Article
citation information:
Labisz, K. Reflectivity of anodised Al-Si alloy surface of belt pulleys used in
combustion engines. Scientific Journal of
Silesian University of Technology. Series Transport. 2021, 112, 125-133. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2021.112.7.10
Krzysztof LABISZ[1]
REFLECTIVITY
OF ANODISED AL-SI ALLOY SURFACE OF BELT PULLEYS USED IN COMBUSTION ENGINES
Summary. The recent trend of
using aluminium alloys instead of steel has reached the transportation
industry, where increasingly, more parts are made of aluminium. An example is
the belt pulley, applied for combustion engines for energy transmission. This
part should be strong, durable, and lightweight. Aluminium-silicon alloys are a
good choice, moreover, even when the surface is anodised, also because of their
moderate inertia control and excellent wear characteristic during mechanical
operations. Since aluminium is lightweight yet mechanically durable and
anodised, it is an ideal belt pulley to use, especially in high-temperature
operations. However, the main question is what type of Al-Si alloy, casting
method and anodisation method should be used in terms
of energy adsorption, having long-term properties for a lifetime, has to be
applied. For this reason, this paper presents the influence of the chemical
composition, casting method and anodising parameters on the structure and
thickness of the anodic layer produced on aluminium alloys, as well as on the
albedo value as an ability to reflect or absorb light. The aluminium alloys, AlSi12Cu1 and AlSi9Cu3, were used
as research materials, obtained using different casting methods. The goal of
this work was to determine the optimal combination of the anodisation
conditions and materials for maximising the reflectivity factor of the surface,
as a very important factor, determining the energy amount absorbed by an
anodised surface. For further improvement of these surface properties as well
as for enhancement of the properties and strengthen the material produced
with different aluminium alloys production methods, different alloying
additives were added. In addition, the mechanical properties of the surface
layer were measured, where a remarkable hardness increase was obtained, and the
best combination in form of AlSi12Cu1 high pressure
cast was found with the highest albedo factor among all tested surface
variants.
Keywords: aluminium, reflectivity, adonisation, surface treatment,
sand cast, high pressure cast, albedo
1. INTRODUCTION
In recent
years, a continuous increase in the use of aluminium alloys across industries has been
observed. Protective and decorative anodic coatings are applied to aluminium household goods, electronic components, parts of
musical instruments, garden furniture, tourist and sports equipment, automotive
accessories and aluminium joinery elements. Anodised aluminium wires are used
as conductors in electrical installations. Oxide coatings are produced on aluminium foil intended for electrodes in capacitors. Hard
anodic coatings are used in the aviation and transportation industries, where
the effect on environmental pollutions is increasingly discussed [1-3].
Anodic oxide coatings permanently bonded to the aluminium
substrate are corrosion resistant [4]. Corrosion resistance can be reduced by
pores in the coating or by the presence of harmful alloying elements and
impurities, especially copper. The intermetallic phases of copper with aluminium dissolve during anodising, which reduces the hardness and
thickness of the coatings, increasing the porosity. During the formation of aluminium oxide, there is a slight increase in the mass of
the workpiece and its volume. The oxide layer is very firmly attached to the
substrate. Dissolution of the oxide layer is possible only in alkaline or
acidic solutions with a pH greater than 8.8 or less than 4.0 [5].
A very important issue concerning the surface properties, beside the
mechanical properties is the reflectivity and albedo. The albedo of an object
or surface (Figure 1) is a measure of the percentage of light reflected from it
for a certain incident light amount. A perfectly-reflecting mirror has an
albedo of 100% (A=1.0) so that if a 100 W of visible light energy fall on its
surface, a 100 watts will be reflected. The most perfect black surface, on the
other hand, has an albedo of 0% (A=0) and will reflect no visible light energy
at all. Instead, it absorbs 100% of the light energy, so if you want to figure
out how hot a surface will get in direct sunlight, the amount of absorbed light
energy is just E = 100% - Reflected, or in terms of A, we have E = Incident
Energy x (1-A) [6, 7].
Albedo currently plays a very important role in world climate
discussions, namely as the world warms up the Earth's albedo shifts. Albedo has also an
influence on the lifetime of engineering materials, especially on surfaces and
layer produced on such working elements. The amount of ice covering the planet
is dropping because of increased temperatures from global warming. This causes
a decrease in the area of white surfaces, leading to less energy to be
reflected and more to be absorbed. This process warms the Earth even more.
Therefore, this study aims to investigate the albedo value based on the
microstructure and thickness of coatings produced on cast aluminium
alloys during the anodisation process and evaluate
the effect of electrolyte and alloy casting method on the anodic layer
reflectivity and their working durability or damaging [8] to ensure long
working time.
Fig. 1. Albedos of different surfaces
2. INVESTIGATION
PROCEDURE
The tests were performed on samples of AlSi12Cu1
and AlSi9Cu3 alloys, used for pulleys production for
combustion engines (Figure 2). The Al-casting was carried out using the high
pressure die casting and sand casting methods applied for both the AlSi12Cu1 and AlSi9Cu3 alloys [9].
The chemical composition of the alloys is given in Table 1. The technological
parameters of the anodising process are presented in Table 2. For anodising of the aluminium
alloys 4% H2SO4, the
electrolyte in mass was applied.
Tab. 1
Chemical composition of AlSi12Cu1 and AlSi9Cu3 alloys
|
Alloy |
Elements mass concentration, % |
||||||
|
Si |
Mg |
Cu |
Mn |
Fe |
Zn |
Al |
|
|
AlSi12Cu11 |
12.5 |
0.05 |
0.05 |
0.5 |
0.6 |
0.1 |
balance |
|
AlSi9Cu3 |
9.5 |
1.5 |
3.0 |
0.5 |
0.9 |
0.5 |
balance |
Tab. 2
Anodising parameters used for treatment
|
Parameter |
Value |
|
Electrolyte, [g/l] |
H2SO4 with concentration of 295 ÷ 315 |
|
Temperature, °C |
– 4 ÷ 2 |
|
Pulse current, [A/dm2] |
2 during 0.25 s, 1 during 0.1 s |
|
Al ions concentration, [g/l] |
6 ÷ 9 |
Fig. 2. Anodised Al-Si belt pulleys used in combustion engines
The metallographic tests were carried out on the OLYMPUS BX60M light microscope equipped with the OLYMPUS DP10 camera. The photos were processed on a PC using the
OLYMPUS image analysis program.
Abrasive wear test was performed using the ABR-8251
supplied by TCD Teknologi ApS. The tests were carried out following the
specifications and requirements of the ISO 8251 standard. A load of 4.9 N was
used for the test, with a slip speed of 40 cycles/min.
The test area is 12 x 30 mm.
To calculate albedo (1), (Table 1) it is necessary to measure with a
light meter the amount of reflected light in lux, and the amount of light that
is falling on the surface in lux according to the relationship below:
albedo [%] = reflected light [lux]/direct light [lux] (1)
If 10,000 lux falls on a surface and 6,000 lux is reflected, the albedo
is 0.6 or 60%. This is about the same as a sheet of ordinary white laser
printer paper. Fresh snow has an albedo of 80%, while fresh asphalt has an
albedo of 10% or 0.1.
Tab. 3
Albedo values for chosen objects
|
Surface |
Direct(lux) |
Reflected(lux) |
Albedo |
|
Street Asphalt
(greyish) |
257,000 |
16,100 |
0.06 |
|
Grass lawn in shade |
16,600 |
431 |
0.03 |
|
White laser printing
paper |
11,350 |
7,000 |
0.62 |
3. INVESTIGATION
RESULTS
As a result of an abrasive wear test, anodisation was found to reduce
abrasive wear. The best wear resistance is achieved when the anode layer
thickness is high (approx. 48 µm). Partial removal of the coating was
observed for all castings made in the high pressure die casting method, where
the coating thickness is lower (approx. 10 µm).
The results in Figure 3 show that anodised samples of sand cast
AlSi9Cu3 alloy have half the mass loss than
non-anodised ones. In the remaining cases, there is a reduction in mass loss
from 30 to 47%.
Fig. 3. Comparison of abrasion of pressure castings
The AlSi12Cu1 alloy
samples show greater weight loss because the test site is close to the
electrode attachment (the layer is probably thinner at this point).
Another variable that can influence the abrasive wear test results is
the surface roughness (Table 4). Based on the obtained results, it was found
that the higher roughness, which is characteristic of sand castings, has a
positive effect on the abrasive wear test results.
As a result of the metallographic tests (Figures 4 - 7), it was found
that the anodised layer produced on pressure and sand castings for both alloys
does not show any gaps, which prevents the appearance of pitting, and the
silicon precipitates contained in it (in the shape of needles) increase abrasion
resistance.
Based on the measurements of the anodised layer thickness presented in
Figure 8, it was found that for both alloys, the thickness of the anodised
layer produced is greater in the case of sand casting and lower for pressure
casting with the same anodising parameters.
The measured reflectivity values (Figure 9) ranges from 243 for high
pressure AlSi9Cu3 cast to even 312 for the AlSi9Cu3 sand cast alloy, which provides the albedo value
of 0.800, thus, seems to be the best solutions in terms of choice for a high
reflectivity surface, which can be applied for the treatment of constructional
elements. The difference in reflectivity reaches even 28%, as they are an
important factor in the anodising treatment of aluminium alloys.
|
|
|
|
Fig. 4. Anodic layer produced on the AlSi9Cu3
alloy, high pressure cast |
Fig. 5. Anodic layer produced on the AlSi9Cu3
alloy, sand cast |
|
|
|
|
Fig. 6. Anodic layer produced on the AlSi12Cu1
alloy, high pressure cast |
Fig. 7. Anodic layer produced on the AlSi12Cu1
alloy, sand cast |
Tab. 4.
Roughness measured during the abrasive wear
test
|
Alloy |
Roughness, Ra μm |
|||
|
Sand cast |
High pressure cast |
|||
|
Non-anodised |
Anodised |
Non-anodised |
Anodised |
|
|
AlSi9Cu3 |
0.38 |
3.26 |
0.28 |
1.93 |
|
AlSi12Cu1 |
0.35 |
1.84 |
0.24 |
1.52 |
Fig. 8. Comparison of anodic layer thickness
Fig. 9. Reflectivity and albedo of the produced anodic layers
3. CONCLUSIONS
Proper anodisation method, as well as
well-selected Al-Si alloys produced by casting, were determined in this paper
in terms of the adsorption possibility of electromagnetic radiation. The
reflectivity/albedo issue is very important for the proper working of car
engine elements, especially in this case, of belt pulleys made of aluminium
–silicon alloys. In general, high albedo value makes it possible to
adsorb a relatively low amount of electromagnetic energy, thus, remarkably
increase the durability and working time of anodised light materials used in
the transportation industry.
During performing of this particular test, the following findings were
determined:
The test results given in Table 3 determined which of the tested alloys in combination with the appropriate production
method (sand or pressure casting) has the highest abrasion resistance. The best
results should be achieved with a combination of AlSi9Cu3
alloy with sand casting and AlSi12Cu1 alloy cast in
die casting. Sand cast AlSi12Cu1 and die-cast AlSi9Cu3 have the lowest wear resistance.
The test results presented in the table show that anodised alloys, both AlSi9Cu3
and AlSi12, observed a lower weight loss compared to
non-anodised alloys. It can be seen that the casting
method also affects the wear resistance; sand cast alloys show a lower weight
loss during the test carried out, therefore, they show higher abrasion
resistance. Finally, the reflectivity value investigation, as an important
factor for long time usage of parts has lead to the
conclusion that the optimal combination for high reflectivity equal 312 lux was
provided by the sand cast AlSi9Cu3 alloy with an
albedo value of 0.8, which can be partially associated with a relatively high
surface roughness. The difference in reflectivity
reaches 28% and is recognised as very important for the determination of proper
anodising treatment of aluminium alloys in terms of environmental issues.
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Received 05.04.2021; accepted in revised form 29.06.2021
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