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Metallurgical structure of A356 aluminum alloy solidified under mechanical vibration: An investigation of alternative semi-solid casting routes
Materials and Design 30 (2009) 3925–3930
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Metallurgical structure of A356 aluminum alloy solidified under mechanical vibration: An investigation of alternative semi-solid casting routes Chaowalit Limmaneevichitr a,*, Songwid Pongananpanya a, Julathep Kajornchaiyakul b a b
Metallurgical Engineering Program, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Prachautit Road, Thonburi, Bangmod, Bangkok 10140, Thailand Foundry Engineering Research and Development Unit, National Metal and Materials Technology Center, Thailand Science Park, Pathumthani 12120, Thailand
a r t i c l e
i n f o
Article history: Received 18 November 2008 Accepted 30 January 2009 Available online 5 February 2009
a b s t r a c t This study investigated the effects of mechanical vibration during solidification on the metallurgical structure of hypoeutectic aluminum–silicon A356. A series of casting trials were conducted. Emphasis was placed on the morphological changes of the primary aluminum phase of the as-cast alloy, which was subjected to different levels of mechanical vibration at various values of pouring temperature and solid fraction. It was found that the average grain size of the primary phase became relatively finer and more globular as the degree of vibration increased. This suggested that during the solidification process, dendrites that formed normally in the liquid alloy were subsequently disturbed and fragmented by the mechanical vibration introduced into the melt. This effect was enhanced when the vibration was introduced into an alloy with a larger solid fraction, as was observed with solidification at lower pouring temperatures. In addition to the macrostructure examination, semi-solid properties were also assessed and reported using the Rheocasting Quality Index. It was shown that the introduction of mechanical vibration into the A356 melt with adequate solid fraction prior to complete solidification successfully resulted in an as-cast structure featuring semi-solid morphology. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction It is well accepted that microstructure is one of major factors that defines the mechanical properties of aluminum alloy [1]. For general casting applications, refining so as to achieve equiaxed and fine grains will increase fluidity [2,3], reduce hot cracking [4], and reduce microporosity [5]. Non-dendritic structures will be obtained if the solidification is well grain-refined for hypoeutectic Al–Si alloy. Semi-solid processing is an attractive process for obtaining globular structures and for minimizing the porosity and segregation problems of conventional casting. However, a specific shear force from external source in semi-solid processing is needed. This is due to the thixotropic characteristics of SSM billets having high viscosity at low stresses but a decreased viscosity when an increased stress is applied [6]. Many new techniques have been developed for creating semisolid structures since the inception of semi-solid processing; for example, electromagnetic stirring [7], ultrasonic vibration [8,9], low temperature casting [10], and gas bubble purging [11]. In this study, A356 aluminum alloy was studied because of its wide variety of applications and because its chemical composition is far below the Al–Si eutectic composition, meaning it has a wide solidification range suitable for semi-solid processing.
* Corresponding author. Tel.: +66 2 470 9188; fax: +66 2 470 9198. E-mail address: [email protected] (C. Limmaneevichitr). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.01.036
In this study a robust method for introducing mechanical vibration was developed, at lower cost than ultrasonic vibration. Wu et al. [12] recently introduced mechanical vibration during isothermal holding period of A356 aluminum alloy to prepare semi-solid slurry. A similar method was also reported by Taghavi et al. [13]; however, they focused only on vibration frequency and time. The effects on microstructures of mechanical vibration acceleration, pouring temperature, and isothermal holding temperature at various solid fractions have been investigated here. 2. Experiment Table 1 summarizes nominal compositions of the A356 alloy ingot used in the present study. The alloy was melted using an induction furnace. For all casting trials, the molten metal was subjected to flux treatment with 0.5 wt.% cleaning and covering flux. Argon purging was used to reduce dissolved hydrogen. In each case, the degassing time was 2 min using a flow rate of 5 L per minute at 0.2 MPa. To introduce the mechanical vibration, a special setup of apparatus was developed as schematically illustrated in Fig. 1. A stainless steel cup, as shown in Fig. 2, was employed as a mold to cast specimens for further metallurgical investigation. The setup was equipped with a salt bath to allow isothermal holding of the melt at a specific temperature to obtain the designed solid fraction. A thermocouple, temperature controller, and data logger were connected to the salt bath to monitor and control the temperature.
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Table 1 Chemical analysis of the A356 aluminum alloy used in the present study. Element
Si
Fe
Mn
Mg
Ni
Zn
Sn
Pb
Ti
Al
wt.%
7.13
0.135
0.005
0.331
0.001
0.013
0.012
0.009
0.111
Balanced
The vibration source is a device equipped with a magnetic coil that operates at a frequency of 55 Hz, and can generate different magnitudes of mechanical vibration acceleration by varying supplied voltage. Values of the voltage supplied to the vibration source and the corresponding magnitudes of acceleration (compared to earth’s gravity: g) are shown in Table 2. In the present study, the corresponding magnitudes of acceleration define the degrees of vibration. These varying degrees of mechanical vibration were introduced into the melt at solid fractions of 13% and 40%. The casting trials, covering solidification conditions at 13% and 40% frac-
tions of solid, were carried out by pouring the liquid alloy at pouring temperatures of 620, 630, and 650 °C into the stainless steel mold, immersing that mold in a salt bath maintained at 580 °C, and then introducing the mechanical vibration to the melt until the melt temperature dropped to 608 ± 2 °C for the 13% solid and 588 ± 2 °C for the 40% solid. The fraction of solid at various temperature was calculated based on the Scheil’s equation, and assuming linear liquidus and solidus lines having partition coefficient at 0.13 [14]. Subsequently the stainless steel mold with the specimen inside was suddenly immersed in water at 30 °C with vigorous agitation.
Table 2 Series of casting trials defining the conditions, i.e. pouring temperatures, solid fractions, and vibration acceleration levels. Pouring temperature (°C)
Fraction of solid (wt.%)
Magnitudes of acceleration (Earth’s gravity: g)
650
13
0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90
40
630
13
40
620
13
40
Fig. 1. Schematic illustration of the apparatus used in the present study (not to scale).
Fig. 2. Stainless steel cup and the plane for microstructure examination.
C. Limmaneevichitr et al. / Materials and Design 30 (2009) 3925–3930
The A356 specimens were then cut into 20 mm sections from the bottom, as also shown in Fig. 2, in preparation for metallurgical examination. The shape factor used was:
SF ¼
3927
For these measurements, the intercept linear average method was used. 3. Results and discussion
P2a ; 4pAa
ð1Þ
where Aa and Pa represent the area and the perimeter of the primary phase in the microstructure respectively. Macro and microstructure were compared using a Rheocast Quality Index [7,15] defined by:
Rheocast Quality Index ¼
Globule size ; Grain size SF
ð2Þ
where the measured globule is the size of the primary phase in the microstructure and the grain size is defined by the macrostructure.
Figs. 3 and 4 show macrostructures of the A356 specimens cast under the different conditions. For the two different solid fraction solidifications, primary aluminum grains were smaller and more globular as the magnitude of mechanical vibration increased. Regarding the influence of pouring temperature, it was found that specimens cast at a pouring temperature of 650 °C (the highest temperature tested) showed the most pronounced change in grain morphology with increased mechanical vibration. The morphological differences in the grain became less significant as the pouring temperature decreased. It should also be noted that the specimens
Fig. 3. Macrostructures from various pouring temperatures and vibration acceleration levels at 13 wt.% solid fraction.
Fig. 4. Macrostructures from various pouring temperatures and vibration acceleration levels at 40 wt.% solid fraction.
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cast at lower pouring temperatures appeared to be less affected by mechanical vibration. Figs. 5 and 6 are micrographs of the A356 specimens cast under the different conditions. Considering the microstructure of the specimen cast at pouring temperature 650 °C and a solid fraction of 13 wt.%, the primary aluminum phase starts like a coarse and long dendrite, then became more equiaxed, then rosette-like, then partly globular over the course of solidification while mechanical vibration was varied at 0, 0.25, 0.80, and 1.90 g, respectively. A similar tendency is observed in the specimens cast from a pouring temperature of 650 °C and solid fraction of 40 wt.%. The dendrites observed in the latter, however, become coarser and more rosette-like as they approach a globular shape. This is due to the fact that the solidification
period is prolonged as the solid fraction increases, leading to the growth of more stable dendrites and remelting of fine and less stable dendrites [16]. As a result, the final dendrites are coarser. As shown in Figs. 5 and 6, the effect of mechanical vibration on the evolution of primary phase morphology appeared to be less significant in the microstructures of specimens cast at 630 °C. The primary phase of all the specimens cast under different magnitudes of mechanical vibration exhibited fairly similar globular morphology. Nevertheless, further investigation revealed that the primary phase became smaller and more globular as the mechanical vibration increased. For a given vibration magnitude, as the solid fraction was increased from 13 to 40 wt.%, the primary phase was relatively more globular and coarser.
Fig. 5. Microstructures from various pouring temperatures and vibration acceleration levels at 13 wt.% solid fraction.
Fig. 6. Microstructures from various pouring temperatures and vibration acceleration levels at 40 wt.% solid fraction.
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Let us now consider the microstructure of the specimens cast at 620 °C. The morphology of the primary phase appeared to be increasingly influenced by the mechanical vibration. In addition to the typical globular and particle-like morphology, some rosette dendrites were also observed. Formation of the rosettes dendrites is fundamentally related to dendrite fragmentation. This is responsible for the stronger influence of mechanical vibration on the primary aluminum grain observed in the specimens cast at the lower pouring temperature. Nonetheless, similar to the specimens cast at the higher pouring temperature, the primary phase became more globular and coarser as the solid fraction was increased from 13 to 40 wt.%. Fig. 7 presents a matrix of the effects of mechanical vibration, solid fraction, and pouring temperature on the Rheocasting Quality Index (RQI), which is used to practically assess the morphological quality of semi-solid structures. Detailed data for the RQI computation is summarized in Table 3. A comparison in terms of the Rheocasting Quality Index (RQI) was made between the results obtained from the present study and typical as-cast structures prepared by other methods. It should be noted that the RQI approaches 1, which indicates the best theoretical value suitable for further semi-solid processing, as the primary aluminum grains become finer and more globular. According to Zoqui et al. [7], the
RQI values of typical as-cast A356 structures are on the order of 0.02–0.03 and rheo-cast structures prepared by the electromagnetic stirring at 600, 900, and 1200 W exhibit the RQI values of 0.19, 0.23, and 0.32, respectively. The best RQI value achieved in the present study was about 0.28, and was attained by pouring at 630 °C at a solid fraction of 40%, and introducing mechanical vibration with magnitudes of acceleration at 1.9 g. Based on the findings reported above, it can be concluded that introduction of mechanical vibration to the liquid A356 alloy prior to complete solidification, under appropriate conditions of pouring temperature and solid fraction, can effectively promote favorable morphology that is suitable for further semi-solid casting. It is believed that the phenomena underlying the diminished dendrite formation involve dendrite fragmentation driven by agitated liquid as well as thermal fluctuation at the liquid–solid interfaces. Small dendrites can be readily re-melted and prohibited from growing [17]. In addition, the unstable boundary layer would become smaller because of convection induced by mechanical vibration resulting in lower constitutional undercooling of the boundary layer [12]. The lower superheat pouring route facilitates partial crystallization of a-Al rosettes more effectively leading to more favorable morphology suitable for further semi-solid casting.
Fig. 7. Effect of mechanical vibration, solid fraction, and pouring temperature on the Rheocasting Quality Index.
Table 3 Parameters for the Rheocasting Quality Index (RQI) calculation. Pouring temperature (°C)
650
Fraction of solid (wt.%)
13
40
630
13
40
620
13
40
Magnitudes of acceleration (Earth’s gravity: g)
Macrostructure Grain size (lm)
Microstructure Globule size (lm)
Shape factor
RQI
0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90
3514 ± 364 2052 ± 287 1294 ± 52 1009 ± 122 3434 ± 371 2002 ± 80 1415 ± 174 1241 ± 65 442 ± 32 409 ± 46 357 ± 13 355 ± 15 388 ± 30 309 ± 17 254 ± 3 228 ± 8 505 ± 113 359 ± 20 351 ± 46 332 ± 14 501 ± 103 380 ± 8 288 ± 6 267 ± 10
154 ± 21 131 ± 14 122 ± 12 103 ± 10 160 ± 17 157 ± 12 143 ± 9 140 ± 11 97 ± 6 90 ± 3 93 ± 3 87 ± 6 116 ± 5 106 ± 3 109 ± 4 103 ± 4 112 ± 13 90 ± 5 71 ± 6 88 ± 5 117 ± 9 112 ± 7 105 ± 7 106 ± 6
6.90 ± 1.97 3.40 ± 0.72 3.13 ± 0.53 3.11 ± 0.41 4.03 ± 0.69 3.08 ± 0.3 2.74 ± 0.41 2.77 ± 0.23 2.16 ± 0.27 1.94 ± 0.15 1.72 ± 0.05 1.77 ± 0.06 1.99 ± 0.16 1.76 ± 0.07 1.59 ± 0.05 1.63 ± 0.04 2.43 ± 0.28 1.83 ± 0.12 1.82 ± 0.10 1.95 ± 0.15 1.81 ± 0.09 1.79 ± 0.15 1.75 ± 0.17 1.68 ± 0.11
0.01 0.02 0.03 0.03 0.01 0.02 0.03 0.04 0.10 0.11 0.15 0.14 0.15 0.19 0.27 0.28 0.09 0.14 0.11 0.14 0.13 0.16 0.20 0.23
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4. Conclusions
References
It was found that, under the range of the solidification conditions investigated in this study, the average grain size of the primary aluminum phase became relatively finer and more globular as the degree of vibration increased. This suggested that during the course of solidification, normal dendrites that were originally formed in the liquid alloy were subsequently disturbed and fragmented by the mechanical vibration introduced into the melt. This effect was markedly enhanced when the vibration was introduced at larger solid fractions and lower pouring temperatures, as observed from the specimen prepared with a solid fraction of 40 wt.% and a pouring temperature of 630 °C. On the other hand, the morphological formation of the primary phase solidified from lower pouring temperatures appeared to be less dependent on the mechanical vibration. In addition to the macrostructure examination, semi-solid properties were assessed and reported using the Rheocasting Quality Index (RQI). It was shown that the introduction of mechanical vibration into the A356 melt prior to complete solidification, with a moderate pouring temperature of 630 °C and a solid fraction of 40 wt.%, successfully resulted in an as-cast structure featuring semi-solid morphology comparable to the structure of a semi-solid slurry ingot obtained via electromagnetic stirring. The RQI values of the former and the latter were 0.15–0.27 and 0.19–0.32, respectively. Conceivably, the solidification route investigated in the present study could be further developed and applied to the production of hypoeutectic aluminum– silicon alloy ingots with controlled primary aluminum phase morphology, without significant dendritic formation, suitable for semisolid casting.
[1] Gabathuler JP, Huber HJ, Erling J. Specific properties of produced parts using the thixocasting process. Metall Ital 1993;86:609–15. [2] Dahle AK, Tondel PA, Paradies CJ, Arnberg L. Effect of grain refinement on the fluidity of two commercial Al–Si foundry alloys. Metall Mater Trans 1996;A27: 2305–13. [3] Mollard FR, Flemings MC, Niyama EF. Aluminum fluidity in casting. J Met 1987;39:34–7. [4] Metz EF, Flemings MC. A fundamental study of hot tearing. In: Flemings C, editor, Symposium on solidification and materials processing, Cambridge, 2000. p. 181–8. [5] McCartney DG. Grain refining of aluminium and its alloys using inoculants. Int Mater Rev 1989;34:247–60. [6] Nafisi DG, Lashkari O, Ghomashchi R, Ajersch F, Charrette A. Acta Mater 2006;54:3503–11. [7] Zoqui EJ, Shehata MT, Paes M, Kao V, Es-Sadiqi E. Morphological evolution of SSM A356 during partial remelting. Mater Sci Eng 2002;A352:38–53. [8] Han Y, Li K, Wang J, Shu D, Sun B. Influence of high-intensity ultrasound on grain refining performance of Al-5Ti-1B alloy on aluminium. Mater Sci Eng 2005;A405:306–12. [9] Jian X, Xu H, Meek TT, Han Q. Effect of power ultrasonic on solidification of aluminium A356 alloys. Mater Lett 2005;59:190–3. [10] Liu Z, Mao W, Zhao Z. Research on semi-solid slurry of hypoeutectic Al–Si alloy prepared by low superheat pouring and weak electromagnetic stirring. Rare Metals 2006;25:177–83. [11] Wannasin J, Martinez RA, Flemings MC. Grain refinement of an aluminium alloys by introducing gas bubbles during solidification. Scr Mater 2006;55: 115–8. [12] Wu S, Xie L, Zhao J, Nakae H. Formation of non-dendritic microstructure of semi-solid aluminum alloy under vibration. Scr Mater 2008;58:556–9. [13] Taghavi F, Saghafian H, Kharrazi YHK. Study on the ability of mechanical vibration for the production of thixotropic microstructure in A356 aluminum alloy. Mater Des 2009;30:115–21. [14] Lashkari O, Nafisi S, Langlais J, Ghomashchi R. The effect of partial decanting on the chemical composition of semi-solid hypo-eutectic Al–Si alloys during solidification. J Mater Process Technol 2007;182:95–100. [15] Zoqui EJ. Morphological analysis of SSM Al-4 5 wt.% Cu measured by the rheocast quality index. J Mater Process Technol 2003;143/144: 195–201. [16] Bouchard D, Kirkaidy JS. Prediction of dendrite arm spacings in unsteady- and steady-state heat flow of unidirectionally solidified binary alloys. Metall Mater Trans B 1997;B28:651–63. [17] Wu SS, Wu XP, Xiao ZH. A model of growth morphology for semi-solid metals. Acta Mater 2004;52:3519–24.
Acknowledgments The present work was supported by the National Research Council of Thailand (2008–2009) and the Higher Education Research Budget of Production Engineering Department, KMUTT.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Metallurgical structure of A356 aluminum alloy solidified under mechanical vibration: An investigation of alternative semi-solid casting routes Chaowalit Limmaneevichitr a,*, Songwid Pongananpanya a, Julathep Kajornchaiyakul b a b
Metallurgical Engineering Program, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Prachautit Road, Thonburi, Bangmod, Bangkok 10140, Thailand Foundry Engineering Research and Development Unit, National Metal and Materials Technology Center, Thailand Science Park, Pathumthani 12120, Thailand
a r t i c l e
i n f o
Article history: Received 18 November 2008 Accepted 30 January 2009 Available online 5 February 2009
a b s t r a c t This study investigated the effects of mechanical vibration during solidification on the metallurgical structure of hypoeutectic aluminum–silicon A356. A series of casting trials were conducted. Emphasis was placed on the morphological changes of the primary aluminum phase of the as-cast alloy, which was subjected to different levels of mechanical vibration at various values of pouring temperature and solid fraction. It was found that the average grain size of the primary phase became relatively finer and more globular as the degree of vibration increased. This suggested that during the solidification process, dendrites that formed normally in the liquid alloy were subsequently disturbed and fragmented by the mechanical vibration introduced into the melt. This effect was enhanced when the vibration was introduced into an alloy with a larger solid fraction, as was observed with solidification at lower pouring temperatures. In addition to the macrostructure examination, semi-solid properties were also assessed and reported using the Rheocasting Quality Index. It was shown that the introduction of mechanical vibration into the A356 melt with adequate solid fraction prior to complete solidification successfully resulted in an as-cast structure featuring semi-solid morphology. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction It is well accepted that microstructure is one of major factors that defines the mechanical properties of aluminum alloy [1]. For general casting applications, refining so as to achieve equiaxed and fine grains will increase fluidity [2,3], reduce hot cracking [4], and reduce microporosity [5]. Non-dendritic structures will be obtained if the solidification is well grain-refined for hypoeutectic Al–Si alloy. Semi-solid processing is an attractive process for obtaining globular structures and for minimizing the porosity and segregation problems of conventional casting. However, a specific shear force from external source in semi-solid processing is needed. This is due to the thixotropic characteristics of SSM billets having high viscosity at low stresses but a decreased viscosity when an increased stress is applied [6]. Many new techniques have been developed for creating semisolid structures since the inception of semi-solid processing; for example, electromagnetic stirring [7], ultrasonic vibration [8,9], low temperature casting [10], and gas bubble purging [11]. In this study, A356 aluminum alloy was studied because of its wide variety of applications and because its chemical composition is far below the Al–Si eutectic composition, meaning it has a wide solidification range suitable for semi-solid processing.
* Corresponding author. Tel.: +66 2 470 9188; fax: +66 2 470 9198. E-mail address: [email protected] (C. Limmaneevichitr). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.01.036
In this study a robust method for introducing mechanical vibration was developed, at lower cost than ultrasonic vibration. Wu et al. [12] recently introduced mechanical vibration during isothermal holding period of A356 aluminum alloy to prepare semi-solid slurry. A similar method was also reported by Taghavi et al. [13]; however, they focused only on vibration frequency and time. The effects on microstructures of mechanical vibration acceleration, pouring temperature, and isothermal holding temperature at various solid fractions have been investigated here. 2. Experiment Table 1 summarizes nominal compositions of the A356 alloy ingot used in the present study. The alloy was melted using an induction furnace. For all casting trials, the molten metal was subjected to flux treatment with 0.5 wt.% cleaning and covering flux. Argon purging was used to reduce dissolved hydrogen. In each case, the degassing time was 2 min using a flow rate of 5 L per minute at 0.2 MPa. To introduce the mechanical vibration, a special setup of apparatus was developed as schematically illustrated in Fig. 1. A stainless steel cup, as shown in Fig. 2, was employed as a mold to cast specimens for further metallurgical investigation. The setup was equipped with a salt bath to allow isothermal holding of the melt at a specific temperature to obtain the designed solid fraction. A thermocouple, temperature controller, and data logger were connected to the salt bath to monitor and control the temperature.
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Table 1 Chemical analysis of the A356 aluminum alloy used in the present study. Element
Si
Fe
Mn
Mg
Ni
Zn
Sn
Pb
Ti
Al
wt.%
7.13
0.135
0.005
0.331
0.001
0.013
0.012
0.009
0.111
Balanced
The vibration source is a device equipped with a magnetic coil that operates at a frequency of 55 Hz, and can generate different magnitudes of mechanical vibration acceleration by varying supplied voltage. Values of the voltage supplied to the vibration source and the corresponding magnitudes of acceleration (compared to earth’s gravity: g) are shown in Table 2. In the present study, the corresponding magnitudes of acceleration define the degrees of vibration. These varying degrees of mechanical vibration were introduced into the melt at solid fractions of 13% and 40%. The casting trials, covering solidification conditions at 13% and 40% frac-
tions of solid, were carried out by pouring the liquid alloy at pouring temperatures of 620, 630, and 650 °C into the stainless steel mold, immersing that mold in a salt bath maintained at 580 °C, and then introducing the mechanical vibration to the melt until the melt temperature dropped to 608 ± 2 °C for the 13% solid and 588 ± 2 °C for the 40% solid. The fraction of solid at various temperature was calculated based on the Scheil’s equation, and assuming linear liquidus and solidus lines having partition coefficient at 0.13 [14]. Subsequently the stainless steel mold with the specimen inside was suddenly immersed in water at 30 °C with vigorous agitation.
Table 2 Series of casting trials defining the conditions, i.e. pouring temperatures, solid fractions, and vibration acceleration levels. Pouring temperature (°C)
Fraction of solid (wt.%)
Magnitudes of acceleration (Earth’s gravity: g)
650
13
0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90
40
630
13
40
620
13
40
Fig. 1. Schematic illustration of the apparatus used in the present study (not to scale).
Fig. 2. Stainless steel cup and the plane for microstructure examination.
C. Limmaneevichitr et al. / Materials and Design 30 (2009) 3925–3930
The A356 specimens were then cut into 20 mm sections from the bottom, as also shown in Fig. 2, in preparation for metallurgical examination. The shape factor used was:
SF ¼
3927
For these measurements, the intercept linear average method was used. 3. Results and discussion
P2a ; 4pAa
ð1Þ
where Aa and Pa represent the area and the perimeter of the primary phase in the microstructure respectively. Macro and microstructure were compared using a Rheocast Quality Index [7,15] defined by:
Rheocast Quality Index ¼
Globule size ; Grain size SF
ð2Þ
where the measured globule is the size of the primary phase in the microstructure and the grain size is defined by the macrostructure.
Figs. 3 and 4 show macrostructures of the A356 specimens cast under the different conditions. For the two different solid fraction solidifications, primary aluminum grains were smaller and more globular as the magnitude of mechanical vibration increased. Regarding the influence of pouring temperature, it was found that specimens cast at a pouring temperature of 650 °C (the highest temperature tested) showed the most pronounced change in grain morphology with increased mechanical vibration. The morphological differences in the grain became less significant as the pouring temperature decreased. It should also be noted that the specimens
Fig. 3. Macrostructures from various pouring temperatures and vibration acceleration levels at 13 wt.% solid fraction.
Fig. 4. Macrostructures from various pouring temperatures and vibration acceleration levels at 40 wt.% solid fraction.
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cast at lower pouring temperatures appeared to be less affected by mechanical vibration. Figs. 5 and 6 are micrographs of the A356 specimens cast under the different conditions. Considering the microstructure of the specimen cast at pouring temperature 650 °C and a solid fraction of 13 wt.%, the primary aluminum phase starts like a coarse and long dendrite, then became more equiaxed, then rosette-like, then partly globular over the course of solidification while mechanical vibration was varied at 0, 0.25, 0.80, and 1.90 g, respectively. A similar tendency is observed in the specimens cast from a pouring temperature of 650 °C and solid fraction of 40 wt.%. The dendrites observed in the latter, however, become coarser and more rosette-like as they approach a globular shape. This is due to the fact that the solidification
period is prolonged as the solid fraction increases, leading to the growth of more stable dendrites and remelting of fine and less stable dendrites [16]. As a result, the final dendrites are coarser. As shown in Figs. 5 and 6, the effect of mechanical vibration on the evolution of primary phase morphology appeared to be less significant in the microstructures of specimens cast at 630 °C. The primary phase of all the specimens cast under different magnitudes of mechanical vibration exhibited fairly similar globular morphology. Nevertheless, further investigation revealed that the primary phase became smaller and more globular as the mechanical vibration increased. For a given vibration magnitude, as the solid fraction was increased from 13 to 40 wt.%, the primary phase was relatively more globular and coarser.
Fig. 5. Microstructures from various pouring temperatures and vibration acceleration levels at 13 wt.% solid fraction.
Fig. 6. Microstructures from various pouring temperatures and vibration acceleration levels at 40 wt.% solid fraction.
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Let us now consider the microstructure of the specimens cast at 620 °C. The morphology of the primary phase appeared to be increasingly influenced by the mechanical vibration. In addition to the typical globular and particle-like morphology, some rosette dendrites were also observed. Formation of the rosettes dendrites is fundamentally related to dendrite fragmentation. This is responsible for the stronger influence of mechanical vibration on the primary aluminum grain observed in the specimens cast at the lower pouring temperature. Nonetheless, similar to the specimens cast at the higher pouring temperature, the primary phase became more globular and coarser as the solid fraction was increased from 13 to 40 wt.%. Fig. 7 presents a matrix of the effects of mechanical vibration, solid fraction, and pouring temperature on the Rheocasting Quality Index (RQI), which is used to practically assess the morphological quality of semi-solid structures. Detailed data for the RQI computation is summarized in Table 3. A comparison in terms of the Rheocasting Quality Index (RQI) was made between the results obtained from the present study and typical as-cast structures prepared by other methods. It should be noted that the RQI approaches 1, which indicates the best theoretical value suitable for further semi-solid processing, as the primary aluminum grains become finer and more globular. According to Zoqui et al. [7], the
RQI values of typical as-cast A356 structures are on the order of 0.02–0.03 and rheo-cast structures prepared by the electromagnetic stirring at 600, 900, and 1200 W exhibit the RQI values of 0.19, 0.23, and 0.32, respectively. The best RQI value achieved in the present study was about 0.28, and was attained by pouring at 630 °C at a solid fraction of 40%, and introducing mechanical vibration with magnitudes of acceleration at 1.9 g. Based on the findings reported above, it can be concluded that introduction of mechanical vibration to the liquid A356 alloy prior to complete solidification, under appropriate conditions of pouring temperature and solid fraction, can effectively promote favorable morphology that is suitable for further semi-solid casting. It is believed that the phenomena underlying the diminished dendrite formation involve dendrite fragmentation driven by agitated liquid as well as thermal fluctuation at the liquid–solid interfaces. Small dendrites can be readily re-melted and prohibited from growing [17]. In addition, the unstable boundary layer would become smaller because of convection induced by mechanical vibration resulting in lower constitutional undercooling of the boundary layer [12]. The lower superheat pouring route facilitates partial crystallization of a-Al rosettes more effectively leading to more favorable morphology suitable for further semi-solid casting.
Fig. 7. Effect of mechanical vibration, solid fraction, and pouring temperature on the Rheocasting Quality Index.
Table 3 Parameters for the Rheocasting Quality Index (RQI) calculation. Pouring temperature (°C)
650
Fraction of solid (wt.%)
13
40
630
13
40
620
13
40
Magnitudes of acceleration (Earth’s gravity: g)
Macrostructure Grain size (lm)
Microstructure Globule size (lm)
Shape factor
RQI
0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90 0.00 0.25 0.80 1.90
3514 ± 364 2052 ± 287 1294 ± 52 1009 ± 122 3434 ± 371 2002 ± 80 1415 ± 174 1241 ± 65 442 ± 32 409 ± 46 357 ± 13 355 ± 15 388 ± 30 309 ± 17 254 ± 3 228 ± 8 505 ± 113 359 ± 20 351 ± 46 332 ± 14 501 ± 103 380 ± 8 288 ± 6 267 ± 10
154 ± 21 131 ± 14 122 ± 12 103 ± 10 160 ± 17 157 ± 12 143 ± 9 140 ± 11 97 ± 6 90 ± 3 93 ± 3 87 ± 6 116 ± 5 106 ± 3 109 ± 4 103 ± 4 112 ± 13 90 ± 5 71 ± 6 88 ± 5 117 ± 9 112 ± 7 105 ± 7 106 ± 6
6.90 ± 1.97 3.40 ± 0.72 3.13 ± 0.53 3.11 ± 0.41 4.03 ± 0.69 3.08 ± 0.3 2.74 ± 0.41 2.77 ± 0.23 2.16 ± 0.27 1.94 ± 0.15 1.72 ± 0.05 1.77 ± 0.06 1.99 ± 0.16 1.76 ± 0.07 1.59 ± 0.05 1.63 ± 0.04 2.43 ± 0.28 1.83 ± 0.12 1.82 ± 0.10 1.95 ± 0.15 1.81 ± 0.09 1.79 ± 0.15 1.75 ± 0.17 1.68 ± 0.11
0.01 0.02 0.03 0.03 0.01 0.02 0.03 0.04 0.10 0.11 0.15 0.14 0.15 0.19 0.27 0.28 0.09 0.14 0.11 0.14 0.13 0.16 0.20 0.23
3930
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4. Conclusions
References
It was found that, under the range of the solidification conditions investigated in this study, the average grain size of the primary aluminum phase became relatively finer and more globular as the degree of vibration increased. This suggested that during the course of solidification, normal dendrites that were originally formed in the liquid alloy were subsequently disturbed and fragmented by the mechanical vibration introduced into the melt. This effect was markedly enhanced when the vibration was introduced at larger solid fractions and lower pouring temperatures, as observed from the specimen prepared with a solid fraction of 40 wt.% and a pouring temperature of 630 °C. On the other hand, the morphological formation of the primary phase solidified from lower pouring temperatures appeared to be less dependent on the mechanical vibration. In addition to the macrostructure examination, semi-solid properties were assessed and reported using the Rheocasting Quality Index (RQI). It was shown that the introduction of mechanical vibration into the A356 melt prior to complete solidification, with a moderate pouring temperature of 630 °C and a solid fraction of 40 wt.%, successfully resulted in an as-cast structure featuring semi-solid morphology comparable to the structure of a semi-solid slurry ingot obtained via electromagnetic stirring. The RQI values of the former and the latter were 0.15–0.27 and 0.19–0.32, respectively. Conceivably, the solidification route investigated in the present study could be further developed and applied to the production of hypoeutectic aluminum– silicon alloy ingots with controlled primary aluminum phase morphology, without significant dendritic formation, suitable for semisolid casting.
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Acknowledgments The present work was supported by the National Research Council of Thailand (2008–2009) and the Higher Education Research Budget of Production Engineering Department, KMUTT.