B4C composite fabricated by accumulative roll bonding (ARB)

Accepted Manuscript Influence of multi-pass FSP on the microstructure, mechanical properties and tribological characterization of Al/B4C composite fabricated by accumulative roll bonding (ARB)

Moslem Paidar, Olatunji Oladimeji Ojo, Akbar Heidarzadeh, Hamid Reza Ezatpour PII: DOI: Reference:

S0257-8972(19)30052-0 https://doi.org/10.1016/j.surfcoat.2019.01.043 SCT 24242

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

16 October 2018 12 December 2018 11 January 2019

Please cite this article as: Moslem Paidar, Olatunji Oladimeji Ojo, Akbar Heidarzadeh, Hamid Reza Ezatpour , Influence of multi-pass FSP on the microstructure, mechanical properties and tribological characterization of Al/B4C composite fabricated by accumulative roll bonding (ARB). Sct (2019), https://doi.org/10.1016/ j.surfcoat.2019.01.043

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Influence of multi-Pass FSP on the microstructure, mechanical properties and tribological

characterization of Al/B4C composite fabricated by Accumulative Roll Bonding (ARB) Moslem Paidar1, Olatunji Oladimeji Ojo2, Hamidreza Ezatpour3, Akbar Heidarzadeh4 Department of Material Engineering, South Tehran Branch, Islamic Azad University, Tehran 1459853849, Iran 2 Department of Industrial and Production Engineering, Federal University of Technology Akure, Nigeria 4 Faculty of Engineering, Sabzevar University of New Technology, Sabzevar, Iran 3 Department of Materials Engineering, Azarbaijan Shahid Madani University, Tabriz, Iran

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* Corresponding author E-mail: [email protected]

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Abstract

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This paper aims to understand the impact of post-multi-pass friction stir processing (FSP) on the microstructure, mechanical, wear and fracture behaviors of the fabricated 10 cycleaccumulative roll bonded Al-2%B4C composites. The increase in the number of tool-passes

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directly improved homogeneity and fragmentation of B4C particles, microhardness, tensile strength (86.84-173.92 MPa) and fracture resilience of the Al-2%B4C composite. The

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tribological properties of the composite are improved with a rise in the number of tool passes. Wear rate, upper boundaries and mid-fluctuation lines of the friction coefficient decreased from 6.198×10-5 to 1.095 × 10-5 mm3/Nm, 0.56 to 0.19, and 0.31 to 0.11 respectively as the

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number of tool passes was varied between 1-8 passes. An increase in the sliding distance

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caused an overshoot of the complete waveform of friction coefficient to be above the midline of fluctuation due to the induced frictional/thermal input emanating from the prolonged surface-surface contact. Homogenous particle dispersion imposes abrasion wear mechanism

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on the composite. Ductile fracture is the predominant failure mode of the composites. Postmulti-pass friction stir processing of the accumulative roll bonded Al-2%B4C composite is an

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effective approach of achieving high performance in Al-B4C composite. Keywords: Accumulative roll bonding; Friction stir processing; Al-2%B4C composite; Mechanical properties; Wear; Microstructure.

1. Introduction Aluminum-based metal matrix composites have evolved as suitable materials for automotive and aircraft industries owing to their reduced weight, excellent fatigue properties, highstrength to weight ratio, high workability/formability, and corrosion resistance [1-3]. Among

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ACCEPTED MANUSCRIPT the varieties of reinforcements, boron carbide (B4C) is unique due to its good attributes such as high stiffness, hardness (exceeded by only cubic boron nitride and diamond), thermal and chemical stability, impact and wear resistance, and neutron absorption/shielding [4-8]. The high-strength to weight ratio and ductility of Al alloys [9-11] combined with that of B4C will produce an attractive synergy of properties (Al-B4C composites). The excellent attributes and

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the low-cost processing methods of Al-B4C composites are expected to widen the

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applications of these composites beyond the conventional automotive (cylinder liner, engine

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piston, brake disc), aerospace, nuclear (for storage of spent fuels), marine, and military fields (armor tank and bulletproof vests) [6,12-14]. Friction stir processing (FSP) and accumulative

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roll bonding (ARB) are solid-state processes capable of producing Al-B4C composites with ultrafine grains. The combination of these severe plastic deformation processes is envisaged

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to be capable of producing a large number of high angle grain boundaries and continuous dynamic recrystallization (CDRX) [15] needed for high-performance Al-B4C composites in

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terms of tribological and mechanical properties. As a result, this paper studies the effect of

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multi-pass FSP on the accumulative roll bonded Al-B4C composite by investigating microstructural modification, mechanical and wear properties.

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The production of ceramic (SiC, ZrO2, Al2O3, WC, TiN, TiC, Ti2B, and B4C) reinforced

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aluminum matrix composites via the use of a single pass friction stir processing (FSP) approach has been well studied in the literature [16-18]. Grain and Orowan strengthening mechanisms, dispersion level and the number of reinforcements have been acknowledged as the phenomena responsible for the improvement of thermal, mechanical and wear properties of composites in single FSP approach [19-22]. Reinforcement particles act as heterogeneous nucleation sites during dynamic recrystallization of grains [23]. The hard B4C reinforcement on the surface of 6061 alloy resisted plastic deformation and strengthened grain boundaries by decreasing the dislocation of atoms. Mirjavadi et al. [24] revealed that grain refinement, 2

ACCEPTED MANUSCRIPT induced dislocation by thermal mismatch and work hardening (due to strain differences) were the strengthening mechanisms of the single-passed friction stir welded AA5083/TiO2 composites. The even dispersion of TiO2 and dislocation strengthening significantly improved the tribological and mechanical properties of the composites. On the other hands, multi-pass FSP does not change the in-nugget metallurgy of welds or composites while only a

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slight change in the HAZ has been reported to ensue owing to overaging effect (as a result of

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multi-pass FSP) [25].

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Multi-pass FSP is adjudged to facilitate the inhibition of particle clusters, porosity, and particle-matrix debonding in Al-B4C composites [26-28]. Chen et al. [29] reported that the

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sizes and shapes of B4C particles in the matrix of 6063 alloy were not significantly altered after a single travel FSW process while heat treatment enforced abnormal grain growth in the

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6063/B4C composite. Homogenization of particles was achieved in the 6061/SiC [30] and 5083/B4C [31] surface composites with an increase in the number of FSP passes.

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Sahraeinejad et al. [32] revealed that an increase in the number of FSP passes enhanced the

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distribution of reinforcement in the upper and lower portions of the SZ. The tensile yield strength of the B4C-reinforced composite was improved while the ductility was reduced to

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about 2.5 % elongation. On the other hand, Yang et al. [33] reported that both the strength and ductility of multi-pass friction stir processed Al3Ti/A356 composites were improved due

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to grain refinement, even particle dispersion, and porosity removal. Multi-pass friction stir processing (from 2 to 8 passes) of AA5083/ZrO2 nanocomposite steadily improved the tensile properties and hardness of the composite owing to the induced continuous dynamic recrystallization (CDRX) (or grain refinement), even dispersion of reinforcement particles and high level of high angle grain boundaries (HAGBs) [15,34]. High traverse and rotational speeds caused inherent defects in the 2024/Al2O3 composite after multi-pass FSP [35]. It was also revealed in the works of Paidar et al. [36] that the fragmentation of particles was greatly

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ACCEPTED MANUSCRIPT controlled by the level of traverse speed while the tool rotational and pinning effects enforced grain refinement in the 5182/WC nanocomposite. ARB process has been adjudged as an effective approach of improving particle distribution in nano-composites due to the increase of layers and elongation (laterally through the rolling

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direction). The increase in the number of ARB cycles promotes the distribution of reinforcement particles in both the normal and rolling directions (in the metal matrix) [37].

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Alizadeh et al. [37] studied the properties of the Al- 4 wt% Al2O3 /B4C nano-composite

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fabricated by accumulative roll bonding (ARB) process. The increase in ARB cycles improved the microhardness and specific strength of the composite when compared to

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monolithic Al. Strain increases with the number of cycles during the ARB process and this

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has been adjudged to improve the strength and elongation of the fabricated composite [38]. Faradonbeh et al. [26] conducted friction stir welding on an accumulative roll bonded Al-B4C composite. The fragmentation and distribution of B4C particles in the stir zone were reported

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to be influenced by FSW parameters. The uniformity of B4C particles in the Al matrix improved with an increase in tool travel speed. Evenly dispersed ultra-fine grains of about 230 nm were obtained after 9 cycles of ARB process on Al/ Al2O3/B4C nano-composite. It

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was reported that grain growth was inhibited by the refined reinforcement particles and the

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refined grains increased the dislocation density [37, 39]. Faradonbeh et al. [26] previously joined accumulative rolled bonded Al-B4C composites via the use of cylindrical, square, triangular and hexagonal pin tools with a single travel pass. It was revealed that the tensile strength and porosity values of the composite improved with the number of ARB cycles. In this present study, the combined effect of accumulative roll bonding and multi-pass FSP on the Al-2%B4C composites was studied with the aim of attaining high composite performance via sufficient particle homogeneity in the Al matrix. Microstructural changes in the processed composites were studied via the use of a scanning 4

ACCEPTED MANUSCRIPT electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The mechanical, fracture and tribological characterization of the Al/B4C composites were also examined.

2. Materials and experimental procedure

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Al-2%B4C composites with 2 mm thicknesses were employed as the base metals for this

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study. The base metal was produced via a 10-cycle accumulative roll bonding (ARB) process

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(without lubrication) by employing boron carbide powders (B4C) (having an average size of 60 µm) as reinforcements. The production procedure of Al-2% B4C composites discussed by

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Faradonbeh et al. [26] was utilized for the production of the aluminum metal matrix composites (AMMCs) or Al-B4C composites of this study. The 10-cycle accumulative roll

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bonded specimens were cut into the dimensions of 150 mm by 930 mm by 2 mm respectively along the rolling direction. These specimens were subsequently cleaned via the use of

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abrasive paper and acetone prior to the multi-pass friction stir process (FSP). The friction stir

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process was carried out by using a H13 steel tool having a hardness and shoulder diameter of 50±2 HRC and 12 mm respectively. The morphology of the welding tool is shown in Fig.1.

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The friction stir processing was performed under constant tool rotational and traverse speeds of 900 rpm and 100 mm/min while the number of tool passes was varied between 1 and 8 in

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order to investigate the combined impact of ARB and friction stir process on the tribological characteristics and mechanical properties of the Al-B4C composites.

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Fig.1. Schematic and image of the FSW tool.

The cross-sections of the friction stir processed Al-B4C composites were obtained and

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subjected to standard metallographic preparation procedures. Keller’s reagent (190 mL distilled water, 5 mL nitric acid, 3 mL hydrochloric acid, and 2 mL hydrofluoric acid) was

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employed as an etchant to reveal the microstructure of the composites after 1-8 multi passes of the tool travel. The microstructure was viewed in both Philips XL30 scanning electron

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microscope (SEM) and transmission electron microscope (TEM). The phases formed in the

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composites (after different tool passes) were analyzed by using X-ray diffraction (XRD) integrated to SEM. The XRD was carried out by employing 40 kV and 40 mA operating

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voltage and Cu-Kα radiation. Linear intercept method [40] was employed to estimate the average grain sizes of the composites. Vickers microhardness was carried out on the

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composites by using a load of 50 g for 10 s at an interval of 1 mm between indentation points. The tensile test was carried out on the processed composites according to ASTM-E8 M standards by utilizing a computer-controlled Hounds field H50KS tensile machine at a 1 mm/min displacement speed. The fracture surfaces of the failed tensile samples were viewed in a scanning electron microscope (SEM) and analyzed. Wear test was performed on the composite samples according to ASTM G99 standards by using the pin on disk method. The wear test samples (surfaces) were cleaned in acetone by using 800 grit silicon carbide paper

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ACCEPTED MANUSCRIPT prior to the wear test. A hardened 52100 steel disk (60 HRC) was used as the counterface material for the wear test. The wear parameters used for the test are pin rotating speed, sliding velocity and applied load/force of 26.04 rpm, 0.35 mm/s and 40 N respectively. The friction coefficients between the specimen and the disk were measured via the measurement of frictional force with a stress sensor. The worn surfaces were examined via the use of a SEM

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while the weight loss and wear rates of the samples were determined.

3. Results and discussion

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3.1 Microstructure

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Fig.2 depicts the secondary electron (SE) mode SEM images of the Al-B4C composites subjected to different tool passes. The appendix shows the micrograph of the 10-cycle

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accumulative roll bonded Al-2%B4C composite prior to FSP. The black/gray dispersions in the whitish Al matrix are the B4C particles. The sizes and area fractions of B4C particles in

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the Al matrix are the notable changes observed on Fig.2 as the number of tool passes is

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increased (from 1 to 8). The area fraction of B4C particles in the micrographs (see Fig.2a-d) increases with the number of tool passes. This indicates that uniform dispersion of

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reinforcements (B4C particles) is induced in the Al matrix owing to the consecutive or multiple stirring actions of the FSP tool. On the other hand, three groups of B4C sizes are

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marked on Fig.2; they are the large (see the red arrows), medium (see the dotted blue enclosures) and fine (see the dotted yellow enclosures) particle sizes respectively. An increase in the number of tool passes reduces the amount of large-sized B4C particles and simultaneously increases the quantity of medium-sized and fine-sized B4C particles in the matrix of the composite. This occurrence is a confirmation that the multi-pass FSP action enforces the fragmentation and dispersion of B4C particles in the 10-cycle accumulative roll bonded composite. These findings corroborate the assertion of Keneshloo et al. [41] that the

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ACCEPTED MANUSCRIPT area/volume fraction and particle size of reinforcements are inversely related. Conclusively, the multi-pass effect of the tool imposes modified distribution and fragmentation of B4C

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particles in the Al matrix as the number of FSP passes increases from 1 to 8.

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Fig.2. SEM images of Al-2%B4C composite after, (a) 1 pass, (b) 2 pass, (c) 4 pass and (d) 8 pass.

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The average grain size of the accumulative roll bonded Al-2%B4C composite before FSP was about 22 µm. Fig.3 reveals the effect of FSP passes on the average grain size of the Al-B4C composite. An indirect relationship exists between the number of FSP passes and the average grain size of the composite. The average grain size of the composite progressively decreased from about 19.21 µm (after 1 pass) to about 3.25 µm (after 8 passes). The dynamically recrystallized grains are retained in the composite owing to the uniformly dispersed B4C particles (offering resistance to grain growth) in the composite subjected to multiple FSP

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ACCEPTED MANUSCRIPT passes. The fragmentation of B4C particles in the composite (owing to the multi-pass) increases the number of grain boundaries (of the Al matrix) and this occurrence is adjudged to have enforced pinning effect on the grain growth. The B4C particulates act as obstacles against the movement of the grain boundary and the counteraction between the B4C particles and the grain boundary enforces Zener pinning effect by retaining the recrystallized grains

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after multiple FSP passes [36]. This agrees with the works of Soleymani et al. [42] as the

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mechanical action of the FSW tool pin was reported to enforce plasticization, dynamic

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recrystallization and grain refinement in the composite’s matrix.

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Fig.3. Correlation between grain size and number of tool passes.

Fig.4 reveals the bright field (BF)-TEM image of the 10-cycle accumulative roll bonded Al2%B4C composite subjected different FSP passes. The dark and gray regions on the images are B4C particles and grains of the Al matrix respectively. The sharp banded contrast on Fig.4 represents high angle grain boundaries [15]. Dynamically recrystallized and refined grains with multiple grain boundaries and dislocations into sub-grains are realized in Fig.4b with some in the Al matrix and the others close to the B4C particles. These dislocations are

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ACCEPTED MANUSCRIPT attributed to the severe plastic deformation process during the FSP. The differential strain rate between the B4C particles and the Al matrix can also be adjudged to have produced these dislocations. This finding agrees with the work of Abraham et al. [43] as dense dislocation, sub-grain boundaries, and ultra-fine grains were in the friction stir processing of 6063/TiO2 composites. The images do not display any proof of diffusion/reaction between the Al matrix

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and the B4C particles due to the discrete nature of the Al/B4C interfaces.

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(b)

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Fig.4. TEM images of the friction stir processed Al-B4C composite (a) 1 pass; (b) 8 passes. Fig.5 shows that no detrimental reaction occurs between the Al matrix and the reinforcement

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as the composite primarily contains Al and the reinforced B4C phases. This observation agrees with the interpretations of the TEM images on Fig.4. However, the phase intensity is

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observed to decrease with the number of FSP passes. This phenomenon has been attributed to the levels of fragmentation and distribution of B4C in the composite [26]. The increase in the number of FSP passes transformed the large-sized B4C particles (from the 10-cycle accumulative roll bonded Al-B4C composites) into medium-sized and fine-sized B4C particles (see Fig.2). This occurrence (reduction in B4C particles and Al grain sizes) is responsible for the decline in the phase intensity of the composites as the number of FSP is increased from 1 to 8 (see Figs. 2 and 3).

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Fig.5. XRD patterns of Al-B4C composites subjected to different number of tool passes.

3.2 Mechanical properties

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3.2.1 Hardness

Fig.6 illustrates a line of indentation marks (hardness measurement path) on the cross-section

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of an Al-B4C composite sample. Fig.7 reveals that an increase in the number of FSP passes progressively increases the hardness values of the 10-cycle accumulative roll bonded Al2%B4C composite. The hardness values along the center line (at a distance of 0 mm) shown in Fig.7 are 40, 44.36, 62.25 and 77 HV in the composites processed with 1, 2, 4 and 8 passes respectively. Meanwhile, the average hardness of the accumulative roll bonded Al-2%B4C composite before FSP was about 50 HV. This finding may be attributed to the level of grain refinement and fragmentation of B4C particles in the composites (as the number of FSP

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ACCEPTED MANUSCRIPT passes is increased). Hall-Petch relationship is corroborated in the observed results shown in Fig.7 (this agrees with Fig.3). Refined grain size, the presence of hard reinforcing and fragmented B4C particles, and higher dislocation density in the friction stir processed composites hindered the softening effect associated with the thermal input of multi-pass FSP [15,44]. This consequently improves the hardness of the composite. However, the hardness

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rise at the HAZ (from the last two points on Fig.7) may be attributed to the direct contact

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effect of hardness indenter on somewhat large B4C particles. This corroborates the findings

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of Huang et al. [45] as higher hardness value was obtained when indentation was on or close to the reinforcement particles.

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The synergy of severe plastic deformation (dynamic recrystallization), phase/particle homogeneity and inhibition of grain coarsening (due to pinning effect) has been reported to

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improve material strengthening in the SZ of 5083/B4C surface composites [46]. This phenomenon may as well be responsible for the improved hardness in the 10-cycle

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accumulative roll bonded Al-2%B4C composites subjected to multi-pass FSP. Thus, the

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resistance of the Al-B4C composite to the indentation load is attributed to the inhibition of the reinforcement particles to dislocation movement [41]. The precipitation kinetics of Al alloy

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was not changed by the presence of B4C particles after a single travel/pass of the tool [29]. As a result, the microhardness of the composite sample produced with a single pass was not as

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high as that of the 8-pass.

Fig.6. Hardness measurement path.

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ACCEPTED MANUSCRIPT The improvement in the hardness of the multi-pass FSP samples may as well be attributed to the presence of submicron-level of B4C particles in the Al-B4C composite [43] after multi-

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pass severe plastic deformation and particle fragmentations.

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Fig.7. Hardness values of the Al- B4C composite after different tool passes.

3.2.2 Tensile strength and fracture

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Fig. 8 shows the stress-strain curves of the 10-cycle accumulative roll bonded Al-B4C

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composites subjected to a different number of FSP passes. The tensile strength of the composite improved from 86.84 to 173.92 MPa as the number of FSP passes is increased from 1 to 8. The area under the stress-strain curves also increases with an increase in the number of FSP passes. This observation shows that the resilience to fracture of the composite is improved as the number of FSP passes is increased. These results may be attributed to the increase in the level of fragmentation and dispersion of B4C particles in the composite, and the tendency of the fragmented B4C particles to resist dislocation movement during the axial loading process. 13

ACCEPTED MANUSCRIPT The multi-pass FSP induces intense material plasticization/flow, dynamic recrystallization, fragmentation and dispersion of B4C within the Al-2%B4C composite. The refined grains (as the number of FSP passes is increased) can be adjudged to increase the volume of grain boundary within the composite. An increase in the volume of the grain boundary is expected to hinder dislocation movement by increasing the dislocation’s hindrance per unit length in

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the composite (produced with 8 passes) during the monotonic axial loading condition [24].

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The reduced grain size is considered to have enforced less dislocation pile up and driving

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force at the grain boundary of the composite during the axial loading process, and as such, a greater amount of applied stress would be necessary to push the dislocation to the subsequent

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grain [36][47]. Dislocation strengthening aided by the homogeneous dispersion and interaction of fragmented B4C particles within the refined Al matrix is thus adjudged to be

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responsible for the improved strength in the 10-cycle accumulative roll bonded Al-B4C composite (as the number of FSP passes is increased).

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According to the works of Huang et al [48], the refined microstructure (owing to multiple

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passes) can activate non-basal slip during axial loading condition. This occurrence will consequently favor deformation harmonization and improved ductility in the Al-B4C

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composites subjected multiple tool pass. This agrees with the works of Wang et al. [49] as both grain refinement and a higher fraction of distributed particles were reported to enhance

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ductility and ultimate tensile strength. Also, stress dispersion (delay in stress concentration) is considered to be enforced in the composites as a result of the refined/fragmented B4C particles in the composites [48]. This phenomenon impedes the likelihood of cavitation/micro-cracks within the composite under axial loading condition and strengthens the composite produced at 8-pass. In addition, mismatched thermal expansion coefficient (between Al matrix and B4C particles) could have aided higher dislocation density in the composite [50] and consequently improved the ultimate tensile strength (UTS) and fracture

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ACCEPTED MANUSCRIPT resistance of the composites after multi FSP pass. Reinforced particles have been reported to bear the load acting on Al matrix because the established interfacial bonding (between matrix and reinforcement) aids easy transfer and distribution of load from the Al matrix to the reinforcement during axial loading [50]. Better interfacial bonding between B4C reinforcement and the Al matrix may thus be responsible for the improved UTS in the

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composites (in Fig.8).

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Fig.8. Engineering stress-strain curves of the Al-2% B4C composites subjected to different FSP passes.

Fig.9 reveals the fracture surfaces of the composites subjected to tensile loading conditions. Large dimples with relatively large embedded/fractured B4C particles are observed on Fig.9a while shallow dimples are observed on Fig.9b-d (see the marked sections). These observations indicate that ductile failure is predominant in the 10-cycle accumulative roll bonded composites subjected to multiple FSP passes. The less amount of dimples in Fig.9a indicates that less plasticity is induced in the sample (produced with a single pass) during the 15

ACCEPTED MANUSCRIPT axial loading condition. This observation justifies the reduced ductility and fracture resilience of the composite subjected to 1-pass in Fig.8. The area fraction of the fracture surfaces with shallow dimples (see the enclosed areas on Fig.9) increases with an increase in the number of FSP passes. This occurrence is attributed to the fragmentation and dispersion levels of B4C and the reduction in the average grain sizes (from 19.21 - 3.25 µm) of the Al matrix. The

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intact Al/B4C interfaces on the fracture surfaces show that the reinforcement-matrix was

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stable under applied axial loading. This occurrence agrees with the works of Li et al. [51].

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Similarly, it can be adjudged that the good interfacial bonding between the B4C particles and the Al matrix (in Fig.9) facilitates even transfer of applied load to the particles [43]. The

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observed particles on the fracture surfaces (see the red arrows) on Fig.9 may be due to failure/fracture while some of the dimple spaces may be due to particle pull out.

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The presence of non-fragmented B4C particles in the samples produced with a single tool pass resulted in the formation of large grain boundaries and sizes (see Fig.3). This could have

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impaired the ductility and the tensile strength of the sample (in Fig.9a) as intact B4C particles

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in the matrix of 6061 alloy was reported to act as void initiation sites in the works of Li et al. [51]. On the other hands, the fragmentation of B4C particles and the reduced average grain

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sizes in the composites due to the effect of multi FSP passes promoted the formation of shallow dimples (Fig.9b-d), improved the tensile strength and resilience to fracture (area

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under the stress-strain curves in Fig.8).

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Fig.9. Fracture surfaces of failed samples (a) 1 pass, (b) 2 passes, (c) 4 passes and (d) 8 passes. 17

ACCEPTED MANUSCRIPT 3.3.3 Wear properties Fig.10 reveals the relationship between weight loss and sliding distance in the 10-cycle accumulative roll bonded Al-B4C composites processed with a different number of FSP passes. The weight loss in this research work implies wear loss in mg. A decline in the amount of weight/wear loss is observed as the number of FSP passes is increased. The wear

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loss (mg) of the composite decreased from 0.028 mg to about 0.019 mg (for a sliding distance

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of 200 m) and from 0.103 mg to about 0.039 mg (for a sliding distance of 1000 m) as the

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number of FSP passes was increased from 1 to 8. The homogeneous dispersion of hard B4C particles after multi-pass FSP provides enough hard and large surface area to counteract

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immediate deformation and material loss during the wear test unlike the composites with predominant and sparely distributed B4C particles (1-pass). This finding agrees with the

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works of Mirjavadi et al. [24] as hardness, dispersion of reinforcement and the extent of grain refinement were identified as the factors controlling the wear resistance of the composites

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subjected to a different number of FSP passes.

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4 passes 8 passes

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2 passes

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Weight Loss (mg)

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Distance (m) Fig.10. Relationship between weight loss and sliding distance.

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ACCEPTED MANUSCRIPT Table 1 revealed that the wear rate of the consecutively processed composites was distinctly lowered than the earlier counterparts as the number of FSP passes was increased. The wear rate decreased from 6.198×10-5 to 1.095 × 10-5 mm3/Nm as the number of FSP passes was increased from 1 to 8. The 10-cycle accumulative roll bonded Al-B4C composites subjected to 8-pass had the lowest wear rate as compared to the other samples with a lower number of

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passes. This agrees with some of the previous works on multi-pass FSP of Al alloys [15, 30].

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The decrease in the wear rate (due to the increase in the number of FSP pass) is attributed to

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the homogeneously dispersed high hardness, increased area fraction and fragmentation, and load-bearing effect of B4C particles in the Al matrix. These factors are adjudged to limit the

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direct contact between the Al matrix and the disk during the wear test. This finding corroborates the works of Paidar et al. [36] as the loadbearing capacity of hard ceramic

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particles was reported to reduce the direct load on the reinforced composite. Table 1. Wear rate as a function of FSW pass Wear rate (mm3/Nm)

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FSP pass

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6.198×10-5 5.698 ×10-5 3.875×10-5 1.095 ×10-5

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Fig.11 reveals the surfaces of the wear tracks or surfaces after the wear test. Whitish patches of shallow grooves cover a significant surface area of the images shown in Fig.11a and 11b while Fig.11d has a fewer number of grooves or pit sizes. This is an indication that more worn debris or wear loss (mg) ensues in Fig.11a and 11b when compared to that of Fig.11c and 11d. Fine loose debris particles were observed on the worn surface of the composite subjected to 8-pass. This observation agrees with the works of Mirjavadi et al. [15] as combined shallow grooves and loose debris particles were observed. The observed forms of shallow grooves in Fig.11 are also referred to as ploughing and fine grooves and they are 19

ACCEPTED MANUSCRIPT adjudged to be formed by abrasive wear mechanism while the occurrence of cavities/craters is associated to local adhesive wears induced by the micro-weld breaking during the sliding action [24]. The discernible scratches on the worn surfaces (Fig.11) show that adhesive wear mechanism is the dominant wear mechanism [15] along the sliding direction. The B4C particles reduce

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plastic deformation by acting as barriers to dislocation movement and adhesive wear along

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the wear tracks. The high hardness attribute of B4C particles supports abrasive wear

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mechanism (owing to the scratches on Fig.11) and this occurrence is adjudged to inhibit the formation of more pronounced worn grooves or pit sizes in the processed samples as the

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number of FSP passes is increased. In fact, an abrasion on the composite’s surfaces and deep penetration of the pin are hindered [24] by the hard B4C particles during the wear test.

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In another viewpoint according to Mirjavadi et al. [24], adhesion of particles promoted abrasion on the worn surface by producing undulated scratches or ups-and-downs wear

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paths/tracks (as seen on Fig.11). Alizadeh et al. [47] revealed that such wear mechanism in

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Figs.11a and 11b was due to the transition from adhesive to delamination-abrasive mechanism while the delamination was associated with the combined accumulation and

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interaction effect. In addition, Soleymani et al. [42] reported that light delamination and abrasion mechanisms occurred concurrently during the wear test of hybrid surface composite

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while delamination wear mechanism was revealed to predominate during the early stages of sliding in the works of Mirjavadi et al. [24].

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Fig.11. Worn surfaces of the composite as a function of the number of passes, (a) 1 pass, (b)

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2 passes, (c) 4 passes and (d) 8 passes.

Fig.12 shows the graphical representations of the relationship between the coefficient of

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friction and sliding distance. The approximate boundaries (upper and lower) and the mid-

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lines across the fluctuations of the coefficient of friction are marked as red and black respectively on Fig.12. It should be noted that one or more extreme fluctuations or amplitudes of the coefficient of friction are outside the marked boundaries. This is permitted in this work because the extreme amplitudes are adjudged as minor regions with local depleted B4C particles in the composites. The approximate upper boundary/amplitude and mid-fluctuation line of the coefficient of friction decreases with an increase in the number of FSP passes. The upper boundaries of the friction coefficient of 0.56, 0.36, 0.34 and 0.19 are obtained at 1, 2, 4, and 8 passes of the tool 21

ACCEPTED MANUSCRIPT respectively while 0.31, 0.28, 0.27 and 0.11 are their respective mid-fluctuation lines. The emergence of large fluctuations (friction coefficient of 0.56) has been attributed to the severe adhesive contact of the Al-B4C composite and the disk in Fig.12a. This occurs due to the uneven dispersion of B4C particles in the structure of the composite. The number of shallow grooves and the amount of wear loss (mg) owing to the transition of wear from delamination

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to abrasion are confirmations for the large fluctuations in Fig.12a. However, the existence of

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homogeneously dispersed and hard B4C particles in a large surface area of the composite

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(multi-pass FSP’ed) protects the Al matrix during the wear test and the amplitude of fluctuation (coefficient of friction) declines. This happening is attributed to the barrier effect

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(load-bearing capacity) of the B4C particles against sliding.

The beginning of the amplitude fluctuation (in Fig.12) shows that higher force is required to

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overcome adhesive contact of opposite materials at the initial stage of wear test and this occurrence leads to a strong rise in the coefficient of friction [15]. This sudden rise in the

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coefficient of friction is more pronounced in Fig.12c, and 12d due to the increase in the area

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fraction and dispersion level of the fragmented B4C particles in the Al matrix. The B4C particles in the Al matrix resist dislocation movement after the initial contact and penetration

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of the pin to prevent further rise of friction coefficient and confine/stabilize the friction coefficient within the marked boundaries. The overlaps of the fluctuation of friction

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coefficient prevent the palpable assessment of the rise in fluctuation (coefficient of friction) in Fig.12a and this happening may be attributed to the inappropriate or inhomogeneous dispersion of hard B4C particles in the Al matrix. The soft Al matrix is deemed to have offered little resistance to wear at the initial stage of the wear test and, as such, intense and overlapped fluctuations of the coefficient of friction ensue at the start of wear test in Fig.12a. The overshoots of the entire waveform (of friction coefficient) above the mid-line of fluctuation as the sliding distance is increased are observed in Fig.12b (above 800 m) and

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ACCEPTED MANUSCRIPT Fig.12c (600-1000 m). The increase in the sliding distance is adjudged to have increased the frictional heat input during the wear test and consequently caused the observed overshoot or rise of friction coefficient above the mid-line. This happening is a confirmation of a rise in wear loss and rate as the sliding distance is increased. It corroborates the findings of Fig.10. The observed overshoot of fluctuation above the mid-line is not as pronounced in the sample

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subjected to 8 FSP passes (Fig.12d). The fine grain size, fragmentation and uniform

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dispersion of hard B4C particles in the Al matrix may be responsible for the observed

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waveform in Fig.12d. This occurrence justifies the reduced amount of wear loss observed in

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the composite produced with 8 passes.

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passes.

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4. Conclusion

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Fig.12. Coefficient of friction during wear test (a) 1 pass, (b) 2 passes, (c) 4 passes and (d) 8

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The composites fabricated with a 10-cycle accumulative roll bonding were successfully subjected to multi FSP passes. The induced microstructural changes arising from the additional severe plastic deformation of FSP were observed to improve the tribological, and mechanical properties of the composite. The attained findings are summarized as follow: 1. The fragmentation and dispersion level (homogeneity) of B4C particles in the Al matrix improve with the increase in the number of FSP passes. No detrimental reaction exist between the Al matrix and the particles as the number of FSP passes is increased from 1 to 8. 24

ACCEPTED MANUSCRIPT 2. An inverse relationship exists between the number of FSP passes and the average grain size of the composite. Grain refinement from 19.21 to 3.25 µm occurs as the number of FSP passes is increased from 1 to 8. 3. The tensile strength of the composite increases from 86.84 to 173.92 MPa as the number of FSP passes is increased from 1 to 8.

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4. Fracture resilience is improved as the number of FSP passes is increased. Ductile

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fracture is the predominant failure morphology of the composites irrespective of the

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number of FSP passes.

5. Abrasion wear is the dominant wear mechanism of the composite owing to the

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dispersion level of B4C particles in the Al matrix.

6. The increase in the number of FSP passes inhibits the wear losses, reduces the wear

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rate (6.198×10-5 - 1.095 × 10-5 mm3/Nm) and friction coefficient. The upper boundaries and mid-fluctuation lines of friction coefficient decrease from 0.56 to 0.19

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and from 0.31 to 0.11 respectively as the number of FSP passes is increased from 1 to

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8.

7. An overshoot of the complete waveform of friction coefficient above the mid-line of

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fluctuation is caused by the increase in the sliding distance or prolonged surfacesurface contact.

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ACCEPTED MANUSCRIPT Yu Li, Qiu-Lin Li, Dong Li, Wei Liu, Guo-Gang Shu, Fabrication and characterization of stir casting AA6061−31%B4C composite, Trans. Nonferrous Met. Soc.

China

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Appendix

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Fig. 13. Particle distribution in the accumulative roll bonded Al-2%B4C composite prior to

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ACCEPTED MANUSCRIPT Highlights  Multi-pass FSP on accumulative roll bonded Al-2%B4C composites was carried out  Fragmentation and dispersion of B4C particles are enhanced by multi-pass FSP  An inverse relationship exists between the number of FSP passes and average grain size

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 Tensile strength and fracture resilience are improved by multi-pass FSP

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 Wear loss, wear rate and friction coefficient are inhibited by multi-pass FSP

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