SOLUBILITY OF GAS IN COPPER
14 Technology
Copper has widespread use in various sectors such as electrical, telecommunications and information technologies as well
as automotive and construction industries.
The physical and chemical properties that make copper the
favored choice include its electrical and thermal conductivity,
formability, corrosion resistance and applicability to soldering,
brazing and welding.
Even though no single property of copper is the most important, the high electrical conductivity and formability or drawability of the wire are oftentimes among its most appreciated
properties due to its various applications within the wire and
cable industry.
The above mentioned general properties of copper,
mentioned above, are somehow influenced by the gas content
which is present. It is interesting to note that the solubility of
oxygen in copper is about 70 ppm at a temperature of 1085°C
(solidification temperature); whereas the maximum solubility of
oxygen in copper at room temperature (25°C) is around 2 ppm
as displayed in Figure 1.
Electrolytic Tough Pitch (ETP) copper rod is manufactured in
such a manner that the copper melt is purposely oxidized;
typical Cu-ETP contains oxygen in the range of 200 ppm to
400 ppm.
1
1084.9
900
Penetration, mm
Temperature
(°C)
Photo 1 – Interstitial cuprous oxides
Į(Cu)
Į(Cu) + Cu2O
20
5
4
.15
3
.10
Penetration, in.
The Cu-ETP as-cast structure consists of solid solution alpha
dendrites. In the eutectic, there are cuprous oxides (Cu2O),
as round precipitates in the solid solution alpha matrix; see
Photo 1. Also, most impurities have been oxidized in the grain
boundaries. The interstitial cuprous oxides will form a very
dangerous element for subsequent cold drawing deformation
and even for operations such as soldering and brazing.
2
700
0.5
1
500
0.1
25
50
Oxygen (ppm)
Figure 1 – Solubility of oxygen in copper
70
0
50
100
150
200
250
Time, minutes
Figure 2 – Hydrogen penetration into the ETP copper hot rolled rod
Photo 2 - Central bursting
induced by copper oxide:
cone portion of the wire break
Photo 3 – ETP Cu, Ø 8.00 mm:
hydrogen embrittlement within
the copper structure, after
hydrogen atmosphere annealing
Technology 15
The cuprous oxides increase the fragility of copper during the
cold drawing processes, especially for fine wires (see Photo 2).
Even if the oxygen is present in the copper, as cuprous oxide, and the metal is exposed to a reducing atmosphere at
high temperature (over 400°C) such as welding, the hydrogen present in the atmosphere will diffuse through the copper
matrix and react with the oxygen and form vapor between
the grain boundaries causing embrittlement.
Weight % Oxygen
Temperature
(°C)
The gas having the greatest tendency to dissolve in copper
is hydrogen. Recent laboratory works indicate the solubility of hydrogen in copper at various temperatures. At room
temperature, the total hydrogen content ranges from 2.5 to
4.5 cm3 per 100 grams. Figure 2 shows the penetration
of the embrittlement in ETP copper at various oxygen
percentages versus temperature.
EFFECT OF OXYGEN CONTENT
From a metallurgical prospective, ETP copper is a binary
alloy, even though the alloying element (oxygen) is present in
very small quantities.
The equilibrium phase diagram presented in Figure 3, indicates that ETP copper is a two phase mixture composed of
a copper-oxygen solid solution and the intermediate cuprous
oxide (Cu2O) phase.
0.002
0.004
0.006
0.008
0.010
1084.9 °C
1050
0.03
1066 °C
(Cu)
As an example, Photo 2 shows central bursting induced by
copper oxide particles – cone portion of the wire break.
Photo 3 shows the Cu-ETP rod sample after exposure to
hydrogen atmosphere annealing. Prior to the hydrogen
annealing test, the sample had the following features:
>> Grain size: 0.025 mm
>> Rm: 230 N/mm2
>> Elongation (A5): 61%
>> Oxygen: 225 ppm
Nitrogen, carbon monoxide and carbon dioxide are practically insoluble in copper. However, they may be entrapped
during solidification.
0
1150
950
(Cu) + Cu2O
850
0
0.01
0.02
0.03
0.04
Atomic % Oxygen
Figure 3 – Equilibrium phase diagram copper-oxygen
Electrical
Conductivity,
% IACS
99.999% Cu
Vacuum Melt Cathode
102.0
OF Rod
101.0
100.0
99.0
The effects of oxygen content upon electrical conductivity
are illustrated in Figure 4, and has considerable commercial
applicability. Conductivity increases for the first 200 ppm of
oxygen and then decreases in a linear manner at higher oxygen concentration.
This important increase in conductivity is caused by the interaction between the oxygen and the impurities to form metal
oxides, thereby reducing the amount of residual elements
remaining in solid solution.
0.0
0.050
0.100
0.150
% Oxygen
Figure 4 – The effects of oxygen content upon electrical conductivity
16 Technology
Annealing
Temperature
Increase
[°F/ppm]
Spiral
Elongation
Decrease
[mm/ppm]
Resistivity
Increase
[μ£cm/ppm]
S
15
10
.0016
Se
15
> 50
.0097
Te
10
20
.0034
Element
Pb
Bi
6
5
15
Figure 5 summarizes the isolated effects of individual metallic
elements on fully annealed wires in reference to:
a Annealing temperature
b Spiral elongation
c Resistivity
The simple addition of these isolated effects result in overall
values that greatly exceed experimentally measured properties from commercial ETP copper. Oxygen also has a profound effect upon most of the physical and mechanical
properties of copper rod as displayed in Figure 6 including:
.0009
>> 30
---
S.E.N.
4
Cuppy breaks
Conductivity
3
3
.00029
6
Area reduction
As
3
4
.00056
7
Twists to failure
5
.00016
8
Energy absorbed
Sn
Fe
1
.0012
Ni
1
.00014
Ag
1
2
The actual choice of oxygen content used in commercial ETP
copper is dictated by a compromise between achieving optimum overall final properties and avoiding drawability problems.
Commercial ETP copper is usually produced by continuous casting and direct rolling systems. All of these systems
take advantage of chemical reactions between oxygen and
either sulfur or hydrogen to form gases in the melt. Consequently, as-cast bars contain some degree of microporosity
which causes slightly reduced bar density. The vertical pouring layout of the Properzi casting machine, however, enables
the optimal evolution of those gases. Residual smaller voids
are eliminated by hot-rolling during the passes in the tandem
rolling mill. by Alberto Greppi
.0002
2
Breaks/ton
0.8
3
Drawing force, kg/mm2
30
20
0.4
10
0.2
0.0
200 400
Oxygen, ppm
102
600
0
800
6
% IACS
200 400
Oxygen, ppm
600
7
Area Reduction, %
100
75
200
50
100
25
60
200 400
Oxygen, ppm
600
8
# Twists to Failure
600
800
0
200 400
Oxygen, ppm
600
800
Energy Absorbed
80
60
40
25
200 400
Oxygen, ppm
0
800
25
50
100
% cuppy breaks
100
300
0
800
4
S.E.N.,mm
400
75
101
0
Drawing force
3
Sb
0.6
5
Breaks per ton
2
5
Figure 5 – Unit effects of individual elements upon
annealing temperature, resistivity and spiral elongation
of fully annealed wires
1
1
20
200 400
Oxygen, ppm
600
800
Figure 6 - Influence of oxygen content upon several different physical
and mechanical properties of copper
0
200 400
Oxygen, ppm
600
800
0
100 200 300 400 500
Oxygen, ppm