Liquid state fabrication of Metal Matrix Composites Liquid state fabrication of Metal Matrix Composites involves incorporation of dispersed phase into a molten matrix molten matrix metal, followed by its Solidification. its Solidification. In order to provide high level of mechanical mechanical properties of the composite, good interfacial good interfacial bonding (wetting) between the dispersed phase and the liquid matrix should be obtained. Wetting improvement may be achieved by coating the dispersed phase particles phase particles (fibers). (fibers). Proper coating not only reduces interfacial energy, but also prevents chemical interaction between the dispersed phase and the matrix. The simplest and the most cost effective method of liquid state fabrication is Stir Casting. Stir Casting
is a liquid state method of composite materials fabrication, in which a Stir Casting dispersed phase (ceramic particles, short fibers) is mixed with a molten matrix metal by means of mechanical stirring. The liquid composite material is then cast by conventional casting conventional casting methods and may also be processed by conventional Metal conventional Metal forming technologies. Stir Casting is characterized by the following features: Content of dispersed phase is limited (usually not more than 30 vol. %). Distribution of dispersed phase throughout the matrix is not perfectly homogeneous: 1. There are local clouds (clusters) of the dispersed particles (fibers); 2. There may be gravity segregation of the dispersed phase due to a difference in the densities of the dispersed and matrix phase. The technology is relatively simple and low cost.
Distribution of dispersed phase may be improved if the matrix is in semi-solid condition. The method using stirring metal composite materials in semi-solid state is called Rheocasting . High viscosity of the semi-solid matrix material enables better mixing of the dispersed phase.
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Infiltration
is a liquid state method of composite materials fabrication, in which a Infiltration preformed dispersed phase (ceramic particles, fibers, woven) is soaked in a molten matrix metal, which fills the space between the dispersed phase inclusions. The motive force of an infiltration process may be either capillary force of the dispersed phase (spontaneous infiltration ) or an external pressure (gaseous, mechanical, electromagnetic, centrifugal or ultrasonic) applied to the liquid matrix phase (forced infiltration) .
Gas Pressure Infiltration
is a forced infiltration method of liquid phase fabrication of Gas Pressur e I nf il tration Metal Matrix Composites, using a pressurized gas for applying pressure on the molten metal and forcing it to penetrate into a preformed dispersed ph ase.
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Gas Pressure Infiltration method is used for manufacturing large composite parts. The method allows using non-coated fibers due to short contact time of the fibers with the hot metal. In contrast to the methods using mechanical force, Gas Pressure Infiltration results in low damage of the fibers. Squeeze Casting Infiltration
is a forced infiltration method of liquid phase fabrication of Squeeze Casti ng I nf il tr ation Metal Matrix Composites, using a movable mold part (ram) for applying pressure on the molten metal and forcing it to penetrate into a performed dispersed phase, placed into the lower fixed mold part. Squeeze Casting Infiltration method is similar to the Squeeze casting technique used for metal alloys casting.
Squeeze Casting Infiltration process has the following steps: A preform of dispersed phase (particles, fibers) is placed into the lower fixed mold half. A molten metal in a predetermined amount is poured into the lower mold half. The upper movable mold half (ram) moves downwards and forces the liquid metal to infiltrate the preform. The infiltrated material solidifies under the pressure. The part is removed from the mold by means of the ejector pin. The method is used for manufacturing simple small parts (automotive engine pistons from aluminum alloy reinforced by alumina short fibers).
Pressure Die Infiltration Pressur e Di e I nf il tration is a forced infiltration method of liquid phase fabrication of
Metal Matrix Composites, using a Die casting technology, when a preformed dispersed phase (particles, fibers) is placed into a die (mold) which is then filled with a molten metal entering the die through a sprue and penetrating into the preform under the pressure of a movable piston (plunger).
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Solid state fabrication of Metal Matrix Composites is a of process, in which Metal Matrix Soli d state f abri cation of M etal M atr ix Composites Composites are formed as a result of bonding matrix metal and dispersed phase due to mutual diffusion occurring between them in solid states at elevated temperature and under pressure. Low temperature of solid state fabrication process (as compared to Liquid state fabrication of Metal Matrix Composites) depresses undesirable reactions on the boundary between the matrix and dispersed (reinforcing) phases. There are two principal groups of solid state fabrication of Metal Matrix Composites: diffusion bonding and sintering. Diffusion Bonding
is a solid state fabrication method, in which matrix in form of foils Dif fusion Bonding and dispersed phase in form of layers of long fibers are stacked in a particular order and then pressed at elevated temperature. The finished laminate composite material has a multilayer structure. Diffusion Bonding is used for fabrication of simple shape parts (plates, tubes).
Variants of diffusion bonding are roll bonding and wire/fiber winding: is a process of combined Rolling (hot or cold) strips of two different metals (e.g. Roll Bonding steel and aluminum alloy) resulted in formation of a laminated composite material with a metallurgical bonding between the two layers. is a process of combined winding continuous ceramic fibers and metallic Wire/fiber Winding wires followed by pressing at elevated temperature. Sinteri ng (Powder M etall ur gy Techn iqu e) is a process, in which a powder of a Sin teri ng fabri cation of M etal M atrix Composites
matrix metal is mixed with a powder of dispersed phase in form of particles or short fibers for subsequent compacting and sintering in solid state (sometimes with some presence of liquid). is the method involving consolidation of powder grains by heating the “green” Sintering compact part to a high temperature below the melting point, when the material of the separate particles diffuse to the neghbouring powder particles. 4
In contrast to the liquid state fabrication of Metal Matrix Composites, sintering method allows obtaining materials containing up to 50% of dispersed phase. When sintering is combined with a deformation operation, the fabrication methods are called: Hot Pressing Fabrication of Metal Matrix Composites H ot Pressin g F abrication of M etal M atrix Composites – sintering under a unidirectional
pressure applied by a hot press;
Hot Isostatic Pressing Fabrication of Metal Matrix Composites
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– sintering under a pressure Hot I sostati c Pressing F abri cation of M etal M atrix Composites applied from multiple directions through a liquid or gaseous medium surrounding the compacted part and at elevated temperature;
Hot Powder Extrusion Fabrication of Metal Matrix Composites
– sintering under a pressure H ot Powder E xtr usion F abrication of M etal M atrix Composites applied by an extruder at elevated temperature.
Metal Matrix Composites may be deformed also after sintering operation by rolling, Forging, pressing, Drawing or Extrusion. The deformation operation may be either cold (below the re-crystallization temperature) or hot (above the re-crystallization temperature). Deformation of sintered composite materials with dispersed phase in form of short fibers results in a preferred orientation of the fibers and anisotropy of the material properties (enhanced strength along the fibers orientation).
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In-situ fabrication of Metal Matrix Composites is a process, in which dispersed (reinforcing) I n situ f abri cation of M etal M atrix Composite phase is formed in the matrix as a result of precipitation from the melt during its cooling and Solidification. Different types of Metal Matrix Composites may be prepared by in situ fabrication method: 1. Particulate in situ MMC – Particulate composite reinforced by in situ synthesized dispersed phase in form of particles. Examples: Aluminum matrix reinforced by titanium boride (TiB2) particles, magnesium matrix reinforced by Mg2Si particles. 2. Short-fiber reinforced in situ MMC – Short-fiber composite reinforced by in situ synthesized dispersed phase in form of short fibers or whiskers (single crystals grown in form of short fibers). Examples: Titanium matrix reinforced by titanium boride (TiB2) whiskers, Aluminum matrix reinforced by titanium aluminide (TiAl3) whiskers. 3. Long-fiber reinforced in situ MMC – Long-fiber composite reinforced by in situ synthesized dispersed phase in form of continuous fibers. Example: Nickel-aluminum (NiAl) matrix reinforced by long continuous fibers of Mo (NiAl-9Mo alloy). Dispersed phases of in situ fabricated Metal Matrix Composites may consist of intermetallic compounds, carbides, borides, oxides, one of eutectic ingredients.
Advantages of in situ Metal Matrix Composites: In situ synthesized particles and fibers are smaller than those in materials with separate fabrication of dispersed phase (ex-situ MMCs). Fine particles provide better strengthening effect; In situ fabrication provides more homogeneous distribution of the dispersed phase particles; Bonding (adhesion) between the particles of in situ formed dispersed phase and the matrix is better than in ex-situ MMCs; Equipment and technologies for in situ fabrication of MMCs are less exp ensive.
Disadvantages of in situ Metal Matrix Composites: Choice of the dispersed phases is limited by thermodynamic ability of their precipitation in particular matrix; The size of dispersed phase particles is determined by solidification conditions; Unidirectional solidification of a eutectic alloy (alloy of eutectic composition) may result in formation of eutectic structure, in which one of the components has a form of long continuous filaments. Scheme of a device for unidirectional solidification of in situ Metal Matrix Composite is shown in the figure:
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Crucible with an eutectic alloy moves downwards (or alternatively the induction coil moves upwards). This movement results in remelting followed by resolidification of the alloy under controlled cooling conditions. Value of heat transfer through the crucible bottom together with the crucible speed (v) and the power of the heating elements (induction coil) determine particular temperature gradient, which provides unidirectional solidification with flat solidification front. The alloy acquires eutectic structure directed along the solidification direction with eutectic components in form of long mono-crystals (fibers). A distance between the fibers (d) is determined by the solidification speed (v) according to the formula: d² ~ v
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Fabrication of Metal Matrix Composites by co-deposition is a process, in which matrix metal is deposited together with the dispersed phase Co-deposition by one of the deposition techniques. The following co-deposition methods are used for ma nufacturing Metal Matrix Composites: Electrolytic co-deposition This method involves Electroplating technique, in which electrolyte solution of matrix metal ions contains suspended particles of dispersed phase. When the matrix metal is deposited on a substrate, the dispersed phase particles are entrapped by the coating, reinforcing the matrix material. Examples of electrolytic co-deposition: Nickel matrix composite materials with various dispersed phases are fabricated by electrolytic co-deposition from Nickel Sulfamate and Watts electrolyte: Ni-Al2O3 - oxidation resistant nickel matrix composite; Ni-SiC – wear resistant nickel matrix composite; Ni-PTFE, Ni-C, Ni-MoS2 – antifriction nickel matrix composites. Anti-friction coating of engine bearings consisting of lead-tin-copper alloy and reinforced by alumina (Al2O3) is fabricated by electrolytic co-deposition from electrolyte solution of lead, tin and copper with alumina particles. Aluminum matrix material reinforced by silica (SiO2) is prepared from AlCl3dimethylsulfone electrolyte containing fine silica particles.
Vapor co-deposition is a group of various methods, utilizing materials in vapor state: Physical Vapor co-depositi on Vapor Deposition (PVD), Chemical Vapor Deposition (CVD), Direct Vapor Deposition (DVD). In these methods coating of solid material is formed as a result of vapor condensation or chemical reaction on a substrate surface. Vapor co-deposition is used for coating fibers, creating multilayer depositions, fabricating nanostructure composite materials.
There are two basic vapor deposition processes: Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD). Physical Vapor Deposition (PVD) is the process involving vaporization of the coating material Physical V apor Depositi on (PVD ) in vacuum, transportation of the vapor to the substrate and condensation of the vapor on the substrate (part) surface. Vaporization of the coating material stock may be made by one of the following methods: Evaporation ; Sputtering ; Arc Vaporization .
is a Physical Vapor Deposition method, utilizing argon ions for bombarding a Sputtering cathodically connected target, made of the coating material. Atoms of the target are knocked out by the high energy ions and deposit on the substrate surface.
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Sputtering process scheme is shown in the picture:
Metals, alloys, ceramics and some polymers may be deposited onto metals, ceramics and polymers by Physical Vapor Deposition method. Applications of PVD: TiN, TiAlN, TiCN and CrN coating for cutting tools; AlSn coating on engine bearings, diamond like coating for valve trains; Coating for forming tools; Anti-stick wear resistant coating for injection molds; Decorative coatings of sanitary and door hardware. Chemical Vapor Deposition (CVD) Chemical Vapor Depositi on (CVD ) – the process, in which the coating is formed on the hot substrate surface placed in an atmosphere of a mixture of gases, as a result of chemical reaction or decomposition of the gases on the substrate material.
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Applications of CVD: Integrated circuits; Optoelectrical devices; Micromachines; Fine powders; Protective coatings; Solar cells; Refractory coating for jet engine turbine blades.
Spray co-deposition This method implements Thermal spraying technique for atomizing molten matrix metal, droplets of which are delivered to a substrate in a high velocity gas stream together with dispersed phase particles supplied to the stream from a separate containe r. The method allows fabrication of near-net-shape forming of Metal Matrix Co mposites. Examples of spray co-deposition: Aluminum matrix material reinforced by silicon carbide (SiC) is produced by spray codeposition followed by Rolling. High Velocity Oxyfuel Spraying (HVOS) method is used for fabrication tungsten carbide-cobalt (WC-Co) composite material, which is conventionally manufactured by more expensive technology of sintering fabrication of Metal Matrix Composites.
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Applications of Metal Matrix Composites Major applications of metal matrix composites are in aerospace and non-aerospace industries. Reduction in the weight of a component is a major driving force for any application in the aerospace field. For example, in the Hubble telescope, pitch based continuous carbon fiber reinforced aluminum was used for wave guide booms because this composite is very light, has a high elastic modulus, and has a low coefficient of thermal expansion. Other aerospace applications of MMC’s involve replacement of light but toxic beryllium. For example, in US Irident missile, beryllium has been replaced b y SiCpAl composite. One of the important applications of MMC’s in automotive area is diesel piston crowns. This application involves incorporation of short fibers of alumina or alumina + silica in the crown of the piston. The conventional diesel engine piston has Al-Si casting alloy with crown made of a nickel cast iron. The replacement of the nickel cast iron by aluminum matrix composite resulted in a lighter, more abrasion-resistant and cheaper product. Another application in the automotive sector involves the use of carbon fiber and alumina fibers in an aluminum matrix for use as cylinder liners. An important potential commercial application of the particle reinforced aluminum composite is to make automatic drive shafts Particulate metal matrix composites, especially with light metal matrix composites such as aluminum and magnesium also find applications in automotive and sporting goods. In this regard, the price per kilogram becomes the driving force for application. An excellent example involves the use of Duralcan particulate MMC’s to make mountain bicycles. A company called Specialized Bicycle in the US sells these bicycles with frames made from extruded tubes of 6061 aluminium containing about 10% alumina particles. The major advantage is the gain in stiffness. An interesting example of a sheet laminate composite is non vibration sheet steel, made by Kawasaki Steel under the trade name Nonvibra. Such a laminated composite muffles noise over a broad range of frequencies, and it can be used in the temperature range of 0-1000C. Examples of applications include oil pans, locker covers, dash board panels, electrical machinery and appliances, and office equipment. Metal matrix composites can be tailored to have a optimal thermal and physical properties to meet the requirements of electronic packaging s ystems. Continuous boron fiber reinforced aluminium composites made by diffusion bonding have been used as heat sink in chip carrier multilayer boards. Unidirectionally aligned pitch-based carbon fibers in an aluminium matrix can have very high thermal conductivity along the fiber direction. The conductivity transverse to the fibers is about two-thirds that of aluminium. Such a C/Al composite can find applications in heat transfer applications where weight reduction is an important consideration, for example in high speed integrated circuit packages for computers and in b ase plates for electronic equipment.
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