Effect of chilling and cerium addition on microstructure and cooling ...

Effect of chilling and cerium addition on microstructure and cooling curve parameters of Al–14%Si alloy V. Vijayan and K. Narayan Prabhu* Al–14%Si alloys, with and without cerium, were cast at varying cooling rates by solidifying them in a crucible and against chills. The effect of melt treatment and chilling on microstructure and cooling curve parameters of the alloy was assessed. Ce treated alloys solidified in clay graphite crucible at a slow cooling rate showed refinement of primary silicon and the formation of Al–Si–Ce ternary intermetallic compound. The addition of Ce to the alloy solidified against chills resulted in simultaneous refinement and modification of primary and eutectic silicon. Nucleation temperatures of both primary and eutectic silicon decreased on addition of cerium. The formation of the intermetallic compound decreased with increase in cooling rate, leading to the modification of the eutectic silicon. The increase in the degree of modification of the eutectic Si was associated with the decrease in the volume fraction of the intermetallic compound formed. On a coule´ des alliages d’Al–14%Si, avec ou sans ce´rium, a` des vitesses variables de refroidissement en les faisant solidifier dans un creuset ou contre des refroidisseurs. On a e´value´ l’effet du traitement du bain et du refroidissement sur la microstructure et sur les parame`tres de la courbe de refroidissement de l’alliage. Les alliages traite´s au Ce et solidifie´s dans un creuset d’argile et graphite a` une faible vitesse de refroidissement montraient un raffinement du silicium primaire et la formation d’un compose´ interme´tallique ternaire d’Al–Si–Ce. L’addition de Ce aux alliages solidifie´s contre les refroidisseurs avait pour re´sultat un raffinement et une modification simultane´s du silicium primaire et eutectique. Les tempe´ratures de nucleátion tant du silicium primaire qu’eutectique diminuaient avec l’addition de ce´rium. La formation du compose´ interme´tallique diminuait avec l’augmentation de la vitesse de refroidissement, menant a` la modification du silicium eutectique. L’augmentation du degre´ de modification du Si eutectique e´tait associeé a` la diminution de la fraction volumique du compose´ interme´tallique forme´. Keywords: Chilling, Cerium, Eutectic silicon, Modification, Primary silicon, Refinement, Thermal analysis

List of symbols TN (PSC) Primary silicon nucleation temperature Tmin(Eutectic) Al–Si eutectic minimum temperature TG(Eutectic) Al–Si eutectic growth temperature DTG The eutectic growth temperature difference between Ce added alloys and base alloy

Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Surathkal, Srinivasnagar, Mangalore 575025, India *Corresponding author, email [email protected]

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ß 2015 Canadian Institute of Mining, Metallurgy and Petroleum Published by Maney on behalf of the Institute Received 1 March 2014; accepted 12 June 2014 DOI 10.1179/1879139514Y.0000000151

DTrecalescence

The difference between eutectic growth temperature and the minimum temperature in each alloy

Introduction Al–Si alloys with Si content greater than 13 wt-% are categorised as hypereutectic Al–Si alloys. Owing to the low coefficient of thermal expansion and high wear resistance properties, hypereutectic Al–Si alloys are generally used for internal combustion engine parts, especially as pistons. The other applications of hypereutectic Al–Si alloys include high performance automobile engine parts such

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1 Schematic sketch of solidification set-up

as connecting rods, rocker arms, cylinder sleeves and piston rings. The cast microstructure of hypereutectic alloy generally consists of coarse and segregated primary silicon crystals along with unmodified eutectic silicon. Generally, primary silicon is refined by phosphorous treatment, but the treatment does not have any effect on the eutectic silicon. To achieve further improvement in the mechanical properties of hypereutectic Al–Si alloys it is important to simultaneously modify eutectic silicon along with the refinement of primary silicon.1–3 Among the alternatives to phosphorous, few rare-earth elements have recently gained attention due to their ability in modifying eutectic silicon. Kowata et al.4 investigated the effect of rare earth (45%Ce, 31%La, 15%Nd, 5%Pr) addition on the refinement of primary silicon crystals in a hypereutectic Al–20 wt-%Si alloy and concluded that the primary silicon crystals were refined with the addition of rare earth elements to Al–Si melt. Chang et al.5 studied the effect of RE addition to Al–21 wt-%Si alloy in a wedge shaped cast iron mould and reported that the RE addition bought simultaneous refinement of both primary and eutectic silicon. The results also showed 12–17uC and 2–7uC depression in the nucleation temperatures of primary silicon and eutectic silicon. Ouyang et al.6 investigated the effect of La addition on Al–18%Si alloy solidified on a preheated metal mould. The La master alloy was added in combination with P. The simultaneous modification of primary and eutectic silicon was achieved on combined La and P addition. The morphology of eutectic silicon changed from long needle-like structure to short rod-like structure. Dai and Liu7 studied the combined and individual effect of P, B and Ce on Al–30%Si and found that Ce has moderate effect on primary and eutectic silicon. The alloy was solidified in preheated (473 K) permanent mould of dimension W35675 mm. They also found that the addition of Ce along with B had good modification

Effect of chilling and cerium addition on Al–14%Si alloy

effect on eutectic Si due to the large undercooling effect. Xing et al.8 found that optimal addition of rare earth element (Er) to Al–17%Si and Al–25%Si alloys yielded a modified microstructure. The melt at 800uC was poured to a metal mould preheated to 200uC. The size of the primary silicon decreased with Er addition and the mechanical properties was found to be highest at the optimal concentration of Er addition. Chen et al.9 studied the complex modification of P and RE on Al– 20Si–1?6Cu–0?7Mn–0?6Mg alloys solidified in a permanent mould preheated at 250uC. They reported that the addition of RE along with P resulted in the refinement and modification of primary and eutectic Si. The primary silicon was refined to 23?3 from 64?4 mm and eutectic silicon was modified to fine fibrous or lamellar form with an average size of 5?3 mm. The tensile strength and elongation of the alloys also improved by 20 and 40% respectively, owing to the refinement and modification achieved. Kores et al.10 studied the effect of Ce addition on cast iron mould solidified Al–17%Si alloy and reported that the addition of 1% Ce resulted in the refinement of primary and eutectic silicon. The primary silicon nucleation temperature decreased from 686 to 591?9uC on Ce addition. However, this was contradictory to the results obtained by Wesis and Loper.11 In their studies, they reported that cerium did not refine primary silicon but it moderately affected the eutectic silicon. Recently, Li et al.12 reported that the Ce could significantly refine and modify primary and eutectic silicon. They studied the effect of Ce addition on the microstructure and tensile properties of Al–20%Si alloy. The alloy was solidified in a 200uC preheated permanent steel mould of 20 mm inner diameter and 50 mm length. The addition of Ce refined the primary silicon size from 94 to 33 mm and transformed eutectic silicon to fine fibrous form. The addition also significantly improved the tensile strength and elongation. From the literature, it is clear that the Ce has significant effect on the microstructure of hypereutectic Al–Si alloys. The existing literature on Ce modification of Al–Si alloy is scant and the reported results are contradictory. One of the reasons for contradiction could be the varying solidification conditions under which experiments were carried out by several researchers. The effects of addition of Ce on the thermal parameter and the degree of modification have not been reported. In the present investigation, attempt was made to study the effect of Ce treatment and cooling rate on cooling curve analysis parameter and evolution of microstructure in Al–14%Si alloy.

Experimental details Al–14%Si–2?6%Cu–0?8%Mg–0?3%Fe alloy was used in the present study. About 350¡50 g of the alloy sample was melted in a clay graphite crucible using an electrical resistance furnace. Cerium [Alfa Aesar, Cerium ingot, 99% pure (REO)] was added to the melt in varying quantities (0?5 wt-%, 1 wt-%, 1?5 wt-% and 2 wt-%) at 750uC. After the addition of Ce, the liquid metal was maintained at 750uC for 30 min. The holding time was 30 min for all the experiments. For slow solidification,


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2 Micrographs of crucible cooled Al–14%Si alloy with varying Ce content: a untreated; b 0?5 wt-%Ce; c 1?0 wt-%Ce (1: primary silicon; 2: eutectic silicon; 3: Ce phase); d 1?5 wt-%Ce; e 2?0 wt-%Ce

the crucible with the melt was removed from the furnace and cooled to room temperature under near equilibrium conditions. For chill solidification, the melt was quickly poured into the stainless tube of 50 mm diameter with a chill at the bottom. Stainless steel tube was selected as it has low thermal conductivity (16 W m21 K21). Copper, brass and cast iron chills with varying thermal conductivities

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were selected to obtain different cooling rates. The chill roughness before experiment was set at 0.5¡0.05 mm. A schematic sketch of the experimental setup is shown in Fig. 1. The temperature data from the castings was recorded using computerised data acquisition system. A K-type thermocouple was inserted at the centre of the crucible to record the cooling behaviour of the alloy in the

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3 Micrographs of Al–14%Si alloy solidified on different chills: a untreated cast iron chill; b 1?5 wt-%Ce cast iron chill; c untreated brass chill; d 1 wt-%Ce brass chill; e untreated copper chill; f 0?5 wt-%Ce copper chill

range of 750–400uC during solidification. The thermocouple was connected to a high speed data acquisition system (NI USB 9162) interfaced with a PC. The temperature was recorded with a time step of 0?1 s and the accuracy of thermocouple used was ¡0?5uC. The experimental set-up was covered using an insulation blanket to maintain constant cooling conditions for all experiments.

For microstructure evaluation, samples were prepared from castings and then polished for metallographic observations. The microstructures of specimens were then examined under a JEOL JSM-6380LA scanning electron microscope. Quantitative measurements of Si particle characteristics were carried out using Axio Vision image analysis software. The eutectic Si


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characteristics like area, length, perimeter, aspect ratio (length/perimeter) and roundness factor (P2/pA, where, P is perimeter and A is the area of silicon particle) are quantitatively measured at different locations of Ce added alloys for all solidifying conditions. The volume fraction of Ce intermetallics was also measured using multiphase analysis module.

Results and discussion Microstructural characterisation Figure 2 shows the micrograph of untreated Al–14%Si alloy solidified in a clay graphite crucible at slow cooling rate. The microstructure consists of irregular shaped primary silicon and long needle-like eutectic silicon in a eutectic matrix. During solidification, the silicon particles grow into a wide variety of shapes depending on the cooling rate and under-cooling achieved. The morphology of the primary silicon particles depend on their nucleation and growth behaviour during solidification. Figure 2b–e shows the effect of Ce addition on the microstructure of the alloy. The addition of Ce transformed the morphology of primary silicon from irregular shape to polyhedral shapes. The size of primary silicon decreased with Ce addition and minimum size was at 1?5 wt-%Ce addition. The microstructures also showed the presence of an intermetallic compound with addition of Ce. With increasing Ce addition, morphology of the intermetallic transformed from needle-like shapes (0?5 wt-%Ce) to block-like shapes (1?0 wt-%Ce). Figure 3 shows the microstructure of the alloy solidified against cast iron, brass and copper chills. The microstructures showed that the size of the primary silicon decreased with an increase in the thermal conductivity of the chill. The average sizes of the primary silicon solidified in crucible and against cast iron, brass and copper chills were found to be 137¡39 and 50¡9, 22¡3, 12¡2 mm respectively. The alloy solidified against copper chill showed finer and well

distributed primary silicon. The size of primary silicon solidified against copper chill was 76% finer than the primary silicon solidified against cast iron chill. The spherical nature of the eutectic Si particle is assessed by the roundness factor and the aspect ratio (Table 1). A roundness factor and aspect ratio of 1 indicates that the particle is perfect sphere. The untreated alloy displays the largest particle size (length, area and perimeter) and a roundness value of 7, indicates that the silicon particle is coarse and acicular nature. The addition of Ce to crucible cooled alloy did not show much effect on the acicular nature of the silicon particle even though the particle length decreased by 38%. On the other hand, the untreated alloy solidified against chills showed a significant decrease in the particle characteristics compared to crucible cooled alloys, but the acicular nature of the eutectic Si was least affected and is seen in Fig 3a, c and d. With the addition of Ce to chilled alloys, the particle parameters decreased significantly. The length of the eutectic particle decreased from 19 to 2 mm on 1?5% Ce addition to cast iron chilled alloys. Similar results were obtained for alloys solidified against copper and brass chills as well. Similarly, the aspect ratio of the particle was found to decrease with Ce addition. This indicates that the acicular Si particles are transformed to fine fibrous form with Ce addition. The roundness factor approached unity with varying Ce content depending on the thermal conductivity of the chill. For example, addition of 1?5% Ce to the alloy solidified against cast iron chill reduced the roundness factor to 1?4 from 4?3. This indicates that the eutectic Si particle has become more spherical with addition of Ce compared to Si in the untreated alloy. Figure 3a–e compares the microstructures of unmodified and Ce modified alloy samples for different chill conditions respectively. The quantity of Ce required for modification varied with the thermal conductivity of the chill used. In the case of cast iron chill, the alloy with 1?5 wt%Ce showed the highest degree of eutectic modification, whereas, for brass and copper chills the maximum

Table 1 Eutectic silicon particle characterstics of Al–14%Si alloy solidified under different cooling conditions

Crucible

Cast iron

Brass

Copper

70

Ce/wt-%

Length/mm

Aspect ratio

Area/mm2

Perimeter/mm

Roundness factor

0 0.5 1 1.5 2 0 0.5 1 1.5 2 0 0.5 1 1.5 2 0 0.5 1 1.5 2

129¡40 92¡50 80¡15 86¡18 95¡25 19¡7 11¡6 5¡2 3¡1 2¡1 10¡2 3.2¡1.5 2.3¡0.5 2.2¡0.5 1.8¡0.2 7.5¡2.5 2¡0.5 1.5¡0.3 1.3¡0.2 1.6¡0.2

22¡15 13¡10 15¡5 16¡5 16¡15 7¡3 6¡3 3¡2 2¡1.1 1.2¡0.5 7¡2.5 2.6¡1.5 1.5¡0.5 1.3¡0.5 1.1¡0.2 5.6¡2.2 1.2¡0.2 1.1¡0.2 1¡1 1.1¡0.2

827¡350 490¡180 301¡65 319¡90 406¡100 52¡18 23¡10 9¡3 5.3¡2.5 3¡0.5 12¡3 3.2¡1 3¡1 2.5¡1 1.9¡0.3 7¡2 3.2¡1.0 1.8¡0.4 1.4¡0.3 2¡0.8

270¡83 200¡100 170¡35 182¡42 210¡50 44¡16 26¡13 13¡5 9¡3 8.6¡2.5 23¡6 9¡3.5 8¡2 8¡2 6.7¡1 18¡6 7.5¡1 5.8¡0.7 5.2¡0.7 6.2¡2

7¡2 6.5¡3 8¡2 8.3¡2 9¡4 5¡1 2.4¡1 1.7¡0.5 1.4¡0.5 1.8¡1.0 3.5¡1 2¡1 1.7¡0.5 1.7¡0.2 1.8¡0.2 4¡1 1.36¡0.1 1.45¡0.2 1.47¡0.2 1.5¡0.5


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4 Representative Si structure morphologies corresponding to AFS chart13

modification was observed at 1?0 wt-%Ce and 0?5 wt%Ce respectively. To assess the effect of chilling on degree of silicon modification, the silicon particle parameter (Roundness factor) was visually compared with AFS standard charts for silicon modification level shown in Fig. 4. The degree of Si modification was found to vary exponentially with

roundness factor and is shown in Fig. 5. The equation was used to find out the Si modification level for varying Ce addition at different solidifying conditions. Figure 6 shows the combined effect of chilling and Ce treatment on the Si modification level. As can be seen, the modification levels more or less remain the same for crucible cooled alloys even after the addition of Ce. The


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5 Quantitative relationship between roundness factor of silicon and AFS modification level

alloys solidified against chills showed significant increase in degree of silicon modification with the addition of Ce.

Thermal analysis Figure 7 shows the cooling and first derivative curves of crucible cooled Al–14%Si alloy without any addition. The primary and eutectic nucleation temperatures of untreated alloy were found to be 612 and 575uC respectively. Figure 8 and Table 2 show the effect of Ce on cooling curves of the alloy under different solidifying conditions. The primary silicon nucleation temperature decreased with the addition of Ce for all conditions. Copper chilled alloy showed a minimum temperature of 582uC on 2% Ce addition. A similar kind of decrease in nucleation temperature of primary silicon was reported by Kores et al.10 They studied the effect of

6 Si modification level vs cerium concentration

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Ce addition on Al–17% Si alloy solidified in cast iron mould and reported that, the primary silicon nucleation temperature decreased from 686 to 591?9uC on 1% Ce addition to the melt. The reason for this kind of decrease in primary silicon nucleation temperature with Ce addition is due to the adsorption of Ce atom to the interface of primary silicon. The fraction solid calculated for Ce added alloys before eutectic nucleation supports this theory. The fraction solid up to the nucleation of eutectic silicon is given in Table 3. The results indicate that the fraction solid formed decreases with the addition of Ce. This implies that the formation of primary silicon is suppressed due to the addition of Ce and as a result, the nucleation temperature is decreased. The effect of Ce addition on eutectic silicon nucleation and growth temperature is shown in Table 2. The result indicated that the thermal analysis parameters decreased on addition of Ce. In the case of crucible cooled alloy, Tmin decreased from 575?0uC (for 0 wt-%Ce) to 573?5uC (2 wt-%Ce) and this corresponds to a moderate modification. In the case of copper chilled alloy, the Tmin decreased to 564?2¡1?2uC on 2% Ce addition, due to the complete eutectic Si modification. DTG and DTrecalescence are used to quantify the effect of Ce addition on thermal parameters for varying solidifying conditions. Figure 9 shows the variation of DTG and DTrecalescence with Ce. According to the results, DTG values increased with the addition of Ce and this indicates that the growth temperature decreased with the addition. The chilled alloys showed significant decrease in growth temperatures compared to the crucible cooled alloys. The recalescence undercooling of eutectic reaction is determined by the difference between eutectic growth temperature and minimum temperature. The effect of Ce addition on recalescense undercooling for different solidifying conditions is


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7 Cooling and first derivative curve for Al–14%Si alloy

shown in Fig. 9b. The undercooling increased with the addition of Ce up to a particular concentration depending on the chill used and then remained constant on excess addition of Ce. The trend remained same for all cooling conditions. The Ce content for eutectic Si modification of alloys solidified against copper, brass and copper chills were found to be 1?5, 1?0 and 0?5 wt-% Ce respectively. The reason for higher growth temperature in crucible cooled alloys in spite of Ce treatment is due to the formation of large Ce ternary compounds as shown in Fig. 2. The presence of Ce bearing compound has been confirmed by X-ray diffraction and is shown in Fig. 10. According to Gro¨bner et al.14 in an Al–Ce–Si system for low concentration of Ce only two phases c1 [Ce(Si1–x Alx)2] and c2 [AlCeSi2] are thermodynamically stable.

Hence in the present study either of these two phases might have been formed during solidification. The formation of Ce intermetallics might have led to the Table 3 Effect of Ce addition on fraction solid formed up to eutectic solidification for Al–14%Si alloy solidified on different chills Fraction solid at eutectic nucleation Ce/wt-%

Cast iron

Brass

Copper

Without addition 0.5 1.0 1.5 2.0

0.16 0.14 0.13 0.11 0.14

0.12 0.10 0.08 0.04 0.07

0.13 0.03 0.05 0.06 0.09

Table 2 Effect of Ce addition on solidification parameters of Al–14% Si alloy for different solidifying conditions

Crucible

Cast iron

Brass

Copper

Si)/uC

Ce/wt-%

TN(Primary

0 0.5 1 1.5 2 0 0.5 1 1.5 2 0 0.5 1 1.5 2 0 0.5 1 1.5 2

612.7¡0.6 591.2¡0.1 590.5¡0.1 589¡0.5 589.7¡0.6 603.5¡2 598¡3 592¡3 587¡1.5 587¡0.5 598.1¡1.2 590¡2 587¡1.5 585¡1 586¡2 595¡2 590.0¡1.5 585.9¡2 583.4¡1 582.8¡0.7

Tmin(Eutectic)/uC

TG(Eutectic)/uC

DTRecalescence/uC

DTG/uC

575.0¡0.4 574.6¡0.2 574¡0.4 573¡0.5 573.5¡0.8 574.6¡0.4 572.2¡0.4 568.3¡0.3 567.3¡0.5 566.2¡0.4 572.8¡0.2 571.7¡0.5 569.3¡0.7 567.8¡1 565.6¡0.9 573¡0.5 571.9¡0.4 570.3¡0.8 566.4¡1 564.1¡0.9

575.64¡0.6 575.6¡0.1 575.2¡0.8 575¡0.2 574.9¡0.4 575¡0.1 573.2¡0.1 569.5¡0.3 568.9¡0.4 567.5¡0.4 573.3¡0.1 572.4¡0.5 570.5¡0.2 569¡1.5 566.6¡0.2 573.3¡0.5 572.5¡0.2 571.0¡1 567.1¡0.7 564.8¡1.5

0.6 1.0 1.2 2.0 1.4 0.4 1.0 1.2 1.6 1.3 0.5 0.7 1.2 1.2 1.0 0.3 0.7 0.7 0.7 0.6

0.0 0.0 0.4 0.6 0.7 0.6 2.4 6.1 6.7 8.1 2.3 3.2 5.1 6.6 9.0 2.3 3.1 4.6 8.5 12.2


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8 Cooling curves of Al–14%Si alloys with varying Ce content solidified on a crucible cooled; b cast iron; c brass and; d copper chills

9 Variation of eutectic solidification charcterstics with addition of Ce: a growth temperature differnce (DTG); b recalescence undercooling (DTrecalescence)

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10 X-ray diffraction pattern of Ce added Al–14% Si alloys

reduction of Ce atoms in eutectic matrix to poison the growth of eutectic Si. In the case of chilled alloys, the degree of eutectic Si modification increased due to the presence of Ce atoms in eutectic matrix. According to Lu and Hellawell,15 the best modifying effect is achieved when the ratio of atomic radius of the modifying agent and atomic radius of Si is closer to 1?65. The radii ratio (Rce/RSi) for cerium and silicon is 1?36 and is near to 1?65. Figure 11 shows the volume fraction analysis of Ce intermetallics formed during solidification of Ce treated Al–14%Si alloy. The results indicate that the Ce intermetallics formed increased with the Ce addition. This is due to the high chemical affinity of Ce in the formation of ternary intermetallics. Under slow solidifying conditions, the Ce atoms from near surroundings diffuse and cluster together to form large block-like particles, making it unavailable for eutectic modification. Figure 11 reveals that the volume fraction of Ce intermetallic was higher in the case of crucible cooled alloy and it decreases with increasing cooling rates. In the case of chilled alloys, the movement of Ce atom is hindered by the fast moving solidification front and hence

12 Growth temperature difference vs modification level

smaller and thinner intermetallics are formed as shown in Fig. 3. Therefore, the presence of the Ce atoms in eutectic matrix will lead to the modification of eutectic Si. For online prediction of eutectic silicon modification, it is necessary to correlate the thermal characteristics with the degree of Si modification. In the present study, DTG was correlated with the corresponding degree of modification estimated using quantitative metallographic technique. DTG is the difference in growth temperature between Ce treated alloys and untreated crucible cooled alloy. The untreated crucible cooled alloy was found to contain eutectic Si with modification level 1. DTG versus modification level for Al–14% Si alloy is shown in Fig. 12. It is clear that the modification level converges to 1 (extremely coarse) when DTG approaches zero. For DTG values greater than 7, the modification level remains constant at 5. The effect of chilling on modification level can be directly found from the curve by determining the growth temperature difference between the treated alloy and the untreated alloy. For example, the DTG for untreated alloy solidified against copper is 2?3uC and the corresponding degree of modification is 2 (Coarse). The alloy with DTG values equal to 5uC showed a completely modified microstructure.

Conclusion

11 Volume fraction of Al–Si–Ce intermetallic formed versus Ce addition (wt-%)

During solidification of hypereutectic Al–Si alloys (Al– 14%Si) in clay graphite crucible and against different chills (cast iron, brass and copper), the effect of Ce melt treatment on microstructure, and thermal analysis parameters was studied. Based on the results, the following conclusions were drawn: 1. Ce additions to Al–14%Si alloy solidified at slow cooling rate in a clay graphite crucible resulted in the transformation of primary silicon from irregular shape to polyhedral shape. The eutectic silicon was found to be modified on Ce addition. The microstructure showed the presence of blocky shaped Al–Si–Ce ternary intermetallics formed due to the addition of Ce.


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Simultaneous refinement and modification of primary and eutectic silicon was obtained with addition of Ce to alloys solidified against chills. The degree of eutectic modification increased with increase in Ce content. The microstructure showed high degree of eutectic modification at 1?5, 1?0 and 0?5 wt-% cerium additions to the alloy solidified against cast iron, brass and copper chills respectively. The roundness factor particle characteristic was used to assess the AFS Si modification level. In the case of chilled alloys, the degree of Si modification increased with cerium addition. An analysis of volume fraction results indicated that the percentage of Al–Si–Ce intermetallics formed decreased with an increase in cooling rate. The morphology of the intermetallic formed transformed from blocky shape to needle shape at higher cooling rates. The lower the volume fraction of intermetallics formed, the higher the degree of eutectic Si modification achieved. Cooling curve analysis results showed that the nucleation temperatures of primary and eutectic Si decreased marginally on Ce addition to slowly cooled alloys in a clay graphite crucible and decreased significantly for chilled alloys. DTG (growth temperature difference) was correlated with the degree of eutectic Si modification.

Acknowledgements V. Vijayan thanks National Institute of Technology Karnataka, (NITK) Surathkal, Mangalore, India for the Research Scholarship. The authors acknowledge the

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help received from Mr Sathish and Mr Dinesha, Technicians at the Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka (NITK), Surathkal, India, during the casting process. The authors also thank Ms Rashmi Banjan, SEM operator, National Institute of Technology Karnataka (NITK), Surathkal, India, for her assistance during scanning electron microscopy.

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