Casting

Question 1

Describe ingot casting

Answer 1

Top loading or bottom loading - liquid alloy put into tapered mould (not straight = easy to remove)
Top- creates turbulent flow = gas trapping in alloy
Bottom - no turbulence as oxides flow to top = purer cast, but takes longer

Question 2

Describe ingot casting solidification

Answer 2

Mould wall extracts heat due to temp gradient between liquid and atmosphere, solid nucleates at wall, convection currents set up due to diff liquid heats
Chill zone - dendritic zone - equiaxed zone with fastest solidification in <100> direction (when parallel to grad)

Question 3

Define partition coefficient and wether a higher or lower value is preferred

Answer 3

K = Cs/Cl
K< 1 is normal, a higher K will mean surviving force to solidify = quicker solidification, lower k = slower solidification = more segregation

Question 4

Describe continuous casting

Answer 4

Ladle, Tundish (passing argon through material causes degassing here), submerged entry nozzle, mould (primary cooling occurs), then onto support rollers (secondary cooling occurs, pieces then flame cut to length

Question 5

Describe the properties of continuous casting materials

Answer 5

Limit segregation as fast solidification, no nucleation on top surface = no gravity segregation, large columnar zones (meet in middle), course grain size (rolling can change later on)

Question 6

Compare continuous and ingot casting

Answer 6

Continuous - continuous = higher production rate, uniform product, can go straight into rolling, have to be big production runs
Ingot - smaller batch casting, cheaper to set up but low production rates, material can’t be rolled straight away, high segregation

Question 7

What are the requirements for a mould?

Answer 7

Higher Tm than casting alloy, warmed to avoid quenching, good surface finish (shapes castings)
Usually expensive and time consuming to produce

Question 8

Describe a temporary mould

Answer 8

Mould made from a permanent pattern, cheap to produce as dont need to be as durable, easily changed pattern, can contain risers for filling system, mould is broken after each casting and needs to be remade (used for lower production runs & cheaper products)

Question 9

What factors need to be considered in mould design?

Answer 9

Shrinkage occurs = mould must be bigger than final shape, where is liquid introduced (top=quicker, bottom = better properties), fluidity of liquid (dictates complexity of mould), Tm of material that is to be cast

Question 10

Define fluidity and how it’s measured

Answer 10

Distance liquid travels before it solidifies, depends on rate of heat loss, supper heat, material properties
Measured: pour metal into spiral mould (metal @ superheat and pressure, mould from mould material), distance travelled before solidifying = fluidity

Question 11

Describe sand casting

Answer 11

Angular sand (appose to round beech sand) is used as has a higher green strength, sand is compacted around pattern and risers (differently and then combined), pattern removed and replaced with alloy, once cooled sand is removed and reused, risers cut off and inserted back into melt pool

Question 12

What are the advantages, disadvantages and improvements to sand casting?

Answer 12

+ - small runs, sand is porous = gas escapes = low porosity, cheap
- - slow process, poor surface finish, sand can wear (poor accuracy), slow cooling = large grains
Imp - automatic filling (inc rate & £), sand and resin (inc accuracy & finish but sand not reusable), degradable core = hollow components

Question 13

Describe lost foam casting

Answer 13

PS beads steamed in permanent mould to form foam and release agent coating added, sand then packed around foam, metal poured onto foam, foam vaporises (low Tm) and porous sand lets gas escape (can’t add resin), san remover &reused but poor surface finish - used instead of sand casting to fill complex shapes

Question 14

Describe investment casting

Answer 14

Wax cast made in permanent mould and waxes assembled to make tree, tree is coated with ceramic and stucco slurry, wax is melted out and ceramic sintered (for strength, metal alloy added, ceramic mould broken off after cooling and cast fettled
Slow production rate, v high cost but good accuracy and surface finish

Question 15

Describe permanent moulds

Answer 15

One mould produced (usually alloy) and component is final shape
Mould must have higher Tm than liquid, good surface finish (as transferred to component), be durable
Hot tearing common (mould strength inc) and more risers needed as mould not porous, and loses heat quicker (higher conductivity)

Question 16

Describe centrifugal casting

Answer 16

Mould is rotated, liquid into mould and forced to outside creating hollow shape, amount of liquid = thickness of wall

Question 17

Compare horizontal and angular centrifugal casing

Answer 17

Angular - quicker solidification, one end solidifies

Both - grains alligned normal to tube length, defects move to inside of tube, risers and fillers not needed

Question 18

Describe die casting

Answer 18

Both - mould splits to be removed, mould prevents shrinkage = hot tearing common, gas trapped = porous, pins to force comp from die = pin marks, good surface finish, tight clamp (avoid flash)
Gravity - liquid fed under gravity, cheaper but slower
Pressure - forced into mould, quicker, more expensive, flash more likely

Question 19

What is meant by flash and hot tearing?

Answer 19

Flash - liquid seeps through mould gaps - needs to be fettled
Hot tearing - outside solidifies 1st = stresses on material = fractures, stopped by removing casting when hot or by reducing radius of casting

Question 20

What are the problems with shaped casting and how are these addressed?

Answer 20

Course grain size (only change is slower cooling)

High SA materials can’t be final shape cast (sheet) so must be continuously/ingot cast and size dramatically reduced