TECHNICAL PAPER TP 2783

Computer Aided Cooling Curve Analysis and Microstructureof Cerium Added Hypereutectic Al–Si (LM29) Alloy

V. Vijeesh • K. Narayan Prabhu

Received: 28 July 2013 / Accepted: 2 January 2014 / Published online: 9 March 2014

� Indian Institute of Metals 2014

Abstract Thermal analysis of LM29 alloy and Ce added

LM29 alloys was carried out. The effect of cerium addition

on solidification parameters and microstructural features of

hypereutectic Al-Si (LM29) alloy was studied using

Newtonian analysis technique. Thermal analysis parame-

ters such as primary and eutectic phase nucleation and

solidus temperatures were determined. The addition of Ce

to LM29 alloy decreased the nucleation temperature of

primary silicon and eutectic silicon. The microstructural

examination of Ce added LM29 alloys revealed the pre-

sence of a polyhedral shaped Al–Si–Ce compound that

might have caused the refinement of primary and eutectic

silicon. The dendrite coherency point temperature of LM29

alloy was found to be suppressed on addition of Ce.

Keywords Hypereutectic Al–Si alloys � Cerium �Thermal analysis � Newtonian analysis �Dendrite coherency point (DCP)

1 Introduction

Aluminum–Silicon alloys with silicon content greater than

13 wt% are classified as hypereutectic Al–Si alloys. Due to

high silicon content, the hypereutectic Al–Si alloys have

low coefficient of thermal expansion and higher wear

resistance compared to other Al–Si alloys. Hypereutectic

Al–Si alloys are therefore extensively used in internal

combustion engine parts, especially in pistons. The alloys

are also used in connecting rods, rocker arms, cylinder

sleeves and piston rings. The cast microstructure of

hypereutectic alloy generally consists of coarse and seg-

regated primary silicon crystals along with unmodified

eutectic silicon. Even though the primary silicon could be

refined by phosphorous treatment, the eutectic silicon

remains unaffected [1–3]. To meet the emission standards

and fuel efficiency, there is a constant demand for high

temperature mechanical properties. Further improvement

in properties of hypereutectic Al–Si alloy can be achieved

by simultaneous modification of eutectic silicon and

refinement of primary silicon [4, 5]. To find an alternative

to phosphorous, hypereutectic alloys were treated with

rare-earth elements. Kores et al. [6] found that Cerium (Ce)

treatment resulted in simultaneous refinement and modifi-

cation of both primary and eutectic silicon at an optimum

quantity of 1 wt%. In addition to this, studies by Harun

et al. [7] and Anasyida et al. [8] revealed that the wear

resistance of the hypereutectic alloy increased with Ce

addition. Since melt treatment by cerium is in the initial

stages of the research, further studies are required to

develop it as an alternative to phosphorous treatment.

Computer aided cooling curve analysis is an online

prediction tool that can be used effectively to determine the

wide range of thermo-metallurgical information related to

metals or alloys. The process involves measuring the

temperature history of the sample with respect to time and

determining the thermal characteristics, fraction solid and

latent heat during melting or solidification [9–11]. Unlike

hypoeutectic Al–Si alloys, the hypereutectic alloys are

rarely characterized using thermal analysis techniques.

Previous studies on hypereutectic alloy reveal that the

thermal analysis technique can be used to detect the

nucleating phases, corresponding temperatures and the

V. Vijeesh � K. N. Prabhu (&)

Department of Metallurgical and Materials Engineering,

National Institute of Technology Karnataka, Srinivasnagar,

Surathkal, Mangalore 575025, India

e-mail: prabhukn_2002@yahoo.co.in

123

Trans Indian Inst Met (2014) 67(4):541–549

DOI 10.1007/s12666-014-0379-6

latent heat of fusion. Thermal analysis study on hypereu-

tectic alloy by Robles and Sokolowski [12], detected the

possible transformation of Si agglomerate to primary sili-

con. Robles carried out a detailed study on hypereutectic

alloys (390.1 and 393.2). The results were used to confirm

the transformation of Si agglomerates into primary silicon

and detect the other phases involved. However, a detailed

investigation is required to understand the effect of process

parameters such as cooling rate, undercooling, nucleation

temperature etc., on the evolution of microstructure and

properties of the hypereutectic alloy. Thermal analysis can

be related to the casting defects and would be a useful input

for simulation of casting solidification [9–12]. One of the

key requirements in the determination of thermal parame-

ters is finding out the amount of heat liberated during

metallurgical transformations. This is commonly done by

fitting a data base line (DBL) to the first derivative curve

(FDC) and then finding out the area under the curves. In

other words, the DBL would be the path taken by the FDC

of the metal or alloy in the absence of any metallurgical

reactions. There are two known techniques namely, New-

tonian and Fourier analysis, by which a DBL is calculated

and fitted to the derivative curve.

In present work the thermal analysis of cerium added

hypereutectic Al–Si (LM29) alloy was carried out using

Newtonian analysis. Cerium being a potential candidate for

simultaneous refinement of both primary and eutectic sili-

con, it is very important to quantify its effect on thermal

parameters. An attempt has been made to assess the effect

of Ce addition on solidification path, nucleation tempera-

ture, fraction solid and dendrite coherency point (DCP) of

hypereutectic Al–Si (LM29) alloy.

2 Theory

Newtonian analysis is based on the lumped heat capaci-

tance method which assumes a negligible thermal gradient

across the sample (Biot number = hL/k \ 0.1). The heat

transfer coefficient from the sample to the surrounding by

convection, conduction and radiation can be a function of

single unique temperature. The Newtonian analysis for

solidification of an alloy is mathematically expressed as

[13–15],

dQ

dt�mCp

dT

dt¼ UAðT� T0Þ ð1Þ

where Q is the latent heat of solidification, Cp is the

specific heat of the metal/alloy, T is the metal/alloy

temperature, U is the overall heat transfer coefficient, A is

the sample surface area and To is the ambient temperature.

In the absence of any metallurgical reaction (dQ/dt = 0),

Eq. 1 can be rewritten as,

dT

dt¼ UAðT � T0Þ

mCp¼ DBL ð2Þ

Equation 2 corresponds to the DBL for the Newtonian

analysis. The DBL for Newtonian analysis was found out

by fitting an appropriate polynomial of order greater than 2

to the portion of FDC, corresponding to the mushy region

(Tliq \ T \ Tsol). The area between the curves was

calculated by subtracting DBL from the FDC. The total

latent heat is calculated and was determined in the form:

L ¼ Cp

Zte

ts

FDC � DBL ð3Þ

where ts and te are the times for start and end of solidifi-

cation and the fraction solid between the duration can be

obtained by finding out cumulative area as the fraction of

total area between the curves.

Computer-aided cooling curve can be used to determine

the DCP. DCP is the point at which the growing dendrites

in metal/alloys start to touch each other and form inter-

connected networks throughout the sample [16]. It is a

point at which the mass feeding of the liquid metal shifts to

inter-dendrite feeding and results in the formation of

casting defects, such as segregation, hot-tearing and

shrinkage porosity. DCP is of great interest in solidification

simulation as it influences the ability of the metal in filling

complex shapes and in the formation of inter-metallic

compounds. The DCP can be determined either by rheo-

logical methods or by thermal analysis. The first method

identifies DCP as the point where there is an abrupt change

in the measured torque due to the change in the mechanical

strength of the solidifying sample [17]. In thermal analysis,

DCP corresponds to the point where the thermal conduc-

tivity of the solidifying sample increases from a minimum

value. It is based on the fact that the thermal conductivity

of the solid is higher than the liquid. The method uses two

thermocouples, one at center (Tc) and another near to the

wall (Tw), to record temperature differences with respect to

the solidification time. The minimum point in the temper-

ature difference (DT = Tw - Tc) curve corresponds to the

DCP [18]. The second derivative of temperature with

respect to time of a single centre thermocouple is also used

to identify the DCP. It is based on the assumption that the

solidification affects the cooling rate and DCP is identified

as point in the second derivative curve, where the curve

shows a deviation [19]. It was found that the second

derivative signals are very weak and it is very difficult to

identify the DCP point from the second derivative curve.

Recently, Djurdjevic et al. [19], based on their studies,

concluded that the FDC itself can be used in identifying the

coherency point. According to Djurdjevic et al. [19] DCP

can be identified by closely examining the loops in the

542 Trans Indian Inst Met (2014) 67(4):541–549

123

temperature versus the FDC. It is chosen as the point where

the derivative curve suddenly deviates from the horizontal

tangent.

3 Experimental Procedure

LM29 (Al–22 % Si–0.8 % Cu–0.8 % Mg) alloy was used

in the present study. About 300 g of the alloy sample taken

in a graphite crucible was melted in an electrical resistance

furnace. Cerium was added to the melt in varying quanti-

ties (0.5, 1, 1.5 and 2 wt%) at 800 �C. After the addition of

Ce, liquid metal was maintained at 800 �C for 30 min. The

holding time was constant at about 30 min for all experi-

ments. The crucible was removed from the furnace and

cooled under near equilibrium conditions. The crucible was

insulated properly using insulator blanket to prevent heat

losses. A K-type thermocouple was inserted at the center of

the crucible (Tc) to record the cooling behavior of the alloy

in the range of 800–400 �C during solidification. The

thermal analysis set-up was designed to ensure a constant

thermocouple height of 10 mm from bottom of the cruci-

ble. Thermocouple was connected to a high-speed data

acquisition system (NI USB 9162) interfaced with a PC.

The scanning rate used for experiments was 10 Hz and the

accuracy of thermocouple used was ±0.5 �C. The experi-

mental set-up was covered with glass-wool blanket to

maintain constant cooling conditions for all experiments

and the cooling rate was found to be 0.2 �C/s. The logged

temperature data was used to generate cooling curves and

derivative curves of the alloy.

Specimens were prepared from the casting for micro-

structural examination. They were polished using silicon

carbide papers of varying grit sizes (180, 220, 400, 800 and

1000). Specimen were washed and dried after each cycle of

polishing. The final polishing was done using rotating disc

polisher with 0.3 lm alumina suspension. The micro-

structures of the specimens were then examined under

JEOL JSM-6380LA scanning electron microscope. Each

experiment was conducted three times to ensure consis-

tency in the experimental results.

4 Results and Discussion

4.1 Calculation of Fraction Solid

The recorded thermal analysis data was used to plot the

cooling and FDCs. For accurate identification of inflection

points, the FDC was smoothed and plotted. Typical cooling

curve and its FDC for LM29 alloy are shown in Fig. 1.

Solidification parameters such as nucleation temperature,

cooling rate, eutectic temperature, start and end of

solidification, total solidification time were calculated from

the cooling and FDCs and the measured values are given in

Table 1. The thermal analysis results provide information

on nucleation temperatures for both primary and eutectic

silicon during solidification of the alloy. The nucleation

temperature for primary silicon was found to be 707.2 �C

and this temperature corresponds to the transformation of

the dissolved Si atom clusters into primary silicon particles.

According to Robles and Sokolowski [12], the Si–Si atoms

segregate or form clusters at temperatures as high as

1,075 �C with a different crystalline structure than that of

solid silicon. Some researchers suggested that these clus-

ters would be beneficial for the formation of primary sili-

con and the morphology and the size of primary silicon

particles are directly dependent on the size of the Si clus-

ters [20, 21].

The nucleation and growth of the primary silicon par-

ticles from the liquid into the surroundings causes rejection

of the aluminum solvent until the local concentration is

sufficient to nucleate a-Al phase. The partitioned Al phase

from the growing crystals accumulates around the Si par-

ticles and appears as halos and restricts the further growth

of Si particles. The precipitation of a-Al phase results in an

increasing Si content in the remaining liquid and eventually

crystallizes as the coupled zone to form eutectic region [22,

23]. The eutectic nucleation temperature and solidus tem-

perature for LM29 alloy were found to be 565.9 and

549.3 �C respectively.

The DBL is of immense importance for the calculation

of latent heat and fraction solid of alloys. In Newtonian

analysis, the linear fitting of DBL to the FDC is done. Few

data points, before and after solidification are used to fit a

linear curve in this technique. Figure 2 shows a typical

FDC merged with the best fit linear curve, which form a

DBL to the FDC.

Fig. 1 Cooling and FDCs for Al–Si (LM29) alloy

Trans Indian Inst Met (2014) 67(4):541–549 543

123

The fraction solid was calculated by finding out the net

area between FDC and DBL curves. The latent heat cal-

culated using Eq. 3 is equal to the product of specific heat

and area between the curves.

Fraction solid is given as:

fs ¼ Lt=Li ð4Þ

where Lt is the total latent heat and Li is latent heat at each

instant. The fraction solid can be therefore rewritten as

fs ¼R t

tsFDC� DBLR te

tsFDC� DBL

¼ Cummulative area till instant of time

Total areað5Þ

Figure 3 shows the solid fraction versus temperature for

Newtonian analysis. The accuracy of the fraction solid

curves depends on the fitting of an appropriate DBL to the

FDC. From Fig. 3 it is clear that the fraction solid up to

eutectic nucleation for the alloy without addition is 0.3. It

implies that 30 % of the alloy is solidified as primary silicon.

4.2 Determination of DCP

The first derivative of the cooling curve was used to

determine DCP of the alloy. Figure 4a illustrates the use of

FDC in DCP determination. Here the first derivative (oTot) is

plotted against temperature and DCP was determined by

identifying the loop in the curve. From Fig. 4a it is clear

that at DCP a loop is formed due to the change in the

cooling rate. Initially due to the formation of dendrite

network the cooling rate has increased, because the thermal

conductivity of the solid network formed is higher than the

liquid. The cooling rate decreased later forming a complete

loop. This decrease in cooling rate can be attributed to the

latent heat liberated due to the formation of solid. Hence at

DCP there is a slight increase in temperature attributing to

the latent heat liberated due to the formation of dendrite

network. Figure 4b shows the increase in temperature with

the increase in fraction solid. The change in slope of the

fraction solid curve indicates the contribution of dendrite

network to the increase in temperature.

4.3 Effect of Cerium

Figure 5 shows the cooling curves of cerium added alloys.

The measured cooling curve parameters are given in

Table 1. From Table, it is obvious that cerium has signif-

icant effect on characteristic solidification temperatures

(nucleation, eutectic, solidus and DCP). The results indi-

cate that the cerium addition has decreased the nucleation

Fig. 2 FDC superimposed on DBLFig. 3 Distribution of fraction solid with temperature calculated

using Newtonian analysis

Table 1 Effect of cerium addition on solidification parameters of LM29 alloy

Alloy Nucleation temperature (�C) Eutectic temperature (�C) DCP temperature (�C) Solidus temperature (�C)

LM29-without Ce 707.2 ± 0.5 565.9 ± 0.5 549.3 ± 0.3 563.5 ± 0.7

LM29?0.5 % Ce 700.2 ± 2 565.1 ± 0.2 548.3 ± 0.7 562.1 ± 1.0

LM29?1 % Ce 695.5 ± 0.5 564.0 ± 1 547 ± 1.0 561.6 ± 0.5

LM29?1.5 % Ce 686.7 ± 3.0 562.7 ± 1.5 546.7 ± 1.0 560.4 ± 1.0

LM29?2 % Ce 692.6 ± 3.0 563.2 ± 2 546.3 ± 1.5 560.4 ± 2

544 Trans Indian Inst Met (2014) 67(4):541–549

123

temperature of primary silicon. For 1.5 wt% Ce added

alloy, the nucleation temperature was found to be mini-

mum (686.7 �C). A similar kind of decrease in nucleation

temperature of primary silicon was reported by Kores et al.

[6]. They studied the effect of Ce addition on cast iron cell

solidified Al–17 % Si alloy and reported that, the primary

silicon nucleation temperature decreased from 686 to

591.9 �C on 1 % Ce addition to the melt.

The effect of cerium addition on the cooling curves is

shown in Fig. 5. The solidification time of the alloy

decreased with the addition of cerium. The time of solid-

ification was lowest at 1.5 wt% of Ce. From the results

given in Table 1 and Fig. 5 it is clear that Ce had a sig-

nificant influence on the nucleation and growth of primary

silicon. The primary silicon nucleation temperature

decreased with Ce addition up to 1.5 wt% and then it was

found to increase with further increase in Ce addition to the

melt. The latent heat of all alloys was calculated using

Eq. 3 to assess the effect of addition of Ce. An average Cp

value of 1.2 J/g for mushy state was used for calculation

[15]. The latent heat evolved for alloys without Ce,

0.5 wt% Ce, 1.0 wt% Ce, 1.5 wt% Ce, 2.0 wt% Ce were

found to be 459, 428, 448, 417, 411 J/g respectively. Fig-

ure 6 shows the effect of Ce on the fraction solidified. At a

given temperature the fraction of solid formed was lower

for Ce added alloys. This is attributed to the ability of Ce

atoms to suppress the nucleation and growth of the primary

silicon.

One of the characteristic features of eutectic modifica-

tion is the depression of eutectic nucleation temperature by

the addition of modifying agent. The present investigation

also shows similar kind of depression in eutectic nucleation

and growth temperatures. From Table 1 it is clear that the

eutectic nucleation temperature decreases with Ce addition

and reaches a minimum value of 562.7 �C at 1.5 wt% Ce.

Further increase in the Ce content to the melt resulted in an

increment in nucleation temperature. From the thermal

analysis results, it can be inferred that the Ce addition has

Fig. 4 a FDC versus temperature curve for DCP determination.

b FDC superimposed on fraction solid curve showing a change in

slope at DCP

Fig. 5 Effect of Ce addition on cooling curves Fig. 6 Effect of Ce addition on solid fraction

Trans Indian Inst Met (2014) 67(4):541–549 545

123

an influence on the nucleation and growth temperatures of

both primary and eutectic silicon. The results showed an

optimum value at 1.5 wt% Ce indicating simultaneous

refinement of primary and eutectic silicon at this concen-

tration of Ce.

To support the above findings, metallographic study was

conducted and the microstructures of cerium added alloys

are shown in Fig. 7. The micrograph also supports the

findings of thermal analysis. It was found that the addition

of Ce to the melt has resulted in a change in morphology

and refinement/modification of primary/eutectic silicon.

Figure 7a shows the micrograph of untreated alloy with

star shaped primary silicon and acicular eutectic silicon. It

was observed that with the addition of Ce, the morphology

of primary silicon transforms from star-like shape to

polyhedral shape. The transformation of primary silicon to

Fig. 7 Microstructures of LM29 alloy with different Ce additions a without Ce b with 0.5 % Ce c with 1 % Ce d with 1.5 % Ce e with 2 % Ce

546 Trans Indian Inst Met (2014) 67(4):541–549

123

polyhedral was complete at 1.5 wt% Ce and further

increase in Ce content resulted in irregularly shaped pri-

mary silicon as shown in Fig. 7c to e. The measured pri-

mary silicon sizes of LM29 alloys without Ce, 0.5 % Ce,

1 % Ce, 1.5 % Ce and 2.0 % Ce were found to be

1,089 ± 70, 851 ± 80, 581 ± 71, 360 ± 85 and

470 ± 75 lm respectively. The corresponding percentage

decrease in primary silicon size with Ce addition is 21, 46,

69 and 56 % for 0.5 % Ce, 1 % Ce, 1.5 % Ce and 2.0 %

Ce respectively. Figure 7d, e shows a microstructure with

the eutectic modification along with the refinement of

primary silicon. The eutectic silicon was found to be

moderately affected at low concentration of Ce additions

(\1.5 wt%). The nucleation temperature of the eutectic

silicon was also suppressed at higher concentration of Ce.

Faraji et al. [24] reported a similar kind of depression in

nucleation temperatures with addition of Sr to Al–19 wt%

Si alloy. They reported that the addition of Sr suppressed

the formation of primary silicon even in the presence of

added P. But Sr additions showed minimum effect on

eutectic nucleation temperature. In an earlier study con-

ducted by the authors, Sr addition yielded fibrous coral-like

structure of eutectic Si and the primary silicon was trans-

formed into a non-faceted structure from a faceted structure

[25]. However, in the present investigation it was found

that the addition of Ce resulted in polyhedral primary Si.

Fig. 8 EDAX results of LM29 alloys. a 0.5 % Ce, b 1.0 % Ce, c 1.5 % Ce and d 2.0 % Ce

Trans Indian Inst Met (2014) 67(4):541–549 547

123

The back scattered scanning electron microscope images

of Ce added alloys shows the presence of Al–Si–Ce ternary

compound. Figure 8 shows the results of EDAX analysis.

The presence of the ternary compound was also confirmed

by X-ray diffraction studies and is shown in Fig. 9.

According to Grobner et al. in an Al–Ce–Si system shown

in Fig. 10 [26, 27], for low concentration of Ce only two

phases u1 [Ce(Si1-xAlx)2] and u2 [AlCeSi2] are thermody-

namically stable. Hence either of these two phases might

have formed and restricted the nucleation and growth of

primary silicon. It is clear from Fig. 7 that the volume

fraction and size of the ternary compound increased with

the increase in Ce. The ternary Ce compounds were found

at the edges of the primary silicon restricting the growth.

The morphology of the ternary compound was observed to

be polyhedral up to 1.5 wt% Ce addition. Further addition

resulted in the transformation of Ce compound to needle

like structure. Hence, either of the Ce ternary phases would

be responsible for modification of eutectic silicon and

refinement of primary silicon. In the present investigation

the primary silicon nucleation temperature decreases up to

1.5 wt% Ce and correspondingly the primary silicon was

refined. Further increment in wt% of cerium resulted in a

increase of primary silicon nucleation temperature and a

decrease in the refinement of primary silicon as well. This

confirms the transformation of one form of Ce phase (u1/u2)

to another Ce phase with increase in Ce content. The for-

mation of needle shaped Ce compound would be due to this

transformation.

The effect of Ce addition on DCP temperature of LM29

alloy is shown in Table 1. It is clear that the Ce has

influenced the DCP temperature to a significant extent. The

DCP temperature decreased with the addition of Ce. The

1.5 wt% Ce added alloy showed the lowest DCP temper-

ature. The change in DCP temperature by Ce shows the

influence of Ce atoms on eutectic growth during solidifi-

cation. According to Wu et al. [28], there is a certain

relationship between nucleation of primary silicon and

eutectic silicon in P treated hypereutectic Al–Si alloys. It

was reported that the addition of P decreased the activation

energy for nucleation of the eutectic leading to the

microstructure refinement. In the present study, it is clear

that the cerium influences the growth of both primary as

well as eutectic silicon. The simultaneous refinement and

modification of primary and eutectic silicon would have

therefore affected the nucleation and growth of a dendrites.

Thus cerium influences the growth of dendrites and hence

the DCP.

Fig. 9 XRD pattern of Ce added LM29 alloys

Fig. 10 a Al–Ce–Si computed isothermal section [26] b Calculated

Al corner of Al–Ce–Si liquidus surface [27]

548 Trans Indian Inst Met (2014) 67(4):541–549

123

5 Conclusion

From the cooling and First Derivative Curves, the nucle-

ation, eutectic and solidus temperatures of LM29 alloy

were found to be 707, 566 and 549 �C respectively.

Thermal analysis of the cerium added alloys revealed that

the cerium had a significant effect on solidification

parameters, starting from nucleation temperature to the

solidus temperature. At 1.5 wt% Ce addition, the primary

silicon nucleation temperature decreased to 687 �C and the

eutectic temperature decreased to 562 �C. In addition to

this, the DCP temperature was suppressed by Ce addition

to the alloy. The thermal analysis results indicated that the

Ce addition restricted the nucleation of primary silicon. Ce

additions to alloy transformed the morphology of primary

silicon from coarse star-shape to polyhedral shape. The

eutectic silicon was moderately refined by Ce additions and

optimum microstructure was obtained at 1.5 wt% Ce.

Scanning electron microscope examination of Ce added

alloys revealed the presence of ternary cerium bearing

compound. With the increase in Ce content, the morphol-

ogy of Ce bearing compound transformed from polyhedral

to needle shape. The increase in primary silicon nucleation

temperature and size of primary silicon with higher con-

centration of Ce was due to the change in morphology of

ternary Ce compound.

Acknowledgments One of the authors (VV) thanks National

Institute of Technology Karnataka for the Research Scholarship.

Authors acknowledge the help received from Mr. Sathish and Mr.

Dinesha, Technicians, Department of Metallurgical and Materials

Engineering, National Institute of Technology Karnataka (NITK),

Surathkal, during the casting process. Authors also thank Ms. Rashmi

Banjan, SEM operator, National Institute of Technology Karnataka

(NITK), Surathkal, for her assistance during scanning electron

microscopy.

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