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Citation: Ge, X.; Jiang, D.; Song, W.;
Wang, H. Effects of Tool Plunging
Path on the Welded Joint Properties
of Pinless Friction Stir Spot Welding.
Lubricants 2023,11, 150. https://
doi.org/10.3390/lubricants11030150
Received: 27 February 2023
Revised: 15 March 2023
Accepted: 17 March 2023
Published: 21 March 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
lubricants
Article
Effects of Tool Plunging Path on the Welded Joint Properties of
Pinless Friction Stir Spot Welding
Xiaole Ge *, Di Jiang, Weiwei Song and Hongfeng Wang
College of Mechanical and Electrical Engineering, Huangshan University, Huangshan 245041, China
*Correspondence: gxl@hsu.edu.cn
Abstract:
Four tool plunging paths including a one-time plunging path and three step-by-step
plunging paths were designed to study the effects of the tool plunging path on the welded joint
properties of pinless friction stir spot welding (PFSSW). The appearance, cross-sectional microstruc-
ture, welding temperature, microhardness, and tensile shear failure load of the PFSSW of thin
copper sheets under different tool plunging paths were explored. Furthermore, the fracture modes
of welded joints under different tool plunging paths were analyzed. Studies showed that path
1 (plunge total depth at one time) produced the largest range of stirring zone, but the grains in
the stirring zone were larger and the width of the thermal-mechanical affected zone was smaller.
Path 1 obtained the highest peak temperature during the welding process, and path 3 (plunge
1/3 total depth + plunge 2/3 total depth) gained the lowest peak temperature. The greater the initial
plunging amount of the tool, the faster the temperature rise rate in the welding stage. The tensile
shear failure loads for path 1, path 2 (plunge 1/2 total depth + plunge 1/2 total depth), path 3, and
path 4 (plunge 2/3 total depth + plunge 1/3 total depth) were 8.65 kN, 8.15 kN, 8.25 kN, and 8.85 kN,
respectively. The tensile shear failure load of path 4 was 2.3% higher than that of path 1. The fracture
modes of welded joints under different tool plunging paths were all nugget pullout fractures. The
fracture morphology indicated that the fracture type was ductile fracture. The step-by-step plunging
path proposed in this work extends the traditional PFSSW process. The findings of this study can
provide a reference for the selection and design of tool plunging paths for PFSSW.
Keywords:
pinless friction stir spot welding; plunging path; thin copper sheet; welded joint
properties; fracture mode
1. Introduction
Friction stir welding (FSW) is a solid-state joining process invented by The Welding
Institute, UK in 1991 [
1
,
2
]. Compared with traditional fusion welding, FSW can produce
high-strength welded joints with smaller distortion due to lower heat input [
3
], and has
been successfully applied in the welding of aluminum alloys [
4
–
6
], magnesium alloys [
7
,
8
],
copper alloys [
9
,
10
], etc. Furthermore, FSW also has significant advantages in the welding
of dissimilar metals [
11
–
13
]. At present, FSW has been effectively used in the fields of
automobile, aerospace, rail transportation, and so on [
14
]. Friction stir spot welding (FSSP)
was developed by the Mazda Motor Corporation based on FSW in 1993 [
3
], and has been
successfully used in the welding of car body structures. Affected by the structure of the
tool with a pin, FSSP will inevitably leave a keyhole at the welded joint, which restricts the
improvement in the welded joint performance to a certain extent, especially for the welding
of thin metal sheets. Subsequently, pinless friction stir spot welding (PFSSW) has been
proposed to solve the keyhole defect [
15
]. The welding process of PFSSW can be divided
into three stages. (i) Plunging stage: The pinless tool plunges to a specified depth at a
certain rotation speed and plunging speed. (ii) Dwell stage: The rotating pinless tool stays
for a specified time after reaching the specified depth. (iii) Retreating stage: The pinless
tool retracts at a certain rotation speed and rising speed to complete the welding process.
Lubricants 2023,11, 150. https://doi.org/10.3390/lubricants11030150 https://www.mdpi.com/journal/lubricants
Lubricants 2023,11, 150 2 of 15
PFSSW has attracted more and more attention due to the absence of keyhole defects and
good welding performance [
16
]. PFSSW has the characteristics of a simple welding process,
environmental protection, and low tool loss, etc. It can avoid problems such as thermal
cracks, pores, and slag inclusions in traditional fusion welding, and has broad application
prospects in the high-quality welding of thin metal sheets [17–19].
Oxygen-free copper sheets, as the basic connector for electrical and thermal conduc-
tivity, are widely used in the electrical industry, electronics industry, aerospace, and other
fields due to its good electrical conductivity, thermal conductivity, and ductility [
20
–
22
]
such as bus bars, conductive strips, radiators, etc. Welding is an important way to achieve
a firm connection of oxygen-free copper sheets. Due to the high thermal conductivity and
high expansion coefficient of oxygen-free copper, it is difficult to weld by fusion weld-
ing [
23
,
24
], and the PFSSW without keyhole defects is a potential option for the high-quality
welding of oxygen-free copper sheets. Therefore, it is significant and valuable to carry out
research on the PFSSW of oxygen-free copper sheets.
Current research on PFSSW has mainly focused on the effects of tool geometry and
process parameters on welded joint performance and numerical simulation modeling.
Bakavos et al. designed five shapes of pinless tools, namely, the flat tool, short flute wiper
tool, long flute wiper tool, scroll tool, and proud wiper tool, and analyzed the forming
mechanism and material flow behavior of PFSSW. The study found that although the
structure of a pinless tool with grooves is simple, the material flow during the welding
process is extremely complex, which increases the penetration between the upper sheet and
the lower sheet [
25
]. Xu et al. investigated the PFSSW of a Mg alloy with Zn interlayer, and
explored the effects of the tool plunging depth, shoulder morphology, and heat treatment
on the welded joint performance. The results showed that an appropriate increase in tool
plunging depth is beneficial for improving the strength of a welded joint [
26
]. Yazdi et al.
conducted friction stir spot welding research on a 6061-T6 aluminum alloy, and analyzed
the influence of a pinless tool and pin tool on the welded joint properties. The study pointed
out that the pinless tool produced a larger bonding width than the pin tool, and the pinless
tool with L-shaped grooves obtained the highest tensile shear strength [
27
]. Furthermore,
Saleh et al. studied the PFSSW of DP600 steel and AA6061 aluminum alloy, and established
the relationship between process parameters and the failure load of welded joints by using
the design of experiments method. The microstructural features and phase evolution of
the welded joint were also analyzed [
28
]. Li et al. proposed the method of PFSSW plus
subsequent FSW to eliminate the hook defect in welded joints. It was found that the tensile
shear strength of the welded joints under this method was greatly improved compared
with the traditional PFSSW [
29
]. Özgül et al. investigated the effect of tool geometry
and rotational speed on the microstructure and mechanical properties of 5754 aluminum
alloy PFSSW joints. In this study, three fracture modes of shear fracture, mixed cleavage
fracture, and nugget pullout fracture were obtained [
30
]. In addition, Suryanarayanan et al.
explored the influence of the PFSSW process parameters on the welded joint properties
of the 5754-6061 aluminum alloy, and optimized the PFSSW process parameters using
the response surface methodology. The study pointed out that the shoulder diameter
was the most important factor affecting the strength of the welded joints [
31
]. Abed et al.
studied the effect of the PFSSW welding process on the mechanical properties of the
welded joints of AA 6061-T6 aluminum alloy. The study showed that the tool plunging
depth and welding time had a significant impact on the joint strength [
32
]. Moreover,
Chu et al. studied the PFSSW of a 2198-T8 aluminum–lithium alloy and analyzed the
microstructure and mechanical properties of the welded joints under different process
parameters. The relationship between the mechanical properties of the welded joints
and the angle between the nugget boundary and the upper surface of the welded joints
were discussed [
33
]. Tchouaha Tankoua et al. performed PFSSW experiments on thin
aluminum–copper sheets and investigated the effect of different tool shoulder diameters
on the welded joint properties [
34
]. Additionally, Park, H. conducted a study on the PFSSW
of a Ti–6Al–4V alloy and analyzed the microstructure, mechanical properties, and fracture
Lubricants 2023,11, 150 3 of 15
modes of the welded joints [
35
]. From the aspect of numerical simulation studies, the
coupled Eulerian–Lagrangian method was adopted to establish the numerical simulation
model of the AA2198 Al–Li alloy by Chu et al. In this study, the material flow during the
welding process was analyzed, and the reliability of the numerical simulation was verified
by experiments [
36
]. Moreover, Jedrasiak et al. simulated the FSSW process of aluminum–
aluminum and aluminum–steel, and analyzed the temperature history during FSSW. The
temperature history was used to predict the growth of intermetallic phases at the welded
joint interface [
37
]. In addition, the numerical simulation models of PFSSW for the 2198-T8
aluminum alloy and AA2024 aluminum–AISI304 stainless steel were also established to
study the temperature change and material flow during the welding process [38,39].
As one of the three key stages of tool movement in PFSSW, the plunging stage has an
important influence on the forming quality of the welded joint. In previous studies, the tool
plunging path in the plunging stage was one-time plunging, that is, the tool plunged to the
specified depth at the same plunging speed at one time, and few studies have explored
the effect of a step-by-step plunging path on the properties of welded joints. The step-
by-step plunging path has different welded joint properties than the one-time plunging
path due to its variable tool movement. Due to the lack of relevant research, the properties
of welded joints under step-by-step plunging paths are not known at present, and the
difference between the properties of welded joints under step-by-step plunging paths and
the one-time plunging path is not yet clear.
The main purpose of this study was to explore the effects of the one-time plunging path
and the step-by-step plunging paths on the welded joint properties of PFSSW. The research
content can provide insights into the relationship between different tool plunging paths
and welded joint properties. The step-by-step plunging path proposed in this paper can
enrich the welding process of PFSSW and can provide design ideas for more combinations
of step-by-step plunging paths.
The rest of this paper is organized as follows. Section 2presents the design ideas
of the tool plunging path, the process parameter settings, and performance test methods.
Section 3discusses the welded joint properties such as appearance features, cross-sectional
microstructure, welding temperature, cross-sectional microhardness, tensile shear failure
load, and fracture mode under different tool plunging paths. Finally, Section 4summarizes
the main findings of this research.
2. Materials and Methods
Oxygen-free copper sheets with a thickness of 1 mm and chemical composition of
(wt.%) Cu + Ag: 99.97, P: 0.002, Bi: 0.001, Sb: 0.002, As: 0.002, Fe: 0.004, Ni: 0.002, Pb: 0.003,
Sn: 0.002, S: 0.004, Zn: 0.003, O: 0.002, impurities: 0.003 were provided by Wenghou Metal
Materials Firm in Hefei’s Shushan District (Hefei, China) and used to carry out the PFSSW
experiments. The dimensions and overlap type of oxygen-free copper sheets are shown in
Figure 1.
Lubricants 2023, 11, x FOR PEER REVIEW 4 of 15
Figure 1. Dimensions and overlap type of copper sheets.
A friction stir processing machine manufactured by Beijing FSW Technology Co.,
Ltd. (Beijing, China) was used to perform the PFSSW experiments. The experimental plat-
form is shown in Figure 2. A pinless tool with a shoulder diameter of 15 mm and three
involute grooves was used to conduct the welding experiments. The depth of involute
grooves was 0.5 mm, and the material of the pinless tool was H13 steel.
Figure 2. Photograph of the experimental platform.
Four tool plunging paths including a one-time plunging path and three step-by-step
plunging paths were designed, as shown in Figure 3. H is the total plunging depth and T
is the total dwell time. The plunging process was automatically controlled by the numer-
ical control program.
Figure 1. Dimensions and overlap type of copper sheets.
Lubricants 2023,11, 150 4 of 15
A friction stir processing machine manufactured by Beijing FSW Technology Co., Ltd.
(Beijing, China) was used to perform the PFSSW experiments. The experimental platform
is shown in Figure 2. A pinless tool with a shoulder diameter of 15 mm and three involute
grooves was used to conduct the welding experiments. The depth of involute grooves was
0.5 mm, and the material of the pinless tool was H13 steel.
Lubricants 2023, 11, x FOR PEER REVIEW 4 of 15
Figure 1. Dimensions and overlap type of copper sheets.
A friction stir processing machine manufactured by Beijing FSW Technology Co.,
Ltd. (Beijing, China) was used to perform the PFSSW experiments. The experimental plat-
form is shown in Figure 2. A pinless tool with a shoulder diameter of 15 mm and three
involute grooves was used to conduct the welding experiments. The depth of involute
grooves was 0.5 mm, and the material of the pinless tool was H13 steel.
Figure 2. Photograph of the experimental platform.
Four tool plunging paths including a one-time plunging path and three step-by-step
plunging paths were designed, as shown in Figure 3. H is the total plunging depth and T
is the total dwell time. The plunging process was automatically controlled by the numer-
ical control program.
Figure 2. Photograph of the experimental platform.
Four tool plunging paths including a one-time plunging path and three step-by-step
plunging paths were designed, as shown in Figure 3. H is the total plunging depth and T is
the total dwell time. The plunging process was automatically controlled by the numerical
control program.
Lubricants 2023, 11, x FOR PEER REVIEW 5 of 15
Figure 3. Schematic diagram of path 1 (a), path 2 (b), path 3 (c) and path 4 (d).
The process parameters under different tool plunging paths are shown in Table 1. In
order to maintain the same experimental conditions, the total cumulative plunging depth
and total cumulative dwell time of the four tool plunging paths were all 0.3 mm and 8 s.
In addition, the rotation speed and the plunging speed of the tool were constant in all
experiments, which were 1200 rpm and 10 mm·min−1, respectively.
Table 1. The process parameters under different tool plunging paths.
Plunging Process Process Parameters Path 1 Path 2 Path 3 Path 4
Step 1 Plunging depth (mm) 0.3 0.15 0.1 0.2
Dwell time (s) 0.8 0.4 0.4 0.4
Step 2 Plunging depth (mm) - 0.15 0.2 0.1
Dwell time (s) - 0.4 0.4 0.4
The research process of this work is shown in Figure 4. A K-type thermocouple with
a diameter of 1 mm was used to measure the temperature change. The diameter and the
depth of temperature measurement hole (P) were 1 mm and 0.6 mm, respectively, as
shown in Figure 1. The appearance features of the center and edge of the welded joints
were observed with a superdepth digital microscope produced by KEYENCE Co., Ltd.
(Osaka, Japan). The welded joints were cut from the center using a wire cutting machine
to observe the macroscopic morphology, microstructure, and microhardness of the cross-
section. A Vickers hardness tester provided by Laizhou Weiyi Experimental Machine
Manufacturing Co., Ltd. (Laizhou, China) was used to test the microhardness of the cross
section. The location of the test points for microhardness was set at 0.7 mm below the
upper surface of the upper sheet, and the horizontal distance between the test points was
0.3 mm. The microhardness test was performed at a pressure of 0.5 kg and a holding time
of 10 s. The tensile tests were adopted to evaluate the strength of the specimens [40,41],
and a universal tensile testing machine was used to perform tensile tests with a tensile
rate of 1 mm·min−1. Three specimens were tested for each tool plunging path, and the av-
erage values were used as the final test results. The fracture morphologies of welded joints
Figure 3. Schematic diagram of path 1 (a), path 2 (b), path 3 (c) and path 4 (d).
The process parameters under different tool plunging paths are shown in Table 1.
In order to maintain the same experimental conditions, the total cumulative plunging
depth and total cumulative dwell time of the four tool plunging paths were all 0.3 mm and
Lubricants 2023,11, 150 5 of 15
8 s. In addition, the rotation speed and the plunging speed of the tool were constant in all
experiments, which were 1200 rpm and 10 mm·min−1, respectively.
Table 1. The process parameters under different tool plunging paths.
Plunging
Process Process Parameters Path 1 Path 2 Path 3 Path 4
Step 1 Plunging depth (mm) 0.3 0.15 0.1 0.2
Dwell time (s) 0.8 0.4 0.4 0.4
Step 2 Plunging depth (mm) - 0.15 0.2 0.1
Dwell time (s) - 0.4 0.4 0.4
The research process of this work is shown in Figure 4. A K-type thermocouple with
a diameter of 1 mm was used to measure the temperature change. The diameter and the
depth of temperature measurement hole (P) were 1 mm and 0.6 mm, respectively, as shown
in Figure 1. The appearance features of the center and edge of the welded joints were
observed with a superdepth digital microscope produced by KEYENCE Co., Ltd. (Osaka,
Japan). The welded joints were cut from the center using a wire cutting machine to observe
the macroscopic morphology, microstructure, and microhardness of the cross-section. A
Vickers hardness tester provided by Laizhou Weiyi Experimental Machine Manufacturing
Co., Ltd. (Laizhou, China) was used to test the microhardness of the cross section. The
location of the test points for microhardness was set at 0.7 mm below the upper surface
of the upper sheet, and the horizontal distance between the test points was 0.3 mm. The
microhardness test was performed at a pressure of 0.5 kg and a holding time of 10 s. The
tensile tests were adopted to evaluate the strength of the specimens [
40
,
41
], and a universal
tensile testing machine was used to perform tensile tests with a tensile rate of 1 mm
·
min
−1
.
Three specimens were tested for each tool plunging path, and the average values were used
as the final test results. The fracture morphologies of welded joints were observed by a
scanning electron microscope produced by Hitachi Ltd. (Tokyo, Japan).
Lubricants 2023, 11, x FOR PEER REVIEW 6 of 15
were observed by a scanning electron microscope produced by Hitachi Ltd. (Tokyo, Ja-
pan).
Figure 4. Flowchart of the research process.
3. Results and Discussion
3.1. Appearance Features
Figure 5 presents the appearance of welded joints under different tool plunging
paths. As can be seen from Figure 5, three interlaced convex peaks formed in the center of
the welded joints due to the stirring effect of the tool with involute grooves, and there was
a circular hole between the three convex peaks, while a circular arc shape feature formed
at the edge of the welded joints due to the material flow. The maximum depths of the
circular hole at the center of the welded joints under path 1, path 2, path 3, and path 4
were 454.26 μm, 414.29 μm, 370.52 μm, and 392.69 μm, respectively. The maximum
heights of the welded joint edges under path 1, path 2, path 3, and path 4 were 260.89 μm,
171.20 μm, 142.94 μm, and 185.13 μm, respectively. It was found that the depth of the
circular hole and the height of the welded joint edge generated by path 1 were the largest,
followed by path 4 and path 2, and the depth of the circular hole and the height of the
welded joint edge of path 3 were the smallest. The depth of the circular hole was closely
related to the material flow at the center of the welded joint. The greater the depth of
circular hole, the stronger the stirring effect of the tool at the center of welded joint. Figure
5 also indicates that the greater the first plunging depth of the tool, the greater the depth
of the circular hole, and the higher the height of the edge of the welded joint.
Figure 4. Flowchart of the research process.
3. Results and Discussion
3.1. Appearance Features
Figure 5presents the appearance of welded joints under different tool plunging paths.
As can be seen from Figure 5, three interlaced convex peaks formed in the center of the
welded joints due to the stirring effect of the tool with involute grooves, and there was a
Lubricants 2023,11, 150 6 of 15
circular hole between the three convex peaks, while a circular arc shape feature formed
at the edge of the welded joints due to the material flow. The maximum depths of the
circular hole at the center of the welded joints under path 1, path 2, path 3, and path 4 were
454.26
µ
m, 414.29
µ
m, 370.52
µ
m, and 392.69
µ
m, respectively. The maximum heights of
the welded joint edges under path 1, path 2, path 3, and path 4 were 260.89
µ
m, 171.20
µ
m,
142.94
µ
m, and 185.13
µ
m, respectively. It was found that the depth of the circular hole and
the height of the welded joint edge generated by path 1 were the largest, followed by path
4 and path 2, and the depth of the circular hole and the height of the welded joint edge of
path 3 were the smallest. The depth of the circular hole was closely related to the material
flow at the center of the welded joint. The greater the depth of circular hole, the stronger
the stirring effect of the tool at the center of welded joint. Figure 5also indicates that the
greater the first plunging depth of the tool, the greater the depth of the circular hole, and
the higher the height of the edge of the welded joint.
Lubricants 2023, 11, x FOR PEER REVIEW 7 of 15
Figure 5. Appearance features of the welded joints under path 1 (a1,a2), path 2 (b1,b2), path 3 (c1,c2),
and path 4 (d1,d2).
3.2. Macroscopic Morphology and Microstructure
Figure 6 shows the cross-sectional macroscopic morphologies of the welded joints
under different tool plunging paths. There were certain differences in the size of the cross-
sectional welded joints under different tool plunging paths, but they all presented typical
characteristics of the stirring zone (SZ), the thermal-mechanical affected zone (TMAZ),
and the heat-affected zone (HAZ). The maximum widths of the stirring zones in path 1,
path 2, path 3, and path 4 were 10.9 mm, 8.9 mm, 7.3 mm, and 9.2 mm, respectively, and
the heights of the stirring zones at the centers of the welded joints under path 1, path 2,
path 3, and path 4 were 1.5 mm, 1.2 mm, 0.8 mm and 1.3 mm, respectively. On the whole,
the range of the stirring zone generated by path 1 was the largest, followed by path 4 and
path 2, and the range of stirring zone formed by path 3 was the smallest, which indicates
that the greater the first plunging depth, the greater the width and depth of the stirring
zone.
Figure 5.
Appearance features of the welded joints under path 1 (
a1
,
a2
), path 2 (
b1
,
b2
), path 3 (
c1
,
c2
),
and path 4 (d1,d2).
Lubricants 2023,11, 150 7 of 15
3.2. Macroscopic Morphology and Microstructure
Figure 6shows the cross-sectional macroscopic morphologies of the welded joints
under different tool plunging paths. There were certain differences in the size of the cross-
sectional welded joints under different tool plunging paths, but they all presented typical
characteristics of the stirring zone (SZ), the thermal-mechanical affected zone (TMAZ), and
the heat-affected zone (HAZ). The maximum widths of the stirring zones in path 1, path
2, path 3, and path 4 were 10.9 mm, 8.9 mm, 7.3 mm, and 9.2 mm, respectively, and the
heights of the stirring zones at the centers of the welded joints under path 1, path 2, path
3, and path 4 were 1.5 mm, 1.2 mm, 0.8 mm and 1.3 mm, respectively. On the whole, the
range of the stirring zone generated by path 1 was the largest, followed by path 4 and path
2, and the range of stirring zone formed by path 3 was the smallest, which indicates that
the greater the first plunging depth, the greater the width and depth of the stirring zone.
Lubricants 2023, 11, x FOR PEER REVIEW 8 of 15
Figure 6. Cross-sectional morphologies of the welded joints under path 1 (a), path 2 (b), path 3 (c),
and path 4 (d).
Figure 7 shows the microstructure of the stirring zone and thermal-mechanical af-
fected zone under different tool plunging paths.
Figure 7. Microstructure of the stirring zone (A1,B1,C1,D1) and thermal-mechanical affected zone
(A2,B2,C2,D2) under path 1 (a), path 2 (b), path 3 (c), and path 4 (d).
It can be seen from Figure 7 that the grains in the stirring zone under path 1 were
significantly larger than those under path 2, path 3, and path 4, which may be due to the
higher temperature generated under path 1, causing the grains in the stirring zone to grow
under the effect of higher temperature [42,43]. Furthermore, path 1 produced the smallest
thermal-mechanical affected zone width of 41.2 μm, followed by path 4, which produced
a width of 96.9 μm, while path 2 and path 3 produced a larger thermal-mechanical affected
zone width. This suggests that the tool plunging path has a great influence on the width
of the thermal-mechanical affected zone. The reason for the larger width of the thermal-
mechanical affected zone produced by the step-by-step plunging path may be related to
the first dwell after the first plunging stage. The material at the bottom of the tool during
Figure 6.
Cross-sectional morphologies of the welded joints under path 1 (
a
), path 2 (
b
), path 3 (
c
),
and path 4 (d).
Figure 7shows the microstructure of the stirring zone and thermal-mechanical affected
zone under different tool plunging paths.
It can be seen from Figure 7that the grains in the stirring zone under path 1 were
significantly larger than those under path 2, path 3, and path 4, which may be due to the
higher temperature generated under path 1, causing the grains in the stirring zone to grow
under the effect of higher temperature [
42
,
43
]. Furthermore, path 1 produced the smallest
thermal-mechanical affected zone width of 41.2
µ
m, followed by path 4, which produced a
width of 96.9
µ
m, while path 2 and path 3 produced a larger thermal-mechanical affected
zone width. This suggests that the tool plunging path has a great influence on the width
of the thermal-mechanical affected zone. The reason for the larger width of the thermal-
mechanical affected zone produced by the step-by-step plunging path may be related to the
first dwell after the first plunging stage. The material at the bottom of the tool during the
first dwell stage is preheated, and when the tool plunges for the second time, the already
softened material is more sensitive to the mechanical stirring effect of the tool, resulting in
a larger range of the thermal-mechanical-affected zone. Comparing Figures 6and 7, it can
be found that although the stirring zone range under path 1 was the largest, the grains in
Lubricants 2023,11, 150 8 of 15
the stirring zone were larger and the width of the thermal-mechanical-affected zone was
the smallest.
Lubricants 2023, 11, x FOR PEER REVIEW 8 of 15
Figure 6. Cross-sectional morphologies of the welded joints under path 1 (a), path 2 (b), path 3 (c),
and path 4 (d).
Figure 7 shows the microstructure of the stirring zone and thermal-mechanical af-
fected zone under different tool plunging paths.
Figure 7. Microstructure of the stirring zone (A1,B1,C1,D1) and thermal-mechanical affected zone
(A2,B2,C2,D2) under path 1 (a), path 2 (b), path 3 (c), and path 4 (d).
It can be seen from Figure 7 that the grains in the stirring zone under path 1 were
significantly larger than those under path 2, path 3, and path 4, which may be due to the
higher temperature generated under path 1, causing the grains in the stirring zone to grow
under the effect of higher temperature [42,43]. Furthermore, path 1 produced the smallest
thermal-mechanical affected zone width of 41.2 μm, followed by path 4, which produced
a width of 96.9 μm, while path 2 and path 3 produced a larger thermal-mechanical affected
zone width. This suggests that the tool plunging path has a great influence on the width
of the thermal-mechanical affected zone. The reason for the larger width of the thermal-
mechanical affected zone produced by the step-by-step plunging path may be related to
the first dwell after the first plunging stage. The material at the bottom of the tool during
Figure 7.
Microstructure of the stirring zone (
A1
,
B1
,
C1
,
D1
) and thermal-mechanical affected zone
(A2,B2,C2,D2) under path 1 (a), path 2 (b), path 3 (c), and path 4 (d).
3.3. Welding Temperature
Figure 8shows the changes in the welding temperature for different tool plunging
paths. The overall temperature change during the PFSSW process showed a trend of
first increasing, then decreasing, and finally stabilizing. During the plunging and dwell
stage of the tool, a large amount of heat is generated in the welding area due to the
friction between the tool and the material and the plastic deformation of material, result-
ing in a continuous increase in the welding temperature. In the stage of tool retraction,
since the tool is separated from the material, no frictional heat is generated, and the
welded joint dissipates heat naturally at room temperature, so the temperature continues
to drop to room temperature. Path 1 obtained the highest peak temperature of 376
◦
C, and
path 3 had the lowest peak temperature of 268
◦
C. The peak temperatures of path 2 and
path 4 were close to each other at 307
◦
C and 306
◦
C, respectively. In path 1, due to the
continuous plunging of the tool, the plastic flow of the material in the welding zone was
greater, and more frictional heat was generated, which is one of the main reasons for
the larger grains under this condition. Figure 8also shows that the greater the initial
plunging amount of the tool, the faster the temperature rise rate during the tool plunging
and dwell stages.
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the first dwell stage is preheated, and when the tool plunges for the second time, the al-
ready softened material is more sensitive to the mechanical stirring effect of the tool, re-
sulting in a larger range of the thermal-mechanical-affected zone. Comparing Figure 6 and
Figure 7, it can be found that although the stirring zone range under path 1 was the largest,
the grains in the stirring zone were larger and the width of the thermal-mechanical-af-
fected zone was the smallest.
3.3. Welding Temperature
Figure 8 shows the changes in the welding temperature for different tool plunging
paths. The overall temperature change during the PFSSW process showed a trend of first
increasing, then decreasing, and finally stabilizing. During the plunging and dwell stage
of the tool, a large amount of heat is generated in the welding area due to the friction
between the tool and the material and the plastic deformation of material, resulting in a
continuous increase in the welding temperature. In the stage of tool retraction, since the
tool is separated from the material, no frictional heat is generated, and the welded joint
dissipates heat naturally at room temperature, so the temperature continues to drop to
room temperature. Path 1 obtained the highest peak temperature of 376 °C, and path 3
had the lowest peak temperature of 268 °C. The peak temperatures of path 2 and path 4
were close to each other at 307 °C and 306 °C, respectively. In path 1, due to the continuous
plunging of the tool, the plastic flow of the material in the welding zone was greater, and
more frictional heat was generated, which is one of the main reasons for the larger grains
under this condition. Figure 8 also shows that the greater the initial plunging amount of
the tool, the faster the temperature rise rate during the tool plunging and dwell stages.
0 20406080
0
50
100
150
200
250
300
350
400
450
Temperature (℃)
Time (s)
Path 1
Path 2
Path 3
Path 4
Figure 8. Welding temperature under different tool plunging paths.
3.4. Mechanical Properties
Figure 9 shows the cross-sectional microhardness of the welded joints under different
tool plunging paths. In path 1, the microhardness at the edge of the welded joint was
lower than that at the center of the stirring zone, and was far from reaching the micro-
hardness of the base metal (89 HV). This was mainly due to the range of the stirring zone
of path 1 being larger, and the range of the heat-affected zone formed under the influence
of higher temperature was also larger, and the limited test points did not reach the base
metal area. The microhardness of path 2, path 3, and path 4 gradually decreased when
moving from the edge of the welded joint to the stirring zone, and rose when it reached
Figure 8. Welding temperature under different tool plunging paths.
3.4. Mechanical Properties
Figure 9shows the cross-sectional microhardness of the welded joints under different
tool plunging paths. In path 1, the microhardness at the edge of the welded joint was lower
than that at the center of the stirring zone, and was far from reaching the microhardness
of the base metal (89 HV). This was mainly due to the range of the stirring zone of path 1
being larger, and the range of the heat-affected zone formed under the influence of higher
temperature was also larger, and the limited test points did not reach the base metal area.
The microhardness of path 2, path 3, and path 4 gradually decreased when moving from
the edge of the welded joint to the stirring zone, and rose when it reached the vicinity
of the stirring zone, which is consistent with the findings of [
44
]. The gradual decrease
in microhardness when moving from the edge of the welded joint to the stirring zone
may be related to the decrease in dislocation density [
45
]. In addition, excessive aging can
also reduce the microhardness of the heat-affected zone [
46
]. Two hard spots appeared
around
−
2 mm and +4 mm of path 3, which were located near the stirring zone and the
thermal-mechanical affected zone. The reason for this may be attributed to the effects of
strain hardening [
27
]. In general, the microhardness of path 3 in the stirring zone was
higher, followed by path 4 and path 2, and the microhardness of path 1 in the stirring zone
was lower, which indicates that the step-by-step plunging path is favorable for increasing
the microhardness in the stirring zone.
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the vicinity of the stirring zone, which is consistent with the findings of [44]. The gradual
decrease in microhardness when moving from the edge of the welded joint to the stirring
zone may be related to the decrease in dislocation density [45]. In addition, excessive ag-
ing can also reduce the microhardness of the heat-affected zone [46]. Two hard spots ap-
peared around −2 mm and +4 mm of path 3, which were located near the stirring zone and
the thermal-mechanical affected zone. The reason for this may be attributed to the effects
of strain hardening [27]. In general, the microhardness of path 3 in the stirring zone was
higher, followed by path 4 and path 2, and the microhardness of path 1 in the stirring zone
was lower, which indicates that the step-by-step plunging path is favorable for increasing
the microhardness in the stirring zone.
Figure 9. Cross-sectional microhardness of the welded joints under the different tool plunging
paths.
Figure 10 presents the tensile shear failure loads of the welded joints under different
tool plunging paths.
1234
0
2
4
6
8
10
Tensile shear failure load (kN)
Tool paths
Path 1 Path 2 Path 3 Path 4
Figure 10. Tensile shear failure loads of the welded joints under different tool plunging paths.
It can be observed from Figure 10 that the tensile shear failure load of path 4 was the
largest at 8.85 kN, and the tensile shear failure load of path 2 was the smallest at 8.15 kN.
Figure 9.
Cross-sectional microhardness of the welded joints under the different tool plunging paths.
Figure 10 presents the tensile shear failure loads of the welded joints under different
tool plunging paths.
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the vicinity of the stirring zone, which is consistent with the findings of [44]. The gradual
decrease in microhardness when moving from the edge of the welded joint to the stirring
zone may be related to the decrease in dislocation density [45]. In addition, excessive ag-
ing can also reduce the microhardness of the heat-affected zone [46]. Two hard spots ap-
peared around −2 mm and +4 mm of path 3, which were located near the stirring zone and
the thermal-mechanical affected zone. The reason for this may be attributed to the effects
of strain hardening [27]. In general, the microhardness of path 3 in the stirring zone was
higher, followed by path 4 and path 2, and the microhardness of path 1 in the stirring zone
was lower, which indicates that the step-by-step plunging path is favorable for increasing
the microhardness in the stirring zone.
Figure 9. Cross-sectional microhardness of the welded joints under the different tool plunging
paths.
Figure 10 presents the tensile shear failure loads of the welded joints under different
tool plunging paths.
1234
0
2
4
6
8
10
Tensile shear failure load (kN)
Tool paths
Path 1 Path 2 Path 3 Path 4
Figure 10. Tensile shear failure loads of the welded joints under different tool plunging paths.
It can be observed from Figure 10 that the tensile shear failure load of path 4 was the
largest at 8.85 kN, and the tensile shear failure load of path 2 was the smallest at 8.15 kN.
Figure 10. Tensile shear failure loads of the welded joints under different tool plunging paths.
It can be observed from Figure 10 that the tensile shear failure load of path 4 was the
largest at 8.85 kN, and the tensile shear failure load of path 2 was the smallest at 8.15 kN.
The tensile shear failure loads of path 1 and path 3 were 8.65 kN and 8.25 kN, respectively.
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The tensile shear failure load of path 4 was 2.3% higher than that of path 1, which shows the
superiority of a reasonable step-by-step plunging path compared to a one-time plunging
path in terms of the welded joint strength. The reason for this phenomenon may be that
path 4 preheats the welding zone during the dwell stage after the first plunging step,
and the softened material at the faying surface produces more severe plastic deformation
and stronger bonding force during the second plunging step. According to the previous
analysis, the maximum width and maximum height of the stirring zone of path 1 was larger
than that of path 4, but the grains of the stirring zone of path 4 were smaller than that of
path 1, and the width of the thermal-mechanical affected zone of path 4 was significantly
higher than that of path 1, which indicates that the welded joint strength of PFSSW was not
positively correlated with the range (width and height) of the stirring zone. The welded
joint strength of PFSSW is related to various factors such as the range of the stirring zone,
the microstructural features, and the range of the thermal-mechanical affected zone.
3.5. Fracture Mode
Figure 11 shows the fracture pictures of the welded joints under different tool plunging
paths (LS means the lower sheet, US-TS means the top surface of upper sheet, and US-
BS means the bottom surface of the upper sheet). It is clear that the welded joints were
completely pulled out from the upper sheet and were tightly bonded to the lower sheet,
which is a typical nugget pullout fracture mode of friction stir spot welding. This fracture
mode was also observed in [
47
]. The size order of the tightly bonded welded joint on
the lower sheet was path 1, path 4, path 2, and path 3, which is consistent with the
variation law of the maximum width of the stirring zone. It can be concluded from the
fracture mode that good metallurgical bonding formed in the welded joints under different
tool plunging paths.
The SEM morphologies of the welded joint fractures of path 1 and path 4 in positions
of P11, P12, P41, and P42 (see Figure 11) are shown in Figure 12.
It can be observed from Figure 12 that the fracture morphologies of path 1 and path 4
were composed of dimples of different sizes, which indicates that the fracture types of the
welded joints in path 1 and path 4 were both ductile fracture [
48
]. Furthermore, the dimple
size of path 4 was smaller than that of path 1, and the distribution was denser, which
indicates that the joint surface of the welded joint of path 4 produced a better bonding
quality. This may be attributed to the finer grain structure and wider thermal-mechanical
affected zone of path 4 [
49
]. Cracks could be observed in the P
12
and P
42
areas of the
upper sheet, which were caused by the continuous elongation of the material in this area
during the stretching process. The cracks in path 4 were larger than that in path 1, which
indicates that the welded joint of path 4 was subjected to greater tensile force during the
tensile process. This is consistent with the changes in the tensile shear failure load of
path 1 and path 4.
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Figure 11. Fracture pictures of welded joints under path 1 (a), path 2 (b), path 3 (c), and path 4 (d).
The SEM morphologies of the welded joint fractures of path 1 and path 4 in positions
of P11, P12, P41, and P42 (see Figure 11) are shown in Figure 12.
Figure 12. SEM morphologies of the welded joint fractures of path 1 (a) and path 4 (b).
Figure 11. Fracture pictures of welded joints under path 1 (a), path 2 (b), path 3 (c), and path 4 (d).
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Figure 11. Fracture pictures of welded joints under path 1 (a), path 2 (b), path 3 (c), and path 4 (d).
The SEM morphologies of the welded joint fractures of path 1 and path 4 in positions
of P11, P12, P41, and P42 (see Figure 11) are shown in Figure 12.
Figure 12. SEM morphologies of the welded joint fractures of path 1 (a) and path 4 (b).
Figure 12. SEM morphologies of the welded joint fractures of path 1 (a) and path 4 (b).
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4. Conclusions
The effects of four kinds of tool plunging paths on the welded joint properties of the
PFSSW of thin copper sheets were explored. The main conclusions are as follows:
1.
The range of stirring zone generated by path 1 (one-time plunging: 0.3 mm) was the
largest, while the range of stirring zone formed by path 3 (step-by-step plunging:
0.1 mm + 0.2 mm) was the smallest. The grains in the stirring zone of a step-by-step
plunging path were smaller than those of the one-time plunging path. The width of
the thermal-mechanical affected zone generated by the one-time plunging path was
the smallest.
2.
Path 1 obtained the highest peak temperature, and path 3 gained the lowest peak
temperature. The greater the initial plunging amount of the tool, the faster the
temperature rise rate during the tool plunging and dwell stages.
3.
The tensile shear failure loads of path 1, path 2 (step-by-step plunging:
0.15 mm + 0.15 mm
), path 3, and path 4 (step-by-step plunging: 0.2 mm + 0.1 mm) were
8.65 kN, 8.15 kN, 8.25 kN, and 8.85 kN, respectively. The tensile shear failure load of path
4, with a step-by-step plunging path, was 2.3% higher than that of the one-time plunging
path 1. The step-by-step plunging path is favorable for increasing the microhardness in
the stirring zone.
4.
The fracture modes of the welded joints under different tool plunging paths were
all nugget pullout fracture, and the fracture types of path 1 and path 4 were both
ductile fracture.
Author Contributions:
Conceptualization, X.G.; Methodology, X.G. and D.J.; Software, X.G.; Inves-
tigation, X.G. and W.S.; Resources, X.G. and H.W.; Writing—original draft, X.G.; Writing—review
& editing, X.G. and W.S.; Visualization, X.G., D.J., W.S. and H.W.; Supervision, H.W.; Funding
acquisition, X.G. All authors have read and agreed to the published version of the manuscript.
Funding:
This research was funded by the Key Research Project of Natural Science in Anhui Higher
Education Institutions, grant number 2022AH051947; the Key Research and Development Project in
Anhui Province, grant number 202004a05020025 and 202104b11020011.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Conflicts of Interest: The authors declare no conflict of interest.
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