Copper-based Conductivity Materials

Copper-nickel-silicon (CuNiSi) alloys can be hardened by cold working and an aged (precipitation) heat treatment process. They combine moderate electrical conductivity (up to 60% IACS) with high tensile strength (up to 800 N/mm2) together with good wear resistance and stress relaxation resistance up to 150 to 200oC.

History

The earliest specification for copper-nickel-silicon alloys is DTD (Directorate of Technical Development) 498 May 1942. DTD specifications were issued by the Ministry of Defence to meet a requirement not covered by an existing British standard for aerospace material. All DTD specifications were obsolete from 1st April 1999 but still may be used as a basis for purchase between suppliers and customers. The technical content of DTD 498 is still valid and forms the basis of a number of UK copper-nickel-silicon alloys.

Copper-nickel-silicon alloys are detailed in British Standards Aerospace Series BS 3B 26:2009. The earliest reference in American UNS specifications is May 1971.

The copper-nickel-silicon alloys were designated as European alloys in the period 1996-1999, namely CuNi1Si (CW109C), CuNi2Si (CW111C) and CuNi3Si (CW112C). In addition to the standard alloys, a number of European propriety alloys have been developed, some with small additions of magnesium which improves high temperature properties.

Heat Treatment

To develop maximum properties, CuNiSi alloys are heated to 750-900oC and water quenched. This is called solution treatment. In this condition the alloy is in its softest condition. Maximum strength is obtained by heating the alloy in the range 425-500oC. This is called ageing.

Mechanical Properties at Room Temperature

Property

CuNi1Si

CuNi2Si

CuNi3Si

Tensile strength (N/mm2)

300-590

300-700

320-800

0.2% Proof strength (N/mm2)

100-570

100-620

120-780

% Elongation

30-5

35-5

30-5

Hardness (HV)

80-220

80-220

80-230

Electrical conductivity (% IACS)

29-60

25-51

35-45

Thermal conductivity (W/moC)

85-260

250

190


The values are indicative of the range possible; they depend upon the form (forging, rod, sheet, tube, wire), dimensions and heat treatment. Liaison with the stockist or manufacturer will be necessary to confirm that the required properties are available in a particular section size.

Properties at Elevated Temperatures

a) Stress relaxation resistance

It is important to maintain an applied stress at temperature e.g. in a contact spring, clip or a tightened nut and bolt. If the stress begins to fall during service this is called stress relaxation. However, copper-nickel-silicon alloys are able to maintain a high percentage of the applied stress up to 150 to 200oC; they have a good stress relaxation resistance. Values may be obtained from manufacturers.

b) The effect of alloy condition and temperature on tensile strength

The table below shows the effect of alloy heat treatment condition on strength with the highest value found in the fully aged and cold-worked condition. At 200oC the strength starts to decrease falling to a low value of 600oC. Values stated are for CuNi2Si. 

Condition

Testing Temperature oC

Tensile Strength N/mm2

Solution treated
20
200
600
265
250
185
Solution treated and age-hardened
20
200
600
605
565
240
Solution treated and age-hardened and cold-worked
20
200
600
740
690
235

Properties at Sub-Zero Temperatures

Copper-nickel-silicon alloys do not suffer embrittlement at sub-zero temperatures and can be used down to -269oC.

Comparison with Alternative Alloys

Copper-nickel-silicon alloys have a superior combination of strength and electrical conductivity compared to alternative alloys such as aluminium-bronze and phosphor-bronze with an electrical conductivity 5 - 6 times greater for similar strength values. This is an important design consideration when selecting alloys for stressed electrical components such as fuse clips, contact springs and rotating electrical machinery parts.

Fabrication

 Process Rating
Cold formability
Good
Hot formability
Excellent
Soldering Good
Brazing Good
Oxyacetylene welding
Good
Gas shield arc welding
Good
Resistance welding
Good

This alloy does not suffer from steam (hydrogen) embrittlement when heated in a hydrogen (reducing) atmosphere.

Machining

This is rated at 20 - 30% which is satisfactory. Free-machining brass is 100%.

Resistance to Corrosion

The resistance to atmospheric and marine corrosion is good and copper-nickel-silicon alloys are insensitive to stress corrosion cracking.

Applications

Copper-nickel-silicon alloys are found in the electrical/electronic industry and utilise the high strength together with the good electrical conductivity, stress relaxation resistance and corrosion resistance of the alloys. Typical applications include:

  • Lead frames
  • Connectors
  • Fuse clips
  • Contact springs
  • Resistance welding electrodes
  • Current carrying formed parts
  • Switchgear
  • Press fit pins
  • Miniature sockets for surface mount devices
  • Power distribution frets in automotive junction boxes

Available Forms

CW109C is available in forgings, profiles, rod and wire.
CW111C is available in forgings, plate, profiles, rod, sheet, strip, tubes and wire.
CW112C is not included in any EN standards for forms.

Specifications

Below are the standards and specifications for Europe and US. Note that for USA, some compositions are not identical. For equivalent standards from other countries visit the Copper Key website.

  • UK: BS 3B 26:2009 (British Standards Aerospace Series).
  • Europe: CW109C (CuNi1Si), CW111C (CuNi2Si), CW112C (CuNi3Si) (European Standard EN designation).
  • USA: C19010, C19015 (CuNi1Si), C70260, C64700 (CuNi2Si) (American Society for Testing and Materials ASTM designation).
Further information on copper-nickel-silicon alloys, and other conductivity materials, is available at the Copper Alloys Knowledge Base.

CuNi1Si
CuNi2Si
CuNi3Si

Application Example 1: Electric Motor and Generator Rotor Slot Wedges

Rotor slot wedges support the rotor coils in the slot during rotation; they are highly stressed and must be made from high strength, temperature-resistant material. To withstand the repeated stopping and starting of the generator, the alloy must also have good fatigue resistance. CuNi2Si has a fatigue strength of approximately one third of its tensile strength after one million cycles and its good electrical and thermal conductivities, making it an excellent choice for this application.

Application Example 2: Measurement While Drilling

Measurement While Drilling (MWD) is a type of well logging that incorporates measurement tools into the drillstring and provides real time information to help with steering the drill. Density, porosity, temperature, radiation levels and rock fluid pressure measurements are taken using sensors in downhole tools situated behind the drill bit. The measurements are processed at the surface and are used to enable safe, accurate and profitable drilling to take place. These measurements involve electricity, and high conductivity copper-nickel-silicon components in the sensors play a vital role in their efficient operation. In addition to its electrical conductivity, copper-nickel-silicon has high strength, wear resistance and good corrosion resistance to withstand the arduous sea bed environment.

Quick Facts

Properties (CuNi2Si)

The properties are shown as a range from the solution treated to the fully aged condition.

  • Tensile strength: 300-700 N/mm2
  • Proof strength: 100-620 N/mm2
  • % Elongation: 35-5
  • Hardness (HV): 80-220
  • Electrical conductivity: 25-51% IACS
  • Thermal conductivity: 250 W/moC

Applications

  • Lead frames
  • Connectors
  • Fuse clips
  • Contact springs
  • Resistance welding electrodes
  • Current carrying formed parts
  • Switchgear
  • Press fit pins
  • Miniature sockets for surface mount devices
  • Power distribution frets in automotive junction boxes

Available Forms

  • Bar
  • Forgings
  • Plate
  • Rod
  • Sheet
  • Strip
  • Tube
  • Wire

Application Example 1: Electric Motor and Generator Rotor Slot Wedges

Slot wedge

Cross section of a generator slot (Courtesy of Siemens Industrial Solutions).

Application Example 2: Measurement While Drilling

Sub-sea drilling operation using Measurement While Drilling (Courtesy of Texas Oil and Gas).

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