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\begin{document}

\title{A comparison study of fatigue behavior in Al-Al and Al-steel refill
friction stir spot welding joint}



\author[1]{Haokun Yang}%
\author[2]{detao Cai}%
\author[1]{Jiaxin Li}%
\author[1]{ChinYung Kwok}%
\author[2]{yunqiao Zhao}%
\author[2]{Yu Li}%
\author[2]{haisheng Liu}%
\author[1]{waiwah Lai}%
\affil[1]{Hong Kong Productivity Council}%
\affil[2]{Guangdong Academy of Sciences}%


\vspace{-1em}



  
  \date{June 16, 2024}


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\textbf{A comparison study of fatigue behavior in Al-Al and Al-steel
refill friction stir spot welding joint}

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 {[}\emph{1} {]}. Several concepts
were developed as: (1) Hybrid sandwich metal-plastic structure
{[}\emph{2} {]}; (2) High strength alloys {[}\emph{3} ,\emph{4} {]}; (3)
Material joining, e.g. self-piercing riveting (SPR) {[}\emph{5} {]},
electric resistance welding (ERW) {[}\emph{6} {]} and friction stir
welding (FSW) {[}\emph{7} {]}.

FSW has achieved outstanding welding properties, but the key-holes
within the joint will bring in a weak bonding interface {[}\emph{8} {]}
and poor mechanical stability {[}\emph{9} {]}. To overcome the above
challenges, refill friction stir spot welding (RFSSW) was proposed and
developed {[}\emph{10} {]}, which works on similar Al-Al and dissimilar
Al-steel metallic joints {[}\emph{7} {]}, as well as metal-polymer
composites {[}\emph{11} {]}. However, the fatigue behavior of the
similar and dissimilar material joints fabricated by RFSSW are seldom
examined. 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 critical
processing parameters are: (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.

After welding, the cross-section of Al-Al and Al-steel joints were
electron spark cut and polished for optical microscope observation. The
hardness test was measured along the welding seam, upper and lower
sheets (1 mm distance from the welding seam). The loading is 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 is 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. 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 outstanding fatigue properties,
compared with other reported results {[}\emph{12} ,\emph{13} {]}. 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 results in the fatigue striations along thickness
direction get more obvious, which 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. 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.

\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 15 months (maybe longer).

\textbf{Reference}

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