y g o l

o Semisolid Processing n h c e T of Metal Matrix

R. KASILINGAM Composite Castings PhD Profesor PhD Research Scholar School of Mechanical & Building Sciences School of Mechanical & Building Sciences VIT University, Chennai VIT University, Chennai

n the recent years, evolution of automotive production has different processes. Structural elements, panels and body parts been driven by competitive materials. The light weight are produced by presswork methods and components with Ialloys are continuously developed as the most sustainable. uniform cross section can be obtained by For many reasons, aluminium is being used as a successful extrusion. The foundry products are in material in automotive applications due to its low weight, general more complex and it higher strength, good corrosion resistance and constitute the engine formability to suit different design requirements. of the car The benefits of using this light alloy are: it (cylinder head, can meet the cost effectiveness, less engine block, environmental pollution (emission pistons, gear of CO2), reduction of fuel boxes, wheels, consumption, safety and suspension recyclability. Each arms, space vehicle has different frame nodes, aluminum parts etc.). obtained by

reinforced with continuous or discontinuous fiber to obtain the seeking a new opportunity for required properties of matrices. The Metal Matrix Composites substantial strength enhancement of purpose of reinforcement is to get Metal matrix composites (MMCs) materials. This is primarily due to superior level of strength and stiffness typically based on Al alloys are the superior mechanical properties found in the composite in a continous fiber- materials of choice for many in this class of materials, including high reinforced composite. The fibers lightweight structural applications. strength and high toughness. For provide virtually all the strength and Recent developments in nano- lightweight structure applications stiffness and in the particulate crystalline (NC) metals and alloys with requiring high strength, an increase in reinforced composites and significant different grain sizes typically smaller material strength consequently leads to i m p r o v e m e n t o b t a i n e d . I n t h a n 1 0 0 n m , h a s a t t r a c t e d a reduction of structure weight. discontinously reinforced composites, considerable research interest in Metal matrix composites are matrix provides a solid form to the

34 November 2013 composite which supports handling used processes are infiltration casting Figure 2 : (c) during manufacturing. In continous and squeeze casting. Silicon carbide is SiC Particle in Platelet-Shaped Geometry phase, the matrix helps the compsite to the most commonly used particle control the transverse properties, reinforcement with aluminum alloy interlaminar strength and better matrix. The benefits of SiC strength at elevated temperatures. reinforcement are more stiffness, Particulate Reinforced Metal Matrix strength, good thermal conductivity, Composites wear resistance, fatigue resistance, Particulate reinforcement in MMCs and thermal expansion. Particle size typically use abrasive grade ceramic and shape are the important factors in grit which includes silicon carbide, determining material properties. alumina, boron carbide and titanium Fatigue strength greatly improved with carbide. Among these, Silicon carbide fine particles and the below Figure.1 offers the best strength and stiffness summarizes the benefits of particle for aluminum matrices,but is slightly size. more expensive than alumina.This is Figure 1 : Effect of Reinforcement Particle Size being used in aerospace and indusctrial applications which offers wide range of attractive propoerties. The interaction between the m e t a l l i c m a t r i x a n d p a r t i c l e reinforcement is the basis for the enhanced material and physical properties. Composite propoerties can be tailored to meet engineering requirements by selecting a particular reinforcement and varying the amount added to the metal matrix. Increasing the reinforcement volume in a composite increases the mechanical propoerties such as elastic modulus, Material properties and formability ultimate strength,and yield strength can be defined based on function of while reducing the thermal expansion reinforcement particle size. The shape and density of the compiste system. of a particle is characterized by its aspect ratio,the ratio of its longest to Al-SiC Metal Matrix Composites shortest linear dimension. Figure.2 The combination of light weight and shows the different shape of SiC useful mechanical properties has made particle. aluminum alloys very popular and this makes aluminum well siuted for use as Figure 2 : (a) SiC Particle in Block Shaped Geometry a metal matrix. Continous fiber reinforcement matrices are more e x p e n s i v e t h a n d i s c o n t i n o u s reinforced (particle or whisker) aluminum MMCs because of their ease Fabrication Methods of manufacture. The most commonly Nearly twice the volume of MMCs used reinforcement materials in are produced by casting and other discontinously reinforced aluminum liquid routes compared to solid state composites are SiC, Al2O3/alumina, fabrication which is being widely used although boron and titanium are being for automotive applications like use in tribological applications. The Figure 2 : (b) SiC Particle in Block Spherical Geometry cylinder block liners and brake liner. most widely used MMC casting alloys Aluminum MMCs can be produced by are based on aluminum-silicon,which conventional casting methods like improves the castability and to reduce sand,gravity,pressure die casting and the chemical interaction with SiC squeeze casting methods. The melting reinforcement during melting. The of matrix is simillar to the unreinforced major portion of the MMC market relies aluminum alloys.But the stirring is upon components in near-net shape i m p o r t a n t t o a v o i d t h e S i C process other than conventional concentration at the bottom because it casting process. The most commonly is more denser than aluminum. 36 Technology November 2013 Figure.3. casting to reaches the metal.When additional so uses voids. results combination formed, t between semi-solid manufacturing the forming hot stress arises n

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of the slurry creates intimate contact between them. Good bonding is achieved by reducing the slurry viscosity as well as by increasing mixing time. Figure 5 : Automotive Application of Semi-solid Casting l Particle Size and Shape : When the CONTROL ARMS FOR STEERING BRAKE MASTER CYLINDER particle size is smaller (nano), the dimensional stability is better than conventional particle size. Because the length changes when the temperature of the matrix increase. Figure.6 shows the change in length Vs temperature with different particle size. l Melting Temperature : Melting temperature control is standard, and in general the pouring temperatures are PISTON similar to those used for unreinforced alloys. The overheating of molten metal will cause formation of aluminum carbide which starts very slowly at temperatures about 750°C and increases about 800°C. This aluminum HYDRAULIC BRAKE VALVE carbide precipitates affects the melt fluidity,wekens the cast material and reduces the corrosion resistance. l Forming / Casting Pressure : The pressure applied during the casting will have effect on the porosity as it will aid to shear the dendrites to small particles. l Viscosity of semisolid metal l Die temperature

Process Control Parameters Involved In Semi Solid Casting Of Metal Matrix Composites l Uniformity of distribution: Uniform distribution of particle will yield better tensile and fatigue strength l Stirring Speed : As SiC particles to the composites. The segregation of are wetted by liquid aluminum,they will particles leads to lower fracture not coalesce into hard mass but will toughness because of the continuous instead concentrate at the bottom of fracture path available in brittle region. the furnace if not stirred propoerly. Also the segregation will increase the The stirring action should be slow to local stress concentration which prevent the formation of vortex at the reduces the fatigue strength. surface of the melt.Continous stirring 38 Technology November 2013 the and reinforcing i properties semisolid c demand. requirement casting material producedbytheconventional suitable r s used. metal aerospace weight Conclusion m e t a r s s p e Due i Change in Length (Micron) t controlling s n r i application t n o g This a g t v matrix n h

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