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Journal of Materials Processing Technology 209 (2009) 54755480

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c

Ultrasonic welding between mild steel sheet and AlMg alloy sheet

Takehiko Watanabe , Hideo Sakuyama, Atsushi Yanagisawa

Department of Mechanical Engineering, Graduate School of Science and Technology, Niigata University, Ikarashi 2-nocho, Niigata 950-2181, Japan

a r t i c l e i n f o

Article history:

Received 9 January 2008

Received in revised form 30 April 2009

Accepted 2 May 2009

Keywords:

Ultrasonic welding

Mild steel sheet

AlMg alloy sheet

Joint strength

Insert metal

Improvement of joint strength

a b s t r a c t

Ultrasonic welding between SS400 mild steel sheet and aluminum alloy sheet containing magnesium

(A5052) wasconducted. In this study, authors investigated the influence of ultrasonicwelding conditions

on the mechanical properties and the interface microstructure of a joint, and the effect of insert metal

was examined to improve the joint strength. The main results obtained in this study are as follows.It was possible to weld ultrasonicallySS400 mild steelsheetto A5052 aluminum alloy sheet containing

magnesium. The strength of the joints welded using various clamping forces and constant welding time

of 1.0s showed themaximumvalue at theclamping force of 588N anddecreased with theclampingforce

over 588 N because the excessively large clamping force reduced the frictional action at the interface.

The strength of the joints welded using the constant clamping force of 588 N and various welding times

showedthe maximum value at thewelding time of 2.5s. However, thestrengthof the joint welded using

thewelding time of3.0 s decreaseddueto theformationof Fe2Al5 intermetalliccompoundat theinterface.

Using the insert metal of commercially pure aluminum,the joint strength wassuccessfully improvedand

the strength of the welded using 3.0 s welding time was about three times as large as that of the joint

without the insert metal.

Crown Copyright 2009 Published by Elsevier B.V. All rights reserved.

1. Introduction

Energy saving and environmental preservation are important

issues for us to be urgently resolved. Since reducing the weight

of vehicles is one of the efficient countermeasures against them,

the use of the combination of steel and aluminum alloy has been

increasing in fabricating the vehicles. Under this situation, many

trials to weld steel to aluminum alloy have been conducted. How-

ever, sound joints have not been produced so far, because hard and

brittle intermetalliccompounds were formed at the weldwhenever

steel was welded to aluminum by fusion welding.

At present, thefollowing bonding methods have been employed

to produce the joint between steel and aluminum, that is to say,

friction welding (Aritoshi andOkita, 2000), resistance spot welding

(Watanabe et al., 2005), rolling (Kohno, 2000) and modified FSW

(Watanabe and Takayama,2003). However, the friction welding hasthe restriction in shape that at least one material to be welded

should be circular in cross-section. The rolling has also shortcom-

ing that it is applicable to only a thin plate. The modified FWS is

a promising method to make a joint between steel and aluminum.

However, we need to develop the other method such as ultrasonic

welding to join them in the near future.

Corresponding author. Tel.: +81 25 262 7006; fax: +81 25 262 7006.

E-mail address: twatanab@eng.niigata-u.ac.jp (T. Watanabe).

The ultrasonic welding has been thought to be one of the solid-

state bonding methods and seems to be promising for joining steelto aluminum. However, there are few studies to join them by ultra-

sonically welding.

In this study, authors tried to weld ultrasonically mild steel

sheet to aluminum alloy sheet containing magnesium and investi-

gated theeffectof welding conditions on themechanical properties

and the interface microstructure of a joint. Furthermore, the effect

of insert metal was investigated to improve the joint proper-

ties.

2. Materials and experimental procedure

Sheetsof 0.8mm thickSS400mildsteel and1.2 mmthick A5052-

H24 (Al2.84at%Mg) aluminum alloy containing Mg (hereafter,

A5052) were welded. The insert metal of commercially pure alu-

minum sheet A1050-24H (hereafter, A1050) with 1.2 mm thickness

was used to investigate the effects on the joint performance. The

ultimate tensile strengths of SS400 base metal, A5052 base metal

and A1050 base metal were about 375 MPa, 250 MPa and 106MPa,

respectively.

The shape and dimension of SS400 and A5052 was rectangu-

lar and 100mm in length and 10mm in width. The insert metal of

A1050 was rectangular and 25 mm in length and 10 mm in width.

The faying surface of the specimen was electrolytically polished

and the average surface roughness of SS400 was about 0.58m

and those of A5052 and A1050 were about 0.34m.

0924-0136/$ see front matter. Crown Copyright 2009 Published by Elsevier B.V. All rights reserved.

doi:10.1016/j.jmatprotec.2009.05.006

http://www.sciencedirect.com/science/journal/09240136http://www.elsevier.com/locate/jmatprotecmailto:twatanab@eng.niigata-u.ac.jphttp://localhost/var/www/apps/conversion/tmp/scratch_13/dx.doi.org/10.1016/j.jmatprotec.2009.05.006http://localhost/var/www/apps/conversion/tmp/scratch_13/dx.doi.org/10.1016/j.jmatprotec.2009.05.006mailto:twatanab@eng.niigata-u.ac.jphttp://www.elsevier.com/locate/jmatprotechttp://www.sciencedirect.com/science/journal/09240136

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5476 T. Watanabe et al. / Journal of Materials Processing Technology 209 (2009) 54755480

Fig. 1. Schematic illustration of the apparatus for ultrasonic welding. Enlarged part

of the specimens shows the location of a thermocouple to measure the interface

temperature.

The output power and the vibration frequency of the ultrason-

ically welding machine were 2400 W and 15kHz, respectively. The

vibration amplitude was about 53m (peak to peak at no loading).

Fig. 1 showsschematically the setting of the welding specimens.

The A5052 specimen was allocated to the upper horn side and was

vibrated to the direction shown in Fig. 1. The clamping force and

application duration of vibration (hereafter, welding time) were

varied. The welded area was approximately 10 mm10 mm. The

welded specimen was processed into U-bent shape using a steel

jig, as shown in Fig. 2, to evaluate the tensile strength of a joint. The

ultimate load in the tensile test was defined as the tensile strength

of a joint in this study.

To estimate the temperature around the joint interface, a ther-

mocouple with the diameter of 0.1 mm was welded at the edge of

the SS400 specimen, as shown in the enlarged circle in Fig. 1.

3. Results and discussion

3.1. The effect of the clamping force on the joint strength

3.1.1. The change in tensile strength

Fig. 3 shows the change of the joint strength when the clamping

force was varied from 343 N to 1764N under the constant weld-

Fig. 3. Relation between tensile load of a joint and clamping force.

ing time of 1.0s. The joint strength welded using 343N clamping

force was low due to the insufficient frictional force at the inter-

face. The joint welded using 588N clamping force showed the

maximum strength of about 412 N, followed by decreasing withthe clamping force. The previous studies report that the exces-

sively large clamping force seemed to generate large friction and

suppress the relative motion at the faying interface, resulting in

decreasingthe jointstrength (Hiraishiand Watanabe, 2003;Electric

Industries Association of Japan, 1995). The similar phenomenon

appears to occur in ultrasonically welding between SS400 and

A5052.

When the clamping forces larger than 882 N were used, it was

frequently observed that not only the faying surface of A5052

adhered to the welding tip on a horn, but also fracture occurred

in the A5052 specimen during welding. Fig. 4 shows the A5052sur-

face appearances of the horn side specimen after welding at the

clamping forces of 588N and 1175 N. The indentations made by

the convex of the knurls on the welding tip are observed on thesurface. The indentations made at 1176N are larger than that at

588 N and are slightly expanded in the directions parallel and per-

pendicularto the vibration. Theseindentation appearances indicate

that the slip occurred between the welding tip and the faying sur-

face of the specimen, resulting in insufficient welding. It has been

reported that the indentation size made by the welding tip was

useful to estimate the relative motion between the faying surfaces

(Hiraishi andWatanabe, 2003). Althoughithasbeenknownthatthe

shape of the knurlsaffects thejoint efficiency(Jahn et al., 2007), we

have no discussions on this issue in this study, because all of the

Fig. 2. Schematic illustration for the tensile test of a joint.

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Fig. 4. The appearances ofscratches onA5052surfaceformed bya weldingtip with

pyramidal projections: (a) clamping force of 588N and (b) clamping force of 1176N.

experiments were conducted using the welding tip with the same

geometry.

3.1.2. The change of the specimen temperature during welding

Fig. 5 shows the change in the temperature of SS400 specimen

during welding when the welding time was changed from 0.1 s to

1.0 s using various clamping forces. The result indicates that the

specimen temperature was below about 100C using 343 N clamp-

ing force because the generation of frictional heat was insufficient,however, the specimen temperature increased up to about 400 C

using the clamping forces of 588 N and 882 N, resulted in the larger

joint strength.

3.1.3. The observation of SS400 fracture surface

Fig.6 shows the photograph andSEM micrographs of the fracture

surface of SS400 in thejointwelded using 588N clampingforceand

1.0 s welding time. The white-looking and black-looking regions are

observed on the fracture surface in (a). The enlarged views of the

Fig. 5. Temperatures of a specimen welded at various clamping forces.

Fig. 6. Fracture surface of SS400 (a). Photographs (b) and (c) are SEM micrographs

of white part (b) and dark part (c) in the photograph of (a), respectively.

white-looking part (b) and the black-looking part (c) are shown in

Fig. 6(b) and (c), respectively. In the fracture surface of (b), dim-

ple pattern indicated by the symbol of W (welded region) and

scratched pattern indicated by the symbol of S (scratched region)

are observed. In the fracture surface of (c), the dimple pattern of

W and flat-looking region indicated by the symbol of EP (elec-

trolytically polished region) are observed. The regions indicated by

the symbols of W, S and EP stand for the region where the weld-ing was completed, the region where faying surface was scraped

in the vibration direction and the region where as-electrolytically

polished surface remained, respectively.

The area ratio of W region, S region and EP region on the frac-

ture surface in the joint welded using various clamping forces are

shown in Fig. 7. The area ratio was calculated using the linear anal-

ysis method on the microphotograph taken with the magnification

of 1000. As shownin Fig. 7, the area of W region shows the maxi-

mum at 588 N clamping force and the change in the W region area

corresponds well to the change in the joint tensile strength shown

in Fig. 3.

Fig. 7. Area ratio of EP, S and W region in fracture surface of SS400 vs. clamping

force.

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Fig. 8. Relation between tensile load of a joint and welding time.

Fig. 9. Arearatioof EP, S and W region in fracturesurfaceof SS400 vs. welding time.

3.2. The influence of welding time on the joint strength

The strength of the joint welded using the various welding

times from 0.5 s to 3.0 s and the constant clamping force of 588 N

is shown in Fig. 8. The joint strength increased with the welding

Fig. 11. Fracture surface of SS400 in the joint made under the conditions of 2.5 s

welding time and 588 N clamping force.

time up to 2.5 s. However, it decreased at the welding time of 3.0 s.

Fig. 9 shows the result of the measurement about the area ratio

of EP, S and W region in the fracture surface of SS400. Although

the area of W region increased with the welding time up to 3.0 s,

the joint strength at 3.0 s welding time decreased as shown in

Fig. 8.

Fig. 10 shows the cross-sectional microstructure and SEMenlarged view of the center part of the joint welded using the

588 N clamping force and 3.0 s welding time. Band-like phase with

about 1m width is observed at the center of the enlarged SEM

photograph. EDS qualitative analysis revealed that the chemical

composition of this phase was 26.9 at%Fe69.8 at%Al3.3 at%Mg.

It appears that the phase is intermetallic compound of Fe2Al5,

referring to the FeAl phase diagram (Masalski et al., 1996).

On the other hand, the intermetallic compound phase was not

observed at the welded interface of the joint made using the

welding time of 2.5s. From this fact, it seems that the reason

why the joint strength welded using 3.0s welding time is low is

attributed to the intermetallic compound formed at the welded

interface.

3.3. The effect of A1050 insert metal on the joint strength

In the study on ultrasonically welding of aluminum alloys con-

taining magnesium, such as A5052and A5086, it has been reported

that the joint strength was decreased due to the magnesium seg-

Fig. 10. Macrostructure and SEM micrograph of the interface of a joint welded under the conditions of 3 s welding time and 588N clamping force. SEM micrograph shows

the intermetallic compound formed at the interface.

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Fig. 12. The influence of preheating A5052 on the tensile load of a joint.

regation onto the faying surface during welding (Hiraishi and

Watanabe, 2002). Fig. 11 shows the SEM photograph of SS400 frac-

ture surface of the joint welded using 588N clamping force and

2.5 s welding time. The inclusion at the bottom of a dimple circled

in the fracture surface was analyzed as MgO using EDS. There-

fore, it appears that the magnesium segregated onto the faying

surface during welding probably decreased the joint strength. To

make sure the harmful effect of the segregated magnesium on the

joint strength, authors ultrasonically welded the A5052 specimen

pre-heated at 100 C for 3.6 ks in air in order to segregate the mag-

nesium onto faying surface before welding. It has been reported

that the A5052 specimen surface was covered with MgO dueto this

pre-heat treatment (Hiraishi and Watanabe, 2002), and it has been

confirmed that the base metal hardness was not changed by this

pre-heat treatment.

The strength of the joint between the pre-heated A5052 and

SS400welded using 588 N clamping force and 2.5 s welding time is

shown in Fig. 12. The area ratio of W, S and EPregion on the fracturesurface of SS400 is shown in Fig. 13. The results in Figs. 12 and 13

prove that the segregated magnesium onto the faying surface by

the preheating lowered the joint strength and the ratio of W

region.

The above-mentioned fact suggests that the joint strength

of SS400/A5052 is certainly increased by inserting a commer-

cially pure aluminum strip containing little amount of magnesium

into the interface. A1050 commercially pure aluminum strip with

dimensions of 25 mm10 mm1.2 mm in thickness was used as

the insert metal after electrolytically polishing to make the joint of

Fig. 13. The influence of preheating A5052 on the area ratio in the fracture surface

of SS400.

Fig. 14. Relation between tensile load of a joint welded using an insert metal and

clamping force.

SS400/A5052. The welding condition is 588 N clamping force and

the welding times of 0.53.0 s. The strength of the joint and the

area ratio of W, S and EP region on the fracture surface of SS400 are

shown in Figs.14 and15, respectively. Thesefigures indicate thatthejoint strength was increased by using the insert metal at the weld-

ing time longer than 1.0s, compared to the result using no insert

metal, as shown in Fig. 9. Especially, the joint strength remarkably

increasedat the welding time of 3.0 s. The joint strength was about

1800N and by about 300% stronger than that without insert metal.

Thewelded region, that is W regionarea, increasedup toabout80%,

as shown in Fig. 15.

Fig. 16 shows the cross-sectional SEM view of the joint welded

using 588N clamping force and 3.0 s welding time. SEM pho-

tographs (a) and (b) are the interface views of A1050/SS400 and

A1050/A5052, respectively. SEM photograph (a) revealed that the

intermetallic compound layer, which was observed in Fig. 10, was

notformed at theinterface between A1050and SS400, even though

the welding time was 3.0 s. It seems that the A1050 insert metal

containing little amount of magnesium effectively prevented the

intermetallic compound of Fe2Al5 from the formation at the inter-

face. The previous paper has already reported that the magnesium

contained in aluminum alloy had the action to increase the Fe

diffusion into Al and resulted in promoting the formation of the

Fe2Al5 intermetallic compound at the interface (Watanabe et al.,

2005). It seems that the interface between A1050 insert metal

and A5052 is completely welded, as shown in SEM photograph

(b).

Fig.15. Arearatio ofEP,S and W regionin fracture surfaceof SS400 ina jointwelded

using an insert metal.

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Fig. 16. SEM micrographs of the joint interface welded using an insert metal at the welding time of 3 s: (a) interface of SS400/A1050 and (b) interface of A1050/A5052.

4. Conclusions

The authors ultrasonically welded the mild steel sheet to the

aluminum alloy sheet containing magnesium and investigated

the effect of welding conditions on the mechanical properties

and the interface microstructure of the joint. Furthermore, the

effect of insert metal was investigated to achieve the joint with

higher performance. The following results were obtained in thisstudy.

(1) It was possible to weld ultrasonically SS400 mild steel sheet

to A5052 aluminum alloy sheet containing magnesium. The

strength of a joint welded using various clamping forces and

constant welding time of 1 s showed the maximum value at

the clamping force of 588 N and the strength decreased with

the clamping force because the excessivelylarge clamping force

reduced the frictional action at the interface.

(2) Thestrength ofa joint weldedusingthe constantclampingforce

of 588 N andvariousweldingtimesshowed themaximumvalue

at the welding time of 2.5s. However, the strength of a joint

welded using the welding time of 3.0 s deceased due to the

formation of Fe2Al5 intermetallic compound at the interface.

(3) Usingthe insertmetal of commercially purealuminumsuccess-

fully improved the joint strength and the joint strength welded

using 3.0 s welding time was about three times as large as that

of the joint without the insert metal.

References

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Hiraishi,M., Watanabe,T., 2003.Improvement of ultrasonic weld strength forAlMgalloy by adhesion of alcohol. Quart. J. Jpn. Weld. Soc. 21 (2), 795801 (in

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