Low-alloy Copper Materials

Copper is a material with a high conductivity level for heat and electricity, excellent corrosion resistance, a medium hardness and good formability.  In some cases the properties of pure copper and not sufficient for the purpose.  These needs have prompted the development of a range of alloys based on copper with low concentrations of additional elements.

Copper alloys with minimal additional elements are refered to as low-alloy coppers.  In most cases the addtional element remain between only 1 to 2 % of the alloy, and overall less than 5%.  Not considered low-alloy coppers are CuZn5, CuSn2, CuSn4, CuSn5, CuAl5As or CuNi2, because these, according to DIN CEN / TS 13388, are usually identified as copper-zinc, copper-tin, copper-nickel and copper-aluminum alloys.

Properties of Low-alloy Coppers

By adding relatively small amounts of other elements, one or more properties of pure copper (such as hardness, softening temperature, and machinability) can be greatly improved, whereas other properties (such as electrical and thermal conductivity and corrosion resistance) remain largely intact.  Elements of this type, either alone or in combinations, include beryllium, chromium, iron, cobalt, magnesium, manganese, nickel, phosphorus, Sulfur, silver, silicon, tellurium, titanium, zinc, tin, and zirconium.  Certain elements such as Manganese and silicon reduce the conductivity even more dramatically, however show increased hardness, weldability, and corrosion resistance compared to other materials.  The influence on the alloy's properties also depends largely on the amount of added elements.

Very high conductivity and good machinability

Although copper has among the highest electrical conductivity, it is not easily machinable on an automatic lathe or turning machine. Machinability can be improved by adding tellurium, sulfur or lead.  A general classification of the machinability of these alloys is only possible if taking into account the cutting procedure (see i018: Guide values for the machining of copper and copper alloys).

 

Copper-Tellurium

Tellurium is only soluable in copper to a very small extent.  Therefore Cu2Te is found dispersed in fine particles throughout the alloy. CuTeP is listed as a phosphordesoxidierte, oxygen-free alloy in DIN CEN/TS 13388 (expandable table).  The oxygen-containing alloy CuTe has not been considered because of the susceptibility to hydrogen embrittlement and higher wear in machining.

CuTeP is a very machinable alloy with high conductivity (90-96% IACS).  With the addition of tellurium, the notched toughness of CuTeP decreases compared to that of copper. It can withstand between roughly 39 to 78 J/cm².  With machined screws, it is particularly important to take note of torque at the maximum point at which the screw can be tightened. Unlike pure copper, it is notable that the hardening temperature is increased to 350° C due to the solubility of small quantities of tellurium in copper. Their presence only affects the electrical conductivity very slightly and the good corrosion resistance does not diminish.  Further mechanical and physical properties of CuTeP can be found in the expandable table.

 

Copper-Lead

The alloy CuPb1 is typically deoxidised with phosphor, whereas the lead in the alloy allows for good machinability.  CuPb1, the only variation of the alloy, is standardised in the DIN CEN/TS 13388.

Leaded machinable copper has the very high conductivity of about 96% IACS *, which is only slightly lower than that of Cu-ETP, and is sufficient for the majority of needs involving current conduction.

Not including pure lead, the melting range of copper-lead alloys lie between 953°C and 1080°C.  The mechanical properties of the standard alloy CuPb1P in rod form (see DIN EN 12164) are similar to those of CuTeP (expandable table).

 

Copper-Sulphur

The solubility of sulphur in copper is minimum.  A varient of CUSP, deoxygenised and oxygen free, is to be found in DIN CEN/TS 13388 (expandable table).  Even small amounts of sulphur will increase the softening temperature of copper to about 300°C.  Because sulphur is only minimally soluable in copper, the termal and electrical conductivity remain nearly the same.  The corrosion resistance of CuSP is also approximately that of unalloyed copper.

 

 

Very high conductivity and high softening temperature

The softening temperature of copper can be increased through the addition of certain alloying elements such as silver or zircon (Figure 2) without seriously compromising the alloy's conductivity (Figure 3).  This is a great advantage in many electrical application, such as in current-carrying parts that are exposed to higher temperatures or mechanical stresses in that they are not softened and misshapened (commutator, armature windings, etc.).

 

Copper-Silver

Kupfer-Silber bildet ein eutektisches System mit einer maximalen Löslichkeit von 8 % Silber bei der eutektischen Temperatur von 779°C.

Copper-silver alloys, with a maximum silver content of 8%, create an eutectic system at (or with) an eutectic temperature of 779°C.

These common commercial alloys contain between 0.03% and 0.12% Ag and have a single-phase structure.  Within this range, the silver serves to increase the softening temperature.  The standards DIN EN 1976 and DIN EN 1977 contain copper-silver alloys with varying silver content and are further differentiate by oxygen-containing, oxygen-free or phosphorus oxidized copper.


Compared to pure copper, copper-silver alloys have another advantage.  Strain hardening accomplished through cold forming remains intact even at relatively high temperatures.  The softening temperature of CuAg0.10 is approximately 300°C.  Another excellent feature of copper-silver alloys is the relatively high creep strength [4], which is particularily useful under strain at high temperatures.  Electrical and thermal conductivity are not greatly influenced by the addition of silver (Figure 3).

 

Copper-Zircon

At 972°C, zircon is approximately 0.17% soluable in copper [20].  Because soluability decreases with sinking temperatures, the alloys are curable.  Only one alloy with 0.1% Zr is listed in DIN CEN/TS 13388.  CuZr is oxygen free and thus not sensitive to hydrogen embrittlement. 

CuZr has both a high conductivity of approximately 54 MS/m (95% IACS) and a hardness value of up to 480 MPa.  This combination is uncommon when compared to other low-alloy coppers.  Additionally, the material has a very high softening temperature which is adventageous soldering (Figure 5).

The actual curing effect is relatively minimal and therefore only brings technological benefits when combined with strain hardening.  Thus, for practical applications, the material is generally heat treated, cold worked and then hardened or is first hardened, and then cold formed.  The effects of curing begin to drop at temperatures above 525 ° C.  After annealing, for example about 1 hour at 425 ° C, the alloys remains solid.  Also notable, is the increased creep strength of copper-zircon alloys, even at significantly higher temperatures.

 

Copper-Zinc

Together, copper and zinc form a homogeneous alloy when the zinc content does not exceed about 37%.  The commercially available low-alloys contain about 0.5 to 0.9% Zn.  Phosphor is only present in traces.  Due to the oxidizing effects of the zinc, the alloys are free of oxygen and, thus, resistant to hydrogen embrittlement.  The alloy CuZn0.5 can be found in the standard DIN CEN/TS 13388.

Depending on the degree of cold forming, the tensile strength of the alloy lies between 220 MPa and 360 MPa.  Although copper-zinc does not reach the high softening temperature of that of CuAg0.10 or CuZr, it begins softening only at temperatures above 250 ° C.  The corrosion resistance is comparable to that of unalloyed copper.  The electrical conductivity of CuZn0.5 lies at approximately 83% IACS which is better than that of Cu-DHP.

High to average conductivity and an average strength

Depending on its material state, pure copper has a tensile strength at room temperature that lies between 200 MPa to slighlty over 400 MPa.  For many uses however, this tensile strength is insufficient.  Its value can often be improved with the addition of other elements such as magnesium, chromium, iron, etc.. This is, for example, necessary for highly stressed electrodes, resistance welding, and other circumstances under which high tensile strength is required. 

Copper-Magnesium

Depending on the conditional diagram of magnesium-copper, a maximum of 3.2% Mg are soluble in copper at 722 ° C.  The solubility of magnesium in the copper mixed crystal decreases as temerature decreases.  Generally alloys are manufactured with a maximum Mg content of 1%.  At room temperature the alloys remain a homogenous structure and do not show any hardening with the artificial aging process.

When compared to pure copper, copper-magnesium alloys exhibit a greater strength under static and dynamic loads at elevated temperatures.  Of particular note is copper-magnesium's good wear performace due to its excellent abrasion resistance.  As the magnesium content increases so do the strength values that result with cold forming.  Additionally, the softening temperature increases with the magnesium content to about 350°C.  The electrical conductivity, on the other hand, decreases somewhat more dramatically with the inclusion of magnesium compare to that of silver. 

Copper-Iron

Iron is soluable in copper up to 4.1% at 1095°C [20].  Because soluability decreases as temperature sinks, copper-iron alloy is mainly useful as a heat-treated alloy system.  The excretion that occurs with the increased strength of the alloy is minimal.  Thus, only when the increase of electrical conductivity and recrystalisation temperature simultaneously increase is the alloy commercially useful.  Both the alloys CuFe2P and CuFe0.1P are commercially available.  CuFe2P is standardized in DIN EN 1654 and DIN EN 1758.  CuFe0,1P can be found in the US (Unified Numbering System) under the designation C19210.

Copper-iron is characterized by a high thermal and electrical conductivity.  Depending on the composition and heat treatment, conductivity values can reach up to 350 W / mK and 90% IACS.  CuFe2P can reach a tensile strength of over 500 MPa at 74% IACS through cold forming.  Depending on the initial state of the alloy, the softening temperature lies between 400°C and 500°C.

 

Copper-Chromium

At 0.65% Cr, the maximum solubility of chromium in copper is very low at the eutectic temperature of 1075 ° C.  This value falls more sharply with decreasing temperature.  At 400°C, the soluability of chromium is under 0.03 %.  As a result, the elimination basis is such that the alloy is curable.  CuCr1-C is the only alloy listed as casting material in DIN EN 1982 (expandable table).  A moldable variation, CuCr1, is also found in DIN CEN/TS 13388.

Both tensile strength and electrical conductivity can be greatly increased by curing.  In the fully cured state, CoCr alloys have a good combination of resistance and electrical conductivity.  A disadvantage of CuCr1 is, in some cases, the relatively high notch sensitivity at elevated temperatures, a problem not found in CuZr for example [9].  The strength values are inversely proportional to the stress-deformation curves such as rolling deformation. The softening resistance of CuCr1 is very high (Figure 13).  At temperatures above 475 ° C, however, the curing effect is reduced due to the redissolution of the chromium particles.

Copper-Chromium-Zircon

With copper-chromium-zircon a hardness greater than that of binary copper-chromium is possible through curing.  The commercially available alloys contain between 0.4 to 1.1% Cr and 0.03 to 0.3% Zr (expandable table), see DIN CEN/TS 13388.  Several advantages of CuZr are its high softening temperature, strong creep resistance [4], and notch insensitivity (even at elevated temperatures).  Its limited hardening effect is a disadvantage.  On the other hand, CuCr shows good strength characteristics after curing.  In certain cases this too is a disadvantage, where CuCr shows a relatively high notch sensitivity at higher temperatures. 

In a ternary alloy of copper-chromium-zirconium (CuCrZr), the favorable properties of CuCr and CuZr can be combined.  CuCrZr exhibits the following properties:

  1. high strength at room temperature,
  2. high softening temperature, and
  3. an improved creep resistance, even at elevated temperatures

In these three areas the properties of CuCr1Zr exceed both those of the binary alloys CuCr and CuZr.  This is possible because the added zirconium increases the solubility of chromium in copper at high temperatures [7].  Under creep strain CuCr1Zr also has a tendency to become brittle due to the formation of grain boundary pores at temperatures above roughly 100 ° C [4].  With regard to curing, CuCr1Zr behaves much like CuCr1.  CuCr1 is unable to reach a Brinell hardness of 160 HB, although, with a combination of hardening and curing, it is possible for CuCr1Zr (see Figure 13).  The other strength characteristics of CuCr1Zr are also more favorable than those of CuCr1, especially at temperatures above 500 ° C where the hardening effect peaks and begins to decrease again (Figure 15).  Figure 16 shows the creep properties of CuCr1Zr compared to two copper-zirconium alloys.  Of the alloys examined, CuCr1Zr has the best properties.  The physical properties of CuCr1Zr correspond approximately to those of CuCr1 (expandable table).  When fully cured, electrical conductivity lies between approximately 78% to 86% IACS.

 

Copper-Nickel-Phosphorus

Copper-nickel-phosphorus can be cured by the addition of small, finely divided particles of nickel phosphides.  Adding nickel phosphides to the alloy improves the mechanical and physical properties of hardness, strength, and electrical and thermal conductivity.  Commerically available alloys contain between 0.8 to 1.2% Ni and 0.15 to 0.25% P (expandable table).  The optimal mass to weight ratio (Ni / P) is 4.3 to 4.8.  Copper and nickel have a complete solubility in both the liquid and solid states.  As phosphorus content increases, the solubility of the formed nickel phosphides decreases greatly in the copper mixed crystal.  In comparison to other low-hardenable copper alloys such as Cu-Ni-Si or Cu-Be, the solubility level of nickel phosphides in the copper mixed crystal is much smaller.  As a result, a higher electrical and thermal conductivity is reached after curing.

The dispersion of fine nickel-phosphide particles and the purity of the copper are both key variables in the microstructure, and thus affect the alloy's strength and conductivity.  Due to their high strength in the cured state, 550 MPa to 700 MPa (see the expandable table), and its relatively good electrical conductivity of 50% to 65% IACS, these alloys are frequently used for the production of electrical contacts.  The optimal paramenters for curing depend on on the preceding cold working, which influences the precipitations kinetics.  As a side effect of the decompression, the breaking elongation can be increased from 3% to up to 20%.  A temperature between 380 ° C and 420 ° C is needed for the most efficient curing.  Cu-Ni-P displays good relaxation (decompression) properties at high operating temperature.

Copper-Tin

Copper-tin alloys are among the oldest recycled copper materials.  Even relatively small amounts of tin can improve the softening properties when compared to those of pure copper.  An alloy with less than 1% tin does not have particularly strong segregation properties, especially when compared to higher alloy bronzes.  Phosphorus can be added to ensure the deoxidation of copper-tin alloy melt.  As a general rule, CuSn alloys contain up to 0.6% Sn and no more than 0.01% phosphorus.  Any more tin would greatly reduce the electrical conductivity of the alloy.  Copper-tin alloys can also contain oxygen, in which the oxygen reacts in the melt to form tin oxide. 

When compared to pure copper, the low alloy copper-tin exibits a slightly greater strength and a good electrical conductivity.  As the tin content increases, so does the strength of the alloy when repeatedly cold formed (Figure 23).  The softening temperature rises to approximately 330 °C.  The electrical conductivity of copper is diministed more by the addition of tin than by the addition of magnesium (see Figure 3).

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Average conductivity and high strength

Some additions to copper such as Beryllium (+ cobalt or nickel), nickel + silicon, nickel + tin, or titanium reduce the electrical conductivity, but show a significant increase of yield strength, tensile strength and hardness.

 

Copper-Beryllium

The curability of copper-beryllium alloy depends on the soluability of Be in copper.  Beryllim has a maximum of 2.1% soluability at high temperatures, but that percentage sinks as the temperature decreases.  The following materials are standardized in DIN CEN/TS 13388:  CuBe2, CuBe2Pb, CuCo1NiBe und CuCo2Be und CuNi2Be (expandable table).  Copper-beryllium materials are curable, which considerably increases the alloy's strength and hardness.  Its electrical conductivity is 43% IACS.  As a solution-annealed material it shows a high elongation factor and deep drawability.  Figure 25 illustrates CuBe2's ranges of strength and electrical conductivity with the curing.  The curing effect peaks at about 350°C.

 

Copper-Cobalt-Beryllium and Copper-Nickel-Beryllium

With the addition of cobalt and nickel, the softening temperature is improved to roughly 550°C.  The commerically available CuCo2Be contains between 2.0 and 2.8% Co and from 0.4 to 0.7% Be.  CuCo2Be can be found standardized in DIN CEN/TS 13388 (expandable table).  Some cobalt in CuCo2Be can be repalced by nickel to form CuCo1Ni1Be.  The properties of copper-nickel-cobalt-beryllium are virtually identical to those of copper-beryllium-cobalt.  The cobalt content can be completely replaced by nickel.  CuNi2Be is also found standardize in DIN CEN/TS 13388.  This is adventageous when a higher electrical conductivity is needed.  The electrical conductivity of CuNiBe surpasses that of CuCoBe.

Compared to the copper-beryllium alloys listed under 4.1, copper-cobalt-beryllium achieved a higher electrical and thermal conductivity and a slightly lower hardness and strength values.  With a systematic combination of cold forming and age treatment, the properties of the material can be customized for different purposes.  Figure 26 illustrates the dependence of the material's properties on the curing period at a given curing temperature.  The curing effect improves up to 500°C and then decreases.

Copper-Nickel-Silicon

The high solubility of silicon in copper is greatly reduced by the addition of nickel at higher temperatures and decreases further as temperature sinks.  Due to the temperature-dependent solubility of the intermetallic compounds, the copper-nickel-silicon alloys Ni2Si, Ni5Si2 (or Ni31Si12) are curable.  The crystal structures of silicon are partially coherent or incoherent with the base material.  CuNi1,5Si, CuNi2Si und CuNi3Si can be found in DIN CEN/TS 13388 (expandable table).


Materials made of copper-nickel-silicon alloys are distinguished by a high tensile strength, thin sheets, for example, have a high tensile strength up to 900 MPa, electrical conductivity to 50% IACS, good corrosion resistance and good formability.  Because of their very good thermal stability, the higher nickel containing materials are used for the construction of spring elements that operate at temperatures above 150 ° C.  Even small additions of magnesium will improve the resistance to stress.  A partial substitution cobalt in place of nickel increases the strength to up to 100 MPa.

 

Copper-Nickel-Tin

When creating the alloy copper-nickel-tin, a portion of the nickel coverts to nickel phosphide.  These fine nickel phosphide particles are distributed throughout the microstructure through a series of rolling and annealing processes.  This yields very good electrical conductivity and strength values, an extremelly useful combination.  The alloys are not normalized under European standards, but several (such as CuNi1Sn0.9, C19025) can be found stardardized in ASTM (B422, B888).  In addition to good conductivity and strength, the alloys display a good resistance to stress relaxation.  Thus, the alloys are particularly well suited for use at higher temperatures (above 150 ° C).  Strength values of over 540 MPa and good bendability can be achieved through the cold forming process, although the success depends on the particular alloy and the scale of the dimensions.  Softening begins at temperatures between 400 and 450 °C.  The materials are particularly suitable for small parts in the electrical and electronics sector.

Materials not requiring a high conductivity.

 

Kupfer-Manganese

Copper and manganese form a homogeneous alloy where Mn levels are at or under 20%.  Because of the deoxidation effect of manganese, these oxygen-free alloys, unlike reducing gases, are not sensitive at high temperatures.  The addition of manganese to copper increases the tensile strength at both room temperature and at higher temperatures (Figure 29).  The softening temperature of CuMn2 and CuMn5 lies between 400°C and 450°C.  The addition of manganese, however, significantly compromises the conductivity of copper (Diagram 3).

 

Copper-Silicon und Copper-Silicon-Manganese 

The solubility of silicon in copper peaks at 5.3% at 842 ° C and decreases as the temperature drops [7].  Industral alloys have a homogeneous structure, however typically contain a maximum of 3.6% Si.  In general, these alloys additionally contain 1.8 to 3.6% Si and 0.3% to 1.3% Mn.  This small amount of manganese does not significantly affect the soluability of the silicon.  CuSi1 and CuSiMn are are both standardized in DIN CEN/TS 13388 (expandable table).  Silicon considerably improves the mechanical properties of copper (Figure 30).  A 3% silicon content has the same strengthening affect on copper comparable to approximately 42% Zn, 8% Al or 6% Sn.  At the same time, silicon improves the forming ability.  Compared to pure copper, the copper-silicon-manganese alloys solidify stronger through cold forming.  The temperature at cold forming influences the resulting strength values.  CuSi3Mn1 is used to illustrate that in Figure 31.  The softening temperature of CuSi3Mn1 is approximately 300°C.  The curves of strength characteristics and impact strength of CuSi3Mn1 in Figure 32 indicate that these materials are also suitable for cryogenic applications.  The electrical conductivity is only about 5 to 10% of that of copper.

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Literatur

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