Stopping the growth of the columnar grains the grains

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stopping the growth ofthe columnar grains. Thegrains are orientatedrandomly, and so theproperties of this regionare isotropic (having aphysical property whichhas the same value when measured in differentdirections).There are many defects introduced to the metalduring casting. One of these is shrinkage. Most metals contract when they solidify, and therefore a cavity forms at the top of the casting.To remedy this, the top of the casting must be cut off or a 'riser' used. Ariser is a pipe on the casting, that gives extra room for liquid metal to flow -then shrinkage occurs in the pipe, and the pipe is later removed from thecasting.The typical grain structure of a casting is not ideal. This is because thegrains are very large and therefore reduce the ductility and there isweakness where the columnar grains meet the equiaxed grains or eachother. To change the grain structure, inoculants or grain refining agents are used.Inoculants encourage nucleation, and therefore there are many moresmaller equiaxed grains in the casting, and the columnar grains areremoved.Advantages of casting:large and complex shapes can be madesome alloys have such low ductility that they cannot be formed by workingit is the most economical forming process
Disadvantages of casting:entrapped gas causes porosity, therefore metal must be degassed before casting to reduce porosityinoculantsmust be added to improve the grain structuresurface finish is generally poordimensional accuracy is usually poorriser must be machined from the partduring solidification, the impurities are rejected to the last liquid to solidify, causing segregationmechanical properties are poorMetal formingIn this section, we will be examining a number of different methods of forming metals and the resulting microstructure and properties resulting from each method.A forming operation is one in which the shape of the metal piece is changed by plastic deformation (leads to the permanent deformation of a material after the release of the applied load. This occurs after the elastic limit, and is related to permanent atomic displacements).The deformation is caused by an external stress, greater than the material's yield strength. These processes are highly suitable for metals because metals are relatively ductile and won't fracture whenplastically deformed.Cold rollingIn cold rolling, a sheet of metal is passed between a set of rolls, which exert a compressive force on the sheet. The compressive force causes a reduction in thickness and an extension in length.In cold rolling, the grains become elongated in the rolling direction. This increases the strength through work hardening, but ductility decreases. The higher the % cold work (ie % reduction in thickness), the lower the ductility.

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