Manufacturing Processes, Chapter 10 notes (Metal Casting)
Manufacturing Processes, Chapter 10 notes (Metal Casting) MIE 375
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This 2 page Class Notes was uploaded by Kirin Anandsongvit on Sunday October 2, 2016. The Class Notes belongs to MIE 375 at University of Massachusetts taught by Dr. Bernd F. Schliemann in Fall 2016. Since its upload, it has received 2 views. For similar materials see Manufacturing Processes in Engineering at University of Massachusetts.
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Date Created: 10/02/16
Fundamentals of Metal Casting -‐ The starting material is either a liquid or is in a highly plastic condition, and a part is created through solidification of the material. o Casting and molding processes dominate this category of shaping operations. -‐ Casting is a process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity. -‐ A variety of shape casting methods are available: o Can be used to create complex part geometries, including both external and internal shapes. o Can be used to create very large parts. o Can be performed on any metal that can be heated to the liquid state. o Some casting methods are quite suited to mass production. -‐ Overview of Casting Technology o Casting is usually carried out in a foundry. § It is a factory for making molds, melting and handling metal in molten form, performing the casting process, and cleaning the finished casting. o The mold contains a cavity whose geometry determines the shape of the cast part. § Actual size and shape of the cavity must be oversized to allow for shrinkage that occurs in the metal during solidification and cooling. § Different metals undergo different amounts of shrinkage, so the mold cavity must be designed for the particular metal to be cast if dimensional accuracy is critical. o To accomplish a casting operation: § The metal is first heated to a temperature high enough to completely transform it into a liquid state. § Open mold: the liquid metal is simply poured until it fills the open cavity. § Closed mold: a passageway, called the gating system, is provided to permit the molten metal to flow from outside the mold into the cavity. § When the temperature drops sufficiently, the metal begins to cool and solidification begins. o Casting Processes divide into tow broad categories: § Expendable mold: the mold in which the molten metal solidifies must be destroyed in order to remove the casting. § Permanent mold: One that can be used over and over to produce many castings. o Sand Casting Molds § By fat the most important casting process. § The mold consists of two halves: cope and drag. • The cope is the upper half of the mold while the drag is the lower half. • The two halves of the mold separate at the parting line. § Contained in a box called a flask. § The gating system in a casting mold is the channel, or network of channels, by which molten metal flows into the cavity from outside the mold. -‐ Heating and Pouring o The metal must flow into all regions before solidifying. § The pouring temperature is the temperature of the molten metal as it is introduced into the mold. § Pouring rate refers to the volumetric rate at which the molten metal is poured into the mold. • If the rate is too slow, the metal will chill and freeze before filling the cavity. § The molten metal flow characteristics are often described by the term fluidity, a measure of the capability of the metal to flow into and fill the mold before freezing. • As viscosity increases, fluidity decreases; the two are inverses of each other. -‐ Solidification and Cooling o Issues associated with solidification include the time for a metal to freeze, shrinkage, directional solidification, and riser design. o The solidification process differs depending on whether the metal is a pure element or an alloy. § A pure metal solidifies at a constant temperature equal to its freezing point, which is the same as its melting point. • The total solidification time is the time taken between pouring and complete solidification. • The rate at which freezing proceeds depends on heat transfer into the mold, as well as the thermal properties of the metal. § Most allows freeze over a temperature range rather than at a single temperature. • Freezing begins at the temperature indicated by the liquidus and is completed when the solidus is reached. § A eutectic composition is a particular composition in an alloy system from which the solidus and liquidus are at the same temperature. o A casting with a higher volume to surface area ratio will cool and solidify more slowly than one with a lower ratio. o Solidification shrinkage occurs in nearly all metals because the solid phase has a higher density than the liquid phase. §
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