Lecture 5 Casting Processes.pdf

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Mechanical Engineering
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Alokesh Pramanik

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Lecture 5: Casting Processes Monday, 28 October 2013 9:54 AM • Introduction • Casting Terminology • Steps of casting processes • The solidification Process • Cast structure • Molten metal problems • Patterns • Types of Casting Processes • Advantages of Casting • Summary Introduction • Production go through a series of processesbefore they are produced: design, material selection, process selection, manufacturing , inspection and evaluation and feedback • Materials processing is the science and technologythat convertsa material into a product of a desired shape in the desired quantity • Four basic categories:casting processes, material removing processes,deformationprocesses, consolidation processes • Decisions should be made after all limitations and alternativesare investigated • Casting is the process that produces objects by filling a mould cavity by gravity or pressure with liquid metal and allowing it to cool and solidify Casting • Casting Process:material is melted, heated to proper temperature,treated to modify its • Casting Process:material is melted, heated to proper temperature,treated to modify its chemical makeup, moltenmetal is poured into a mould, solidifies • Casting can product a large variety of parts • Big parts: engine blocks and cylinder heads for automotivevehicles, wood burning stoves, machine frames, railway wheels, pipes, church bells, big statues, pump housings • Small parts: dental crowns, jewellery, small statues, frying pans • Pattern: approximateduplicate of the part to be cast • Moulding material: material that is packed around the pattern to provide the mould cavity • Flask: rigid frame that holds the moulding aggregate • Cope: top half of the flask • Drag: bottom half of the flask • Core: sand or metal shape that is inserted into the mould to create internal features • • Mould cavity:combination of the mould material and cores • Riser: additional void in the mould that provides additional metal to compensatefor shrinkage • Gating system:network of channels that delivers the molten metal to the mould • Pouring cup: portion of the gating system that controls the delivery of the metal • Spruce: vertical portion of the gating system • Runners: horizontal channels • Gates: controlled entrances • Parting line: separates the cope and the drag • Draft: angle or taper on a pattern that allows for easy removalof the casting from the mould • Casting: describes both the process and the product when moltenmetal is poured and solidified Casting Process • Mould cavity is produced, having the desired shape and size of the part; takes shrinkage into account; single use or permanent mould • Melting process: provides molten material at the proper temperature • Pouring technique: molten metal is poured into the mould at a proper rate to ensure that erosion or defected are minimized • Solidification process: controlled solidification allows the product to have desired properties, mould should be designed so that shrinkage is controlled • Mould removal:The casting is removedfrom the mould; single-use moulds are broken away from the casting, permanent moulds must be designed so that removaldoes not damage to part • Cleaning, finishing and inspection operations: excess material along parting lines may have to be removed Solidification Process: • Molten material is allowed to solidify into the final shape. Gas porosity and shrinkage occur during solidification. The two stages of solidification are nucleation and grain growth. • Nucleation: ○ Stable particles form from the liquid metal ○ Occurs when there is a net release of energy from the liquid Undercooling is the difference between the melting point and the temperatureat which ○ Undercooling is the difference between the melting point and the temperatureat which nucleation occurs ○ Each nucleation event produces a grain ○ Nucleation is promoted(more grains) for enhanced material properties ○ Inoculation of grain refinement is the process of introducing solid particles to promote nucleation • Grain Growth ○ Occurs as the heat of fusion is extractedfrom the liquid ○ Direction, rate and type of growth can be controlled ○ Controlled by the way in which heat is removed ○ Rates of nucleation and growth control the size and shape of the crystals ○ Faster cooling rates generally produce finer grain sizes • Cooling Curves: useful for studying the solidification process; cooling rate is the slope of the cooling curve; solidification can occur over a range of temperaturesin alloys; beginning and end of solidification are indicated by changes in slope. Prediction of solidification time: Chvorinov’sRule • Ability to removeheat from a casting is related to the surface area through which the heat is removedand the environmentthat it is rejecting heating to • Chvorinov’sRule: ; n=1.5->2.0,t s time from pouring to solidification, B = mould constant, V = volume of casting, A = surface area through which heat is replaced Fluidity and Pouring Temperature • Metal should flow into all regions of the mould cavity and then solidify • Fluidity is the ability of a metal to flow and fill a mould • Affects the minimum section thickness, maximum length of a thin section, fineness of detail, ability to fill mould extremities • Dependent on the composition,freezing temperature,freezing range and surface tension • Most important controlling factor is pouring temperature Cast Structure • Three distinct regions or zones: chill zone, columnar zone, equiaxed zone • Chill zone: Rapid nucleation that occ
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