> 55

> 120

> 130

> 150

350

CuZr

R 260a

R 370a R 400b R 420b

> 260

> 370

> 400

> 420

> 100

> 270

> 280

> 400

35

12

12

10

> 55

> 100

> 105

> 115

280

CuCr1Zr

R 200a

R 400b

R 450b

> 200

> 400

> 450

> 60

> 210

> 360

30

12

10

> 70

> 140

> 155

420

Copper-Zirconium Alloys

The solubility of Zirconium in copper is 0.15 wt% Zr at the eutectic temperature of 980°C Figure 10. Copper-zirconium materials have a similar properties spectrum compared to the one for copper-chromium materials. At room temperature the mechanical properties of copper-zirconium are less suitable than those of copper chromium, its temperature stability is however at least the same.

Copper-Chromium-Zirconium Alloys

The earlier used CuCr and CuZr materials have been partially replaced over the years by the capitation hardening three materials alloy CuCr1Zr. This material exhibits high mechanical strength at elevated temperatures and good oxidation resistance as well as high softening temperatures. In its hardened condition CuCr1Zr has also a high electrical conductivity Figure 11. Their usage extends from mechanically and thermally highly stressed parts such as contact tulips in high voltage switchgear to electrodes for resistance welding.

Figure 10 Copper corner of the copper- zirconium for up to 0.5 wt% zirconium

Figure 11 Softening of CuCr1Zr after 1 hr annealing and after 90% cold working

Copper corner of the copper- zirconium for up to 0.5 wt% zirconium
Softening of CuCr1Zr after 1 hr annealing and after 90% cold working

References

References