3rd Japan-China Joint Symposium on Fatigue of Engineering Materials and Structures, Nov. 7-8, 2014, Takayama, Japan

Fatigue Behavior of Dissimilar Al Alloy/Galvanized Steel Friction Stir Spot
Welds Fabricated by Scroll Grooved Tool without Probe
Ishak IBRAHIM*,1,**, Yoshihiko UEMATSU*, Toshifumi KAKIUCHI* and Yasunari TOZAKI**** Department of Mechanical and Systems Engineering, Gifu University,
1-1 Yanagido, Gifu 501-1193, Japan**School of Mechatronic Engineering, Universiti Malaysia Perlis,
02600 Arau, Perlis, Malaysia.
***Gifu Prefectural Research Institute for Machinery and Material,
1288 Oze, Seki 501-3265, Japan
1) Present address: Department of Mechanical and Systems Engineering, Gifu University,
1-1 Yanagido, Gifu 501-1193, Japan
Abstract: In this study, the tensile-shear fatigue tests were performed to investigate the effect of plated zinc layer to the fatigue
strength of dissimilar FSSW joint. The Aluminum alloy and galvanized steel are successfully joined by the FSSW technique.
FSW scroll grooved tool was used and inserted only into aluminum plate. The tensile-shear strength and fatigue strength of thejoint are 3300N and 1250N, respectively. Both of the static and fatigue fracture were interface fracture mode. Detailed observation of the plated zinc layer characteristic using a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy(EDS) shows that the interface layer was formed between the aluminum alloy surface and steel surface. The EDS analysis of this newformed interface layer was compromised of Fe and Al elements, while the zinc was not detected by the EDS analysis. These indicate that during the FSSW process, the plated zinc layer was melted and has been peeled off from the base steel surface. As theresult, the melted plated zinc layer was squeezed out from the joint area by the plastic flow of the Al alloy. Then, this melted plated
zinc layer was cumulative at the weld periphery forming a new layer coating surrounding the FSSW joint which is providing the
FSSW joint a stress relaxation at the weld notch enhancing the joint strength.
Key words: Fatigue, Friction Stir Spot Welding, Dissimilar weld, Aluminum alloy, Galvanized steel

1. INTRODUCTION
The efforts among the automobile manufacturer in
reducing the vehicle weights due to the demanding for
better fuel consumptions while maintaining the highstrength to weight ratio have lead the increasing of thealuminum [1], meanwhile the steel is also used as used tobe because of its high reliability. Therefore the dissimilar weld of Al/Fe is required in the manufacturing sector
in order to achieve this goal.
The weld ability of dissimilar material has been well
explored in conventional fusion bonding, such as resistantance spot welding, laser welding and etc. A large
temperature difference between types of metal used in thedissimilar weld may cause mechanical property degrading due to melting and re-solidification during the welding process.
Furthermore, intermetallic compounds can easily beformed at the interface between the dissimilar metal inconventional fusion bonding, where this cannot secure asufficient strength of the joint because of brittle fracture
mechanism of the intermetallic compound itself. Since
the formation of the intermetallic compound can be controlled in the solid bonding phase as the FSSW
methods[2], it is possible to avoid the brittle fracture ofdissimilar weld.
Recently a number of researches have been made so
far for dissimilar between aluminum alloy and galvanizedsteel by using friction stir spot welding (FSSW). The
result shows that the plated zinc layer has improved thestatic fracture strength of dissimilar weld.[3] However,
the joint between dissimilar metals is used in general

structure, it is necessary to understand the fatigue behavior of the dissimilar welds interface , but detailed research on the interface fatigue of the dissimilar FSSW
joint has not yet been investigated.
In this study, the dissimilar lap welds between aluminum alloy sheet and galvanized steel sheet were fabricated by FSSW method using a scroll grooved tool without probe and tensile shear fatigue tests were conducted.
The fatigue strengths of these joining were evaluated andfracture mechanisms were discussed based on microstructure analysis of the weld zone.
2. EXPERIMENTAL DETAILS
2.1 Materials and Specimen Configuration
As base material, an automotive standard used material
SGCA galvanized steel and A6022 aluminum alloy wereused for the FSSW joining, whose thickness of each sheet
are 1.4mm and 2.0 mm, respectively. The chemical compositions of the base material and plated zinc layer weresummarized in Table 1.
The microstructures of the SGCA and A6022 on the
cross-section are revealed in Fig. 1. Fig. 2 shows theplated zinc layer thickness is almost 8 µm.
The dissimilar FSSW joint strength was evaluated byusing tensile-shear lap specimen. The dimensions of thespecimens are according to JIS. Specimen were made byusing two sheets 40mm×150mm sheets with a 40mm×40mm overlap area as shown in Fig. 3. The upper andlower sheets are aluminum alloy and galvanized steel,respectively.

3rd Japan-China Joint Symposium on Fatigue of Engineering Materials and Structures, Nov. 7-8, 2014, Takayama, Japan

interface structure and element distribution of the joint
were analyzed using scanning electron microscopy(SEM) equipped with energy dispersive X-ray spectroscopy (EDS) analysis system.

(a)(b)
Fig. 1 Microstructures of materials on the cross-section(a) SGCA270C, (b) A6022-T4

Fig. 3 Lap shear specimen configuration.

Fig.2 Thickness of the plated zinc layer
Table 1 Chemical composition of materials (mass %)
Fe
0.15

Mn
0.07

Mg
0.56

Zn
0.01

Ti
0.02

Al
Bal

C
0.01

Si
0.01

Mn
0.16

P
0.13

S
0.06

Fe
Bal

Plated zinc layer

Fe
9-11

A6022

SGCA270Si
1Al
0.2-0.3Zn
Bal

2.2 Tool Geometry and Welding Conditions
The scroll grooved tool without probe is shown in
Fig.4 where it’s performance is describe in the work Tozaki et al [4].
The aluminum alloy and galvanized steel sheets wereoverlap and tightly clapped. The dissimilar weld tensile-shear and fatigue test specimens were spot lap joint
using automated friction welding machine
In the FSSW, the important processing variables aretool rotational speed, tool holding time, and tool plunge
depth. It should be noted that the tool has no probe, but a
0.5 mm depth scroll grooved on its shoulder surface.
Therefore, the insertion depth of the shoulder part was
defined as the tool plunge depth.
In this study, pre-tensile test were conducted to define
those variables and fixed as the tool rotational speed of3000 rpm, the tool holding time of 5 s and the shoulder
plunge depth of 0.7 mm. In all specimens, the tool wasinserted from the Al alloy sheet side. Then the rotating
tool was reacted from the Al alloy side and left the joint
cool by air.
2.3 Experimental Procedures
The joint mechanical properties were evaluated using
harness test, tensile-shear test and fatigue test. After thewelding the specimen was cut as such as cross-section isperpendicular to the welding direction for Vickers hardness test under load 4.9 N with dwell time for 30 s. The(a)(b)
Fig. 4 The scroll design tool;[5](a) side view, (b) bottom view
The fatigue tests were conducted with an electro-hydraulic fatigue testing machine under load control
at room temperature and different loading amplitudes.
The test operating at a sinusoidal frequency of 10 Hz andstress ratio of R = 0.1.
After fatigue tests, the fracture surfaces were analyzed
in detail using scanning electron microscopy (SEM)
equipped with energy dispersive X-ray spectroscopy(EDS).
3. EXPERIMENTAL RESULT
3.1 Microstructure in the Weld Zone
After the FSSW process, a detail examination wasmade on the vertical cross-section of the FSSW dissimilar weld.
The macroscopic view of the verticalcross-section of the FSSW dissimilar weld is shown in
Fig. 5.
Gray layer
Al
Steel

Fig. 5 Macroscopic appearance of the cross-section of theweld zone
The upper side is the Al alloy sheet and the lower sideis the steel sheet side. The interface between the upperand lower sheet is clearly been seen which indicates thatstirring of material occurred only in the upper sheet,
A6022. The microstructures of the weld zone are simi-

3rd Japan-China Joint Symposium on Fatigue of Engineering Materials and Structures, Nov. 7-8, 2014, Takayama, Japan

lar to as reported by Uemastu et al[5].
The EDS line analysis revealed the presence of Al andFe across the interface layer, which indicates that the intermetallic compound (IMC) layer was formed at the
Al/Fe interface during FSSW but the presence of zinc
was not detected as shown in the corresponding EDS line
analysis of the image in Fig. 6.

are shown in Fig. 8. The hardness distribution in the stirzone on the al-side is similar to the base metal (HV 78)
but the Vickers hardness on TMAZ is reducing due to thedissolution of fine precipitations in the Al sheet. In despite, the hardness distribution in the stir zone on the steelside is higher than the base metal (HV 120) due to work
hardening induced by the stirring heat during the FSSW
process.

Al

IMC
5µm

CountsSteel

Fig. 7 EDS analysis of the cross-section of the weld zone
: (A) gray layer, (B) center, (C) weld periphery in Fig. 5,
respectively.

AlFe

In order to observe and understand the plated zinc layer flow behavior during the FSSW process, a higher
magnification and EDS analysis of the interface at region
A, B, and C in Fig. 5 are shown in Fig. 7. During the
FSSW process the interface temperature rises up until
430℃ [3] and the plated zinc layer are melted at the joint
interface. While the stirring process removed the melted
zinc from the steel surface and leaving a newly fresh steel
surface and allowing an interface layer forming, consistof Fe and Al elements, as shown in Fig. 7 (B).
Further observation, the melted plated zinc layer weresqueezed out to the periphery region of the FSSW joint
due to the plastic flow of the Al alloy during the FSSW
process, as shown in Fig. 7. A symmetrical gray layer
pattern which is rises up from the joint interface toward
the Al sheets near the weld periphery. This layer is consisting of zinc, as shown in Fig. 7 (A). Figure 7 (C)shows this melted plated zinc layer were solidified at theweld periphery forming a new layer coating surrounding
the FSSW joint which providing the FSSW joint a geometry effect at the weld notch .
3.2 Vickers Hardness Distribution
The hardness distributions of the dissimilar welds were
measured along the line + 0.01mm from the interface andit is measured from the center of the joint with +10mm
range. The Vickers hardness distributions of weld zone

Vickers hardness, HV

Position, µm
Fig. 6 EDS line analysis of the interface

150
SGCA

100
A6022

50

0

BASE
METAL

-10

TMAZ

-5

STIR ZONE

0

TMAZ

5

BASE
METAL

10

Distance from joint center D, mm
Fig. 8 Vickers hardness distributions in the cross-sectionof weld zone.
3.2 Tensile-Shear Strength under Static Loading
The average tensile-shear strengths of three specimens
in the dissimilar are 3.3 kN. Typical macroscopic appearances of tensile-shear fracture surface of dissimilar
welds are shown in Fig. 9. The static loading fracturesurface was observed in order to evaluate the static fracture mechanism. It is appeared that the fracture occurred along the interface layer.
(a)

(b)

Fig. 9 SEM macrographs showing tensile-shear fracturesurface of dissimilar FSSW joint: (a) Al side (b) steel side

3rd Japan-China Joint Symposium on Fatigue of Engineering Materials and Structures, Nov. 7-8, 2014, Takayama, JapanA detail observation on the tensile-shear fracture surface is conducted using an EDS analysis. The results of
EDS on analysis of region (a), (b) and (c) in Fig. 9 areshown in Fig. 10. Al adhesive was observed on almost
the entire joint fracture surface. In the meantime, zinc
was not found at the center of the joint fracture surface
but only at the periphery region, as shown in Fig. 10 (b)and Fig. 10 (c).

surface using EDS analysis shows that Al adhesive were
not observed on almost of the joint fracture surface.
This indicates that fatigue crack grows between the lower
steel sheet and interface layer, as show in Fig 13. In themeantime, zinc was not found at the weld zone but onlyat the periphery of the fracture surface as shown in Fig.
12 (b) and Fig. 12 (c).
(a)(b)
Secondarycrack

Fig. 12 SEM macrographs of tensile fracture surface ofdissimilar weld: Pmax= 2000N, Nf = 1.0x106 cycle,(a) al side (b) steel side.

Fig. 10 EDS analysis of the tensile-shear fracture surface:(a) Center, (b) left, and (c) top side in Fig. 9, respectively.
3.4 Tensile-Shear Strength under Fatigue Loading
The fatigue test results of the dissimilar FSSW for
Al-SGCA is shown in Fig. 11.
As a reference,
A6061-SPCC fatigue test result by Uematsu et al[5] isalso plotted. The fatigue strengths of the dissimilar
welds of Al-SGCA are higher than those welds ofA6061-SPCC despite that the tensile-shear strength ofA6061-SPCC is 5.4kN as report by Uematsu et al which
is higher than that of A6022-SGCA dissimilar weld.
The fatigue strengths of A6022-SGCA dissimilar weld is
at 1250N where all the fractures are interface fracturemode.

Maximum load Pmax, N

3500○A6022-SGCA△A6061-SPCC
R= 0.01

3000
2500
2000
1500

Fig. 13 EDS analysis of the fatigue fracture surface:(a) Center, (b) edge, and (c) secondary crack of joint in
Fig. 12, respectively.
It should be noted that final fracture are occurred along
the interface at all loading condition, while a secondary
crack grew into the upper sheet as shown in Fig.12. The
secondary crack was observed in all specimens. The SEM
images of secondary crack fracture surface are shows in
Fig. 14 and magnified view of the crack initiation site areshows in Fig. 14(a), Fig. 14(b) and Fig. (c). The crack
was initiated at the interface then grew into the al sheet.

1000
500
0

104

105

106

107

108

Number of cycles to failure Nf, cycle
Fig. 11 Relationship between maximum loads, Pmax, andnumber of cycles to failure, Nf.
Typical macroscopic appearances of fatigue fracturesurface of dissimilar welds are shown in Fig. 12. The
fatigue fracture occurred along the interface between theupper and lower sheets. This failure mode is similar tothat under static loading condition as in Fig. 9
However, a detail observation on the fatigue fracture

[a]
[a]

[c]

[b]
[b]

500µm

[c]

100µm

100µm

Fig. 14 SEM micrograph of the secondary crack fracturesurface: Pmax= 2000N, Nf = 1.0x106 cycles:
Magnified view at region [a], region [b] and region [c].

3rd Japan-China Joint Symposium on Fatigue of Engineering Materials and Structures, Nov. 7-8, 2014, Takayama, Japan

Figure 15 is a sample of cross-section Al side weldzone that parallel to the loading direction shows that thesecondary crack occurred in TMAZ region, as the region
where the lower Vickers hardness is measured and not thezinc layer that flow towards Al sheet.

[a]
1mm

(a)

(b)

[b]
100µm

500µm

Von Misess Stress, Mpa

Fig. 15 SEM micrograph of the secondary crack fracturesurface: Pmax= 2500N, Nf = 6.0x105 cycles:
Magnified view at region (a) and region (b).
Al
Steel

6

Plated zinc
Non plated

4. CONCLUSIONSThis study investigated the dissimilar lap welds between galvanized steel and aluminum alloy sheets. It
aimed at characterizing the intermetallic compounds(IMC) formed during spot welding and the plated zinclayer behavior in the FSSW joint.
(1) EDS analysis results indicate that the intermetallic
compound (IMC) layer was formed at the Al/Fe interface
during FSSW.
(2) The fatigue crack grows between the lower steel sheet
and interface layer and grew through the interface layerand steel sheet.
(3) The fatigue strength of Al-SGCA dissimilar weld ishigher than Al-SPCC especially at higher fatigue loading
condition. The substantially improved of fatigue resistance AL-SGCA was the result to the stress relaxation
surrounding the weld nugget.
Acknowledgment – We would like to express our appreciation for the financial support from RING! RING!
Project organized by Public Interest Incorporated Foundation "JKA" (Grant Number 26-112)
REFERENCES[1]

Y. C. Chen, T. Komazaki, Y. G. Kim, T.
Tsumura,andK.
Nakata,“Interface
microstructure study of friction stir lap joint of
AC4C cast aluminum alloy and zinc-coatedsteel,” Mater. Chem. Phys., vol. 111, no. 2–3, pp.
375–380, Oct. 2008.

[2]

S. Bozzi, a. L. Helbert-Etter, T. Baudin, B. Criqui,
and J. G. Kerbiguet, “Intermetallic compounds in
Al 6016/IF-steel friction stir spot welds,” Mater.
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Jun. 2010.

[3]

K. Feng, M. Watanabe, and S. Kumai,“Microstructure and Joint Strength of Friction
Stir Spot Welded 6022 Aluminum Alloy Sheets
and Plated Steel Sheets,” Mater. Trans., vol. 52,
no. 7, pp. 1418–1425, 2011.

[4]

Y. Tozaki, Y. Uematsu, and K. Tokaji, “A newly
developed tool without probe for friction stir spot
welding and its performance,” J. Mater. Process.
Technol., vol. 210, no. 6–7, pp. 844–851, Apr.
2010.

[5]

Y. Uematsu, K. Tokaji, Y. Tozaki, and Y.
Nakashimac, “Fatigue behaviour of dissimilar
friction stir spot weld between A6061 and SPCC
welded by a scrolled groove shoulder tool,”
Procedia Eng., vol. 2, no. 1, pp. 193–201, Apr.
2010.

4
2
0

-10

-5

0

5

10

Distance from joint center D, mm
Fig. 16 Von Misses stress of both sheet at the interface.
FEM analysis was conducted regarding the solidified
plated zinc layer at the weld periphery. Fig. 16 showed
the Von Misses stress results of both sheet with andwithout the solidified plated zinc layer condition. The
maximum Von Misses stress for non-plated steel sheet
higher than the plated zinc layer steel. Also, the solidified
plated zinc layer gives a larger bonded area and lowering
the stress distribution in the lap joint during the fatigue
testing. As indicates in Fig. 12 and Fig. 13, it is possibly that the interface fatigue crack initiated between the
IMC and steel surface at the steel side.
In the present welding condition a joint of FSSW joint
has been obtained successfully. The fatigue strength of
Al-SGCA dissimilar weld is higher than Al-SPCC especially at higher fatigue loading condition. The substantially improved of fatigue resistance AL-SGCA was theresult to the stress relaxation surrounding the weld nugget.