Download Effect of SiC Grain Refining on Wear Resistance of Mg

UCTEA Chamber of Metallurgical & Materials Engineers
Effect of SiC Grain Refining on Wear Resistance of Mg-Al
Alloys
Erdem Karakulak¹, Norbert Hort², Yusuf Burak Küçüker1
Abstract:
different average grain sizes. Grain refinement of Mg-Al
alloys were obtained with SiC inoculation. SiC addition
is an effective way of refining grain size of magnesium
alloys, especially alloys containing aluminium as
alloying element or impurity. Effect of grain refinement
with SiC on microstructure, hardness, wear resistance
was investigated in detail.
Different amounts of SiC particles were added to Mg-Al
alloys to refine grain size of the samples cast with
permanent mold direct chill casting technique.
Microstructures of the cast materials investigated and
grain size measurements were conducted. Vickers
hardness tests and dry sliding wear tests were realized on
the specimens with different grain size. Addition of SiC
and increasing aluminum content in the alloy caused a
decrease in the grain size of the specimens. Refinement
of microstructure increased hardness and wear resistance
of the cast samples. Worn surfaces of the specimens were
investigated by scanning electron microscope to
understand wear mechanisms.
1. Introduction
Magnesium is the lightest structural metal with a density
of 1,738 g/cm3 [1,2]. Low density and good castability of
magnesium and its alloys make them candidate materials
for numerous applications especially in automotive
industry to lower fuel consumption and CO2 emissions
[3,4]. However limited mechanical properties of
magnesium alloys are the main problems to overcome to
widen the usage of these materials in different
applications [5]. Wear damage is one of the main
problems for moving parts in automotive production. In
general magnesium and its alloys show poor wear
resistance [6]. There are different ways to increase wear
resistance of Mg alloys like plasma electrolytic oxidation
(PEO) [7], surface mechanical attrition treatment [8],
ceramic coatings with physical vapour deposition (PVD)
[9]. Grain refining is a process where hardness and wear
properties of material can be improved without any
alloying addition or coating. In this study wear tests were
conducted on different binary Mg-Al alloys with
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¹Kocaeli University,
²Helmholtz-Zentrum Geesthacht - Türkiye,Germany
2. Experimental
Melting of different Mg alloys were realized using an
electric resistance furnace under protective atmosphere of
0,3 % SF6 +Ar. Chemical compositions of the used alloys
are given in Table 1. When the magnesium is molten
alloying elements and SiC (average particle size 2 μm)
was added to the melt. Then liquid metal is transferred to
a cylindrical steel mold (100 mm in diameter and 230
mm in height). This mold is then placed into the
permanent mold direct chill casting machine. In this
device the melt is kept at 680 °C for 30 minutes for all
alloys. After waiting for 30 miutes two TP1 samples
were taken and solidified according to the standard for
grain size measurements. After that the mold with
remaining melt inside slowly lowered in to water to be
solidified. Grain size measurements were conducted
using line intercept method. Hardness tests were realized
using a Vickers hardness tester, using 5 kg load and 30 s
of loading duration. All given hardness values are an
average of 10 measurements. Wear tests were conducted
on a Nanovea ball-on-disc type tribometer at room
temperature, using 25 N normal load and AISI 52100
steel ball with 5 mm diameter as counter surface. Sliding
distance was kept constant at 500 m for all tests.
Specimens were cleaned with alcohol and weighed
before and after wear tests to obtain weight loss data.
Obtained weight loss data is used to calculate wear rate
of the specimens. The fallowing equation was used to
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calculate wear rate of specimens: W = M/D, where W is
the wear rate (mm3/m) M denotes mass loss (g), and (g/mm3) and D (m) are the density and sliding distance
respectively [10]. Worn surfaces of the specimens after
wear tests were investigated under SEM to understand
wear mechanism operated during wear tests.
Table 1. Results of chemical analyses of the cast samples
Element
Pure Mg
Mg-1Al
Mg-3Al
Al
0,11
0,92
2,83
Fe
0,02
0,03
0,03
Mn
0,03
0,03
0,03
Zr
0,004
0,004
0,003
Mg
Bal.
Bal.
Bal.
(b)
3. Results
3.1. Grain Size Measurements
Addition of SiC to pure Mg and Mg-Al alloys refined the
grain size of the material. Representative microstructures
and measured grain size values are given in Fig. 1 and
Fig. 2 respectively. Increasing SiC in the alloy decreases
the grain size of the cast materials. But the grain refining
effect is almost faded after % 0,4 SiC addition. Addition
of higher amount of SiC has little effect on grain size.
Also increasing aluminium content of the alloy decreases
grain size of the material, which is a result of growth
restriction effect of aluminium in magnesium alloys [11].
(c)
Figure 1. Microstructures of (a) Pure Mg, (b) Mg-1Al
and (c) Mg-3Al without SiC addition
(a)
Figure 2. Effect of SiC addition on grain size of pure Mg
and Mg-Al alloys
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3.2. Hardness Tests
To understand the effect of aluminium addition and SiC
grain refining on the hardness of the materials hardness
tests were conducted on all specimens. Results of
hardness tests can be seen on Fig. 3. With increasing SiC
and Al content hardness of magnesium increases as
expected. Addition of SiC dramatically decreases grain
size of the material especially for pure magnesium.
Decrease of grain size causes an increase in the hardness
of material. Addition of aluminium to pure magnesium
also increases hardness both with grain refining and
alloying effects.
Figure 4. Change of wear rate of alloys with SiC
addition
3.4. Worn Surface Investigations
Figure 3. Effect of SiC and Al content on hardness
3.3. Wear Tests
Worn surfaces of the specimens were investigated under
SEM to understand the wear mechanism. Representative
images of worn surfaces can be seen on Fig. 5. Wear
mechanism operated during the wear tests of specimens
were mainly abrasion. Small amount of plastic
deformation was also reported especially on the edges of
the wear track. Another important finding was the cracks
formed on the worn surface during wear tests. The reason
of these cracks is the low deformation ability of
magnesium because of its hexagonal lattice structure.
Weight loss data of the specimens were obtained by
weighing cleaned specimens before and after wear tests.
Weight loss data is used to calculate wear rate values of
the specimens. Variation of wear rate values of alloys
with different SiC addition is given in Fig. 4. As can be
seen on the image wear tests results are in good harmony
with hardness test results. With increasing hardness wear
rate of alloys decreases resulting with a lower wear rate.
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[3] Y. C. Lee, A. K. Dahle, D. H. StJohn, Meallurgical
and Materials Transactions A, 31A (2000) 2895-2906
[4] H. Friedrich, S. Schumann, Journal of Materials
Processing Technology, 117 (2001) 276-281.
[5] K. B. Nie, K. K. deng, X. J. Wang, W. M. Gan, F. J.
Xu, K. Wu, M. Y. Zheng, Journal of Alloys and
Compounds, 622 (2015) 1018-1026.
[6] C. Taltavull, A. J. Lopez, B. Torres, J. Rams, Surface
& Coatings Technology, 236 (2013) 368-379.
Figure 5. SEM images of worn surface of Mg-3Al alloy
with 0,6 SiC addition in different magnification
[7] H. Li, S. Lu, W. Qin, L. Han, Z. Wu, Acta
Astronautica, 116 (2015) 126-131.
4. Conclusions
[8] Y. Liu, B. Jin, D. J. Li, X. Q. Zeng, J. Lu, Surface &
Coatings Technology, 261 (2015) 219-226.
Effect of SiC on grain size, hardness and wear properties
of three different magnesium alloys were investigated in
this study. Fallowing conclusions were drawn according
to the results of the experimental work.
•
•
•
•
Addition of SiC to pure Mg and Mg-Al alloys
decreases grain size. After 0,4 % addition grain
refining effect becomes less effective.
Grain refining of pure Mg and Mg-Al alloys
results with an increase in the hardness of
material. The increment of hardness is much
higher in pure Mg, because of higher grain
refining effect.
Increasing hardness with grain refining also
increases wear resistance of alloys. Wear rate of
all alloys were decreased with SiC addition.
Main wear mechanism was abrasion but crack
formation and propagation was also reported on
the worn surfaces of the specimens.
[9] Y. Mao, Z. Li, K. Feng, X. Guo, Z. Zhou, J. Dong, Y.
Wu, Applied Surface Science, 327 (2015) 100-106
[10] R. Yamanolu, E. Karakulak, A. Zeren, M. Zeren,
Materials ad Design 49 (2013) 820-825.
[11] D. H. StJohn, M. Qian, M. A. Easton, P. Cao, Z.
Hildebrand, Metallurgical and Materials Transactions A,
36A (2005) 1669-1679.
Acknowledgment
Authors thank to The Scientific and Technological
Research Council of Turkey (TUBITAK) for their
financial support under 2219 program.
References
[1] A. A. Luo, Journal of Magnesium and Alloys, 1
(2013) 2-22.
[2] C. Taltavull, P. Rodrigo, B. Torres, A. J. Lopez, J.
Rams, Materials and Design 56 (2014) 549-556.
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