SCRATCH FREE AND LOW WEAR ALUMINIUM PANEL: FROM SCRAP TO BOEING
R.M. Nasir 1School of Mechanical and Aerospace Engineering,
Engineering Campus Universiti Sains Malaysia(USM), 14300, Nibong Tebal, Seberang
Perai Selatan, Pulau Pinang, Malaysia
2Cluster of Polymer Composite (CPC), Science and Engineering Research Centre (SERC), Engineering Campus, Universiti Sains Malaysia, Nibong Tebal,
14300, Malaysia
A.Y.Saad School of Mechanical Engineering,
Engineering Campus Universiti Sains Malaysia(USM), 14300, Nibong Tebal, Seberang Perai Selatan, Pulau Pinang,
Malaysia
ABSTRACT Recycled aluminium is known as secondary aluminium, but
maintains the same physical properties as primary aluminium.
Secondary aluminium is produced in a wide range of formats and
is employed in 80% of alloy injections. Another important use is
for extrusion. To improve aluminium recycling, one can use the
process such as physical separation, magnetic, air separation,
eddy current separation and sink float separation. Recycled
aluminium had been studied due to its wear and friction
resistance, strength, energy consumption and environmental load
lesser than the primary metal. In this work, wear rate, coefficient
of friction, hardness, wear mechanism and elemental analysis or
chemical characterization of the pure and recycled aluminium
were monitored using pin-on-disk tester, Vickers hardness test,
scanning electron microscope (SEM) and Energy-dispersive X-
ray spectroscopy (EDX) respectively. The outcome verified that
the percentage increase in wear resistance of recycled aluminum
compared with pure aluminum is better by 33% at 20N, 27% at
35N and 11% at 50N load imposed. While the average coefficient
of friction for recycle aluminum decreases linearly whereas for
pure aluminium is non-linear. There is also an increase in
hardness of recycled aluminum ranging from 10 to 23 %
compared to pure aluminuim due to high concentration of ferrous
element (maximum of 22 wt. %).
INTRODUCTION
Aluminium recycling brings many environmental and
economical benefits, such as reduce in greenhouse gas
emission, increase in energy efficiency and increase in
material efficiency. Recycling of aluminium involves melting
the scrap, a process that requires only 5% of the energy used
to produce aluminium from ore. 42% of beverage cans, 85%
of construction materials and 95% of transport vehicles was
reported using recycled aluminium[1]. In general, the non-
recycled aluminium has better material properties than the
recycled aluminium due to the present of impurities and other
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November 14-20, 2014, Montreal, Quebec, Canada
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materials [1]. Besides that, the recycling process may also
cause porosity in the aluminium. Hypothetically, the
mechanical properties and wear resistance of recycled
aluminium should be better than non-recycled aluminium. In
this project, the work is focused on finding out the wear and
friction as well as the hardness of recycled aluminium and
then the results was to be compared the properties to the non-
recycled aluminium. Besides that, hypothetically to prove that
properties of recycled aluminium that undergone many
treatment are better than the aluminium that is not recycled
and undergone lesser treatment.
Nowadays, wear properties are significant as metals are
applied for many applications, for examples; automobile,
sports, medical apparatus unto aircrafts and space vehicles.
Wear is always been refer to the material removal from the
surface due to the relative motion such as sliding with/on
another material and it is caused by some mechanisms such as
abrasion, fatigue, erosion, oxidation and scratching [2]. The
factors that affect wear rate of metals are type of material
used, speed of sliding, roughness of surface, temperature,
magnitude of normal load, friction and vibration[3]. It was
found that as the normal load and sliding speed are increasing,
the value of wear rate of metals increases with noticeable
incremental in thermal and frictional values depending on the
hardness value of metal alloys, the velocity employed and
contact time[4]. This is because at higher velocity, there are
more relative pressure moving in the normal direction, hence,
more upward force produced to the upper surface and this will
increase the separation between the contact surfaces [5].
Stephen L. Rice found that the flat-ended specimen of
aluminium has been impacted on stainless steel counter-face
under normal impact and transverse impact shows the
hardness of metals depends on the microstructure of the
specimen[6]. By increasing normal impact force and
transverse impact velocity causing in the weight loss of
specimen [7]. The schematic view of pin-on-disk testing
machine was detailed by the author in previous work [8].
Fig. 1 Schematic view of pin-on-disk testing machine [8]
Coefficient of friction can be determined according to the
applied load and the frictional force [8-10].The conditions of
the pin-on-disk wearing test without lubricant, at constant
rotational speed, with specimens of the same size and was
cleaned and dried. Wear rate normally calculated as in
equation 1.
(1)
where Ai= area above the profile at four different sections (i=
1-4) in mm2.
Hardness depends on the grain size of metals. As the grain
size becomes smaller, the layers between the grain boundaries
increase the slippage-resistance in the metal microstructure
and vice-versa. In order to increase the hardness, treating the
metal to re-crystallization temperature is needed to reduce its
grain size. During dry sliding, the frictional force transforms
into heat energy, therefore, the thermal conductivity of the
specimens must be avoided to prevent thermal shock[9].
Hence, in order to abrade/polish/cut the metal, the hardness of
the abrading/polishing/cutting tool must be as least 1.2 times
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harder than the metal[10]. Most of the Vickers hardness
testing machine such as diamond indentor, has square shaped
pyramid with 136 degree angle between opposite surfaces[11-
13]. In Vickers hardness test, the hardness is measured as in
equation
2.
(2)
EXPERIMENT Pure aluminium was obtained from XXX and recycled
aluminium was prepared using sand casting method. Firstly,
fabrication of casted recycle aluminium from used aluminium
beverage cans was done using high temperature oven furnace
around 1400oC. The aluminum scraps, sand and binders were
melted in the furnace. The mixer was stopped for several times
and a piece of metal was used to mix the sand and binder at
the bottom of the container to form homogeneous molten
metal. Powder was put on the mold and the rack. Next,
mixture was poured into the mold. The mixture was punched
using puncher so that it is fixed to the mold. Some holes were
made on the mixture using the metal stick. The carbon dioxide
was allowed to enter the holes twice to cure the binder through
a pipe. The gas was inlet to fill in the holes for 10 seconds for
the first time. Then, the gas was left to fill in the holes for
another 5 seconds. From the other side of the mold, the ingot
was poured again. Steps 3 to 9 were repeated. Next, the ingot
and mold were removed. Molten aluminium was poured into
the sand mold and allowed to cold down for half a day. Then
the sand mold was removed and recycled aluminium was been
taken out from the cast as in Fig. 2 (a+b). Afterwards, both
pure and recycled aluminium was cut using CNC machine into
smaller pieces with dimensions of 10mm x 10 mm x 30mm.
Then, both pure and recycled aluminium samples were
polished and cleaned prior to wear, friction and hardness tests
as in Fig.2 (c+d). Afterwards, the analysis and comparison
was made between recycled aluminium and non-recycled
aluminium. Three samples were ran for each wear specimen
was tested at five different speeds (100rpm, 200rpm, 300rpm,
400rpm and 500 rpm) and different loads (10N, 20N, 30N,
40N and 50N) using Ducom Tribo-tester as in Fig.3.
The specimens were indented by a square-shaped pyramid
diamond indenter subjected to a load for 5 to 10 seconds. The
two diagonals of the indentation are measure using
microscope and the average values of Vickers hardness were
obtained. Then, the abraded specimens were placed into the
Hitachi S-3400N Scanning Electron Microscope (SEM) for
scanning its morphology. The magnification used were 250x
and 500x. EDAX was determined to identify the elemental
compositions present in the specimens.
(
(a)
Fig. 2(a) Pure aluminium (b) recycled aluminium (c+d) shape of specimens after cut
(c)
(
(a)
(
(a)
( (b) (a) (d)
(
(a)
(
(a)
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Fig.3 Pin-on-disk test machine
Fig. 4 Wear rate for pure Aluminium at 400 rpm and at 500 rpm in comparison to recycled Aluminium at 400 rpm and at 500 rpm
RESULTS AND DISCUSSION Effect of speed and load on wear rate In order to make a comparison on the effect of speed and load
on the wear rate of pure and recycled aluminium, different
load parameters were applied at 1400th
. second and at 400 rpm
and 500 rpm respectively as shown in Figs. 4The results of the
specimens of pure and recycled aluminium at 400 rpm are
closer to the ideal results. The results of specimen of pure and
recycled aluminium at 500 rpm are not ideal due to the
composition of the recycled aluminium.
In general, the wear rates of both type of aluminium increase
as the speed of the wearing surface between the specimen and
the rotating disk. This is mainly due to the fact that greater
speed creates greater sliding force (Fs) that will remove the
specimen significantly. Hence, the wear rate increases.
Besides, from the results, the wear rate of both aluminium
increases as the load applied increases. Similarly, greater load
applied to the specimen creates higher pressure per velocity
(P/v) to overcome the frictional force between the surfaces.
Hence, the greater the load, the greater the wear rate of the
specimen.
0
50000
100000
150000
200000
250000
300000
10N 20N 30N 40N 50N
We
ar r
ate
(m
3 /s)
Load (N)
400rpm-pure
500rpm-pure
400rpm-recycled
500rpm-recycled
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Fig. 5 Comparative results of wear rate against load at 400 rpm
In Fig. 5, comparative results was extracted from
both type of aluminium at 400 rpm, the specimen of
recycled aluminium shows greater resistance to wear
compared to the pure aluminium under the same
speed and same load. Therefore, the recycled
aluminium has better resistance to wear compared to
the pure aluminium under the same speed and load.
For example, under load of 50N, the wear rate of
specimen of recycled aluminium is 180000 mm3/s,
but the wear rate of specimen of pure aluminium is
200000mm3/s.
Effect of speed and load on average coefficient of friction In order to compare the effect of speed and load on
the average coefficient of friction of pure and
recycled aluminium, the average value of coefficient
of friction at 200 rpm and 500 rpm at different loads
was monitored as shown in Figs.6 The average value
of frictional force depends on the contact surface
between the specimens and the rotating disk. So,
greater frictional force needed to be overcome
between the surfaces if the value of coefficient of
friction is greater. The results of specimen as shown
in Fig 6 show pure and recycled aluminium at 200
rpm and 500 rpm is linear. The values of average
frictional force decreases as the speed and loading of
the wearing surface and the rotating disk increases.
This is because the greater speed creates inertia that
helps to overcome some of the frictional force
between the surfaces. Hence, the value of frictional
force decreases as the speed and load increased.
Fig. 6 Average Frictional Force for pure Aluminium at 200 rpm and at 500 rpm in comparison to recycled
Aluminium at 200 rpm and at 500 rpm
y = -44.28x + 562.74 R² = 0.9722
0
100
200
300
400
500
600
10N 20N 30N 40N 50N
CO
F (x
10
-3)
Load (N)
400rpm-pure
500rpm-pure
400rpm-recycled
500rpm-recycled
Linear (400rpm-recycled)
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Fig. 7 AFF against load at 500 rpm
As extracted from Fig. 7 for both type of aluminium
at 500 rpm, the specimen of recycled aluminium has
greater value of average frictional force compared to
pure aluminium under the same speed and same load.
Therefore, the recycled aluminium has better anti-
friction to wear. For example, at 500 rpm, under load
of 10N, 20N, 30N and 40N, the value of average
frictional force of recycled aluminium is greater than
the pure aluminium. The hardness values of pure and
recycled aluminium are determined using Vickers
hardness test machine as shown in Table 1 and
plotted in Fig. 8. Generally, the hardness values of
the specimens of recycled aluminium are having
greater value compared to the specimens of pure
aluminium. This shows that the hardness properties
of recycled aluminium are better than the pure
aluminium hence its wear resistance, as argued by
author previously [8].
Table 1 Hardness value for pure and recycled aluminium
Speed (rpm.) Average hardness value for pure
aluminium
Average hardness value for
recycled aluminium
100 137.3 144.5
200 145.8 163.1
300 135.9 157.3
400 150.3 196.4
500 140.9 146.9
Fig. 8 Vickers Hardness Value
Scanning electron microscope (SEM) test and EDX
analysis were carried out on the worn surface of the
specimens of pure and recycled aluminium at 400
rpm and 500 rpm as shown in Figs. 9(a-d) and EDAX
analysis in Figs.10 (a-d).
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Fig. 9 Scanning electron microscope of (a) pure aluminium at 400 rpm (b) pure aluminium at 500 rpm (c) recycled
aluminium at 400 rpm (d) recycled aluminium at 500 rpm (arrow indicates direction of sliding)
Fig. 10 EDAX analysis of specimen (a) pure aluminium at 400 rpm (b) pure aluminium at 500 rpm (c) recycled
aluminium at 400 rpm (d) recycled aluminium at 500 rpm
In Figs. 9 (a-d), the abraded surfaces shows wear mechanism
with intragranular fractures and adhesive wear. In energy
dispersive x-ray spectroscopy (EDAX) analysis on the area of
adhered surface in Figs. 10 (a-d), show the composition of
aluminium is higher in the pure aluminium and the
composition of ferum is higher in the recycled aluminium. The
existence of higher amount of ferum in the composition of
recycled aluminium increases the wear resistance of the
recycled aluminium. The existence of ferum in the
microstructure increases the resistance between the layers of
grain boundaries. The grain boundaries need more force to
overcome the frictional force or friction. Hence, the recycled
aluminium has greater wear resistance compared to the pure
aluminium. Besides, there are other materials exist in the
(a) (b) (c) (d)
(a)
(b)
(d)
(c)
Intragranullar fracture
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composition of recycled aluminium which also improve the
wear resistance of recycled aluminium as their existence
increase the resistance of the layers between grain boundaries.
In some regions, as the one of Fig. 9(b), a similar thin adhered
layer is found, while in the others the surfaces appear clearer,
as in the Fig. 4 (a and d), due to continuous formation and
removal of the tribolayer [14].
High ratio of composition for aluminium/ferum in the
specimens causes less resistance of the layers between the
grain boundaries, therefore, there is less force needed for the
layers to sliding over each other (as shown in Fig.11). Hence,
pure aluminium is easily polished compared to recycled
aluminium. Besides that, there are less composition of other
material exist in the composition of pure aluminium specimen
which also contribute to the weak Van de Waals bonding
leading to initiation of slippage, crackage and breakage.
Fig. 11 Effect of impurities on grain boundaries
CONCLUSION The study of wear resistance properties of recycled and pure
aluminium depends significantly on the normal load applied
and the sliding velocity of the contacted surfaces. The wear
rate of pure and recycled aluminium increase when the applied
load and the sliding speed. Recycled aluminium shows the
better wear resistance compared to the pure aluminium ranges
from 11-33% based on applied load from 20 to 50N. Mean
while for average frictional force reduced linearly (3.5428) for
recycled aluminium compared to pure aluminium which
shows drastic exponential reduction of COF gradient. This is
mainly due to impurities and thermal treatment process in
fabrication hence microscopically has altered the composition
in the recycled aluminium that improves its wear resistance. In
Vickers hardness test, the hardness value of recycled
aluminium is greater compared to pure aluminium by 23%
maximum due to the present of 22 wt.% of ferrous element in
the recycled aluminium leading to better hardness compared to
pure aluminium. ACKNOWLEDGEMENT
The authors gratefully acknowledge the Universiti Sains
Malaysia for the funding of this entire research with the grant
number of 304/PMEKANIK/60311052.
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Estimated of eight million pounds of recycled aluminum is to be melted and reshaped into new aerospace materials by
Boeing's sub-contractors. Alcoa Aerospace, Transportation and Industrial Rolled Products director of supply chain Leslie
Shuman quoted: "This programme will maximise the value of aluminum scrap materials throughout the supply chain while
also reducing waste."
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