Precipitation Hardening Copper Alloys

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, that increases 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 decreases. 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 Figure 2, Figure 3, Figure 4).

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. Development efforts for alternative precipitation hardening materials without toxic and declarable additives are underway, e.g. CuNiCoSi as a substitute for 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

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 - 8.3b9.1c	110	17	125a b	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

_a solution annealed, and cold rolled

_b solution annealed, cold rolled, and precipitation hardened

_c solution 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

_a solution annealed, and cold rolled

_b solution annealed, cold rolled, and precipitation hardened

_c solution 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 and 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

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

_a solution annealed, and cold rolled

_b solution 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 or 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

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