Fabrication and hardness investigation of Al-15%Mg2Si-3%Cu in-situ cast composite

1. Fabrication and hardness investigation of Al-15%Mg2Si-3%Cu
in-situ cast composite
Hossein Ramezanalizadeh and S. Reza Iyzi
Department of Materials and Polymer Engineering, Hakim Sabzevari University, Sabzevar, Iran
ABSTRACT
The goal of this study was production of an Al–15%Mg2Si metal
matrix composite (MMC) containing 3 wt.% copper and also study
microstructural and hardness properties. For this, pure Al, Mg, Si
and Cu ingots were used to make composite via in situ route. The
composite was characterised by X-ray diffraction, optical micro-
scope (OM) and scanning electron microscope equipped with
energy-dispersive X-ray spectroscopy (EDX) analysis.
Microstructural observations explored that Mg2Si particles form all
over the Al matrix. According to the OM characterisation, the size of
primary Mg2Si particles was estimated to be ~20 μm. This amount
could be decreased to ~7 μm (65% reduction) due to the presence
of copper, mostly because of the formation of Cu-rich intermetallic
phases during solidification, as detected by EDX analysis. Further
results showed that these phases lead to an MMC with more hard-
ness compared to the counterpart without Cu. In addition, because
of Mg2Si phases in these systems which are even lighter than Al, it
has high specific stiffness and strength. Therefore, it could be
widely used as a lightweight material in aerospace and automobile
fields, such as safety-belt pretensioner, steering wheel frame, cross-
beams, wheel rims, car frame and shock absorbing cover.
ARTICLE HISTORY
Accepted 24 March 2021
KEYWORDS
Metal matrix composites
(MMCs); hardness; Al; Mg2Si;
Cu-intermetallics
1. Introduction
Recently, the pollutants entering from fuel consumption have become a serious problem for
environments. One of the best solutions for reducing fuel consumption in the car and
aerospace industry is weight loss as it has taken a diversity of research areas [1–3]. In addition,
one of the most important factors for selection of a metal as a matrix is lightness and so
aluminium due to low density could be suitable for use as matrix in metal matrix composites
(MMCs) [4]. On the other hand, due to the favourable properties such as low density, nice
cast ability, good tribology resistance and low costs of final counterparts of aluminium matrix
composite reinforced with Mg2Si particles, they are increasingly considered as good candi-
dates for MMCs. It should be noted that Mg2Si compound is known as a hard intermetallic
with a high melting temperature (1085°C), low density and low coefficient of thermal
expansion (CTE) along with an advisable high elastic modulus. Therefore, that is why it
has been made a good candidate as a reinforcing agent [5]. It is reported that there are many
CONTACT Hossein Ramezanalizadeh h.ramezanalizadeh@hsu.ac.ir Department of Materials and Polymer
Engineering, Hakim Sabzevari University, Sabzevar, P.O. Box: 397, Iran
ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES
https://doi.org/10.1080/2374068X.2021.1909331
© 2021 Informa UK Limited, trading as Taylor & Francis Group

2. similarities between Mg2Si and Si, and between Al–Mg2Si and Al–Si systems from point of
view of properties and solidification behaviour. Figure 1 depicts the equilibrium phase
diagram of Al–Mg2Si [6]. According to Figure 1, during solidification of Al–15%Mg2Si
compound, Mg2Si intermetallic is created as the initial phase. Then, α-Al and secondary
Mg2Si phases simultaneously form from the liquid phase in the ternary phase region. This
pseudo-eutectic reaction is completed at 583.5°C.
The whole reactions from one-phase region of melt to ambient temperature are as
follows:
L ! L1þMg2SiP ! Mg2SiPþ Al þ Mg2Si
E
(1)
where E denotes a eutectic, P refers to a primary and L1 represents a liquid in a two-phase
area.
Some benefits are enumerated for in situ process of production of MMCs, such as
uniform dispersion of second phase, well reinforcement matrix, thermodynamically
stable condition and much lower prices of fabrication compared with their parts from
ex situ processes [7]. Due to coarse primary Mg2Si phase, MMCs reinforced by Mg2Si
usually have weak mechanical properties. It is therefore critical to refine Mg2Si by adding
alloying elements like Sc [8], Ce [9] and Gd [10]. The goal of this research is fabrication of
Al–15%Mg2Si–3%Cu in situ composite and investigation of Cu on the microstructural
and hardness properties.
Figure 1. Pseudo-binary phase diagram of Al–Mg2Si [6].
2 H. RAMEZANALIZADEH AND S. R. IYZI

3. 2. Experimental procedure
The Al–15% Mg2Si–3%Cu composite ingot was produced by using industrially pure
metals (Al, Mg, Si and Cu) as starting materials. All materials were heated in an electrical
resistance furnace using a 6 kg SiC crucible. Due to the importance of elemental loss
during the preparation of the melt, amount of weight loss was selected to be 5, 5, 10 and
15% for Al, Cu, Si and Mg, respectively. Note that the amount of weight loss for Mg
(15%) was due to the high amount of oxidation of this element in the temperature
range used for preparation of the melt. First, pure Al (~2 kg) was melted at 750ƕ
C in
crucible. Then, Si was added to the melt to try to submerge its pieces. After 10 min, Mg
was added to the melt and the solution was hand stirred with a graphite rod for about
1 min to ensure complete mixing. After stirring and cleaning off the dross, molten
MMCs were poured into the cast iron mould (45 mm diameter and 70 mm height,
Figure 2) and left to cool in the air. This sample was considered as base MMC (Cu-free)
sample.
For preparation of Al–15% Mg2Si–3%Cu, after doing the mentioned steps and before
casting, pure Cu (99.99%) was added in small increments to the melt. In addition, 10 min
after Cu addition (3.0 wt.%) the molten MMC was hand stirred for 1 min and finally
poured to mould. For achieving near full density, the cast samples were then subjected to
extrusion procedure in 500ƕ
C and 6:1 ratio. Table 1 shows the chemical composition of
hypereutectic Al–15% Mg2Si-3%Cu in situ (MMCs).
Figure 2. Schematic drawing of cylindrical cast iron mould.
Table 1. Chemical composition of Al–15% Mg2Si–3%Cu (wt.%).
Materials Si Mg Fe Ni Zn Mn Cu Ti Cr
Al–15%Mg2Si 5.71 9.85 0.14 0.01 0.01 0.01 3.1 0.01 0.01
ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 3

4. The sample surfaces from perpendicular section to extrusion direction were selected
for microstructural studies. The cut sections were polished and then etched by HF (1%)
to study the structure. Quantitative data on the microstructures were determined using
an optical microscope equipped with an image analysis system (Clemex Vision Pro.
Ver.3.5.025). Additional microstructural characteristics of the specimens were examined
by scanning electron microscope (SEM) equipped with the energy-dispersive X-ray
analysis (EDX) accessory. Phase composition of samples was also evaluated by X-ray
diffractometer (XRD) (Philips PW-1730) using Cu-Kα radiation.
3. Results and discussion
3.1. Microstructural characterisation
The typical as-cast microstructure of Al–15% Mg2Si composite is shown in Figure 3. It is
clear from the phase diagram (Figure 1) that the composition of the sample is located in
the hyper section of the diagram. This means that the microstructure consisted of
primary Mg2Si particles in a matrix of α-Al and pseudo-eutectic cells (Figure 3). In
addition, according to the quantitative investigations from many pictures like Figure 3,
the size of primary Mg2Si particles was estimated to be almost 20 μm.
Figure 4 shows a SEM picture from microstructure of Al–15% Mg2Si composite. As
seen clearly from this figure, there are three discriminate parts on the base of colour:
dark, grey and bright. As mentioned before, the microstructure of Al–15% Mg2Si
composite consists of primary Mg2Si, α-Al grains in a matrix of Al–Mg2Si eutectic
cells. On the other hand, primary Mg2Si particles will act naturally as heterogeneous
sites for the nucleation of α-Al in order to decrease the interfacial energy [5]. Therefore, it
Figure 3. Optical image of Al–15% Mg2Si composite.
4 H. RAMEZANALIZADEH AND S. R. IYZI

5. could be concluded that the dark faceted particles are primary Mg2Si which is sur-
rounded by a layer of bright α-Al and the grey part is a matrix of Al–Mg2Si eutectic cells.
Figure 5 shows the XRD pattern of the as-cast Al–Mg2Si–Cu composite. The result
reveals that the components of the composite consist of Al, Mg2Si and MgO phases, as
expected. The lack of Cu phases in XRD graph is due to the small amount that XRD
method is unable to characterise them in this condition [11,12].
Figure 4. A backscatter SEM image of Al–15% Mg2Si composite.
Figure 5. XRD pattern of as-cast Al–Mg2Si–Cu composite.
ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 5

6. Figure 6 is a SEM image of MMC containing 3 wt.% Cu. As is clear from this figure,
with the addition of Cu, the microstructural changes were found to be associated with the
formation of new intermetallic phase. To recognise this intermetallic, EDX analysis of the
sample containing 3% Cu was redirected (Figure 6(b)).
Qin et al. [13] have also reported that Cu may have an effect on the relative content of
Mg2Si due to a change in the synchronise equilibrium phase fields. Copper is now used as
a vital alloying element in Al alloys, depending on its amount, and many intermetallic
phases have been reported in the Al–Mg–Si–Cu alloy systems [14]. In addition, Q phase
is the quaternary intermediate phase, which has been given different descriptions and
forms with different stoichiometries [14,15]. Moreover, as shown in Figure 7, it exists in
all three tetrahedrons phase fields. The structure of Q phase is hexagonal type with lattice
parameters of c = 0.405 nm and a = 1.04 nm and 21 atoms in a unit cell [14]. The exact
syntax of Q phase is uncertain. However, it has been defined as Al5Cu2Mg8Si6, Al4CuMg5
Si4, Al4Cu2Mg8Si7 and Al3Cu2Mg9Si7 [14]. Q phase, which formed during solidification,
has a complex honeycomb-type morphology as shown in the backscattered electron SEM
image for a 2014 alloy (Figure 8). It is great to mention that the introduction of Cu to Al–
Mg–Si alloys not only creates the Q phase, but it also develops θ phase (Al2Cu) with
body-centred tetragonal (BCT) structures [14].
3.2. Hardness properties
To study the mechanical properties of Al–15%Mg2Si with addition of 3 wt.% Cu, the
Brinell hardness test was done and measurements showed an 85 HBN amount for
Al–15%Mg2Si–3%Cu, which was more than a 70 HBN level for Al–15%Mg2Si
without Cu. The formation of intermetallics (Q and θ phases) in the microstructure
and its refining is recognised to be an important factor on hardness enhancement. It
should be mentioned that these phases have high hardness and high elastic modulus
values. Therefore, these intermetallics are assumed to be strong reinforcements in
the composite [16–18]. As mentioned earlier, the size of primary Mg2Si particles
decreases by addition of Cu. Lowering the reinforcing particle size at a constant
Figure 6. A backscatter SEM image of Al–15% Mg2Si–3% Cu composite, showing Cu intermetallic
(white colour phase) and (b) corresponding EDX analysis of Cu-containing intermetallic.
6 H. RAMEZANALIZADEH AND S. R. IYZI

7. amount decreases the distance between them. This behaviour is described by
Equation (2). According to this equation, as the reinforcement particle size
decreases, the distance between the particles will also decrease [18].
Figure 7. Line diagram of stable equilibrium phase fields in Al–Mg–Si–Cu system at room temperature
[14].
Figure 8. SEM backscattered electron images of 2014 ingot sample showing the honeycomb-type
structure of the Q phase [14].
ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 7

8. λ ¼
4ð1 f Þr
3f
(2)
where λ is the distance between the reinforcement particles, f is the particle volume
fraction and r is the particle radius, assuming them spherical. In other words, according
to Equation (3) decreasing the distance between the Mg2Si particles will increase the
required tension for dislocations movement between them, leading to an increase in the
composite strength.
τ0 ¼
Gb
λ
(3)
where τ0 is the required tension for forcing dislocations to move among reinforcement
particles, G is the material’s elastic modulus and b is the Berger’s vector [18].
This phenomenon is also explained by the Halle–Petch relationship.
σ0¼σiþKD1=2
(4)
where σo is the flow stress, σi is the stress opposing the movement of dislocations, K is
constant and D is the grain size. According to Equation (4), as the grain size becomes
smaller, flow stress also increases, leading to high strength in the composite [18]. It
should be mentioned that there is a direct relationship between strength and hardness of
a material.
4. Conclusion
Ɣ An Al-based composite containing 15%Mg2Si was fabricated in an in situ manner.
Ɣ The initial microstructures consisted of coarse primary Mg2Si particles (~20 μm)
surrounded by α-Al and pseudo-eutectic cells.
Ɣ Addition of 3 wt.% Cu resulted in Cu-rich intermetallics which could reduce the size
of primary Mg2Si to ~7 μm.
Ɣ The hardness of composite could be improved by addition of Cu.
Disclosure statement
No potential conflict of interest was reported by the author(s).
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