casting: materials gauntlet

aluminum/ zinc alloys

• process = ingot casting

• shapes = looks like gold bars

• features/microstructure = small grains at mold wall (chill zone) / surface promotes crystallisation/ grains grow inwards along maximum temp grad/ large grains in the core

• application =

ingot casting

• taking a ladle from a furnace, which is filled with molten metal and heated to high temperatures in a furnace

• then removed from the furnace and positioned over a platform which holds the molds on a base plate

• so the mold is a container made from different materials or refractory bricks/ ceramics/ cast iron/ graphite mold which allows the molten phase to cool quickly

• mold needs to withstand the melting point of the material casted

• there is 2 processes = uphill teeming or downhill teeming (teeming = to pour)

• open a valve in the ladle and molten phase just pours into the basin and flows below and up into the mold

• leave to cool in air and remove mold

continuous casting

• used to produce large scale slabs

• continuously pouring and solidifying molten metal into desired shape

• have to be post processed - hardened, worked, formed

aluminum slabs/ steel billets/ blooms/ slabs

• process = continuous casting

• features/ microstructure = fewer defects and more consistent microstructure

sand casting

• is a 2 part mold made from a core and a drag and use a pattern to form a cavity in green sand and pour the molten metal

• pattern can be used again

• has shrinkage

• non uniform grains and no direction associated with the crystal axis when in the center of the cast.

• column of grains at the edge

investment casting

• can give finer details than sand casting

• used for complex shapes with finer features

• wax is injected in the die and separated to get the pattern.

• the pattern than be stuck to a wax tree to cast multiple objects at the same time

• dipped into slurry and refractory sand is then thrown on it building a thick layer for 6-7 mm

• the autoclave then hardens the ceramic and creating a hard shell and the wax is loss due to the high heats

• cool at room temp and remove the ceramic coating (via high pressure washing or vibration)

die casting

• mass production process

• used for non ferrous metal (zinc, aluminum magnesium)

• uses a reusable metal mold (high thermal conductivity) and water cooling

• 3 types = gravity die casting/ pressure die casting low/ high pressure die casting

3D metal objects/ any metal alloy

• process = sand casting

• features = surface finish is rough and the pits are associated with the sand in the molds

• application =

aluminum tree / turbine blades

process = investment casting - use this method because it you can make alot at the same time and they are all consistent

features/ microstructures = tree shape with multiple components on it / has a good surface finish as good as wax pattern

• for blades they need excellent creep resistance

• single crystal = no grain boundaries = no creep

gearheads

process = die casting

features/microstructure = metal mold and water cooling cause small uniaxial grains to be produced

gears/ cylinder heads/ pipe fittings

• process = gravity die casting

• can reuse the mold, do not have to make a new 2 half of the mold

• gives a good surface finish

high and low pressure die casting

look out for sand casting

look out for investment casting

look out for gravity die casting

look out for low pressure die casting

look out for high pressure die casting

when to do sand casting, investment casting or die casting?

• determined by the melting temperature/ type of metal you are using/ part size you want to produce/ detail you want to achieve/ surface finish you want/ how much time and money needed in order to prepare

ship anchor

• process = sand casting

• doesn’t need a good surface finish and is made from a ferrous material

what to make only a few so sand casting is used

cooker hob/ ring burner

• process = die casting

• surface finish = good surface finish as it will be in someone’s kitchen

want to make multiple, therefore die cast is used

casting considerations

• solidification and shrinkage

• gas porosity

• want consistently sized parts / uniform mechanical properties / uniform grain size

• want to avoid porosity and segregation