Materials

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Materials and Design 29 (2008) 20–27

& Design

Advantages of the centrifugal casting technique for the productionof structural components with Al–Si alloys

G. Chirita, D. Soares, F.S. Silva *

Mechanical Engineering Department, School of Engineering, Minho University, Campus de Azurem, 4800-058 Guimaraes, Portugal

Received 12 June 2006; accepted 12 December 2006Available online 28 December 2006

Abstract

This work discusses the mechanical properties advantages of using the vertical centrifugal casting technique for the production ofstructural components when compared to traditional gravity casting. An analysis of the most important features that occur in mechanicalproperties due to the centrifugal force is made. A comparison of mechanical properties of specimens obtained by both centrifugal castingtechnique and gravity casting technique is made.

It is observed that the centrifugal force may increase rupture strength in �35%, and rupture strain in about 160% over the gravitycasting technique. The Young modulus also increases in about 18%. The fatigue life experiences an increase of about 1.500% and thefatigue limit increases in about 45%. The centrifugal casting process is, therefore, much more effective in terms of obtained mechanicaland fatigue properties as compared to gravity casting.

The previous effects vary within the casting, according to the relative position from where the specimen is taken from the casting. Thehigher the distance in relation to the rotation centre (higher centrifugal force or higher G) the bigger the increase in mechanical prop-erties. Thus, a functional graded material, with properties changing along one axis is obtained. This effect may be useful for the produc-tion of components where different specifications are needed in different parts of the component.

An example of a potential application is an engine piston. In this paper it will also be shown how this technique would be very advan-tageous for the production of these structural components.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Centrifugal casting; Al–Si alloy; Mechanical; Fatigue properties

1. Introduction

The use of aluminium–silicon casting alloys as structuralmaterials is determined by their physical properties (pri-marily influenced by their chemical composition) and theirmechanical properties (influenced by chemical compositionand microstructure). The high specific tensile strength ofaluminium alloys is very strongly influenced by theircomposed poly-phase microstructure. The mechanicalproperties of a specific alloy (hypoeutectic, eutectic orhypereutectic) can be attributed to the individual physicalproperties of its main phase components (a-aluminium

0261-3069/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.matdes.2006.12.011

* Corresponding author. Tel.: +351 253 510220; fax: +351 253 516007.E-mail address: fsamuel@dem.uminho.pt (F.S. Silva).

solid solution and silicon crystals) and to the volume frac-tion and morphology of these components. According to[1] the tensile properties and fracture behavior of cast alu-minum alloys A356 and A357 strongly depend on second-ary dendrite arm spacing (SDAS), Mg content, and, inparticular, the size and shape of eutectic silicon particleand Fe-rich intermetalics. Thus, mechanical properties ofAl–Si cast alloys depend not only on chemical compositionbut, more important, on micro structural features such asmorphologies of dendritic a-Al, eutectic Si particles andother intermetalics that are present in the microstructure.

There are different solutions to control these microstructural features, for example by introducing specialelements [2,3] to refine the grain. However the most com-mon solution to improve mechanical properties of cast

Fig. 1. Schematic presentation of both (a) gravity and (b) verticalcentrifugal casting procedures.

Fig. 2. Position from where tensile specimens were taken for both gravityand centrifugal castings, in the three different positions studied (arrowindicates the pouring direction).

Fig. 3. Position from were fatigue specimens were taken for both gravityand centrifugal castings (arrow indicates the pouring direction).

G. Chirita et al. / Materials and Design 29 (2008) 20–27 21

aluminium–silicon alloys is by cast technology [4]. Eachtechnology has particular aspects that interfere on micro-structure and consequently on mechanical properties.

Traditionally the centrifugal casting process has beenmainly used for obtaining cylindrical parts. There areessentially two basic types of centrifugal casting machines:the horizontal types, which rotate about horizontal axis,and the vertical type, which rotates about a vertical axis.Horizontal centrifugal casting machines are generally usedto make pipe, tube, bushing, cylinder sleeves (liners), andcylindrical or tubular casting that are simple in shape.The range of application of vertical centrifugal castingmachines is considerably wider: gear blanks, pulley sheaves,wheels, impellers, electric motor rotors, valve bodies, plugs,yokes, brackets. Casting that are not cylindrical, or evensymmetrical, can be made using vertical centrifugal casting.Centrifugally cast parts have a high degree of metallurgicalcleanliness and homogeneous microstructures, and they donot exhibit the anisotropy of mechanical properties evidentin rolled/welded or forged parts [5,6].

Centrifugal casting is also a very common method forobtaining functionally graded materials, mainly compositematerials such as SiC reinforced Al-based FGMs or metal-lic materials which has high differences of density and lowsolubility on different phases or different materials of thesame alloy [2]. However there are almost no studies empha-sizing the fact that the centrifugal process can be success-fully used for obtaining functionally graded materialsalso for metallic materials (alloys) with high solubilityand small differences of density of the different phases, suchas Al–Si alloys. The aim of this paper is to emphasize thatthe centrifugal technique is also effective to obtain FGMs-functionally graded materials and that these materials andtechnique, in particular Al–Si alloys, may be very useful asa tool in the design of structural components.

Furthermore, presently, there are no reliable and inex-pensive way of fabricating of FGMs that allows for bulkproduction of parts. Current methods of fabricationinclude solidification processing, chemical vapour deposi-tion, spray atomization and co-deposition, and powdermetallurgy techniques. Of these perhaps the most econom-ical and attractive processing route to obtain FGMs wouldbe by centrifugal casting [7].

2. Experimental methods and materials

2.1. Materials

The material used for castings was an aluminium alloy AS12UN withthe following composition: Fe-0.75%, Si-(11.50–13.00)%, Zn-0.20%, Mg-(0.75–1.30)%, Ni-(0.80–1.30)%, Pb-0.10%, Sn-0.05%, Ti-0.20%.

2.2. Methods

The material was melted at 670 �C and poured into the permanentmould which was preheated at 130 �C.

A high frequency induction centrifugal casting furnace (Titancast700 mP Vac, from Linn High Term, Germany), equipped with a vacuumchamber, was used for melting and casting the samples. This system

enables positioning of both the melt (crucible) and the mould in a centrif-ugal arm (see Fig. 1b) which rotates around a central axis. A charge of�140 g of material was used in each experiment, which was always per-formed under vacuum.

For centrifugal casting while the mould was rotating around the cen-tral axis of the casting machine, the molten aluminum was poured intothe mould cavity. A schematic representation of the casting process is pre-sented in Fig. 1. For gravity casting was used the same induction vacuummelting equipment used in centrifugal casting, but in this case the melt waspoured into the mould manually (Fig. 1).

Three tensile specimens from each casting were cut in order to comparethe mechanical properties of the aluminium alloy in different places of theingot (Fig. 2).

Two round specimens were machined from each casting for the fatiguetests (Fig. 3).

Fig. 4. Micrographics of both gravity and centrifugal castings on position 2 (from where fatigue specimens and position 2 static specimens where cut).

22 G. Chirita et al. / Materials and Design 29 (2008) 20–27

Tensile tests were done in a Dartec tensile testing machine at roomtemperature and fatigue tests were done at room temperature on a rotatingbending machine, at R = �1 and different maximum stress levels to obtainS–N curves.

Two micrographics were taken from position 2 of both castings, cen-trifugal and gravity. Position 2 of tensile tests is coincident with the posi-tion from where the fatigue specimens were taken. SDAS was measured byusing an optical microscope.

3. Results

The obtained results are shown in Figs. 5a–b and 6.The tensile results can be summarized as follows:

0

50

100

150

200

250

300

Rup

ture

str

engt

h (M

Pa)

centrifugal casting

gravity casting

0

0.5

1

1.5

2

2.5

1 2 3

Position

Rup

ture

str

ain

(%)

centrifugal casting

gravity casting

1 2 3Position

Fig. 5. (a) Ultimate strength and (b) rupture strain results for bothcentrifugal and gravity castings in different positions of the casting.

� In centrifugal castings the rupture strength shows a ten-dency to increase from position 3 to position 1. In grav-ity castings there is no substantial difference among thedifferent positions (Fig. 5a).� In centrifugal castings the rupture strain shows a huge

tendency to increase from position 3 to position 1. Ingravity castings there is not, again, a substantial differ-ence among the different positions (Fig. 5b).� In centrifugal castings the Young modulus shows also a

tendency to increase from position 3 to position 1. Ingravity castings there is not, again, a substantial differ-ence among the different positions (Fig. 6).� All mechanical properties results show higher values for

the centrifugal casting process in any position, whencompared to gravity castings (Figs. 5a–b and 6).� The previous difference reaches in position 1, an increase

in about 35% (rupture strength), 160% (rupture strain),and 18% (Young’s modulus).

Fig. 7 show the S–N curve for both casting techniques.The fatigue results can be summarized as follows:

� Centrifugal casting specimens have a higher fatigue lifethen gravity casting specimens for all load levels (Fig. 7);

20

40

60

80

100

1 2 3

Position

You

ng M

odul

us (

GP

a)

centrifugal casting

gravity casting

Fig. 6. Young’s modulus results for both centrifugal and gravity castingsin different positions of the casting.

S-N curves

0.0

0.5

1.0

1.5

2.0

10.000 100.000 1.000.000 10.000.000

Life (Number of Cycles)

σ a /

w

gravitycentrifugal

σ

Fig. 9. S–N (for ra/rw) curves for both centrifugal and gravity castings. ra

– stress amplitude; rw – fatigue limit.

S-N curves

80

100

120

140

160

180

200

10.000 100.000 1.000.000 10.000.000

Life (Number of Cycles)

σa (

MP

a)

gravitycentrifugal

Fig. 7. S–N curves for both centrifugal and gravity castings. ra – stressamplitude.

SDAS

0

2

4

6

8

10

12

14

16

18

1 2 3Position

SDA

S (μ

m)

centrifugal castinggravity casting

Fig. 8. SDAS for both centrifugal and gravity castings as a function ofspecimen position.

G. Chirita et al. / Materials and Design 29 (2008) 20–27 23

SDAS is shown in Fig. 8 for the two casting techniquesand for the three different positions.

SDAS results can be summarized as follows:

� SDAS is lower for centrifugal castings then for gravitycastings;� SDAS increases in both casting techniques from posi-

tion 1 to position 3.

4. Discussion

It is clear by the results that the centrifugal techniquesubstantially improves the mechanical and fatigue proper-ties of the material. It is significant to verify that theimprovement is observed in the whole casting and not onlyin one side. It is also important to observe that the absolutevalue of the improvement in properties gives to this tech-nique a very important position as a substitute of the tradi-tional gravity casting. As a fact, when mechanicalproperties need to be improved, other techniques are usedand one of the most frequent is squeeze casting. In Ref. [8]can be observed that when using squeeze casting as a

replacement of gravity casting, for Zn–Al alloys, ultimatestrength increases in about 11.7–40%, depending on thealloy, and the fatigue limit increases in about 8–40%. Inanother study [9], in an Al–Si (AA356) alloy it has beenshown that squeeze casting increases rupture strength inabout 15% and rupture strain in about 720%, when com-pared to gravity casting. By using the centrifugal process,in the study of this paper, tensile strength increased inabout 35% in one side of the casting and about 28% inthe other side. Rupture strain increased in about 160%and the fatigue limit increased in about 45% (in position2). In position 1 the difference would be bigger (see expla-nation below).

In terms of fatigue life, S–N curves and mainly fatiguelimit, rw, there are basically three theories [10]. The firstand probably the most common, correlates rw (fatiguelimit) with ultimate static strength, ru, of the materials.The second correlates rw with Vickers Hardness, Hv, ofthe materials. The third correlates rw with the size, (interms of projected

parea) of pores, impurities, or some

fragile phases, of the materials. For aluminium alloys, thisthird theory correlates rw with the size, (

parea) of pores

and, for some alloys, with the size of the silicon particles.

1st theory: rw @ 0.5ru;2nd theory: rw @ 1.6Hv + 0.1Hv;3rd theory: rw @ (

parea)

In the alloy studied in this paper it is clear that there is agood correlation of the fatigue limit, rw, with the ultimatestrength, ru (see Fig. 9). The values of tensile tests consid-ered for the analysis are those from position 2 because theposition on castings from where the fatigue specimens weretaken is the same of position 2 of the tensile tests.

Based on this correlation and as the tensile static resultsgive better properties in every position for the centrifugalcasting as compared to the gravity casting it is expectedthat every position in the centrifugal castings also have bet-ter fatigue properties then gravity castings.

24 G. Chirita et al. / Materials and Design 29 (2008) 20–27

This is particularly evident in one side of the castingwhere the fatigue life may increase in about 300% andthe fatigue limit increases in about 50%.

If compared to other casting techniques, in Ref. [11] canbe seen that squeeze casting increases the fatigue limit, rw,in about 18% in a Al–7Si–0.3Mg–T6 alloy, as compared togravity casting. In the study of this paper, the centrifugaltechnique promotes an increase in the fatigue limit in about45%, as seen before (Fig. 7).

Although it is not possible to compare different alumi-num alloys because each alloy has different sensitivities toeach casting technique, it is clear that the increase in prop-erties due to the centrifugal casting process is very substan-tial and the centrifugal casting technique may be elected asa substitute of traditional gravity casting, at least for somealuminum alloys.

In this paper is not developed a quantified explanationof the effects of the centrifugal casting technique on themicrostructure and on the mechanical properties of thealloys. An observation of the microstructures in micro-graphics in Fig. 4 shows that there is a difference on phases(dendrites, eutectic silicon and intermetalics) between cen-trifugal and gravity castings. However, firstly, there is nota standard correlation between phases and mechanicalproperties. And secondly there is not yet a good explana-tion for the microstructure changes due to the centrifugaleffect. Some studies state that there exist a correlationbetween dendrite arm spacing (DAS) and ultimate strength[2,3,12] and between DAS and fatigue [12]. Some otherstudies [13] state that the eutectic silicon, the intermetallics,and the silicon phase shape also play a role. However, afirst look to the secondary dendrite arm spacing (SDAS)and tensile results show a clear correlation between SDASand both rupture strength and rupture strain. As widelyaccepted rupture strength and rupture strain increase whenSDAS decreases. In Fig. 8 can be observed that SDAS islower (this means better rupture strength and strain) forthe centrifugal casting technique then for gravity casting.Furthermore both rupture strength and strain increasefrom position 3 to position 1 and SDAS decrease in thesame sense. Thus SDAS seems to have a good correlationwith mechanical properties. However, further studies areneeded to have a deeper understanding of the relationshipbetween mechanical properties and phases present in thealuminium alloys.

The understanding of the centrifugal phenomena isbeing carried out by the authors of this paper on a widerstudy where the influence of different variables on micro-structure is being assessed, namely:

– Fluid dynamics behaviour by measurement of tempera-ture profile on different points of the mould duringcastings.

– Fluid dynamics analysis through finite element method.

Furthermore, the influence of the microstructure on themechanical properties is also being studied through:

– Chemical analysis along the castings.– Quantitative metallurgical analysis of the different

phases along the centrifugal direction on the castings.– Mechanical properties finite element prediction based on

the metallurgical phases.

Although a quantitative analysis is not provided yet, inthis paper it will be made a description of the particularfeatures that occur in centrifugal castings, and its possibleeffect on mechanical properties will be discussed.

4.1. Particular features of centrifugal castings

It is assumed in this paper that the centrifugal effect oncastings may be divided in three main features: centrifugalpressure, intrinsic vibration of the process, and fluiddynamics. The effect of each of these variables on mechan-ical properties will be briefly discussed subsequently.

4.2. Centrifugal pressure

The centrifugal effect is not so elevated to cause a shift ofsolidification diagram due to high pressure like in squeezecasting [14]. Thus the solidification diagram remains thesame on the solidification during the centrifugal process.

However the centrifugal pressure is important on thepositioning of the densest phases during solidification.The densest phases have a tendency to move to the outerpart of the casting [6]. Thus it is expected that in alloyswhere there are phases or metals with different densitiesthe densest ones will concentrate on the outer part of thecasting. This is what happens with most of composite par-ticle reinforced materials obtained by centrifugal casting[15] (also called functionally graded materials).

This effect, in terms of mechanical properties can beschematically represented by Fig. 10b.

4.3. Vibration

Although vibration is not directly associated with thecentrifugal castings, it is always associated with this centrif-ugal process because of the inherent vibration of the equip-ments, mainly on vertical equipments. Thus it is importantto take into consideration that this feature may also havean important effect on mechanical and metallurgical prop-erties of the castings. The eventual effect of vibration wouldbe, according to [16] an increase in the overall mechanicalproperties of the casting due to more refined grains (this isattributed to the higher density of germens of solidifica-tion). Schematically and again comparing to gravity cast-ing it would be obtained a shift on mechanical propertiessuch as that shown in Fig. 10a.

4.4. Fluid dynamics

The dynamics of the fluids is expected to be very differ-ent under gravity pressure (gravity casting) and under

Fig. 10. Scheme of the mechanical properties variation with the followingvariables: (a) vibration or fluid dynamics; (b) centrifugal pressure; (c)combined effect of (a) and (b).

G. Chirita et al. / Materials and Design 29 (2008) 20–27 25

centrifugal pressure + gravity pressure (centrifugal cast-ing). It is expected that the turbulence of the melt in thecentrifugal process is much higher inside the mould.According to [17] the first melt that reaches the wall surfaceof the mould (where the first germens of solidificationappear) will came backwards to the inside part of the cast-ing and this will increase the number of solidification sitesin the inner part of the casting and consequently promote a

quicker solidification and a refined microstructure, as com-pared to the gravity casting process.

This effect, in terms of mechanical properties, would besimilar to the vibration effect and can be schematically rep-resented by the same figure (Fig. 10a).

If both the centrifugal pressure and at least vibration orfluid dynamics were active at the same time, the schematiceffect would be that shown in Fig. 10c.

Based on these assumptions, the results obtained in thisstudy may be explained in terms of all effects presentedabove: centrifugal pressure and vibration, and/or fluiddynamics. As it is observed there is a sharp increase ofthe properties (rupture strength and strain) in the centrif-ugal direction (from position 3 to position 1). This couldbe explained based on the pressure effect. However itwould be expected based only on the centrifugal pressureeffect that the mechanical properties would decrease in theother side (position 3). But this does not happen. Anexplanation could be if it is considered that one of theother two effects (vibration or fluid dynamics) is also pres-ent. This effect would promote an increase in the overallproperties of the casting (in positions 1, 2, and 3) in sucha way that even the inside part in casting (position 3) themechanical properties are higher then the propertiesobtained on gravity casting. If the active mechanism isthe vibration or the fluid dynamics it is not known at thisstage of the study. Thus, the obtained results may beexplained if at least two of the previous effects are activeduring the centrifugal process (pressure + vibration) or(pressure + fluid dynamics). The most probable is thatthere is a combined effect of the three variables (pressure,fluid dynamics and vibration).

The strong effect of the centrifugal force on mechanicaland fatigue properties may be useful in design of structuralcomponents where different specifications exist for differentparts of the component. One type of component where thisdesign philosophy may be applied is an engine piston.

5. Example of an application – an engine piston

The piston is one of the most stressed components of anentire vehicle pressures at the combustion chamber mayreach about 180–200 bar [18] – a few years ago this valuewas common only for heavy-duty trucks but nowadays itis usual in HDSI engines. Speeds reach about 25 m/s andtemperatures at the piston crown may reach about400 �C [18].

As one of the main components in an engine, pistonstechnological evolution is expected to continue and theyare expected to be more and more strong, light, thin anddurable. The main reason is because the mechanical effi-ciency of an engine is still low and only about 25% of theoriginal energy is used in brake power [19].

The basic design of the piston is still pretty much thesame. So what has changed? – The operating environment.Todays engines run cleaner, work harder and run hotterthan ever before. At the same time they are expected to last

Fig. 11. Typical stress distribution due to loads on engine pistons: (a) diesel engine piston; (b) gasoline engine piston.

Fig. 12. Example of casting of an engine piston using the verticalcentrifugal casting technique.

26 G. Chirita et al. / Materials and Design 29 (2008) 20–27

longer and with minimal maintenance. Developments havebeen achieved in different fields: examples may be found onpiston geometry/combustion flow; materials/mechanicaland thermal behaviour; materials/wear and lubrication(coatings); analytical tools – FEA; and processing technol-ogies [20].

The use of the centrifugal casting technique, as describedon this paper, seems to be adequate to produce a function-ally graded component.

Notwithstanding this technological evolution there arestill a significant number of damaged pistons. Damagesmay have different origins: mechanical stresses; thermalstresses; wear mechanisms; temperature degradation, oxi-dation mechanisms; etc. On [20] it can be found that thereis one region where the damaging of pistons occurs. Thesame is to say that there is one region where pistons areparticularly stressed. Fig. 11a–b show examples of mechan-ical stresses that exist on an engine piston (by using FEManalysis). It can be seen that the most stressed regions arealways located on the half top part of the piston – regionbetween the top part of the pin hole and the head of theengine. The same half top part of the piston is where ther-mal stresses occur [20]. As both thermal and mechanicalstresses are cyclical it is expected that these regions are alsofatigue resistant. Thus it seems that it would be useful touse a technology that would be able to produce betterproperties on the alloy at the top of the piston.

In Fig. 12, if the piston mould is placed on the positiondescribed, a functionally graded piston, with propertiesincreasing from the skirt to the piston head would beachieved. The higher mechanical and fatigue functionalperformance requirements are located just where thoseproperties achieve higher values.

Of course that different design variable must be takeninto consideration in order to decide which casting tech-nique should be used to produce a component, for examplemanufacturing costs or others. However, based only onimprovement of mechanical and fatigue propertiesobtained on the whole casting and with special emphasisin one side of the component, when the vertical centrifugal

G. Chirita et al. / Materials and Design 29 (2008) 20–27 27

casting technique is used, it seems that this technique maybe useful for engine pistons and other structural compo-nents where the specifications are different in different posi-tions of the component.

6. Conclusions

The main conclusions of this work are

� The centrifugal casting process due mainly to centrifugalforce may be effective even for materials with similarphase densities or metal densities in the same alloy.� Much better mechanical and fatigue properties are

obtained along the whole casting, as compared to thegravity casting process.� It seems that the process could be explained based on at

least two variables: centrifugal pressure + vibration orcentrifugal pressure + fluid dynamics.� The process may be useful in design of structural com-

ponents and in particular when the specifications are dif-ferent in different positions (FGMs are ideal).� In terms of mechanical and fatigue properties, engine

pistons would be obtained by vertical centrifugal castingwith obvious advantages over the traditional gravitycastings.

Acknowledgement

This work is being supported by an FCT Grant (SFRH/BD/19618/2004).

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