Ultrasonic Spot Welds of Gas Diffusion Layer to Proton

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Ultrasonic Spot Welds of Gas Diffusion Layer to Proton Exchange Membrane of

Fuel Cells

Quang Khoa Dang1, Pei-Lun Chang1, Thien Ngon Dang2, Fangbor Weng3, Jun-Yen Uan4,

Dung-An Wang1*,

1Graduate Institute of Precision Engineering, National Chung Hsing University,

Taichung 40227, Taiwan, ROC 2Faculty of Mechanical Engineering, Ho Chi Minh City University of Technology and

Education, Vietnam. 3Department of Mechanical Engineering, Yuan Ze University, Taoyuan 320, Taiwan,

ROC 4Department of Materials Science and Engineering, National Chung Hsing University,

Taichung 40227, Taiwan, ROC

October 4, 2018

Abstract

Ultrasonic spot welding of gas diffusion layer to proton exchange membrane of fuel cells

is investigated under various processing parameters. Key parameters are found to be the

welding time and the pressure. The optimum welding time and the peak pressure are

0.25 sec and 2.75 MPa, respectively. The holding time has no significant effect on the

bonding strength of the welds and the holding time 0.2 sec seems to be a reasonable

selection. Mechanical interlocking is found to be the major contributing factor of the

ultrasonic bonding. Ultrasonic weld of the carbon paper and the proton exchange

membrane for membrane exchange assemblies of fuel cells is demonstrated as a viable

technology to replace hot-pressing, which may be time consuming and energy inefficient.

Keywords: Gas diffusion layer; Proton exchange membrane; Ultrasonic spot welding

* Corresponding author. Tel.:+886-4-22840531; fax:+886-4-22858362

E-mail address: [email protected] (Dung-An Wang).

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1. Introduction

Fuel cells have a high potential in powering portable devices, stationary devices

and transportation vehicles. Satyapal (2018) reported that proton exchange membrane

(PEM) fuel cell has seen the greatest increase in total power shipped by fuel cell

industries among the various fuel cells. Romdhane and Louahlia-Gualous (2018)

described that PEM fuel cells have high-power density and efficiency, and fast start-up

time due to their low operating temperature. These characteristics make PEM fuel cells a

superior power source for real life applications. Tang et al. (2008) also noted that PEM

fuel cells are considered as an alternative power source for transportation, due to their

high-energy efficiency and clean operation. The Toyota MIRAI powered by PEM fuel

cells has started sales in December 2014 (Yoshida and Kojima, 2015). Owens and

McGuinness (2015) reported that PEM fuel cells with sizes ranging from 1 W to 20 kW

were used in recreational settings (campers, boats, lighting) and skid mounted generators.

Fuel Cells Bulletin (2017) revealed that Toshiba had received an order to supply a 100

kW PEM fuel cell system to Japanese chemical engineering company Showa Denko, for

installation in a new hotel.

Since late 1980s, various fabrication methods of PEMs were developed and

optimized to accelerate applications of PEM fuel cells. Litster and McLean (2004)

provided an overview of several fabrication methods that have been shown to produce

effective electrodes and have high future potential. Hack et al. (2018) reported that hot-

pressing is the most common means of fabricating membrane exchange assemblies

(MEAs) for PEM fuel cells. Meyer et al. (2017) have explored the optimum temperature

and pressure for hot-pressing of MEA components based on microstructure observations

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and electrochemical performance. They pointed out that certain pressure and temperature

are required to achieve optimum interfacial contact for effective lamination of MEA

layers. Kim et al. (2010) stated that hot-pressing method may cause structural

deformation in the porous catalyst layers and gas diffusion layers (GDLs) and induce

slow gas transportation and high electrical resistance. Towne et al. (2007) reported that

applying hot-pressing step in the catalyst-coated membrane based method can improve

the MEA performance and has a positive effect on MEA’s performance. However, Beck

et al. (2013) stated that the viability of hot-pressing of MEA components for PEM fuel

cells is strongly correlated with process cycle time and energy consumption. However,

Kim et al. (2010) reported that one disadvantage of hot-pressing method is a large

amount of catalyst may immerse into the catalyst coated GDL and be wasted when the

catalyst layer is heat bonded into the Nafion membrane. Given the fact that relatively

long process cycle time and high energy consumption during thermal bonding of the

MEAs, a cost and time efficient means for bonding MEA components may be developed

to address the above cycle time and energy consumption issues.

Ultrasonic welding is a promising technology for joining similar and dissimilar

materials where ultrasonic vibrations are locally applied to parts being held together

under pressure to form a solid state weld. Rani and Rudramoorthy (2013) mentioned that

ultrasonic welding is an efficient and economical process and the welding time is usually

less than a second for bonding of a variety of industrial thermoplastic components.

Truckenmuller et al. (2006) demonstrated ultrasonic welding of polymer parts and

reported that processing time of ultrasonic welding is relatively short and an effective

bonding can be achieved without adding additional substance, such as adhesives or

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solvents. Nguyen et al. (2014) have shown that the vibration during ultrasonic welding

generates sufficient heat at the joint area and the adjacent material is softened and melted,

then cooled to create a fast, strong and clean bond. Kelly et al. (2014) suggested that

ultrasonic bonding may occur well below the material melting temperatures, where

interfacial friction and shearing bring material from both surfaces into intimate contact to

form a solid-state bond. Due to low cycle time and energy efficiency, ultrasonic welding

has been used to join parts in the manufacturing sector. There is a potential to lower the

energy cost and process time by adopting ultrasonic welding for bonding of MEA

components for PEM fuel cells. Beck et al. (2012) reported that MEAs produced by

ultrasonic bonding have comparable performance to those made using thermal bonding.

Bonding mechanisms and mechanical strength of the ultrasonic welds of MEA

components have not been revealed in their studies.

In this paper, processing parameters for ultrasonic bonding of a GDL and a PEM

were investigated by spot welding of lap-shear specimens. Bonding strength and failure

mechanisms of the ultrasonic spot welds of a GDL and a PEM in lap-shear specimens

were examined based on experimental observations. An ultrasonic spot welding machine

with an ultrasonic horn of 12 m peak to peak vibration amplitude and 35 kHz resonant

frequency was used. Effects of the processing parameters of the ultrasonic spot welding

on the bonding strength of the lap-shear specimens were investigated. Micrographs of

ultrasonic spot welds in lap-shear specimens before and after testing were obtained.

Failure mechanisms for the ultrasonic spot welds were then examined. Finally, the

optimal processing parameters for the ultrasonic spot welds of the MEA components

were presented.

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Fig. 1. Schematic of an ultrasonic welding system.

2. Ultrasonic welding system

Fig. 2. A close up view of a horn and a specimen.

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Fig. 1 is a schematic of an ultrasonic welding system for bonding of the MEA

components. The vibration generated by a piezoelectric converter is amplified by a

booster and an ultrasonic horn. The MEA components are held by a fixture. The high

frequency oscillations of the ultrasonic horn ensure effective bonding at the interfaces of

the MEA components. In this investigation, spot welding is utilized to examine the

bonding strength of weld-bonded joints of the MEA. Fig. 2 is a schematic close up view

of the horn and the components of the MEAs to be welded. A typical single-cell PEM

fuel cell consists of two flow fields, two GDLs and one PEM. One side of the GDL is

coated with the catalyst. The GDL acts as an interface of the flow field and the catalyst.

The catalyst coated side of the GDL is in touch with the PEM. As shown in Fig. 2, a

PEM is sandwiched between two carbon paper GDLs in the experimental setup. The

catalyst side of the GDLs is facing the PEM. Thin Kapton tapes are placed between the

upper GDL and the horn and between the lower GDL and the fixture bed to avoid

sticking. Fig. 3 schematically shows the ultrasonic spot welding process. The horn tip is

pressured into the top surface of the upper GDL. The lower GDL is supported by the

fixture bed. The ultrasonic vibration of the horn is switched on and the pressure is kept at

an adequate level for a period of time to generate enough mechanical disruptions at the

joint interface. Entanglement of fiber/polymer chains occurs across the interface. The

ultrasonic bonding causes fiber disruption locally near the interface and the integrity of

the material is maintained. The frictional heat does not cause glass transition or melting

of the material and a solid state bond at the interface is created. Then, the ultrasonic horn

is drawn out of the upper GDL.

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Fig. 3. Three step operation of the ultrasonic spot welding.

3. Experimental procedures

Fig. 4. A schematic of a lap-shear specimen and the applied force P shown as the bold

arrows.

Fig. 4 shows dimensions of a lap-shear specimen employed in this investigation.

The circular indentation on the top surface of the upper GDL is caused by the plunging of

the ultrasonic horn into the upper GDL of the specimen. The welded zone is idealized as

a cylinder. The GDLs have a thickness of 0.08 mm, a width of 10 mm, and a length of 35

mm. The length, width and thickness of the PEM are 35 mm, 10 mm and 0.05 mm,

respectively. The indentation diameter is approximately 5 mm. The overlap length of the

lap-shear specimen is 15 mm. Selection of the specimen dimensions and testing

procedure are based on ASTM D3163-73, “Standard Test Method for Determining the

Strength of Adhesively Bonded Rigid Plastic Lap-Shear Joints in Shear by Tension

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Loading”. The direction of the tension loading during tests is indicated in the figure. The

GDLs and PEMs used in this investigation were recycled from used PEM fuel cells. The

physical and chemical properties of the materials are not known. This investigation

focuses on the feasibility study of ultrasonic welding of a typical GDL to a typical PEM.

Mechanical bonding strength of the welds and failure mechanism of the weld specimen

under tensile tests are evaluated to examine the effects of the processing parameters on

the weld quality.

Fig. 5. Process parameters.

The welding time and the peak pressure are selected as the processing parameters

for the ultrasonic spot welding process. These parameters and their levels were found to

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affect the weld quality of the welds on the basis of trial and error testing. A controller of

the ultrasonic machine controls the welding time. The peak pressure is controlled by

adjusting the pressure of a pneumatic system. Fig. 5 schematically shows a typical

pressure history of the welding process. After the delay time, the ultrasonic oscillation is

activated. The pressure increases, then decreases during the welding period. Then, the

MEA is held under pressure while the ultrasonic vibration is stopped during the holding

period. The pressure is gradually decreased almost to zero during the holding period.

The ultrasonic horn is then extracted from the specimen. As shown in the figure, it

represents the time that the ultrasonic horn starts to move downward toward the specimen.

1t is the time when the horn contacts to the top surface of the upper GDL. The time

when the vibration of the ultrasonic horn stops is denoted by 2t . The time when the horn

removes from the top surface of the upper GDL is represented by ft . The welding time

equals 2t - 1t , and the holding time is ft - 2t .

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Fig. 6. (a) Dimensions of an ultrasonic horn. (b) A cross sectional view of an ultrasonic

weld. The bonded region is represented by the thick lines at the interfaces between the

PEM and the GDLs.

A ultrasonic welding machine (35K, Maxwide Ultrasonic Co., Ltd.) was

employed to create the welds. Fig. 6(a) is a photo of an ultrasonic horn used in this

investigation. The dimensions of the horn are indicated in the figure. Fig. 6(b) shows a

horn extracted from a welded MEA after ultrasonic welding is finished. The bonded

region is represented by the thick lines at the interfaces between the PEM and the GDLs.

The bonding strength of the specimen was measured by a tensile test. During tensile

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tests, one end of the specimens was pulled by a force gauge (FG5020, Lutron Electronic

Enterprise Co., Ltd., Taiwan). The quasi-static tensile strength of the specimen was

recorded by the force gauge at a monotonic displacement rate of 5.8 mm per minute.

Tests were terminated when the upper GDL was separated from the lower GDL. The

maximum load recorded by the force gauge represents the bonding strength of the lap-

shear specimen under tensile loading.

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Fig. 7 (a) A schematic of experimental setup for ultrasonic spot welding. (b) A photo of

the experimental setup.

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Fig. 7(a) is a schematic for measurement of the pressure during welding of the

specimens. The specimen is placed on top of a fixture. The top plate of the fixture is free

to move along longitudinal direction of the ultrasonic horn. The pressure applied to the

specimen is measured by a load cell (LTZ-50kA, Kyowa Electronic Instruments Co., Ltd.)

below the top plate of the fixture. The pressure data of the load cell was acquired by a

data acquisition unit (DBU-120A, Kyowa Electronic Instruments Co., Ltd.) with a

sampling rate up to 31020 samples/sec. Fig. 7(b) is a photo of the experimental setup

for the pressure measurement. Temperature of the specimens during welding were taken

using a thermal imaging camera (F30W, NEC Avio Infrared Technologies Co., Ltd.,

Japan). The camera has a measurement range ranging from -20 °C to 350 °C, a

resolution of 0.2 °C and a frame rate of 8.5 frames/sec. The output frequency of the

ultrasonic welding machine is 35 kHz. The horn has a vibration amplitude of 12 m at

35 kHz. The material of the GDL is a carbon paper, and a DuPont’s Nafion membrane is

selected as the material of the PEM.

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Fig. 8. (a) A photo of a horn and a specimen. (b) An infrared image of the horn and the

specimen.

4. Results

Fig. 8(a) and (b) show a photo of a specimen and a horn and the corresponding

infrared image of the specimen and the horn during ultrasonic welding, respectively, with

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a welding time of 0.25 sec, a holding time of 0.2 sec and a peak pressure of 2.75 MPa.

The maximum temperature at the surface of the specimen near the horn tip is 50 oC. The

temperature of the horn near the horn tip is 94.8 oC. In fact, the real maximum

temperature of the specimen should occur inside the specimen and may be a bit higher

than 50 oC, but well below the typical glass transition temperature of PEMs, considering

the porous nature of the GDLs and the PEMs. For example, Lin et al. (2009) reported

that a Nafion material used in their experiments has a glass transition temperature of

nearly 125°C. Yazdanpour et al. (2012) pointed out that a microstructure change in the

Nafion membranes can happen when the processing temperature is much higher than the

glass transition temperature gT , therefore an irreversible water uptake loss may ensue.

Osborn et al., (2012) reported that mechanical stability of Nafaion membranes may

deteriorate at temperatures higher than gT . Mathur et al. (2007) and Kusoglu et al.

(2007) suggested that mechanical behavior of PEMs is critical in operation of PEM fuel

cells. From the material viewpoint, an polymer’s stiffness may decrease significantly

when its temperature is increased above its glass transition temperature. Besides, Jung

and Kim. (2012) revealed that thermal heating may cause PEMs to develop a crystalline

morphology that led to reduced water uptake and proton conductivity. In this

investigation, the maximum temperatures of the specimens during ultrasonic welding

were well below the typical glass transition temperature of Nafion materials, 100°C. It is

recognized that the temperature measured in this study represent a measurement over a

certain area and may not be the maximum. Considering the thickness of the specimen

0.21 mm and the porous nature of the materials, it is believed that the maximum

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temperature of the specimen does not exceed twice that of the measured, 50 oC, and thus

is still far below that of the typical glass transition temperature of Nafion materials.

The tensile test described in the section of Experimental procedures was used as

the criterion of a "good" weld; a "good" weld is defined as one in which the whole nugget

is pulled, leaving a similar sized hole in the other member being joined. Optical

micrographs of the cross sections of the specimens before and after tensile testing were

utilized to examine the quality of the ultrasonic spot welds. There are two competing

failure modes of the welds, the circumferential failure mode (nugget put-out) and the

interfacial failure mode. In the circumferential failure mode, the failure of the spot weld

may be initiated in the upper or lower GDL near the middle of the circumference of the

circular weld, and then propagates along the thickness direction of the weld. In the

interfacial failure mode, the failure occurs via crack propagation through the interface

between the GDL and the PEM. The failure load of the circumferential failure mode is

higher than that of the interfacial failure mode. In this investigation, the ultrasonic spot

welds failed in the circumferential mode are considered successful. The welding time

and the peak pressure are taken as the processing parameters to be optimized for the

ultrasonic welds to fail in the circumferential failure mode under tensile tests.

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Fig. 9. Micrographs of the cross sections of an ultrasonic spot weld in a lap-shear

specimen before (a) and after (b) tensile testing.

A micrograph of the cross section of an ultrasonic weld before testing is shown in

Fig. 9(a). There is an indentation with a length scale of the diameter of the horn tip.

Outside of this bonding zone, the upper sheet (upper GDL) and the PEM are slighted

bended upward due to the plunging of the ultrasonic horn. The bending of the upper

sheet creates a gap between the PEM and the lower sheet (lower GDL). Near the

intersection of the faying surfaces between the upper sheet and the lower sheet, notches

appear to be formed. Fig. 9(b) shows a micrograph of the cross section of the ultrasonic

spot weld in the failed lap-shear specimen. The direction of the tensile force is indicated

by arrows in the figure. The circumferential failure mode can be seen in the upper sheet

of the specimen. Near the upper left portion of the weld, a crack (marked as break point)

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in the upper GDL appears to emanate from the tip of the notch tip, and then propagates

through the thickness direction of the upper GDL. However, no breakage can be seen in

the lower right portion of the weld. This can be attributed to the different geometries of

the weld in the upper and the lower GDL.

Fig. 10 A SEM image of a cross section parallel to welding direction of a lap-shear

specimen.

Due to the magnification limit of the available microscope, the optical micrograph

has a low resolution. SEM images of the cross section of the welded specimen were

taken in order to examine the continuity of the weld at the interface between the GDLs

and the PEM. Fig. 10 shows a SEM image (scale: 10 m ) of a cross section of an

ultrasonic spot weld in a lap-shear specimen welded using a welding time of 0.25 sec, a

holding time of 0.2 sec and a peak pressure of 2.75 MPa. The upper GDL, the PEM and

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the lower GDL are marked in the figure. The interfaces between the GDLs and the PEM

in the bonding zone appeared nearly flat and mechanical interlocking can be seen from

the saw-tooth like interface. The mechanical interlocking is considered as an indication

of a sound weld.

\

Fig. 11. Bonding strength of the ultrasonic welds in the lap-shear specimens as a function

of welding time, where the peak pressure and the holding time are 2.75 MPa and 0.2 sec,

respectively.

Long welding time may cause physical or thermal damages of the MEA

components. A too short welding time may cause poor mechanical bonding of the welds.

Fig. 11 shows the averaged bonding strength of the welded specimens as a function of the

welding time with a peak pressure of 2.75 MPa and a holding time of 0.2 second. The

experiments were repeated five times for each set of the welding parameters. For the

welding time ranging from 0.15 sec to 0.35 sec, the bonding strength of the ultrasonic

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welds increases initially, reaches the maximum bonding strength, and then decreases.

The optimum welding times is 0.25 sec under the peak pressure of 2.75 MPa and the

holding time of 0.2 second.

Liang et al. ( 2007) reported that bonding strength between MEA components

depends on the applied pressure. Large deformation of the electrodes occurs under high

pressure and therefore induces porosity loss and large oxygen transfer resistance in the

cathode catalyst layer. Mathur et al. (2007) pointed out that functioning of fuel cells

requires the GDLs to be porous, electronically and ionically conducting, and

electrochemically active. It is critical to investigate the effects of the pressure on the

ultrasonic welded specimens. Fig. 12 shows the bonding strength of the welds as a

function of the peak pressure ranging from 1.27 MPa to 3.36 MPa with a welding time of

0.25 sec and a holding time of 0.2 sec. The value was obtained by averaging the bonding

strengths of five welding tests with the same welding parameters. The optimum peak

pressure is found to be 2.75 MPa. The pressure selected in this investigation is well

below the sealing pressure 4.5 MPa used by Beck et al. (2012) in their ultrasonic bonding

of MEAs.

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Fig. 12. Bonding strength of the ultrasonic welds in the lap-shear specimens as a

function of the peak pressure, where the welding time and the holding time are 0.25 sec

and 0.2 sec, respectively.

In the experiments, the holding time was set to be 0.2 sec based on an initial

parameter study in a trial and error manner. The assumption that the holding time has

minor effects on the bonding strength of the ultrasonic weld should be evaluated. The

relation between the holding time and the bonding strength of the ultrasonic spot welds in

the lap-shear specimens is shown in Fig. 13. The peak pressure and the welding time

were set as 2.75 MPa and 0.25 sec, respectively. Five tests with the same welding

parameters were performed to compute the averaged results. The averaged bonding

strength of the welds within the range of the holding time, 0.1 sec to 0.3 sec, does not

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vary to any significant extent. The influence of the holding time on the bonding strength

is minor. The 0.2 sec holding time seems to be a reasonable selection. Holding time

was intended to be long enough for the weld to cool and harden. Given the low

temperature of the specimen during the welding, the holding time was conceived to be a

less significant factor.


function of the holding time, where the peak pressure and the holding time are 2.75 MPa


Ram et al. (2007) reported that ultrasonic bonding is generally attributed to one or

more of the following mechanisms: mechanical interlocking, interfacial melting,

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interfacial atomic forces, and interfacial chemical reactions. In the content of the

ultrasonic welds of the polymeric materials, GDLs and PEMs, mechanical interlocking is

believed to be responsible for the excellent bonding of the GDLs and PEMs. Interfacial

melting due to friction heat generation between the dissimilar materials may not be the

major contributor of the bonding with the low bonding temperature observed in the

experiments (see the infrared image shown in Fig. 8(b), where the maximum temperature

at the surface of the specimen near the horn tip is 50 oC, well below the typical glass

transition temperature of Nafion materials, 100°C. Hazlett and Ambekar (1970) reported

that interfacial atomic forces (nascent bonding) is evidenced in the bonds in which the

interface disappeared. Disappearance of interface is not observed in the specimens and

therefore the interfacial atomic forces were not considered as the cause of the bonding

mechanism in this particular study. The presence of the interfacial chemical reaction is

indicated by diffusion with a full range of phases (Hazlett and Ambekar, 1970). It

appeared that diffusion did not occur across the interface in the specimens. It may be

concluded that the ultrasonic bonding between the particular GDL and PEM was not

caused by interfacial chemical reactions.

Based on the parametric study, the optimum values of the peak pressure and the

welding time were 2.75 MPa and 0.25 sec, respectively. With the aid of the ultrasonic

vibration of 35 kHz, short welding time and low power consumption for bonding GDLs

to PEMs were achieved. The mechanical bond was examined by SEM images. These

electron micrographic studies indicate that, for the particular GDL and PEM, there was

limited evidence of melting across the bond interface and there was mechanical

interlocking of the GDL and the PEM achieved by the short welding time.

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5. Conclusions

Two important processing parameters in determining ultrasonic weld quality of a

carbon paper and a proton exchange membrane of fuel cells as determined by the tensile

test are the welding time and the pressure.

Using the ultrasonic welding system in this investigation, the optimum welding time

and the peak pressure are 0.25 sec and 2.75 MPa, respectively. The holding time has no

significant effect on the bonding strength of the welds in the lap-shear specimens and the

holding time 0.2 sec seems to be a reasonable selection.

Base on observations from the SEM image of the weld, mechanical interlocking is

conceived as a major contributing factor of the ultrasonic bonding.

Acknowledgement

This work is supported by a grant from Ministry of Science and Technology,

Taiwan, ROC, under Grant: MOST 105-2221-E-005-060. The assistance from the

National Center for High-Performance Computing (NCHC), Taiwan is appreciated.

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28

List of figures

Fig. 1 Schematic of an ultrasonic welding system.

Fig. 2 A close up view of a horn and a specimen.

Fig. 3 Three step operation of the ultrasonic spot welding.


arrows.




PEM and the GDLs.

Fig. 7. (a) A schematic of experimental setup for ultrasonic spot welding. (b) A photo

of the experimental setup.


specimen.



Fig. 10. A SEM image of a cross section parallel to welding direction of a lap-shear

specimen.



respectively.


of the peak pressure, where the welding time and the holding time are 0.25 sec and 0.2

sec, respectively.


of the holding time, where the peak pressure and the holding time are 2.75 MPa and 0.2

sec, respectively.

29

Fig. 1. Schematic of an ultrasonic welding system.

30

Fig. 2. A close up view of a horn and a specimen.

31

Fig. 3. Three step operation of the ultrasonic spot welding.

32


arrows.

33


34



PEM and the GDLs.

35

Fig. 7 (a) A schematic of experimental setup for ultrasonic spot welding. (b) A photo of

the experimental setup.

36


specimen.

37



38

Fig. 10 A SEM image of a cross section parallel to welding direction of a lap-shear

specimen.

39

\



respectively.

40


function of the peak pressure, where the welding time and the holding time are 0.25 sec


41


function of the holding time, where the peak pressure and the holding time are 2.75 MPa


Documents

Ultrasonic Spot Welds of Gas Diffusion Layer to Proton