PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Investigation of the applicability of acoustic emission andvibration analysis to describe the thermo-mechanical mechanism
during ultrasonic metal welding
Elie ABI RAAD1; Isabel BALZ2; Uwe REISGEN2; Michael VORLÄNDER1
1

Institute of Technical Acoustics, Germany
2

Welding and Joining Institute, Germany

ABSTRACTUltrasonic Metal welding (USMW) is particularly suitable for connecting electrotechnicalcomponents. Despite industrial spread, unexplainable process fluctuations may occur in USMW. Thislargely originates from the lack of profound scientific knowledge and understanding of theinterdependency between tools and joining members. The aim of this research is to derivecharacteristic features, which correlate with the mechanisms during bond formation and joint
properties, to achieve a more profound understanding, and in the future to open up new possibilities
for online process control monitoring. In this study, copper sheets (CW008A) were ultrasonicallywelded at 20 kHz to study their effects on the thermo-mechanical mechanism during welding. Twolaser vibrometers recorded the tool vibrations of the anvil and horn, directly connected to the sheets,
and a ¼’ microphone recorded the airborne sound. Additional T-peel tensile testing and laser scanningmicroscopy of the fractured surface at different welding stages are used for validation. The results ofthis paper provide a more profound understanding of the thermo-mechanical mechanism during
USMW and will introduce a prospective improvement in quality and process stability.
Keywords: vibroacoustics, ultrasonic metal welding, bond formation

1. INTRODUCTION
Ultrasonic metal welding (USMW) is a solid state welding process that uses high frequency
oscillations and a static clamping force. It is widely used in various industry applications such as
joining in electric motors, strand welding in harness production or in the assembly and connection ofbattery cells (1). Since bonding is formed in solid state, common geometric and metallurgicalproblems encountered in fusion welding of aluminum and copper, such as gas porosity, intermetallics
and embrittlement (2) are avoided. Further reasons for the industrial widespread of USMW are its low
energy consumption, the short welding times and its good automation capacity. Although there isextensive research on the bond formation in USMW, there is still a lack of profound understanding of
the thermo-mechanical mechanism during welding, which is essential for weld optimization.
In USMW, various binding mechanisms have been proposed such as mechanical interlocking,melting, adhesion, recrystallization and even diffusion (1, 3, 4) whereby in the last studies (4, 5)
mainly dynamic recrystallization (DRX) is regarded as the primary binding mechanism. The bondformation is described by different process stage models. There is agreement that periodic shear
deformations at the interfaces are significantly responsible for the bond formation in USMW (5). The
temperature increase with increasing welding time is caused by friction and deformation (5). The
friction conditions and the development of plastic deformations are strongly in fluenced by welding
parameters (7, 8). Thus, friction work increases with increasing oscillation amplitude and is in non linear relation to the welding pressure (6, 7, 9). According to (5, 6, 10), friction energy dominates theinitial welding process and is mainly responsible for the breaking of oxides and surface cleaning. As
welding time increases, the temperature of the joining members rises and results in material softening(5, 6). This leads to a larger plastic deformation and favors the bond formation. Theoret ical andexperimental analyses (6, 11) have shown that a thermal-stationary stage is established at the end of
welding in which no further plastic deformation occurs and the maximum bonding area is achieved.
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elie.abiraad@akustik.rwth-aachen.deThere are different theories regarding the description of an ideal end of process. Some researchers (4,6) describe a stagnation of the joint strength, whereby others (8, 11) observe a decrease in the joint
strength exceeding a critical welding time or welding energy. This decrease in strength is explainedby material fatigue (8, 11). Other studies (9) observe a slip between the horn and the upper joining
member during over-welding, whereby tool sticking reduces the final joint strength. Other authors (3,5) describe over-welding as a breakage of previously bonded regions by further oscillating excitation.
In-situ measurement during USMW as a highly dynamic process is still challenging. Therefore,
most of USMW research results are based on post-weld characterizations. As a result, there are barely
any direct observations about bond formation, which are essential for a fundamental unde rstanding.
Laser Doppler Vibrometer (LDV) measurement of single spot (12) or multiple spots (5) was used
to determine the oscillations during USMW. Lu et al. (5) used a Photonic Doppler Velocimetry (PVD)
system to study the relative motion between the horn, aluminum foils and anvil. It was shown that thewelding process consists of four different welding stages: The first two stages - slip [1] and slip-stick[2] - are characterized by a large relative motion between both foils [1] and between horn and upper
foil [2] which are essential to disperse oxides and contaminations as well as to increase fr ictional heat.
The stick stage [3] is mentioned as the key stage where bond development and DRX increase withwelding time. The maximum strength is obtained when horn and foils oscillate with similar velocity.
Over-welding [4] is characterized by a quick drop in bottom and top foil velocities which can becaused by tool sticking or breakage of previously bonded regions.
The above-mentioned studies have shown initial results on relative motions of the tools and thejoining members during USMW. However, there is still a lack of characteristic variables from sensor
signals, which correlate with the mechanism during bond formation and introduce the prospective toapply monitoring criteria for USMW.
In this paper, acoustic emissions and characteristic vibrations of the horn and anvil produced during
welding are measured to study their relationship to bond formation in ultrasonic welding and to joint
quality. Acoustic emissions have already proved useful for tool condition monitoring and online
process control (13, 14). This is possible because friction produces acoustic emissions characteristicof the condition of the surfaces in friction (15, 16, 17). As for the vibrations, they should take into
account mode coupling, in which different tools with different modal behaviors affect each other’svibrational patterns (15). In our case, this means that each of the horn and anvil, when coupled during
the welding process, will behave differently than they would if they were vibrating alone.

2. EXPERIMENTAL SETUP
2.1

Metrological test setup for acoustic emission and vibration monitoring

Figure 1 – (a) Schematic illustration of measurement setup and (b) zoomed-in view showing themeasuring positions on horn and anvil during copper sheet welding
In this study two Compact Laser Vibrometer “CLV-2534” from Polytec and a ¼’ GRAS microphone
were used to record the vibrations of the horn and the anvil along the welding direction and theairborne emissions during welding. Data from all three of those sensors was synchronously recorded
at a sampling rate of 250 kHz. The microphone was placed 50 cm away from the joining area anddirected towards it. Both LDV use a 633 nm wavelength helium-neon-source. The locations of the4701

laser spots are illustrated in Figure 1. Despite the small spot size (~ 110 μm), measurement errors due
to process emissions, such as metal particles and dust, increasingly dominate d at the measurement
position 2, close to the weld zone. Therefore, the majority of the experimental measurements werecarried out in the middle of the horn (position 1).
In order to estimate the temperature development during bond formation, two K -type
thermocouples (Ø 0.51 mm) were ultrasonically welded at the top of the upper specimen at 10 mm(TC 2) and 17 mm distance (TC 3) from the horn. A third K-type thermocouple (Ø 0.13 mm) was
placed between the two sheets in the middle of the welding zone (TC 1), as shown on Figure 2. The
temperature measurements were carried out three times for over-welding and in addition to the main
experiments.
2.2 Ultrasonic welding setup
For the investigation, similar lap welds made of copper (CW-008A, R240) sheets with 0.8 mm
thickness were prepared by USMW. The dimensions of the samples were 45 mm x 125 mm. The
rolling direction (RD) of the sheets was in the oscillation direction (OD) as shown in Figure 2. A
Schunk Sonosytems LS-C longitudinal welding system with a 20 kHz operating frequency and amaximum power of 4 kW was used for welding. The maximum vibration amplitude under no -loadconditions was 28 μm. The dimension of the horn surface was 8 mm x 8 mm knurled with 0.8 mmbase side, 0.3 mm height and 90° apex angle. In order to reduce inconsistency in weld quality, energycontrolled mode was mainly used in the present study. The welding parameter were set bas ed on
preliminary experimental tests. The maximum joint strength can be achieved with a clamping forceof 2250 N (pcylinder = 4.5 bar), an oscillation amplitude of 26 μm (‫ؙ‬100%) and a weld energy of1800 Ws (~870 ms). In order to be able to analyze the effects of over-welding as well, the welding
tests were carried out with an increased weld energy of 2100 Ws (~1030 ms). In addition, the welding
process was interrupted every 50 ms for sampling to evaluate the bond formation.

Figure 2 – Geometric specifications of weld specimen
2.3

Evaluation of the bondT-peel tensile testing based on DIN EN 14270 as shown in Figure 2 was performed to evaluate
weld quality and to observe the fractured surface. After welding, the sheets were bent by 90° in two
directions to allow the specimen to be clamped. A load was then applied to peel the weld sample witha displacement rate of 10 mm/min. The maximum T-peel load from the load-displacement curve andthe failure mode was recorded. For each investigated weld time, 3 replicates were evaluated.
In order to analyze the welded area (microwelds) with increasing welding time, the fracturesurfaces of the peeled samples were measured with a laser confocal microscope “VK -X100”(Keyence) at 10x magnification. Furthermore the microscope was used to determine the penetration
depth of the horn (5x magnification) at different welding times.

3. EXPERIMENTAL RESULTS
3.1 Joint strength and weld formation
As the maximum peel forces are good indicators for bond formation, they are plotted as a functionof welding time in Figure 3. First, the joint strength increases with weld time and interfacial failure4702

(“cold weld”) is observed for welding times lower than about 0.5 s. Within this welding time range,
the fracture surface (see Figure 4) changes from a few scratch marks within the first 150 ms toextensive plastically deformed and bonded areas with increasing weld time. At welding times between
0.5 ms to 0.7 ms the fracture mode changes and interfacial separation with scattered tear inside thewelding zone can be observed. At the same time the joint strength stagnates in this time range. With
increasing welding time up to 0.9 ms the breaking forces but also the variance rise by reaching thehighest bonding area. With further welding time the joint strength decrease (“over-weld”). Here anugget pull-out fracture can be observed due to thinning and breakage of welding spec imens byexcessive plastic deformation.

Figure 3 – Maximum peel load as a function of welding time

Figure 4 – Fracture surfaces measured with laser confocal microscope for different welding times
Figure 5 (a) shows an exemplary temperature curve during a typical over-welding process at
different thermocouple positions. The temperatures within the welding zone (TC 1) rise very strongly
at the beginning of the welding process until the temperature rise slows down at ~0.2 s. At 0.6 s afurther temperature rise is visible, which causes the joining temperature to rise to the maximum of
500°C by the end of the welding process. The other measuring positions on the upper sheet show asimilar curve, but reach maximum temperatures of 280°C (TC 2) and 230°C (TC 3) respectively.
The penetration depth of the horn during over-welding can be determine by the vertical4703

displacement of the horn given by the welding system as well as by measuring the horn imprints on
the surface of the upper sheet after different welding times. For comparison, the determined
penetration depth curves from the two methods can be seen in Figure 5 (b). Both curves show a similar
trend: in the initial stage of the welding process (< 0.15 s) there is a large material compaction,
between 0.15 s - 0.6 s the compaction rate slows down and increases again at 0.6 s.

Figure 5 – (a) Temperature development and (b) penetration depth of the horn during overwelding
3.2 Vibroacoustic results
The following figures describe the behavior of the horn and anvil, as well as the sound received
by the microphone, during both a free run and during a typical welding. A free run is defined as having
the horn vibrating alone, without any connection to the anvil or to metal sheets. The laser vibrometers
were found to be sensitive to vibrations from the environment such as passing cars and other machines
operating nearby. The data was therefore filtered using a high-pass filter with a 1 kHz cutoff frequency.
In addition, since the vibrometers measure the velocity of vibration of a surface, the data was thenintegrated once to get the vibrations of the horn and anvil surfaces in time.
Figure 6 shows spectrograms of the acoustic emissions recorded by the microphone and thevibrations of the horn during a free run. As can be seen, the airborne sound and the horn vibrations
contain more than one frequency. For the microphone, the pressure is concentrated in bands centeredaround harmonics of the welding frequency, 20 kHz, as well as harmonics of 10 kHz. For the horn,
the vibrations are centered around 20 kHz and its harmonics. The anvil vibrations are not shown since,
during a free run, the anvil is not excited by the horn and thus does not vibrate.
Figure 7 shows spectrograms of the airborne sound and horn during welding. Looking at the horn
spectrogram, we can see the same frequency bands as during free run. However, the amplitude at
20kHz is smaller and the amplitudes of the other frequency bands are larger than during free run .
Figure 8, Figure 9 and Figure 10 show time plots of the airborne sound and the horn and anvil
vibrations filtered in different frequency bands. Time plots were chosen instead of spectrograms tobetter show the amplitude variations of different frequency bands in time. The frequency bands were
10 kHz wide and centered on the first four harmonics of the welding frequency.

(a)

(b)
Figure 6– Spectrograms of (a) airborne sound and (b) horn displacement during free run

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(a)

(b)
Figure 7– Spectrograms of (a) the airborne sound and (b) horn vibrations during welding

(a)

(b)
Figure 8- Amplitude of airborne sound during (a) free run and (b) welding

(a)

(b)
Figure 9- Amplitude of horn and anvil vibrations at (a) 20 kHz and (b) 40 kHz

(a)

(b)
Figure 10- Amplitude of horn and anvil vibrations at (a) 60 kHz and (b) 80 kHz

4. DISCUSSION
The spectrograms show that the vibrations of the horn and the sound emissions during free run aremostly constant and distributed into multiple frequency bands. Hypothetically, different stages of
welding would act as different boundary conditions between the metal sheets, since the bond strength
between the metal sheets would be different, leading to different frequency contents in the acoustic
emissions in the process. However, since the free vibrations of the horn, the source of vibrations in
the welding process, already contain many frequency bands, finding the different welding frequencies
becomes more complicated: one must now separate the frequencies produced by the different stages

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of welding and the input frequencies. Though the anvil and horn would couple and affect each other’s
modal response during welding, it is not clear exactly how, and the possibility of a frequency comingfrom the horn rather than from the welding process cannot be neglected. In addition, the vibrational
measurements do not take into account the modal behavior of the horn and anvil at the point of
measurement, which would explain the very low amplitudes for the horn vibrations at 40 kHz.
Based on the findings from this study, the initial stage of the welding process (< 0.2s) is a couplingstage in which the welding tools and the metal sheets connect better. There is a large increase invibrations in the horn and anvil at the welding frequency, with the horn vibrations increasing before
the anvil vibrations, and with a steeper slope. This increase in vibration leads to the increase intemperature, due to an increase in friction work between the horn, the sheets, and the anvil. This isalso confirmed by the low joint strengths and the observed scratch marks on the fracture surfaces. The
rising temperatures lead to a softening of the copper, favoring the horn penetration into the material,
thus the knurls of the horn completely plunged into the upper sheet at the end of this initial stage. This
stage continues until approximately 0.2s, at which point the anvil and horn reach a sort of steady s tateof the vibrations at 20 kHz.
The better coupling between the horn and the upper specimen causes lower overall friction workand thus slows the temperature rise down. Therefore the breaking forces stagnate at a first joint
strength level (~ 260 N) as well as the penetration depth of the horn in a time range from 0.2s to 0.5s.
For the vibrations, there is an increase of the horn vibrations at 40, 60 and 80 kHz, a decrease of anvil
vibrations at 40 kHz, and an increase of anvil vibrations at 60 and 80 kHz. This could indicate slipping
between the bottom sheet and the anvil at 40 kHz.
The increasing plastic deformation within the welding time range of 0.5s to 0.7s, identifiable by
the further penetration of the horn, leads to a further growth of the bonded area and to a second joint
strength level (~ 400 N), in which in addition to the previously observed adhesive failure, interfacial
separation with scattered tear may occur. In accordance with the findings fr om previous study (6),friction heat is reduced with increasing material softening, causing temperatures to stagnate slightly.
Meanwhile, the horn vibrations and the anvil vibrations at 20 kHz do not change much. At 40, 60 and
80 kHz, the anvil vibrations decrease, with an inflection point around 0.7s. The decrease in anvil
vibrations could indicate slipping somewhere between the horn and the anvil. At this stage, it is most
likely to be slipping between the bottom sheet and the anvil.
Significant material softening occurs after approximately 0.7s, which leads to a typical bending of
the upper sheet around the heat affected zone. The additional deformation work is considered as acause of the renewed temperature rise. This comes with a decrease in the horn vibrations at 20, 40 and
60 kHz and in the anvil vibrations at 20, 40 and 80 kHz, as well as an increase in horn vibrations at
80 kHz and in anvil vibrations at 60 kHz. These changes can be seen as a continuation of the state ofvibrations in the previous phase: further increase in the bonding area leads to a better coupling of themetal sheets, which helps in getting a better coupling of the horn and anvil, and leads to further
decreases in the amplitude of vibrations of the anvil and horn. This is different from the initial stage,
when better coupling meant higher vibration amplitudes of the anvil. In the beginning, a better
coupling leads to better excitation of the anvil, but for a very strong coupling, the horn would carryso much of the mass of the anvil that the vibrations of the horn would decrease in amplitude.
At a welding time of 0.9s the largest bonding areas are reached, and the maximum breaking forces(~ 600 N) can be observed. There is also an inflection point in the plots of the vibrations of the horn
and anvil in all frequencies.
If welding time exceeds approximately 0.9 s the specimens are thermally and mechanicallyoverstrained (“over-weld”). Temperatures and penetration depth of the horn reach their highest levels
resulting in nugget pull-out failure and a reduction in joint strength due to excessive plastic
deformation of the upper sheet. This is accompanied by noticeable changes in the vibrations of theanvil, with a strong increase at 60 and 80 kHz and a slowing down of the decrease at 40 kHz. This
indicates an increase of the vibrations received by the anvil at those frequencies. This could be due tothe fatigue failure of microwelds in the overwelding stage, which would release high -frequency
vibrations into the anvil.
The sound pressure was not used much in this discussion, as its inflection points do not correlatewith the results from the other sensors. This could be due to the presence of at least one other soundsource in the room other than the vibrations of the horn, anvil, and copper sheets. Further research is
required before using the airborne sound.

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5. CONCLUSIONS
During ultrasonic metal welding, the interaction between the anvil and the horn is complex, andgreatly depends on the transfer of vibration from the horn, to the top sheet, to the bottom sheet, andfinally to the anvil. These connections depend on the type of frictional coupling between each element,
as well as the state of the weld between the two sheets. In this paper, we have tried to find features to
describe the different stages of the welding process by looking at data recorded by various sensors
during welding and by post-weld testing. Correlations between the temperature, the vibrations of thetools, the penetration depth, the bond strength and the bond area could be found at different points intime, and were linked to the different stages of welding. However, some elements of vibrational
behavior and the airborne sound could not be properly explained yet.

ACKNOWLEDGEMENTS
The authors would like to thank the German Research Foundation DFG for the support of theresearch work under grant number RE2755/52-1. For the sponsorship and the support, we wish to
express our sincere gratitude.

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