Strengthening and toughening mechanisms in refilled friction stir spot welding of AA2014 aluminum alloy reinforced by graphene nanosheets

Journal Pre-proof Strengthening and toughening mechanisms in refilled friction stir spot welding of AA2014 aluminum alloy reinforced by graphene nanosheets Shuai Wang, Xiao Wei, Jijin Xu, Jie Hong, Xuefeng Song, Chun Yu, Junmei Chen, Xiaoqi Chen, Hao Lu PII:

S0264-1275(19)30650-1

DOI:

https://doi.org/10.1016/j.matdes.2019.108212

Reference:

JMADE 108212

To appear in:

Materials & Design

Received Date: 6 July 2019 Revised Date:

22 August 2019

Accepted Date: 12 September 2019

Please cite this article as: S. Wang, X. Wei, J. Xu, J. Hong, X. Song, C. Yu, J. Chen, X. Chen, H. Lu, Strengthening and toughening mechanisms in refilled friction stir spot welding of AA2014 aluminum alloy reinforced by graphene nanosheets, Materials & Design (2019), doi: https://doi.org/10.1016/ j.matdes.2019.108212. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 The Author(s). Published by Elsevier Ltd.

Credit Author Statement Shuai Wang: Investigation; Methodology; Visualization; Writing-original draft. Jijin Xu and Hao Lu: Resource; Funding acquisition; Writing-review & editing; Supervision. XiaoWei, Jie Hong: Formal Analysis. Xuefeng Song, Chun Yu, Junmei Chen, Xiaoqi Chen: Conceptualization.

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Strengthening and toughening mechanisms in refilled friction

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stir spot welding of AA2014 aluminum alloy reinforced by

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graphene nanosheets

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Shuai Wanga, XiaoWeia, Jijin Xu a,*, Jie Honga, Xuefeng Songa, Chun Yua, Junmei

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Chena, Xiaoqi Chena,b, Hao Lu a,**

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a

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and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China

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b

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Engineering and Technology, Swinburne University of Technology, Hawthorn, Australia

Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science

Department of Mechanical and Product Design Engineering, Faculty of Science,

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* Corresponding author.

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** Corresponding author.

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Tel: +86 21 3420 2548

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Fax: +86 21 3420 2543

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E-mail: [email protected]

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[email protected]

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Abstract: Refilled friction stir spot welding is proposed to replace friction stir spot welding to

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solve the problem of keyhole in joining of light metals. However, it is rather challenging to ensure

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the mechanical property of refilled friction stir spot welding joints because of hook defect. In this

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study, a novel method was proposed to use graphene nanosheets to strengthen the tip of hook

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defect and improve overall mechanical properties of joint. The joints of AA2014 aluminum alloy

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with 0.6%wt graphene nanosheets were fabricated by refilled friction stir spot welding. The results

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show that the tensile/shear strength of joint increases by 31% and the fracture toughness also is

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improved by nearly 20% with graphene nanosheets. Extension in fatigue life was achieved on

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joint with graphene nanosheets. Microstructural observations, fracture characteristics and electron

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back scattered diffraction analysis were systematically investigated to clarify the mechanism for

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improvement of strength and toughness. The synergetic effect including grain refinement, load

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transfer, dislocation strengthening and crack bridging, caused by graphene nanosheets, leads to

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hardening behavior of graphene nanosheets pinned zone and a significant crack deflection during

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fracture process, which contributes greatly to improving strength and toughness of joint.

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Keywords: Refilled friction stir spot welding; Graphene nanosheets; Strengthening and

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toughening mechanism; Hook defect; AA2014 aluminum alloy

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1. Introduction

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Friction stir spot welding (FSSW), a solid-state joining technique, is suitable for joining of

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light metals in aerospace and automotive fields. However, it still is a great challenge for

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conventional resistance spot welding because of solidification defects[1, 2]. The remaining

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keyhole after welding seriously affects the joint strength. Then a modified technology called refill

43

friction stir spot welding (RFSSW) is proposed by German factory GKSS based on FSSW to

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eliminate the keyhole defect [3]. However, the strength of RFSSW joint is then greatly influenced

45

by another crucial defect, i.e. hook defect [4].

46

Hook defect is an unavoidable defect in lap joints. The formation of hook defect originates at

47

the interface between the two welded sheets. The schematic illustration of RFSSW process is

48

shown in Fig. 1a. During plunge process, the sleeve moves downward while the pin moves

49

upward to contain the material squeezed by the sleeve. Then the sleeve retracts upward while the

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pin moves downward to push the materials to refill the cavity created by the sleeve. In the retract

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process, materials around the sleeve will flow upwards when the sleeve retracts to form hook

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defect. As shown in Fig. 1b and Fig. 1c, hook defect inside the joint is a partially metallurgical

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bond region or an unbound region in the RFSSW joints. Consequently, hook tip tends to be a crack

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initiation point and crack propagation occurs along the hook line, which does harm to the tensile

55

and fatigue performance [5, 6].

56

Many studies have reported that the tensile/shear strength of RFSSW joints has a close

57

correlation with the hook geometry [7-9]. At present, there are two main approaches to optimize

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the dimensions and curvature of hook defect by adjusting the welding parameters [9-11] and

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changing the geometry of stir tool [12-15]. However, the adverse effects of hook defect can only

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be alleviated but not completely eliminated.

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Fig.1. (a) schematic illustration of the RFSSW processes, (b) the physical map corresponding to

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the region marked in (a) and (c) the hook defect in the RFSSW joint

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For now, further alleviating the impact of hook defect is still a great challenge. Therefore, it is

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meaningful to develop an innovative method to solve the adverse effects of hook defect. In fact,

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since hook tip tends to be the initiation point of fracture, the overall strength of joint can be

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increased by improving critical strength of hook tip. At present, there are many researches on

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strengthening and toughening of metals matrix materials. Among them, carbonaceous

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nanomaterials, such as graphene, are attractive reinforcements for fabricating light weight

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metal-matrix composites with high strength and toughness, due to the unique two-dimensional

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structure producing a maximum value of surface-to-volume ratios [16, 17]. Many works relate to

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the effect of graphene on strengthening and toughening of metal matrix materials. The addition of

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graphene in metal matrix composites has three main strengthening effects: (i) grain refinement [18,

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19]; (ii) dislocation strengthening [20-23]; (iii) stress transfer [22, 24, 25]. In addition, because of

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its unique two-dimensional structure, the crack hinder, deflection and bridge effect greatly

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enhance the toughness of matrix and improve the fatigue performance during fracture process [26,

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27].

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In fact, Graphene nanosheets (GNSs), which consist of multilayer graphene, possess similar

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properties with single layer graphene to be a more suitable reinforcement in practice. In addition,

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GNSs are more low-cost and easier to produce than single layer graphene [28]. However, since the

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influence of Van der Waals force, GNSs are easily agglomerated and have a bad influence on the

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mechanical properties of metal matrix composite. Therefore, various methods have been

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developed such as powder metallurgy [29-31], spark plasma sintering [24, 32], multi-turn high

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pressure rolling [18, 33] and friction stir processing (FSP) [19, 34-37] to make GNSs dispersed

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uniformly. Among these approaches, FSP has received significant attention owning to high

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efficiency and well dispersibility. As a matter of fact, RFSSW is a variation of FSP. Therefore, it

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provides the possibility of adding graphene to strengthen the hook defect tip during RFSSW

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process. However, most researches related to addition of pristine graphite to be reinforcement

89

[36-38], and few works were reported on the application of GNSs in RFSSW. More importantly,

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the mechanism for improving mechanical performance of joints has not been investigated

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systematically.

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In this work，we proposed an innovative method to improve overall mechanical properties of

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RFSSW joint by adding GNSs to strengthen the tip of hook defect. The results show that the

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tensile/shear strength of RFSSW joints increases by 31% and the fracture toughness also is

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improved by nearly 20% with addition of GNSs. Fatigue life obtains significant extension on joint

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with graphene nanosheets. Strengthening and toughening mechanisms were discussed on the basis

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of microstructural observations, load-displacement curves, fracture characteristics and EBSD

98

analysis. The obtained findings may provide guidance towards the application of novel material in

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RFSSW joint, which is essential for the fabrication of welding joints with high strength and

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toughness.

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2. Experiment

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2.1 Preparation of GNSs

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A modified Hummers' method [39] was used to synthesize GOs with pristine graphite

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powders (>99.99%). GNSs were obtained by hydrothermal method. The solution for hydrothermal

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method, which contains 15mL EG, 0.1g GOs, 0.32 g PVP, and 0.201 g [C16MMIm]Br, was stirred

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for 1 h at room temperature. Then, the solution was heated at 160 °C for 6 h in an autoclave. After

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that, the autoclave was cooled to room temperature naturally. The final black precipitate was

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washed with ethanol and deionized water for three times and then dried at 60 °C in Ar flow

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overnight.

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2.2 Welding experiment

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In this study, AA2014-O of 2 mm thickness, which is broadly used in the aviation industry,

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was selected for RFSSW. The detailed chemical compositions are listed in Table 1. RFSSW joints

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were fabricated using two 75mm×25mm×2mm sheets with an overlapped area of 25×25 mm2, as

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shown in Fig. 2. Considering the geometry of stir tool and the volume of GNSs, a ring-like groove

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was designed to control GNSs content and distribution precisely. The RFSSW tool with a clamp

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ring 14 mm in diameter, a 9mm sleeve and a 5.2mm probe was used. A set of suitable multi-step

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welding parameters were adopted, including one-step plunge and two-step retract, which can make

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material flow more sufficiently to achieve uniform dispersion of GNSs. Welding process

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parameters are chosen based on the aluminum alloy resistance spot welding standards [40]. The

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Surface appearance and mechanical properties of the joint can meet the relevant standards. The

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detailed processing parameters are listed in Table 2. The weight percent of GNSs is calculated

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based on the mass of stirred aluminum alloy. The content of GNSs was determined to be 0.6 wt%.

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0.6 wt% of GNSs was filled in the groove, which is compatible with the volume of groove. The

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joints with GNSs and without GNS were obtained with the same welding parameters.

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Table1 Chemical compositions (wt%) of AA2014 aluminum alloy. Alloy

Cu

Si

Mg

Mn

Zn

Fe

AA2014

3.9-4.8

0.6-1.2

0.4-0.8

0.4-1.0

<0.3

0-0.7

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Table2 Processing parameters of RFSSW. Rotation Speed

Plunging Depth

Dwell Time

(rpm)

(mm)

(s)

Start

500

0

0

Plunge

1800

2.6

2.5

Retract-I

1800

1.3

1

Retract-II

1500

0.1

1

Finish

500

0

0

Step

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129 130

Fig.2. Schematic of AA2014 specimen for RFSSW: (a) pre-processing of a single sheet and (b) the

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positions of samples for characterization in joint with GNSs

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2.3 Characterization

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The morphology of as-prepared GNSs powders were characterized using scanning electron

134

microscopy (SEM) and transmission electron microscopy (TEM). The phases of the powders were

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identified by X-Ray diffraction (XRD) and Raman, respectively.

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After welding, the tensile/shear test was performed with a constant displacement rate of

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0.5mm/min by using Zwick 2500. It is worth noting that it is not necessary to completely separate

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the two lap sheets after the joint fails for subsequent failure analysis. In order to further assess the

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effect of GNSs on the quality of RFSSW joint, fatigue tests of two joints were also conducted at

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room temperature under the stress-control mode with a stress ratio of 0.1 and a frequency of 10 Hz.

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According to the tensile/shear strength of joint with GNSs, the maximum stress levels are 1600N,

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1800N and 2000N, respectively.

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In addition, in order to understand fracture process better, the specimens at the 1st (unbroken

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position), 2nd (total broken position) and 3rd (partial broken position) locations in Fig. 2b were

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cut and mechanically ground using abrasive papers (400#, 800#, 1200#, 1500#, 2000#) followed

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by a fine polishing step using a 0.05 µm colloidal silica. The morphology and GNSs survivability

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of the specimens at the 1st position were observed by SEM after etched by Keller's reagent (4 ml

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HF, 6 ml HCl, 10 ml HNO3, and 180ml H2O). The hardness test and nano-indenter test were also

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performed in the specimens of the 1st position. The fracture morphology and crack paths were

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investigated for the specimens at the 2nd position and the 3rd position, respectively. Electron back

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scattered diffraction (EBSD) data was obtained by scanning the selected zone in the specimens at

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the 3rd position and analyzed by using standard HKL-EBSD Channel 5 software package. In

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addition, the detailed microstructures of GNSs and aluminum matrix were characterized by high

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resolution transmission electron microscopy (HRTEM) where the TEM specimens were prepared

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by the double jet thinning at -25 °C at 16 V in a 10% perchloric acid and 90% ethanol solution.

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3. Results and discussion

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3.1 Characterization of as-prepared GNSs

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Fig. 3a shows SEM image of the prepared GNSs. Obviously the prepared GNSs have

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extremely thin lamellar structure and large surface-to-volume ratio. In order to obtain more details,

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TEM was used to observe single layer or few layers graphene. The higher magnification image in

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Fig. 3b shows wrinkles of GNSs. From the XRD pattern (Fig. 3c), a broad diffraction peak of

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graphene was observed at about 23.4° which is significantly different from the pristine graphite

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(26.6°) and GOs (10.8°) [41, 42], further confirming the obtained graphene. The typical Raman

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spectrum of graphene is shown in Fig. 3d, including D band at 1350 cm-1 and G band at 1590 cm-1,

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and the ratio of ID/IG is 0.85, which proves that graphene oxide is reduced effectively.

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Fig.3. Characterization of as-prepared GNSs: (a) SEM image, (b) TEM image, (c) XRD pattern

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and (d) Raman spectrum

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3.2 Distribution of GNSs in RFSSW joint

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Fig. 4 shows the microstructures of joint without GNSs and joint with GNSs after etching.

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Compared with the corrosion morphologies in Fig. 4a, more and finer voids appear with addition

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of GNSs in Fig. 4b. Through further observation, GNSs distribute in the voids, which can be

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confirmed by SEM images and EDS results, depicted in Fig. 4c and Fig. 4d. The peak of carbon

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element marked by red line indicates that the sheets sandwiched between grains are GNSs. The

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thickness of GNSs is about 50-100 nm. It is worth noticing that a tight bond still remains between

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GNSs and the matrix after the chemical etching process, as marked by white arrows. A strong

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interfacial bonding can be produced, which means more effective stress transfer from matrix to

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GNSs and subsequently much higher strengthening effect.

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Fig.4. Morphology of specimens at the 1st position after etching: (a) joint without GNSs, (b) joint

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with GNSs, (c) the distribution of GNSs at grain boundary and (d) the EDS results of GNSs and

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the interface bond between GNSs and aluminum matrix in joint with GNSs.

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The aluminum matrix around GNSs and Al2Cu is more susceptible to corrosion because

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GNSs and Al2Cu act as the cathode to accelerate the dissolution of adjacent matrix [43]. It can be

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inferred that bigger voids are leaved when Al2Cu peels off from the surface, while finer voids

186

result from the peeling of GNSs. Residual GNSs are observed in finer voids. The distribution of

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GNSs in the joint indicates that the GNSs can be dispersed in the joint by friction stir processing.

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3.3Mechanical properties of joint

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The tensile/shear strength of the joint is significantly improved by inducing GNSs as shown

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in Fig. 5a. The maximum tensile shear strength of joint with GNSs is to be 5212 N, which is 31%

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higher than that of joint without GNSs (3973 N). Moreover, the displacement of joint fracture is

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increased by nearly 20%. The area under the load-displacement curve is significantly increased

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due to the improved shear strength and displacement, which corresponds to the toughness of the

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joint.

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Vickers hardness for half of the joint was tested at the mid-depth position because of the

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symmetry of RFSSW joints, as shown in Fig. 5b. Microhardness profiles of two samples show

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similar trends. The hardness reaches the maximum value in the boundary of the stir zone (SZ) and

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the thermal-mechanical affected zone (TMAZ). Then the hardness value gradually decreases

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toward both sides. Due to the addition of GNSs, the microhardness near the stir zone is

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significantly increased compared with that of joint without GNSs. The microhardness distributions

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in the stir zone also confirm that the GNSs are mixed into the stir zone and drastically enhanced

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the mechanical properties of the joint. This phenomenon has an important influence on the

203

deflection of the crack tip, which will be discussed later.

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The number of cycles to failure of joints with GNSs and joints without GNSs were illustrated

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in Fig. 5c. It is apparent that the fatigue life of joints with GNSs is far inferior to that of joints

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without GNSs when the same cyclic loads are applied. This observation is valid for proving the

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positive effect of GNSs on the dynamic mechanical properties of the joints. Therefore, the role

208

played by GNSs in the process of fracture should be discussed in detail, which will be very helpful

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to understand the reason for improvement of fatigue performance.

210

211 212

Fig.5. Comparison of mechanical performances between joint without GNSs and joint with GNSs:

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(a) tensile/shear results, (b) micro hardness distributions and (c) fatigue life for R=0.1

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3.4 Effect of GNSs on hardening behavior of matrix

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Young's modulus is a physical quantity that only depends on the physical properties of the

216

material itself. The value of Young's modulus indicates material stiffness, so the improvement in

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Young's modulus can present strengthening effect of GNSs. The Halpin-Tsai model [46] can be

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used to predict the elastic modulus of composites. The elastic modulus enhancement attributed to

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disperse GNSs can be calculated by the following formulas:

220 221 222




 =
  × 



(   )

 

 =



+  ×

( /!" )



( /!" )# $ ( /!" )

 % = (

 /!" )

 

 



(1) (2) (3)

223

where
 ,
 , and
& are the Young's modulus of aluminum/GNSs, AA2014 alloy (72.4 GPa),

224

and GNSs (~800 GPa) [44, 45], respectively; fv is the volume fraction of GNSs (0.6%wt is about

225

6.7%vol); p is the aspect ratio (d/t ~100, diameter of GNSs is about 10 µm and thickness of GNSs

226

is in the range of 50–100 nm, respectively);  and  % are the strengthening efficiency

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coefficients for GNSs oriented along the longitudinal and transverse directions with respect to the

228

external loading conditions, respectively. Assuming that GNSs are uniformly dispersed in the joint

229

as reinforcement phase, the predicted elastic modulus of the prepared joint with GNSs is

230

95.84GPa.

231

Experimental data of elastic modulus are also obtained from sub-micron indentation testing

232

with Berkovich pyramidal indenters based on an elastic solution to simulate the contact process

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[46]. Load and unload curves are depicted in Fig. 6. According to the unloading load-displacement

234

data, the mean elastic modulus values of joint with GNSs and joint without GNSs are calculated,

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92.8GPa and 76.9GPa.

236 237

Fig.6. Load and unload versus indenter displacement curves of joint without GNSs and

238

GNSs/aluminum joint

239

There is a good agreement between theoretical prediction and experimental results.

240

Improvements in the modulus of aluminum matrix with incorporation of GNSs during RFSSW are

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attributed to the high strength and elastic modulus of GNSs. The increase in elastic modulus

242

means that the capability of joint with GNSs to resist deformation obtains enhanced, which is

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caused by the interaction of GNSs inside the grain and on the grain boundary, as shown in Fig. 7a

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and Fig. 7b. In Fig. 7c, the severe plastic deformation during stirring process brings a firm

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interface, where the C and Al atoms accommodate each other. The distance of layer-to-layer GNSs

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is ~0.33nm. Fig. 7d shows that a large number of dislocations are generated around GNSs after

247

deformation, which further validates the pinning effect of GNSs. The accumulation of dislocations

248

will lead to an increase in strength. Corresponding selective area diffraction (marked by yellow

249

rectangle) shows obvious crystal and amorphous diffraction characteristics.

250 251

Fig.7. TEM images of joint with GNSs: (a) existence of GNSs at the grain boundaries, (b)

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existence of GNSs inside grain, (c) the incorporation interface between GNSs and aluminum

253

matrix, (d) the dislocations around GNSs after deformation.

254

3.5 Role of GNSs in deformation process

255

It has been demonstrated that GNSs play a vital role in determining the strength of aluminum

256

matrix. The strengthening effect of GNSs during deformation process should not be neglected.

257

The specimens at the 3rd position were selected for EBSD and the scanned areas are near the main

258

crack, which have experienced a strong plastic deformation. Grain sizes, Schmid factor and

259

geometrical necessary dislocation (GND) have been calculated. From Fig. 8g, it can be calculated

260

that the mean grain size of SZ decreases from ~1.09 µm down to ~0.85 µm after the addition of

261

GNSs, which means grain refinement occurs during dynamic recrystallization process.

262

Strong strain hardening phenomenon after deformation can be reflected by the maps of

263

Schmid factor (seen in Fig. 8b and Fig. 8e). The grains with a low Schmid factor and high GND

264

density usually behave as “hard” grains, which will result in high strain hardening state at the

265

early stage of deformation for such grains [47]. From Fig. 8h, Schmid factors of grains in joint

266

without GNSs mostly range between 0.4 and 0.5, while those of joint with GNSs are more

267

uniform, between 0.3 and 0.5. It is evident that the grains surrounding crack initiate point

268

possesses lower average Schmid factor with existence of GNSs. This means that the strain

269

hardening occurs remarkably. When the crack is in the expansion stage, a larger driving force is

270

required to penetrate the local hardening zone.

271

The strengthening effect in the deformation process mainly comes from the strain hardening

272

caused by the accumulation of dislocation and stress transfer mechanism due to the tight bond

273

between GNSs and aluminum matrix, which can be proved by the comparison of GND density in

274

two joints (Fig. 8c and Fig. 8f). It must be noted GND density can only represent total dislocation

275

density partly, as some of the dislocations exists in statistically stored dislocation (SSD) form. The

276

increase of GND density mainly results from pinning effect of GNSs. GNSs strengthen aluminum

277

alloy by resisting dislocation motion and forming dislocation fostering during RFSSW process and

278

tensile-shear process. For GNSs existing inside grains (Fig. 7a), they can increase the number of

279

dislocation pinning points. For GNSs existing in grain boundaries (Fig. 7b), the grain boundaries

280

resistance to dislocation glide gets enhanced greatly. Thus, incorporation of GNSs promotes

281

higher dislocation density, which attributes to higher strength of joint with GNSs through effective

282

resistance to dislocation slip.

283 284

Fig.8. EBSD analysis: (a) IPF map, (b) Schmid factor map and (c) GND density map of joint with

285

GNSs, (d) IPF map, (e) Schmid factor map and (f) GND density map of joint without GNSs, (g)

286

corresponding grain sizes distribution, (h) corresponding Schmid factor distribution.

287

3.6 Influence of GNSs on improvement of fracture toughness

288

Crack deflection, crack bridging and crack block are the vital toughening mechanisms in

289

metal matrix composite. In these mechanisms, crack deflection and bridging are main mechanisms

290

for obtaining enhanced toughness in joint with GNSs [48].

291

3.6.1 Crack bridging mechanism

292

Crack bridging is a way of intrinsic toughening to improve the fracture toughness, by

293

affecting the inherent resistance to microstructural damage and fracture ahead of the crack tip. A

294

crack bridging phenomenon is shown in Fig. 9, where fractured GNSs bridge the propagated crack

295

behind the crack tip. Fig. 9a shows typical fracture morphologies of RFSSW joint. Hook defect tip

296

tends to be the initiation point of the crack because there is no effective bonding formed, so an

297

irregular bedded structure is presented. Afterwards plentiful dimples appear, showing ductile

298

fracture. More detailed features of hook defect and ductile are shown in Fig. 9b. Fig. 9c presents

299

the bridging GNSs with a layered morphology are embed in aluminum matrix near the crack

300

initiation zone. And in Fig. 9d, the corresponding EDS results confirm the presence of GNSs by a

301

significant increase of carbon content.

302

When the plane of the sheets is perpendicular to the plane of the fracture surface, the energy

303

required to tear a sheet is greater than that of joint without GNSs due to the GNSs/Al layers

304

debonding and the prolonged crack pathway. For the sake of simplicity, the energy dissipation of

305

joints can be correlated with the area under the load–displacement curve, depicted in Fig. 6a,

306

which is the total energy absorbed by the joint during the whole fracture process. After integral

307

calculation, energy consumed by the complete fracture of the joint with GNSs is 14469.95 mJ

308

while that of joint without GNSs is 8601.71mJ. Due to GNSs, the fracture absorbed energy of

309

joint with GNSs increases by more than 60%. It is clear that the addition of GNSs increases the

310

energy consumed by crack propagation during fracture process, which means an improvement of

311

fracture toughness.

312

Similarly, the improvement of fatigue life for joint with GNSs also can be explained by the

313

bridging and energy dissipating mechanism brought by GNSs. During cyclic tension, GNSs can

314

effectively bear part of the load to reduce matrix damage. Because of bridging effect, the number

315

of cycles required for separation of adjacent grains is extended. Therefore, the increase in fatigue

316

life is expected.

317 318

Fig.9. Fracture morphologies of joint with GNSs: (a) the fracture surface of joint with GNSs, (b)

319

microstructures of hook defect zone and crack zone, (c) bridging GNSs embed in aluminum

320

matrix, (d) corresponding EDS results at point A and point B in (b).

321

3.6.2 Crack deflection mechanism

322

Fig.10 depicts the fracture modes and paths of two joint. From Fig. 10a and Fig. 10b, the

323

fracture mode of both joints is plug fracture. However, their fracture paths present significant

324

difference. In joint without GNSs, the hook tip is the origin position of fracture and then the main

325

crack extends along the boundary between HAZ and TMAZ. But for joint with GNSs, the crack

326

also starts from the tip of hook defect. Because of the presence of GNSs, the crack path shows a

327

deflection angle of 40o to migrate away from the GNSs-pinned region, and then continues to

328

expand. In order to further confirm difference in the fracture process, the 3rd position is selected,

329

where two joints are not completely broken, as shown in Fig. 10c and Fig. 10d. This phenomenon

330

is consistent with the results of the total fracture specimen.

331 332

Fig.10. Fracture modes and paths of (a) joint without GNSs at the 2nd position, (b) joint with

333

GNSs at the 2nd position, (c) joint without GNSs at the 3rd position and (d) joint with GNSs at the

334

3rd position.

335

Crack deflection is a way of extrinsic toughening to determine the fracture toughness [49],

336

which aims to reduce the local stress intensity actually experienced at or behind the crack tip. As

337

shown in Fig. 10, the crack has a significant deflection. When the crack propagates to the region

338

where GNSs distribute, it is hindered or retarded by GNSs. And crack is difficult to continue to

339

expand along the original direction so it can only be deflected toward the area without GNSs.

340

Therefore, the fracture path in joint with GNSs is formed. As noted above, cross-section of crack

341

shows that extensive deflection of the crack path is about 40°. Quantitatively, the effect of

342

toughening mechanism can be estimated on the basis of crack-deflection mechanics [50]. The

343

local mode-I and mode-II linear elastic stress intensity, k1 and k2, at the tip of a deflected crack,

344

can be stated in terms of the applied stress intensities (KI and KII), as followed by:

345

' = (

())*+ + (  () )*++

(4)

346

' = ( ())*+ + ( () )*++

(5)

347

where KII=0, cij (α) are mathematical functions of the deflection angle α. The effective stress

348

intensity at the tip of the deflected crack tip (Kd), can be calculated by summing the mode-I and

349

mode-II contributions in terms of the strain-energy release rate, as followed by: *, = -'  + '

350

(6)

351

It can be calculated that Kd of joint with GNSs is 0.83 KI, which suggests that the value of the

352

stress intensity at the crack tip is reduced locally by 17% due to crack deflection, compared with

353

an undeflected crack in joint without GNSs.

354

4. Conclusion

355

In summary, we carried out RFSSW to fabricate joint with GNSs and joint without GNSs.

356

The effects of the GNSs on strengthening and toughening mechanism of joint with GNSs have

357

been discussed mainly based on the load-displacement curve, microhardness, fracture feature as

358

well as microstructural analysis. Major findings of this work include:

359

1. Joint with GNSs was successfully fabricated using RFSSW processing. GNSs are

360

fragmented during stir processing and disperse in aluminum matrix as suitable welding parameters

361

are adopted. Improved hardness, tensile/shear strength and fatigue life of joint are obtained with

362

addition of GNSs.

363

2. The increase of strength of joint with GNSs can be attributed to grain refinement, strain

364

hardening, dislocation strengthening and stress transfer. Grain refinement is resulted from

365

suppress effects for grain growth associated with GNSs during dynamic recrystallization. Strain

366

hardening and dislocation strengthening contribute to the pinning effect of GNSs during

367

deformation process. And stress transfer mechanism is derived from a strong C-Al interface bond

368

and load-share effect of GNSs.

369

3. The improvement on fatigue life and fracture toughness of joint is originated from the

370

combined effect of crack deflection and bridge, which results from tight incorporation between

371

GNSs and aluminum matrix. They can significantly increase the fracture absorbing energy and

372

reduce the effective stress intensity of crack tip.

373

374

Reference:

375

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[49] R.O. Ritchie, Mechanisms of fatigue crack propagation in metals, ceramics and composites: role of crack tip shielding, Mater. Sci. Eng. A 103 (1988) 15-28. [50] B. Cotterell, J.R. Rice, Slightly curved or kinked cracks, Int. J. Fract. 16 (1980) 155-169.

506

Fig.1. (a) schematic illustration of the RFSSW processes, (b) the physical map corresponding to

507

the region marked in (a) and (c) the hook defect in the RFSSW joint

508

Fig.2. Schematic of AA2014 specimen for RFSSW: (a) pre-processing of a single sheet and (b) the

509

positions of samples for characterization in joint with GNSs

510

Fig.3. Characterization of as-prepared GNSs: (a) SEM image, (b) TEM image, (c) XRD pattern

511

and (d) Raman spectrum

512

Fig.4. Morphology of specimens at the 1st position after etching: (a) joint without GNSs, (b) joint

513

with GNSs, (c) the distribution of GNSs at grain boundary and (d) the EDS results of GNSs and

514

the interface bond between GNSs and aluminum matrix in joint with GNSs.

515

Fig.5. Comparison of mechanical performances between joint without GNSs and joint with GNSs:

516

(a) tensile/shear results, (b) micro hardness distributions and (c) fatigue life for R=0.1.

517

Fig.6. Load and unload versus indenter displacement curves of joint without GNSs and

518

GNSs/aluminum joint

519

Fig.7. TEM images of joint with GNSs: (a) existence of GNSs at the grain boundaries, (b)

520

existence of GNSs inside grain, (c) the incorporation interface between GNSs and aluminum

521

matrix, (d) the dislocations around GNSs after deformation.

522

Fig.8. EBSD analysis: (a) IPF map, (b) Schmid factor map and (c) GND density map of joint with

523

GNSs, (d) IPF map, (e) Schmid factor map and (f) GND density map of joint without GNSs, (g)

524

corresponding grain sizes distribution, (h) corresponding Schmid factor distribution.

525

Fig.9. Fracture morphologies of joint with GNSs: (a) the fracture surface of joint with GNSs, (b)

526

microstructures of hook defect zone and crack zone, (c) bridging GNSs embed in aluminum

527

matrix, (d) corresponding EDS results at point A and point B in (b).

528

Fig.10. Fracture modes and paths of (a) joint without GNSs at the 2nd position, (b) joint with

529

GNSs at the 2nd position, (c) joint without GNSs at the 3rd position and (d) joint with GNSs at the

530

3rd position

531

Table1 Chemical compositions (wt %) of AA2014 aluminum alloy

532

Table2 Processing parameters of RFFSW

Highlights: 1. An innovative method is proposed to improve overall strength and fatigue life of refilled friction stir spot welding joint through using graphene nanosheets to strengthen the tip of hook defect. 2. High strength and pinning effect of graphene nanosheets can contribute to strengthening of the tip of hook defect. 3. Crack deflection and bridging mechanism provided by the robust C-Al interface can be effective to improve toughness of joint.