Compocasting of an Al-Si-SiC p Composite Using Powder Injection Method

Compocasting of an Al-Si-SiCp Composite Using Powder Injection

Method

M. Ghahremainian1,a and B. Niroumand2,b 1,2Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran,

84156-83111

[email protected],

[email protected]

Keywords: Compocasting, Al-Si-SiCp Composite, Powder Injection, Particle Incorporation, Wettability.

Abstract. In this work Al-7wt%Si-10wt%SiCp composites were produced by injection of the

reinforcement in the form of SiCp particles or a specially made particulate composite powder of

aluminum and SiCp into the melt of proper composition at a temperature corresponding to 10%

solid fraction. This paper presents the results of the investigation on the effects of reinforcement

addition form, reinforcement addition temperature, stirring speed and magnesium addition on the

incorporation and distribution of the reinforcement particles. The results showed that incorporation

of SiCp particles was considerably improved by their injection in the form of milled Al/SiCp

composite powder. Better particle wetting, improved particle dispersion and reduced particles size

were achieved by injection of milled Al/SiCp composite powder instead of SiCp powder.

Magnesium addition and high temperature injection were necessary for achieving good

incorporation. Reinforcement incorporation was improved by increasing the stirring speed up to 500

rpm, after which the incorporation decreased slowly.

Introduction

Particle metal-matrix composites (PMMCs) have found good commercial applications due to their

improved properties as well as lower cost and ease of fabrication compared to other metal matrix

composites (MMCs). They are now commonly used as attractive structural materials for some

automotive, defense and aerospace applications [1].

Among a variety of PMMCs produced in last few decades, aluminum matrix composites

reinforced with SiCp particulates (Al-SiCpp) have drawn the attention of many researchers owing to

their high specific strength, high specific modulus, low density, excellent wear resistance and higher

service temperature compared to the matrix material alone [2-4].

There are many different liquid and solid state processes for synthesis of Al-SiCp composites.

The liquid state processes include stirring the reinforcing particles in metallic melts, infiltration of

particulate or hybrid preforms by squeeze casting, vacuum infiltration or pressure infiltration,

reaction infiltration of particulate preforms or in-situ synthesis of the particulate reinforcement in

the melt [5, 6].

Compocasting is one of the liquid state processes in which the reinforcement particles are added

to a solidifying melt within its liquid-solid zone while being vigorously agitated. Once the

reinforcing particles are introduced into the semi-solid slurry, they are entrapped mechanically by

primary solid particles. This will reduces the possibility of their segregation and results in a better

distribution of reinforcement particles. The lower porosity contents observed in the castings have

been attributed to the better wettability between the matrix and the reinforcement particles as well as

lower volume shrinkage of the matrix alloy [5, 7-10]. The final microstructure of a compocast Al-

SiCp composite consists of coarse non dendritic primary particles of αAl, fine dendritic secondary

particles of αAl, low melting eutectic constituents of αAl and Si phases and dispersed SiCp particles.

Solid State Phenomena Vols. 141-143 (2008) pp 175-180online at http://www.scientific.net© (2008) Trans Tech Publications, Switzerland

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In this work Al-7wt%Si-10wt%SiCp composite was produced by a special compocasting method

and effects of some processing parameters on the incorporation of reinforcement particles into the

matrix were investigated.

Materials and experimental methods

Al-7wt%Si-10wt%SiCp composites were produced by a special compocasting method using Al-Si

alloys of proper Si contents, SiCp particles with average size of about 5µm and pure Al powder with

average size of about 60 µm as starting materials. Fig. 1 illustrates the scanning electron microscope

(SEM) images of the starting SiCp and Al particles.

(a) (b)

Fig. 1: SEM images of (a) SiCp and (b) Al particles before milling.

Fig. 2 schematically shows the experimental set-up used in production of the composites. It

consisted of a graphite crucible placed in a muffle furnace, a coated injection tube for introduction

of the reinforcement into the melt and a graphite stirrer. The crucible was equipped with baffles and

bottom pouring arrangement. The temperature of the melt was measured using a K-type

thermocouple dipped about 25 mm into the melt.

Fig.2: Schematic of the experimental set-up used in this investigation.

Argon was used as the carrier gas for injection of the reinforcement into the melt. The

reinforcement particles were injected into the melt in two different forms, i.e. as the original

purchased SiCp and as a specially-made particulate composite of aluminum and SiCp. In the latter

case, the composite slurry was produced by injection of the desired amounts of the particulate

composite powder into the melts of proper Si content so that after the completion of the injection,

Al matrix composites of 7wt% Si and 10wt% of SiCp were produced.

176 Semi-Solid Processing of Alloys and Composites X

The particulate composite powder was produced by low energy ball milling of equal volumes of

the pure aluminum powder and SiCp to achieve a mechanically interacting Al-SiCp powder. A good

interaction of the particles was achieved after 24 hours of milling.

The reinforcement was injected into the melt at a temperature above its liquidus temperature

while it was being isothermally stirred. After completion of the injection, the slurry was

continuously cooled at an average rate of 4.2 ºC/min until reaching 610 ºC corresponding to 0.1

solid fraction (according to Scheil equation) and cast into a steel die placed below the furnace.

The variables of the experiments included the reinforcement form, reinforcement injection

temperature, stirring speed and magnesium content of the melt. 1wt%Mg was added to the melt in

some experiments to increase the wettability between the matrix and the reinforcement. Table 1

shows the experimental conditions used in different experiments.

Table 1: Experimental conditions used in different experiments.

Sample

No.

SiCp

[Wt%]

Reinforcement

form

Reinforcement

injection temp.

[ºC]

Casting

temp.

[ºC]

Stirring

speed

[rpm]

1wt%Mg

addition

1 10 Milled Al/SiCp 650 610 500 Yes

2 10 Milled Al/SiCp 700 610 500 No

3 10 Milled Al/SiCp 700 610 500 Yes

4 10 SiCp powder 700 610 500 No

5 10 SiCp powder 700 610 500 Yes

6 10 Milled Al/SiCp 700 610 0 Yes

7 10 Milled Al/SiCp 700 610 250 Yes

8 10 Milled Al/SiCp 700 610 750 Yes

9 10 Milled Al/SiCp 700 610 1000 Yes

Microscopic observations were carried out following standard metallographic techniques on the

samples cut from the middle of the castings.

It was observed that normally a portion of the injected powder could not get incorporated into the

melt and was separated from the slurry. The remained powder could be collected after casting from

the crucible and the die. In this study the weight of the unincorporated powder in each experiment

was measured and was taken as a criterion for the effectiveness of the process. Therefore a

reinforcement incorporation factor (RIF) was defined as the percentage of the weight of the

reinforcement incorporated into the melt to the total weight of the reinforcement injected to the

melt. The larger the RIF, the more effective the process.

Results and discussions

Fig.3 illustrates the SEM images of the Al-SiCp composite powder after 24 hours milling. Fig 3(a)

and (b) reveal that after 24 hours milling, not only the SiCp particles have been pushed into the outer

surfaces of the Al particles, but many of the reinforcing particles have managed to enter the bulk of

the Al particles creating a relatively uniform distribution of the reinforcement. The figures show that

a proper mechanical interaction between Al and SiCp powders has been achieved.

The average size of SiCp particles was reduced to about 3 microns after 24 hours milling, while

their average aspect ratio (the ratio of length to width of each SiCp particle) remained almost

unchanged at about 1.6.

It has been evidently shown that small size particles have high tendency for clustering at various

stages during processing [11, 12]. These clusters may survive the shear forces of mixing,

particularly if they find their ways to the less turbulent zones of the crucible (e.g. dead zone at the

bottom of crucible or near the crucible walls).

Solid State Phenomena Vols. 141-143 177

In the preliminary stages of the experiments, it was observed that a considerable portion of the

injected particles floated on the melt surface and were pushed by the radial flow of the stirrer toward

the crucible wall where they could stick to the wall. Use of baffles improved the incorporation and

distribution of the reinforcement in the slurry by modifying the flow pattern inside the crucible from

a so-called solid body rotation (tangential flow) to a mainly axial flow [13] and creating a narrow

high speed flow along the crucible wall [14]. The particles clustering which occurred behind the

baffles could also be eliminated by occasional manual stirring next to the baffles by a rod. This was

particularly effective when the slurry reached its semi-solid temperature range.

The reinforcement incorporation factors (RIF) of the experiments are shown in Table 2.

(a) (b)

Fig 3. SEM image of 24 hours milled composite powder: a) outer surface of a particle and b) cross

section of the mounted powder.

Table 2: The reinforcement incorporation factor (RIF) of the experiments outlined in Table 1.

Effects of Mg addition are evident from comparison of RIFs of samples 4 and 5 and RIFs of

samples 2 and 3. In both cases incorporation of reinforcement is not acceptable without Mg

addition. It has been shown that magnesium improves the wetting of the SiCp particles, enhances the

interfacial bonding and strengthens the Al matrix [15, 16].

Comparison of RIFs of samples 2 and 4 and RIFs of samples 3 and 5 shows that considerable

improvement can be achieved by injection of the reinforcement in the form of milled Al/SiCp

composite powder instead of SiCp powder. Although the milling has reduced the average size of the

reinforcing particles, but their retention in the slurry is significantly superior than normal injection

of larger sized particles.

Particles agglomeration and gas entrapment between the reinforcing particles, which results in

poor wettability, are two of the major draw backs of traditional particle injection methods. Fig. 3(b)

displays a uniform distribution of SiCp particles in the Al matrix of the milled powder. There is

good contact between the SiCp particles and the matrix and no agglomeration of SiCp particles or

Sample No. RIF [%]

1 15

2 25

3 86

4 10

5 67

6 7

7 52

8 80

9 70

178 Semi-Solid Processing of Alloys and Composites X

any air gap between the constituents. Therefore, it is expected that when each of the composite

particles comes into contact with the melt, the Al matrix of the particle starts to melt from outside

inward and to gradually release the SiCp particles into the melt resulting in a better distribution of

reinforcement. On the other hand, due to the already existing close contact between the

reinforcement surfaces and the melting Al matrix (which is akin to the bulk of the melt) in each

composite particles, the wetting of the SiCp particles by the melt is improved and there will be less

chance of agglomeration or flotation of the reinforcement.

Fig. 4 illustrates the typical microstructures of composites produced by injection of milled

Al/SiCp powder. Although these figures may imply that particles have agglomerated at grains

boundaries, higher magnification observation reveals a uniform distribution of SiCp particles in the

matrix with very little agglomeration (Fig. 5). In fact dark patches shown in Fig. 4 are the result of

particles segregation between the growing dendrites rather than agglomeration. Particle pushing has

been shown to be the dominant mechanism in solidification of Al-7Si-SiCp composites [18].

(a) (b)

Fig. 4: Typical microstructures of composites produced by injection of milled Al/SiCp powder: a)

good distribution of SiCp particles, b) segregation of SiCp particles.

Fig 5: Uniform distribution of SiCp particles in the matrix.

Comparison of RIFs of samples 1 and 3 underlines the importance of reinforcement injection

temperature. Increasing the injection temperature from 650 ºC to 700 ºC has drastically improved

the particles incorporation. Higher temperatures favor wetting of SiCp particles [17]. However,

studies on Al-SiCp systems have shown that at temperatures above 727.8 ºC, the reaction between

Al and SiCp results in the formation of Al4C3 phase at the interface which is hydrophilic and hence

brittle and sensitive to moisture contact and degrades the properties of the composite [18].

Effect of stirring speed on reinforcement incorporation is demonstrated by comparison of RIFs of

samples 6, 7, 3, 8 and 9. It seems that the injected Ar gas bubbles are able to carry a large portion of

the reinforcement to the surface of the melt. Increasing the stirring speed provides a strong flow

which can prevent particles flotation or retune the floated particles into the slurry. However the

50 µm 50 µm

Incorporated SiCp

particles

25 µm

Solid State Phenomena Vols. 141-143 179

results show that the highest incorporation is achieved at the stirring speed of 500 rpm. Further

increase in the stirring speed results in a slow decrease in the incorporation of the reinforcement.

It was observed that in practice that while baffles eliminated the vortex formation, but at high

stirring speeds the surface of the slurry became turbulent. This is believed to result in the

entrapment and dispersion of small air bubbles into the slurry which helps the particle floatation.

Nevertheless, it must be mentioned that microstructural studies showed a better distribution of

reinforcement and smaller particles segregation or agglomeration at higher stirring speeds.

Conclusion

1 - Incorporation of SiCp particles was considerably improved by their injection in the form of

milled Al/SiCp composite powder.

2 - Better particle wetting, improved particle dispersion and reduced particles size were achieved by

injection of milled Al/SiCp composite powder instead of SiCp Powder.

3 - Magnesium addition and high temperature injection were necessary for achieving good

incorporation.

4 - Reinforcement incorporation was improved by increasing the stirring speed up to 500 rpm, after

which the incorporation decreased slowly.

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