Title of Invention

"A METHOD FOR FABRICATING FUNCTIONALLY GRADED HYBRID METAL MATRIX COMPOSITES "

Abstract

The present invention relates to the fabrication of functionally graded hybrid metal matrix composites by centrifugal casting, based on aluminum and its alloys as matrix material and silicon carbide, alumina, graphite and primary silicon as reinforcements, wherein both the hybrid particles, denser and lighter reinforcements with respect to matrix alloy are distributed at the inner periphery of the hollow cylindrical casting. In general, during centrifugal casting the lighter and denser reinforcements than the matrix alloy move towards the inner and the outer periphery of the casting respectively. On the other hand, in the present invention, both the denser and lighter reinforcements are distributed with a gradient near the inner periphery by controlling the material and solidification parameters such as melt viscosity, composite pouring temperature, mould temperature, heat extraction rate and mould spinning speed.

Full Text

The present invention relates to the field of composite materials and casting technology and more particularly to a method for processing functionally graded hybrid metal matrix composites wherein a hollow cylindrical casting formed by centrifugal casting having both low and high density reinforcement is formed near the inner periphery of casting. Background of the Invention Functionally graded materials (FGM) have emerged as one of the advanced materials with multifunctional behaviour and performance. It finds wide application in the field of automotive, aerospace, defence, electronics, biomedical and power engineering. FGMs exhibit gradual transitions in the microstructure and/or the composition in a specific direction, the presence of which leads to variation in the functional performance within a part. FGMs are in their early stages of evolution and expected to have a strong impact on the design and development of new components and structures with better performance. The specific properties of functionally graded metal ceramic composites are high temperature surface wear resistance, surface friction and thermal properties, adjustable thermal mismatching, reduced interfacial stresses, increased adhesion at metal-ceramic interface, minimized thermal stresses, increased fracture toughness and crack retardation. Among the wide variety of processing methods available for the manufacturing of metal-based FGMs, centrifugal casting has emerged as the simplest and cost effective technique for producing large-scale engineering components. The composition gradient is formed mainly from the difference in the centrifugal force produced by the density difference between the molten metal and phases or particles present in it. Centrifugal Casting of MMC involves casting of MMC synthesised by stir casting method followed by solidification under the centrifugal force leading to the formation of gradients in microstructure. When particle-containing slurry is subjected to centrifugal force, two distinct zones of particle enriched and depleted are formed. The extent of particle segregation and relative locations of enriched and depleted particles zones within the casting are mainly dictated by the densities of the particles and liquid metal. The other factors influencing the particle segregation are the melt temperature, melt viscosity, cooling rate, the, particle size and magnitude of centrifugal acceleration. Depending on the density of particles, the lighter particles segregate towards the axis of rotation, while the denser particles move away from the axis of rotation. In the case of aluminium alloy, the particle enriched zone of the heavier particles such as SiC, alumina and zircon are at the outer periphery and that of the lighter particles such as graphite, mica and micro balloons of carbon are at the inner periphery of horizontally spun cylindrical centrifugal castings. The thickness of particle enriched zone decreases with increasing pouring temperature and speed of rotation. As known from the work of B.P. Krishnan, H.R. Shetty and P.K. Rohatgi, (Trans AFS, 1976, 84, 73) and P.K. Rohatgi, R. Asthana and S. Das, (Inter. Mater. Reviews, 1986, Vol 31, No. 3, 115-139) centrifugally cast aluminum-graphite composite cylinders have already been developed where lighter graphite particles are segregated to the inner periphery of the casting, where they give anti-friction properties for bearing applications, whereas the graphite free outer zone provides a strong and tough backing material. Centrifugal casting of Aluminum alloy cylinders dispersed with mica [Deonath and P.K. Rohatgi, J. Compos., 1981, 12(2), 124] and porous alumina [M.K. Surappa, Ph.D. thesis, Indian Institute of Science, Bangalore, 1979] has resulted in similar segregation of reinforcements near the inner periphery. When Al alloy containing zircon particles is centrifugally cast, the heavier zircon particles segregate near the outer periphery of the hollow cylinder providing abrasion resistant surface [P.K. Rohatgi, R. Asthana and S. Das, Inter. Mater. Reviews, 1986, Vol 31, No. 3, 115-139]. Reference may also be made to Noordegraaf et al, US Patent 5,002,115, March 26, 1991, which describes a process for casting of shaped objects by subjecting a mould to a centrifugal acceleration directed to the bottom of the mould and introducing a molten pure metal or alloy into the mould, wherein a dispersed filler is placed before adding the melt and retained immovably during the addition of the melt by using a cover plate. Alternatively, Chamlee et al, US Patent 5,980,792, 1999, has applied centrifugal casting to establish a gradient profile of reinforcement particulate densities in a carrier matrix of a polymer resin and in an aluminum alloy. Cast disks of silicon carbide (SiC) reinforced resin and Al-Si/SiC particulate reinforced MMCs have been produced with a gradient of SiC increasing from a denuded region adjacent to an axis of rotation of the disk to a maximum density at the periphery of the disk. When solidification accompanies centrifuging, the segregation of particles due to particle movement is slowed down as a result of decreasing melt temperatures during cooling and crowding of particles occurring in progressively narrow zones. As known from Velhinho etal, Nuclear Instruments and Methods in Physics Research B 200 (2003) 295-302, functionally graded SiCp-reinforced aluminium matrix composites produced by centrifugal casting shows that SiC particles are partially clustered, with some pores due to imperfect wetting of ceramic particles by the molten aluminium alloy. Hence, proper wetting of particles with the matrix is necessary to reduce the clustering of particles. Functionally gradient Al (A359)/SiCp composites have been fabricated by centrifugal casting by Rodriguez etal. [R. Rodriguez-Castro, R. C. Wetherhold and M. H. Kelestemur, Mater. Sci. and Engg. A, 323 (1-2 ), 2002, 445-456] and it is observed that the concentration of particles varies smoothly throughout the reinforced region, with a decreasing SiC volume fraction as a function of position giving the functionally graded microstructure. Al-SiC FGMCC fishing boat cable pulleys are fabricated successfully by centrifugal casting method [A. Nordmark, Stoberiet, , July 1992, 6-9]. In-situ synthesized and centrifugally cast metal-intermetallic FGMs of Al-Al3Ni | Y. Fukui, K. Takashima and C.B.Y. Ponton, J Mater Sci 1994, 29, 2281-8, Y. Fukui, JSME Int. J., Ser. Ill, 1991, 34, 144-148, Y. Fukui, N. Yamanaka and Y. Enokida, Composites Part B 28B (1997) 37-43 and Matsuda K, Watanabe Y, Fukui Y., Ceramic Trans 2001 ;114:491-8.], Al-Al2Cu [Y. Watanabe and S. Oike, Acta Materilia, 2005, 53, 1631-1641] and Al- (A 13Ti+A 13Ni) [Y. Watanabe and T. Nakamura] fabricated by centrifugal casting creates a compositional gradient distribution of primary intermetallic phase in an aluminum matrix, near the outer periphery of the casting. Copper alloy- graphite composites with graded distribution of graphite near the inner periphery have been synthesized by centrifugal casting for tribological applications [J.K. Kirn, M. Kestursatya and P.K. Rohatgi, Metall. Mater. Trans., 2000, Vol. 31 A, 1283-1293 and M. Kestursatya, J.K. Kim and P.K. Rohatgi, Metall. Mater. Trans., 2000, Vol. 32A, 2115-2125]. Selectively Reinforced Casting (SRC), is a process developed by Pacific Northwest National Laboratory on the basis of centrifugal casting method to produce non-homogenously reinforced / functionally graded material (FGM) components from stir-cast paniculate metal matrix composites (MMC) |http://www.technet.pnl.gov/dme/materials/casting.stm]. The process uses the radial forces generated from centrifugal casting to segregate a second discrete phase from the matrix of MMC materials. Selectively Reinforced Casting can produce parts with a high density of particles in the reinforced region. Reinforcement density can reach levels > 50% by volume, with starting feedstock levels of only 10%. This can reduce the material cost of a MMC component, by minimizing the amount of costly reinforcement phase needed in the feedstock material. The inventions made hitherto describe the fabrication of functionally graded metal matrix composites by centrifugal casting with single type of reinforcements wherein with respect to matrix alloy, the lighter particles segregate near the inner periphery and the denser particles towards the outer periphery of the casting. The presence of graphite particles alone at the inner periphery of the hollow composite cylindrical casting gives better adhesive wear resistance and poor abrasive wear resistance. Hard particles such as SiC or alumina have to be dispersed along with graphite to have better abrasive wear resistance. In general, during centrifugal casting of composite melt low density particles segregate to the inner periphery and the high-density particles towards the outer periphery. In view of the above the present invention provides a process for manufacturing functionally graded hybrid composites with high density hard particles such as SiC and alumina and low density soft particles such as graphite / mica / microballons of carbon at the inner periphery of a hollow cylindrical casting. These composites find application in making cylinder liner for automotive application. Summary of the present Invention It is therefore the primary object of the invention to fabricate a hybrid functionally graded metal ceramic composite with low density soft particle such as graphite and high density hard particle such SiC / alumina to the inner periphery providing both adhesive and abrasive wear resistance. In order to obtain the functionally gradient hybrid composite, the hybrid composite mixture processed by stir casting method is taken and centrifugally cast under controlled and selective processing conditions. Accordingly the invention provides a process for fabricating functionally graded hybrid composite hollow casting of light metal matrix alloys reinforced with high and low density and/or hard and soft hybrid particles, in which hybrid particles are distributed near the inner periphery of the hollow casting, the process comprising the steps of (a) Pre-treatment of reinforcements at 400 - 800 °C in order to remove the volatile matters from its surface and to artificially oxidize the SiC surface, (b) Melting of light metal matrix alloy at a temperature of 500 - 1000 °C and adding the pretreated reinforcement to it so as to obtain a composite slurry, (c) Pouring of the composite slurry into a preheated rotating mould in a horizontal / vertical centrifugal casting machine rotating at an rpm of 1000 to 2500 to enable the formation of particle distribution gradient of hybrid particles near the inner periphery of the casing, and (d) Solidifying the above composite slurry in the rotating mould by compressed air cooling, water-cooling, compressed air & water mix cooling or atmospheric cooling. These and other objects, features and advantages of the present invention will become more readily apparent from a reading of the following detailed description taken in conjunction with the drawings In the drawings accompanying this specification Figure 1 represents the machined surface of vertical centrifugal cast LM25-10%SiC-5%Graphite FGM hollow cylinder Figure 2 represents the machined surface of vertical centrifugal cast LM13-10%SiC-5%Graphite FGM hollow cylinder Figure 3 represents the optical micrographs of centrifugally cast LM13-10%SiC-5%Graphite hybrid composite hollow cylinder by vertical centrifugal casting Figure 4 represents the optical micrographs of LM 13-10% SiC-5%Gr hybrid composite hollow cylinder by horizontal centrifugal casting Figure 5 represents the optical micrographs of vertical centrifugally cast Al-20% Si -5% Graphite FGM hollow cylinder Detailed description of the invention It should be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore the details disclosed herein are not to be interpreted as limiting but merely as the basis for the claims and as a basis for teaching one skilled in the art how to make or use the invention. The present invention provides methods for fabricating functionally graded hybrid aluminium matrix composites by centrifugal casting of stir cast hybrid composite followed by having high density and low density particles as well as hard and soft particles near the inner surface. The hybrid particles may be added ex-situ or formed in-situ. The steps involved in the fabrication of functionally graded hybrid aluminium matrix composites are synthesis of hybrid aluminium matrix composites by liquid metal stir casting followed by fabrication of hybrid functionally graded composites by centrifugal casting. (i) Synthesis of hybrid aluminium matrix composite slurry Synthesis of hybrid aluminium matrix composites involves the heat treatment of reinforcements, prior mixing of reinforcements and addition of mixed reinforcements into the molten aluminium. The particles are preheated separately in the range of 400 - 1000 °C and mixed thoroughly. The mixed particles heated again in the above range are added into the molten metal in the semisolid or above liquidus temperature. The liquid metal is then stirred in the range of 300-1000 rpm. The volume fraction of the particles may be from 1 - 30 %. (ii) Fabrication of hybrid functionally graded composites by centrifugal casting The centrifugal casting technique used may be either vertical or horizontal. The composite slurry containing the hybrid particles is poured into a mould rotating at a speed of 500-5000 rpm. The mould may be at room temperature or at a preheated temperature of 30-500 °C. The mould is rotated for 1-30 minutes. The Figures 1 and 2 show the photographs of LM25 / 356 (7Si, 0.35 Mg, Bal. - Al) and LM13 aluminium alloys (0.7-l.SCu, 10-12SJ, 0.8- l.SMg, 0.5 max Mn, 1.0 max Fe, 0.2 max Ti, 1.0-l.SNi, 0.5 max Zn, 0.1 max Pb, 0.1 max Sn, Bal.- Al) reinforced with 10%SiC and 5% Graphite hybrid functionally graded composite hollow cylinder fabricated by vertical centrifugal casting. The dark shaded inner periphery of composites shows the presence of SiC and Graphite hybrid particles distribution. Example 1 Processing of LM13-10%SiC-5%Graphite functionally graded hybrid composite hollow cylinder by vertical centrifugal casting LM13-10%SiC-5%Graphite functionally graded hybrid composite is synthesized hy stir casting method. LM13 alloy is melted in a clay-graphite crucible above its liquidus temperature. The preheated particles of silicon carbide of 23 microns average particle size (APS) and graphite of 75 microns APS are mixed and added to the molten metal, which is stirred using a motor driven impeller. The composite slurry is then poured into a preheated rotating mild steel mould of 2000 rpm in vertical centrifugal casting machine driven by electric motor. The mould is then rotated for 5 minutes. The macrostructure (Figure 2) shows a dark ring of particle segregation near the inner periphery of casting. Figure 3 shows the optical micrograph of LM13-10%SiC-5%Graphite hybrid composite hollow cylinder. A graded distribution of SiC and graphite near the inner periphery is observed as depicted in Figures 3(c) and 3(d), whereas there is no particle distribution in between the layer near inner periphery and outer periphery (Figure 3(b)). Outer periphery (Figure 3(a)) shows a very thin layer of SiC particle solidified initially. The presence of SiC and graphite particles near the inner periphery of casting aids in improving abrasive and adhesive wear resistances of the composite components such as cylinder liner. Example 2 Processing of LM 13-10% SiC-5%Gr hybrid composite hollow cylinder by horizontal centrifugal casting LM13 alloy is melted in a clay-graphite crucible in a resistance furnace and the preheated hybrid particles are added to the melt in the temperature range of 720 - 750 °C. '1'he hybrid particles are mixed using an impeller driven by a motor at a speed of 600-750 rpm. The particle addition time is 20 minutes with a post addition stirring for 15 minutes, the hybrid composite slurry is then poured into a rotating mould kept on the rollers of a horizontal centrifugal casting machine through a pouring cup. The mould is rotated at 1000 rpm for 5 minutes. Figure 4 shows the optical photomicrograph of LM13-10%SiC- 5%Graphite hybrid composite cylinder. Hybrid particles of SiC and graphite are distributed near the inner periphery of the casting as shown in Figures 4(c) and 4(d). Figure 4(b) shows only very few particles in between the layer near inner and outer periphery. Outer periphery (Figure 4(a)) shows only a thin layer of SiC particle solidified initially. Example 3 Processing of AI-20% Si -5% Graphite hybrid composite FGM cylinder by vertical centrifugal Casting Al-20%Si alloy is melted in a clay-graphite crucible in a resistance furnace and the graphite particles are added to the melt at 740-760 °C. The liquid metal stir casting method is used for making the composite. Here, the hybrid composite is formed by ex-situ addition of graphite particles and in-situ formation of primary silicon particle during solidification. The composite slurry containing the graphite is poured into a mould rotating at 2000 rpm of a vertical centrifugal casting machine. Figure 5 shows the optical photomicrographs of vertical centrifugally cast Al-20% Si -5% Graphite hybrid FGM hollow cylinder. The graphite and in-situ formed primary silicon particles are distributed near the inner periphery of the casting, where as there is no graphite or primary silicon particles away from inner periphery making the composite hollow cylinder a functional graded component. Advantages The invention provides a simplest method for making a functionally graded composite placing both hard and soft as well as denser and lighter reinforcements near the inner periphery of a hollow cylindrical casting. The presence of hard and soft reinforcements can offer both abrasive and adhesive wear resistance. The present process uses the conventional centrifugal casting technique with controlled processing and material parameters. Further, the quantity of reinforcement required to achieve a graded distribution will be minimum compared to the homogenous composites. While it is apparent that the invention herein disclosed is well calculated to fulfil the objects above stated, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention 1. A process for fabricating functionally graded hybrid composite hollow casting of light metal matrix alloys reinforced with high and low density and/or hard and soft hybrid particles, in which hybrid particles are distributed near the inner periphery of the hollow casting, the process comprising the steps of (a) Pre-treatment of reinforcements at 400 - 800 °C in order to remove the volatile matters from its surface and to artificially oxidize the SiC surface. (b) Melting of light metal matrix alloy at a temperature of 500 - 1000 °C and adding the pretreated reinforcement to it so as to obtain a composite slurry. (c) Pouring of the composite slurry into a preheated rotating mould in a horizontal / vertical centrifugal casting machine rotating at an rpm of 1000 to 2500 to enable the formation of particle distribution gradient of hybrid particles near the inner periphery of the casing, and (d) Solidifying the above composite slurry in the rotating mould by compressed air cooling, water-cooling, compressed air & water mix cooling or atmospheric cooling. We claim: 1. A method for fabricating functionally graded hybrid metal matrix composites which are hollow casting of light metal matrix alloys reinforced with high and low density and/or hard and soft hybrid particles, in which hybrid particles are distributed near the inner periphery of the hollow casting, the process comprising the steps of: (e) pre-treatment of hybrid particles at 400 - 800°C in order to remove the volatile matters from its surface and to artificially oxidize the Sic surface, (f) melting of light metal matrix alloy at a temperature of 500 - 1000 "C and adding the pretreated hybrid particles of step a) to it so as to obtain a composite slurry, (g) pouring of the composite slurry into a preheated rotating mould in a horizontal / vertical centrifugal casting machine rotating at an rpm of 1000 to 2500 to enable the formation of particle distribution gradient of hybrid particles near the inner periphery of the casting, and ) solidifying the above composite slurry in the rotating mould by compressed air cooling, water-cooling, compressed air & water mix cooling or atmospheric cooling to obtain hybrid metal matrix composites. 2. A method as claimed in claim 1, wherein the matrix alloys used are based on aluminum, magnesium, titanium or copper and the hybrid particles are silicon carbide, alumina, spinels, graphite, fly ash, mica, zircon, zirconia and micro balloons of carbon and silica. 3. A method as claimed in claim 1, wherein the centrifugal casting is horizontal, vertical or semi centrifugal. 4. A method as claimed in claim 1, wherein the hybrid particle is added either above liquid femperature or at semisolid temperature and poured above liquid temperature. 5. A method as claimed in claim 1, wherein the mould used is vertical, horizontal or rotated in any axis. 6. A method as claimed in claim I, wherein the mould material is metallic, ceramic or sand lined.

Documents:

903-del-2006-Abstract-(12-03-2013).pdf

903-del-2006-abstract.pdf

903-del-2006-Claims-(12-03-2013).pdf

903-del-2006-claims.pdf

903-del-2006-Correspondence Others-(12-03-2013).pdf

903-del-2006-correspondence-others..pdf

903-del-2006-correspondence-others.pdf

903-del-2006-Description (Complete)-(12-03-2013).pdf

903-del-2006-description (complete).pdf

903-del-2006-Drawings-(12-03-2013).pdf

903-del-2006-drawings.pdf

903-del-2006-form-1.pdf

903-del-2006-form-18.pdf

903-del-2006-form-2.pdf

903-del-2006-Form-3-(12-03-2013).pdf

903-del-2006-form-3.pdf

903-del-2006-form-5.pdf