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Characterization of centrifugal cast functionally graded aluminum-silicon carbide metal matrix composites
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 9 2 3 –9 2 8
available at www.sciencedirect.com
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Characterization of centrifugal cast functionally graded aluminum-silicon carbide metal matrix composites T.P.D. Rajan⁎, R.M. Pillai, B.C. Pai Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology, CSIR, Thiruvanathapuram - 695 019, India
AR TIC LE D ATA
ABSTR ACT
Article history:
The present investigation is on characterization of functionally graded composites based on
Received 14 May 2007
356 cast and 2124 wrought aluminum alloys reinforced with SiC particles of 23 μm average
Received in revised form 30 May 2010
particle size processed by liquid metal stir casting followed by horizontal centrifugal casting.
Accepted 3 June 2010
A maximum of 45 and 40% SiC particles are obtained at the outer periphery of the Al(356)-SiC and Al(2124)-SiC FGMMC casting respectively. The maximum hardness obtained at the outer
Keywords:
periphery after heat treatment for Al(356)-SiC and Al(2124)-SiC FGMMC are 155 BHN and
Functionally graded material (FGM)
145 BHN respectively. The freezing range of the matrix alloy has been found to dictate the
Metal matrix composites (MMC)
nature of transition from particle enriched to depleted zone. These composites are suitable
Aluminum
for making engineering components, which require very high surface hardness and wear
Silicon carbide
resistances with high specific strength.
Centrifugal casting
1.
Introduction
Functionally graded materials (FGM) are the emerging new class of advanced materials, which exhibit gradual transitions in the microstructure and/or the composition in a specific direction, and hence different functional performance with in a part. FGM are in their early stages of evolution and are expected to have a strong impact on the design and development of new components and structures with better performance. FGMs are mostly composites having continuous variation in composition along a certain direction [1]. Functionally graded metal matrix composites (FGMMC) are FGM with metal and ceramic constituents, which are one of the most potential and prominent system for the design and fabrication of components and structures with gradient properties. FGMMC have superior capabilities for materials design and development of advanced engineering components. The specific properties obtained by the use of FGMMC are high temperature surface wear resistance, surface friction and thermal properties, adjusted thermal mismatching, reduced interfacial stresses, increased adhesion at metal–ceramic
© 2010 Elsevier Inc. All rights reserved.
interface, minimized thermal stresses, increased fracture toughness and crack retardation [2,3]. Among the various processing methods available, solidification route is preferred for FGMMC because of its economics and capability to make large size products [4]. The conventional centrifugal casting method can be used for making functionally graded metal matrix composites. This process involves synthesis of MMC by stir casting method followed by centrifugal casting to form the gradient in microstructure due to centrifugal force. When particle-containing slurry is subjected to centrifugal force, two distinct particle enriched and depleted zones are formed. Depending on the density, the lighter particles segregate towards the axis of rotation, while the denser ones move away from axis of rotation. In the case of aluminium alloy, the particle enriched zone of heavier particles such as SiC, alumina and zircon is at the outer periphery, while that of lighter particles such as graphite and mica is at the inner periphery of horizontally cast centrifugal castings. The extent of particle segregation and relative locations of enriched and depleted particles zones within the casting are mainly dictated by the melt temperature, metal viscosity, cooling rate, the densities of the particle and liquid, particle size and magnitude of centrifugal
⁎ Corresponding author. Tel.: +91 471 2515327; fax: +91 471 2491712. E-mail address: [email protected] (T.P.D. Rajan). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.06.002
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MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 9 2 3–9 2 8
acceleration. The thickness of particle enriched zone decrease with increasing pouring temperature and speed of rotation. Many studies have been reported on the use of centrifugal casting method as an efficient technique for making wide range of functionally graded metal matrix composites [1,3–11]. Most of the investigations are only on selected cast alloys (359, 356, Al–Ni and Al–Cu alloys). The present investigation is on characterization of functionally graded aluminum–silicon carbide composites using 356 cast and 2124 wrought aluminium alloys processed by liquid metal stir casting technique cum horizontal centrifugal casting method. The study also intends to evaluate the effect of variation in freezing range of the alloys on particles distribution.
2.
Materials and Methods
The two different matrix alloys used for synthesizing the FGMMC are 356 (Al-7.5Si-0.35 Mg) cast and 2124 (Al-4.5Cu1.6 Mg-0.25Zn-0.2Si) wrought aluminum alloys. Green variety SiC particles of 23 μm average particle size have been used as reinforcement. Initially, the Al(356)–15%SiC and Al(2124)–15% SiC composite melts are synthesized by liquid metal stir casting method and later shaped into hollow cylinder in a horizontal centrifugal casting machine. The composite melt at 750–760 °C is poured into a coated and preheated (250± 10 °C) metal mould, which is rotated at 1100 rpm. Figs. 1 and 2 show the schematic diagram of horizontal centrifugal casting equipment used and the typical functionally graded cylindrical castings made respectively. The dimensions of the castings made (weighing 5 kg) are 380 mm length and 120 mm diameter with a wall thickness falling between 15 and 17 mm. Rings are first cut from the casting and later sliced to specimens of 20 mm height and 40 mm length for microstructural evaluation, heat treatment and hardness testing (insert of Fig. 2). The standard heat treatment procedures of 356 alloy (solution treatment at 535 °C for 10 h, quenching in warm water and artificially aging at 165 °C for 8 h) and 2124 alloy (solution treated at 495 °C for 4 h quenching in warm water and aging at 190 °C for 5 h) are followed for the respective composites. Microstructural characterization is carried out using Leica optical microscope and Leica Qwin image analyser has been used for the measurement of volume fraction of the silicon carbide particles in the matrix. Brinell hardness measurements from outer to the inner periphery of the as-cast and heat treated specimen have been made using Indentec hardness tester.
Fig. 1 – Schematic diagram of horizontal centrifugal casting equipment used for fabricating FGMMC.
Fig. 2 – Typical functionally graded cylindrical castings made by centrifugal casting and schematic diagram from where specimen is cut (insert).
3.
Results and Discussion
3.1.
Al(356)-SiC FGM
The microstructures in Fig. 3 show the graded distribution of SiC particles in Al(356) alloy matrix. The outer periphery of the casting shows higher concentration of SiC particles than interior of the casting. The image analysis results depicted in Fig. 4 shows that the outer periphery of the cylindrical casting contains a maximum of 45 vol.% SiCp followed by a graded and reduced SiC volume percentage of 43, 37, 33 and 30 at 2, 3, 4 and 4.5 mm away from the outer periphery respectively. After 5.5 mm, the volume fraction drops sharply reaching zero at 6.5 mm. By subjecting a homogenous composite melt of Al (356)–15%SiC to centrifugal force, a maximum volume fraction of 45% has been obtained at the outer periphery leading to selective improvement in specific properties such as hardness and wear resistance. The inner periphery of the casting at 15.5 mm from outer edge has shown the presence of gas porosity and few agglomerated particles (Fig. 3f). The gas bubbles present in the melt are thrown towards the inner periphery of the casting by the centrifugal force due to their lower density. The agglomerates constituting partially wetted or non-wetted particles or both and gases having lower overall density are also pushed towards the inner periphery. Further, the movement of gas bubbles from the outer periphery towards the inner during the rotation can hinder the particle movement in the opposite direction and carry away few particles. The gas and shrinkage
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 9 2 3 –9 2 8
925
Fig. 3 – Microstructures of Al(356)-SiC FGMCC hollow cylinder fabricated by horizontal centrifugal casting. Numbers are the distances from the outer to inner periphery in mm. [(a) 1.5 mm; (b) 3.5 mm; (c) 5.5 mm; (d) 6.5 mm; (e) 12 mm; (f) 15 mm].
Fig. 4 – Graded distribution SiC particle from the outer periphery of the functionally graded Al(356)-SiC centrifugal cast ring.
Fig. 5 – Variation in hardness from outer edge of as-cast and heat treated Al(356)-SiC functionally graded composites.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 9 2 3–9 2 8
porosities pushed to the inner periphery can be removed by machining to obtain a sound casting. The microstructural features of the matrix alloy also vary from outer to inner periphery. The size of the primary aluminum phase in particle enriched zone is very fine (25 μm average particle size) [Fig. 3a and b], which becomes coarser towards interior (120 μm average particle size) [Fig. 3e]. This variation in microstructure is caused by different phenomena taking place during solidification under centrifugal force. The presence of high volume fraction of SiC particles inhibits growth of primary aluminum and also the shear caused by movement of ceramic particles during solidification can break the arms of dendrites to form fine structure. Studies by Rohatgi et al have also shown that the SiC particles have the tendency in refining matrix microstructure [12,13]. Further, in the presence of high volume fraction of particles and cen-
trifugal force, the effect is magnified. In the case of particle depleted zone also, the fine equiaxed primary phases are observed. However, these equiaxed primary phases are larger (80 μm average particle size) than those observed in the particle rich zone (25 μm average particle size). In the absence of particles, the large centrifugal force can cause the breaking of dendrites due to a column or a cantilever loading state and to a lesser degree by the shearing action of the liquid at the solid–liquid interface [8,14]. The other major observation in the matrix microstructure is the presence of higher amount eutectic silicon phase at the inner periphery of the casting (Fig. 3e). The freezing range of 356 alloy is between 615 °C (liquidus) and 564 °C (solidus) [15]. In the presence of particles, the liquidus temperature will be usually reduced by 10–12 °C [16–18]. After the solidification of primary aluminum, the remaining eutectic liquid will move
Fig. 6 – Microstructures of Al(2124)-SiC FGMCC hollow cylinder fabricated by horizontal centrifugal casting. Microstructures are given from the outer periphery towards the inner periphery at different positions (in mm) [(a) 1 mm; (b) 1.5 mm; (c) 2.5 mm; (d) 5 mm; (e) 12.5 mm].
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 9 2 3 –9 2 8
towards the inner periphery leading to a higher amount of eutectic silicon near the inner periphery. Moreover, a modified eutectic silicon structure is observed due to the agitation caused by centrifugal force in the eutectic liquid during solidification. Fig. 3 (c–d) also show the presence of iron intermetallics in the microstructure and their concentration is higher towards outer periphery. These iron intermetallic phases are formed from the iron present in the matrix alloy as well as its pick up from the dissolution of stirrer blade during particle mixing. No platelet shaped iron intermetallic phases are observed in the particle enriched zone. These phases are formed in particle free zone. As the density of these iron phases is higher than the matrix, these have a tendency to move towards the outer periphery. However, these iron intermetallic phases are formed only after the formation of primary aluminum phases. Hence, complete segregation of this phase towards outer periphery is not possible due to the hindrance for its movement from the primary aluminum phase. The hardness values measured from the outer to the inner periphery for as cast and heat treated Al(356)-SiC are shown in Fig. 5. As expected, the maximum hardness values are observed at the outer periphery of the casting due to the presence of higher volume fraction of SiC particles. In as cast Al(356)-SiC FGMMC, the outer edge hardness having a composition of Al(356)–44%SiC is 98 BHN compared to 58–60 BHN for the particle free zone. The hardness profile of heat treated Al(356)–SiC FGMMC is different from that of the as cast composite. The region A in the graph corresponds to the particle enriched zone showing a maximum hardness of 148–155 BHN. The area near to the outer periphery in region A (at 1–2 mm) slightly lower hardness is observed, which could be due the over ageing of the matrix alloy caused by higher volume fraction of SiC and presence high dislocation density. In region B, the presence of iron intermetallics in the matrix as evident from Fig. 3c and d and ageing results in higher hardness. The region C corresponds to the hardness value of normal 356 alloy matrix, although the peak value (95 BHN) is lower than the standard alloy (100–105 BHN). The region D
Fig. 7 – Graded distribution SiC particle from the outer periphery of the Al(2124)-SiC centrifugal cast ring.
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shows an increase in hardness value due to the presence of higher amount of eutectic silicon and Mg2Si precipitates.
3.2.
Al(2124)–SiC FGM
The microstructures (Fig. 6) show the graded distribution of SiC particles in Al(2124) alloy matrix. The image analysis results (Fig. 7) show that the outer periphery of the cylindrical casting contains a maximum of 40 vol.% of SiC particles followed by graded and reduced SiC volume fraction of 27, 24 and 23 at 1.5, 2 and 2.5 mm from the outer periphery respectively. The volume fraction decreases below 10% SiC at 4 mm reaching zero at 9 mm. This decrease is gradual compared to Al(356)–SiC FGM. About 1–2 vol.% of SiC particles are observed in few places towards the inner periphery of the casting. A maximum volume fraction of about 40% has been obtained by subjecting a homogenous composite melt of Al(2124)–15%SiC to centrifugal force. Like Al(356)–SiC FGMMC, the inner periphery of the casting at 16 mm from the outer edge shows the presence of gas porosity and few agglomerated particles. Comparison of the distribution pattern of silicon carbide particles in the centrifugal cast 356 and 2124 Al alloys (Figs. 3 and 6) shows that there is a sharp transition between the SiC enriched and depleted zones in 356 alloy matrix, whereas a gradual or smooth transition is seen in 2124 matrix alloy. This is obviously due to the presence of varying amount of eutectic liquid, i.e. 356 alloy contains more eutectic liquid compared to 2124 alloy, the difference in freezing range (longer freezing range of 2124 alloy — 637–490 °C than 356 alloy — 615–564 °C) and viscosity of the alloy [15]. Hence, the freezing range of the matrix alloy dictates the nature of transition from particle enriched to depleted zone. In Al(2124)–SiC FGMMC, the hardness near the outer surface is 115 BHN against 90 BHN for the particle free zone (Fig. 8). After heat treatment, peak hardness obtained at outer periphery is 145 BHN compared 115–118 BHN for the base alloy. Further the reduction in hardness is smooth from outer to inner periphery in both as cast and heat treated conditions. The enormous increase in the hardness at the outer periphery can provide significant improvement in the high temperature surface wear
Fig. 8 – Variation in hardness from outer edge of as-cast and heat treated Al(2124)-SiC functionally graded composites.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 9 2 3–9 2 8
resistance. Similarly, the high volume fraction of SiC in the outer surface can enhance the stiffness of the composite [9].
4.
Conclusions
Al-SiCp functionally graded metal matrix composites with graded distribution of SiC particles near the outer periphery of the casting have been successfully processed by centrifugal casting method using the homogenous melts containing 356 and 2124 aluminum alloys reinforced with 15% SiC particles. Maximum of 45 and 40% SiC particles are obtained near the outer periphery of the Al(356)-SiC and Al(2124)-SiC FGMMC casting respectively. The maximum hardness obtained at the outer periphery after heat treatment for Al(356)-SiC and Al (2124)-SiC FGMMC are 155 BHN and 145 BHN respectively. The longer the freezing range of the matrix alloy, the smoother will be the transition from SiC particle enriched to depleted zone. These composites can be successfully used for manaufacturing engineering components which require very high surface hardness and wear resistances with high specific strength.
Acknowledgements The authors would like to thank the Director, NIIST, CSIR, Thiruvanathapuram for his encouragement and support and, DST and CSIR for the funding. The authors are grateful to Dr. V. John, Mr. V. Antony and Mr. K.S. Shibu for their help during MMC synthesis and centrifugal casting, Mr. S.G.K. Pillai for metallography and Mr. K. Sukumaran and Mr. K.K. Ravi Kumar for hardness testing.
REFERENCES [1] Mortensen A, Suresh S. Functionally graded metals and metal–ceramic composites: Part 1, Processing. Inter Mater Rev 1995;40(6):239–65. [2] Schulz U, Peters M, Bach W, Tegeder G. Graded coatings for thermal, wear and corrosion barriers. Mater Sci Engg A 2003;362:61–80. [3] Kevorkijan Varuan. Functionally graded aluminum-matrix composites. Am Ceram Soc Bull 2003;82(2):60–4.
[4] Rajan TPD, Pillai RM, Pai BC. Solidification processing of functionally graded metal–ceramic composites. Indian Foundry J 2003;49(9):19–30. [5] Watanabe Yoshimi, Nakamura Tatsuru. Microstructures and wear resistance of hybrid Al-(Al3Ti + Al3Ni) FGMs fabricated by a centrifugal method. Intermetallics 2001;9:33–43. [6] Forster MF, Hamilton RW, Dashwood RJ, Lee PD. Centrifugal casting of aluminium containing in situ formed TiB2. Mater Sci Tech 2003;19(9):1215–9. [7] Wang Qudong, Wei Yinhong, Chen Wenzhou, Zhu Yanping, Ma Chunjiang, Ding Wenjiang. In situ surface composites of (Mg2Si + Si)/ZA27 fabricated by centrifugal casting. Mater Letts 2003;57(24-25):3851–8. [8] Rodriguez Castro R, Kelestemur MH. Processing and microstructural characterization of functionally gradient Al A359/SiCp composite. J Mater Sci 2002;37:1813–21. [9] Rodriguez Castro R, Wetherhold RC, Kelestemur MHR. Microstructure and mechanical behavior of functionally graded Al A359/SiCp composite. Mater Sci Engg A 2002;323:445–56. [10] Biesheuvel PM, Verweij H. Calculation of the composition profile of a functionally graded material produced by centrifugal casting. J Am Ceram Soc 2000;83(4):743–9. [11] Watanabe Y, Fukui Y. Fabrication of functionally graded aluminium materials by the centrifugal method. Aluminum Trans 2000;2(2):195–208. [12] Rohatgi P, Asthana R, Yarandi F. Formation of solidification microstructures in cast metal matrix particle composites. In: Rohatgi P, editor. Solidification of metal matrix composites. Warrendale: TMS; 1990. p. 50–75. [13] Rohatgi P, Asthana R. The solidification of metal-matrix particulate composites. JOM 1991;43(3):35–41. [14] Vaidhyanathan C.H., Ph.D. dissertation, (University of Wisconsin-Madison, 1994) [15] Rajan TPD, Pillai RM, Pai BC. Thermal analysis of SiC reinforced cast and wrought aluminum alloy matrix composites. Proceedings of International Conference on Non-Ferrous Metals-2002, Hyderabad; June, 2002. p. 125–30. [16] Rajan TPD, Narayan Prabhu K, Pillai RM, Pai BC. Solidification characteristics of discontinuous aluminum matrix composites. Compos Sci Technol 2007;67:70–8. [17] Rajan TPD, Pillai RM, Pai BC. Computer aided cooling curve analysis of aluminum matrix composites. Proceedings of 4th Pacific Rim International Conference on Modelling of Casting and Solidification Processes, Seoul, South Korea; Sept. 1999. p. 399–407. [18] Gowri S, Samuel FH. Effect of cooling rate on the solidification of Al-7 Pct Si-SiCp metal-matrix composites. Metall Trans A 1992;23A:3369–76.
available at www.sciencedirect.com
www.elsevier.com/locate/matchar
Characterization of centrifugal cast functionally graded aluminum-silicon carbide metal matrix composites T.P.D. Rajan⁎, R.M. Pillai, B.C. Pai Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology, CSIR, Thiruvanathapuram - 695 019, India
AR TIC LE D ATA
ABSTR ACT
Article history:
The present investigation is on characterization of functionally graded composites based on
Received 14 May 2007
356 cast and 2124 wrought aluminum alloys reinforced with SiC particles of 23 μm average
Received in revised form 30 May 2010
particle size processed by liquid metal stir casting followed by horizontal centrifugal casting.
Accepted 3 June 2010
A maximum of 45 and 40% SiC particles are obtained at the outer periphery of the Al(356)-SiC and Al(2124)-SiC FGMMC casting respectively. The maximum hardness obtained at the outer
Keywords:
periphery after heat treatment for Al(356)-SiC and Al(2124)-SiC FGMMC are 155 BHN and
Functionally graded material (FGM)
145 BHN respectively. The freezing range of the matrix alloy has been found to dictate the
Metal matrix composites (MMC)
nature of transition from particle enriched to depleted zone. These composites are suitable
Aluminum
for making engineering components, which require very high surface hardness and wear
Silicon carbide
resistances with high specific strength.
Centrifugal casting
1.
Introduction
Functionally graded materials (FGM) are the emerging new class of advanced materials, which exhibit gradual transitions in the microstructure and/or the composition in a specific direction, and hence different functional performance with in a part. FGM are in their early stages of evolution and are expected to have a strong impact on the design and development of new components and structures with better performance. FGMs are mostly composites having continuous variation in composition along a certain direction [1]. Functionally graded metal matrix composites (FGMMC) are FGM with metal and ceramic constituents, which are one of the most potential and prominent system for the design and fabrication of components and structures with gradient properties. FGMMC have superior capabilities for materials design and development of advanced engineering components. The specific properties obtained by the use of FGMMC are high temperature surface wear resistance, surface friction and thermal properties, adjusted thermal mismatching, reduced interfacial stresses, increased adhesion at metal–ceramic
© 2010 Elsevier Inc. All rights reserved.
interface, minimized thermal stresses, increased fracture toughness and crack retardation [2,3]. Among the various processing methods available, solidification route is preferred for FGMMC because of its economics and capability to make large size products [4]. The conventional centrifugal casting method can be used for making functionally graded metal matrix composites. This process involves synthesis of MMC by stir casting method followed by centrifugal casting to form the gradient in microstructure due to centrifugal force. When particle-containing slurry is subjected to centrifugal force, two distinct particle enriched and depleted zones are formed. Depending on the density, the lighter particles segregate towards the axis of rotation, while the denser ones move away from axis of rotation. In the case of aluminium alloy, the particle enriched zone of heavier particles such as SiC, alumina and zircon is at the outer periphery, while that of lighter particles such as graphite and mica is at the inner periphery of horizontally cast centrifugal castings. The extent of particle segregation and relative locations of enriched and depleted particles zones within the casting are mainly dictated by the melt temperature, metal viscosity, cooling rate, the densities of the particle and liquid, particle size and magnitude of centrifugal
⁎ Corresponding author. Tel.: +91 471 2515327; fax: +91 471 2491712. E-mail address: [email protected] (T.P.D. Rajan). 1044-5803/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2010.06.002
924
MA TE RI A L S CH A R A CT ER IZ A TI O N 61 ( 20 1 0 ) 9 2 3–9 2 8
acceleration. The thickness of particle enriched zone decrease with increasing pouring temperature and speed of rotation. Many studies have been reported on the use of centrifugal casting method as an efficient technique for making wide range of functionally graded metal matrix composites [1,3–11]. Most of the investigations are only on selected cast alloys (359, 356, Al–Ni and Al–Cu alloys). The present investigation is on characterization of functionally graded aluminum–silicon carbide composites using 356 cast and 2124 wrought aluminium alloys processed by liquid metal stir casting technique cum horizontal centrifugal casting method. The study also intends to evaluate the effect of variation in freezing range of the alloys on particles distribution.
2.
Materials and Methods
The two different matrix alloys used for synthesizing the FGMMC are 356 (Al-7.5Si-0.35 Mg) cast and 2124 (Al-4.5Cu1.6 Mg-0.25Zn-0.2Si) wrought aluminum alloys. Green variety SiC particles of 23 μm average particle size have been used as reinforcement. Initially, the Al(356)–15%SiC and Al(2124)–15% SiC composite melts are synthesized by liquid metal stir casting method and later shaped into hollow cylinder in a horizontal centrifugal casting machine. The composite melt at 750–760 °C is poured into a coated and preheated (250± 10 °C) metal mould, which is rotated at 1100 rpm. Figs. 1 and 2 show the schematic diagram of horizontal centrifugal casting equipment used and the typical functionally graded cylindrical castings made respectively. The dimensions of the castings made (weighing 5 kg) are 380 mm length and 120 mm diameter with a wall thickness falling between 15 and 17 mm. Rings are first cut from the casting and later sliced to specimens of 20 mm height and 40 mm length for microstructural evaluation, heat treatment and hardness testing (insert of Fig. 2). The standard heat treatment procedures of 356 alloy (solution treatment at 535 °C for 10 h, quenching in warm water and artificially aging at 165 °C for 8 h) and 2124 alloy (solution treated at 495 °C for 4 h quenching in warm water and aging at 190 °C for 5 h) are followed for the respective composites. Microstructural characterization is carried out using Leica optical microscope and Leica Qwin image analyser has been used for the measurement of volume fraction of the silicon carbide particles in the matrix. Brinell hardness measurements from outer to the inner periphery of the as-cast and heat treated specimen have been made using Indentec hardness tester.
Fig. 1 – Schematic diagram of horizontal centrifugal casting equipment used for fabricating FGMMC.
Fig. 2 – Typical functionally graded cylindrical castings made by centrifugal casting and schematic diagram from where specimen is cut (insert).
3.
Results and Discussion
3.1.
Al(356)-SiC FGM
The microstructures in Fig. 3 show the graded distribution of SiC particles in Al(356) alloy matrix. The outer periphery of the casting shows higher concentration of SiC particles than interior of the casting. The image analysis results depicted in Fig. 4 shows that the outer periphery of the cylindrical casting contains a maximum of 45 vol.% SiCp followed by a graded and reduced SiC volume percentage of 43, 37, 33 and 30 at 2, 3, 4 and 4.5 mm away from the outer periphery respectively. After 5.5 mm, the volume fraction drops sharply reaching zero at 6.5 mm. By subjecting a homogenous composite melt of Al (356)–15%SiC to centrifugal force, a maximum volume fraction of 45% has been obtained at the outer periphery leading to selective improvement in specific properties such as hardness and wear resistance. The inner periphery of the casting at 15.5 mm from outer edge has shown the presence of gas porosity and few agglomerated particles (Fig. 3f). The gas bubbles present in the melt are thrown towards the inner periphery of the casting by the centrifugal force due to their lower density. The agglomerates constituting partially wetted or non-wetted particles or both and gases having lower overall density are also pushed towards the inner periphery. Further, the movement of gas bubbles from the outer periphery towards the inner during the rotation can hinder the particle movement in the opposite direction and carry away few particles. The gas and shrinkage
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 9 2 3 –9 2 8
925
Fig. 3 – Microstructures of Al(356)-SiC FGMCC hollow cylinder fabricated by horizontal centrifugal casting. Numbers are the distances from the outer to inner periphery in mm. [(a) 1.5 mm; (b) 3.5 mm; (c) 5.5 mm; (d) 6.5 mm; (e) 12 mm; (f) 15 mm].
Fig. 4 – Graded distribution SiC particle from the outer periphery of the functionally graded Al(356)-SiC centrifugal cast ring.
Fig. 5 – Variation in hardness from outer edge of as-cast and heat treated Al(356)-SiC functionally graded composites.
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porosities pushed to the inner periphery can be removed by machining to obtain a sound casting. The microstructural features of the matrix alloy also vary from outer to inner periphery. The size of the primary aluminum phase in particle enriched zone is very fine (25 μm average particle size) [Fig. 3a and b], which becomes coarser towards interior (120 μm average particle size) [Fig. 3e]. This variation in microstructure is caused by different phenomena taking place during solidification under centrifugal force. The presence of high volume fraction of SiC particles inhibits growth of primary aluminum and also the shear caused by movement of ceramic particles during solidification can break the arms of dendrites to form fine structure. Studies by Rohatgi et al have also shown that the SiC particles have the tendency in refining matrix microstructure [12,13]. Further, in the presence of high volume fraction of particles and cen-
trifugal force, the effect is magnified. In the case of particle depleted zone also, the fine equiaxed primary phases are observed. However, these equiaxed primary phases are larger (80 μm average particle size) than those observed in the particle rich zone (25 μm average particle size). In the absence of particles, the large centrifugal force can cause the breaking of dendrites due to a column or a cantilever loading state and to a lesser degree by the shearing action of the liquid at the solid–liquid interface [8,14]. The other major observation in the matrix microstructure is the presence of higher amount eutectic silicon phase at the inner periphery of the casting (Fig. 3e). The freezing range of 356 alloy is between 615 °C (liquidus) and 564 °C (solidus) [15]. In the presence of particles, the liquidus temperature will be usually reduced by 10–12 °C [16–18]. After the solidification of primary aluminum, the remaining eutectic liquid will move
Fig. 6 – Microstructures of Al(2124)-SiC FGMCC hollow cylinder fabricated by horizontal centrifugal casting. Microstructures are given from the outer periphery towards the inner periphery at different positions (in mm) [(a) 1 mm; (b) 1.5 mm; (c) 2.5 mm; (d) 5 mm; (e) 12.5 mm].
M A TE RI A L S CH A RACT ER IZ A TI O N 61 ( 20 1 0 ) 9 2 3 –9 2 8
towards the inner periphery leading to a higher amount of eutectic silicon near the inner periphery. Moreover, a modified eutectic silicon structure is observed due to the agitation caused by centrifugal force in the eutectic liquid during solidification. Fig. 3 (c–d) also show the presence of iron intermetallics in the microstructure and their concentration is higher towards outer periphery. These iron intermetallic phases are formed from the iron present in the matrix alloy as well as its pick up from the dissolution of stirrer blade during particle mixing. No platelet shaped iron intermetallic phases are observed in the particle enriched zone. These phases are formed in particle free zone. As the density of these iron phases is higher than the matrix, these have a tendency to move towards the outer periphery. However, these iron intermetallic phases are formed only after the formation of primary aluminum phases. Hence, complete segregation of this phase towards outer periphery is not possible due to the hindrance for its movement from the primary aluminum phase. The hardness values measured from the outer to the inner periphery for as cast and heat treated Al(356)-SiC are shown in Fig. 5. As expected, the maximum hardness values are observed at the outer periphery of the casting due to the presence of higher volume fraction of SiC particles. In as cast Al(356)-SiC FGMMC, the outer edge hardness having a composition of Al(356)–44%SiC is 98 BHN compared to 58–60 BHN for the particle free zone. The hardness profile of heat treated Al(356)–SiC FGMMC is different from that of the as cast composite. The region A in the graph corresponds to the particle enriched zone showing a maximum hardness of 148–155 BHN. The area near to the outer periphery in region A (at 1–2 mm) slightly lower hardness is observed, which could be due the over ageing of the matrix alloy caused by higher volume fraction of SiC and presence high dislocation density. In region B, the presence of iron intermetallics in the matrix as evident from Fig. 3c and d and ageing results in higher hardness. The region C corresponds to the hardness value of normal 356 alloy matrix, although the peak value (95 BHN) is lower than the standard alloy (100–105 BHN). The region D
Fig. 7 – Graded distribution SiC particle from the outer periphery of the Al(2124)-SiC centrifugal cast ring.
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shows an increase in hardness value due to the presence of higher amount of eutectic silicon and Mg2Si precipitates.
3.2.
Al(2124)–SiC FGM
The microstructures (Fig. 6) show the graded distribution of SiC particles in Al(2124) alloy matrix. The image analysis results (Fig. 7) show that the outer periphery of the cylindrical casting contains a maximum of 40 vol.% of SiC particles followed by graded and reduced SiC volume fraction of 27, 24 and 23 at 1.5, 2 and 2.5 mm from the outer periphery respectively. The volume fraction decreases below 10% SiC at 4 mm reaching zero at 9 mm. This decrease is gradual compared to Al(356)–SiC FGM. About 1–2 vol.% of SiC particles are observed in few places towards the inner periphery of the casting. A maximum volume fraction of about 40% has been obtained by subjecting a homogenous composite melt of Al(2124)–15%SiC to centrifugal force. Like Al(356)–SiC FGMMC, the inner periphery of the casting at 16 mm from the outer edge shows the presence of gas porosity and few agglomerated particles. Comparison of the distribution pattern of silicon carbide particles in the centrifugal cast 356 and 2124 Al alloys (Figs. 3 and 6) shows that there is a sharp transition between the SiC enriched and depleted zones in 356 alloy matrix, whereas a gradual or smooth transition is seen in 2124 matrix alloy. This is obviously due to the presence of varying amount of eutectic liquid, i.e. 356 alloy contains more eutectic liquid compared to 2124 alloy, the difference in freezing range (longer freezing range of 2124 alloy — 637–490 °C than 356 alloy — 615–564 °C) and viscosity of the alloy [15]. Hence, the freezing range of the matrix alloy dictates the nature of transition from particle enriched to depleted zone. In Al(2124)–SiC FGMMC, the hardness near the outer surface is 115 BHN against 90 BHN for the particle free zone (Fig. 8). After heat treatment, peak hardness obtained at outer periphery is 145 BHN compared 115–118 BHN for the base alloy. Further the reduction in hardness is smooth from outer to inner periphery in both as cast and heat treated conditions. The enormous increase in the hardness at the outer periphery can provide significant improvement in the high temperature surface wear
Fig. 8 – Variation in hardness from outer edge of as-cast and heat treated Al(2124)-SiC functionally graded composites.
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resistance. Similarly, the high volume fraction of SiC in the outer surface can enhance the stiffness of the composite [9].
4.
Conclusions
Al-SiCp functionally graded metal matrix composites with graded distribution of SiC particles near the outer periphery of the casting have been successfully processed by centrifugal casting method using the homogenous melts containing 356 and 2124 aluminum alloys reinforced with 15% SiC particles. Maximum of 45 and 40% SiC particles are obtained near the outer periphery of the Al(356)-SiC and Al(2124)-SiC FGMMC casting respectively. The maximum hardness obtained at the outer periphery after heat treatment for Al(356)-SiC and Al (2124)-SiC FGMMC are 155 BHN and 145 BHN respectively. The longer the freezing range of the matrix alloy, the smoother will be the transition from SiC particle enriched to depleted zone. These composites can be successfully used for manaufacturing engineering components which require very high surface hardness and wear resistances with high specific strength.
Acknowledgements The authors would like to thank the Director, NIIST, CSIR, Thiruvanathapuram for his encouragement and support and, DST and CSIR for the funding. The authors are grateful to Dr. V. John, Mr. V. Antony and Mr. K.S. Shibu for their help during MMC synthesis and centrifugal casting, Mr. S.G.K. Pillai for metallography and Mr. K. Sukumaran and Mr. K.K. Ravi Kumar for hardness testing.
REFERENCES [1] Mortensen A, Suresh S. Functionally graded metals and metal–ceramic composites: Part 1, Processing. Inter Mater Rev 1995;40(6):239–65. [2] Schulz U, Peters M, Bach W, Tegeder G. Graded coatings for thermal, wear and corrosion barriers. Mater Sci Engg A 2003;362:61–80. [3] Kevorkijan Varuan. Functionally graded aluminum-matrix composites. Am Ceram Soc Bull 2003;82(2):60–4.
[4] Rajan TPD, Pillai RM, Pai BC. Solidification processing of functionally graded metal–ceramic composites. Indian Foundry J 2003;49(9):19–30. [5] Watanabe Yoshimi, Nakamura Tatsuru. Microstructures and wear resistance of hybrid Al-(Al3Ti + Al3Ni) FGMs fabricated by a centrifugal method. Intermetallics 2001;9:33–43. [6] Forster MF, Hamilton RW, Dashwood RJ, Lee PD. Centrifugal casting of aluminium containing in situ formed TiB2. Mater Sci Tech 2003;19(9):1215–9. [7] Wang Qudong, Wei Yinhong, Chen Wenzhou, Zhu Yanping, Ma Chunjiang, Ding Wenjiang. In situ surface composites of (Mg2Si + Si)/ZA27 fabricated by centrifugal casting. Mater Letts 2003;57(24-25):3851–8. [8] Rodriguez Castro R, Kelestemur MH. Processing and microstructural characterization of functionally gradient Al A359/SiCp composite. J Mater Sci 2002;37:1813–21. [9] Rodriguez Castro R, Wetherhold RC, Kelestemur MHR. Microstructure and mechanical behavior of functionally graded Al A359/SiCp composite. Mater Sci Engg A 2002;323:445–56. [10] Biesheuvel PM, Verweij H. Calculation of the composition profile of a functionally graded material produced by centrifugal casting. J Am Ceram Soc 2000;83(4):743–9. [11] Watanabe Y, Fukui Y. Fabrication of functionally graded aluminium materials by the centrifugal method. Aluminum Trans 2000;2(2):195–208. [12] Rohatgi P, Asthana R, Yarandi F. Formation of solidification microstructures in cast metal matrix particle composites. In: Rohatgi P, editor. Solidification of metal matrix composites. Warrendale: TMS; 1990. p. 50–75. [13] Rohatgi P, Asthana R. The solidification of metal-matrix particulate composites. JOM 1991;43(3):35–41. [14] Vaidhyanathan C.H., Ph.D. dissertation, (University of Wisconsin-Madison, 1994) [15] Rajan TPD, Pillai RM, Pai BC. Thermal analysis of SiC reinforced cast and wrought aluminum alloy matrix composites. Proceedings of International Conference on Non-Ferrous Metals-2002, Hyderabad; June, 2002. p. 125–30. [16] Rajan TPD, Narayan Prabhu K, Pillai RM, Pai BC. Solidification characteristics of discontinuous aluminum matrix composites. Compos Sci Technol 2007;67:70–8. [17] Rajan TPD, Pillai RM, Pai BC. Computer aided cooling curve analysis of aluminum matrix composites. Proceedings of 4th Pacific Rim International Conference on Modelling of Casting and Solidification Processes, Seoul, South Korea; Sept. 1999. p. 399–407. [18] Gowri S, Samuel FH. Effect of cooling rate on the solidification of Al-7 Pct Si-SiCp metal-matrix composites. Metall Trans A 1992;23A:3369–76.