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\begin{document}

\title{Fatigue behavior of Al-Al and Al-steel refill friction stir spot welding
joints}



\author[1]{Haokun Yang}%
\author[2]{detao Cai}%
\author[1]{Jiaxin Li}%
\author[1]{ChinYung Kwok}%
\author[3]{yunqiao Zhao}%
\author[3]{Yu Li}%
\author[3]{haisheng Liu}%
\author[1]{waiwah Lai}%
\affil[1]{Hong Kong Productivity Council}%
\affil[2]{Hunan University State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body}%
\affil[3]{Guangdong Academy of Sciences}%


\vspace{-1em}



  
  \date{May 29, 2024}


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\textbf{Fatigue behavior of Al-Al and Al-steel refill friction stir spot
welding joints} Haokun Yang\textsuperscript{1a*}, Detao
Cai\textsuperscript{2,3a}, Jiaxin Li\textsuperscript{1}, Chin Yung
Kwok\textsuperscript{1}, Yunqiang Zhao\textsuperscript{3}, Yu
Li\textsuperscript{4}, Haisheng Liu\textsuperscript{4}, Waiwah
Lai\textsuperscript{1}

1 Smart Manufacturing Division (SMD), Hong Kong Productivity Council
(HKPC), Hong Kong 999077, People's Republic of China

2 State Key Laboratory of Advanced Design and Manufacturing for Vehicle
Body, Hunan University, Changsha, Hunan 410082, People's Republic of
China

3 China-Ukraine Belt and Road Joint Laboratory on Materials Joining and
Advanced Manufacturing, China-Ukraine Institute of Welding Guangdong
Academy of Sciences, Guangzhou, Guangdong 510650, People's Republic of
China

4 Center for Industrial Analysis and Testing, Guangdong Academy of
Science, Guangzhou, Guangdong 510650, People's Republic of China

a These authors contribute equally

Corresponding author: Haokun Yang, +852 27885679,\emph{hkyang@hkpc.org}

Keywords: Refill friction stir spot welding, Fatigue fracture, Aluminum,
Steel

\textbf{1. Introduction}

Lightweight is one of the most important directions in the global
development of new energy mobility \textsuperscript{\emph{1}} . Several
concepts were developed as: (1) Hybrid sandwich metal-plastic
structure\textsuperscript{\emph{2}} ; (2) High strength/weight ratio
alloys\textsuperscript{\emph{3} ,\emph{4}} ; (3) Material joining, e.g.
self-piercing riveting (SPR)\textsuperscript{\emph{5}} , electric
resistance welding (ERW)\textsuperscript{\emph{6}} and friction stir
welding (FSW)\textsuperscript{\emph{7} ,\emph{8}} .

FSW has achieved outstanding welding properties, but the key-holes
within the joint will bring in a weak bonding
interface\textsuperscript{\emph{9}} and poor mechanical
stability\textsuperscript{\emph{10}} . To overcome the above challenges,
refill friction stir spot welding (RFSSW) was proposed and
developed\textsuperscript{\emph{11}} , which works on similar Al-Al and
dissimilar Al-steel metallic joints\textsuperscript{\emph{7}} , as well
as metal-polymer composites\textsuperscript{\emph{12}} . Owing to the
welded metallic structure often suffers vibration and other damages
during long-term service, the fatigue properties and failure mode of
RFSSW joint needs to be careful examination before real
application\textsuperscript{\emph{13}} . Therefore, the fatigue testing
and failure analysis shall be carried out. As the physical properties
between Al and steel are different, the joint interface shall have
different effects on the fatigue behavior. The present research shall
accelerate the design of similar and dissimilar materials joining under
RFSSW process.

\textbf{2. Experimental procedure}

The studied materials include Aluminum (Al 1060) and steel (DIN 1623)
sheets, cleaned and grinded, with thickness of 2.0 mm and area of 50 x
150 mm. The welding was conducted using RFSSW machine provided by
Aerospace Engineering Equipment (Suzhou) Co., Ltd. The sequence of the
welded joint from up to down is Al and steel, as well as the overlap
area is 50 x 50 mm. The critical processing parameters and processes
were: (1) rotation speed: 2000 RPM; (2) penetration depths 2.40 and 1.85
mm for Al-Al and Al-steel joints; (3) penetration speed: 1 mm/s. It
shall be noted that the refill material comes from the upper sheet. The
joint locates in the center position of the overlap area, and the joint
diameter is 9 mm.

After welded, the cross-section of Al-Al and Al-steel joints were
electron spark cut and polished for hardness test, where hardness was
measured along the welding seam, upper and lower sheets (1 mm distance
from the welding seam) using Vickers indenter. The load was 0.245 N and
a dwell time of 10 s. The Al-Al and Al-steel specimens were tensile
tested by Zwick/Roell Z250 with a strain rate of 1mm/min. The
tension-tension fatigue tests were conducted using sinusoidal wave in
load control mode with R=0, and the testing frequency was 30 Hz by
Instron 8801. The stress amplitude
(\selectlanguage{greek}Δσ\selectlanguage{english}=(\selectlanguage{greek}σ\selectlanguage{english}\textsubscript{max}-\selectlanguage{greek}σ\selectlanguage{english}\textsubscript{min})/2) was set as 0.2, 0.3
and 0.4 of the joint shear strengths (\selectlanguage{greek}σ\selectlanguage{english}\textsubscript{b}) to
quantitative analysis and comparison. After the fatigue test, all the
specimens were stretched to fracture for observing fatigue damages.

\textbf{3. Results and discussion}

The hardness distribution of Al-Al joint is shown in Fig. 1(a). In the
regions of welding seam, as well as upper and lower sheets, the hardness
value varies from 23 to 28 HV. However, the hardness distribution of the
Al-steel joint varies much as displayed in Fig. 1(b). The welding seam
and lower steel sheet exhibit higher hardness between 120 and 150HV,
while the upper Al sheet's hardness is only about 30 HV, resulting in
inhomogeneous hardness distribution.

The static overlap-shear strengths (\selectlanguage{greek}σ\selectlanguage{english}\textsubscript{b}) of Al-Al and
Al-steel joints are 4691 N and 3846 N, respectively. The higher shear
strength of Al-Al joint shall be resulted from the higher penetration
depth and more solid mixture of similar materials. To quantitatively
compare the fatigue behaviors of Al-Al and Al-steel joints, the stress
amplitudes (\selectlanguage{greek}Δσ\selectlanguage{english}) were set as 0.2, 0.3 and 0.4 of the joint shear
strength. The testing results are illustrated in Figs. 1(c\&d), where
both the Al-Al and Al-steel RFSSW joints exhibit reliable fatigue
properties, compared with other reported
results\textsuperscript{\emph{14} ,\emph{15}} . It is noted that the
Al-steel joint with \selectlanguage{greek}Δσ/σ\selectlanguage{english}\textsubscript{b}=0.2 does not suffer obvious
damages after 10\textsuperscript{5} cycles.

\textbf{3.1 Al-Al joint}

Figures 2 (a\&b) illustrate the detailed OM observation of the fatigue
fracture with \selectlanguage{greek}Δσ/σ\selectlanguage{english}\textsubscript{b}=0.2. Two fatigue patterns are found:
(1) the fatigue striations exhibit circumference direction along the
RFSSW joint, where commonly observed in the front of joint, i.e. the
fatigue crack initiation region (Fig. 2(a)); (2) the fatigue striations
have the path penetrating the thickness direction, which close to the
torn fracture region (Figs. 2(b).

With \selectlanguage{greek}Δσ/σ\selectlanguage{english}\textsubscript{b}increasing to 0.3 as shown in Figs. 2(c\&d),
the fatigue striations also exhibit two different types with
circumference and thickness directions. However, the fatigue striations
along thickness direction get more obvious and these striations approach
to the fatigue crack initiation region. With \selectlanguage{greek}Δσ/σ\selectlanguage{english}\textsubscript{b}
further increasing to 0.4 (shown in Figs. 2(e\&f)), the fatigue
striations along thickness direction is detected in both crack
initiation and torn fracture regions.

The present observation of fatigue behaviors of Al-Al RFSSW supports
that the joint significantly influenced by stress amplitude. The higher
stress amplitude is, the more obvious fatigue striations, along
thickness direction, are. Besides, the fatigue striations along
thickness direction cover the fatigue striations with circumference
direction located in the crack initiation region. This fatigue behavior
transition shall be attributed to the higher stress concentration around
the RFFSW joint., rather than the interface between Al-Al layers.

\textbf{3.2 Al-steel joint}

Figures 3(a\&b) illustrate the OM observation of both the Al sheet and
steel sheet fatigue fractures with \selectlanguage{greek}Δσ/σ\selectlanguage{english}\textsubscript{b}=0.3,
respectively. It is found that fatigue striations exist in the edge of
the joint, starting from the Al part of the joint. With
\selectlanguage{greek}Δσ/σ\selectlanguage{english}\textsubscript{b} increasing to 0.4, the fatigue striations are both
found from the Al sheet and steel sheet parts as show in Figs. 3(c\&d).
The present observation supports that the effect of secrete hardness
distribution within the Al-steel joint plays a more important role in
fatigue crack initiation position rather than that of the parameter of
\selectlanguage{greek}Δσ/σ\selectlanguage{english}\textsubscript{b} does.

\textbf{4. Conclusions}

The fatigue behavior of Al-Al and Al-steel RFSSW joints are systemically
studied, and several conclusions are listed as below:

1. Al-Al RFSSW joint has homogeneous hardness distribution across the
welding seam, but Al-steel RFSSW joint has discrete hardness
distribution.

2. With stress amplitude or \selectlanguage{greek}Δσ/σ\selectlanguage{english}\textsubscript{b} increasing, the stress
concentration within the joint of Al-Al joint grows, resulting in the
fatigue striations with thickness direction gets more obvious in the
crack initiation region.

3. The similar Al-Al RFSSW joint is significantly influenced by stress
amplitude, while the fatigue behavior of dissimilar Al-steel joint is
insensitive to stress amplitude.

4. Regardless of stress amplitude or \selectlanguage{greek}Δσ/σ\selectlanguage{english}\textsubscript{b} varying,
Al-steel RFSSW has discrete hardness distribution, leading to fatigue
crack initiation from edge of Al-steel joint.

The current observation of the fatigue properties and behavior of
similar Al-Al and dissimilar Al-steel RFSSW joints shall accelerate the
lightweight design for vehicle industry.

\textbf{Declaration of competing interest}

The authors declared that they have no conflicts of interest in this
work.

\textbf{Data availability statement}

The datasets generated during and/or analysed during the current study
are available from the corresponding author on reasonable request.

\textbf{Author Contribution Statement}

H.K. and D.T designed the experiments and wrote the manuscript. J.X. and
C.Y. performed optical microscope observation. Y. and H.S. carried out
the tensile and fatigue test. W.W. helped shape the research, analysis
and manuscript.

\textbf{Acknowledgments}

The Innovation and Technology Fund (ITF) ITP/021/19AP financially
supported this work. Y.H.K. thanks for his wife M.T.'s support on his
career during the custom sealing off period between Hong Kong and Shen
Zhen in the past 18 months (maybe longer).

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