G. CAO et al.: A NEW TYPE OF PLUG-IN FRICTION-STIR LAP WELDING BASED ON THE 6061-T6 ALUMINUM ALLOY
103–110
A NEW TYPE OF PLUG-IN FRICTION-STIR LAP WELDING
BASED ON THE 6061-T6 ALUMINUM ALLOY
RAZISKAVA NOVE VRSTE VRTILNO-TRENJSKEGA VARJENJA
ZLITINE NA OSNOVI ALUMINIJA VRSTE 6061-T6
Gaowei Cao, Jin Wang
*
, Bo Gao, Kaicheng Mu, Hui Zhang, Baoge Li
School of Mechanical and Automotive Engineering, Qingdao University of Technology, Qingdao 266520, China.
Prejem rokopisa – received: 2023-03-29; sprejem za objavo – accepted for publication: 2023-12-21
doi:10.17222/mit.2023.839
In this study a new type of plug-in friction-stir lap welding (PFSLW) is proposed to prepare welded joints based on 4-mm-thick
6061-T6 aluminum alloy sheet. The differences in the cross-sectional morphology, microstructure, cross-sectional hardness and
shear properties between the PFSLW joint and the normal friction-stir lap-welding (FSLW) joint are discussed. The results show
that the cross-sectional morphology of the PFSLW joint has undergone changes. The PFSLW joint has a mechanical interlock-
ing structure on the advancing side that is beneficial to the connection strength of the joint. The grain structure differs at the
boundary between the thermo-mechanically affected zone (TMAZ) and the heat-affected zone (HAZ), and the PFSLW joints
show a more pronounced bending deformation of the grain organization near the boundary. The microhardness of PFSLW joints
was increased in the TMAZ and HAZ areas, and the lowest hardness is further away from the center of the weld. The failure
load of the PFSLW joint has been improved, the microcracks part of the PFSLW joint has a ridge-like structure. In addition, the
actual welding width of PFSLW joints was improved.
Keywords: Plug-in friction-stir lap welding, mechanical interlocking, microhardness, failure load.
V ~lanku avtorji opisujejo nov tip varjenja s pomo~jo trenja, ki so ga poimenovali priklju~no torno vrtilno lepalno varjenje
(PFSLW; angl.: plug-in friction stir lap welding). Pri tem se izraz lepanje tehni{ko pojmuje kot fino ploskovno bru{enje trdih
materialov. Za medsebojno varjenje (spajanje) s tem postopkom so pripravili vzorce iz 4 mm debele plo~evine iz Al zlitine vrste
6061-T6. Nato so obravnavali oziroma primerjali razlike, med PFSLW zvari in zvari izdelanimi s konvencionalnim tornim
vrtilno-lepalnim varjenjem (FSLW), v morfologiji presekov zvarnih spojev, med mikrostrukturami, med trdotami vzdol` pre~nih
presekov in mehanskimi lastnostmi pod stri`nimi obremenitvami. Rezultati preimerjave so pokazali pomembne spremembe
morfologije preseka pri PFSLW zvarih. Ti zvari imajo mehansko spojeno strukturo na strani, ki je pomembna za trdnost
zvarnega spoja. Kristalna struktura se razlikuje na meji med term-mehansko vplivano cono (TMAZ; angl.: thermo-mechanically
affected zone) in toplotno vplivano cono (HAZ; angl.: heat-affected zone). PFSLW zvarni spoji imajo bolj izrazito deformacijo
zaradi upogibne obremenitve blizu meje. Mikrotrdote PFSLW zvarnih spojev so bile o~itno vi{je v TMAZ in HAZ in ni`je v
podro~jih dale~ stran od sredine zvarnih spojev. Obremenitev pri poru{itvi PFSLW je bila precej vi{ja oz. izbolj{ana v
primerjavi z FSLW in nastale mikrorazpoke so imele togo strukturo. Poleg tega so bile dejanske {irine zvarnih spojev PFSLW
mo~no (opazno) izbolj{ane.
Klju~ne besede: priklju~no vrtilno torno-lepalno varjenje, mehansko spajanje, mikrotrdota, obremenitev pri poru{itvi
1 INTRODUCTION
Friction-stir welding (FSW) is a patented technology
proposed by The Welding Institute (TWI) in the 1990s.
1
It was first applied to the welding of aluminum alloys
with low melting points, and then gradually expanded to
the welding of the same or dissimilar materials.
2
FSW is
a solid-state joining technology, which can avoid defects
such as porosity and cracks produced by the traditional
fusion-welding process. It has been applied in structural
manufacturing in aerospace, automobiles, shipbuilding
and other fields, and has shown good prospects for engi-
neering applications.
3, 4
FSW has a variety of joint forms
such as butt, lap, angle joint, and T-joint.
1,5,6
The
high-performance butt joints that can be achieved by
friction-stir welding have been confirmed in many stud-
ies.
7
Friction-stir lap welding (FSLW) joints are also an
important form of joints, and lap joints are widely used
in the assembly of various parts and products in the aero-
space and automotive industries to replace riveted lap
joints.
8
FSLW is extensively utilized in the fabrication of
structural components for aircraft, including wings, tail
sections, and fuselage segments. In the electric-vehicle
sector, FSLW is employed in the manufacturing of bat-
tery casings, ensuring the structural integrity and safety
of the battery compartment.
There has been a lot of research work on FSLW and
many researchers have conducted a lot of research in or-
der to improve the performance of lap joints. Ge et al.
9
studied the effects of pin length and welding speed on
the quality of FSLW joints of dissimilar aluminum al-
loys. The research results show that optimizing the com-
bination of pin length and welding speed can effectively
improve the tensile strength of the joint. Sharma et al.
10
studied the addition an interlayer of graphene
nanoplatelets (GNPs) at the lap interface of 6061 alumi-
num alloy. The results showed that the addition of the
GNP interlayer increased the weld strength and percent-
Materiali in tehnologije / Materials and technology 58 (2024) 1, 103–110 103
UDK 621.791:661.862 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(1)103(2024)
*Corresponding author's e-mail:
jinwangqut@163.com (Jin Wang)

age elongation by 121 % and 53 %, respectively, com-
pared to the weld without the GNP interlayer. Paran-
thaman et al.
11
used a modified-interlocking
friction-stir-welding lap joint to join AA8011-AA7475
with different w/% of SiC particles. The results showed
that the joints made with SiC particles exhibited better
static properties and the hardness, tensile strength and
elongation of AA8011-AA7475 joints were increased
when the SiC content was 2 w/%. Kesharwani et al.
12
es-
tablished a method for optimizing the groove width of
AA6061-T6 plates filled with Al2O3 powder particles.
They found that the tensile and yield strengths of speci-
mens increased by 7 % and 20 %, respectively, for a
groove width of 1 mm. Abegunde et al.
13
machined
V-grooves in aluminum plates to fill titanium carbide
particles and studied the effects of process parameters on
the microstructure, microhardness and tensile properties
under this condition.
In previous studies, many experimenters have tried to
improve the strength of lap joints by changing the pro-
cess parameters or adding interlayers at the lap interface.
Some researchers have also grooved the surface of the
sheet before welding, but the main purpose was to place
particles or interlayers in the groove, and the research fo-
cus did not pay attention to the effect of the groove itself
on the joint. In this study, a plug-in friction-stir lap weld-
ing (PFSLW) in which a groove is made at the lap inter-
face and the two plates are plugged together is proposed.
In this lap-joint mode, the contact area of the interface
between the upper and lower plates is increased and
there is mechanical occlusion before welding, and it also
has the characteristics of a butt connection on the side of
the groove. In this paper, the cross-sectional morphology,
microstructure, microhardness and shear properties of
the PFSLW are investigated. The research results are also
of significance for related research that requires grooving
to add intermediate materials.
2 EXPERIMENTAL PART
The schematic diagram of the plate is shown in Fig-
ure 1a. 6061-T6 aluminum alloy plate is used as the base
material, and the plate size is 120 mm × 60 mm×4mm.
A 2-mm width and 2-mm depth groove are cut in the
plate, and a 13-mm-width and 2-mm-depth side groove
are cut along the edge to complete the plug-in. The lap
width of the two plates is 30 mm, and the welding line
along the straight line in the middle of the two grooves is
also the center line of the lap overlap area, as shown in
Figure 1b. Figure 1c shows the stirring tool used, the
pin is left-threaded and made of H13 steel, the pin length
is 4.7mm. During the experiment, the stirring tool tilt an-
gle of 2.5°, rotation speed of 1200 min
–1
, welding speed
of 120 mm/min, and the shoulder is inserted to a depth of
0.2 mm.
After the welding was completed, the lap shear speci-
mens and metallographic specimens were cut out using
an electrical discharge cutting machine, and the flash
generated during the welding process was removed be-
fore cutting. The metallographic sample was polished
with sandpaper, and etched with Keller reagent after pol-
ishing, and the metallographic organization and welding
defects were observed with an optical microscope. Joint
cross-section microhardness measurements were per-
formed under a load of 100g and a dwell time of 10 s.
The lap shear specimen standard is ASTM D3164, Fig-
G. CAO et al.: A NEW TYPE OF PLUG-IN FRICTION-STIR LAP WELDING BASED ON THE 6061-T6 ALUMINUM ALLOY
104 Materiali in tehnologije / Materials and technology 58 (2024) 1, 103–110
Figure 1: Dimensions of FSLW equipment and lap shear specimens: a) Base material, b) Schematic of PFSLW process, c) Friction stir welding
tool, d) FSLW lap shear specimen, e) PFSLW lap shear specimen

ure 1d and 1e shows the FSLW lap shear specimens and
PFSLW lap shear specimens with a width of 20 mm and
a length of 90 mm, lap width of 30 mm, the two speci-
mens differ only in the thickness of the lap area, but the
rest of the parameters are identical, the specimens are cut
along the vertical direction of the weld. Two shims are
used at both ends of the specimen to make the thickness
of both ends of the specimen consistent with the thick-
ness of the lap area. The lap shear experiments were per-
formed on an electronic universal tensile testing machine
with a constant speed of 1 mm/min during the test, and
the average value of three specimens was taken as the re-
sult for discussion. The fracture forms were observed
and the fracture morphology was analyzed using a scan-
ning electron microscope (SEM).
3 RESULTS AND DISCUSSION
3.1 Surface and cross-sectional morphology
During the FSW process, the high-speed rotating pin
and shoulder drive the plasticized material on the AS to
flow in the direction of the RS, and some of the material
is extruded out of the joint due to the squeezing effect of
the shoulder to form flash.
14
Figure 2 shows the typical
surface morphology of the FSLW and PFSLW under the
same welding parameters, and both modes successfully
prepared lap joints with good surfaces. In comparison,
the surface morphology of the joint obtained under
PFSLW conditions is smoother, and the shaft shoulder
marks are not obvious. This is due to the presence of un-
avoidable gaps in the plug-in mode, and the material fills
into the gaps during the plastic-flow process, resulting in
less overflowing material.
Figure 3a and Figure 4a show the macroscopic im-
ages of the FSLW joint and the PFSLW joint cross-sec-
tions, and it can be seen that the joint has a bowl shape
due to the joint action of the shoulder and the pin, and
four typical zones can be observed in both cross-sec-
tions: the SZ, the thermo-mechanically affected zone
(TMAZ), the heat-affected zone (HAZ) and the BM.
9
HD and CLD are typical features of lap joints,
1
which
are caused by the penetration of the tool through the
lower plate at a certain depth, the original plate interface
on both sides of the weld is slightly bent upward or
downward depending on the tool geometry and the weld-
ing parameters,
15
the defects appearing on the AS side
are called HD and the defects on the RS side are called
CLD. It has been shown that the extension direction of
the hook defect can be either upward or downward.
16
Figure 3b and Figure 4b show the AS side hook
structure of the FSLW and PFSLW. A complex and tortu-
ous interface geometry appears in the PSFLW hook
structure, as shown in Figure 4b and Figure 5a, where
the upper and lower plates are inserted and occluded
G. CAO et al.: A NEW TYPE OF PLUG-IN FRICTION-STIR LAP WELDING BASED ON THE 6061-T6 ALUMINUM ALLOY
Materiali in tehnologije / Materials and technology 58 (2024) 1, 103–110 105
Figure 2: Joint surface morphology: a) FSLW, b) PFSLW
Figure 3: Cross-sections of the FSLW joints: a) Cross-sectional macroscopic morphology, b) HD, c) CLD

with each other, forming a mechanical interlocking struc-
ture, which is not found in the normal FSLW. It has been
shown that similar such interlocking structure increases
the effective contact area between the upper and lower
plates, which is beneficial for improving the weld joint’s
strength,
17
so the appearance of such mechanical inter-
locking structure may be able to be a useful structure to
improve the joint strength of lap joints. At the same time,
microcracks appear at the end of the joint interface,
forming part of the hook structure in PFSLW, which is
also not present in the AS side hook structure of normal
FSLW joints. Figure 3c and Figure 4c show the RS side
cold lap structures of FSLW and PFSLW.
Figure 5b and 5c show the SEM images of
microcracks on the AS side and RS side of the PFSLW,
where it can be observed that some of the microcracks
are fully connected, indicating that although the
microcracks are defect extensions and are likely to be-
come crack extensions during fracture, they can improve
the joint strength of the joint to a certain extent com-
pared to the defects without connected parts.
3.2 Microstructure
Figure 6 and Figure 7 show the microstructure of
different areas of the FSLW and PFSLW joints. Elon-
gated slate-like organization and equiaxed grains can be
observed in the TMAZ region, with a clearer boundary
between the HAZ and TMAZ on the AS side (Figure 6b
and Figure 7b), while in the RS-side TMAZ (Figure 6d
and Figure 7d), the tissue evolution is more complicated
due to the complex flow of extruded metal, and no obvi-
ous boundary occurs.
18
The TMAZ was subjected to less
mechanical stirring, which caused only a large bending
deformation in this part of the tissue, while partial
recrystallization occurred by a local reversion reaction
due to the thermal cycling.
19
In TMAZ at the same dis-
tance position from the upper plate surface, the down-
G. CAO et al.: A NEW TYPE OF PLUG-IN FRICTION-STIR LAP WELDING BASED ON THE 6061-T6 ALUMINUM ALLOY
106 Materiali in tehnologije / Materials and technology 58 (2024) 1, 103–110
Figure 4: Cross-sections of the PFSLW joints: a) Cross-sectional macroscopic morphology, b) HD, c) CLD
Figure 5: A(a), B(b), C(c) in Figure 4: a) Mechanical interlocking structure, b) SEM image of microcracks on the AS side, c) SEM image of
microcracks on the RS side

G. CAO et al.: A NEW TYPE OF PLUG-IN FRICTION-STIR LAP WELDING BASED ON THE 6061-T6 ALUMINUM ALLOY
Materiali in tehnologije / Materials and technology 58 (2024) 1, 103–110 107
Figure 6: FSLW joint microstructure: a) SZ, b) TMAZ on AS, c) HAZ, d) TMAZ on RS
Figure 7: PFSLW joint microstructure: a) SZ, b) TMAZ on AS, c) HAZ, d) TMAZ on RS

ward elongated slat-like tissue around the boundary line
in PFSLW has more obvious bending deformation,
which can be explained by the presence of the down-
ward-bending hook structure here, which is subjected to
less deformation resistance when affected by thermal
machine action.
SZ was simultaneously subjected to strong mechani-
cal stirring and thermal cycling at higher temperatures,
and the strong plastic deformation and high temperature
led to dynamic recrystallization of the original slate-like
organization of the BM and the formation of refined
equiaxed grains (Figure 6a and Figure 7a).
20
HAZ tissue
was only subjected to weaker thermal cycling during the
welding process, and no significant deformation oc-
curred, forming a rough slate-like organization similar to
that of the BM (Figure 6c and Figure 7c).
3.3 Microhardness
The results of the FSLW and PFSLW microhardness
tests are shown in Figure 8, and the hardness-test loca-
tions are both located at 3 mm from the upper plate. The
hardness distribution of both models showed a typical
W-shaped feature, which is similar to the distribution re-
sults of the previous study content.
21
Microhardness val-
ues in the weld area as well as in the weld-affected area
with varying degrees of decrease compared to the base
material. SZ area due to the strong mechanical stirring
and high temperature makes the dissolution of the pre-
cipitation phase during the welding process, reducing the
hardness of the SZ area, but the hardness values here are
higher than HAZ and TMAZ because of the dynamic
recrystallization that occurs here to generate finer
grains.
18
The material is subjected to weaker thermal and
mechanical effects in the TMAZ region, where the grains
undergo bending deformation and partial recryst-
allization causing an increase in dislocation density, and
an increase in hardness in the TMAZ toward the center
of the weld.
22
In the HAZ region, the material undergoes
thermal cycling, which changes the organization and me-
chanical properties, resulting in the lowest hardness val-
ues all occurring here.
23
In the TMAZ and HAZ regions, the hardness values
of the PFSLW joints are higher than the hardness values
of the same region in the FSLW joints, which is particu-
larly significant in the TMAZ region. In the HAZ region,
where the hardness values are relatively low, the location
of the lowest hardness values on the AS side and RS side
of the PFSLW joint occur further from the center of the
weld than in the FSLW joint. It is understood from the
available studies that the roughness and contact area of
the lap interface can have a significant effect on the
heat-transfer efficiency.
24
In the PFSLW process, the
presence of the plug-in slot increases the interface con-
tact area between the upper and lower plates, which will
enhance the heat-transfer efficiency between the upper
and lower plates compared to the normal FSLW, and the
enhanced thermal circulation can lead to faster dissolu-
tion of the precipitated phase in the SZ, resulting in a
slight decrease in the hardness value in the SZ region.
The presence of the plug-in slot also allows the
heat-transfer efficiency to be enhanced in the lateral di-
rection, and the thermomechanical effect produces a
wider range of influence in the lateral direction, so that
the hardness values in both the TMAZ and HAZ regions
of the PFSLW joint are increased to a certain extent, and
the location of the lowest hardness value generation is
shifted laterally to the outside accordingly.
3.4 Shear performance
The load-displacement curves of the FSLW joint and
PFSLW joint during the experiments are shown in Fig-
ure 9. The failure load of the PFSLW joint is generally
larger than that of the FSLW joint. The average failure
load of the FSLW joint is 7.434KN and the maximum
G. CAO et al.: A NEW TYPE OF PLUG-IN FRICTION-STIR LAP WELDING BASED ON THE 6061-T6 ALUMINUM ALLOY
108 Materiali in tehnologije / Materials and technology 58 (2024) 1, 103–110
Figure 9: Load-displacement curves Figure 8: Microhardness curves and location chart

failure load is 7.476 kN, while the average failure load of
the PFSLW joint is 9.67 kN and the maximum failure
load reaches 10.138 kN, and the failure load of the
PFSLW joint gets a 30 % increase.
Figure 10a and 10d show the macroscopic morphol-
ogy of the fractures of the FSLW specimen and the
PFSLW specimen. To further clarify the fracture charac-
teristics of the FSLW and PFSLW joints, the fracture
morphology of the AS side of both joints is given in Fig-
ure 10. Many dimples can be observed in the diagram,
indicating that both joints fracture in a ductile fracture
mode
25
. The size of the dimples in Figure 10b is rela-
tively uniform, while the presence of larger size dimples
can be found in Figure 10e, indicating that the ductile
fracture morphology is enhanced in the PFSLW joint
fracture,
1
and this occurs probably because of the differ-
ent thermal influences on the reinforcement of the joint.
Figure 10c shows the fracture morphology of the HD
side of the FSLW joint, where some of the smaller dim-
ples and tear-like structures can be observed, indicating
that there is a potential crack-propagation area here,
which belongs to the weakening area of the connection
strength. Figure 10f shows the fracture characteristics of
the microcracks (Figure 5b) on the AS side of the
PFSLW joint, it can be observed here that there is a layer
of ridges without typical fracture characteristics, no ef-
fective metallurgical bond is formed here, and the bond-
ing force is generated on this side mainly by the mechan-
ical interlocking structure observed in Figure 5a.
4 CONCLUSIONS
The following conclusions can be drawn from this
work:
• PFSLW mode joints have better joint surface quality
and produce a complex mechanical interlocking
structure appears on the AS side of the PFSLW joint,
and the presence of this mechanical interlocking
structure is beneficial for improving the joint
strength.
• PFSLW joints have significantly higher microhard-
ness in the TMAZ and HAZ regions than normal
FSLW joints, PFSLW specimens have a wider effec-
tive welding width, the lowest hardness values occur
farther from the weld centerline of PFSLW joints.
• The maximum failure load of PFSLW joints is 30%
higher than that of FSLW joints, no effective metal-
lurgical bond formed on the microcracked side of the
PFSLW fracture, the mechanical interlocking struc-
ture plays the main connecting role here.
Acknowledgment
This work was supported by Key R&D projects of
Shandong Province (Grant numbers 2019GGX102023).
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