Matrix Metal Matrix Composite (MMC) The matrix is the monolithic material into which the reinforcement is embedded, and is completely continuous. This means thatt there is apath throughh the matrix ti to anypoint it in the material, unlike two materials sandwiched together. In structural applications the matrix is usually a lighter metal such as aluminum, magnesium, or titanium, and provides a compliant support for the reinforcement. In high temperature applications, cobalt and cobalt-nickel alloy matrices are common.
Reinforcement Metal Matrix Composite (MMC) The reinforcement can be either continuous, or discontinuous. Discontinuous MMCs can be isotropic, and can be worked with standard metalworking techniques, such as extrusion, forging or rolling. Continuous reinforcement uses monofilament wires or fibers such as carbon fiber or silicon carbide. Because the fibers are embedded into the matrix in a certain direction, the result is an anisotropic structure in which the alignment of the material affects its strength. Discontinuous reinforcement uses "whiskers", short fibers, or particles. The most common reinforcing materials in this category are alumina and silicon carbide.
Metal Matrix Composite (MMC) Fabrication Schematic overview of the approaches employed in the fabrication of MMCs
Metal Matrix Composite (MMC) Technology Liquid State Fabrication: Stir Casting Infiltration Gas Pressure Infiltration Squeeze Casting Infiltration Pressure Die Infiltration These processes are suitable for discontinuous reinforcement ( i.e. Whiskers and particles)
Stir Casting Stir Casting is aliquid state method of composite materials fabrication, in which a dispersed phase (ceramic particles, short fibers) is mixed withamoltenmatrixmetalby means of mechanical stirring. A concern is to ensure that good particle wetting occurs. Itisthesimplestand themostcosteffectivemethod of liquid state fabrication The method using stirring metal composite materials in semi-solid solid state is called Rheocasting. High viscosity of the semi-solid matrix ti material enables btt better mixing of the dispersed phase.
Stir Casting Disadvantages Stir casting difficulties can arise from the increase in viscosity on adding particles to a melt. Microstructural inhomogeneities can arise, notably particle agglomeration and sedimentation in the melt. Redistribution as a result of particle pushing by an advancing solidification front can also be a problem. This is reduced when solidification is rapid. Interfacial reaction: Stir casting usually involves prolonged liquid-ceramicid i contact, t which h can cause substantial ti interfacial i reaction.
Stir Casting Disadvantages Microstructures (Lloyd 1991) of Al-7Si/ 20 vol.% SiC p a) Stir Casting (slow cooling) b) Pressure e die decs casting g( (rapid dcooling). At the slower cooling rate SiC particles have been pushed into the interdendritic regions by the growing dendrites, causing severe clustering.
Infiltration Liquid metal is injected into the interstices of an assembly of short fibers or particles, usually called a preform. Thepreform is designed with a specific shape to form an integral part of a finished product in the as-cast form. Preforms are commonly fbi fabricated tdby sedimentation ti of short fibers from liquid suspension. The process can also be adapted for production of particulate MMCs. In order for the preform to retain its integrity and shape, it is often necessary for a binder to be used.
Infiltration 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 liquidid matrix phase (forced infiltration). Pressure less infiltration is less common but it is possible. This technique is very appropriate for metal matrix compared to polymer or glass matrix.
Infiltration Gas Pressure Infiltration ti is a forced infiltration method of liquid phase fabrication of MMCs, using a pressurized gas for applying pressure on the molten metal and forcing it to penetrate into a heated preform. Gas Pressure Infiltration method is used for manufacturing large composite parts. It results in low damage of the fibers..
S Squeeze Casting Infiltration Infiltration Squeeze Casting Infiltration is a forced infiltration method of liquid phase fabrication of Metal Matrix Composites, using a movable mold part (ram) for applying pressure on themolten metal tl The method is used for manufacturing simple small parts (automotive engine pistons from aluminum alloy reinforced by alumina short fibers).
Pressure Die Infiltration Pressure Die Infiltration 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
Advantages of Infiltration Low damage of the reinforcement. Gas infiltration is more suitable than squeeze casting for near net shape product. Reduce subsequent machining to produce near net shape composites, which is very difficult in some composites systems. Plastic deformation processes can used to reduce porosity and improve the mechanical properties.
Solid State fabrication Solid state fabrication of MMCs is the process, in which MMCs are formed as a result of bonding matrix metal and reinforced 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 MMCs, depresses undesirable reactions on the boundary between the matrix and reinforcing phases. Metal Matrix Composites may be deformed also by sintering operation followed by rolling, forging, pressing, drawing or extrusion. The deformation operation may be either cold or hot.
Solid State fabrication Diffusion Bonding (continuous reinforcement) Powder Metallurgy (discontinuous reinforcement) Hot Pressing Hot Isostatic Pressing Hot Powder Extrusion
Diffusion Bonding Diffusion Bonding is a solid state fabrication method, in which a matrix in form of foils and a dispersed phase in form 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).
Hot Pressing (cont. reinforcement) Hot pressing is a solid state fabrication method, in which a matrix in form of foils and a reinforcement in form of long fibers. The reinforcement and the matrix are consolidated with heat and pressure as shown below. Flowchart of B-Al composite fabrication
Hot Pressing (cont. reinforcement) The reinforcement is usually Boron, SiC or metal wire. They held together with a binder, or woven with metal wire or ribbon. The matrix is normally in foil form with thickness in the range of 50-150 μm arestacked in aparticular order and then pressed at elevated temperature. Typical the fiber spacing are in order of 4-10 per millimeter. Temperature is raised first to offgas the binder under light pressure and vacuum inside the chamber. After the binder is removed, the temperaturet and pressure are raised to produced metal flow around the fibers.
After the diffusion bonding occurs between the foils and the fibers the pressure is released and the chamber isallowedtocooldownto room temperature. Hot Pressing (cont. reinforcement) Cross section of filaments and foils
Metal Matrix Composite (MMC) Fabrication Schematic overview of the approaches employed in the fabrication of MMCs
Powder Metallurgy P/M is a process, in which a powder of a 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. Cold compaction is a process that powder materials is compressed in a temperature region where high temperature deformation mechanics like diffusional creep can be neglected. Cold compressing is the most important compaction method in powder metallurgy. It starts from bulk powders containing very small amounts of lubricant or binder additions.
Powder Metallurgy Sintering is a method involving consolidation of powder particles by heating the green compact part to a high temperature below the melting point, when the material of the separate particles diffuse to the neighboring powder particles. In contrast to the liquid state fabrication of MMCs, sintering allows obtaining composites containing up to 50% reinforcement.
Mixing Some common equipment geometries for mixing or blending powders: (a) cylindrical, (b) rotating cube, (c) double cone, and (d) twin shell.
Compaction Powder Metallurgy (a) Compaction of metal powder to form a bushing. The pressed powder part is called green compact. (b) Typical tool and die set for compacting a spur gear.
Sintering i Burn off stage Powder Metallurgy High temperature stage (occurs at 0.7 to 0.9 melting temperature) approximately 1120 C Bonding Cooling stage SOLID STATE DIFFUSION LIQUID STATE SINTERING
Powder Metallurgy Sintering can be considered to proceed in three stages. During the first, neck growth proceeds rapidly but powder particles remain discrete. During the second, most densification occurs, the structure recrystallizes and particles diffuse into each other. During the third, isolated ltdpores tend to become spherical and densification continues at a much lower rate. The words Solid State in Solid State Sintering simply pyrefer to the state the material is in when it bonds, solid meaning the material was not turned molten to bond together as alloys are formed.
Powder Metallurgy Hot and Cold Isostatic Pressing Schematic diagram of cold isostatic pressing, as applied to forming a tube. The powder is enclosed in a flexible container around a solid core rod. Pressure is applied isostatically to the assembly inside a highpressure chamber. Schematic illustration of hot isostatic pressing. The pressure and temperature variation vs. time are shown in the diagram.
Hot Isostatic Pressing Hot Isostatic Pressing of MMCs sintering under a pressure applied from multiple directions through a liquid or gaseous medium surrounding the compacted part and at elevated temperature.
Hot Pressing Hot Pressing of MMCs sintering under a unidirectional pressure applied by a hot press. Hot pressing lowers the pressures required to reduce porosity and speeds welding and grain deformation processes. Also, it permits; Better dimensional control of the product, Lessened sensitivity to physical characteristics of starting materials. Allows powder to be driven to higher densities than with cold pressing, resulting in higher strength.
Hot Powder Extrusion Hot Powder Extrusion Fabrication of Metal Matrix Composites sintering under a pressure applied by an extruder at elevated temperature.
Optical microscope photographs of copper-matrix titanium diboride platelet composites fabricated by powder metallurgy. a 15 vol% TiB2, coated filler method; b 15 vol% TiB2, admixture method; c 60 vol% TiB2, coated filler method; d 60 vol% TiB2, admixture method.
In-situ fabrication of Metal Matrix Composites In situ fabrication of Metal Matrix Composite is a process in which dispersed In situ fabrication of Metal Matrix Composite is a process, in which dispersed (reinforcing) phase is formed in the matrix as a result of precipitation from the melt during its cooling and Solidification.
In-situ fabrication of Metal Matrix Composites Outstanding high temperature properties can be achieved with directionally solidified eutectics, natural composites consisting of high strength fibers in a ductile matrix. Eutectics, are thermodynamically in equilibrium even at melting point. This technique offer unique solution to the problem of fiber/matrix incompatibility.