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Influence of Sc modification on the fluidity of an A356 aluminum alloy
Journal of Alloys and Compounds 487 (2009) 453–457
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Influence of Sc modification on the fluidity of an A356 aluminum alloy Wattanachai Prukkanon a , Nakorn Srisukhumbowornchai a , Chaowalit Limmaneevichitr b,∗ a b
School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, 126 Pracha-Utid Road, Bangmod, Tungkhru, Bangkok 10140, Thailand Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha-Utid Road, Bangmod, Tungkhru, Bangkok 10140, Thailand
a r t i c l e
i n f o
Article history: Received 18 March 2009 Received in revised form 25 July 2009 Accepted 29 July 2009 Available online 5 August 2009 Keywords: Fluidity Aluminum–silicon alloys Scandium Grain refinement Modification
a b s t r a c t Influence of Sc modification on the fluidity of an A356 aluminum alloy was investigated. To use Sc as a practical and effective eutectic Si modifier, it is crucial to understand the effects of Sc on castability, including fluidity. Higher fluidity can reduce the temperature needed to fill a mold. The fluidity increased with higher mean shape factor, which was from the effect of Sc modification. This increase of fluidity was evident at all tested temperatures, especially within the higher temperature range of 690–720 ◦ C. We found that Sc also effectively refined the grain size. The fluidity of these reduced grain size samples is higher than that of non-grain refined samples. However, the sample with the finest grain size, 0.4 wt.% Sc, did not have better fluidity than samples with less grain refined grains (0.2 wt.% Sc and 0.2 wt.% Sc + 0.2 wt.% Zr). © 2009 Published by Elsevier B.V.
1. Introduction Sc adding in aluminum has shown promise as a new way to increase strength, reduce hot tearing susceptibility, and increase corrosion resistance for wrought aluminum alloys [1]. Sc also increases the recrystallization temperature of aluminum alloys to around 600 ◦ C [2]. Fujikawa [3] showed that adding Sc to pure aluminum enhanced its mechanical properties, thermal properties, and weldability. Verma and co-workers [4] concluded that Sc addition to pure aluminum reduces grain size 50% further than adding Al–5Ti–1B at the same concentration. Addition of Sc together with Zr refines grain size very effectively and also increases thermal stability [5]. Recently, Prukkanon et al. [6] reported that Sc addition in A356 modified the eutectic silicon to a fibrous structure. However, the effect of Sc addition on castability of aluminum alloys has not been studied. One of the major factors affecting castability of foundrygrade aluminum alloys is fluidity. Fluidity is defined as the ability of molten metal to fill a mold at a given temperature before the molten metal is stopped by solidification. It is necessary to obtain molten aluminum with high fluidity to avoid the need for overheating. Overheating aluminum increases the chance of successive problems, including gas porosity, solidification shrinkage, and dross formation. The fluidity is determined by changes occurring at the initial stage of solidification. The addition of eutectic modifying
∗ Corresponding author. Tel.: +66 2 470 9188; fax: +66 2 470 9198. E-mail addresses: [email protected], [email protected] (C. Limmaneevichitr). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2009.07.169
agents in Al–Si alloys would not be expected to have a significant effect on fluidity [7] unless the Si content is close to the eutectic composition. Many investigators have studied the relationship between alloy modification and fluidity. Kotte [8] found that both Na and Sr lower fluidity. Na reduced fluidity more than Sr, because Na affects the surface tension of Al–Si alloys. Venkateswaran et al. [9] observed a reduction in fluidity of Al–Si alloys modified with Na, Na + Sr, Ti, Na + Ti, Na + Sr + Ti and an increase upon addition of S, Sb, Sb + Ti, S + Ti. After the modification of an Al–12Si alloy, Seshadri and Ramachandran [10] observed a decrease in fluidity by 5–7% in a sand mold and 2–3% in a cast iron mold. Sabatino et al. [11] reported that an increase in the concentration of Mg in A356 decreased the fluidity of the melt, when combined with the addition of Sr. Ti, as a grain refiner, does not improve the fluidity of the melt. However, Ware et al. [12] reported that modification by Sr seemed to give a significant improvement in fluidity in their experiments. Further addition of Na, P and Ca did not cause a large difference to the flow length of the metal. However, they suggested that further trace amounts of Na, Ca or P are detrimental to the fluidity of Sr modified alloys. However, more data is required. Prukkanon and Limmaneevichitr [13] presented a preliminary result of the effect of Sc on the fluidity of A356 (Al7Si0.25Mg) and A380 (Al7Si3Cu1Fe). They found that 0.4 wt.% Sc addition increased the fluidity of A356 but decreased the fluidity of A380. However, they did not correlate the fluidity of A356 with levels of modification. Before the practical implementation of Sc as a modifier for aluminum–silicon alloys, there are many factors that need to be studied, including fluidity. Limited knowledge related to the effect of Sc on fluidity has been presented. This study describes an effort to understand Sc as a modifier relationship of grain size and
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Table 1 Chemical compositions of A356 aluminum ingots (wt.%). Alloys
Element (wt.%)
A356 A356 + 0.2 wt.% Sc + 0.2 wt.% Zr A356 + 0.4 wt.% Sc A356 + 0.2 wt.% Sc
Si
Mg
Cu
Sc
Zr
Zn
Fe
Al
7.01 6.94 6.99 6.97
0.28 0.26 0.26 0.26
0.017 0.014 0.011 0.017
<0.001 0.22 0.40 0.23
<0.001 0.23 <0.001 <0.001
0.02 0.04 0.03 0.02
0.14 0.16 0.16 0.09
Bal. Bal. Bal. Bal.
mold, immediately after the mold was filled with molten aluminum. The other end of the quartz tube was connected to a plastic tube that was in turn connected to a container with a volume of 9500 cm3 . This container was connected to a vacuum pump to make a vacuum container. A vacuum gauge was installed to determine the internal pressure inside the container. The pressure inside the container was set at 5 cmHg. Once the temperature decreased to a set-point of 660, 690, or 720 ◦ C, valve A was immediately opened and molten aluminum was drawn up. The height of the aluminum was measured from the top of the mold to the end of the aluminum sample inside the quartz tube. This height was taken as indicative of the fluidity of the molten aluminum. The fluidity tests were performed in triplicate. Each of the aluminum samples inside the tubes from these fluidity tests were taken out and cut in half along the longitudinal axis for microstructural analysis with an optical microscope. The etchant was Poulton’s reagent. The top portion of the sample was used for microstructure observation to determine modification effects. The bottom part of the sample was used for study of the macrostructure to determine grain refinement effects (Fig. 1). The effect of modification was quantitatively determined using the shape factor. To compare the effect of Sc modification on fluidity, it is important to quantify the modification of eutectic silicon. The mean shape factor (S) was used to represent the eutectic silicon modification level and the appearance probability of plate-like eutectic silicon. This technique has been previously used by other investigators [6,15,16]. If the mean shape factor is small, it implies that the eutectic structure is plate-like. The mean shape factor is calculated as follows: n
s=
Ai
√
2
Ai Pi
i=1
n
(1) Ai
i=1
Fig. 1. Experimental setup for the fluidity test.
eutectic Si shape factor to fluidity in Sc-modified A356 aluminum alloys. 2. Experimental procedure A356 is an aluminum alloy that is widely used for both sand and permanent mold casting processes. 800 g of A356 was melted in a silicon carbide crucible with a 12-kW induction furnace. A covering flux (45 wt.% NaCl–45 wt.% KCl–10 wt.% NaF) was added at the beginning of melting. The weight of this flux was 0.5% of the weight of charge. Master alloys of Al–2 wt.% Sc and Al–20 wt.% Zr were used to adjust the chemical composition. The chemical compositions of alloys used in this study were determined by emission spectrometry (Table 1). After composition adjustment, argon was purged into the melt through a stainless steel tube (6 mm inside diameter) with a zircon coating. This purge was performed to degas hydrogen. The degassing time was 3 min in each experiment with a flow rate of 4 L/min at 0.2 MPa. The experimental setup for fluidity testing is shown in Fig. 1. This experimental setup has been used successfully in previous work [14]. The molten aluminum alloy was poured into a mold made of CO2 silicate sand lined with a zircon coating. This mold had been preheated to 400 ◦ C with an electrical resistant furnace. The tip of a K-Type thermocouple probe was set 10 cm from the top of the mold to measure the temperature of molten aluminum inside the mold. This position was adjacent to the end of a quartz tube with 7 mm ID, 9 mm OD, and 650 mm length. The end of the quartz tube was inserted at the center of the mold, 10 cm below the top of the
where S is the mean area weighted shape factor of the silicon phase; Ai is the area of a single silicon structure and/or plate; Pi is the perimeter of a single silicon structure and/or plate, and n is the number of structures and/or plates in a single field. These parameters were determined from an area with a 1-mm diameter on each sample. At least 25 points were used at 100× magnification for three pieces of each alloy to obtain the average results. The mean shape factor (S) of samples without Sc addition was determined identically except that 10 points were used rather than 25.
3. Results and discussions The height of the aluminum drawn up from molten aluminum the melt is indicative of the fluidity of the molten aluminum. Table 2 summarizes fluidity data for all samples. To verify the reproducibility of the fluidity test, we used the linear least squares fitting technique to determine the slopes (a) and intercepts (b) of each line. We also determined the correlation coefficient (R2 ) and the standard deviations of each test series to verify that the experimental setup for this fluidity test was sufficiently reliable for this research. A similar analytical technique was successfully implemented previously [17]. As shown in Fig. 2, the fluidity of all samples increased with increasing temperature. The trend of increasing fluidity after addi-
Table 2 Summary of fluidity data. Alloys
A356
A356 + 0.2 wt.% Sc
A356 + 0.4 wt.% Sc
A356 + 0.2 wt.% Sc + 0.2 wt.% Zr
Mean value
Slope, a (cm ◦ C−1 ) Intercept, b (cm) R2 S.D. (%)
0.0944 −21.056 0.87 0.71
0.13 −44.3 0.94 2.08
0.1583 −63.75 0.97 1.23
0.1583 −63.528 0.92 0.72
0.925 1.185
W. Prukkanon et al. / Journal of Alloys and Compounds 487 (2009) 453–457
455
Fig. 2. Fluidity of molten A356 aluminum with different chemical compositions at different temperatures.
tion of 0.2 wt.% Sc, 0.4 wt.% Sc, and 0.2 wt.% Sc with 0.2 wt.% Zr is especially obvious at higher temperatures, i.e., 690 and 720 ◦ C. The increase in fluidity was not as significant at the lower temperature (660 ◦ C). The A356 with no Sc and Zr addition had the lowest fluidity. Adding 0.2 wt.% Zr with 0.2 wt.% Sc did not significantly increase the fluidity compared to A356 + 0.2Sc and A356 + 0.4Sc samples. To correlate the grain sizes with Sc and Sc + Zr additions, we examined the macrographs and micrographs of all samples. Fig. 3 shows the macrographs of samples obtained from the fluidity experiments, showing grain size. In this study, it is not possible
to determine the grain size according to ASTM E112-88 because of the size limitation of samples. However, it can be qualitatively observed that the Sc and Sc with Zr additions can refine grain size. Samples with the addition of 0.4 wt.% Sc additions displayed the finest grain size at all temperatures. The higher temperature samples have larger grain sizes, but this effect was less significant than changing the alloyed composition. Dahle et al. [18] reported that finer grain size should improve fluidity of molten aluminum. If this is true for grain refinement due to Sc addition, it would be expected that the samples with 0.4 wt.% Sc should have the highest fluidity.
Fig. 3. Macrographs from the fluidity samples at different temperatures and compositions.
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We found that the fluidity of samples with 0.4 wt.% Sc was not significantly higher than the fluidity of samples with 0.2 wt.% Sc, or 0.2 wt.% Sc and 0.2 wt.% Zr. However, the samples with no Sc addition (with the largest grain sizes) did have the lowest fluidity. The diminishing difference in fluidity between the base alloy and the grain refined alloy at high pouring temperature can be explained by the small difference in grain size at the flow tip of both alloys. The few large dendrites in the flow tip of the base alloy can be broken and multiplied when the alloy flows a long distance. This phenomenon can make grain sizes at the flow tip almost as small as they are in the grain refined alloy. Based on grain size, we still cannot explain why samples with 0.4 wt.% Sc did not have the highest fluidity. Thus, we investigated the sample microstructures further, as shown in Fig. 4. Micrographs in Fig. 4 show the morphology of modified eutectic Si. The eutectic silicon morphology in the unmodified alloy is relatively coarse with a lamellar structure and sharp edges as shown in Fig. 4 samples with no Sc addition at all temperature tests. We found that Sc addition modified the eutectic Si from a plate-like structure to a lamellar structure. For the 0.2 wt.% Sc and 0.2 wt.% Zr treatment, the eutectic morphology was finer and lamellar. The addition of Zr with Sc increases the efficiency of modification of Sc. If Zr is completely soluble in Al3 Sc, a stable formation of Al3 (Scx Zr1−x ) phase might be expected and results in an increase in impurity on the interface of silicon and liquid aluminum [5]. However, further research is needed to clarify and confirm the above explanation. Fig. 5 shows the clear relationship between the mean shape factor and the fluidity of all samples at different temperatures. The fluidity increased as the mean shape factor increased at all tested temperatures. In addition, the fluidity improved appreciably in the higher temperature range. The increase in fluidity after modification with 0.2 wt.% Sc and 0.2 wt.% Sc + 0.2 wt.% Zr additions was completely different from the modification by Sr and Na, which tended to lower the fluidity [8]. Further study is needed to determine the major reason for this difference, including the effect of Sc on surface tension and viscosity in comparison with Sr and Na.
Fig. 5. Fluidity of molten A356 aluminum with different mean shape factors at different temperatures.
4. Conclusions The major conclusions and suggestions drawn from this work are as follows: 1. Modification by Sc does not diminish the fluidity of the melt. 2. The fluidity of A356 samples modified with 0.2 wt.% Sc, 0.4 wt.% Sc, and 0.2 wt.% Sc + 0.2 wt.% Zr increased with increasing temperature. 3. Sc and Sc + Zr additions can also effectively reduce the grain size of cast aluminum samples. The fluidity of these reduced grain size samples is higher than that of non-grain refined samples. However, the sample with the finest grain size, 0.4 wt.% Sc, did not have better fluidity than samples with less grain refined grains (0.2 wt.% Sc and 0.2 wt.% Sc + 0.2 wt.% Zr).
Fig. 4. Micrographs from the fluidity samples at different temperatures and compositions.
W. Prukkanon et al. / Journal of Alloys and Compounds 487 (2009) 453–457
4. The fluidity increased as the mean shape factor increased at all tested temperatures. In addition, the fluidity appreciably improved within the higher temperatures, i.e., 690 and 720 ◦ C. Acknowledgements This research has been funded by the National Research Council of Thailand (B.E. 2551-2552). The scholarship for Mr. Prukkanon from University of the Thai Chamber of Commerce is greatly appreciated. References [1] M.G. Mousavi, C.E. Cross, ∅. Grong, Sci. Technol. Weld. Joining 4 (1999) 381–388. [2] Z. Ahmad, JOM 55 (2003) 35–39. [3] S. Fujikawa, J. Japan Inst. Light Met. 49 (1999) 128–144. [4] K. Venkateswarlu, L.C. Pathak, A.K. Ray, G. Das, P.K. Verma, M. Kumar, R.N. Ghosh, Mater. Sci. Eng. A 383 (2004) 374–380.
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[5] A.F. Norman, R.S. McEwen, Acta Mater. 46 (1998) 5715–5732. [6] W. Prukkanon, N. Srisukhumbowornchaia, C. Limmaneevichitr, J. Alloy. Compd. 477 (2009) 454–460. [7] A.K. Dahle, L. Bæckerund, L. Arnberg, Castability of Aluminum Foundry Alloys, Final Report for AFS (1997). [8] B. Kotte, Mod. Cast. (1985) 33–35. [9] S. Venkateswaran, R.M. Mallya, M.R. Seshadri, AFS Trans. 94 (1986) 701–708. [10] M.R. Seshadri, A. Ramachandran, Mod. Cast. 21 (1965) 110–122. [11] M.D. Sabatino, S. Shankar, D. Apelian, The 2005 TMS Annual Meeting Shape Casting the John Campbell Symposium, 2005, pp. 193–202. [12] T.N. Ware, A.K. Dahle, S. Charles, M.J. Couper, Advances in Aluminum Casting Technology II, USA (2002) 159–168. [13] W. Prukkanon, C. Limmaneevichitr, The 9th Asian Foundry Congress, Hanoi, Vietnam, 2005, pp. 7–14. [14] C. Limmaneevichitr, R. Sirichaivetkul, K. Puparatanapong, The 8th Aluminium Cast House Technology Conference, Brisbane, Australia, 2003, pp. 127–135. [15] M. Djurdjevic, H. Jiang, J.H. Sokolowski, Mater. Charact. 46 (2001) 31–38. [16] X. Chen, H. Geng, Y. Li, Mater. Sci. Eng. A 419 (2006) 283–289. [17] Y.D. Kwon, Z.H. Lee, Mater. Sci. Eng. A 360 (2003) 372–376. [18] A.K. Dahle, L. Arnberg, P.A. Tondel, C.J. Paradies, Met. Mater. Trans. A 27 (1996) 2305–2313.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Influence of Sc modification on the fluidity of an A356 aluminum alloy Wattanachai Prukkanon a , Nakorn Srisukhumbowornchai a , Chaowalit Limmaneevichitr b,∗ a b
School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, 126 Pracha-Utid Road, Bangmod, Tungkhru, Bangkok 10140, Thailand Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha-Utid Road, Bangmod, Tungkhru, Bangkok 10140, Thailand
a r t i c l e
i n f o
Article history: Received 18 March 2009 Received in revised form 25 July 2009 Accepted 29 July 2009 Available online 5 August 2009 Keywords: Fluidity Aluminum–silicon alloys Scandium Grain refinement Modification
a b s t r a c t Influence of Sc modification on the fluidity of an A356 aluminum alloy was investigated. To use Sc as a practical and effective eutectic Si modifier, it is crucial to understand the effects of Sc on castability, including fluidity. Higher fluidity can reduce the temperature needed to fill a mold. The fluidity increased with higher mean shape factor, which was from the effect of Sc modification. This increase of fluidity was evident at all tested temperatures, especially within the higher temperature range of 690–720 ◦ C. We found that Sc also effectively refined the grain size. The fluidity of these reduced grain size samples is higher than that of non-grain refined samples. However, the sample with the finest grain size, 0.4 wt.% Sc, did not have better fluidity than samples with less grain refined grains (0.2 wt.% Sc and 0.2 wt.% Sc + 0.2 wt.% Zr). © 2009 Published by Elsevier B.V.
1. Introduction Sc adding in aluminum has shown promise as a new way to increase strength, reduce hot tearing susceptibility, and increase corrosion resistance for wrought aluminum alloys [1]. Sc also increases the recrystallization temperature of aluminum alloys to around 600 ◦ C [2]. Fujikawa [3] showed that adding Sc to pure aluminum enhanced its mechanical properties, thermal properties, and weldability. Verma and co-workers [4] concluded that Sc addition to pure aluminum reduces grain size 50% further than adding Al–5Ti–1B at the same concentration. Addition of Sc together with Zr refines grain size very effectively and also increases thermal stability [5]. Recently, Prukkanon et al. [6] reported that Sc addition in A356 modified the eutectic silicon to a fibrous structure. However, the effect of Sc addition on castability of aluminum alloys has not been studied. One of the major factors affecting castability of foundrygrade aluminum alloys is fluidity. Fluidity is defined as the ability of molten metal to fill a mold at a given temperature before the molten metal is stopped by solidification. It is necessary to obtain molten aluminum with high fluidity to avoid the need for overheating. Overheating aluminum increases the chance of successive problems, including gas porosity, solidification shrinkage, and dross formation. The fluidity is determined by changes occurring at the initial stage of solidification. The addition of eutectic modifying
∗ Corresponding author. Tel.: +66 2 470 9188; fax: +66 2 470 9198. E-mail addresses: [email protected], [email protected] (C. Limmaneevichitr). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2009.07.169
agents in Al–Si alloys would not be expected to have a significant effect on fluidity [7] unless the Si content is close to the eutectic composition. Many investigators have studied the relationship between alloy modification and fluidity. Kotte [8] found that both Na and Sr lower fluidity. Na reduced fluidity more than Sr, because Na affects the surface tension of Al–Si alloys. Venkateswaran et al. [9] observed a reduction in fluidity of Al–Si alloys modified with Na, Na + Sr, Ti, Na + Ti, Na + Sr + Ti and an increase upon addition of S, Sb, Sb + Ti, S + Ti. After the modification of an Al–12Si alloy, Seshadri and Ramachandran [10] observed a decrease in fluidity by 5–7% in a sand mold and 2–3% in a cast iron mold. Sabatino et al. [11] reported that an increase in the concentration of Mg in A356 decreased the fluidity of the melt, when combined with the addition of Sr. Ti, as a grain refiner, does not improve the fluidity of the melt. However, Ware et al. [12] reported that modification by Sr seemed to give a significant improvement in fluidity in their experiments. Further addition of Na, P and Ca did not cause a large difference to the flow length of the metal. However, they suggested that further trace amounts of Na, Ca or P are detrimental to the fluidity of Sr modified alloys. However, more data is required. Prukkanon and Limmaneevichitr [13] presented a preliminary result of the effect of Sc on the fluidity of A356 (Al7Si0.25Mg) and A380 (Al7Si3Cu1Fe). They found that 0.4 wt.% Sc addition increased the fluidity of A356 but decreased the fluidity of A380. However, they did not correlate the fluidity of A356 with levels of modification. Before the practical implementation of Sc as a modifier for aluminum–silicon alloys, there are many factors that need to be studied, including fluidity. Limited knowledge related to the effect of Sc on fluidity has been presented. This study describes an effort to understand Sc as a modifier relationship of grain size and
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Table 1 Chemical compositions of A356 aluminum ingots (wt.%). Alloys
Element (wt.%)
A356 A356 + 0.2 wt.% Sc + 0.2 wt.% Zr A356 + 0.4 wt.% Sc A356 + 0.2 wt.% Sc
Si
Mg
Cu
Sc
Zr
Zn
Fe
Al
7.01 6.94 6.99 6.97
0.28 0.26 0.26 0.26
0.017 0.014 0.011 0.017
<0.001 0.22 0.40 0.23
<0.001 0.23 <0.001 <0.001
0.02 0.04 0.03 0.02
0.14 0.16 0.16 0.09
Bal. Bal. Bal. Bal.
mold, immediately after the mold was filled with molten aluminum. The other end of the quartz tube was connected to a plastic tube that was in turn connected to a container with a volume of 9500 cm3 . This container was connected to a vacuum pump to make a vacuum container. A vacuum gauge was installed to determine the internal pressure inside the container. The pressure inside the container was set at 5 cmHg. Once the temperature decreased to a set-point of 660, 690, or 720 ◦ C, valve A was immediately opened and molten aluminum was drawn up. The height of the aluminum was measured from the top of the mold to the end of the aluminum sample inside the quartz tube. This height was taken as indicative of the fluidity of the molten aluminum. The fluidity tests were performed in triplicate. Each of the aluminum samples inside the tubes from these fluidity tests were taken out and cut in half along the longitudinal axis for microstructural analysis with an optical microscope. The etchant was Poulton’s reagent. The top portion of the sample was used for microstructure observation to determine modification effects. The bottom part of the sample was used for study of the macrostructure to determine grain refinement effects (Fig. 1). The effect of modification was quantitatively determined using the shape factor. To compare the effect of Sc modification on fluidity, it is important to quantify the modification of eutectic silicon. The mean shape factor (S) was used to represent the eutectic silicon modification level and the appearance probability of plate-like eutectic silicon. This technique has been previously used by other investigators [6,15,16]. If the mean shape factor is small, it implies that the eutectic structure is plate-like. The mean shape factor is calculated as follows: n
s=
Ai
√
2
Ai Pi
i=1
n
(1) Ai
i=1
Fig. 1. Experimental setup for the fluidity test.
eutectic Si shape factor to fluidity in Sc-modified A356 aluminum alloys. 2. Experimental procedure A356 is an aluminum alloy that is widely used for both sand and permanent mold casting processes. 800 g of A356 was melted in a silicon carbide crucible with a 12-kW induction furnace. A covering flux (45 wt.% NaCl–45 wt.% KCl–10 wt.% NaF) was added at the beginning of melting. The weight of this flux was 0.5% of the weight of charge. Master alloys of Al–2 wt.% Sc and Al–20 wt.% Zr were used to adjust the chemical composition. The chemical compositions of alloys used in this study were determined by emission spectrometry (Table 1). After composition adjustment, argon was purged into the melt through a stainless steel tube (6 mm inside diameter) with a zircon coating. This purge was performed to degas hydrogen. The degassing time was 3 min in each experiment with a flow rate of 4 L/min at 0.2 MPa. The experimental setup for fluidity testing is shown in Fig. 1. This experimental setup has been used successfully in previous work [14]. The molten aluminum alloy was poured into a mold made of CO2 silicate sand lined with a zircon coating. This mold had been preheated to 400 ◦ C with an electrical resistant furnace. The tip of a K-Type thermocouple probe was set 10 cm from the top of the mold to measure the temperature of molten aluminum inside the mold. This position was adjacent to the end of a quartz tube with 7 mm ID, 9 mm OD, and 650 mm length. The end of the quartz tube was inserted at the center of the mold, 10 cm below the top of the
where S is the mean area weighted shape factor of the silicon phase; Ai is the area of a single silicon structure and/or plate; Pi is the perimeter of a single silicon structure and/or plate, and n is the number of structures and/or plates in a single field. These parameters were determined from an area with a 1-mm diameter on each sample. At least 25 points were used at 100× magnification for three pieces of each alloy to obtain the average results. The mean shape factor (S) of samples without Sc addition was determined identically except that 10 points were used rather than 25.
3. Results and discussions The height of the aluminum drawn up from molten aluminum the melt is indicative of the fluidity of the molten aluminum. Table 2 summarizes fluidity data for all samples. To verify the reproducibility of the fluidity test, we used the linear least squares fitting technique to determine the slopes (a) and intercepts (b) of each line. We also determined the correlation coefficient (R2 ) and the standard deviations of each test series to verify that the experimental setup for this fluidity test was sufficiently reliable for this research. A similar analytical technique was successfully implemented previously [17]. As shown in Fig. 2, the fluidity of all samples increased with increasing temperature. The trend of increasing fluidity after addi-
Table 2 Summary of fluidity data. Alloys
A356
A356 + 0.2 wt.% Sc
A356 + 0.4 wt.% Sc
A356 + 0.2 wt.% Sc + 0.2 wt.% Zr
Mean value
Slope, a (cm ◦ C−1 ) Intercept, b (cm) R2 S.D. (%)
0.0944 −21.056 0.87 0.71
0.13 −44.3 0.94 2.08
0.1583 −63.75 0.97 1.23
0.1583 −63.528 0.92 0.72
0.925 1.185
W. Prukkanon et al. / Journal of Alloys and Compounds 487 (2009) 453–457
455
Fig. 2. Fluidity of molten A356 aluminum with different chemical compositions at different temperatures.
tion of 0.2 wt.% Sc, 0.4 wt.% Sc, and 0.2 wt.% Sc with 0.2 wt.% Zr is especially obvious at higher temperatures, i.e., 690 and 720 ◦ C. The increase in fluidity was not as significant at the lower temperature (660 ◦ C). The A356 with no Sc and Zr addition had the lowest fluidity. Adding 0.2 wt.% Zr with 0.2 wt.% Sc did not significantly increase the fluidity compared to A356 + 0.2Sc and A356 + 0.4Sc samples. To correlate the grain sizes with Sc and Sc + Zr additions, we examined the macrographs and micrographs of all samples. Fig. 3 shows the macrographs of samples obtained from the fluidity experiments, showing grain size. In this study, it is not possible
to determine the grain size according to ASTM E112-88 because of the size limitation of samples. However, it can be qualitatively observed that the Sc and Sc with Zr additions can refine grain size. Samples with the addition of 0.4 wt.% Sc additions displayed the finest grain size at all temperatures. The higher temperature samples have larger grain sizes, but this effect was less significant than changing the alloyed composition. Dahle et al. [18] reported that finer grain size should improve fluidity of molten aluminum. If this is true for grain refinement due to Sc addition, it would be expected that the samples with 0.4 wt.% Sc should have the highest fluidity.
Fig. 3. Macrographs from the fluidity samples at different temperatures and compositions.
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We found that the fluidity of samples with 0.4 wt.% Sc was not significantly higher than the fluidity of samples with 0.2 wt.% Sc, or 0.2 wt.% Sc and 0.2 wt.% Zr. However, the samples with no Sc addition (with the largest grain sizes) did have the lowest fluidity. The diminishing difference in fluidity between the base alloy and the grain refined alloy at high pouring temperature can be explained by the small difference in grain size at the flow tip of both alloys. The few large dendrites in the flow tip of the base alloy can be broken and multiplied when the alloy flows a long distance. This phenomenon can make grain sizes at the flow tip almost as small as they are in the grain refined alloy. Based on grain size, we still cannot explain why samples with 0.4 wt.% Sc did not have the highest fluidity. Thus, we investigated the sample microstructures further, as shown in Fig. 4. Micrographs in Fig. 4 show the morphology of modified eutectic Si. The eutectic silicon morphology in the unmodified alloy is relatively coarse with a lamellar structure and sharp edges as shown in Fig. 4 samples with no Sc addition at all temperature tests. We found that Sc addition modified the eutectic Si from a plate-like structure to a lamellar structure. For the 0.2 wt.% Sc and 0.2 wt.% Zr treatment, the eutectic morphology was finer and lamellar. The addition of Zr with Sc increases the efficiency of modification of Sc. If Zr is completely soluble in Al3 Sc, a stable formation of Al3 (Scx Zr1−x ) phase might be expected and results in an increase in impurity on the interface of silicon and liquid aluminum [5]. However, further research is needed to clarify and confirm the above explanation. Fig. 5 shows the clear relationship between the mean shape factor and the fluidity of all samples at different temperatures. The fluidity increased as the mean shape factor increased at all tested temperatures. In addition, the fluidity improved appreciably in the higher temperature range. The increase in fluidity after modification with 0.2 wt.% Sc and 0.2 wt.% Sc + 0.2 wt.% Zr additions was completely different from the modification by Sr and Na, which tended to lower the fluidity [8]. Further study is needed to determine the major reason for this difference, including the effect of Sc on surface tension and viscosity in comparison with Sr and Na.
Fig. 5. Fluidity of molten A356 aluminum with different mean shape factors at different temperatures.
4. Conclusions The major conclusions and suggestions drawn from this work are as follows: 1. Modification by Sc does not diminish the fluidity of the melt. 2. The fluidity of A356 samples modified with 0.2 wt.% Sc, 0.4 wt.% Sc, and 0.2 wt.% Sc + 0.2 wt.% Zr increased with increasing temperature. 3. Sc and Sc + Zr additions can also effectively reduce the grain size of cast aluminum samples. The fluidity of these reduced grain size samples is higher than that of non-grain refined samples. However, the sample with the finest grain size, 0.4 wt.% Sc, did not have better fluidity than samples with less grain refined grains (0.2 wt.% Sc and 0.2 wt.% Sc + 0.2 wt.% Zr).
Fig. 4. Micrographs from the fluidity samples at different temperatures and compositions.
W. Prukkanon et al. / Journal of Alloys and Compounds 487 (2009) 453–457
4. The fluidity increased as the mean shape factor increased at all tested temperatures. In addition, the fluidity appreciably improved within the higher temperatures, i.e., 690 and 720 ◦ C. Acknowledgements This research has been funded by the National Research Council of Thailand (B.E. 2551-2552). The scholarship for Mr. Prukkanon from University of the Thai Chamber of Commerce is greatly appreciated. References [1] M.G. Mousavi, C.E. Cross, ∅. Grong, Sci. Technol. Weld. Joining 4 (1999) 381–388. [2] Z. Ahmad, JOM 55 (2003) 35–39. [3] S. Fujikawa, J. Japan Inst. Light Met. 49 (1999) 128–144. [4] K. Venkateswarlu, L.C. Pathak, A.K. Ray, G. Das, P.K. Verma, M. Kumar, R.N. Ghosh, Mater. Sci. Eng. A 383 (2004) 374–380.
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[5] A.F. Norman, R.S. McEwen, Acta Mater. 46 (1998) 5715–5732. [6] W. Prukkanon, N. Srisukhumbowornchaia, C. Limmaneevichitr, J. Alloy. Compd. 477 (2009) 454–460. [7] A.K. Dahle, L. Bæckerund, L. Arnberg, Castability of Aluminum Foundry Alloys, Final Report for AFS (1997). [8] B. Kotte, Mod. Cast. (1985) 33–35. [9] S. Venkateswaran, R.M. Mallya, M.R. Seshadri, AFS Trans. 94 (1986) 701–708. [10] M.R. Seshadri, A. Ramachandran, Mod. Cast. 21 (1965) 110–122. [11] M.D. Sabatino, S. Shankar, D. Apelian, The 2005 TMS Annual Meeting Shape Casting the John Campbell Symposium, 2005, pp. 193–202. [12] T.N. Ware, A.K. Dahle, S. Charles, M.J. Couper, Advances in Aluminum Casting Technology II, USA (2002) 159–168. [13] W. Prukkanon, C. Limmaneevichitr, The 9th Asian Foundry Congress, Hanoi, Vietnam, 2005, pp. 7–14. [14] C. Limmaneevichitr, R. Sirichaivetkul, K. Puparatanapong, The 8th Aluminium Cast House Technology Conference, Brisbane, Australia, 2003, pp. 127–135. [15] M. Djurdjevic, H. Jiang, J.H. Sokolowski, Mater. Charact. 46 (2001) 31–38. [16] X. Chen, H. Geng, Y. Li, Mater. Sci. Eng. A 419 (2006) 283–289. [17] Y.D. Kwon, Z.H. Lee, Mater. Sci. Eng. A 360 (2003) 372–376. [18] A.K. Dahle, L. Arnberg, P.A. Tondel, C.J. Paradies, Met. Mater. Trans. A 27 (1996) 2305–2313.