casting fundamentals Flashcards
what is casting of metals
process in which molten metal flows by gravity or some force until it solidifies in the shape of mould cavity
advantages of casting
complex part geometries, external and internal shapes. can make large parts. some casting methods good for mass production
disadvantages of casting
limitations on mechanical properties
poor dimensional accuracy and surface finish for some processes like sand casting
safety hazards because of hot metal
environmental problems
what parts can be made by casting
big parts: engine blocks, stoves, machine frames, train wheels, pipes, church bells, big statues, pump housings
small parts: dental crowns, jewellery, small statues, frying pans
what is a foundry
factory equipped for making moulds, melting and handling metal, performing casting process, cleaning finished casting
temp v time when solidifying?
after temp drops to frfeezing point, temp stays constant while latent heat of fusion is given off
what is local solidification time
the actual freezing time/horizontal portion
what is total solidification time
time taken between start of pouring and complete solidification
how does metal get colder wrt mould wall?
chilling action of mould wall means thin skin of solid metal initially formed at interface immediately after ppouring.
thickness of skin increases to form a shell, as solidification progresses inward.
rate at which freezing proceeds depends on heat transfer into mould, as well as thermal properties of the metal
grain structure of solidification?
at mould walls, fine equiaxed (randomly oriented grains) form. (thin skin area)
grains grow in opposite direction to heat transfer out through mould.
columnar grains grow, they have favourable orientations. the grains that have substantially different orientations are blocked from further growth.
difference between freezing pure metal and alloy
most freeze over a temperature range. freezing temperature spread between liquidus and solidus.
what happens when freezing starts for alloys
when freezing starts, thin skin forms. dendrites form growing away from the mould wall.
advancing zone (liquid and solid coexist) forms due to temperature spread between liquidus and solidus. called MUSHY ZONE
what can mushy zone be?
narrow, with fast heat transfer rate from metal to mould. happens from small difference between liquidus and solidus temp.
exist throughout casting, with slow heat transfer rate. wide difference between liquidus and solidus temp
why can composition affect solidification of alloys
some regions freeze first from being richer in one component, and the remaining molten alloy is deprived by time freezing occurs in interior. general segregation
grain structure of solidification of alloys
similar to pure metal, except equiaxed zone forms in centre from the other metal.
