JOM, Vol. 64, No. 3, 2012
DOI: 10.1007/s11837-012-0266-8
Ó 2012 TMS
Ultrasonic Welding of Wires and Cables
STEFAN HEINZ,1,2 GUNTRAM WAGNER,1 and DIETMAR EIFLER1
1.—Institute of Materials Science and Engineering (WKK), University of Kaiserslautern, P.O. Box
3049, 67653 Kaiserslautern, Germany. 2.—e-mail: heinz@mv.uni-kl.de
In the automobile industry, ultrasonic metal welding is an established
method. At the Institute of Materials Science and Engineering (WKK) at
the University of Kaiserslautern, Germany, systematic investigations of the
weldability of Al-wires and flat flexible copper cables were carried out. In the
case of Al-wires, joints with cross-sectional area of up to 80 mm2 and tensile
shear load of about 3500 N were finally realized. Furthermore, methods to
reduce unintentional adherence between the sonotrode coupling face and the
Al-wires were developed. To realize FFC joints, ultrasonic spot welding
systems and ultrasonic torsion welding systems were used. A central purpose
of these investigations is the development of a system to enable welding
through the insulation of the FFC without weakening the base material.
INTRODUCTION
In the automotive industry, a continuously
increasing demand to integrate electrical and electronic components can be observed.1 As a result the
length of the cable harness has increased to a few
kilometers, leading to an extreme weight increase of
this component. On the other hand, modern vehicle
design causes a restriction of the installation space for
the cable harness, especially in the interior. For these
reasons the automotive industry is searching for
possibilities to substitute conventional copper wires.
Approaches to solve this problem are use of lighter
aluminum wires or flat flexible copper cables (FFC).
In the case of FFC, there is the possibility to reduce
the required installation space because of their flat
design. The low specific density of aluminum enables
a considerable reduction of the weight of the cable
harness. Furthermore, both possible solutions are
characterized by excellent ultrasonic weldability.2
STATE OF THE ART
Ultrasonic metal welding systems are always
based on the same components.2 To realize a
mechanical oscillation amplitude between 10 lm and
40 lm, an ultrasonic generator (1) converts the
50 Hz main voltage into a high-frequency alternating voltage output between 20 kHz and 60 kHz
(Fig. 1). By the use of the inverse piezoelectric effect,
the converter (2) transforms the alternating voltage
into a mechanical oscillation of the same frequency.
(Published online March 2, 2012)
The required amplitude, which depends on the
intended application, is ensured by an appropriate
design of the booster (3) and the welding tool, called a
sonotrode (4). Finally, the sonotrode transmits the
oscillation to the joining partners (5). To avoid relative movement between the sonotrode and the upper
joining partner, or between the anvil and the lower
joining part, the surface of the sonotrode as well as
the anvil (6) are profiled. This ensures the necessary
relative movement between the joining parts to
realize the weld. Besides the amplitude, the inputted
welding energy and the welding force, which is
applied perpendicular to the joining partners on the
anvil, are important machine parameters. Central
material parameters which influence the ultrasonic
weld are the chemical, physical, and technological
properties of the materials, the thickness of the
upper joining partner, and the surface structure of
the joining components.
In the industry, ultrasonic welding technology is
mostly used to join copper wires.2 In the case of aluminum wires, the problem is the distinct tendency of
aluminum to adhere to the sonotrode coupling face,
which makes an industrial manufacturing process
unprofitable.3 In the case of joining of FFC, ultrasonic
welding technology is only rarely used and not systematically investigated.1
Experimental Setup
Investigations of aluminum wire joints were carried out at WKK using wires with cross-sectional
421
422
Heinz, Wagner, and Eifler
Fig. 1. Principle of an ultrasonic metal welding system.
Fig. 2. Ultrasonic welded cables and wires.
area of 12.9 mm2 formed by 731 single conductors of
pure aluminum (diameter of single wire: 0.15 mm)
(Fig. 2a). The material is annealed and possesses
ultimate tensile strength of about 90 MPa and
hardness of 43 HV 0.2. The ultrasonically welded
aluminum wire knots have cross-section of 25 mm2.
The used welding system was a commercial ultrasonic metal welding machine to manufacture
Cu-wire joints. Besides a conventional sonotrode,
two sonotrodes with TiN and TiAlN coatings on the
coupling faces were applied to reduce adherence.
Flat flexible cables have several parallel-arranged, insulation-embedded flat Cu conductors.1
Investigations of FFC joints were divided into two
steps. First, joints were welded with FFC exhibiting
a welding area stripped of insulation. To illustrate
the possible industrial applications, FFC/copper
sheet joints and FFC/FFC joints with overlap angles
of 0° and 90° were realized. The thickness of the Cu
conductors varied between 35 lm and 200 lm. The
dimensions of the sheet were 20 mm 9 60 mm 9
1 mm. The welded layouts of FFC are shown in
Fig. 2b. Ultrasonic spot welding and ultrasonic
torsion welding systems were used.
Afterwards, FFC/sheet joints using FFC without
a stripped joining zone were welded. In this case the
thickness of the FFC conductor was always 100 lm
and it was embedded in polyvinylchloride (PVC)
insulation. To realize these joints, the ultrasonic
welding machine was upgraded with a control system developed at WKK which allows the joining
parameters to be varied during the weld.
The strengths of the aluminum wire joints and
the FFC welds were characterized by tensile shear
load tests.
RESULTS
Ultrasonic Welding of Aluminum Wires
After preliminary investigations to determine an
appropriate welding parameter window, welds were
carried out using a conventional sonotrode with
oscillation amplitude of 25 lm, welding force
between 700 N and 1300 N, and welding energy in the
range of 800 Ws up to 1200 Ws.3 The investigations
showed that different adherence effects occurred at
the coupling face. After a few welds, a thin Al-layer
develops on the surface of the sonotrode without a
negative influence on the welding behavior. Extensive adherence with completely covered pyramids of
the coupling face appears rarely but leads to damage of the joints and inhibits further welds with the
clogged sonotrode. In this case, the sonotrode was
cleaned using a chemical cleaning process based on
sodium hydroxide solution before further welds
were carried out. Figure 3 shows some examples,
where the correlation between the degree of adherence at the sonotrode coupling face and the
decreasing tensile shear load is clearly shown. If
more than 15% of the sonotrode coupling face is
clogged, the weld fails.
To understand the adherence process, the temperature at the surface of the sonotrode coupling
face and in the welding zone was measured using
Ultrasonic Welding of Wires and Cables
thermocouples, as well as the sonotrode penetration
depth and the welding power. Figure 4 shows the
time courses of these parameters for a successful
weld (a) with tensile shear load of 597 N and a weld
that failed (b) due to intense adherence. Comparison
of the courses of the welding power, penetration
depth, and temperature in the welding zone reveals
no remarkable differences, but the maximum temperature at the sonotrode coupling face is 85°C
higher for the failed weld than for the successful
weld. This higher maximum temperature is a sign of
unwanted relative movement between the sonotrode and the upper joining partner. This results in
intense plastification of the aluminum wires,
resulting in the adherence to the coupling face.
The influence of the elevated temperature at the
coupling face can be clarified by a cross-section
through the knot of the failed weld (Fig. 5). The
overview (Fig. 5a) shows high compaction at the
sonotrode side. In the detailed view, extreme
deformation of the single wires can clearly be seen
(Fig. 5b). The cable has lost its original structure, and the material looks mixed. The intensive
423
material deformation in combination with the high
temperature finally causes extensive adherence of
aluminum to the sonotrode coupling face.
To reduce the adherence tendency of aluminum,
the sonotrode was coated with TiN and TiAlN
(Fig. 6). For the TiN coating the average tensile
shear load increased by about 5% up to F = 586 N
(Fig. 6a). Furthermore, the standard deviation r was
reduced by about 60%, and the occurring adherence
is maximum 35%. The number of failed welds was
reduced but not completely excluded. Use of the
TiAlN-coated sonotrode led to the intended result
(Fig. 6b). Al adherence was nearly zero for all welds.
At the same time, the average tensile shear load
increased to 634 N, representing an increase of 14%
in comparison with welds made using the uncoated
sonotrode. Furthermore, the standard deviation
decreased to 89 N, representing a reduction of 60%
relative to the uncoated sonotrode. As a result, the
use of TiAlN-coated sonotrodes now enables welding
of knots with cross-sectional area up to 120 mm2 for
industrial applications without adherence problems.
Ultrasonic Welding of Flat Flexible Cables
Fig. 3. Correlation between tensile shear load and the degree of
adherence using a conventional sonotrode, according to Ref. 3.
The investigations of the FFC joints were realized
in two steps. Systematic optimization of the three
machine parameters oscillation amplitude u, welding force FUS, and welding energy WUS was carried
out first, with the purpose of achieving the maximum tensile shear load of the joints. Therefore, a
statistical model named ‘‘central composite design
circumscribed (CCC)’’ was used. This method
enables significant reduction of the number of welds
required for optimization in comparison with the
conventional one-factor-at-a-time method.4 A typical
parameter space of a three-dimensional (3D) model,
which consists of 15 different parameter triples, is
shown in Fig. 7a. The orthogonal structure of the
model is achieved by the star distance and ensures
the independence of the influencing parameters.
An exemplary result of the parameter optimization of ultrasonic spot welded FFC/sheet joints with
Fig. 4. Comparison of the process parameters: (a) successful weld, and (b) failed weld, according to Ref. 3.
424
Heinz, Wagner, and Eifler
Fig. 5. Cross-section of an ultrasonic welded aluminum wire joint: (a) overview, and (b) detail, according to Ref. 3.
Fig. 6. Tensile shear load and degree of adherence for welds made using: (a) a sonotrode with TiN coating, and (b) a sonotrode with TiAlN
coating, according to Ref. 3.
Fig. 7. (a) Parameter space of the CCC model. (b) 3D contour plot for constant amplitude of u = 10 lm for FFC/sheet joints.
conductor thickness of 100 lm is shown as a 3D
contour plot in Fig. 7b. It shows the attainable
tensile shear load as a function of the welding force
and the welding energy for constant oscillation
amplitude of 10 lm, ascertained to be the most
appropriate amplitude in previous investigations.
As a result of the contour plot, the highest tensile
shear load of 36 N can be expected for the welding
parameter triple: u = 10 lm, FUS = 150 N, and
WUS = 50 Ws. Verification tests using this parameter triple even yielded 40 N, which corresponds to
100% of the tensile load of the flat conductor.
Figure 8 presents an overview of the achieved
average tensile shear loads for different FFC joints.
In every case, tensile shear load of almost 100% of
the tensile load of the conductors was realized.
Ultrasonic Welding of Wires and Cables
425
Fig. 8. Tensile shear load of ultrasonically welded FFC joints with a stripped joining zone.
Fig. 9. Ultrasonic welding of FFC without a stripped joining zone: (a) time courses of the generator power and the voltage of the contact signal,
and (b) tensile shear load and parameter combinations.
Furthermore, the FFC joints welded with optimized
parameters possessed excellent values of electrical
contact resistance and corrosion behavior.
Subsequently, FFC/sheet joints without the bothsided stripped joining area were carried out in the
same way as welds with stripped areas. However,
these welds led to low tensile shear loads and high
standard deviations. To improve the quality of these
joints, a two-stage welding process was developed at
WKK. The special welding process is illustrated in
Fig. 9a. In the first step the insulation is pressed out
of the joining zone with machine parameters specially adjusted to the melting behavior of the plastic
coating of the FFC. During this process, the first
electrical contact between the joining partners
appears, which is used to trigger the change of the
parameter settings. After a time lag to ensure sufficient displacement of the plastic coating, the
welding parameters are switched to the optimized
parameters for Cu-conductor welding to realize
high-strength joints. The short low-power impulse
at the end of the welding is a shake-off impulse.
Figure 9b shows a few example parameter sets for
the welding of FFC/sheet joints without stripped
joining area and the resulting tensile shear loads.
In comparison with the tensile load of the Cu
conductor, the maximum average tensile shear
loads of the joints achieve 80%. A negative influence
of the remaining insulation in the joining zone on
the electrical contact resistance could not be observed.
CONCLUSIONS
For the first time, systematic statistical investigations of aluminum wires with knot cross-section
of 25 mm2 have been carried out. When using conventional sonotrodes, a considerable adherence
tendency of the Al-wires was observed. Hence, different coated sonotrodes were tested, and for a
sonotrode with TiAlN coating it was possible to
avoid completely failed welds caused by Al adherence. Based on this coating, it was also possible to
realize welds with cross-section up to 120 mm2 for
industrial applications.
Also, FFC joints with strengths on the level of the
embedded Cu conductor could be realized by optimization of the machine parameters using the statistical CCC method. In addition, the welds
possessed excellent electrical contact resistance and
good corrosion behavior. The investigations have
also shown that ultrasonic welding of FFC without a
stripped welding zone is possible, by use of the
426
Heinz, Wagner, and Eifler
two-stage welding process developed at WKK. This
offers possibilities for new cost-efficient industrial
applications of ultrasonic welding.
13.285 N and 15.112 N) and the German Research
Association for Welding and Allied Processes (DVS).
ACKNOWLEDGEMENTS
1. J. Czabaniski and J. Krieger, Draht 3, 54 (2000).
2. M. Greitmann, T. Adam, H.G. Holzweißig, D. Stroh, G.
Wagner, P. Wiesner, and R. Züst, Weld. Cut. 55, 268 (2003).
3. M. Gutensohn and G. Wagner, DVS-Berichte 237, 452 (2005).
4. T. Hill and P. Lewicki, Statistics: Methods an Applications
(Tulsa: StatSoft, Inc., 2005).
The authors appreciate support of these investigations by the German Federal Ministry of Economic Affairs and Technology via the Study Group
of Industrial Research Associations (AiF-Number:
REFERENCES