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Effect of SiC Particles Treatment and Mg Additionon Improvement of Microstructural and MechanicalProperties of Al356/SiCp Composite Using SemisolidProcessHamidreza Ghandvara, Saeed Farahanya & Jamaliah Idrisa

a Department of Materials, Manufacturing and Industrial Engineering, Faculty of MechanicalEngineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, MalaysiaAccepted author version posted online: 18 Feb 2015.

To cite this article: Hamidreza Ghandvar, Saeed Farahany & Jamaliah Idris (2015): Effect of SiC Particles Treatment and MgAddition on Improvement of Microstructural and Mechanical Properties of Al356/SiCp Composite Using Semisolid Process,Materials and Manufacturing Processes, DOI: 10.1080/10426914.2015.1004687

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1

Effect of SiC particles treatment and Mg addition on improvement of

microstructural and mechanical properties of Al356/SiCp composite using semisolid

process

Hamidreza Ghandvar1, Saeed Farahany

1, Jamaliah Idris

1

1Department of Materials, Manufacturing and Industrial Engineering, Faculty of

Mechanical Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Malaysia

Corresponding author E-mail: hghandvar@yahoo.com, ghamidreza4@live.utm.my

Abstract

The effects of SiCp treatment and magnesium addition on microstructural and

mechanical properties of Al356/20wt. % SiCp semisolid composites were investigated.

The results showed that cleaning and oxidizing of SiCp and addition of 1 wt. % Mg

resulted in improving wettability, incorporation and uniform distribution of SiCp in A356

matrix. Consequently, ultimate tensile strength (UTS) value increased by 19% and 32%

when the SiC was treated and also Mg was added respectively compared to as received

SiCp. In addition, hardness value increased from 69.7 HV in as received SiCp to 94.8 HV

after SiCp treatment and addition of Mg.

KEYWORDS: Metal, matrix, composite, aluminium, wettability, casting,

microstructure, properties

INTRODUCTION

Aluminium Matrix Composites (AMCs) draw more and more attention to automotive and

aerospace industries because of their desirable properties, containing high specific

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strength, low density, excellent wear resistance, high specific stiffness, and controllable

expansion coefficient [1-3].Soft aluminium alloys can be relatively made highly resistant

by introducing predominantly hard but brittle particles such as SiC, Al2O3, B4C and CNT

[4,5] . A356 aluminium alloy is used comprehensively as the matrix in MMCs owning to

ease of handling, low cost, resistance to atmospheric corrosion and good strength and

ductility. SiC and Al2O3 particles are regularly used in the aluminium composites as

reinforcement phases [6, 7]. For fabrication of the MMCs; stir casting and compocasting

are generally accepted as encouraging routes and recently practiced commercially. Their

advantages implicate their simplicity, flexibility, and usability to large quantity

production as well as minimizing the final cost of the product [8-11]. Generally,

combination of the MMCs includes the insertion of the reinforcement material into the

melt with solidification controlling of the melt to achieve the preferred distribution of the

spread phase in the matrix.

The wettability of the reinforcement through the molten metal is especially important to

create a strong adequate interface. The interface between reinforcement and matrix plays

a critical role in mechanical behavior of metal matrix composites (MMCs). In MMCs the

reinforcements support most of the applied load while that of the matrix bind the

reinforcements together. Enhancement of wetting is required to provide greater

mechanical properties and to induce a strong interface to allow transmission of load from

the matrix to the reinforcements without failure [11]. In addition, efficient incorporation

of solid ceramic particles into the melt requires that the melt wets solid ceramic phase.

Unfortunately, in general, the wettability of ceramic particles with liquid aluminium

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alloys is insignificant. Subsequently, the insertion of reinforcement into a liquid matrix is

tough [11]. Moreover, large surface to volume ratio of very fine particles and differences

in density between reinforcement particles and matrix alloy favour their easy

agglomeration and clustering. Therefore, dispersion of the ceramic particles in the matrix

alloy is not uniform with liquid metal routes [12] that affect directly the quality and

mechanical properties of composites. In this respect, several methods have been adopted

to advance the wetting of reinforcement particles with molten matrix alloy, containing

pre-treatment of particles [13], ceramic particles subjecting to coating or oxidizing [14],

addition of some surface-active elements [15] into the matrix and utilizing nonpresure

infiltration [16]. Moreover, Flemings and other co-researchers [14], have also studies the

properties and application of semisolid rather than fully liquid alloy slurries. They

concluded that the introduction of non-metallic particles into partially solidified alloy

with high viscosity prevents the particles from settling, floating or agglomerating. Which

increasing the mixing time, the particles may interact with the liquid matrix to develop

bonding. However, there is still a need to investigate the effect of wettability

improvement of SiCp on mechanical properties of Al356/SiC semi solid composites. The

relevance of present paper is immense with respect to automotive industry as the typical

parts have included pistons, cylinder liners and connecting rods, etc... This study is

related to fabrication of A356-20 wt. %SiC in which the improvement of SiC

reinforcement wettability and corresponding mechanical properties by applying different

preparations during semi solid process has been investigated.

MATERIALS AND METHODS

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A356 Al-Si alloy with chemical composition given in Table 1 was used as the matrix

material. Thermal analysis was performed to determine characteristic temperatures of

A356 alloy for semi-solid process [17]. SiCp/A356 composites with 20 wt. % SiCp were

produced by semi-solid material (SSM) stirring technique. The process included

following steps: (1) Pieces of aluminium ingot were heated to approximately 750°C in an

electric furnace and then melt temperature dropped to retain the slurry in semi-solid state

(605±5°C). (2) SiC particles in the range size of 32-80 µm were added, whereas slurry

was stirring with speed of 600 rpm for 20 minutes using DC motor with stainless steel

stirrer [18]. (3) Composite slurry was reheated to 650°C and further stirred at 200 rpm for

about 10 minutes to particles from depositing. (4) The slurry was poured at 630±5°C into

a permanent mould to produce tensile bars. Three different mixtures (Table 2) were

prepared as follows:

Set A: As received SiC was used without any surface treatment and preheating.

Set B: SiC particles were dipped in acetone solution and placed inside the ultrasonic

cleaning bath, and washed with distilled water. Afterward, they were dried at 100°C for

24 h. The SiC particles were then heated to a temperature of 900°C within 2 hours in a

muffle furnace.

Set C: After cleaning and heating the SiCp (similar to set B), 1wt. % pure magnesium

(98.08%) was wrapped in an aluminium foil and added to the molten aluminium, and

mixture of aluminium, magnesium, and SiCp was mixed properly.

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Samples for microstructural analysis were cut from the runner of tensile bars mould.

Cross sections of specimens were grounded and polished by standard metallographic

procedures. Samples were observed by optical microscope (Nikon-LV150) and field

emission scanning electron microscope (Supra-35VP, Carl Zeiss, Germany) equipped by

Energy Dispersive Spectroscopy (EDS). Samples for tensile testing were produced by

permanent metal mould to fabricate the cylindrical tensile bars according to the ASTM

B-108 type. Tensile test specimens were prepared according to the standards (ASTM-

557M) with gauge length and diameter of 45 and 9 mm, respectively. Tensile testing was

accomplished on prepared test bars by an Instron universal mechanical testing machine

(model 5982). Tensile tests were performed at room temperature at cross-head speed of 2

mm/min equipped with a data-acquisition system. Hardness test was done by Vickers

micro hardness. Hardness value was taken from three sections of specimens to get more

accurate results. Applied load was 5kg for15s.

RESULTS AND DISCUSSION

Thermal Analysis

According to recorded cooling curve and its first derivative curve (Fig. 1a), it was found

that the liquidus and solidus temperatures of A356 alloy was 620ºC and 527ºC. The

variation of solid fraction at different stage is important to apply semi-solid process.

Since the solid fraction must be below 20-30%, 605ºC was selected for semi-solid

process (Fig. 1b).

Microstructure Analysis

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Optical micrograph of the Al356-SiCp (set A) specimen is shown in Fig. 2 (a). It shows a

low SiC incorporation within matrix and non-uniform distribution of SiC reinforcement

in A356 matrix. It can be observed that the pores are associated with clustered SiC

particles. It is worth noting that SiC particles are very susceptible to agglomeration and

clustering owning to their low wettability, their large surface to volume ratio and van der

Waals forces between them [11, 19]. Moreover, extensive particle free regions beside the

large SiCp clusters are good signs of low wettability with molten aluminium.

Fig. 2 (b) shows microstructure of Al356-SiCp in that SiC particles are cleaned by

acetone and heated at 900°C for 2 hours (set B). In comparison with Fig. 2(a),

distribution uniformity of particles got better in composites produced with pre-treated

particles. This improvement is most likely associated to the succeeding causes: (1)

Acetone removes impurities which are on particle surface, leads to wettability

improvement between SiC particles and Al356 matrix alloy. Indeed, a surface cleaning

offers better chance for the interaction between melt and particle; it enhances the

wettability Furthermore, agglomeration of SiC particles in raw material is fragmented by

ultrasonic vibration through ultrasonic cleaning procedure that improves incorporation

and distribution uniformity of reinforcement in composites. (2) Heating operation can

create a film of SiO2 on surface of SiC particle that reacts with Aluminum to form Al2O3

during stirring through the following reaction [11, 20]:

2 2 34Al 3SiO 2Al O 3Si (1)

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Silica layer improves wettability through aluminum that results from the reaction

between aluminum and SiO2 layer [21]. Although distribution of SiC was better than

untreated sample, distribution of SiC particles was not uniform throughout the matrix.

This might be related to float of particle on the top or gathered at the bottom of crucible

during stirring, it was found that after pouring, some portion of particles still gathered at

bottom of crucible.

Fig. 2(c) shows optical micrographs of Al356/SiC composite in that magnesium element

is added, and SiC particles are cleaned by acetone and heated at 900°C for 2 hours

(similar to set B). It can be seen that incorporation of SiC particles into the melt and

distribution homogeneity of composites improved. This observation is in good agreement

with [22] that the Mg increases the wettability between matrix and reinforcement.

Furthermore in the study which is carried out by Ghazi [23], although the SiCp treatment

is not adopted, Mg addition to matrix alloy improved wettability and facilitated

homogeneous particle distribution. The addition of Mg into aluminum melt progresses

wetting due to lower surface tension of Mg (0.599 N/M) compared to that of aluminum

(0.760N/M) [24]. Furthermore, Mg addition reduces solid-liquid interfacial energy

through forming new MgAl2O4 compound at Al/SiC interface that can improve

wettability. Oxygen is on particle surface, Mg as a powerful scavenger of oxygen reacts

with oxygen on the surface of particles, diminishing gas layer and developing the

wetting, and reducing agglomeration susceptibility [22]. After pouring composite melt, it

was found that most of SiC particles were incorporated in the matrix. It means that less

particles were left at the bottom of crucible.

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Mechanical Properties

Hardness

Hardness values of different fabricated A356-SiC composite are presented in Fig. 3. As it

can be seen, Al/as received SiC composite shows the lowest value (69.7 HV) in

comparison with Al/treated SiC composite. In addition, composite manufactured by

A356-(SiC treated-Mg) have greater hardness (94.8 HV) than those fabricated by pre-

treated and untreated SiC particles. Uniformity of SiCp distribution has an important

effect on hardness of composites as explained by Jamaati et al. [25]. During hardness test,

limitations to plastic deformation of matrix depended on distribution of SiC particles in

matrix. Another factor that affected hardness of Al356/SiCp composites was extent of

SiCp incorporation into matrix. Due to wettability improvement through the addition of

the Mg, increasing incorporation of SiCp into the melt increased amount of SiC particles

in matrix. This leads to increased dislocation density owning to thermal expansion

mismatch among matrix and reinforcement at interfaces between particles and matrix

[26]; it led to enhanced hardness. However, as SiCp incorporation to the melt increased,

distance between particles decreased. The requisite tension increased for dislocations

movement among SiC particles through plastic deformation caused by hardness test.

Tensile Properties

A stress-strain curve obtained for three different A356/SiC composites is shown in Fig. 4

(a). Results showed that Ultimate Tensile Strength (UTS) of A356/SiCp composite

produced with pre-treated SiC particles were clearly improved (96.3 MPa) compared to

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untreated SiC (81MPa) due to better wettability and distribution of pre-treated SiCp with

Aluminum matrix (Fig. 4(b)). Attendance of porosity and clustering of SiC particles in

A356/as received composites with untreated SiC is supposed to be liable for their weak

tensile properties. Such deficiencies operate act as decohesion nucleation sites or crack

leading to fail the composite at low stress levels [27]. It is evident to Fig. 4(a) that UTS of

composite fabricated with pre-treated SiC, and magnesium addition increased to 107.2

MPa that was higher than that of composites made by untreated and pre-treated SiC,

respectively. Concerning elongation to fracture (Fig. 4(b)), results showed that elongation

increased from 0.9% to 1.4% with pre-treatment process. Moreover, addition of Mg to

pre-treated SiC increased value of elongation to 1.5 %. Strengthening mechanisms might

be related to the following three major effects which are enhanced by greater wettability

(incorporation) of SiCp with matrix (Fig. 2(c)) [19, 28, 29]:

1. Presence of SiC particles in the soft Al356 matrix generates dislocation pile-up in their

neighborhood. Therefore, dislocation density and dislocation–dislocation interactions in

the matrix near the matrix-reinforcement interfaces increases and enhance the strength of

the composite.

2. During the tensile test, SiC particles act as barriers to dislocation movement, further

enhancing the strength.

3. With addition of Mg; interfacial bonding between matrix and SiC particles is

enhanced, resulting in increase of interfacial bonding strength and optimized transferring

load from matrix to SiC particles.

Fracture Analysis

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Elemental mapping image of fracture surface of Al356-(SiC treated-Mg) (Fig. 5) revealed

that SiC reinforcements occupied a large area of fracture surface of composite. FESEM

micrographs of tensile fracture surface features of A356/SiC composite fabricated with

untreated SiC particles are presented in Fig. 6(a)-(c). Porosity on fracture surface is

clearly observed in Fig. 6 (a) that is consistent with optical micrographs observed for

A356/as received SiC composite in Fig. 1. Fig. 6 (b) shows that SiC particles

agglomerate in some area on fracture surface. This accompanies with low strength and

elongation of composite as before was illustrated in Fig. 4. Clustering of particle

increases local stresses, and offers sites for crack nucleation and low-energy propagation

paths across the joined brittle particles [30]. Cracks initiated from agglomerated SiC

particles are marked by an arrow in Fig. 6(c). Failure can be described by higher stresses

created in these areas. Nevertheless, matrix as expected undergoes the mixed fracture due

to refining the eutectic Al-Si through fast cooling rate in permanent mould casting (Fig.

6(c)).

Figs. 6(d)-(f) show fracture surface of composites fabricated by pre-treated SiC particles.

There are only some micro-pores on fracture surface (Fig. 6 (d)). Difference in fracture

surface in comparison with A356/as received SiC composite is predominately attributed

to different particle spreading and no agglomeration area (Fig. 6(e)). This is compatible

with tensile properties (Fig. 4), indicating that uniform distribution of particle can

reinforce matrix alloy because of effective prevent of crack propagation by particle-rich

region, but particle-poor region proffers canals for crack propagation [31]. Large

concentration on SiC particles is shown in Fig. 6(e), suggesting a pushing mechanism of

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the reinforcement to grain boundaries during solidification [32]. Matrix fractured in

ductile-brittle mode is shown in Fig. 6(f) where cleavage planes are observed. Fracture

surface of Al356-(SiC treated-Mg) composite is shown in Figs. 6(g)-(i). Some micro-

pores are observed on fracture surface (Fig. 6(g)). However, both of area fraction and size

are lower than that of Al-as received SiC and Al-pretreated SiC composites. It can be

seen that composite mostly fractures from matrix; however, decohered SiC reinforcement

is observed (Fig. 6(h)). In fact, strength of the interface of SiC reinforcement and the

matrix is relatively good. Cleavage planes are observed on the fracture surface.

Moreover, sign of ductile fracture and dimples are detected at high magnification as

shown in Fig. 6(i). Therefore, matrix fractures in ductile-brittle mixed mode.

Fracture profile of A356/as received SiC composite observed under optical microscope is

presented in Fig. 7(a). Presence of pores and agglomerated SiC in A356/as received SiC

composite also facilitate brittle intergranular crack growth (Fig. 7(a)). Main crack crosses

a two-phase zone in eutectic Al-Si. Fig. 7(b) shows cross-section of failed A356/pre-

treated composite. As it can be seen, A356/pre-treated composite failed in transgranular-

intergranular mode [33]. Fracture profile of A356-(SiC treated-Mg) composite is shown

in Fig. 7(c). It can be observed that more SiC incorporate in fractured surface. It may be

assumed that in the A356-(SiC treated-Mg) composites, most of the reinforcing particles

are engulfed by the growing dendrites because of their improved wetting through the

melt. The engulfed particles form stronger interfaces with the matrix and can resist larger

stresses and strains before a decohesion mechanism is activated to pull the particles and

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the matrix apart. Thus, these composites are less prone to attack to intergranular crack

propagations.

CONCLUSIONS

20wt. %SiCp/Al356 composites were produced by semi-solid stirring technique with

three different conditions namely (a) as received SiC, (b) cleaned and heated SiC, and (c)

cleaned and heated SiC as well as the addition of Mg. Microstructural and mechanical

properties of fabricated composites were examined. Main conclusions can be stated as

follows:

1) SiC particles pre-treatment improved wettability among SiC particle and Al356

alloy that helped in realizing particle distribution.

2) Distribution uniformity of SiCp particles in matrix was further improved by

superior wettability between reinforcement and matrix and enhancing interface bonding

strength with the addition of 1% Mg into the melt.

3) Tensile strength, elongation, and hardness of composite with addition of 1% Mg

were higher than pre-treated and untreated SiC/Al composites.

ACKNOWLEDGMENTS

The authors would like to thank Universiti Teknologi Malaysia (UTM) for provision of

research facilities.

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26. Amirkhanlou, S.; Niroumand, B. Fabrication and characterization of Al356/SiCp

semisolid composites by injecting SiCp containing composite powders. Journal of

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27. Amirkhanlou, S.; Rezaei, M.R.; Niroumand, B.; Toroghinejad, M.R. High-strength

and highly-uniform composites produced by compocasting and cold rolling processes.

Materials and Design 2011, 32(4), 2085-2090.

28. Beffort, O.; Long, S.; Cayron, C.; Kuebler, J. Alloying effects on microstructure and

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by squeeze casting infiltration. Composites Science and Technology 2007, 67(3), 737-

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29. Jayakumar, K.; Mathew, J.; Joseph, M.A.; Suresh Kumar, R.; Shukla, A.K.; Samuel,

M.G. Synthesis and Characterization of A356-SiCp Composite Produced through

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30. Barekar, N.S.; Tzamtzis, S.; Babu, N.H.; Fan, Z . Processing of ultrafine-size

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32. Schultz, B.F.; Ferguson, J.B.; Rohatgi, P.K . Microstructure and hardness of Al2O3

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33. Casari, D.; Fortini, A.; Merlin, M. Fracture behaviour of grain refined A356 cast

aluminium alloy: tensile and Charpy impact specimens; Convegno Nazionale IGF XXII,

Rome, Italy, July 1-3, 2013.

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Table 1 Chemical composition of the A356 aluminum alloy

Element Si Fe Cu Mn Mg Cr Ni Zn Ti Pb Sn Al

Wt.% 7.07 0.20 0.034 0.017 0.410 0.034 0.003 0.014 0.070 0.002 0.000 Bal.

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Table 2 Materials for wettability experiments

Sample Composition

A Aluminum+20%SiC(as received condition)

B Aluminum+20%SiC(Surface treated)

C Aluminum+20%SiC(Surface treated)+1%Mg

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Figure 1 Cooling and first derivative curves of A356 alloy (a), and variation of solid

fraction as a function of temperature for A356 alloy (b).

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Figure 2 Microstructure of cast A356/SiC composite: (a) without treatment, (b) the

cleaned and heated SiC, and (c) added the magnesium in addition to the cleaned and

heated SiC.

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Figure 3 Hardness values for different A356/SiC composite specimens.

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Figure 4 (a) Stress-strain curves (b) variation of ultimate tensile strength (UTS) and

elongation to fracture for different fabricated composites.

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Figure 5 Elemental mapping of fracture surface of A356-(SiC treated-Mg) composite.

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Figure 6 Fracture surfaces of (a, b, c) the A356/as received SiC, (d, e, f) the A356/treated

SiC, and (g, h, i) the A356-(SiC treated-Mg) samples failed in tensile test.

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Figure 7 Fracture profile of specimen after static tensile test: (a) A356/as received SiC,

(b) A356/treated SiC, and (c) A356-(SiC treated-Mg).

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