International Journal of Mechanical Engineering and Technology (IJMET)
Volume 10, Issue 04, April 2019, pp. 661–670, Article ID: IJMET_10_04_064
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=4
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
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STUDY ON MORPHOLOGY AND MECHANICAL
PROPERTIES OF STOICHIOMETRICALLY
DEVELOPED AL-CU-MG CAST ALLOY
Rakesh Kumar and Varinder Sahni
Department of Mechanical Engineering
Sant Longowal Institute of Engineering and Technology, Longowal, INDIA
ABSTRACT
The present study was carried out to investigate the mechanical properties of
newly developed cast aluminum alloy with the (1.0 at. %) of copper and (1.0 at. %)
magnesium addition in the mole ratio of 1:1:1 of Al-Cu-Mg in as - cast and thermally
aged conditions. The atomic weight (at. %) selection of Cu-Mg on the basis of
stoichiometric calculations, 1 unit (at. %) of copper and 1 unit (at. %) of magnesium
preheated at 200C were mixed in liquid aluminium base material, heated in an
electric furnace and melting temperature was kept at 750C for about 10 min alone
and this melt was hold at 7300C with alloying additions of copper and magnesium for
about 30 min to ensure complete homogenisation, Further, this liquidous aluminium
metal matrix was stirred at 800 rpm for five minutes and poured at temperature
7000C 10 0C in permanent mild steel mould preheated at 2000C in order to achieve
as- cast aluminium alloy.
The solution treatment at temperature 5000C for 2h followed by a quenching in
water at 60 °C. Further, it was thermally aged at temperature 1600C for 6 h followed
by quenching at room temperature. The effect of solution and aged temperature on
mechanical properties by changing metallurgical morphology and further the role of
intermetallic compounds on mechanical properties of as-cast alloy have been studied.
The optical microscopy and scanning electron microscopy (SEM) equipped with
energy dispersive spectroscopy (EDS) were used to identify the intermetallic phases
and formation of different precipitates were studied by using X-ray diffraction (XRD).
The improvement in hardness values 35.55% at 1% Cu &Mg as-cast alloy with
solution temperature 500 0C,2h and 56.66 % have been recorded and reported at 1%
Cu &Mg as-cast alloy with 160 0C,6h.
Key words: 6xxx Al alloys, Mechanical properties, Solution treatment Temperature,
Thermal Aged temperature
Cite this Article: Rakesh Kumar and Varinder Sahni, Study on Morphology and
Mechanical Properties of Stoichiometrically Developed Al-Cu-Mg Cast Alloy,
International Journal of Mechanical Engineering and Technology 10(4), 2019, pp.
661–670.
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Study on Morphology and Mechanical Properties of Stoichiometrically Developed Al-Cu-Mg Cast Alloy
1. INTRODUCTION
Aluminum alloys are next to steels in use as space vehicles, aircrafts as well as many types of
surface and water borne vehicles [1]. In automobiles, the components such as engine blocks,
head, pistons and wheels etc. are generally aluminum based cast alloys [2]. The low cost and
continuous demands for weight reduction and improvements in fuel efficiency have increased
pace of research in developing aluminum-based cast alloys [3].
The Al-Cu-Mg alloys offer high hardness and strength. Its components contribute the high
degree of damage tolerance [6-8]. The first age-hardening of aluminum alloy as reported in
literature was performed by Alfred Wilm in year 1909 who patented duralumin of casting
components containing Cu and Mg substances [3]. The steps consist of solution treatment,
quenching and artificial aging mechanism is responsible for strengthening is based on the
formation of intermetallic compounds during decomposition of a metastable supersaturated
solid solution by performing solution treatment and quenching [10-13]. Additionally,
hardness and mechanical strength can be increased by aging heat treatment [14-15]
The composition of the alloying elements and casting conditions influence the state of
intermetallic phases and finally the mechanical properties of the alloy [ 16-19]. To improve
the mechanical properties of this aluminium alloys are frequently heat treated by a two-step
process i) solution heat treatment and ii) Thermal aging [20].
The copper and magnesium in combination have been used for improving the aging
characteristic of the alloys. Some of the investigators have taken compositions of alloying
elements in weight fraction or volume fraction but in arbitrary manner. They have used design
of experiments for material compositions as input parameters and different mechanical
properties as responses, and finally optimum values of alloying elements are suggested for the
given objective.
In place of taking the fraction of alloying elements in arbitrary manner, a pattern based on
stoichiometric weight fraction is explored and presented in the paper. Study has been made on
Al-Cu-Mg casting and results in terms of metallurgical and mechanical properties have been
presented. Therefore, this study undertakes the effect of the addition of up to1 at.% of Cu-Mg
and solution treatment time of 2 h on Al-Cu -Mg alloy to understand the resultant
microstructure and its respective hardness. The variation of microstructure, size, morphology
and distribution of precipitates, as well as Brinell Hardness are presented and discussed .
2. EXPERIMENTATION
2.1. Methodology
Initially to start with stoichiometric ratio of Al-Cu-Mg as 23.496 -55.338 -21.165. Such a high
percentage of Cu and Mg cannot be a suitable condition for formation of the Al based alloy,
hence 1 (at. %) each of Cu and Mg as per stoichiometric ratio were taken for mixing with the
base metal.
A graphite crucible of 2-kg capacity was used in electric resistance furnace, and the
melting temperature was kept at 750 0C [7] for 5 min. The Cu and Mg as decided above were
preheated at 200C for 30 minutes and mixed in liquidous aluminium metal and stirred for
five minutes. The melts were hold at 7300C for about 30 min [17] to ensure complete
homogenisation and poured after degassing into a permanent mould. It refines the
microstructure of metallic material which change the morphology and distribution of
intermetallic particulates to enhance the mechanical properties of as-cast aluminium alloy.
Finally, metallographic and mechanical properties of the as-cast alloy have been analysed.
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2.2. Metallographic tests
The cast specimens were polished as per standard metallographic procedure by using emery
paper of progressive finer grades 400,600,800,1000,1200,1500,2200,2500 and 3000 grit size
with single disk machine. They were polished with alumina powder to obtain a mirror like
surface by solvyt polishing cloth and continue water supply. Kroll’s etchant (Distilled water
(92 ml) +Nitric acid (6ml) +Hydrofluoric acid (2ml) was applied for 15 seconds to reveal
microstructure. Optical microscopy was used to capture microstructures of as-cast and
thermally aged aluminium alloy shown in figure-4. Fractography was done to determine type
of fracture for tensile and CVN samples using scanning electron microscopy (SEM) JEOL
(JSM-6510LV).
2.3. Testing for mechanical properties
Tensile specimen samples were tested on a servo hydraulic based digital controlled tensile
testing machine of having capacity 50 kN (make: Tinius Olsen, UK, Model-H50KS) and
ultimate tensile strength for as- cast alloy and thermally aged cast base alloys was determined.
The tensile specimens were prepared in accordance with ASTM E08/E8M-09 standard as
shown in Figure 3. The Charpy V-notch (CVN) test was done to measure the fracture
toughness of as-cast and aged as-cast material. The sample for Charpy V-notch test was
prepared as per ASTM E23-12c standard and shown in Fig. 2. Micro hardness measurements
were taken using Brinell hardness tester (make-: Miraj, Model-B3000(O) with 10 mm
diameter indenter at load 500kgf .
Figure 1. Test specimen extracted from as-cast plate as per ASTM standard
Figure 2. Impact Test Specimen
Figure 3. Tensile Test Specimen
3. RESULTS AND DISCUSSION
3.1. Spectro analysis
The chemical compositions were tested as per ASTM E 1251-17a using spark emission
spectrometer analysis (Model No: LMF-01, Spectromaxx, Make: Germany). The chemical
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Study on Morphology and Mechanical Properties of Stoichiometrically Developed Al-Cu-Mg Cast Alloy
compositions of as-cast alloy and thermally aged alloy were tested using spectrometer and
shown in Table 1.
Table 1. Alloying elements composition (wt.%)
1% Cu
&Mg ascast alloy
with
solution
temperat
ure 500
0
C,2h
1% Cu
&Mg ascast alloy
with 160
0
C,6h
Si
0.42
2
Cu
0.28
07
Mg
0.56
99
Fe
0.60
6
Zn
0.00
56
Ni
0.00
16
Mn
0.01
07
Cr
0.00
27
Ti
0.13
96
Sn
0.00
38
Pb
0.00
41
V
0.007
9
Al
97.9
3
0.47
3
0.31
44
0.61
14
0.64
2
0.00
70
0.00
24
0.01
09
0.00
26
0.15
36
0.00
51
0.00
44
0.008
3
97.7
5
The chemical composition 1% Cu &Mg as-cast alloy with solution temperature 500 0C,2h
and 1% Cu &Mg as-cast alloy with 160 0C,6h from spectrometer analysis on addition (1.0 at.
%) copper and magnesium in the base material, there is significant development of alloying
elements in aluminium metal matrix.
3.2. X-Ray Diffraction (XRD)
The X-ray diffraction (XRD) technique was used to show different intermetallic precipitate
formations in as-cast and thermally aged alloy which has been observed in XRD spectra as
shown in figure 4(a)&(b).
In as-cast base aluminium alloy, AlCu compound has less strength and Al12Mg17
compound contributes to hardness of as-cast alloy. However, when aging was done to 160C,
formation of Al5Cu6Mg, sigma phase takes place which increases the tensile strength of the
material. At the elevated temperature coarse precipitate AlCuMg was also formed. The aging
treatment dissolves the AlCu compound and forms complex precipitate compounds which
enhance the strength of the matrix.
The complex compounds occur and formation of coarse precipitates such as Al2CuMg and
Al14Mg13, which enhances the toughness and strength at high temperatures. Sigma phase
precipitation was noticed in as cast and aged aluminium alloyed with copper and magnesium
which enhances their strength as compared to cast base material.
Figure 4 (a)
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Rakesh Kumar and Varinder Sahni
Figure 4 (b)
Figure 4. Intermetallic precipitate formation as per XRD diffraction patterns for (a) 1% Cu &Mg ascast alloy with solution temperature 500 0C,2h (b) 1% Cu &Mg as-cast alloy with 160 0C,6h.
3.3. Optical Microscopy (OM) studies
a
b
d
c
Figure 5. Photomicrographs at 200X for (a) as-cast aluminium, (b) aged as-cast base aluminium, (c)
1% Cu &Mg as-cast alloy with solution temperature 500 0C,2h and (d) 1% Cu &Mg as-cast alloy with
160 0C,6h.
This microstructure consists primarily of α-Al with dendritic morphology, eutectic Si, AlCu phases. The photographs of microstructure of as-cast aluminium fig.(a), aged as-cast base
aluminium fig.(b), as-cast aluminium with 1% Cu, Mg fig.(c) and aged & as-cast aluminium
with 1% Cu, Mg fig.(d)are shown in Figure 5. There was negligible precipitate formation
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Study on Morphology and Mechanical Properties of Stoichiometrically Developed Al-Cu-Mg Cast Alloy
along the grain boundaries in as-cast aluminium but after aging, nucleation of precipitates at
most of the grain boundaries were noticed which enhances the strength. Alloying with copper
and magnesium leads to fine precipitate formation. However, as aging was done, these fine
precipitates transform into coarser precipitates at high temperature and nucleation at grain
boundaries were enhanced.
3.4. SEM-EDS Analysis
The Fe- SEM-EDS samples were analysis for 1% Cu &Mg as-cast alloy with solution
temperature 500 0C,2h and 1% Cu &Mg as-cast alloy with 160 0C,6h Mg has been revealed.
Element
Wt.%
At. %
Mg K
0.68
0.77
Al K
97.19
98.32
Cu K
2.13
0.91
Totals
100.00
Figure 6 .(a) SEM micrograph, EDS mapping and measurement of chemical composition
corresponding to solution treatment at 5000C,2h
Element
Wt.%
At. %
Mg K
0.73
0.82
Al K
96.42
97.95
Cu K
2.85
1.23
Totals
100.00
Figure 6. (b) SEM micrograph, EDS mapping and measurement of chemical composition
corresponding to aged temperature at 1600C for 6 h.
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Rakesh Kumar and Varinder Sahni
3.5. Tensile and microhardness studies
Tensile strength & microhardness values of as-cast metal at different stages are shown in
Table 2. Their tensile fractography are shown in Fig. 7. dimple morphology was observed in
the fractography. In as-cast base metal, small dimples comprising of precipitates were
observed but after aging of this as-cast base, the small dimples coalesce together to form large
dimples which contain hardened precipitates. These hardened participates contribute to
increase in ultimate tensile strength of the aged cast. Copper and magnesium act as inoculants
when alloyed with aluminium base metal and induce grain refining in matrix by nucleating
more nucleation positions for precipitation. Thus, precipitation hardening was induced in
alloyed metal matrix while casting which improves the ultimate tensile strength of cast metal.
The resulting improvement of the mechanical properties after heat treatment is correlated to
the interdendritic and the precipitation hardening as previously reported.
Table 2. Hardness and ultimate tensile strength values
S.
No.
1.
2.
3.
as-cast alloy stage
as-cast base aluminium
1% Cu &Mg as-cast alloy
with solution temperature
500 0C,2h
1% Cu &Mg as-cast alloy
with 160 0C,6h
Area of
tensile test
specimen,
mm2
36
36
Ultimate tensile strength
(N/mm2)
Hardness
(BHN)
140
157
33.5
52.80
36
192
77.30
a
b
Fine dimple
formation
Coarse dimple
formation
Figure-(b) Aged cast base Metal at 500X
Figure-(a) Cast Base Metal at 500X
c
d
Fine dimple
formation
Fine dimple
formation
Figure-(d) 1% Cu &Mg as-cast alloy with 160
0
C,6h at 500X
Figure(c) 1% Cu &Mg as-cast alloy with
0
solution temperature 500 C,2h
Figure 7. Tensile fractography at 500X for (a) as-cast aluminium, (b) aged as-cast base aluminium, (c)
1% Cu &Mg as-cast alloy with solution temperature 500 0C,2h and (d) aged as-cast base Aluminium
with 1% Cu, Mg.
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3.5. Impact studies
The results of impact strength in terms of joules of energy to estimate fracture toughness of
cast materials was shown in Tab.3. and their impact fractography are shown in Fig. 8.
Table 3. Impact strength values of cast base and Metal matrix alloy in as cast and aged condition
S. No.
1.
2.
3.
4.
as-cast alloy stage
as-cast base aluminium
Aged as-cast base aluminium
1% Cu &Mg as-cast alloy with
solution temperature 500 0C,2h
1% Cu &Mg as-cast alloy with 160
0
C,6h
Impact Strength (joules)
15
18
34
14
b
a
Coarse dimple
formation
Figure-(a) as-cast base Metal at 500X
Figure-(b) Aging cast base Metal at 500X
d
c
Fine dimple
morphology in metal
matrix alloy
Figure-(c) 1% Cu &Mg as-cast alloy with solution
0
temperature 500 C,2h
0
Figure-(d) 1% Cu &Mg as-cast alloy with 160 C,6h
Figure 8. Impact fractography at 500X for (a) as-cast aluminium, (b) aged as-cast base aluminium, (c)
as-cast aluminium with 1% Cu, Mg and (d) aged as-cast base Aluminium with 1% Cu, Mg.
In cast base metal, dimple fracture was noticed which corresponds to ductile fracture.
When alloyed with copper and magnesium, cast aluminium base metal possess very small
dimples (Fig. 8.c&d) because of refinement of grain structure of base metal which ultimately
improves the fracture toughness of material.
4. CONCLUSIONS

When copper and magnesium were added in as-cast base aluminium change in wt.% were
noticed as per spectroscopy analysis. These elements enhance intermetallic precipitate
formation at the grain boundary of cast base aluminium alloy.

Significant improvement in tensile strength, impact strength and microhardness were observed
in copper and magnesium alloyed as-cast alloy and thermally aged condition. The hardness
values improved 35.55% BHN for 1% Cu &Mg as-cast alloy with solution temperature 500
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0
C,2h 36.55% and increased 56.66% BHN for 1% Cu &Mg as-cast alloy with 160 0C,6h. This
is because of increase in size of dimples and increased nucleation of precipitates in aluminium
alloy.
ACKNOWLEDGEMENTS
This work was carried out with the financial support from Sant Longowal Institute of
Engineering and Technology, Longowal under Ministry of Human Resource Development,
Government of India. The XRD facilities of IIT Ropar and SEM facilities of IIT Roorkee
&SAIL, Thapur University Patiala were used to record metallurgical morphology.
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