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Metal Matrix Composites, Recycling of
Metal Matrix Composites, Recycling of$ MSH Bhuiyan, University of Malaya, Kuala Lumpur, Malaysia HP Degischer, Technische Universität Wien, Austria r 2016 Elsevier Inc. All rights reserved.
Recycling of a material is defined as submitting the material to secondary processing steps that produce the same material with the same properties as the original primary product. Recycling can in some instances be conducted several times, the material undergoing, after primary processing, several cycles of secondary processing, service, and recovery without significant degradation. This is, for instance, the case for conventional aluminum alloys (Krüger, 2000; ASM, 1999), which can be remelted and processed with full restoration of the primary microstructure, resulting in identical material properties. Recycling can be done within the production line without any service exposure of the material, or after re-collection of components already out of service. In the latter case some degradation of the material is expected, which would have to be repaired by the recycling process. Alternatively, a material can be processed by reclamation, in which case it is processed to a different form or microstructure to produce a different, but still useful, material. In a real recycling process of composite materials, the first step is the dissolution of the organic matrix (e.g., in supercritical water). During the second step of the recycling process, the recycled fibers are in the form of pieces of dry fabric (without sizing). Then, dry carbon fabric has to be unwoven and the recovered carbon fibers have to be realigned and reshaped (Zimmermann and Zattera, 2013). Several types of recycling method used in recycling metal matrix composites, for example, chemical recycling, microwave heating, mechanical recovery, and fluidized beds, etc., are the usual approaches used at lab scale; pyrolysis is the furthest advancement in recycling processes, which has now reached early stages of commercialization (Witik et al., 2013). Primary processing of metal matrix composites is essentially the bonding of the composite ingredient materials, metallic matrix, and reinforcement, so as to form a percolating metallic matrix containing the reinforcement as an embedded second constituent. The main issues in primary processing of metal matrix composites are to achieve bonding between the two usually nonwetting ingredient materials and to avoid deleterious interface reactions after contact between these constituent materials is established. Frequently, primary metal matrix composite processing yields near-net-shape products, at times by the very nature of the process, and also often because these composites are very difficult to machine. Depending on the composite and the component at hand, the material can be recycled, or alternatively more profitably reused in another form after reclamation. Recycling of metal matrix composites takes advantage of the already achieved bonding between matrix and reinforcement. Recycling of metal matrix composites is attractive, given that it provides the commercial benefit of avoiding the frequently costly primary processing, and on some occasions saves expensive reinforcements. There are, however, some specific issues that must be taken into account in the recycling of these materials. Namely, it is important to avoid excessive thermal exposure in the recycling process if the constituents are mutually prone to chemical interaction. Also, because metal matrix composites are prone to accumulation of internal damage during deformation, their recycling or reuse will often require severe nondestructive testing to verify that the microstructural integrity has been preserved. Another concern that is specific to the recycling of metal matrix composites is that the reinforcement of metals is frequently only used for partial, localized, reinforcement of metal-based components. With such components, an additional effort is necessary to separate the composite from the unreinforced section, in order to allow recycling of both materials separately. Reclamation of the composite ingredient materials requires work to separate the two constituents of the metal matrix composite. Although the interface free energy of the reinforcement in the composite often exceeds its surface free energy in air, matrix/ reinforcement separation does not occur spontaneously because the complementary free surface energy of the matrix has to be formed anew in the process. This results in a positive total free energy barrier, which must be overcome in the reclamation process. This barrier can be overcome by several methods, one being the use of insoluble flux materials having a smaller surface energy with the reinforcement than the matrix. Other processes for metal matrix composite reclamation rely on gravity for separation of the composite constituents. These can be aided by blowing bubbles of an appropriate gas through the molten composite, so as to separate the reinforcement by attachment and flotation with gas bubbles (this process is akin to froth flotation processes used in ore preparation). An additional issue in reclamation of expensive reinforcements like fibers is that their properties are often degraded in the process. In some cases, when the reinforcement is far more costly than the matrix, reclamation of the reinforcement alone can be envisaged; this is achieved with relative ease by dissolving the matrix chemically. Given the relatively wide variety of metal matrix composites and components made therefrom, recycling and reclamation of these materials raises issues that depend on the materials class. Issues specific to the two principal metal matrix composite classes are therefore more specifically addressed in what follows, together with questions specific to reclamation of composite ingredients.
☆ Change History: August 2015. M.S.H. Bhuiyan added Abstract and Keywords, extended the Introduction and the subsection 'Reclamation of Ingredients,' and updated the reference section.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.03648-1
1
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Metal Matrix Composites, Recycling of
Particle Reinforced Metals Cast primary metal matrix composite products containing relatively low volume fractions of discontinuous reinforcements, like foundry ingots, extrusion billets, or rolling slabs, can be recycled with comparative ease. If the recovered metal matrix composite cannot be used for the same component and application at equal performance quality, reuse can be envisaged for different applications allowing a certain degree of downgrading of the performance of the material. In the case of components selectively reinforced by particulates, remelting results in dilution of the reinforcement, yielding a different composite; here again, the resulting material could be reused in a different application. With particulate reinforcements at volume fractions less than 30%, recycling of the composite can be achieved by remelting if the constituents are not overly reactive. The foundry composite alloy AlSi9Mg/SiC/10–20p (Lloyd, 1989) provides an example where in-house return rates in the production line may amount to up to 50% without property degradation (Klimowicz, 1994). Some specific measures are recommended for the recycling procedure: dry preheated scrap and tools, no overheating of the melt above 750 1C, impellers to produce convection beneath the dross without formation of a vortex, and moderate fluxing with argon or SF6 (Provencher et al., 1992; Chamberlain and Bruski, 1998). Careful sorting and cleaning is required before the recycling of used particle reinforced metal components in a similar remelting process not yet established industrially. Aluminum alloys of the 2xxx, 6xxx, and 7xxx series reinforced with alumina particles react at the interface even close to the solidus temperature. Therefore, any remelting process can potentially alter the composite constituent phases and properties. In the case of AA6061/Al2O3/10–20p it is reported (Schuster et al., 1993) that the amount of interface spinel formation stabilizes without significant influence on the mechanical properties of the composite. Recycling of Duralcan wrought alloys is so far only executed on a large scale by the producer (Klimowicz, 1994). Composite systems such as Mg–SiC, Al–SiC with little silicon content, Al–B4C, and Cu–SiC, are too reactive to allow remelting without severe interface reactions. Iron-base alloys with Ti(C,N) reinforcement stabilize at the solubility limit, which depends on the concentration of other carbide-forming alloying elements. It has been proposed to reuse particle reinforced aluminum alloys for the production of aluminum foam (Gergely et al., 2000). Indeed, several closed-cell aluminum foam production processes rely on the presence of ceramic particles within the melt for stabilization of the foam structure during the process (Jin et al., 1992; Ruch and Kirkevag, 1991; Gergely and Clyne, 1999). One advantage of this recycling route is that property requirements placed on composites to be used for the production of metallic foams may not be as stringent as for bulk composites. Particle reinforced metals also have the attribute that they can be disintegrated into smaller fragments without disintegration of their microstructure. Machining-chips of these materials can thus be reused to fabricate MMC components by solid state compaction methods. This solid state process has the added advantage of minimizing interfacial reactions between matrix and reinforcements.
Melt-Infiltrated Preforms Metal matrix composites with short or continuous fibrous reinforcements are usually produced by infiltration of preforms, in a process that also defines the final shape of the metal matrix composite, and which can also produce fully or partially reinforced components. Recycling of such composites would thus require preservation of the shape of the component. For example, the alumina short fiber preform ring reinforcing the crown of an aluminum alloy piston can be recuperated during remelting. Although the matrix is molten, such MMCs are mechanically stable up to about 850 1C (Burkhard, 1995) and can be handled. The integrity of such MMC rings can be confirmed by nondestructive testing and inserted again into the casting of a piston, which has only to achieve bonding between the composite and the matrix, as the primary casting process had already infiltrated the preform. Recycling of MMC sections of defined shape can be envisaged for alumina fiber reinforced aluminum (or magnesium alloys, provided that recuperation of the composite from the melt is rapid enough to restrict interface reactions), SiC reinforced aluminum–silicon alloys, carbon fiber reinforced magnesium, and, with some reservations, for monofilament reinforced titanium alloys. Particle reinforced metals with particulate volume fractions greater than 40% are also frequently prepared by melt infiltration of powder preforms. It may be envisaged to reshape simple geometries of these composites to some extent by forming processes; if remelted, the volume fraction of reinforcement might be increased if some of the matrix is squeezed out during secondary pressure casting (Burkhard, 1995). Alternatively, these composites could be used as a basis for the production of lower volume fraction composites by remelting and dilution by a matrix melt (Klier et al., 1991).
Reclamation of Ingredients Isolated ceramic particulates or fibers can be removed from aluminum or magnesium melts by conventional salt addition or fluxing techniques as executed to remove oxide films (Provencher et al., 1992). By gravity settling, the fluxed ceramic ingredients
Metal Matrix Composites, Recycling of
3
float to the dross at the top of the aluminum melt, or accumulate as sediment at the bottom of a magnesium alloy. Rotary salt furnace technology is an established reclamation process to recover aluminum from various mixtures, including particle reinforced metals; however, this requires 20–50wt.% salt (Duralcan, 1990). Both wrought and foundry alloys can be recovered using this technique, even machining-chips of fiber reinforced MMC. Typically, about 80% recovery of the available aluminum can be achieved (Klimowicz, 1994). The efficiency of particle removal by fluxing is related to the probability of contact between the ceramic constituent and the flux. Duralcan, as a supplier of particulate reinforced aluminum, proposes to incorporate the salt into the melt, agitated by injecting gas (Duralcan, 1996). Thus dewetting is achieved by much smaller salt additions (less than 1wt.% for alumina and about 1.5wt.% for SiC) and the combination with adsorption of gas to the reinforcement also accelerates particle separation by floating. This gas injection technique can also be applied to melts incorporating fiber reinforced components (Kainer, 1996). Gas bubbles are adsorbed sufficiently to the fiber preforms to make these float to the surface of an aluminum melt; however, this process proves more difficult with magnesium melts. With these, dewetting can be achieved by stirring in a suitable salt; here, the preform with the incorporated salt will then sediment to the bottom of the crucible instead of floating (Kiehn et al., 1996). Nishida et al. (1999) reported on different flux systems for 10–20 vol.% alumina reinforcement in pure aluminum and in AlSiCuMg alloys (AA6061); these allow reclamation of up to 50% of the matrix. Only 20% of the AA6061 matrix could, however, be reclaimed from a 25 vol. % SiC-whisker reinforcement, presumably due to severe interface carbide formation. The preforms are partly infiltrated by the flux, causing these to float on the aluminum melt where they can be skimmed off. Burkhard (1995) reports on centrifugal filtering of aluminum alloys reinforced by particulates, and also by short or continuous alumina fibers. Using medium-grade graphite filters in a centrifugal plasma torch furnace at accelerations above 2000 m s 2, more than 50% of the matrix melt can be separated from ceramic reinforcements. Further reuse can also be imagined for the highly enriched filter cakes containing up to 40 vol.% particulates or short fibers. A melt extraction technique was reported (Lotze et al., 1996) to recover the aluminum melt by producing rapidly solidified needles thereof. In this method, remelted particulate or short fiber reinforced aluminum are held above the liquidus for times sufficient to segregate the heavier reinforcements to the bottom. A cooled copper disk with grooves is then dipped into the melt to extract flakes or metallic needles by fast rotation; these can then be submitted to powder compaction techniques. Again the sediment is enriched in discontinuous reinforcement and could be reused. The reclamation of the reinforcement free of metal matrix can be achieved by dissolving the matrix chemically. The necessary effort will increase with the volume fraction of the inert reinforcement, because convection is progressively hindered. Such leached reinforcement, especially fibers, could be chopped for reuse as discontinuous reinforcements. Klimowicz (1994) mentions that from the segregates of discontinuously reinforced metals resulting from the gas injection method or from of the plasma furnace, a ceramic powder may also be recovered. Gillet et al. (2015) have recycled wooden posts to extract Cr, Cu, and As. The extraction of metal particle from the wood post through acid leaching has presented satisfactory results for both acid concentrations used. Though there were some upside and downside in terms of efficiency and mechanical properties in both the process, the extracted metal particles have shown better properties among composites.
References American Society of Materials 1999. Proc. 4th ASM Int. Conf. Recycling of Metals, Vienna, 17th/18th June 1999 ASM International Metals Park, OH. Burkhard, R. 1995. Recycling von Metallen aus metallhaltigen industriellen Rohstoffen mittels Plasma. Ph.D. thesis, ETH-Zurich. Chamberlain, B., Bruski, R., 1998. Melting and Recycling of DURALCANs MMC Material. CA: Duralcan-USA San Diego. Duralcan, 1990. Duralcans Composite Casting Guidelines. CA: Duralcan-USA San Diego. Duralcan, 1996. Aluminium Recovery from Metal Matrix Composite Scrap. CA: Duralcan-USA San Diego. Gergely, V., Clyne, T.W., 1999. A novel melt-based route to aluminium foam production. In: Banhart, J., et al. (Eds.), Proc. Metal Foams and Porous Metal Structures. Bremen Germany: Verlag MIT Publishing, pp. 83–89. Gergely, V., Degischer, H.P., Clyne, T.W., 2000. Recycling of MMC and production of metallic foams. In: Clyne, T.W., et al. (Eds.), Comprehensive Composite Materials, 3: Metal Matrix Composites. Oxford: Pergamon, pp. 797–820. Gillet, A., Mantaux, O., Cazaurang, G., 2015. Characterization of composite materials made from discontinuous carbon fibres within the framework of composite recycling. Composites Part A: Applied Science and Manufacturing 75, 89–95. Jin, I., Kenny, L.D., et al., 1992. Stabilized metal foam body. US Pat. 5 (112), 697. Kainer, K.U., 1996. Konzepte zum Recycling von Metall-MatrixVerbundwerkstoffen. In: Leonhardt, D., et al. (Eds.), Proc. Recycling von Verbundwerkstoffen. Oberursel Germany: Deutsche Ges.f. Materialkunde, pp. 39–45. Kiehn, J., Oldörp, F., 1996. Wiederverwertung von Kreislaufmaterial aus Kurzfaserverstärkten Magnesiumlegierungen. In: Leonhardt, D., et al. (Eds.), Proc. Recycling von Verbundwerkstoffen. Oberursel/Germany: Deutsche Ges. f. Materialkunde, pp. 51–56. Klier, E.M., Mortensen, A., Cornie, J.A., Flemings, M.C., 1991. Fabrication of cast particle reinforced metals via pressure infiltration. J. Mater. Sci. 26, 2519–2526. Klimowicz, T.F., 1994. The large scale commercialization of aluminum matrix composites. J. Met. 46, 49–53. Krüger, J., 2000. Aluminium Recycling Verein Deutscher Schmelzhuetten e.V., Duesseldorf Germany. Lloyd, D.J., 1989. The solidification microstructures of particulate reinforced Al/SiC composites. Compos. Sci. Tech. 35, 159–180. Lotze, G., Lehnert, F., Stephani, G., Kostmann, C., Büttner, T., 1996. Rezyklieren des metallischen Anteils von Al-MMC mittels Tiegelschmelzextraktion. In: Leonhardt, D. (Ed.), Proc. Recycling von Verbundwerkstoffen. Oberursel Germany: Deutsche Ges. f. Materialkunde. 45−50. Nishida, Y., Izawa, N., Kuramasu, Y., 1999. Recycling of aluminum matrix composites Metall. Mater. Trans. 30A, 839–844.
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Provencher, R., Riverin, G., et al., 1992. Recycling of duralcan aluminium metal matrix. Composites Proc. Adv. Production and Fabrication of Light Metals and Metal Matrix Composites, Montreal. PA: TMS Warrendale. Ruch, W.W., Kirkevag, B. 1991. A process of manufacturing particle reinforced metal foam and product thereof. Pat. WO 9101387, 7.2.1991. Schuster, D.M., Skibo, M.D., et al., May 1993. The recycling and reclamation of metal−matrix composites. J. Met. 45, 26–30. Witik, R.A., Teuscher, R., Michaud, V., Ludwig, C., Månson, J.-A.E., 2013. Carbon fibre reinforced composite waste: An environmental assessment of recycling, energy recovery and landfilling. Composites Part A: Applied Science and Manufacturing 49, 89–99. Zimmermann, M.V.G., Zattera, A.J., 2013. Recycling and reuse of waste from electricity distribution networks as reinforcement agents in polymeric composites. Waste Management 33 (7), 1667–1674.
Recycling of a material is defined as submitting the material to secondary processing steps that produce the same material with the same properties as the original primary product. Recycling can in some instances be conducted several times, the material undergoing, after primary processing, several cycles of secondary processing, service, and recovery without significant degradation. This is, for instance, the case for conventional aluminum alloys (Krüger, 2000; ASM, 1999), which can be remelted and processed with full restoration of the primary microstructure, resulting in identical material properties. Recycling can be done within the production line without any service exposure of the material, or after re-collection of components already out of service. In the latter case some degradation of the material is expected, which would have to be repaired by the recycling process. Alternatively, a material can be processed by reclamation, in which case it is processed to a different form or microstructure to produce a different, but still useful, material. In a real recycling process of composite materials, the first step is the dissolution of the organic matrix (e.g., in supercritical water). During the second step of the recycling process, the recycled fibers are in the form of pieces of dry fabric (without sizing). Then, dry carbon fabric has to be unwoven and the recovered carbon fibers have to be realigned and reshaped (Zimmermann and Zattera, 2013). Several types of recycling method used in recycling metal matrix composites, for example, chemical recycling, microwave heating, mechanical recovery, and fluidized beds, etc., are the usual approaches used at lab scale; pyrolysis is the furthest advancement in recycling processes, which has now reached early stages of commercialization (Witik et al., 2013). Primary processing of metal matrix composites is essentially the bonding of the composite ingredient materials, metallic matrix, and reinforcement, so as to form a percolating metallic matrix containing the reinforcement as an embedded second constituent. The main issues in primary processing of metal matrix composites are to achieve bonding between the two usually nonwetting ingredient materials and to avoid deleterious interface reactions after contact between these constituent materials is established. Frequently, primary metal matrix composite processing yields near-net-shape products, at times by the very nature of the process, and also often because these composites are very difficult to machine. Depending on the composite and the component at hand, the material can be recycled, or alternatively more profitably reused in another form after reclamation. Recycling of metal matrix composites takes advantage of the already achieved bonding between matrix and reinforcement. Recycling of metal matrix composites is attractive, given that it provides the commercial benefit of avoiding the frequently costly primary processing, and on some occasions saves expensive reinforcements. There are, however, some specific issues that must be taken into account in the recycling of these materials. Namely, it is important to avoid excessive thermal exposure in the recycling process if the constituents are mutually prone to chemical interaction. Also, because metal matrix composites are prone to accumulation of internal damage during deformation, their recycling or reuse will often require severe nondestructive testing to verify that the microstructural integrity has been preserved. Another concern that is specific to the recycling of metal matrix composites is that the reinforcement of metals is frequently only used for partial, localized, reinforcement of metal-based components. With such components, an additional effort is necessary to separate the composite from the unreinforced section, in order to allow recycling of both materials separately. Reclamation of the composite ingredient materials requires work to separate the two constituents of the metal matrix composite. Although the interface free energy of the reinforcement in the composite often exceeds its surface free energy in air, matrix/ reinforcement separation does not occur spontaneously because the complementary free surface energy of the matrix has to be formed anew in the process. This results in a positive total free energy barrier, which must be overcome in the reclamation process. This barrier can be overcome by several methods, one being the use of insoluble flux materials having a smaller surface energy with the reinforcement than the matrix. Other processes for metal matrix composite reclamation rely on gravity for separation of the composite constituents. These can be aided by blowing bubbles of an appropriate gas through the molten composite, so as to separate the reinforcement by attachment and flotation with gas bubbles (this process is akin to froth flotation processes used in ore preparation). An additional issue in reclamation of expensive reinforcements like fibers is that their properties are often degraded in the process. In some cases, when the reinforcement is far more costly than the matrix, reclamation of the reinforcement alone can be envisaged; this is achieved with relative ease by dissolving the matrix chemically. Given the relatively wide variety of metal matrix composites and components made therefrom, recycling and reclamation of these materials raises issues that depend on the materials class. Issues specific to the two principal metal matrix composite classes are therefore more specifically addressed in what follows, together with questions specific to reclamation of composite ingredients.
☆ Change History: August 2015. M.S.H. Bhuiyan added Abstract and Keywords, extended the Introduction and the subsection 'Reclamation of Ingredients,' and updated the reference section.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.03648-1
1
2
Metal Matrix Composites, Recycling of
Particle Reinforced Metals Cast primary metal matrix composite products containing relatively low volume fractions of discontinuous reinforcements, like foundry ingots, extrusion billets, or rolling slabs, can be recycled with comparative ease. If the recovered metal matrix composite cannot be used for the same component and application at equal performance quality, reuse can be envisaged for different applications allowing a certain degree of downgrading of the performance of the material. In the case of components selectively reinforced by particulates, remelting results in dilution of the reinforcement, yielding a different composite; here again, the resulting material could be reused in a different application. With particulate reinforcements at volume fractions less than 30%, recycling of the composite can be achieved by remelting if the constituents are not overly reactive. The foundry composite alloy AlSi9Mg/SiC/10–20p (Lloyd, 1989) provides an example where in-house return rates in the production line may amount to up to 50% without property degradation (Klimowicz, 1994). Some specific measures are recommended for the recycling procedure: dry preheated scrap and tools, no overheating of the melt above 750 1C, impellers to produce convection beneath the dross without formation of a vortex, and moderate fluxing with argon or SF6 (Provencher et al., 1992; Chamberlain and Bruski, 1998). Careful sorting and cleaning is required before the recycling of used particle reinforced metal components in a similar remelting process not yet established industrially. Aluminum alloys of the 2xxx, 6xxx, and 7xxx series reinforced with alumina particles react at the interface even close to the solidus temperature. Therefore, any remelting process can potentially alter the composite constituent phases and properties. In the case of AA6061/Al2O3/10–20p it is reported (Schuster et al., 1993) that the amount of interface spinel formation stabilizes without significant influence on the mechanical properties of the composite. Recycling of Duralcan wrought alloys is so far only executed on a large scale by the producer (Klimowicz, 1994). Composite systems such as Mg–SiC, Al–SiC with little silicon content, Al–B4C, and Cu–SiC, are too reactive to allow remelting without severe interface reactions. Iron-base alloys with Ti(C,N) reinforcement stabilize at the solubility limit, which depends on the concentration of other carbide-forming alloying elements. It has been proposed to reuse particle reinforced aluminum alloys for the production of aluminum foam (Gergely et al., 2000). Indeed, several closed-cell aluminum foam production processes rely on the presence of ceramic particles within the melt for stabilization of the foam structure during the process (Jin et al., 1992; Ruch and Kirkevag, 1991; Gergely and Clyne, 1999). One advantage of this recycling route is that property requirements placed on composites to be used for the production of metallic foams may not be as stringent as for bulk composites. Particle reinforced metals also have the attribute that they can be disintegrated into smaller fragments without disintegration of their microstructure. Machining-chips of these materials can thus be reused to fabricate MMC components by solid state compaction methods. This solid state process has the added advantage of minimizing interfacial reactions between matrix and reinforcements.
Melt-Infiltrated Preforms Metal matrix composites with short or continuous fibrous reinforcements are usually produced by infiltration of preforms, in a process that also defines the final shape of the metal matrix composite, and which can also produce fully or partially reinforced components. Recycling of such composites would thus require preservation of the shape of the component. For example, the alumina short fiber preform ring reinforcing the crown of an aluminum alloy piston can be recuperated during remelting. Although the matrix is molten, such MMCs are mechanically stable up to about 850 1C (Burkhard, 1995) and can be handled. The integrity of such MMC rings can be confirmed by nondestructive testing and inserted again into the casting of a piston, which has only to achieve bonding between the composite and the matrix, as the primary casting process had already infiltrated the preform. Recycling of MMC sections of defined shape can be envisaged for alumina fiber reinforced aluminum (or magnesium alloys, provided that recuperation of the composite from the melt is rapid enough to restrict interface reactions), SiC reinforced aluminum–silicon alloys, carbon fiber reinforced magnesium, and, with some reservations, for monofilament reinforced titanium alloys. Particle reinforced metals with particulate volume fractions greater than 40% are also frequently prepared by melt infiltration of powder preforms. It may be envisaged to reshape simple geometries of these composites to some extent by forming processes; if remelted, the volume fraction of reinforcement might be increased if some of the matrix is squeezed out during secondary pressure casting (Burkhard, 1995). Alternatively, these composites could be used as a basis for the production of lower volume fraction composites by remelting and dilution by a matrix melt (Klier et al., 1991).
Reclamation of Ingredients Isolated ceramic particulates or fibers can be removed from aluminum or magnesium melts by conventional salt addition or fluxing techniques as executed to remove oxide films (Provencher et al., 1992). By gravity settling, the fluxed ceramic ingredients
Metal Matrix Composites, Recycling of
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float to the dross at the top of the aluminum melt, or accumulate as sediment at the bottom of a magnesium alloy. Rotary salt furnace technology is an established reclamation process to recover aluminum from various mixtures, including particle reinforced metals; however, this requires 20–50wt.% salt (Duralcan, 1990). Both wrought and foundry alloys can be recovered using this technique, even machining-chips of fiber reinforced MMC. Typically, about 80% recovery of the available aluminum can be achieved (Klimowicz, 1994). The efficiency of particle removal by fluxing is related to the probability of contact between the ceramic constituent and the flux. Duralcan, as a supplier of particulate reinforced aluminum, proposes to incorporate the salt into the melt, agitated by injecting gas (Duralcan, 1996). Thus dewetting is achieved by much smaller salt additions (less than 1wt.% for alumina and about 1.5wt.% for SiC) and the combination with adsorption of gas to the reinforcement also accelerates particle separation by floating. This gas injection technique can also be applied to melts incorporating fiber reinforced components (Kainer, 1996). Gas bubbles are adsorbed sufficiently to the fiber preforms to make these float to the surface of an aluminum melt; however, this process proves more difficult with magnesium melts. With these, dewetting can be achieved by stirring in a suitable salt; here, the preform with the incorporated salt will then sediment to the bottom of the crucible instead of floating (Kiehn et al., 1996). Nishida et al. (1999) reported on different flux systems for 10–20 vol.% alumina reinforcement in pure aluminum and in AlSiCuMg alloys (AA6061); these allow reclamation of up to 50% of the matrix. Only 20% of the AA6061 matrix could, however, be reclaimed from a 25 vol. % SiC-whisker reinforcement, presumably due to severe interface carbide formation. The preforms are partly infiltrated by the flux, causing these to float on the aluminum melt where they can be skimmed off. Burkhard (1995) reports on centrifugal filtering of aluminum alloys reinforced by particulates, and also by short or continuous alumina fibers. Using medium-grade graphite filters in a centrifugal plasma torch furnace at accelerations above 2000 m s 2, more than 50% of the matrix melt can be separated from ceramic reinforcements. Further reuse can also be imagined for the highly enriched filter cakes containing up to 40 vol.% particulates or short fibers. A melt extraction technique was reported (Lotze et al., 1996) to recover the aluminum melt by producing rapidly solidified needles thereof. In this method, remelted particulate or short fiber reinforced aluminum are held above the liquidus for times sufficient to segregate the heavier reinforcements to the bottom. A cooled copper disk with grooves is then dipped into the melt to extract flakes or metallic needles by fast rotation; these can then be submitted to powder compaction techniques. Again the sediment is enriched in discontinuous reinforcement and could be reused. The reclamation of the reinforcement free of metal matrix can be achieved by dissolving the matrix chemically. The necessary effort will increase with the volume fraction of the inert reinforcement, because convection is progressively hindered. Such leached reinforcement, especially fibers, could be chopped for reuse as discontinuous reinforcements. Klimowicz (1994) mentions that from the segregates of discontinuously reinforced metals resulting from the gas injection method or from of the plasma furnace, a ceramic powder may also be recovered. Gillet et al. (2015) have recycled wooden posts to extract Cr, Cu, and As. The extraction of metal particle from the wood post through acid leaching has presented satisfactory results for both acid concentrations used. Though there were some upside and downside in terms of efficiency and mechanical properties in both the process, the extracted metal particles have shown better properties among composites.
References American Society of Materials 1999. Proc. 4th ASM Int. Conf. Recycling of Metals, Vienna, 17th/18th June 1999 ASM International Metals Park, OH. Burkhard, R. 1995. Recycling von Metallen aus metallhaltigen industriellen Rohstoffen mittels Plasma. Ph.D. thesis, ETH-Zurich. Chamberlain, B., Bruski, R., 1998. Melting and Recycling of DURALCANs MMC Material. CA: Duralcan-USA San Diego. Duralcan, 1990. Duralcans Composite Casting Guidelines. CA: Duralcan-USA San Diego. Duralcan, 1996. Aluminium Recovery from Metal Matrix Composite Scrap. CA: Duralcan-USA San Diego. Gergely, V., Clyne, T.W., 1999. A novel melt-based route to aluminium foam production. In: Banhart, J., et al. (Eds.), Proc. Metal Foams and Porous Metal Structures. Bremen Germany: Verlag MIT Publishing, pp. 83–89. Gergely, V., Degischer, H.P., Clyne, T.W., 2000. Recycling of MMC and production of metallic foams. In: Clyne, T.W., et al. (Eds.), Comprehensive Composite Materials, 3: Metal Matrix Composites. Oxford: Pergamon, pp. 797–820. Gillet, A., Mantaux, O., Cazaurang, G., 2015. Characterization of composite materials made from discontinuous carbon fibres within the framework of composite recycling. Composites Part A: Applied Science and Manufacturing 75, 89–95. Jin, I., Kenny, L.D., et al., 1992. Stabilized metal foam body. US Pat. 5 (112), 697. Kainer, K.U., 1996. Konzepte zum Recycling von Metall-MatrixVerbundwerkstoffen. In: Leonhardt, D., et al. (Eds.), Proc. Recycling von Verbundwerkstoffen. Oberursel Germany: Deutsche Ges.f. Materialkunde, pp. 39–45. Kiehn, J., Oldörp, F., 1996. Wiederverwertung von Kreislaufmaterial aus Kurzfaserverstärkten Magnesiumlegierungen. In: Leonhardt, D., et al. (Eds.), Proc. Recycling von Verbundwerkstoffen. Oberursel/Germany: Deutsche Ges. f. Materialkunde, pp. 51–56. Klier, E.M., Mortensen, A., Cornie, J.A., Flemings, M.C., 1991. Fabrication of cast particle reinforced metals via pressure infiltration. J. Mater. Sci. 26, 2519–2526. Klimowicz, T.F., 1994. The large scale commercialization of aluminum matrix composites. J. Met. 46, 49–53. Krüger, J., 2000. Aluminium Recycling Verein Deutscher Schmelzhuetten e.V., Duesseldorf Germany. Lloyd, D.J., 1989. The solidification microstructures of particulate reinforced Al/SiC composites. Compos. Sci. Tech. 35, 159–180. Lotze, G., Lehnert, F., Stephani, G., Kostmann, C., Büttner, T., 1996. Rezyklieren des metallischen Anteils von Al-MMC mittels Tiegelschmelzextraktion. In: Leonhardt, D. (Ed.), Proc. Recycling von Verbundwerkstoffen. Oberursel Germany: Deutsche Ges. f. Materialkunde. 45−50. Nishida, Y., Izawa, N., Kuramasu, Y., 1999. Recycling of aluminum matrix composites Metall. Mater. Trans. 30A, 839–844.
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Provencher, R., Riverin, G., et al., 1992. Recycling of duralcan aluminium metal matrix. Composites Proc. Adv. Production and Fabrication of Light Metals and Metal Matrix Composites, Montreal. PA: TMS Warrendale. Ruch, W.W., Kirkevag, B. 1991. A process of manufacturing particle reinforced metal foam and product thereof. Pat. WO 9101387, 7.2.1991. Schuster, D.M., Skibo, M.D., et al., May 1993. The recycling and reclamation of metal−matrix composites. J. Met. 45, 26–30. Witik, R.A., Teuscher, R., Michaud, V., Ludwig, C., Månson, J.-A.E., 2013. Carbon fibre reinforced composite waste: An environmental assessment of recycling, energy recovery and landfilling. Composites Part A: Applied Science and Manufacturing 49, 89–99. Zimmermann, M.V.G., Zattera, A.J., 2013. Recycling and reuse of waste from electricity distribution networks as reinforcement agents in polymeric composites. Waste Management 33 (7), 1667–1674.