Casting.docx - Casting Introduction Humans have been using...

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Casting IntroductionHumans have been using and forming metals for centuries. During 5,000-6,000BC, copper was used by the Sumerians and Chaldeans in Mesopotamia. Theprimary uses were arrows, spear heads and chisels. The bronze age began3500-3300 BC. From 1000 BC iron began to be introduced.Then, the metals were heated and beaten into the desired shape. Now there aremany processes used to form metals, and a lot more is known about the effect ofthe forming processes on the properties.CastingCasting is a simple process used for many applications. Molten metal is pouredinto a mould, where it solidifies to form the required shape. Casting is used where the shape is very complicated, the alloy has such low ductility that it can't be formed by any other process or where low cost is required. The mechanical properties produced are inferior to those made by other processes.Examples of products that are cast are automotive cylinder blocks, turbine blades.Forging metalForging is another process used to form metals. In forging,one piece of normally hot metal is mechanically worked byrepetitive blows or squeezing. Forged articles have verygood mechanical properties.Examples of products that are forged are automotive crankshafts, wrenches and connecting rods.RollingIn rolling, a piece of metal is passed between two rolls, resulting ina reduction in thickness of the metal, because of the compressivestress exerted by the rolls.Mechanical properties are good, but areanisotropic((of an objector substance) having a physical property which has a differentvalue when measured in different directions. An example is wood,which is stronger along the grain than across it).Examples of products produced by rolling are sheet, foil, I-beams(from grooved rolls).
ExtrusionIn the process of extrusion, hot metal is pushed through a die bya ram exerting a compressive force on the metal. The length thatemerges has the shape of the die orifice and a reduced section. Examples of extruded products are pipes and window frames.DrawingIn the process of drawing, metal is pulled through a die by a tensileforce applied on the exit side of the die.The length that emerges has the shape of the die orifice, a reducedsection and increased length.Examples of drawn products are tubes, wire and rod.In casting, molten metal is poured into a mould to solidify in the required shape.Initially in the casting process, small crystals form at the mould wall. This is because heat is conducted away through the mould walls and therefore the liquid next to the mould wall solidifies first, and also because nucleation of solid grains is easier at the mould wall. These arecalled chill crystals.Some chill crystals are oriented suitably to give rapid growth, and grow inwards from the mould wall to become elongated or columnar grains. As the grains grow in one direction, the properties of the columnar grains areanisotropic - they are dependent on the direction.
Usually equiaxed grains(rounded grains) form inthe centre of the casting,

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