influence of magnesium on structure and properties of al-si alloy
TRANSCRIPT
Influence of Magnesium on Structure and Properties of Al-Si Alloy
M. M. Haquea,A. A. Khanb
Department of Manufacturing and Materials Engineering, Kulliyyah of Engineering International Islamic University Malaysia, Gombak, 53100 Kuala Lumpur, Malaysia
Keywords: Thermal, Intermetallic, Non-ductile, Deformation, Eutectic.
Abstract. Aluminium-silicon alloys have low density, high electrical and thermal conductivity and
high resistance to corrosion at ambient temperature. However, these alloys usually contain
numerous alloying and impurity elements, which consist essentially of a fairly ductile matrix of
alpha aluminium solid solution with a variety of non-ductile particles of silicon and various
intermetallic compounds. The shape and distribution of these constituents largely control the
deformation behaviour of the alloy. The addition of magnesium makes the alloys lighter and harder,
but its hardening effect is fully responsive only after proper heat treatment. Therefore, in the present
study, microstructures and properties of the alloys have been evaluated on the as-cast and heat-
treated conditions. Results show that the addition of magnesium to aluminium-silicon eutectic base
alloy refines microstructure up to certain level and increases the strength and hardness at the
expense of ductility.
Introduction
Aluminium-silicon alloys are very suitable for aerospace structural applications, automobile
industry and military applications. However, these alloys usually contain numerous alloying and
impurity elements, which consist essentially of a fairly ductile matrix of alpha aluminium solid
solution with a variety of non-ductile particles or networks of silicon and various intermetallic
compounds. The primary plate shaped silicon crystals are very hard and brittle. They do not respond
in the same way, as does the ductile aluminium matrix of the alloy. At the same time, the presence
of alpha-aluminium phase provides the necessary paths to easy deformation and shear [1]. Other
elements like copper, magnesium, iron and manganese form complex compounds such as CuAl2,
Mg2Si and Fe and Mn rich phases such as (FeMn)3Si2Al15. The shape and distribution of these
constituents largely control the deformation behaviour of the alloy [2]. The two most important
hardening constituents in aluminium-silicon alloys are the intermetallic compounds, magnesium
silicide (Mg2Si) and copper aluminide (CuAl2). On application of a suitable heat treatment, the
presence of either of these compounds in the alloy is capable of improving the mechanical
properties and exerts its own individual effects. The strength of the aluminium-silicon binary alloy
depends less on the composition than on the distribution and shape of the silicon particles as well as
other compounds or networks either primary or eutectic [3]. The large primary crystal has an
unfavourable effect on the castability as well as on the technological properties of aluminium-
silicon alloy [4]. However, the structure and properties of aluminium-silicon alloys are very much
dependent upon the composition, cooling rate, modification and heat treatment. The addition of
magnesium makes the alloy lighter and harder, but its hardening effect is fully responsive only after
proper heat treatment, since it is a useful solid-solution strengthening element [5]. Thus, in this
investigation, the effect of Mg addition in the as-cast and heat treated Al-Si alloy would be studied.
Therefore, the general theme of the present investigation is to observe the structure and properties
of aluminium-silicon alloys containing various amounts of magnesium in the as-cast and heat-
treated conditions.
Advanced Materials Research Vol. 23 (2007) pp 291-294Online available since 2007/Oct/03 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.23.291
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Experimental Details
In order to produce round cast to shape and size tensile test bars, green sand was used as main
moulding material. The moulding mixture consisted of natural sand having sub-angular grain shape
(A.F.S. Fineness No. 110) and 10% bentonite clay to which about 6% water was added.
The charge material was a basic Al-Si eutectic alloy, whose composition is given in Table 1. The
required amount of the charge material was kept in a mild steel crucible and heated in an electric
furnace (Nobertherm N81/13) for melting and superheating the alloy.
Table 1, Chemical composition (wt %) of Al-Si eutectic alloy.
Mg Si Cu Fe Mn Ni Sn Pb Zn Ti Al
0.10 12.20 0.10 0.51 0.29 0.01 0.01 0.01 0.01 0.02 Bal.
When the melt was superheated to about 750
o C, the crucible was taken out from the furnace and Mg
was added in to it. Pure Mg in the form of foil was heated in an oven at about 300o C and then it was
put in a pre-heated perforated mild steel box especially prepared for Mg addition. A long mild steel
handle was fixed with a circular mild steel lid, which was attached with the perforated box. The lid
covered the cylindrical crucible in such a way that the perforated box made a gap of about 12.0 mm
from the bottom of the crucible. At the same time, it immersed fully within the molten alloy and
made a gap of about 25.0 mm from the top. When Mg was added with the molten Al-Si alloy at 750o
C, there was an explosion inside the crucible and Mg mixed thoroughly with the molten alloy. The
lid covered the crucible in such a way that the molten metal could not come out from the crucible,
thus preventing any splitting of the molten alloy around it.
The molten alloy was then stirred and poured in to the green sand mould at about 700o C in a
continuous stream. After about 15 minutes, the mould was broken to get the tensile test specimens.
Several experiments were carried out with 0.1%, 0.2%, 0.3% and 0.4% Mg in Al-Si alloy. In order
to compare the densities of test pieces containing various magnesium contents, the densities of the
specimens were determined by the buoyancy method using an Oertling balance (accuracy of ±0.1
mg). The balance had computerized memory facility to calculate and display the density of each
specimen in gm/cc.
The full heat-treatment operations, i.e. solution and ageing/precipitation treatments were carried
out with 50% of the tensile specimens produced by casting process. The solution treatment was
performed in a furnace (CMTS Lab Furnace, type L3/250, electrically operated) at 535o C (±5
o C)
for constant 8 hours. The specimens were transferred from 535o C in to hot water at 60
o C for
quenching and were kept there for 15 minutes. They were then dried and kept at - 10o C for
overnight in the deep freezer. The specimens were dried again and heated in the electric furnace up
to 170o C (±2
o C) for constant 8 hours for ageing or precipitation treatment. They were then
removed from the furnace and cooled in normal room temperature [6]. The specimens were then
ready for testing and micro-examination. Tensile and hardness tests were conducted with Instron
Universal Testing Machine (Instron 5482) and Mitutoyo Digital Hardness Testing Machine
(Mitutoyo ARK 600), respectively. The metallographic specimens were polished in the usual
manner and final polishing was carried out with fine magnesia powder by hand over velvet cloth.
The specimens were then etched in 0.5% aqueous HF acid for about 20 seconds. A Universal
microscope (Olympus CK40M) having computerized monitoring system and digital photograph
taking arrangement was used for the examination and recording of the representative
microstructures.
Results and Discussion
Fig. 1(a) shows the as-cast structure of aluminium-silicon eutectic alloy containing 0.1% Mg. It
consists of α-aluminium, plate shape primary silicon and eutectic silicon needles. When it is heat
treated, the structure becomes finer compared to the as-cast one (Fig. 1e). However, it is noticed
from the Figs. 1(a) to 1(h) that as magnesium addition is increased to the alloy up to 0.3%, the
292 Materials and Technologies
structure becomes more finer. However, beyond that point, its refining action is reduced for both as-
cast and heat treated alloys. It can be seen from Figs. 1(d) and 1(h) that the refining action of Mg
addition to the alloy is fading more in the as-cast alloy compared to the heat treated alloy. These
structural changes have also been reflected in the properties of the alloys (Fig. 2).
(a) (b) (c) (d)
(e) (f) (g) (h)
Fig. 1, Microstructures of Al-Si eutectic alloy, as-cast (a - d) and heat treated (e - h).
It can be seen from Fig. 2 that as the magnesium content is increased in the alloy, the ultimate tensile
strength and hardness values are also increased at the expense of ductility for both as-cast and heat
treated alloys. The strength and hardness increments in case of heat-treated alloy are tremendous
compared to the as-cast alloys. In the present study, it was also observed that as magnesium content
increased in the alloy, the density decreased, making the alloy lighter, but harder. This is because;
small additions of magnesium to the alloy induce significant age-hardening through precipitation of
Mg2Si in the aluminium matrix [1]. However, the highest increment in strength and hardness was
achieved at 0.3% Mg and beyond that point, the rate of increment is less.
0
50
100
150
200
250
300
0.1 0.2 0.3 0.4
Magnesium (%)
UTS (MPa)
As-Cast
Heat treated
0
1
2
3
4
5
6
7
0.1 0.2 0.3 0.4
Magnesium (%)
Elongation (%)
As-Cast
Heat treated
0
20
40
60
80
100
120
0.1 0.2 0.3 0.4
Magnesium (%)
Hardness (HRB)
As-Cast
Heat treated
(a) (b) (c)
Fig. 2, Properties of as-cast and heat treated Al-Si alloy, (a) UTS, (b) Elongation and (c) Hardness.
The overall structures and properties of the aluminium-silicon eutectic base alloy containing various
amounts of magnesium show that the magnesium addition has refined the structures and thus, has
reduced the stress concentration regions of the alloy. This has markedly improved the strength and
hardness of the alloy for both as-cast and heat treated conditions. However, this structural
Advanced Materials Research Vol. 23 293
refinement did not improve the elongation or ductility of the alloy. The reason might be that the
alloy used in the present study contains numerous alloying and impurity elements (Table 1), which
form complex compounds and intermetallic phases, producing deleterious effects on the properties
of the alloy. Kim and Heine [7] report that the addition of 0.073% Mg gives rise to the formation of
primary silicon crystals in the aluminium-silicon eutectic alloy and they do not deform in harmony
with the matrix which results in lower ductility. It is also reported [8] that the elongation, as
measured by fit back varies inversely with increase in magnesium concentration to the alloy from
0.33 to 0.74 weight percent. At the same time, aluminium-silicon alloy containing more than 0.4%
Mg responded very poorly to the modification [9]. Again, the new concept proposed by Campbell
(10) is a defect structure in the alloy, in which the defects are constituted by fragments of the
entrained liquid surface. The liquid surface is commonly covered with an oxide, and the
entrainment mechanism is a folding action, the entrainment defects are double oxide films. They are
necessarily folded dry side to dry side, entrapping a layer of air, and thus experiencing no bonding
between the two films, known as bifilms, causing properties to fall significantly. Therefore, this may
be one of the probable reasons why the alloy in study containing 0.4% Mg showed inferior
properties compared to the lesser amount of Mg.
Conclusions:
The following conclusions can be drawn from the work of the present investigation:
(a) Magnesium addition to aluminium-silicon eutectic base alloy refines the microstructure up to
0.3% and beyond that point refining action starts to fade.
(b) Magnesium addition increases the strength and hardness of the alloy at the expense of
ductility for both as-cast and heat-treated conditions.
(c) Proper magnesium addition minimizes the sources of stress concentration regions by refining
the silicon phase either eutectic or primary, but the heat treated one responded more pronouncedly.
Acknowledgements
The authors are grateful to the Research Centre of International Islamic University Malaysia (IIUM)
for approving the project and sanctioning the necessary funds to carry out the present investigation.
Authors are also indebted to Dean, Kulliyyah of Engineering, IIUM for allowing them to use
Engineering Workshop and other facilities in the Kulliyyah.
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