The mechanical response of Al/Si/Mg/SiCp composite: influence ofporosity

C. Tekmen a,*, I. Ozdemir a,b, U. Cocen a, K. Onel a

a Department of Metallurgical and Materials Engineering, Faculty of Engineering, Dokuz Eylul University, Bornova, Izmir 35100, Turkeyb Material Process Laboratory, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku 468-8511, Japan

Received 10 February 2003; received in revised form 2 June 2003

Abstract

The effect of porosity on the mechanical and fracture behaviour in Al/Si matrix alloy and composites reinforced with SiCparticles of 10 and 20 vol.% in the as-cast state and after extrusion process has been studied. Matrix alloy and composites were

fabricated by compocasting and extrusion. Samples were characterized by optical microscopy, image analyzer, scanning electron

microscopy and tensile tests. The results demonstrate that hot extrusion considerably reduces the porosity, while size and

# 2003 Elsevier B.V. All rights reserved.

Materials Science and Engineering A360 (2003) 365/371

www.elsevier.com/locate/mseaKeywords: Metal matrix composite; Porosity; Strength; Extrusion

1. Introduction

The use of Al/Si alloys in the manufacture ofautomotive engine components, such as cylinder blocks,

cylinder heads, pistons and piston rings, has dramati-

composites [8]. Porosity formation can be attributed to

two factors: (a) shrinkage coupled with a lack of

interdentritic feeding during mushy zone solidification

and (b) evolution of hydrogen gas bubbles due to a

sudden decrease in hydrogen solubility during solidifica-distribution of the reinforcement particles are also affected. In the point of fracture behaviour, the existence of large porosity is more

effective.cally increased. The principal usage of these alloys is

their manufacturability, high wear resistance, low ther-

mal expansion coefficient, good corrosion resistance,

low density and improved elevated temperature proper-

ties [1/4]. It is generally known that the addition ofceramic particulates to these alloys increases the high

temperature strength, strength and wear resistance at

ambient temperature [5,6]. Nevertheless, some defects in

the cast microstructure will occur during the manufac-

ture of particulate metal matrix composites (MMCs)

using the processing techniques, such as melt-stirring or

powder metallurgy. Defects will undermine the perfor-

mance characteristics and casting quality [7].

An important defect is porosity, which tends to cause

a reduction in mechanical and fatigue properties of the

tion [7/9]. The solubility is a function of temperature,pressure and alloy composition. After solidification is

complete, these bubbles become micropores. Once

porosity forms, the pores will grow until they have

reached equilibrium between the forces acting on them

to include pressure, hydrogen solubility and interfacial

energy. On the other hand, it has been observed that

reinforcement particles have a tendency to associate

themselves with porosity, thereby giving rise to particle-

porosity clusters [1]. The melt stirring method is

economical, easy to apply and convenient for mass

production. However, in this technique the mixing of

reinforcement with the molten metal has problems, such

as low wettability and particle settling. Increasing the

liquid temperature, coating or oxidizing the reinforce-

ment particles, adding some surface-active elements

such as magnesium and lithium into the matrix [10]

and stirring of molten matrix alloy for an adequate time

period during incorporation are some viable ways used

to eliminate the defects during casting. The porosity in

* Corresponding author. Tel.: /90-232-388-2880; fax: /90-232-388-7864.

E-mail address: cagri.tekmen@deu.edu.tr (C. Tekmen).

0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0921-5093(03)00461-1

composites that enters the melt along with the reinforce-

ment during stir mixing can be eliminated by applying

pressure during solidification [11] and the application of

plastic forming processes to the composites [12/16]. Inother words, thermomechanical processes, such as

forging, extrusion, and rolling, are important methods

that can be used to improve the mechanical properties of

MMCs and produce standard products having stable

properties. It has been reported that improvements in

strength and ductility are observed with the application

of plastic forming processes to the composites [12,16/19]. The observed improvement in ductility and proper-ties is attributed to (a) the decrease in porosity content,

(b) better interfacial bonding between particle and

matrix and (c) the refinement of the matrix structure.

Studies conducted on the effect of reinforcement on

porosity showed that the porosity size and shape was

affected by the presence of reinforcement [7]. In general,

SiC particles tend to restrict the growth of pores. Also,

increasing the reinforcement volume fraction signifi-cantly increases the pore count [1,7]. This could be

attributed to the longer stirring time that causes a larger

amount of gas dissolved in the molten metal vortex.

The formation of porosity and its effect on mechan-

ical properties of MMCs have been the matter of several

studies [20,21]. It is generally accepted that tensile

properties decrease with an increase in porosity content.

However, the effect of parameters, such as porositytype, volume fraction, size and distribution, process

variables (e.g. mould and pouring temperature) and

hydrogen content on mechanical properties and fracture

behaviour are still incomplete. In this study, the effect of

porosity on mechanical properties and fracture beha-

viour, in addition to the role of reinforcement on

porosity formation in Al/Si/Mg/SiC composites, havebeen investigated using optical microscopy, imageanalyzer, scanning electron microscopy and tensile

testing.

2. Experimental

The chemical composition of the matrix alloy is given

in Table 1. The matrix alloy was reinforced with 10 and

20 vol.% of silicon carbide particles (SiCp) by using the

compocasting technique. The production of the matrix

alloy and composites used in the present study was

carried out as follows: SiC particles peroxidised at

900 8C were added to the semi-solid matrix alloy at600 8C. An argon atmosphere was maintained over themelt to reduce oxidation. The mixture was rapidlyheated to 750 8C and the composite slurry was pouredinto a preheated permanent iron die (150 8C). In orderto minimize solidification porosity and obtain better

mechanical properties, the samples were hot extruded.

Extrusion of the ingots was performed at an extrusion

ratio of 10:1.

It has been observed that the produced samples

contain various types of porosity. Composite sampleswith high and low content porosity were studied and an

average porosity content of the produced samples in as-

cast and extruded conditions was determined using the

Archimedean method. Microstructural characterization

studies were carried out in order to determine porosity

size distribution, area fraction and distribution of the

porosity by using the optical microscopy and image

analyzer. Tensile samples were machined from theextruded bars, having a gauge length of 16 mm and

diameter of 6 mm. Tensile tests were conducted at a

cross head speed of 0.5 mm s1. The fractured surfaces

were characterized by scanning electron microscope

(SEM) and optical microscope.

3. Results and discussion

3.1. Porosity in as-cast condition

Typical microstructures of the composites in as-cast

condition are shown in Fig. 1. It is evident from Fig. 1that different types of porosity, such as gas bubbles and

particle-porosity clusters, are present in the structure,

which are inevitable when the composites produced by

casting technique.

In order to determine the overall porosity content,

density measurements were conducted on unreinforced

and composites reinforced with 10 and 20 vol.% SiC

particles and the results are given in Table 2. Thedifference between the calculated density (Dc), which

was obtained by using chemical composition of the

composites and experimental density (De) is due to the

existence of porosity in the structure. The effect of

reinforcement volume fraction on observed porosity of

the composite samples in as-cast condition is illustrated

Table 1

Chemical composition of the matrix alloy used in the present study

Material Composition (wt.%)

Si Mg Cu Mn Ni Zn Fe Al

Matrix alloy 6.62 0.67 0.013 0.027 0.008 0.08 0.298 Bal.

C. Tekmen et al. / Materials Science and Engineering A360 (2003) 365/371366

in Fig. 2. The size distribution of porosity in the as-cast

samples are shown in Fig. 3. An increase in reinforce-

ment volume fraction increases the porosity content

(Table 2). In contrast, Bindumadhavan et al. [1] and

Samuel et al. [7] evaluated on particle cluster type

porosity and found that the presence of a larger volume

fraction of SiC particles physically restricts the growth

of porosity and thus reduces the overall porosity

content. Difference in results can be attributed to the

fact that measurements in this study were carried out on

both gas bubbles and shrinkage type porosity, thus the

overall porosity size was found greater.

Reinforcement particles have a tendency to associate

themselves with porosity and give rise to particle-

porosity clusters [1,7]. In the present study, compared

to 10 vol.% SiCp composite, a greater number of

particle-porosity clusters were found for the 20 vol.%

SiCp composite (see Fig. 4). In contrast, Bindumadha-

van et al. [1] studied a A356 matrix alloy reinforced with

SiC particles having an average particle size of 47 mm,observed that the tendency for formation of particle-

porosity clusters was greater in low volume fraction

composites. They explained that in high volume fraction

composites, the geometric capturing of particles restricts

their movement inside the melt during solidification.

The difference in test results can be attributed to the

following reasons: (i) for the same reinforcement volume

fraction, due to lower reinforcement particle size (12 mm,in this study) causes an increase in nucleation sites for

porosity at the SiC/matrix interface; and (ii) due to

improper feeding, where fluidity of liquid metal is

insufficient to fill the gaps between adjacent particles.

Fig. 1. Optical micrograph of 20 vol.% SiCp composite in as-cast

conditions (a) typical microstructure, (b) particle-porosity clusters and

(c) gas porosity.

Table 2

Density and porosity values of the matrix alloy and composites in as-cast and extruded conditions

Materials Calculated density Dc (g cm3) Experimental density De (g cm

3) Porosity (%)

As-cast Extruded As-cast Extruded

Matrix alloy 2.6917 2.6558 2.6670 1.33 0.92

Al/10% SiCp 2.7427 2.6162 2.7135 4.61 1.06Al/20% SiCp 2.7942 2.4342 2.7610 12.88 1.19

Fig. 2. The change of porosity content of the samples as a function of

SiCp volume fraction.

C. Tekmen et al. / Materials Science and Engineering A360 (2003) 365/371 367

3.2. The effect of extrusion process on porosity

The difference between the as-cast and hot-extruded

composite microstructures is that the SiCp clusters

initially present in some areas in the as-cast composite

have disappeared, giving a more uniform distribution of

SiCp, as shown in Fig. 5. The microstructures revealed

that the number of resolvable pores is reduced, some

particle fragmentation is noticeable and some particle

orientation in the direction of extrusion has taken place

during hot-extrusion. A detailed microstructural char-

acterization and X-ray diffraction analyses of the matrix

alloy and composites produced in the same manner was

demonstrated in the previous study [18].

Fig. 3. The porosity size distribution of the composite samples in (a, b) as-extruded and (c, d) as-cast states.

Fig. 4. Typical particle-porosity cluster formed in 20 vol.% SiCpcomposite.

Fig. 5. Optical micrograph of typical microstructure of 20 vol.% SiCpcomposite in extruded conditions.

C. Tekmen et al. / Materials Science and Engineering A360 (2003) 365/371368

Porosity volume fractions were found to vary between

1.33 vol.% for the unreinforced alloy and 12.88 vol.%

for the 20 vol.% SiCp composite in the as-cast condition.

Following hot-extrusion, the porosity levels were found

to be 0.92 vol.% for the unreinforced alloy and 1.19

vol.% for the 20 vol.% SiCp composite (Table 2).

Consequently, extrusion decreases the porosity content

while also changing the shape of the porosity. Similar

results also obtained from image analyses (Table 3). In

addition, the average porosity size after extrusion is

considerably reduced compared to the as-cast condition

(Fig. 3; Table 3).

Application of the secondary deformation processes

to discontinuously reinforced composites results in to

break up of particle or whisker clusters, reduction or

elimination of porosity and improved bonding charac-

teristics between particle and the matrix. Therefore, the

secondary process is very important in improving the

mechanical properties of MMCs, while it is an essential

step in engineering applications of MMCs for producing

standard products having stable properties. Rozak et al.

reported that the porosity level of A356 alloy based SiCpreinforced composites can be reduced by plastic work-

ing. Moreover, if the amount of applied deformation is

around 90%, porosity can be eliminated [12]. Zhang et

al. observed that after applying isothermal hot indirect

extrusion to cast Al/2024/SiCp composites, the contentof porosity reduced from 5.56 to 0.56% for an extrusion

ratio of 39 [22]. As reported earlier, Ozdemir et al.

investigated the effect of the hot forging on microstruc-

ture and mechanical properties of SiCp reinforced Al/Sialloy based composites having similar compositions to

those used in the present work. It was pointed out that

the porosity level of the composites could be reduced to

below 1.5 wt.% by application of hot-forging [16]. This

is a higher value than obtained during extrusion of the

composite. Although extrusion decreases the porosity

level, as shown in Fig. 3, it is not effective in eliminating

porosity, which exceeds a critical size. On the other

hand, by increasing the extrusion ratio, it is possible to

eliminate the large-sized porosity but this could ad-

versely affect mechanical properties due to particle

fracture and particle/matrix debonding, which is con-

ducive for void formation. Further, the extrusion

process affects the shape of the porosity by flattening

the porosity along the extrusion direction, as shown in

Fig. 6.

3.3. The effect of porosity on mechanical behaviour

When the composites are fabricated by melt-stirring,

the bonding strength maybe lowered by porosity andsegregation at the interface between the matrix and the

reinforcement. When pores are located at the boundary

of matrix and particles, they cause debonding of

particles from the matrix under low stress and reduce

the ability of load transfer to the particle and minimal

strengthening is achieved. The second type of porosity

located away from the particles tends to reduce the

effective area supporting the load and is detrimental tostrength [21,22]. When porosity or an equivalent defect

is present in a sample, the load bearing area is reduced.

It can be safely assumed that the defective region will

yield first, thereby concentrating the strain. On the other

hand, voids present in the cross section create a multi-

axial stress state and cause local strain concentrations in

their vicinity [8]. It is clear from Fig. 7 that an increase in

porosity content decreases both yield and ultimatetensile strength values of the produced samples.

Fig. 8 illustrates the micrographs near the fracture tip

of 10 vol.% SiCp composite having low porosity content.

As seen from Fig. 8, fracture damage of the 10 vol.%

SiCp composite occurred as particle cracking (PC) and

interfacial debonding (ID) for the 20 vol.% SiCpcomposite having high porosity content, the existence

Table 3

The results of porosity measurements of the composite samples in as-extruded and as-cast conditions

Materials Measured area (mm2) Mean size (mm) Min. size (mm) Max. size (mm) Porosity area fraction (%)

As-extruded Al/10% SiCp 3.4/107 25.1 1.7 117 1.2

Al/20% SiCp 3.7/107 35.6 10.9 180 2.2

As-cast Al/10% SiCp 5.8/107 30.8 6.2 158 4.5

Al/20% SiCp 6.1/107 65.3 14.9 276 9.6

Fig. 6. The effect of extrusion on porosity shape in 20 vol.% SiCpcomposite.

C. Tekmen et al. / Materials Science and Engineering A360 (2003) 365/371 369

of a large size porosity causes fracture, as shown in Fig.

9. The micrograph clearly shows that almost all the

particles are actually broken, since SiC particles were

observed on the fracture surface and the surface

contains large-sized porosity. Caceres and Selling [8]

carried out studies on Al/Si/Mg alloys and found thetensile strength to show little or no correlation with bulk

porosity content, while the mechanical performance

decreased with an increase in area fraction of defects

on the fracture surface of the samples. Also, for some

samples it was observed that having low porosity

content the mechanical results are lower compared to

samples having a higher porosity content. This result isattributed to the extrusion process, which is effective in

eliminating porosity lower than a critical size. For the

case of large-sized porosity, the extrusion process aids in

reducing the size and changing the shape of the pores. It

is possible to draw a conclusion that the average

porosity content is not a reliable parameter to predict

the mechanical properties. Further, large-sized pores,

which cannot be eliminated through extrusion, is animportant factor that promotes fracture while reducing

the strength of the composites.

4. Conclusions

The conclusions obtained are summarized in the

following paragraphs.

The increase in reinforcement volume fraction in-

creases the overall porosity content in both as-cast and

extruded conditions. With the application of the extru-

sion process, the porosity content and size is substan-

tially reduced to low levels. However, the extrusionprocess is only effective to eliminate porosity, which is

lower than a certain size.

The increase in porosity content decreases both the

yield and ultimate tensile strength values of the pro-

duced samples. The average porosity content is not a

reliable parameter to predict the mechanical results. In

the point of fracture behaviour, the existence of large

pores is more effective than the overall porosity content.

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