Interface stability and fracture mechanism of Al/Steel friction stir lap joints by novel designed tool

Abstract

A novel welding tool, characterized by the enlarged pin design was developed to solve the hook feature or insufficient interface deformation for Al/steel friction stir lap welded joints. The welding tool enhanced the interface deformation effect and eliminated hook feature. The thickness of the intermetallic compound (IMC) layer at the interface decreased from 3.3 μm to 0.46 μm, when the welding speed increased from 30 mm/min to 300 mm/min. The laminated structure composed of IMCs and fine steel grains gradually disappeared, with the interface gradually changing from serrated to straight as welding speed increased. The nanohardness value of the microstructure reached 9.4 ± 0.3 GPa at a distance of 10 μm from the interface layer. Due to the larger metallurgical bonding area, the best line load 499.4 N/mm reached 52 % of the 3 mm 6082-T6 alloy, which was obtained at the rotational speed of 1200 r/min and the welding speed of 50 mm/min. Four different interface failure modes were found and established during the shearing process. The strain concentration phenomenon of the successive occurrence, development and transfer of interface failure presented obvious interval characteristics in time and space.

Full text

Journal of Materials Processing Tech. 300 (2022) 117425
Available online 9 November 2021
0924-0136/© 2021 Elsevier B.V. All rights reserved.
Interface stability and fracture mechanism of Al/Steel friction stir lap joints
by novel designed tool
Jinglin Liu
a
,
b
,
1
, Zilin Hao
a
,
1
, Yuming Xie
a
, Xiangchen Meng
a
, Yongxian Huang
a
,
Long Wan
a
,
b
,
*
a
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China
b
Harbin World Wide Welding Co., Ltd, Harbin, 150001, China
ARTICLE INFO
Associate Editor: R. Mishra
Keywords:
Al/steel
Friction stir lap welding
Novel tool design
Interface formation
Mechanical properties
Fracture mechanism
ABSTRACT
A novel welding tool, characterized by the enlarged pin design was developed to solve the hook feature or
insufcient interface deformation for Al/steel friction stir lap welded joints. The welding tool enhanced the
interface deformation effect and eliminated hook feature. The thickness of the intermetallic compound (IMC)
layer at the interface decreased from 3.3
μ
m to 0.46
μ
m, when the welding speed increased from 30 mm/min to
300 mm/min. The laminated structure composed of IMCs and ne steel grains gradually disappeared, with the
interface gradually changing from serrated to straight as welding speed increased. The nanohardness value of the
microstructure reached 9.4 ±0.3 GPa at a distance of 10
μ
m from the interface layer. Due to the larger
metallurgical bonding area, the best line load 499.4 N/mm reached 52 % of the 3 mm 6082-T6 alloy, which was
obtained at the rotational speed of 1200 r/min and the welding speed of 50 mm/min. Four different interface
failure modes were found and established during the shearing process. The strain concentration phenomenon of
the successive occurrence, development and transfer of interface failure presented obvious interval character-
istics in time and space.
1. Introduction
Hybrid structures consisting of dissimilar alloys are widely used to
satisfy the increasing demands of reducing energy consumption and
protecting environment, as reported by Meng et al. (2021). However,
the welding technology of the Al/steel hybrid structures as an emerging
challenge in lightweight design has not been effectively solved, as re-
ported by Thom¨
a et al. (2018). Solid-state welding technology has
inherent advantages in overcoming the welding difculties caused by
the differences in physical and chemical properties of dissimilar mate-
rials, as reported by Yu et al. (2021). Therefore, Friction stir lap welding
(FSLW) is becoming a promising technique to fabricate Al/steel joints, as
reported by Zhou et al. (2020). For the FSLW process of Al/steel dis-
similar materials, Zheng et al. (2016) stated that many factors inuence
the joint formation and mechanical properties. Joint performance
optimization and interface intermetallic compounds (IMCs) control have
been investigated, as reported by Wan and Huang (2018).
The design of welding tool plays a signicant role to the joint quality.
Kaushik and Dwivedi (2020) stated that the geometric size and shape of
the welding tool have an important inuence on the formation quality of
the weld and the plastic ow behavior of the materials. Conventional
cylindrical and conical tools were unable to ensure the full ow of
materials with poor wear resistance, as reported by Shamsujjoha et al.
(2015). Researchers have developed scribe tool (Wang et al., 2018b),
milling shape tool (Patterson et al., 2016) and pinless tool (Sun et al.,
2013) to avoid the severe wear of welding tools and promote the ow
behavior of softed materials. Huang et al. (2019) stated that a slight
penetration depth of the lower sheet is benecial for the rotational pin to
break the oxide lms on the surface of the lower sheet, leading to ma-
terials mixing between the metal surfaces and the upper sheet materials
without oxide lms, the IMCs and welding defects can be effectively
inhibited. Therefore, the design of welding tools can effectively promote
the uniform mixing of materials at the Al/steel lap interface, ensuring
the improvement of the joint strength, as reported by Wang et al.
(2018a). In conventional Al/steel lap joints, the welding pin plunges into
the lower sheet, and the steel with higher hardness of the lower sheet
* Corresponding author at: State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China.
E-mail address: wanlong@hit.edu.cn (L. Wan).
1
These authors contributed equally to this work.
Contents lists available at ScienceDirect
Journal of Materials Processing Tech.
journal homepage: www.elsevier.com/locate/jmatprotec
https://doi.org/10.1016/j.jmatprotec.2021.117425
Received 29 August 2021; Received in revised form 2 November 2021; Accepted 6 November 2021

Journal of Materials Processing Tech. 300 (2022) 117425
2
migrated upwards and formed hook features under the effect of severe
plastic deformation, as reported by Wang et al. (2019a). Hook features
deteriorate the interface stability and in turn inuence the mechanical
properties, as reported by Wang et al. (2019b).
Based on the above researches, novel FSLW tool was designed to
enhance the interface deformation effect, broaden the metallurgical
bonding area of the interface, improve the adaptability of the welding
tool, and eliminate the hook feature. The effect of welding parameters
on the macroscopic formation of Al/steel FSLW interface was studied, a
process optimization window was established to reveal the correlation
between interface structure and load-bearing properties of Al/steel
FSLW joints and the fracture behavior.
2. Experimental procedures
2.1. Materials and methods
The materials used in the experiment were 3-mm-thick 6082-T6 Al
alloy sheets and 2-mm-thick QSTE340TM ne-grained structural steel
sheets. The chemical compositions and mechanical properties of base
materials (BM) were listed in Table 1. The dimensions of the sheets were
all 300 mm ×100 mm, and the welding direction was perpendicular to
the rolling direction of the sheets. Al sheet was the upper sheet and was
placed on the right side, the welding tool rotated counterclockwise
(Fig. 1). During the welding processes, the rotational pin was slightly
plunged into the lower steel sheet surface with a depth of 0.1 mm. The
plasticized steel material under the interface was severely deformed
under the effect of friction and shearing provided by the enlarged pin top
of the rotational tool, and the steel at the interface owed with the
movement of the rotational pin. To conrm that the joints obtained by
novel designed tool with enlarged pin top and concave structure have
excellent formation stability under various welding parameters. The
geometric parameters of welding tool was listed in Table 2. A series of
experiments were carried out under the parameter of unchanged 0.1 mm
plunge depth and 2.5◦tilt angle, while rotational speed ranged from
700–1200 r/min and welding speed ranged from 30–250 mm/min.
The optical microscope (OM) and scanning electron microscope
(SEM) were used to observe the macrostructure morphology and analyze
the joint formation, microstructure composition, joint defect charac-
teristics, and fracture location. The width of tensile shear specimens is
15 mm, and the tensile shear tests were performed at a crosshead speed
of 0.5 mm/min. Three samples were tested under each welding
parameter. Digital image correlation (DIC) was used to record the ac-
curate local strain changes to obtain deformation behavior during the
tensile tests, and the distance value of several marking points at different
regions were recorded to investigate the strain concentration micro-
mechanism of the interface failure fracture process. Nanoindentation
measurements were taken in the interface microstructure near the steel
side under a load of 9.8 mN for 10 s.
2.2. Novel tool design
The novel tool (Fig. 1) was designed to enhance the interface
deformation effect, broaden the interfacial metallurgical bonding area,
improve the adaptability of the welding tool and eliminate the hook
feature of the Al/steel FSLW joint. The enlarged pin with triple
circumferential notches can effectively broaden the interfacial effective
metallurgical bonding area and accelerate ow behavior of the softening
materials compared with conventional cylindrical and conical pins. The
concave design on the pin top made the interfacial plasticized material
produce a centripetal ow force, which can effectively eliminate or
reduce the generation of the interface hook feature and drive the plas-
ticized Al alloy to the bottom of the concave region to prevent the
stirring pin contact with the steel sheet directly for the purpose of
avoiding tool abrasion.
3. Results
3.1. Joint formation stability
Fig. 2 shows the macrostructure of the Al/steel lap joint interface
under different welding parameters by the novel designed tool. Under
different welding parameters, the difference in the macroscopic
appearance of the joints was not obvious. The interfaces were all tightly
bonded. Compared with the lap joint obtained by Ji et al. (2019) and
Shamsujjoha et al. (2015), no hook features formed on the advancing
side (AS), indicating that the hook feature prone to form in conventional
FSLW joints can be effectively eliminated under the effect of the
enlarged pin of the novel welding tool, which was helpful to improve the
bearing capacity of the joints.
The interface formation of the Al/steel FSLW joints under different
welding parameters is shown in Fig. 3. With the welding speed
increasing, the laminated structure composed of Al/Fe IMCs and steel
ne-grain layers at the lap interface gradually disappears, and the
interface gradually changes from serrated shape to straight shape.
Because higher welding speed leads to lower thermal input of the joint
interface, and the high-temperature residence time is also shortened, as
reported by Tang and Shen (2017). The deformation range and degree of
the steel surface are shortened, and the plastic ow of the materials is
insufcient and discontinuous, which leads to the incompleteness of the
joint interface. When the welding speed is xed, the friction stir degree
and plastic deformation at the interface are increased with the rotational
speed increasing. Thus, the grains near the interface of the steel side, the
IMCs layer, and the laminated structure are all rened.
Fig. 4 shows the comparison of the thickness of the IMCs layers at the
Al/steel interfaces when the welding speed is in the range of 30~350
mm/min at a xed rotational speed of 1200 r/min. As the welding speed
increases, the thickness of the IMCs at the interface decreases from 3.3
μ
m at 30 mm/min to 0.46
μ
m at 300 mm/min. The IMCs layer disap-
pears when the welding speed exceeds 300 mm/min. (Liu and Dong,
2021) stated that the increase in rotational speed could change the
chemical composition of the IMCs layer, and the increase in welding
speed reduced the thickness of the IMCs layer due to the shortening of
the high-temperature residence time.
The comparison of the thickness of the IMCs layers at the interface of
Al/steel FSLW joints at different speeds is shown in Fig. 5. When the
xed welding speed is 50 mm/min, as the rotational speed increases, the
IMCs layer at the lap interface changes from a discontinuous structure to
a continuous layered structure with the thickness increasing. The ma-
terials ow behavior at the interface changes to be more complicated,
and a laminated structure appears at the interface. However, when the
welding speed reaches 350 mm/min, increasing the rotational speed
within a certain range cannot make the interface produce an IMCs layer,
and the plastic deformation degree of the near-surface layer on the steel
Table 1
Chemical compositions and mechanical properties of 6082-T6 Al alloy and QSTE340TM steel.
Base material Chemical compositions (wt. %) Mechanical properties
Fe Al Si Mg C Mn Cu Cr Ti Line Load (N/mm) Elongation (%)
6082-T6 0.5 Bal. 1.0 0.8 – 0.6 0.1 0.25 0.1 960 15.5
QSTE
340TM
Bal. – 0.35 – 0.12 1.3 – – – 1080 25.0
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Journal of Materials Processing Tech. 300 (2022) 117425
3
side under the interface increases severely, the grains are signicantly
rened. When the rotational speed furtherly increases to 1200 r/min,
the surface layer structure of the steel under the interface shows an
obvious phenomenon of peeling off.
3.2. Mechanical properties and fracture process
The steel surface layer under the FSLW joint interface can be divided
into three different regions: single IMCs layer, IMCs layer and steel ne
grain mixed layer, and severe plastic deformation of steel layer (Fig. 6).
Nanoindentation was used to investigate the inuence of IMCs on the
mechanical properties. Results showed that the region near the interface
layer had a higher nanohardness value. The nanohardness value of the
microstructure reached 9.4 ±0.3 GPa at a distance of 10
μ
m from the
interface layer, and the IMCs layer and steel ne grain mixed layer at a
distance of 50~100
μ
m from the interface layer reached 3.9 ±0.2 GPa.
In the severe plastic deformation of the steel layer, the nanohardness
value of the deformed steel structure reached 2.5 ±0.13 GPa, which was
38.9 % higher than that of the steel BM, as shown in Fig. 6d. The elastic
modulus of the deformed structure kept relatively stable, indicating that
the elastic modulus of the ne-grained steel structure was not sensitive
to the deformation degree. Under the combined effect of the concave
structure at the top of the enlarged pin and the thread design, the Al
materials at the upper part of the interface and the steel materials at the
Fig. 1. Illustration of Al/steel FSLW process and novel designed tool.
Table 2
Geometric parameters of welding tools.
Category Material Shoulder diameter
(mm)
Pin length
(mm)
Pin root diameter
(mm)
Pin front diameter
(mm)
Notch depth
(mm)
Triple circumferential notches WC-Co alloy steel 14 2.85 6.83 Max 8
Min 5.7
1
Fig. 2. Macrostructures of Al/steel joints under different welding parameters. (a-d) The rotational speed of 700 r/min under welding speeds of 30, 50, 100, and 250
mm/min. (e-h) The rotational speed of 900 r/min under welding speeds 30, 50, 100, and 250 mm/min.
J. Liu et al.

Journal of Materials Processing Tech. 300 (2022) 117425
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Fig. 3. Interfaces at Al/steel lap joints under different welding parameters. (a) 700 r/min-50 mm/min. (b) 700 r/min-200 mm/min. (c) 700 r/min-350 mm/min. (d)
1200 r/min-50 mm/min. (e) 1200 r/min-200 mm/min. (f) 1200 r/min-350 mm/min.
Fig. 4. Inuences of welding speeds on the thickness and morphology of IMCs layer. (a) 30 mm/min. (b) 50 mm/min. (c) 200 mm/min. (d) 300 mm/min. (e) 350
mm/min. (f) Thickness of IMCs layer.
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Journal of Materials Processing Tech. 300 (2022) 117425
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lower part of the interface converged at the interface. Under the effect of
high temperature and high pressure, a metallurgical reaction occurred
between Al and Steel, and IMCs layers were produced, which resulted in
a sharp increase in the nanohardness value at the interface.
Fixed the tilt angle of welding tool as 2.5 ◦, and plunge depth as 0.1
mm. Al/steel FSLW comparative experiments were conducted at the
rotational speed of 700~1200 r/min range and the welding speed of
30~400 mm/min range to explore the inuence of parameters on the
mechanical properties of joints, and the results are shown in Fig. 7.
Under the condition of a xed rotational speed, with the welding speed
increasing, the line load of the joints generally shows a trend of rst
rising and then falling. The change of rotational speed also had an
important impact on the line load performance of lap joints and the
optimization parameter range of welding speed. When the rotational
speed was increased from 700 r/min to 900 r/min, the optimization
range of welding speed was narrowed from 50~250 mm/min to
Fig. 5. Inuences of rotational speeds on the thickness and morphology of IMC layer. (a) 700 r/min-50 mm/min. (b) 900 r/min-50 mm/min. (c) 1200 r/min-50 mm/
min. (d) 700 r/min-350 mm/min. (e) 900 r/min-350 mm/min. (f) 1200 r/min-350 mm/min.
Fig. 6. Microstructure and nanohardness in the interfacial mixed layer. (a) Overall structure. (b) Mixed layer. (c) Severely deformed steel layer. (d) Nanohardness
and elastic moduli at the interface.
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Journal of Materials Processing Tech. 300 (2022) 117425
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50~200 mm/min, when the rotational speed was further increased to
1200 r/min, the optimization range of welding speed was further nar-
rowed. The best shearing performance was shown at the welding speed
of 50 mm/min, and the value was 499.4 N/mm, which reached 52 % of
the 3 mm 6082-T6 alloy BM.
The actual deformation and failure process of a typical Al/steel lap
joint in the DIC test are shown in Fig. 8. When initial load was added,
stress concentration was rst generated at the periphery of the shoulder
effect area of the AS, which propagated along the lap interface to the lap
region on the AS. However, crack propagation was hindered because
strong mechanical and metallurgical bonding effects produced contin-
uous IMCs at AS lap interface, higher interface bonding strength was
achieved. With the load gradually increasing, the periphery of the
shoulder effect area of the retreating side (RS) began to crack along the
interface and expanded to the lap interface of the RS. During this pro-
cess, the crack propagation was hindered by the metallurgical bonding
between RS and the plasticized deformed Al alloy. High-stress concen-
tration was generated at both AS and RS of the joint interface
simultaneously. As the load further increased, cracks propagated from
RS and AS to the central area of the interface. Finally, under the com-
bined effect of tensile shear load and corresponding torsion, the joint
fractured from the interface.
4. Discussion
4.1. Joining mechanism
Fig. 9 shows the mixed interfacial zone (MIZ) of the Al/steel FSLW
joint. The MIZ is composed of IMCs layer, laminated structure, and
swirls structure due to the stirring and mixing effect of the welding tool
at the interface. The hook feature at the interface was effectively elim-
inated because of the concave design of the enlarged pin; the circum-
ferential triple-notch design at the pin top accelerate the ow behavior
of materials near the interface. The serrated structure appeared at the
joint interface, which proved severe plastic deformation occurred near
the top surface of the steel plate. Therefore, the micromechanical
interlocking effect of the lap interface was increased. In the Al/steel
FSLW process, the enlarged pin was slightly plunged into the upper
surface of steel sheet, the plasticized deformed Al alloy and the steel
surface layered structure undergone plastic ow mixing process,
resulting in the production of ne striped structures. The MIZ at the
interface extended about 200
μ
m to the steel surface (Fig. 9c and d).
Furthermore, compared with the joint obtained by Huang et al. (2019)
and Coelho et al. (2008), the interface effective lap width was increased
by about 17 % and 94 % in this study, respectively.
The materials at the lap interface was subjected to the combined
effects of thermal cycling and mechanical stirring, and Fe atoms diffused
into Al matrix during the Al/steel FSLW process. It can be observed that
the atomic content of the Al element in the layered structure below the
interface varied from 20~65 %, as shown in Fig. 9f and g, indicating that
the layered structure was composed of Fe-Al IMCs. MIZ was mainly
composed of equiaxed
α
-Fe crystal grains with an average grain size of
0.3~0.5
μ
m and Fe-Al IMCs.
The enlarged pin design can effectively suppress the forming of hook
feature, and the schematic diagram is shown in Fig. 10. Elrefaey et al.
(2005) found that under the effect of conventional cylindrical welding
tool, the interface presented an irregular non-linear shape, hook feature
was produced at the outer edge of the pin on the AS, as shown in
Fig. 10b. Ogura et al. (2012) stated that the hook feature can improve
the lap strength of the joint under certain circumstances, but it is also a
Fig. 7. Inuences of rotational speed and welding speed on line load.
Fig. 8. Interfacial fracture behavior acquired at different time.
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Journal of Materials Processing Tech. 300 (2022) 117425
7
potential defect and stress concentration region, which often becomes
the rst cracking area at the lap interface, as reported by Song et al.
(2014). Compared with the conventional cylindrical pin (Fig. 10a), the
enlarged pin with triple circumferential notches has a more severe
stirring effect on the material. The steel materials at the interface was
softened and owed under the effect of the rotational pin. The enlarged
structure at the pin top has a certain effect to avoid the upward ow of
the materials. Under the squeezing effect of the lower surface of the
enlarged pin, the material tends to ow along a horizontal direction to
both sides of the WNZ, thereby avoiding the occurrence of hook feature.
At the same time, the sealing structure has a strong promoting effect on
the horizontal ow of the Al alloy, which is composed of softened Al
alloy of the upper sheet between the upper surface of the enlarged
structure and the shoulder surface.
4.2. Fracture analysis
Fig. 11 shows the strain cloud of the area of interest (AOI) along the
tensile direction of a typical Al/steel FSLW joint interface, with the
highest strain level reaching 28.8 %. In the initial unloaded stage of the
test, the strain eld distribution in the AOI zone of the Al/steel lap
interface was relatively balanced. As the load increased, strain concen-
tration began to appear in the outer edge area of the shoulder on the AS
of the joint, the strain concentration became more serious with the
loading time increasing. With the load further increasing, strain con-
centration also began to appear in the outer edge of the shoulder area on
the RS of the joint. The strain concentration phenomenon indicated the
existence of the concentration of stress here, as reported by Leitao et al.
(2016). But no strain increase in the central area of the lap interface was
recorded, showing that the interface had formed an effective metallur-
gical bond with excellent quality. As the tensile load increased, the strain
concentration area began to expand and spread to the middle area. The
severity of strain concentration occurred alternately on AS and RS of the
joint. This irregular development of the strain eld revealed that the
failure of the specimens started from the outer edge of the shoulder ef-
fect area on AS of the joint and then alternately expanded to the center
area. As the tensile load continued to increase, the fracture occurred
suddenly when the specimens were unable to bear the load.
Fig. 9. Microstructure characteristics near the Al/steel lap interface. (a) MIZ Macrostructure. (b-c) Laminated structure. (d) Swirls structure. (e) Serrated interface.
(f) SEM photograph. (g) Line scanning result.
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Journal of Materials Processing Tech. 300 (2022) 117425
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Fig. 12 shows the overall cross-section morphologies of the failed
specimens. The upper Al alloy was torn directly from the steel sheet
under the effect of tensile and shear force. However, by observing the
characteristics of the near-region at the interface, four different fracture
path behaviors for interface failure were found. When the thermal input
was too large, and the interface deformation degree was too high (a
combination of high rotational speed and low welding speed), the
interface was prone to form an excessively thick IMCs layer. The stress
value at the interface was too high, which results in cracking directly
from the IMCs layer (Fig. 12a). Due to the inconsistency of the expansion
coefcients of Al alloy and steel, and excess IMCs layer caused internal
stress, IMCs layer separates from the steel surface and deteriorate the
load-bearing performance of the joint. A large crack started from the
IMCs layer in the lower part of the weld nugget zone (WNZ) and
extended to the AS at the lap interface, the continuously distributed
IMCs layer with large thickness provided a channel for the propagation
of the crack, which proved the negative effects of the thick IMC layers.
The inconsistency between the monoclinic Fe
4
Al
13
IMCs and the cubic
structure of Al and steel may increase its cracking tendency, as reported
by Patterson et al. (2016). When the thermal input was too low (a
combination of low rotational speed and high welding speed), the peak
temperature and deformation degree at the interface were insufcient.
The high-temperature residence time was short, the Fe and Al atoms at
the interface were not sufciently diffused to form an effective metal-
lurgical bond, which caused the crack to propagate along the lap
interface (Fig. 12b). When the welding parameters were in the optimal
range, IMCs with appropriate thickness was formed at the lap interface,
and an effective metallurgical bond was formed so that the interface
would be effectively strengthened. The specimen fractured at WNZ of
the upper Al alloy instead of cracking along the interface (Fig. 12c), the
highest tensile-shear strength was obtained. When the welding param-
eters were not fully matched, some regions at the interface formed a
metallurgical bond, but some regions could not form a metallurgical
bond, the upper Al alloy and the lower steel sheet joined weakly,
alternate intermittent cracking along with the upper Al alloy WNZ and
interface occurred during the tensile-shearing process, which could be
called hybrid fracture mode (Fig. 12d).
Fig. 13 shows the specic marking point distance changes with time
in AS, RS, and central area of three different interface failure behaviors
in the DIC test. In the initial stage, the distance between the marking
points on AS and RS increased. With the load further increasing, the
distance increasing trend between the marking points on AS and RS kept
unchanged (Figs. 13a and d). When the thermal input decreased, the
increase of distance was alternately reciprocated (Fig. 13b and e). And as
Fig. 10. Interface formation mechanism. (a) Enlarged pin with circumferential notches. (b) Cylindrical pin.
Fig. 11. Strain distribution maps acquired during the tensile shear test.
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Journal of Materials Processing Tech. 300 (2022) 117425
9
the thermal input further decreased, the maximum increase of the dis-
tance between the marking points on both sides was basically the same.
(Fig. 13c and f). These results revealed that with the increase of welding
speed, the joint lap interface changed from an effective metallurgical
bond to a partially inadequate metallurgical bond, and nally, no
effective metallurgical bond was formed at the lap interface. The strain
concentration phenomenon of the successive occurrence, development,
and transfer of interface and failure presented obvious interval charac-
teristics in time and space. The strain concentration micro-mechanism of
the interface failure fracture process of Al/steel FSLW joints can be
explained as: due to the joining inconsistency of the AS, central, and RS
of the lap interface, different local elements of the interface along the
tensile direction showed incompatibility of microscopic deformation,
strain concentration phenomenon occurred. This made the occurrence
position and time of interface fracture failure have specic orders. The
failure of different interfacial local regions did not occur at the same
time. The failure started from a local strain peak of the interface, then
the local strain peak transferred to another local unit to cause fracture
and failure, the process lasted alternately and reciprocatingly, and
nally, the whole lap joint interface failed entirely.
Fig. 12. Inuences of welding parameters on fracture behavior of lap joints. (a) 1200 r/min-30 mm/min. (b) 700 r/min-300 mm/min. (c) 1200 r/min-50 mm/min.
(d) 1200 r/min-300 mm/min.
Fig. 13. Inuences of welding parameters on specic points distance at different time. (a, d) 1200 r/min-50 mm/min. (b, e) 1200 r/min-300 mm/min. (c. f) 700 r/
min-300 mm/min.
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Journal of Materials Processing Tech. 300 (2022) 117425
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4.3. Analysis of shearing line load
Fig. 14 shows the effect of rotational speed on the shear performance
of Al/steel lap joints under different welding speeds. When the welding
speed was lower than 100 mm/min, increasing the welding speed was
benecial to improve the line load of the lap joint, and the shear per-
formance can be optimized by adjusting the rotational speed. When the
welding speed exceeded 250 mm/min, increasing the welding speed can
improve the joint line load to a certain extent, but the optimization space
and scope were not large. When the welding speed was in the range of
100~250 mm/min, the line load of the lap joint decreased with the
increase of rotational speed, indicating that low rotational speed com-
bined with high welding speed can achieve the optimization of joint
performance. The thermal-mechanical coupling effect in the FSLW
process made the Al alloy and steel form a dual effect of mechanical
coupling and metallurgical bonding at the interface. The change of
welding speed affected the high-temperature residence time of the
thermal cycle process, the change of rotational speed affected the degree
of interface deformation and the thermal input. When the rotational
speed was reduced within a certain range, the thermal input of the joint
and the deformation degree of the interface could be effectively reduced,
the thickness of the IMCs layer at the interface could be controlled, and
the interface mechanical performance could be signicantly improved.
As the welding speed increased, the thermal input of the joint and the
degree of deformation at the interface decreased, which in turn reduced
the peak temperature, high-temperature residence time, and the defor-
mation degree of the lap interface, reducing the interfacial IMC thick-
ness and the stress value of the microstructure near the interface,
thereby improving the performance of the joint. However, when the
welding speed was increased to a certain extent, the deformation degree
and thermal input generated near the interface were insufcient, and the
high-temperature residence time was shortened. It was difcult to pro-
mote the diffusion of interfacial elements to form an effective metal-
lurgical bond, the interface performance was in turn deteriorated. On
the other hand, when the rotational speed was increased, the interface
thermal input would increase, the degree of microstructure deformation
near the interface would be improved at the same time. This indicated
that the shear performance of the joint could be improved to a certain
extent, but it could not be effectively promoted at a high welding speed,
even if the rotational speed of the welding tool was increased.
5. Conclusion
This paper studied the effect of novel designed tool with enlarged pin
head on Al/Steel FSLW process. The joint formation, interface stability,
processing window and fracture modes were systematically investi-
gated. The main results are as follows.
(1) Under different parameters, the interfaces were all tightly bonded
without hook feature, indicating that the hook feature prone to
form in conventional FSLW joints can be effectively eliminated
under the effect of novel designed enlarged pin, which was
helpful to improve the bearing capacity of the joints.
(2) The IMCs layer at the interface disappeared when the welding
speed exceeded 300 mm/min. As the welding speed increased,
the IMCs layer at the lap interface changed from a discontinuous
shape to a continuous layered structure with the thickness
increasing.
(3) When the rotational speed was increased from 700 r/min to 900
r/min, the optimal range of welding speed dropped from 50~250
mm/min to 50~200 mm/min. When the rotational speed further
increased to 1200 r/min, the optimal range of welding speed was
further narrowed.
(4) The optimal shear performance was obtained at the rotational
speed of 1200 r/min and the welding speed of 50 mm/min. The
strain concentration phenomenon of the successive occurrence,
development, and transfer of interface failure presented obvious
interval characteristics in time and space.
CRediT authorship contribution statement
Jinglin Liu: Conceptualization, Methodology, Investigation, Writing
- original draft. Zilin Hao: Conceptualization, Methodology, Investiga-
tion, Writing - original draft. Yuming Xie: Methodology, Investigation,
Writing - review & editing. Xiangchen Meng: Methodology, Investi-
gation, Writing - review & editing. Yongxian Huang: Conceptualiza-
tion, Writing - review & editing, Supervision. Long Wan:
Conceptualization, Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgments
The work was supported by the Youth Program of National Natural
Science Foundation of China (No. 52001099), Natural Science Foun-
dation of Heilongjiang Province (No. JJ2020JQ085), National Natural
Science Foundation of China (No. 52175301), Heilongjiang Postdoctoral
Foundation (No. LBH-Z20055).
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