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Original Article
Wear and mechanical properties of Al chips and Al chips composites recycled by high-pressure torsion
Mohamed Ibrahim Abd El Aal a,b
a Mechanical Engineering Department, College of Engineering, Wadi Addawaser, Prince Sattam Bin Abdulaziz
University, Saudi Arabia b Mechanical Design & Production Department, Faculty of Engineering, Zagazig University, Egypt
a r t i c l e i n f o
Article history:
Keywords:
composites
a b s t r a c t
Pure Al chip, Al chip-20% Al2O3, and Al chip-20% SiC samples were recycled using high-
pressure torsion (HPT). Influence of the HPT processing on the feasibility of the consoli-
dation, the microstructure evolution, the hardness, and the wear properties of pure Al chip,
Al chip-20% Al2O3, and Al chip-20% SiC samples were investigated and compared with
those of the as-received Al and HPTed Al solid samples. The HPT processing successfully
produces approximately fully dense ultrafine-grained (UFG) microstructure Al and Al
composite samples with relative densities ranged from 99.7 to 98.3%. The HPT processing
of the Al chip and Al chip composites samples effectively refine and fragmented the Al
matrix and reinforcement particles, and so the hardness increases. The increase of the
hardness enhances the wear resistance and frictional properties of the Al after recycling
the Al chip and Al chip composites via HPT processing. The enhancement of the hardness
and so the wear resistance affects the wear mechanism of the different samples. Worn
surface morphology and analysis of the wear samples and WC ball and so support the wear
and friction results.
© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC
BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
aluminum and its alloys are used extensively to produce many
products, from food cans to transportation means and air-
planes. Aluminum is produced from its ores or themelting of Al
scraps [1]. However, the extraction of one ton from Al from its
oresneedsabout ten timesofpower required toproduce1 tonof
steel [2]. The recent world agreement about global warming,
high-power consumption, and CO2 emission limits the process
oo.com, [email protected]
of producingAl extraction from its ores. So, the production ofAl
was directed to depend more on Al scarp recycling [3]. Nowa-
days, 26%ofaluminumworldproduction isobtainedbyAl scrap
recycling, and that amount will increase up to 50% soon [3].
Most of the recycledAl products are producedby traditional
recycling techniques (melting of scrap and then casting into
semi-finish and finished products). Unfortunately, the con-
ventional recycling techniques have different disadvantages
such as high power consumption, losing around 46e48%of the
Al scrap during the casting process [4e6], and emission of
harmful gases (mainly CO2) to humans and the environment
du.eg, [email protected].
an open access article under the CC BY-NC-ND license (http://
due to the burning of the fuel during the scrap re-melting
process [7]. Therefore, the need for a new technique that can
overcome the disadvantages of the conventional recycling
techniques used in the recycling of the Al scrap was the moti-
vation of different previous works [4,5,7e10]. A new noncon-
ventional recycling technique of Al scrap was performed
through cold compaction of the Al scrap or even Al composites
followed by the hot extrusion and so-called solid-state recy-
cling technique to obtain further energy saving [4,5,7e10].
Therefore, Al scrap canbe recycledwith ahighdegreeof power
saving and a low pollution level.
Themetalcuttingprocess isoneof theAlandAlalloysscrap's primary resources, which produces about 5% of the Al and Al
alloys' overall scrap amount in the world [11]. The machining
process produces scrap that so-called the chip that comes from
removing the excess material to produce semi-finish and
finished products. Interestingly, the machining chips have
unique features relative to the other types of scrap types. The
chiphasnanometeror (UFG) sizemicrostructuresbecauseof the
shear strain imposed in the machining process [12e14]. There-
fore, recycling of Al and Al alloys chips using the conventional
(casting)ornonconventional (solid-state) techniquesunderhigh
temperature contributes to the grain growth of the recycled
chips [8]. Therefore, the need for a new recycling technique that
can save the chip nanometer or UFG grain size microstructures
of the chip or even introduce a further refinement of the chip
grain size is still required.
Severe plastic deformation (SPD) processes can be consid-
eredoneof themost effective recyclingmetal chipsmethods [7].
However, the equal channel angular pressing (ECAP) and high-
pressure torsion (HPT) processes still the most popular SPD
methods [15,16] used in the recycling of metal chips. Interest-
ingly, most of the previous works related to the metals chip's recycling process using ECAP have been performed under high
temperatures [17e21]. The Ti, Mg alloy, Al AA6060 alloy, and
pure Al chips were consolidated using ECAP and a combination
of ECAP and extrusion under high temperature up to 590 C [17e21]. Themetal chips' ECAP processing produced fully dense
samples with superior mechanical properties comparable with
the reference material properties [20,21]. Nevertheless, the
metal chips' consolidation under high temperature produced
large grain sizes [17e20].
On the other hand, metal chips recycling using the HPT
process can be considered more effective in conserving the
metal chips' fine-grain microstructure due to the low recycling
temperature. HPT processing of Cu and Al chips was performed
under room temperature (RT) and a temperature that did not
exceed 300 C [14,22e24]. More recently, the HPT was also used
effectively in producing magnesium chip-alumina composite
[25]. Therefore, using HPT processing can be considered a
promising method in recycling metal chips and metal chip
composite because of the significantly high strain of the HPT
process relative to the ECAP [15,16]. Therefore, the high strain
imposed during the metal chips' HPT processing under (RT)
conserved the chip grain size or even contributed to a further
decrease inrecycledchipsamplegrainsize [14,22e24].However,
with the promising results of the chip and chip composites
recycling using the HPT process, there is an apparent lack in
previous works related to this topic. Therefore, a further inves-
tigation about theHPTprocessing efficiency in recycling pureAl
chip and pure Al chip composites with comparable or superior
properties relative to pure Al solid samples is still required.
In contrast to the increase inmost mechanical properties of
thepureAlsolidandpureAlchip-recycledsamplesprocessedby
deteriorateddue to the lackofworkhardeningcapacity after the
SPDprocessing [21,26e28]. However, pureAlwear and frictional
properties were enhanced through the formation of Al com-
posites [29e33]. The pure Al wear and mechanical properties
reinforced by ceramics particles, especially Al2O3 and SiC with
micro andnanoparticle sizes,were improvedobviously [29e33].
Interestingly the previous observation of the decrease of the
wear resistance of Al processed by SPD was overcome through
the formation of the AleSiC composites through accumulative
roll bonding (ARB) [33]. Therefore, the need to form pure Al chip
composites to improve the pure Al mechanical and wear prop-
erties is needed.
The objectives of the current study can be listed as follows:
1. Study the feasibility of using the HPT processing to
consolidate pure Al chip, pure Al chip-Al2O3, and pure Al
chip-SiC composites into fully dense samples.
2. Investigate HPT processing's influence on the microstruc-
ture evolution of pure Al chips, pure Al chip-Al2O3, and
pure chip-SiC samples (the grain size refinement and
ceramic particles fragmentation).
3. Study the HPT processing influence on the hardness of the
recycled Al chips, pure Al chip-Al2O3, and pure Al chip-SiC
samples.
4. Investigate the effect of the HPT processing and the for-
mation of pure Al chip-Al2O3 and pure Al chip-SiC com-
posites on the pure Al's wear and friction properties.
2. Experimental work
A pure aluminum 1080A provided by ALU Misr Company [34]
was used in the current study. The 1080A samples were dry
turned (to avoid any contamination of the produced chip if
lubrication used) by the center lathe machine, as shown in
Fig. 1(a). The turning process performed using a single-point
cutting tool with a rake, clearance, tool angles, and tool nose
radius of 40, 6, 44, 0.9 mm, respectively. The turning pro-
cess was performed under a speed, feed, and depth of cut of
120 m/min, 0.21 mm/rev, and 5 mm. The cutting conditions
used in the current study were selected based on those rec-
ommended by ASM stander [35]. The turning process pro-
duced a continuous chip with a length ranges from 2 to
35 mm, as indicated in Fig. 1(b). The turning process with the
selected cutting tool geometry occurs under a shear angle of
13 according to the following equation (1) [36].
tan∅ ¼ r cos a
1 r sin a
(1)
where a and r are the rake angle, and the chip thickness ratio,
the value of r can be obtained from equation (2) [36].
r¼ t0 tc
j o u r n a l o f m a t e r i a l s r e s e a r c h and t e c hno l o g y 2 0 2 1 ; 1 4 : 4 0 7e4 2 7 409
where t0 and tc are the chip thicknesses before and after the
cutting process, respectively. In the current study, the values
of chip thicknesses before and after the cutting process were
0.3 and 1.2 mm, respectively. So, the chip thickness ratio r
value in the present work is 0.25. Therefore, based on the
values of different variables of equation (3) [36], the turning
process induces a shear strain of 3.83.
g¼ tanð∅ aÞ þ cot ∅ (3)
Themachined continuous chipwas cleanedbyacetone inan
ultrasonic bath for 1 h. The chipwas then comminuted into fine
particles with an average size of 85 mmand particle size ranging
from 30.5 to 212.2 mm, as shown in Fig. 1(c) and (d). The
comminuted chip was thenmixedwith flake-shaped Al2O3 and
SiC powders with the average particle sizes of 41.4 and 25.5 mm,
as shown in Fig. 1(e) and (f), respectively. Themixtures of the Al
chip-Al2O3 and Al chip-SiC were further mixed using a roller
mixer for 10 h to obtain Al chip-20% Al2O3 and Al chip-20% SiC
Fig. 1 e Schematic diagram of the HPT processing of Al-ch
(Volume% of Al2O3 and SiC) composites powders, as shown in
Fig. 1(g) and (h). TheAl chip-20%Al2O3 composition andAl chip-
20% SiC composites powders were proved by the X-ray spec-
troscopy (EDS) analysis, as shown in Fig. 1(g) and (h).
The comminuted chip and different composites mixtures
were compacted at (RT) under a pressure of 500 MPa into
samples with a 10mmdiameter and a thickness of 2.5 mm, as
shown in Fig. 1(i). The compacted pure Al chip, chip com-
posite, and pure solid Al alloy samples were carefully ground.
Then the compacted pure Al chip, chip composite, and pure
solid Al samples were HPTed for 20 revolutions under an
applied pressure of 9 GPa, and a speed of 1 rpm at (RT) using
HPT die with 10 mm diameter [14,16], as shown in Fig. 1(j) and
(k).
The as-received Al, HPTed Al chip, and HPTed chip com-
posites sampleswere ground andpolished up to shiny surfaces.
Theexperimental and relativedensities of thedifferent samples
weremeasured.Moreover, themirror-likesurfacesampleswere
further polishedusingmixturesof colloidal silica andethanol to
investigate the different samples' microstructure. The micro-
structureobservationswerecarriedoutusinganFE-SEMwithan
misorientation ineach sample.On theotherhand, the tracingof
the Al2O3 and SiC particles distribution and fragmentation after
the HPT processing performed by field emission scanning elec-
tron microscope (FE-SEM; model JEOL JSM-6330F, JEOL, Japan).
Therefore, the HPTed Al chip-20% Al2O3 and Al chip-20% SiC
samples were further etched using Keller's reagent. The Vickers microhardness of the different samples was
measured using a Mitutoyo microhardness tester under an
applied load of 100 gf and a dwell time of 15 s. The microhard-
nessmeasurementswerecarriedoutalongthesamplediameter
with a spacing of 0.5 mm between each two measurement
points. The microhardness measurements were obtained
through 5 different diameters, and the average value of the five
microhardness measurements in the same position was used
and plotted. The standard deviation of the microhardness
Fig. 2 e (a) Density and relative density of the different sample
compacted (b) Al chip, (c) Al chip-20% Al2O3, and (d) Al chip-20%
chipechip particles or between the chip and Al2O3 or SiC partic
measurements was calculated to assess the deformation in-
homogeneity index. A ball-on-flat surface reciprocating wear
test of the solid Al, recycled Al chip, and recycled Al chip com-
posites samples was conducted at RT under sliding distance
distances of 315e1260 m, loads of 2.5e20 N, and a stroke with a
length of 7 mm. The effects of the formation of Al chip com-
posites, the load, and distance on the wear rate and friction
behavior were investigated. The surface morphologies and
analysisof thewearsamplesand theWCballwere studiedusing
FE-SEM. The area X-ray spectroscopy (EDS) was used in the
analysis of the reinforcement powders, Al as-received, Al chips,
and worn surfaces samples throughout the current study.
3. Results and discussion
Fig. 2(a) shows the experimental and relative densities of as-
received Al, HPTed Al solid, HPTed chip Al, HPTed Al chip-
s and the optical microscope micrographs of the cold
SiC samples (the white arrows indicate, the pores between
20% Al2O3, and HPTed Al chip-20% SiC samples. The Al as-
received and HPTed Al solid samples experimental and rela-
tive densities are equal with relative densities of 100%, as in-
dicates in Fig. 2(a). On the other hand, the HPTed Al chip
sample has a relative density of 99.7%, according to equation
(4). Considering the theoretical and experimental densities of
HPTed Al chip sample are 2.7 and 2.68 g/cm3.
rrel ¼ rexp
100 (4)
where rrel, rexp, and rtheo are the values of the sample's rela-
tive, experimental, and theoretical densities. The HPT Al chip
sample's relative density indicated that HPT processing
effectively consolidated Al chip into approximately fully
dense samples with void content, not more than 0.3%. The
HPTed chip Al sample's relative density was near or even
higher than those of Al powder consolidated by HPT of
99.5e99.99% under a pressure of 1.5e6 GPa for 6e50 revolu-
tions [37e40]. Although different, other solid-state recycling
methods can provide approximately fully dense Al, and Al
alloys recycled samples using cold compaction followed by
sintering, hot extrusion, and ECAP [9,20,21]. Nevertheless, it
Fig. 3 e Color-coded orientation map images, color-coded grain
angle < 15 red line) and EDS analysis of (aec) as-received Al an
must consider that recycling of the AlMg2, AA6060, and
commercially pure Al samples was performed under high
temperature ranged from 200 to 550 C [9,20,21]. Therefore, the
HPT of the Al pure chip effectively produces fully dense
samples without any needing for heating.
The HPTed Al chip sample density increased after the
addition of the Al2O3 and SiC particles. The HPTed Al chip
sample experimental density increased from 2.68 up to 2.9
and 2.77 g/cm3 in the case of the HPTed Al chip-20% Al2O3 and
HPTed Al chip-20% SiC samples. On the other hand, the Al
chip-20% Al2O3 and HPTed Al chip-20% SiC samples' theoretical densities were calculated depending on the rule of the
mixture (5) [41].
(5)
where rmatrix, rreinforcement, rmatrix, Vreinforcement, are the den-
sity and volume fractions of the Al matrix and the Al2O3 or SiC
reinforcement.
Considering the Al matrix, Al2O3 and SiC densities are 2.7,
3.95, and 3.21 g/cm3, respectively. Moreover, the Al matrix,
Al2O3, and SiC volume fractions are 80, 20, and 20%,
boundaries map images (high angle ≥ 15 blue line and low
d (def) Al chip samples.
respectively. Depending on the values of the theoretical den-
sities of the Al chip-20% Al2O3 and HPTed Al chip-20% SiC
samples of 2.95 and 2.80 g/cm3. It can be noted that the
theoretical densities of the Al chip-20% Al2O3 and HPTed Al
chip-20% SiC samples are 98.3 and 98.7%, as shown in Fig. 2(a).
The relative density of the HPTed Al chip-20% Al2O3 and
HPTed Al chip-20% SiC samples in the present work of 98.3 and
98.7% were near, or even higher than those of HPTed Al
powder-carbon nanotubes (CNT) and Al powder-10, 20 and 30%
nanoAl2O3 composites of 96e98.4% [37e40]. Therefore, the HPT
recycling of chip composite can be useful in producing
approximately fully dense samples. Through comparing the
present results with the relative density of HPTed Al-nano
Al2O3 composites with volume fractions from 10 to 30% and
Fig. 4 e Color-coded orientation map images and color-coded gr
low angle < 15 red line) of (aeb) HPTed Al as-received, (ced) HPT
Al chip-20% SiC samples.
Al2O3 particle size of 30 nm. It is observed that the chip recycled
composite samples reinforced with micro-size particles can be
more effective in reducing the agglomeration of the reinforce-
ment particles [38e40] that contribute to improving the sample
density. Interestingly this observation was also noted by
comparing the relative density of the HPTed AlSiCu-5% SiC, Ti-
18% Al2O3, and the density of Al-nano Al2O3 composites
[38e40,42,43]. The relative density of HPTed AlSiCu-5% SiC, Ti-
18% Al2O3 reinforced with an initial particle size of SiC and
Al2O3 of 53 and 1 mm of 99.4 and 100% [42,43] were also higher
than those of the HPTed Al-nano Al2O3 composites [38e40].
The HPT processing's effectiveness in recycling Al chip and
Al chip composites into approximately fully dense samples
was also confirmed through the cold compacted samples'
ain boundaries map images (high angle ≥ 15 blue line and
ed Al chip, (eef) HPTed Al chip-20% Al2O3, and (geh) HPTed
HPTed Al chip-20% Al2O3 and (ced) HPTed Al chip-20% SiC samples.
microstructure observation. The cold compacted Al chip, Al
chip-20% Al2O3, and Al chip-20% SiC samples microstructure
are shown in Fig. 2(bed). The cold compacted Al chip and Al
chip composites' microstructure indicate large-sized pores
between the chip particles or between chip particles and the
reinforcement particles, as indicated by the arrows in
Fig. 2(bed). Therefore, the cold compaction under 500 MPa is
not sufficient to obtain fully dense samples. This observation
was consistentwith that previously observed of Al scrap's cold compaction with different sizes from 0.5 to 2 mm using a
pressure of 360 MPa, which produces samples with a relative
density of 90% [5]. Therefore, improving the density of the
recycled Al chips and powders or their composites was ob-
tained by applying sintering and sintering followed by hot
extrusion under temperatures up to 650 C [8e10,29e32]. So
HPT recycling of the Al chip and Al chip composite at (RT)
without heating is an effective and promising solid-state
recycling to produce fully dense recycled samples.
3.2. Microstructure evolution
The EBSD color-coded orientation and grain boundaries maps
images (high angle 15 and low angle 15 shown by blue
line and red colors) of as-received Al are shown in Fig. 3(a) and
(b). The as-received Al sample has equiaxed grains with an
average grain size of 414 mm and high angle grain boundaries.
Moreover, (EDS) analysis shown in Fig. 3(c) did not indicate the
presence of any oxygen.
The grains of the as-received Al sample were refined after
the turning process, as shown in Fig. 3(d). The chip sample has
an elongated grain with a grain size ranging from 0.13 to
3.9 mm and an average grain size of 0.97 mm with approxi-
mately 70% of high angle grain boundaries (HAGBs), as shown
in Fig. 3(d) and (e). The Al chip grain size decrease after the
turning process is due to the strain imposed during the
machining. The imposed strain during the turning process of
2.2 is calculated based on the shear strain obtained from
equation 3 of 3.8 [36]. The imposed strain on the turned Al
sample is equal to that imposed through 2 passes of ECAP,
according to Y. Iwahashi's equation [44]. Similar results of the
formation of UFG microstructures were noted after the
machining of different aluminum alloys [11e14]. Reverse to
that observed in the as-received Al sample oxygen, with a
percentage of 2.8%, was found in the chip sample's EDS
analysis, as shown in Fig. 3(f). The presence of oxygen in the
chip sample can be explained by the oxidation that occurred
during the machining process due to the high temperature
generating in the turning process.
The microstructure of HPTed Al solid, Al chip, Al chip-20%
Al2O3, and Al chip-20% SiC composites are shown in Fig. 4. The
HPTed Al solid, Al chip, Al chip-20% Al2O3, and Al chip-20% SiC
samples consist of approximately equiaxed grain. Therefore the
HPT processing of the Al chip, Al chip-20% Al2O3, and HPTed Al
chip-20% SiC samples effectively transformed the microstruc-
ture of the initial chipwith elongated grains into equiaxed grain
microstructure. The Al solid as-received sample average grain
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 0
20
40
60
80
100
120
140
160
180
200
220
Al as-received HPTed Al as-received HPTed Al chip HPTed Al chip-20%Al
2 O
Fig. 6 e (a) Microhardness distribution across the samples
diameter and (b) average microhardness and
microhardness inhomogeneity index of the different
samples.
Fig. 7 e SEM photomicrographs of the wear scar of (a) Al as-
received, (b) HPTed Al as-received, (c) HPTed Al chip, (d)
HPTed Al chip-20% Al2O3, and (e) HPTed Al chip-20% SiC
samples tested under a sliding distance of 1260 m and an
applied load of 20 N, and (f) cross-sectional profiles of the
wear scars of the samples.
j o u r n a l o f ma t e r i a l s r e s e a r c h a nd t e c h no l o g y 2 0 2 1 ; 1 4 : 4 0 7e4 2 7414
size of 414 mm decreased down to 0.61 mm with a grain size
ranging from 0.05 to 1.25 mm after HPT processing, as shown in
Fig. 4(a). Moreover, the HPTed Al solid sample has a percent of
79.5%of (HAGBs) and an average grainboundarymisorientation
angleof32.57, asshowninFig. 4(b).Themicrostructure features
and grain size of theHPTed Al solid samplewere close to that of
solid Al-1080, Al 99.7, and Al-1050 HPTed samples [45e47].
The HPTed chip sample has an average grain size of 0.54 mm
with a grain size ranging from 0.07 to 2.39 mm, as shown in
Fig. 4(c). The HPTed Al chip sample has a percent of 80.9% of
HAGBs and an average grain boundary misorientation angle of
33.42, as shown in Fig. 4(d). The HPTed chip samples grain size
was smaller than those of different HPTed pure solid Al sam-
ples in the present and previous works [45,46]. Moreover, the
grain size of the HPTed Al chip was very near that of 0.5 mm of
the HPTed Al powder disc shape samples [37,40]. However, the
combination of ball milling and HPT processing of Al powered
is capable of producing an Al sample with a grain size of
0.16 mm [39].
Through the microstructure observations of the HPTed
processed solid, powder, and chip pure Al samples in the cur-
rent and previous works [37e40,46,47]. It can be noted that the
HPT of Al powders and chips can revile microstructures with
smaller grain sizes and HAGBs than the HPTed solid samples.
The smaller grain size with a high percentage of HAGBs in the
case of the HPTed Al powder and chip is due to the presence of
the oxide layer in the surface of the processed Al powders and
chips, as indicated by the EDS analysis shown in Fig. 3(f) and
previously noted [37]. The oxide particles in the form of
alumina particles are good sites that can hamper the disloca-
tion motion, and so the dislocations are accumulated in those
sites. The dislocation accumulation then contributed to the
formation of subgrain boundaries. Furthermore, due to thehigh
strain imposed in the HPT process, enormous numbers of dis-
locations are generated, and so the mutual interaction of dis-
locations becomes enhanced, and the subgrain boundaries
further evolve into grain boundaries with HAGBs.
The HPTed Al chip-20% Al2O3 and Al chip-20% SiC com-
posites microstructures are shown in Fig. 4(eeh). The micro-
structure of the Al matrix of both composites was amixture of
micro, UFG, and nano grain sizes as that observed in the
HPTed Al solid and chip samples. The HPTed Al chip-20%
Al2O3 and Al chip-20% SiC composites samples have average
grain sizes of 0.44 and 0.24 mm respectively. The trimodal
(c) 1260 m.
microstructure samples' formation with a combination of
different grain sizes conserving a high ductility degree with
reasonable high strength (hardness), as noted previously in
HPT processing AA5083eB4C and CueSiC composites [48,49].
The formation of trimodal microstructure occurred due to the
dislocation density's difference through the deformed mate-
rial combined with the non-homogenized deformation during
HPT processing. The good bonding between the Al matrix and
both the Al2O3 and SiC after the HPT processing contributes to
hampering the dislocation's motion. Therefore, localized
deformation and dislocation activities occur, and the grain
size is refined to the nano or UFG sizes in regions of high
dislocations density and remains with coarse sizes in the re-
gions of low dislocation activities.
Interestingly, the Al matrix average grain sizes and grain
size ranges of the HPTed Al chip-20% Al2O3 and Al chip-20%
SiC samples were smaller than those of the HPTed Al solid
and chip samples. The smaller Al matrix average grain sizes
and grain size range of the composites are due to the presence
of Al2O3, SiC, and oxide (Al2O3 noted in the case of the chip)
particles. Moreover, the presence of such hard particles in-
creases the number of sites, those obstacles the dislocation
motion. So the dislocations accumulated in those sites
become more apparent. Therefore, the dislocation accumu-
lation becomes higher, so the formation of subgrain bound-
aries that evolved into grain boundaries with HAGBs becomes
faster than that in the case of the solid Al and Al chip samples.
The HPTed chip-20% Al2O3 and Al chip-20% SiC composites
samples have a percentage of HAGBs of 81.7 and 82.9% with
average grain boundaries misorientation angles of 34.22 and
35.1, respectively, as shown in Fig. 4(f) and (h). Therefore, the
percentage of HAGBs and average grain boundaries misori-
entation angles of the HPTed Al chip composites were higher
than those of the HPTed Al and Al chip samples. Those ob-
servations of the higher percentage of HAGBs and average
grain boundaries misorientation angles of the HPTed Al chip
composites confirmers the effect of the addition of ceramic
particles and their fragmentation on the evolving grain
boundaries into HAGBs.
Thesmallergrainsizeof theHPTof thecompositesrelative to
those of HPTed solid or powder metal samples was also previ-
ously noted [37e40,42,43,47,49,50]. The Al matrix average grain
size of the HPTed AleCNT and AleAl2O3 composites of 0.1 and
0.16e0.48mmweresmaller than thoseof0.5and1.2mmofHPTed
size of the HPTed Cue20% SiC, TieAl2O3, and Ale15%Sie2.5%
Cue0.5%Mge5%SiC composites of 0.3, 0.1 and 0.06e0.07 mm
were smaller than those of the HPTed Cu, Ti, and Ale15%
Sie2.5%Cue0.5%Mg solid and powder samples of 0.95, 0.15, and
0.1e0.3 mm [34,35,48,49]. Therefore, the previous results of
obtaining smaller matrix grain size in the HPTed composites
[37e40,42,43,47,49,50] supports the present work results.
Although the Al2O3 and SiC particles in the cold compacted
sample are still not fragmented, as shown in Fig. 2(c) and (d).
However, Al2O3 and SiC particles were fragmented and
redistributed after the HPT processing of the Al chip
Fig. 9 e Variation of COF under different applied loads and sliding distances of 315 m of (a) Al as-received, (b) HPTed Al as-
received, (c) HPTed Al chip, (d) HPTed Al chip-20% Al2O3, and (e) HPTed Al chip-20% SiC samples.
composites, as shown in Fig. 5. The particle size and ranges of
the Al2O3 and SiC were decreased to 3.2e0.15 and
2.97e0.19 mm. Moreover, the Al2O3 and SiC reinforcements
particle sizes decreased to 0.92 and 0.47 mm, as shown in Fig. 5.
The fragmentation and redistribution of Al2O3 and SiC parti-
cles during the HPT processing occurs as follows. First, the
high-applied pressure and strain of HPT processing fragment
the Al2O3 and SiC particles. Consequently, the rotation during
the torsion process redistributes the fragmented Al2O3 and SiC
particles in the Al matrix.
The lowfracture toughnessof theAl2O3 andSiCparticles' can alsoexplain their fragmentationduring theHPTprocessing.The
applied pressure and strain used in the present work are suffi-
cient or even higher than the pressure and strain values used in
the fragmentation of the Al2O3 and SiC during the HPT of
different metal matrix composites, as previously noted
[42,43,49]. The HPT processing in the present work under a
pressure of 9 GPa, which is higher than the critical pressure
value observed in the previous studies [42,43,49,51e54] of 5 GPa
that needed to the fragmentation of the hard reinforcement
particles to ultrafine and nano size particles, as shown in Fig. 5.
Generally, theHPTof the composites reinforcedwithmicro-size
particles can effectively fragment the reinforcement particles
down toUFGparticle size under pressure equal to 5 GPa ormore
[42,43,49,51e54]. Therefore, theHPTpressure and thenumberof
revolutions can be considered the critical factors determining
thedegreeof fragmentationof thehard reinforcementparticles,
Through the comparison between the previous works
[42,43,49,51e54], it can be noted that the pressure has a more
noticeable influence on the reinforcement particles fragmen-
tation. The HPT using pressure with a value of 5 GPa or less can
fragment the Al2O3 and SiC particles but to a limited degree, as
the particle size still in the micro size [43,51,52]. However, HPT
under pressure higher than 5 GPa has successfully fragmented
the Al2O3 SiC, Cr, and W particles to UFG and nanoparticle
particle size [42,9,53,54]. So the HPT processing of Al chip-20%
Al2O3 and Al chip-20% SiC composites in the present study
can be considered efficient in producing UFG Al composites.
The SEM photomicrographs indicate the HPT processing's effectiveness in improving the reinforcement particles distri-
bution with the chip composite's consolidation without
Fig. 10 e Variation of COF under different applied loads and sliding distances of 1260 m of (a) Al as-received, (b) HPTed Al as-
received, (c) HPTed Al chip, (d) HPTed Al chip-20% Al2O3, and (e) HPTed Al chip-20% SiC samples.
apparent content of the voids, as shown in Fig. 5. The Al2O3
and SiC particles have a homogeneous distribution, as shown
in Fig. 5(b) and (d). The HPT processing's effectiveness in
improving the reinforcement particles distribution in the Al
chip composite samples can also confirm by comparing the
opticalmicroscopemicrographs of the cold compacted Al chip
composites samples shown in Fig. 2(c) and (d). The optical
microscope micrographs of the cold compacted Al chip com-
posites indicated the large voids with a high degree of the
Al2O3 and SiC particles agglomeration. Therefore, the HPT
processing can effectively produce approximately fully dense
with a homogenized distribution of the reinforcement Al chip-
20% Al2O3 and Al chip-20% SiC composites samples.
3.3. Microhardness results
eter of the different samples'. The hardness distribution of the
as-received sample was constant around its average hardness
value of 27.6 Hv, as shown in Fig. 6(a). The Al as-received
sample inhomogeneity index was 0.2 that confirms the
constant distribution of the sample's hardness in the form of a
straight line, as shown in Fig. 6(b). Then hardness distribution
becomes nonhomogeneous after the HPT processing, as
hardness increased from the sample center to the edge.
Hardness values increased from 61.5, 96, 151, and 175 Hv in
the sample center to 71, 110, 179.1, and 203.8 Hv at the edge of
the HPTed Al solid, Al chip Al chip-20% Al2O3, and Al chip-20%
SiC samples, respectively, as shown in Fig. 6(a). The hardness
distribution pattern of the HPTed sample results in a differ-
ence between the hardness values in the sample center and its
edge of 9.5, 14, 28.1, and 28.8 Hv in the HPTed Al solid, Al chip
Al chip-20% Al2O3, and Al chip-20% SiC samples, respectively.
This kind of difference between hardness values produces a
low hardness area in the sample center with a diameter of 0.5,
1, and 2 mm in the HPTed Al solid, Al chip, Al chip-20% Al2O3,
and Al chip-20% SiC samples, respectively. The hardness of
theHPTed samples is due to the increase of the imposed strain
from the center to the sample's outer surface [17].
Similar observations of the hardness distribution were also
observed after the HPT processing of solid, chip, powder, and
composites samples of different metals
and (c) 1260 m.
[14,25,37e40,42,45e47,49]. Interestingly similar to that noted
in the current study, the HPT processed solid samples have a
smaller low hardness area in the sample center
than that noted in the HPTed composites samples
[25,37e40,42,43,49,50]. This observation is due to the hard
reinforcement particles that hinder the deformation process
with the lower imposed strain in the sample center relative to
that at its edge. This observation can be proved by assessing
the deformation inhomogeneity index in the form of the
stander deviation of the hardness values in each case. The
hardness inhomogeneity index was increased from 2.2 in the
HPTedAl solid samples to 3.1, 9.5, and 9.6 in theHPTedAl chip,
Al chip-20% Al2O3, and Al chip-20% SiC samples, respectively,
as shown in Fig. 6(b). Therefore, the presence of the Al2O3 and
the SiC particles decreases the deformation homogeneity of
the samples. However, it can be noted that the composite
samples in the present work have smaller low hardness areas
in the sample centers relative to those noted in the previous
works [25,37e40]. The smaller low hardness area noted in the
current study is due to the high applied pressure of 9 GPa for
20 revolutions used. Applying the current conditions con-
tributes to reaching the grain refinement and particle frag-
mentation with smaller sizes than observed previously, and
so a higher degree of hardness distribution homogeneity is
acquired.
The values of the average hardness are shown in Fig. 6(b).
The Al solid sample's average hardness was increased from
27.6 to 67.8 Hv after the HPT. The hardness of the HPTed solid
Al sample in the current study was very closed to those of the
commercially pure Al 1080, Al 1070, andAl 1050 of 62.5, 61, and
65 Hv under the number of revolutions of 8e5 and applied
pressure of 1e8 GPa [45e47]. The increase of the hardness in
solid Al sample after the HPT processing is due to the grain
refinement, as shown in the microstructure section through
Figs. 3 and 4. According to the HallePetche relation shown in
equation (6) [55,56], the hardness of a material H is generally
related to the grain size.
H ¼ H0 þ KH d 1 2 (6)
where d is the grain size, and H0 and KH are constants.
Moreover, the increase of the dislocation density also
contribute to the increase of hardness H and strength s after
HPT processing according to the Taylor equation (7) [57,58].
s ¼ s0 þ aMGbr 1 2 (7)
where a is a constant, G is the shearmodulus, b is the length of
the Burgers vector of dislocation, M is the Taylor factor, and r
is a dislocation density.
The HPTed Al chip,Al chip-20% Al2O3, and Al chip-20% SiC
samples average hardness were higher by 55, 149.1, and
187.2% of the HPTed Al solid, respectively. The higher hard-
ness of the HPTedAl chip sample relative to the HPTed Al solid
sample is due to the presence of the Al2O3 particles, as shown
in Fig. 3. Interestingly, the addition of the Al2O3 or SiC particles
contributes to a further increase in the hardness of the HPTed
Al chip-20% Al2O3, and Al chip-20% SiC composites over that
of the HPTed Al solid and chip samples. The higher hardness
of the Al chip-20% Al2O3 and Al chip-20% SiC samples over
that of the HPTed Al solid sample is due to the refinement and
Fig. 12 e SEM photograph of the worn surface morphology of (a) Al as-received, (b) HPTed Al as-received, (c) HPTed Al chip,
(d) HPTed Al chip-20% Al2O3, and (e) HPTed Al chip-20% SiC samples corresponding to a sliding distance of 315 m and an
applied load of 2.5 N.
fragmentation of the Al, Al2O3, and SiC grains and particles,
the high dislocation density, and the contribution of the fine
Al2O3 and SiC particles in the obstruction of dislocation
motion.
3.4. Wear properties
3.4.1. Wear scars and rate results The SEMmicrographs and profiles of the wear scars are shown
in Fig. 7. The Al HPTed sample scar width and depth were
smaller by only 5.5e1.4% than those of the Al as-received sam-
ple. Although the Al sample hardness increased after the HPT
processingdue to thegrainrefinement, asshown inFigs. 4and6,
the HPTed Al wear resistance did not improve. The present re-
sults confirm the previous one that the SPD of the pure Al
decrease or even did not improves its wear and friction prop-
erties due to the lack of work hardening [21,26e28].
In the HPTed chip sample, the wear scar width and depth
were smaller by 29e18 and 33e19.2% than that of the Al as-
received and HPTed samples. Therefore, the high hardness
of the recycled Al chip sample improves the Al wear resis-
tance. The presence of the oxide particles and the grain
refinement of the chip sample during the machining and HPT
processing enhance the HPTed chip sample's wear resistance
over that of the Al as-received and HPTed.
The influence of hardness increase on the wear scar width
and depth becomes clearer in the case of the HPTed Al chip
composite samples, as shown in Fig. 7(d), (e), and (f). The wear
scar width and depth of the HPTed Al chip composites sam-
ples were smaller by 68.4e51% and 75e62.7% than that of Al
as-received and HPTed Al chip samples. Therefore, the HPT
processing of the Al chip composites increases the hardness
that decreases the wear scar width and depth. The influence
of hardness on the wear resistance of Al composites samples
applied load of 20 N.
was also observed through the comparison between the
HPTed Al chip-Al2O3 and SiC composites wear scars. As the
wear scarwidth and depth of theHPTedAl chip-SiC composite
sample were smaller than those of the Al chip-Al2O3 com-
posite sample by 23.7 and 13.6%.
The effect of hardness increased due to the formation of the
metal composites, and the HPT processing of metal and metal
composites on the wear scars width and depth were also noted
previously [29,32,40,62e64]. The wear scar width decrease by
24e40% after ball milling and sintering of the 99.5% pure Al
powder reinforcedwith 1.5%and5%powderwax andAIN those
increase hardness of the 99.5% pure Al powder from 27 to 95 Hv
and then to 120 Hv [29]. Moreover, the wear scars size of
AleAl2O3 samples with different volume fractions of Al2O3 was
decreasedobviously [32].However, theHPTprocessingofCuand
Al composites and alloys was more effective in increasing the
wear resistance. The HPT processing of Cu and Al powders and
alloys decreases thewear scarwidth anddepth of Cupowder by
33.3e75% and 57.7e68.8%, respectively [40,59e61]. Moreover,
HPT processing of the Cu powder reinforced with SiC particles
decreases the wear scar width and depth of Cu powder by
63.9e83.3% [59,60]. Therefore, the HPT processing of the Al chip
can consider effective in increase the pure Al wear resistance
with further increasedafter theformationofAlchipcomposites.
Further investigation of the effect of the recycling of the Al
chip and Al chip composites on the wear resistance of the Al
was obtained through the determination of thewear rate under
different loads and distances, as shown in C 8. The wear rate
increased by 31e70, 31e71, 27e59, 25e54, and 19e45%, with the
increasing the applied load under different sliding distances of
as-received Al, HPTed Al, HPTed Al chip, HPTed Al chip-20%
AleAl2O3, and HPTed Al chip-20% SiC samples, respectively.
Moreover, the wear rate increases by 14e31, 14e33, 12e29,
10e17, and 6e14%, with increasing the sliding distance under
different applied loads in the case of as-received Al, HPTed Al,
HPTed Al chip, HPTed Al chip-20% AleAl2O3, and HPTed Al
chip-20% SiC HPTed samples, respectively. The increase of the
wear rate after increasing the sliding distance and applied load
applied load of 20 N.
is due to the severe wear conditions that occurred, as previ-
ously noted [29e33,40,59e62].
The HPT processing of the as-received Al samples did not
improve the wear resistance, as the wear rate of the HPTed Al
samples was lower only by 1e4% than that of the as-received
Al sample due to the lack of work hardening capacity of Al
after the HPT processing [21,26e28]. On the other hand, the
HPT recycling of the Al chip, Al chip-20% Al2O3, and Al chip-
20% SiC samples decreased the wear rate by 10e40, 84e91,
and 87e95% under different sliding distances and loads after
the recycling of the Al chip, chip-20% Al2O3, and Al chip-20%
SiC samples, respectively. Therefore, the increase of the
HPTed recycled chip and chip composites samples' wear
resistance is due to the hardness increase that contributes to
the decrease of the wear volume removed, according to
Archard's law [63].
(8)
Here V, N, L, K, and Hv are the volumetric wear loss, the
applied load, the total sliding distance, the wear coefficient,
and the wear surface's hardness. The present results are
congruent with those previously noted of different pure
aluminum metal matrix composites. The wear rate of the Al
99.5, Al 1100, Al 99.97 samples reinforced with AIN, Al4C3,
Al2O3, and SiC particles with different particle sizes under
sliding distance from 100 to 400 m and applied load up to 10 N
decreased by 54.2e65.2, 46.1e56.9, 43.6e45.9, and 66% [29e32].
Moreover, the SPD of Al 1050-2% SiC samples using cumula-
tive roll bonding contributes to the grain refinement of the Al
grains with the fragmentation of the SiC particles improves
the wear resistance of the pure Al [33]. The wear resistance of
pure aluminum reinforcedwith 2% SiC particles [33] increased
by 50e100% relative to the wear resistance of pure aluminum
[26,27] after the cumulative roll bonding processing. There-
fore, reinforcement and SPD of the pure aluminum in the
present and previous work [33] effective in improving the
Fig. 15 e SEM photograph of the worn surface morphology of the WC ball after the sliding against (a) Al as-received, (b)
HPTed Al as-received, (c) HPTed Al chip, (d) HPTed Al chip-20% Al2O3, and (e) HPTed Al chip-20% SiC samples corresponding
to a sliding distance of 1260 m and an applied load of 20 N.
wear resistance and overcome the effect of the decrease of the
pure aluminum wear resistance due to the lack of work
hardening capacity after the SPD processing [21,26e28].
The hardness of the HPTed Al chip-20% Al2O3 and Al chip-
20% SiC samples was higher by 75e100% over the hardness of
differentialpureAlcomposites [29e33].Thissuperiorityof theAl
relativelyhigherwear resistancerelativetothatof themicrosize
or nano Al composites processed by casting and powder met-
allurgywith andwithout and ballmilling and even SPD through
cumulative roll bonding [29e33].Thesuperiorwear resistanceof
theHPTedAlchip-20%Al2O3andAlchip-20%SiCover thatof the
different Al composites is in agreement with the results of the
HPTedCueSiCcomposites relative to thatofas-castandpowder
3.4.2. Friction properties Thebehaviorof thecoefficientof friction (COF)with thedistance
for the different loads is shown in Figs. 9 and 10. The COF of all
samples increased from zero to its maximum value in the first
50m of the sliding distance. Then the COF oscillates around the
COF maximum value in each case. However, the COF behavior
andvariation rangewerevaried fromonesample toanotherdue
to the effect of the material properties (hardness), load, and
distance, as shown in Figs. 9 and 10. AS, theCOF variation range
of the as-received and HPTed Al samples decreased from
1.46e0.7 to 0.6e0.51, 0.6e0.48, and 0.45e0.35 after the HPT pro-
cessing of Al chip, Al chip-20% Al2O3, and Al chip-20% SiC
samples, respectively.Therefore, the increaseof the recyclingAl
chip and Al chip composites hardness decreases the COF oscil-
lation range. The reduction in the COF oscillation range from
0.65 to 0.1 also noted after the Al hardness increase after the
cumulative roll bonding of AleSiC composites [33]. The influ-
ence of hardness increase on the decrease of theCOFoscillation
range was also observed after the HPT of Cu, CueSiC compos-
ites, Al 6061, and Ale7% Si samples [59e62].
The variation of the average OF values under different
sliding distances and loadswas also traced, as shown in Fig. 11.
The COF average values decreased by 5e32 and 3e33% with
increasing the load under sliding distances of 315 and 1260 m
for the different samples. The decreases of average COF can
attribute to Newton's relation that indicates an inverse rela-
tionship between COF and the load as noted previously
[59e61,64]. Reverse to load effect on the COF, the increase of the
sliding distance increases the average values of the COF, as
shown in Fig. 11, due to the damage of the samples andWCball
surfaces during the wear test. The increase of the COF with
increasing the sliding distancewas also previously observed for
different metals and metal composites [59e61,64].
The HPT recycling process of the Al chip, Al chip-20% Al2O3,
and Al chip-20% SiC samples, contributes to the decrease of the
average COF of Al by 10e25, 34e51, and 40e53% respectively
under different conditions. As the hardness increase that re-
sults from the HPT recycling, changes the wear mechanism
from adhesive wear into abrasive one that reduces the sticking
of Al with the WC ball and reduces COF as noted in the surface
morphology part and noted previously [29,33,59e61,64]. The
increase of the hardness that combined the formation of Al, Cu
composites and further SPD processing of Al and Cu samples
and composites samples contributes to the decrease of the COF
by 14.5, 71, 66% after the formation and SPD processing of
AleAIN, AleSiC, and CueSiC samples [29,33,59,60]. The for-
mation of Al and Cu composites reinforcedwith Al2O3, SiC, and
AIN increases the hardness that consequently decreases the
degree of the delamination and plastic deformation wear
mechanisms and so decreases the COF values [29,33,59,60].
Interestingly the Al chip-20% Al2O3 and Al chip-20% SiC sam-
ple's wear degree was much lighter than that of the Al com-
posites processed by the conventional method such as powder
metallurgy and casting [29,31e33].
It can be noted that the HPT processing of as-received Al
does not affect the COF. The COF of the as-received Al and
HPTed Al sampleswere very close to each other, as noted after
the ECAP and HPT of the Al samples [21,28]. However, the
presence of the Al2O3 particles in the chip samples and the
reinforcing of the chip sampleswith Al2O3 and SiC contributes
to enhancing the hardness. Therefore, the HPT recycling of
the Al chip composites overcome the decrease in the strain
hardening of the Al HPTed samples and increase the hardness
and so decrease the COF [28].
3.4.3. Worn surface morphology of wear samples and WC ball 3.4.3.1. Worn surface morphology of wear samples. The wear
samples' surface morphology and analysis are shown in Figs.
12e14 and Table 1. The wear mechanism of the as-received
Al sample under different conditions was a mixture of delam-
ination, plastic deformation, and adhesive wear, as shown in
Figs. 12 (a), 13(a), and 14(a). The damages of the as-received Al
samples surface increased with increasing both the load and
distance, as shown in Figs. 12(a), 13 (a), and 14 (a). The EDS
analysis of the as-received Al sample worn surface indicates
the presence of oxygen, with an increase in the oxygen content
with both increasing the load and sliding distance that confirm
the occurrence of oxidation in the as-received Al samples, as
indicated in Table 1. The severe wear characteristics noted in
the surface morphology of the as-received Al samples support
the results of their high wear rate and COF. Interestingly, the
HPTed Al sample worn surface morphology characteristics
were similar to those noted of the as-received Al sample. The
wear morphology was a mixture of delamination, plastic
deformation, adhesive, and oxidation wear mechanisms, as
shown in Figs. 12(b), 13(b), and 14(b) and further indicated in
Table 1. Therefore, the worn surface morphology of the HPTed
Al samples confirms their low wear resistance (indicated by
high wear rate). Therefore, the HPT (or any other SPD process)
cannot consider a technique that can be used in improving the
wear resistance of the pure Al samples, as previously noted
[21,26e28].
The surface morphology of the recycled HPTed Al chip wear
samples shown in Figs. 12(c), 13(c), and 14(c) indicates the
improvement of theAl sampleswear resistanceafter theAl chip
recycling process. Those observations are compatible with the
wear rate results shown in Fig. 8. Although the HPTed Al chip
samples'wearmechanismwas similar to that of theas-received
and HPTed Al samples. However, the recycled Al chip sample's high hardness decreases the degree of plastic deformation,
delamination, and oxidation. The oxygen content reduced from
19.4 to 30.4 and 17.2 to 23.6% in the as-received and HPTed Al
samples down to 7.1e16.5% in the case of the HPTed Al chip
sample under different conditions, as indicated in Table 1.
Therefore, the HPT recycling of the Al chip was effective in
improving the wear resistance of the Al, as noted through the
wear rate results and confirmed by the wear sample surface
morphology and analysis.
The higher wear resistance of the HPT recycled Al chip
composites over that of the Al chip recycled sample was also
confirmed through the wear surface morphology results, as
shown in Figs. 12e14. The wear mechanism of the Al chip-20%
Al2O3 and Al chip-20% SiC was mainly a mixture of adhesive
and abrasive wear. The delamination and the plastic deforma-
tion shear bands were disappeared under a distance and a load
of 315 m and 2.5 N, respectively, as shown in Fig. 12. However,
with the increase of thedistance and the load, the delamination
was occurred with a slight degree, as shown in Figs. 13 and 14.
The oxidation as a wear mechanism was also noted in the
HPTed Al chip-20% Al2O3 and Al chip-20% SiC samples that
confirm it as a wear mechanism in the Al and Al composites
samples. But the oxidation degree in the Al chip composites
wasmuch lower relative to that in the Al, HPTed Al, and HPTed
Al chip samples, as indicated in Table 1. Interestingly, the Al2O3
and SiC particles can be noted obviously in the surface of the
HPTed Al chip-20% Al2O3 and Al chip-20% SiC samples with the
same size range that observed in the SEM micrographs before
thewear test, as noted in Fig. 5. Therefore, the wear test did not
contribute to any further fragmentation of the Al2O3 and SiC
particles. Moreover, the worn surface of the HPTed Al chip-20%
Al2O3 and Al chip-20% SiC samples was approximately free
from any groves. This observation confirmed the strong
bonding between Al2O3 and SiC particles and the Almatrix that
increase the hardness and wear resistance.
Furthermore, the good bonding overcomes the Al2O3 and
SiC particles takeoff and so overcome the occurrence of the
three action mechanism (that include the fraction between
the sample, the WC ball and the Al2O3 or SiC particles) and so
increase the wear rate that noted previously [29,31,32].
Through the EDS analysis of the different worn surfaces, it can
be noted the presence of W with increasing the load and the
The samples type Wear test conditions Elements content Wt.%
Al O Si C Co W
Al as-received A sliding distance of 315 m and an applied load of 2.5 N 80.6 19.4 e e e e
HPTed Al as-received 82.8 17.2 e e e e
HPTed Al chip 92.9 7.1 e e e e
HPTed Al chip-20% Al2O3 87.5 12.5 e e e e
HPTed Al chip-20% SiC 81.6 1.9 6.2 10.3 e e
Al as-received A sliding distance of 315 m and an applied load of 20 N 75.84 24.16 e e e e
HPTed Al chip-20% Al2O3 74.2 12.8 e e 2.3 10.7
HPTed Al chip-20% SiC 62.5 2.4 13.0 5.6 2.3 14.2
Al as-received A sliding distance of 1260 m and an applied load of 20 N 69.6 30.4 e e e e
HPTed Al chip-20% Al2O3 70.4 13.2 e e 2.9 13.5
HPTed Al chip-20% SiC 55.7 3.1 13.2 6.8 2.9 18.3
Table 2 e EDS' analysis of worn surface of the WC ball after the sliding against different samples corresponding to a sliding distance of 1260 m and an applied load of 20 N.
The samples type Elements content Wt.%
Al W Co O
HPTed Al as-received 5.1 2.8 38.4 53.7
HPTed Al chip 5.3 3.8 48.7 42.2
HPTed Al chip-20% Al2O3 4.2 2.7 63.2 29.9
HPTed Al chip-20% SiC 3.7 3.2 69.2 23.9
distance only in the case of the Al chip composites, as indi-
cated in Table 1. This observation also further confirms that
the HPT recycling of Al chip-20% Al2O3 and Al chip-20% SiC
samples effectively increases the hardness, and so wear
resistance of the Al that can be capable of removing particles
from the hard WC ball those further bounded to the com-
posites samples surface.
formation of the wear mechanism from delamination, adhe-
sive, and plastic deformation into adhesive and abrasive wear
after the HPT recycling of the Al chip and chip composites can
be explained by the increase of the hardness. Similar obser-
vations of the effect of the hardness increase on the change of
the wear mechanism and so reduce the wear rate (increase
wear resistance) through the formation of the Al composites,
and the SPD processing of the Al and Cu composites was pre-
viouslynoted [29,31e33,61,62]. But, amore improvement in the
wear resistance and so decrease the degree of the delamina-
tion, plastic deformation, and adhesive wear was noted after
the accumulative roll bonding and HPT processing of AleSiC
and CueSiC composite [33,61,62]. The HPT of the Al compos-
ites increases the hardness through fragmentation and ho-
mogenous redistribution of the hard particles across the Al
matrix that combines with the grain refinement of the Al ma-
trix grain. Therefore, the recycled HPTed Al chip composites
morphology of the worn samples surfaces confirms the supe-
riorwear resistanceof theHPTedAl chip composites relative to
that of the Al, HPTed Al, or Al chip samples or even than those
previously noted of different Al composites [29,31e33].
3.4.3.2. Surface morphology and analysis of WC ball surface. Fig. 15 and Table 2 show the SEM micrographs of surface
morphology and analysis of theWCball tested under a distance
anda loadof1260mand20Nfor thedifferent samples.A layerof
Al adhered to the WC ball surface was noted in all cases, as
shown in Fig. 15. The size of theAl layer adhered to theWCballs
surfaces varied from one ball to another depending on the
sample the ball tested against it. The Al layer coversmost of the
WCball surface in the case of the Al as-received, HPTed Al, and
HPTed Al chip samples, as shown in Fig. 15(aec). The Al layer
adhered to theWCball after thewear test with soft or relatively
soft samples (Al as-received, HPTed Al, and HPTed Al chip
samples) has some delaminated parts that indicate the large
thicknessofAladhered layer.However, thehighhardnessof the
HPTed Al chip composites decreased the size of the adhered Al
layer, asshown inFig. 15(dee). Inaddition,clearshearbandscan
be observed across the WC balls surfaces, as indicated by the
white arrows in Fig. 15(dee). Therefore, the HPTed Al chip
composites with high hardness have an evident effect on the
WC ball surface.
EDS analysis of the wear test samples and WC ball proves
the effect of the hardness of tested samples on the surface
morphology of theWC ball, as indicated in Tables 1 and 2. Two
notes confirm the results of the WC ball surface morphology
shown in Fig. 15. First, the Al content on the WC ball was
decreasedwith the increase of the tested sample hardness. The
EDX analysis indicates the decrease of the Al content from 59.4
to 53.7% in the case of Al as-received and HPTed Al samples
down to 42.2, 29.9, and 23.9% in the case of theHPTedAl chip, Al
chip-20% Al2O3, and Al chip-20% SiC samples, as indicated in
Table 2. Therefore, the increase of the tested sample hardness
reduces its softening degree during the wear test, which de-
creases the Al amount transfer to the ball surface. Second, it
can note that an amount of theW and Co was transferred from
the WC ball into the sample surface only in the case of the Al
chip-20% Al2O3 and Al chip-20% SiC with the high hardness
under severe wear conditions, as indicated in Table 1. This
observation supported the WC ball surface morphology that
indicates the presence of shear bands on the ball afterwear test
against Al chip-20% Al2O3 and Al chip-20% SiC that, as shown
and indicted bywhite arrows in Fig. 15(d) and (e). Therefore, the
increase of the hardness of theAl through the formation HPTed
Al chip-20% Al2O3 and Al chip-20% SiC can be the dominant
factor that affects the surface morphology and analysis of the
samples and WC ball.
4. Conclusions
In the current study of Al pure chip and Al pure chip com-
posites recycling, the following conclusions were obtained.
1. The HPT recycling of the pure Al chip, Al chip-20% Al2O3,
and Al chip-20% SiC effectively produced approximately
fully dense samples with relative densities ranging from
99.7 to 98.3%.
2. The HPT processing of the Al chip and Al chip composites
conserve the UFG microstructure of the chip with further
refinement of the chip grain size and fragmentation of the
Al2O3 and SiC particles. Therefore, HPT processing effec-
tively produces UFG recycled bulk Al chip and Al chip
composites with trimodal microstructure.
3. The HPT of Al chip-20% Al2O3 and Al chip-20% SiC com-
posites effectively produced UFG Al composites with Al2O3
and SiC average particle sizes of 0.92 and 0.47 mm with Al
matrix average grain sizes of 0.44 and 0.24 mm.
4. The HPTed Al solid, Al chip, Al chip-20% Al2O3, and Al chip-
20% SiC samples have similar hardness distribution pat-
ternswith the increaseofhardness fromthesamplecenter to
its edge. Thedeformation inhomogeneity indexbased on the
hardness distribution increased from 2.2 in the HPTed Al
solid samples to 3.1, 9.5, and 9.6 in the case of the HPTed Al
chip, Al chip-20% Al2O3, and Al chip-20% SiC samples.
5. The HPT recycling of the Al chip, Al chip-20% Al2O3, and Al
chip-20% SiC increases the hardness by 55, 149, and 187.2%
over that of the HPTed Al solid sample.
6. The HPT recycling of the Al chip and Al chip-20% Al2O3 and
Al chip-20% SiC enhances thewear resistance (reducewear
rate) and decreases the values of the COF of the pure Al.
7. HPT recycling of the Al chip, Al chip-20% Al2O3, and Al
chip-20% SiC samples has an obvious influence on their
wear mechanism. The wear mechanism transformation
results were supported by the wear surface morphology
and analysis of the wear samples and WC ball.
Declaration of Competing Interest
relationshipswhichmaybe considered aspotential competing
research project 2020/01/11731 supported by the Deanship of
Scientific Research at Prince Sattam Bin Abdulaziz University,
Kingdom of Saudi Arabia.
Deanship of Scientific Research at Prince Sattam Bin Abdula-
ziz University, Kingdom of Saudi Arabia.
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1. Introduction
3.4.2. Friction properties
3.4.3. Worn surface morphology of wear samples and WC ball
3.4.3.1. Worn surface morphology of wear samples
3.4.3.2. Surface morphology and analysis of WC ball surface
4. Conclusions

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