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Cooling slope casting and thixoforming of hypereutectic A390 alloy
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 200–203
journal homepage: www.elsevier.com/locate/jmatprotec
Cooling slope casting and thixoforming of hypereutectic A390 alloy ¨ Yucel Birol ∗ Materials Institute, Marmara Research Center, Tubitak, 41470 Gebze, Kocaeli, Turkey
a r t i c l e
i n f o
a b s t r a c t
Article history:
A390 alloy offers outstanding wear resistance, high strength, low thermal expansion,
Received 9 January 2007
excellent castability and low density and is thus the material of choice for heavy wear
Received in revised form
applications. Thixoforming of this alloy has received some attention recently since the con-
15 May 2007
ventional die casting route poses serious problems. In the present work, A390 alloy feedstock
Accepted 18 December 2007
produced with cooling slope casting was thixoformed successfully at a temperature of 844 K, which is much lower than the typical die casting temperatures employed for this alloy. The thixoformed part was metallurgically sound, free from porosity and attained a hardness
Keywords:
level as high as 144 HB in T6 temper. © 2007 Elsevier B.V. All rights reserved.
Aluminium alloys Thixoforming Cooling slope
1.
Introduction
Outstanding wear resistance, high strength and low thermal expansion, together with excellent castability and reduced density, make hypereutectic Al–Si alloys such as A390 very competitive in heavy wear applications (Pratt, 1973; Rohatgi et al., 1986; Lasa and Rodriguez-Ibabe, 2002). Silicon is the first phase to form during solidification in these alloys, producing hard silicon particles throughout the aluminium matrix which are credited for the superior wear resistance (Lasa and Rodriguez-Ibabe, 2002). While A390 alloy is often die cast, die cast components tend to contain residual porosity, which is extremely detrimental to performance. The use of die cast A390 alloy has been further restricted owing to its high latent heat and consequent long solidification time resulting in die wear, to the segregation and excessive growth of primary silicon particles, and to its unfavourable shrinkage behaviour (Kapranos et al., 2003; Midson et al., 2002). Thixoforming was recently considered to be a viable alternative in the production of this alloy, as it can help to largely overcome these adversities
∗
Tel.: +90 262 6773084; fax: +90 262 6412309. E-mail address: [email protected]. 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.071
(Kapranos et al., 2003; Midson et al., 2002). In thixoforming, the casting temperature and heat content are much lower since the alloy is only partially liquid, the primary silicon is fine and uniformly distributed and the shrinkage is much less than that of a fully molten alloy (Ward et al., 1996). In addition, the die filling process can be controlled to eliminate porosity, thanks to the high viscosity of semisolid alloys. The present work was undertaken to identify a sound practice for the manufacture of A390 components via thixoforming. Microstructural features and hardness of the thixoformed A390 part were compared with those of die-cast counterpart.
2.
Experimental procedures
The chemical composition of the A390 alloy used in this study is given in Table 1. The cooling slope (CS) casting process was employed to produce the non-dendritic feedstock (Haga and Kapranos, 2002; Haga and Suzuki, 2001; Birol, 2007). A390 ingot was melted in an induction furnace set at 1023 K. The melt was
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 200–203
Table 1 – Chemical composition of the A390 alloy used in the present work Si Fe Cu Mg Zn Al
16.83 0.29 4.12 0.53 0.09 Balance
Fig. 1 – DSC curve of the A390 alloy.
then allowed to cool to the pouring temperature. In the usual CS casting process, the pouring temperature is selected so as to limit the superheat of the melt, i.e. slightly above the liquidus temperature, in order to facilitate partial crystallization of ␣-Al rosettes on the cooling plate. The pouring temperature in the present work was selected below the liquidus temperature obtained from the differential scanning calorimetry (DSC) data (Fig. 1). Formation of primary silicon crystals was already underway at the start of pouring in this practice which was adopted intentionally to limit the super heat of the eutectic melt and thus to reduce the solidification time of the matrix. The CS casting involved pouring the partially molten alloy
201
with a small fraction of Si crystals over a 0.05 m wide and 0.5 m long, inclined steel plate into a permanent mold with a diameter of 0.03 m and a depth of 0.15 m. The cooling plate was adjusted at 60◦ with respect to the horizontal plane and was cooled with water circulation underneath (Fig. 2). The best as-cast microstructural features were obtained with a pouring temperature of 858 K and a cooling length of 0.3 m. The ingots thus obtained were sectioned into 35 mm long slugs. A medium frequency induction coil (9.6 kHz, 50 kW) placed right underneath the die was used to heat these slugs in situ, into the semisolid state. Temperature was monitored with a K-type thermocouple inserted in a 0.003 m diameter hole drilled in the center of the slugs. Measures were taken to achieve rapid heating (2.5 K/s) to prevent undesirable grain growth. The slugs were then held at 844 ± 1 K for 300 s to allow spheroidization of the grains. The thixoforging operation was carried out with a laboratory press. A pneumatic cylinder was used to provide the forging load (15 kN) and the maximum speed of the ram was 0.5 m/s. The die was pre-heated to 723 K. The thermocouple was withdrawn from the sample just before forming and the slurry was forged into an arbitrary shape part (Fig. 3). The thixoformed samples were heat treated in an air furnace. The T5 temper practice involved ageing at 448 K for 21.6 ks while T6 heat treatment employed an additional solutionizing step at 773 K for 21.6 ks, followed by forced air cooling before ageing. The thixoformed and heat-treated samples were sectioned transversely and were prepared with standard metallographic practices. These samples were etched with a 0.5% HF solution before they were examined with an optical microscope. The hardness of the thixoformed and heat-treated samples were measured in Brinel units with a load of 306.25 N and a 0.0025 m diameter indenter.
3.
Results and discussion
The CS-cast ingot is characterized by a dispersion of primary Si particles in a matrix of Al–Si eutectic and some intermetallic
Fig. 2 – Cooling slope casting unit used in the present work.
202
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 200–203
Fig. 3 – Vertical mid-sectional view of the arbitrary shape part produced by thixoforming. particles (Fig. 4a). Si was shown by XRD analysis to be the predominant phase while reflections of the intermetallic phases were too small to be identified (Fig. 5). The primary Si particles exhibited polyhedral morphologies and were of sizes ranging from several to 3 × 10−5 m. Low pouring temperature and high cooling rates are responsible for the small size of the primary Si particles which are almost invariably surrounded by ␣-Al rosettes (Fig. 4a). Si depletion around primary particles in the melt before pouring started is believed to be responsible for the shift in local chemistry which facilitates the formation of primary aluminium dendrites–rosettes. The features of the thixoformed samples are typical of semisolid-state processing with predominantly ␣-Al globules and Si particles sitting in between these globules (Fig. 4b and c). Aluminium rosettes which have formed upon CS casting were found to undergo substantial spherodisation and coarsening during reheating before thixoforming. Some coarsening of primary Si particles is also noted. At a reheating temperature of 844 K, one would expect the primary Si particles and ␣-Al rosettes to remain solid while the Al–Si eutectic to melt with time. The Al–Si eutectic has indeed melted and the molten eutectic was rearranged during holding. A fraction of the dissolved Si and Al are believed to have deposited on the primary Si particles and ␣-Al rosettes, respectively, contributing to the coarsening process during holding in the semisolid state. The remaining melt was apparently forced to move around Si particles and ␣-Al rosettes during forming, as inferred from the rounding of both species. Finally, solidification of the semisolid alloy has produced a mixture of silicon particles and aluminum globules dispersed in a eutectic matrix. The latter was largely modified, thanks to the reduced latent heat and the rapid solidification which prevail under thixoforming conditions. The aluminum globules were found to vary between 5 × 10−5 and 10 × 10−5 m in diameter. The thixoformed A390 part was perfectly sound, free of porosity, in contrast to the die cast grade of the same alloy which revealed both micro and macroporosity (Fig. 4d). The matrix structure in the latter, however, was much finer. It should be noted that the size of the primary Si particles in the CS-cast ingot, which remain unmelted during rehating, was largely retained after thixoforming.
Fig. 4 – Microstructural features of (a) the CS cast ingot, (b and c) thixoformed and (d) die cast A390 part.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 200–203
203
formed part thus produced was metallurgically sound, free from porosity and revealed a uniform dispersion of fine Si particles in a homogeneous matrix. Heat treating the thixoformed part to the T6 temper produced a substantial increase in hardness. The thixoforming temperature employed in the present work, is much lower than the range of die casting temperatures (973–1033 K) used for A390. Thixoforming is thus offered as an attractive alternative in the production of these alloys, as it can overcome the problems encountered in die casting of hypereutectic Al–Si alloys.
Acknowledgements The author is indebted to O. C ¸ akır and F. Alageyik for their help in the experimental part of the work. State Planning Organization of Turkey is thanked for the financial support.
Fig. 5 – XRD pattern of the A390 alloy.
references
Table 2 – Hardness values of the die cast and thixoformed parts Process
Temper
Die casting Thixoforming Thixoforming Thixoforming
F F T5 T6
Hardness (HB) 80 90 101 144
± ± ± ±
12 4 8 3
The hardness values measured in the thixoformed A390 part in F, T5 and T6 tempers are listed in Table 2. The hardness of the thixoformed part in F temper is relatively higher than that of the die-cast part. Artificial ageing shortly after thixoforming (T5 temper) provided only a slight increase in hardness while an additional solutionizing treatment before ageing (T6 temper) resulted in significant hardening. The T6 processing of the die cast part, on the other hand, resulted in excessive blistering and is thus not applicable. While T5temper processing is attractive cost-wise, the T6 temper may be necessary when higher hardness levels are targeted. Substantial savings in both solutionizing and ageing times were claimed to be possible in thixoformed hypereutectic alloys, without a penalty in mechanical properties (Kapranos et al., 2000).
4.
Conclusions
A390 alloy feedstock produced with CS casting with a pouring temperature of 858 K and cooling length of 0.3 m, was thixoformed successfully at a temperature of 844 K. The thixo-
Birol, Y., 2007. A357 thixoforming feedstock produced by cooling slope casting. J. Mater. Process. Technol. 186, 94–101. Haga, T., Kapranos, P., 2002. Simple rheocasting processes. J. Mater. Process. Technol. 130–131, 594–598. Haga, T., Suzuki, S., 2001. Casting of aluminium alloy ingots for thixoforming using a cooling slope. J. Mater. Process. Technol. 118, 169–172. Kapranos, P., et al., 2000. In: Chiarmetta, G.L., Rosso, M. (Eds.), Proceedings of the Sixth International Conference on Semi-Solid Processing of Alloys and Composites. Turin, Italy, September 27–29. Edimet Spa, Brescia, Italy, pp. 85–90. Kapranos, P., Kirkwood, D.H., Atkinson, H.V., Rheinlander, J.T., Bentzen, J.J., Toft, P.T., Debel, C.P., Laslaz, G., Maenner, L., Blais, S., Rodriguez-Ibabe, J.M., Lasa, L., Giordano, P., Chiarmetta, G., Giese, A., 2003. Thixoforming of an automotive part in A390 hypereutectic Al–Si alloy. J. Mater. Process. Technol. 135, 271–277. Lasa, L., Rodriguez-Ibabe, J.M., 2002. Effect of composition and processing route on the wear behaviour of Al–Si alloys. Scr. Mater. 46, 477–481. Midson, S., Keist, J., Svare, J., 2002. Semi-solid metal processing of aluminum alloy A390. In: SAE 2002 World Congress, Detroit, Michigan, March 4–7, 2002-01-394. Pratt, G.C., 1973. Material for plain bearings. Int. Met. Rev. 18, 1. Rohatgi, P.K., Asthana, R., Das, S., 1986. Solidification, structures, and properties of cast metal–ceramic particle composites. Int. Mater. Rev. 131, 115–139. Ward, P.J., Atkinson, H.V., Anderson, P.R.G., Elias, L.G., Garcia, B., Kahlen, L., Rodriguez-ibabe, J.-M., 1996. Semi-solid processing of novel MMCs based on hypereutectic aluminium–silicon alloys. Acta Mater. 44, 1717–1727.
journal homepage: www.elsevier.com/locate/jmatprotec
Cooling slope casting and thixoforming of hypereutectic A390 alloy ¨ Yucel Birol ∗ Materials Institute, Marmara Research Center, Tubitak, 41470 Gebze, Kocaeli, Turkey
a r t i c l e
i n f o
a b s t r a c t
Article history:
A390 alloy offers outstanding wear resistance, high strength, low thermal expansion,
Received 9 January 2007
excellent castability and low density and is thus the material of choice for heavy wear
Received in revised form
applications. Thixoforming of this alloy has received some attention recently since the con-
15 May 2007
ventional die casting route poses serious problems. In the present work, A390 alloy feedstock
Accepted 18 December 2007
produced with cooling slope casting was thixoformed successfully at a temperature of 844 K, which is much lower than the typical die casting temperatures employed for this alloy. The thixoformed part was metallurgically sound, free from porosity and attained a hardness
Keywords:
level as high as 144 HB in T6 temper. © 2007 Elsevier B.V. All rights reserved.
Aluminium alloys Thixoforming Cooling slope
1.
Introduction
Outstanding wear resistance, high strength and low thermal expansion, together with excellent castability and reduced density, make hypereutectic Al–Si alloys such as A390 very competitive in heavy wear applications (Pratt, 1973; Rohatgi et al., 1986; Lasa and Rodriguez-Ibabe, 2002). Silicon is the first phase to form during solidification in these alloys, producing hard silicon particles throughout the aluminium matrix which are credited for the superior wear resistance (Lasa and Rodriguez-Ibabe, 2002). While A390 alloy is often die cast, die cast components tend to contain residual porosity, which is extremely detrimental to performance. The use of die cast A390 alloy has been further restricted owing to its high latent heat and consequent long solidification time resulting in die wear, to the segregation and excessive growth of primary silicon particles, and to its unfavourable shrinkage behaviour (Kapranos et al., 2003; Midson et al., 2002). Thixoforming was recently considered to be a viable alternative in the production of this alloy, as it can help to largely overcome these adversities
∗
Tel.: +90 262 6773084; fax: +90 262 6412309. E-mail address: [email protected]. 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.12.071
(Kapranos et al., 2003; Midson et al., 2002). In thixoforming, the casting temperature and heat content are much lower since the alloy is only partially liquid, the primary silicon is fine and uniformly distributed and the shrinkage is much less than that of a fully molten alloy (Ward et al., 1996). In addition, the die filling process can be controlled to eliminate porosity, thanks to the high viscosity of semisolid alloys. The present work was undertaken to identify a sound practice for the manufacture of A390 components via thixoforming. Microstructural features and hardness of the thixoformed A390 part were compared with those of die-cast counterpart.
2.
Experimental procedures
The chemical composition of the A390 alloy used in this study is given in Table 1. The cooling slope (CS) casting process was employed to produce the non-dendritic feedstock (Haga and Kapranos, 2002; Haga and Suzuki, 2001; Birol, 2007). A390 ingot was melted in an induction furnace set at 1023 K. The melt was
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 200–203
Table 1 – Chemical composition of the A390 alloy used in the present work Si Fe Cu Mg Zn Al
16.83 0.29 4.12 0.53 0.09 Balance
Fig. 1 – DSC curve of the A390 alloy.
then allowed to cool to the pouring temperature. In the usual CS casting process, the pouring temperature is selected so as to limit the superheat of the melt, i.e. slightly above the liquidus temperature, in order to facilitate partial crystallization of ␣-Al rosettes on the cooling plate. The pouring temperature in the present work was selected below the liquidus temperature obtained from the differential scanning calorimetry (DSC) data (Fig. 1). Formation of primary silicon crystals was already underway at the start of pouring in this practice which was adopted intentionally to limit the super heat of the eutectic melt and thus to reduce the solidification time of the matrix. The CS casting involved pouring the partially molten alloy
201
with a small fraction of Si crystals over a 0.05 m wide and 0.5 m long, inclined steel plate into a permanent mold with a diameter of 0.03 m and a depth of 0.15 m. The cooling plate was adjusted at 60◦ with respect to the horizontal plane and was cooled with water circulation underneath (Fig. 2). The best as-cast microstructural features were obtained with a pouring temperature of 858 K and a cooling length of 0.3 m. The ingots thus obtained were sectioned into 35 mm long slugs. A medium frequency induction coil (9.6 kHz, 50 kW) placed right underneath the die was used to heat these slugs in situ, into the semisolid state. Temperature was monitored with a K-type thermocouple inserted in a 0.003 m diameter hole drilled in the center of the slugs. Measures were taken to achieve rapid heating (2.5 K/s) to prevent undesirable grain growth. The slugs were then held at 844 ± 1 K for 300 s to allow spheroidization of the grains. The thixoforging operation was carried out with a laboratory press. A pneumatic cylinder was used to provide the forging load (15 kN) and the maximum speed of the ram was 0.5 m/s. The die was pre-heated to 723 K. The thermocouple was withdrawn from the sample just before forming and the slurry was forged into an arbitrary shape part (Fig. 3). The thixoformed samples were heat treated in an air furnace. The T5 temper practice involved ageing at 448 K for 21.6 ks while T6 heat treatment employed an additional solutionizing step at 773 K for 21.6 ks, followed by forced air cooling before ageing. The thixoformed and heat-treated samples were sectioned transversely and were prepared with standard metallographic practices. These samples were etched with a 0.5% HF solution before they were examined with an optical microscope. The hardness of the thixoformed and heat-treated samples were measured in Brinel units with a load of 306.25 N and a 0.0025 m diameter indenter.
3.
Results and discussion
The CS-cast ingot is characterized by a dispersion of primary Si particles in a matrix of Al–Si eutectic and some intermetallic
Fig. 2 – Cooling slope casting unit used in the present work.
202
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 200–203
Fig. 3 – Vertical mid-sectional view of the arbitrary shape part produced by thixoforming. particles (Fig. 4a). Si was shown by XRD analysis to be the predominant phase while reflections of the intermetallic phases were too small to be identified (Fig. 5). The primary Si particles exhibited polyhedral morphologies and were of sizes ranging from several to 3 × 10−5 m. Low pouring temperature and high cooling rates are responsible for the small size of the primary Si particles which are almost invariably surrounded by ␣-Al rosettes (Fig. 4a). Si depletion around primary particles in the melt before pouring started is believed to be responsible for the shift in local chemistry which facilitates the formation of primary aluminium dendrites–rosettes. The features of the thixoformed samples are typical of semisolid-state processing with predominantly ␣-Al globules and Si particles sitting in between these globules (Fig. 4b and c). Aluminium rosettes which have formed upon CS casting were found to undergo substantial spherodisation and coarsening during reheating before thixoforming. Some coarsening of primary Si particles is also noted. At a reheating temperature of 844 K, one would expect the primary Si particles and ␣-Al rosettes to remain solid while the Al–Si eutectic to melt with time. The Al–Si eutectic has indeed melted and the molten eutectic was rearranged during holding. A fraction of the dissolved Si and Al are believed to have deposited on the primary Si particles and ␣-Al rosettes, respectively, contributing to the coarsening process during holding in the semisolid state. The remaining melt was apparently forced to move around Si particles and ␣-Al rosettes during forming, as inferred from the rounding of both species. Finally, solidification of the semisolid alloy has produced a mixture of silicon particles and aluminum globules dispersed in a eutectic matrix. The latter was largely modified, thanks to the reduced latent heat and the rapid solidification which prevail under thixoforming conditions. The aluminum globules were found to vary between 5 × 10−5 and 10 × 10−5 m in diameter. The thixoformed A390 part was perfectly sound, free of porosity, in contrast to the die cast grade of the same alloy which revealed both micro and macroporosity (Fig. 4d). The matrix structure in the latter, however, was much finer. It should be noted that the size of the primary Si particles in the CS-cast ingot, which remain unmelted during rehating, was largely retained after thixoforming.
Fig. 4 – Microstructural features of (a) the CS cast ingot, (b and c) thixoformed and (d) die cast A390 part.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 200–203
203
formed part thus produced was metallurgically sound, free from porosity and revealed a uniform dispersion of fine Si particles in a homogeneous matrix. Heat treating the thixoformed part to the T6 temper produced a substantial increase in hardness. The thixoforming temperature employed in the present work, is much lower than the range of die casting temperatures (973–1033 K) used for A390. Thixoforming is thus offered as an attractive alternative in the production of these alloys, as it can overcome the problems encountered in die casting of hypereutectic Al–Si alloys.
Acknowledgements The author is indebted to O. C ¸ akır and F. Alageyik for their help in the experimental part of the work. State Planning Organization of Turkey is thanked for the financial support.
Fig. 5 – XRD pattern of the A390 alloy.
references
Table 2 – Hardness values of the die cast and thixoformed parts Process
Temper
Die casting Thixoforming Thixoforming Thixoforming
F F T5 T6
Hardness (HB) 80 90 101 144
± ± ± ±
12 4 8 3
The hardness values measured in the thixoformed A390 part in F, T5 and T6 tempers are listed in Table 2. The hardness of the thixoformed part in F temper is relatively higher than that of the die-cast part. Artificial ageing shortly after thixoforming (T5 temper) provided only a slight increase in hardness while an additional solutionizing treatment before ageing (T6 temper) resulted in significant hardening. The T6 processing of the die cast part, on the other hand, resulted in excessive blistering and is thus not applicable. While T5temper processing is attractive cost-wise, the T6 temper may be necessary when higher hardness levels are targeted. Substantial savings in both solutionizing and ageing times were claimed to be possible in thixoformed hypereutectic alloys, without a penalty in mechanical properties (Kapranos et al., 2000).
4.
Conclusions
A390 alloy feedstock produced with CS casting with a pouring temperature of 858 K and cooling length of 0.3 m, was thixoformed successfully at a temperature of 844 K. The thixo-
Birol, Y., 2007. A357 thixoforming feedstock produced by cooling slope casting. J. Mater. Process. Technol. 186, 94–101. Haga, T., Kapranos, P., 2002. Simple rheocasting processes. J. Mater. Process. Technol. 130–131, 594–598. Haga, T., Suzuki, S., 2001. Casting of aluminium alloy ingots for thixoforming using a cooling slope. J. Mater. Process. Technol. 118, 169–172. Kapranos, P., et al., 2000. In: Chiarmetta, G.L., Rosso, M. (Eds.), Proceedings of the Sixth International Conference on Semi-Solid Processing of Alloys and Composites. Turin, Italy, September 27–29. Edimet Spa, Brescia, Italy, pp. 85–90. Kapranos, P., Kirkwood, D.H., Atkinson, H.V., Rheinlander, J.T., Bentzen, J.J., Toft, P.T., Debel, C.P., Laslaz, G., Maenner, L., Blais, S., Rodriguez-Ibabe, J.M., Lasa, L., Giordano, P., Chiarmetta, G., Giese, A., 2003. Thixoforming of an automotive part in A390 hypereutectic Al–Si alloy. J. Mater. Process. Technol. 135, 271–277. Lasa, L., Rodriguez-Ibabe, J.M., 2002. Effect of composition and processing route on the wear behaviour of Al–Si alloys. Scr. Mater. 46, 477–481. Midson, S., Keist, J., Svare, J., 2002. Semi-solid metal processing of aluminum alloy A390. In: SAE 2002 World Congress, Detroit, Michigan, March 4–7, 2002-01-394. Pratt, G.C., 1973. Material for plain bearings. Int. Met. Rev. 18, 1. Rohatgi, P.K., Asthana, R., Das, S., 1986. Solidification, structures, and properties of cast metal–ceramic particle composites. Int. Mater. Rev. 131, 115–139. Ward, P.J., Atkinson, H.V., Anderson, P.R.G., Elias, L.G., Garcia, B., Kahlen, L., Rodriguez-ibabe, J.-M., 1996. Semi-solid processing of novel MMCs based on hypereutectic aluminium–silicon alloys. Acta Mater. 44, 1717–1727.