Lightweight Copper/Aluminum Composites – Next Generation

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Lightweight Copper/Aluminum Composites – Next Generation Conductors
for the Aerospace Market
Emilio I. Cerra
VP Product Development & Engineering
IWG High Performance Conductors
Abstract
Weight reduction is a never-ending challenge on an aircraft and
the latest generation of fuel efficient airplanes has placed even
more pressure on manufacturers to reduce weight.
Unfortunately, in the realm of aerospace cables, there has been
precious little improvement in weight reduction over the past
decade, and what has occurred has been primarily due to
insulation system improvements. The electrical conductors used
in these cables have not changed significantly during that time.
This paper will explore a new conductor construction that,
utilizing both copper and aluminum strands, has the potential for
reducing cable weight without significantly impacting resistance
and, more importantly, without changing the methods with
which said conductors are terminated.
1. Introduction
Electrical conductors used in aerospace cables have remained
virtually unchanged since the introduction of advanced alloys
such as PD135 (Tensile-Flex®) and CS95® by the Hudson Wire
Company more than thirty years ago. In more recent years, EC
aluminum and copper clad aluminum (CCA) ropes have been
used in power feeder applications at both Boeing and Airbus.
These conductors, however, require special care during
manufacture and termination in order to avoid potential
electrical failures.
After examining several alternative constructions and materials,
High Performance Conductors (HPC) settled on a composite
configuration that utilized both copper and aluminum strands in
the conductor.
2. Composite Constructions
Composite constructions are used in the wire and cable industry
when properties are desired that are not available in existing
materials; for example, ACSR power cables that use a steel core
for tensile strength and aluminum alloys for electrical
conductivity and weight savings.
The composite conductors described in this article utilize a core
made of aluminum strands surrounded by an outer layer of
copper wires. Constructions typically contain 19 or 37 wires (or
members, in the case of a rope). As a point of reference, the
evaluation sample referenced elsewhere in this document was a
1/0 gauge rope, containing 37 members with 7 strands of 24
gauge each (37x7/24). Aluminum and copper were not mixed in
the members nor in the layers; the 19 member core was made of
1350 EC aluminum and the outer 18 members were made of
nickel plated ETP copper. The copper strands were coated with
nickel at the request of the OEM testing the cable; silver or tin
coatings could also be used.
EC Aluminum
The introductions of popular new airframes such as Boeing’s
787 Dreamliner and Airbus’s A350 and A380 that promise
significantly lower fuel consumption and operating costs have
placed enormous pressure on aerospace engineers to reduce
weight in all areas of the aircraft. One such conductor solution –
nickel plated aluminum and/or nickel plated copper clad
aluminum has the unfortunate tendency to form cracks in the
nickel coating. To date these voids have been managed via
careful control of the manufacturing and assembly processes;
nevertheless the potential for trouble exists.
Approximately six years ago IWG High Performance
Conductors Inc. embarked on a project to develop a conductor
with significantly lower weight per thousand feet relative to an
equivalent copper conductor. The project included the following
targets:
Weight savings of 10-20%
Economical alterative to existing products
Easy to install; capable of utilizing
connector/crimp technologies
Designed for manufacturability
ETP Copper
existing
Figure 1. 37 member composite copper
& aluminum rope construction
19 wire constructions on the other hand, are made with 7 inner
strands (or members) of aluminum and 12 outer strands of
copper. These geometric constraints drive the physical
characteristics of the cables; therefore a 37 wire cable will
contain more aluminum as a percentage of the total conductor
(51.4%) than a 19 wire cable (36.8%). This same 37 wire
conductor will weigh less and be lower in conductivity than a
similar sized cable using 19 wires.
An alternate construction that would reduce the weight variation
between 19 and 37 wire conductors by using alternating strands
of copper and aluminum was also considered but ultimately
discarded. This construction would have been more uniform in
composition (52.6% and 51.4% aluminum respectively in 19
wire and 37 wire constructions), but would have contained
aluminum strands in the outer layer, requiring the use of special
connectors.
In order to simplify production and reduce costs, as well as
avoid the surface quality issues known to occur with nickel
plated aluminum and copper clad aluminum, it was decided to
leave the aluminum conductors unplated. However, since
galvanic corrosion of the aluminum strands is a potential
concern in a construction of this nature the copper wires are
plated to inhibit any incipient corrosion.
3. Weight Savings
The primary purpose of these constructions is weight reduction;
Tables 1 and 2 show expected weight and direct current
resistance (DCR) values for a representative sample of nickel
plated cables and ropes in both conventional copper and
copper/aluminum constructions.
Table 1. Composite Copper/Aluminum
Conductor Properties
Size
22
20
18
16
14
12
10
8
6
4
2
1
1/0
2/0
3/0
4/0
Construction
19 x 34
19 x 32
19 x 30
19 x 29
19 x 27
37 x 28
37 x 26
19 x 7 / 29
19 x 7 / 27
19 x 7 / 25
19 x 35 / 30
19 x 43 / 30
19 x 55 / 30
19 x 70 / 30
37 x 45 / 30
37 x 57 / 30
Weight
(lbs/kft)
1.61
2.60
4.13
5.25
8.25
11.06
17.92
37.10
58.70
95.40
146.00
173.00
228.00
292.00
316.00
397.00
DCR
(Ω/kft)
18.480
11.250
7.060
5.520
3.470
2.440
1.530
0.803
0.504
0.318
0.205
0.167
0.131
0.103
0.088
0.069
Generally speaking, a 19 wire composite construction will be
26% lighter than an equivalent copper conductor, with DCR
16% higher than the same. A 37 wire composite construction
will be 36% lighter but 24% higher in resistance than an
equivalent copper conductor.
Table 2 – Conventional Copper
Conductor Properties
Size
22
20
18
16
14
12
10
8
6
4
2
1
1/0
2/0
3/0
4/0
Construction
19 x 34
19 x 32
19 x 30
19 x 29
19 x 27
37 x 28
37 x 26
19 x 7 / 29
19 x 7 / 27
19 x 7 / 25
19 x 35 / 30
19 x 43 / 30
19 x 55 / 30
19 x 70 / 30
37 x 45 / 30
37 x 57 / 30
Weight
(lbs/kft)
2.17
3.50
5.56
7.06
11.10
17.22
27.90
49.90
79.00
128.30
196.00
233.00
307.00
393.00
492.00
618.00
DCR
(Ω/kft)
16.000
9.770
6.100
4.770
3.000
1.980
1.240
0.694
0.436
0.275
0.177
0.144
0.113
0.089
0.071
0.056
In applications where resistance is of primary importance and
DCR values must be maintained, increasing overall conductor
size by approximately 6% in 19 wire and 9% in 37 wire
composite constructions will ensure that resistance be
unchanged. These cables would still be 14% and 21% lighter
than their equivalent 19 & 37 wire counterparts, although the
size change would preclude one from using existing connectors
during termination.
4. Evaluation Sample
A 37 member, 1/0 gauge evaluation sample (37x7/24) was
submitted to a top-tier aerospace cable manufacturer to be
insulated and tested, then sent to an end user for further
corrosion, crimp and thermal shock tests.
The insulated conductor was tested at the OEM to Douglas
specification DMS 2340. With the exception of flexure
endurance, all conductor related tests, including bend radius,
stiffness, tensile and elongation passed the test requirements. All
other insulation related tests passed as well.
Table 3 lists flexure endurance test results for both the
copper/aluminum composite and a nickel plated copper
standard. In the case of the composite rope, as would be
expected, aluminum strands began failing in the core by the
500th cycle, with all strands broken (both aluminum and copper)
by the 2,875th cycle (see figures 2 & 3). In contrast, the 1/0
gauge nickel plated copper rope did not exhibit strand failures
until the 5,000 cycle mark.
Since there is no industry standard for flexure endurance failure,
it will be necessary for individual OEMs and organizations such
as ASTM and SAE to determine appropriate failure levels for
these conductors going forward.
Table 3 – Flexure Endurance
(Boeing Standard BSS 7324, Sec. 7.26)
Cycle Members
500 Copper
Aluminum
1,000 Copper
Aluminum
1,600 Copper
Aluminum
2,600 Copper
Aluminum
2,700 Copper
Aluminum
2,875 Copper
Aluminum
5,000 Copper
Aluminum
Results
NP Composite
NP ETP Copper
All strands intact
All strands intact
Few broken strds
n/a
All strands intact
All strands intact
Most strds broken
n/a
Few broken strds
All strands intact
All strands broken
n/a
Few broken strds
All strands intact
All strands broken
n/a
Most strds broken
All strands intact
All strands broken
n/a
All strands broken
All strands intact
All strands broken
n/a
All strands broken
Few broken strds
All strands broken
n/a
5. Next Steps
Additional tests will be needed to gauge the ability of other
commonly used plating materials such as silver and tin in
reducing or eliminating galvanic activity. It will also be useful to
see if other types of aluminum alloys are more or less
susceptible to corrosion.
Additionally, crimpability testing must be performed, including
Mil-T-7928 terminal lug pull tests under a variety of
environmental and aging conditions, to confirm the absence of
cold creep in the terminations of these cables.
Finally, other construction types and sizes should be tested; in
particular 19 strand unilay conductors of the type commonly
used in airframe interconnect cables. HPC has prepared several
19 strand unilay conductors, most recently a 22 gauge conductor
using 7 strands of 5254 aluminum alloy in the core with 12
strands of tin plated ETP copper in the outer layer. These
conductors needs to be insulated and tested in a similar fashion
to the 1/0 gauge rope described in the preceding section.
Figure 2. Flexure endurance after 1,258 cycles;
most aluminum strands broken
Figure 4. Cross-section of 19 strand composite
unilay conductor, 22 gauge
6. Conclusions
Composite copper/aluminum conductors as presented in this
document exhibit considerable promise for use in aerospace
applications.
Figure 3. Flexure endurance after 2,500 cycles;
all aluminum and some copper strands broken
The cables are lightweight; in some cases as much as a third
lighter than their copper-only counterparts, while producing a
manageable 15 to 25% increase in resistance. In those cases
where resistance is critical, 15 to 20% weight savings can be
achieved with a modest diameter size increase of 6 to 9%.
If, however, higher flex life or tensile strength is required,
alternative materials such as aluminum alloy 5254 can be used
that will significantly improve performance, albeit with a
noticeable increase in resistance.
They are easy to use and install; by restricting the use of
aluminum strands exclusively to the inner layers of the
conductor, it is expected that problems commonly associated
with terminating aluminum will be avoided.
They are also cost competitive; by utilizing standard
manufacturing techniques and by avoiding the potential pitfalls
of plated aluminum strands, these conductors can be produced at
prices that are competitive on a per foot basis with existing
products.
In summary, composite conductors containing a mixture of
aluminum and copper strands can be a valuable tool in the
aerospace engineer’s weight reduction toolbox. Although
additional research is needed to better understand the effects of
galvanic corrosion and creep, the data to date suggests a very
bright future for these conductors.
7. Acknowledgements
The author would like to thank the following individuals for
their assistance during the preparation and testing of the
conductors described herein: Zhimin Yang, Stephen Childers
and Bill Dorcas of IWG High Performance Conductors; Glen
Terry, Bill Brown and John Kim of Judd Wire Inc; James
Likoray of Bombardier Aerospace.
Flexure endurance photographs are courtesy of Judd Wire Inc.
8. Contact
Emilio I. Cerra
VP Product Development & Engineering
IWG High Performance Conductors
1570 Campton Rd.
Inman, SC 29349
phone: 864-472-0410
email: [email protected]
Mr. Cerra has been Vice President of Product Development &
Engineering at IWG High Performance Conductors in Inman,
South Carolina since 2004. During the nine years prior to that,
Mr. Cerra was responsible for all manufacturing and engineering
activities at HPC. He began his wire career with Phelps Dodge
in 1986 as a process engineer in their International Wire &
Cable division, and has worked in numerous countries such as
Mexico, Chile, Venezuela and China. Mr. Cerra has a Bachelor
of Science degree in Mechanical Engineering from the
University of Missouri and is an ASQ trained Six Sigma Back
Belt as well as a lifetime member ASME and a voting member
of the SAE AE-8D technical committee.
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