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MEC 322 > Casting > Flashcards

Casting Flashcards

Class notes

1
Q

what is casting?

A

heating up/melting any material to mold it into something. The shape of the mold determies the shape of the casting

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2
Q

what is the importance of casting

A

versatility and its not limited to its size, any material can be casted, which is important when some manufacturing processes are difficult to come about

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3
Q

casting is a _____ state process

A

single/ net shape
near net shape is like a finish or an extra step

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4
Q

what are the 2 types of casting?

A

expendable mold: we need a mold for every part. Meant for a very specific design (single use)

permanent: can be reused. meant for more general use so more can be used for it.
- used for high volume manufacturing

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5
Q

define foundry

A

a factory equipped for making molds and handling all steps of the casting process

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6
Q

what are the steps of casting

A
  1. make mold
  2. melt metal
  3. pour into mold
  4. let it freeze
  5. remove from mold
    - expandable : break mold
    - permanent: bend it and be gentle
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7
Q

what are the benefits of casting?

A
  1. complex parts can be manufactures
  2. makes both external and internal shapes
  3. no size limit
  4. less waste
  5. versatiles
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8
Q

what are some cons

A
  1. limits on mechanical properties ( the mech properties can change as it cools)
  2. some (expandable casting) and finish have poor dimensional accuracy
  3. shrinkage/porosity/cracks
    (going from liquid to solid)
  4. bad for workers and environments cuz of fumes and melting metal
  5. expensive for permanent tool casting
  6. microstructures are difficult to control
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9
Q

what are the things that make up the mold?

A

Mold: Consists of 2 halves:
- cope = upper half
- Drag = lower have

the two halves separate ate the parting line

mold is contained in a box called a flask

You do not want turbulence/a lot of air bubbles, which is why you got a pouring cup which creates a uniform pour

downsprue through which molten metal enters a runner

runner takes the molten material into the casting

internal shape can be defines as core

external can be defined as cast metal in cavity

riser creates pressure so the molten material moves into every crevice, and sends excess material into the space when shrinkage occurs
- riser must be designed so it solidifies last

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10
Q

What is shrinkage

A

shrinkage means the mold has to be a little bigger than required

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11
Q

what is a pattern?

A

pattern defined as final shape which helps create the mold. it is just slightly larger than required for the actual size to compensate for shrinkage

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12
Q

what are the 2 kinds of molds?

A

open molds: a container like shape

closed mold: to create complex shapes

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13
Q

why do mold cavities have a draft?

A

draft is a taper that is meant to remove the pattern

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14
Q

what is the pouring temp?

A

highers than the melting point of metal so it doesnt immediately solidify

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15
Q

when does solidification/freezing begin?

A

at melting point. this is when freezing begins

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16
Q

describe the TST cooling curve for Pure metal

A

total solidification time = time taken between pouring and complete solidification

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17
Q

what happens during solidification

A

a skin is formed, whihc slowly inscreases untile heat is transferred

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18
Q

grain structure for pure metals

A

at the walls - chill zone/skin/shell of fined equiaxed grains
dendritic growth. - grains grow inwards, in the opposite direction of heat transfer

columnar grains- aligned towards the centre of casting, orientation due to dendritic growth

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19
Q

which element will be the first to freeze for alloys?

A

the first element to freeze will be the one with a high melting point. The last one to freeze will be the one with a low melting point. For alloys there is a freezing range

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20
Q

grain structure for alloys

A

there will be a chill zone at the wall of the molds

dendritic growth favours metals with a higher melting point so in an alloy, so there will be an imbalance with the composiiton metal

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21
Q

effects of cooling rate

A

slow : big dendritic structures - not as strong

high: small dentrites
- stronger, ductiltiy increases, decreases porosity
cost is higher

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22
Q

micro segregation alloy

A

segregation of alloy in micro scale
low melting point alloy will be stuck in between the spines dendrites microstructure of high melting point metal

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23
Q

describe how columnar dendrites look like

A

in line with eachother

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24
Q

describe how eqiaxed dentrites look like

A

evenly spaced and congruent shape, more circular and random formation

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25
Q

what is normal segregation

A

it is when the lower melting point metal in an alloy moves to the centre

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26
Q

what is inverse segregation (macro)

A

has low concentration of alloying element at the centre (liquid metal with high concentration of alloying element, enters the cavity developed from solidification shrinkage in the dendrite arms)

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27
Q

gravity segregation( alloy macro segregation)

A

due to gravity high density compounds sinks and lighter elements float

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28
Q

convection

A

how to control cooling rate as we go from one state to another

promotes formation of chill zone and refines the grain size

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29
Q

what happens with increased convection

A

dentrites arm seperate i.e dentrities multiplication. increasing convention can make grains stronger

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30
Q

what happens with reducing/eliminating convection

A

it will result in coarse and longer columnar dentritic grains

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31
Q

how to prevent macro segregation

A

nucleation agent: we do not want segregation so it minimizes it by adding intermetalic compounds

create more grains through vibration and electromagnetic stirring

32
Q

is micro or macro segregation easier to control

A

macro segregation is easier to control then micro

33
Q

what is fluidity

A

A measure of the capability
of a metal to flow into and fill the
mold before freezing.

34
Q

what is viscosity

A

Fluidity Decreases as viscosity (resistance to flow) and the
viscosity index (its sensitivity to
temperature) increases

35
Q

surface tension

A

High surface tension reduce fluidity

36
Q

inclusion

A

Inclusion (Insoluble
particles) have adverse effect on
fluidity

37
Q

freezing rage( mostly impacts alloys)

A

shorter the freezing range higher the fluidity (alloys with long freezing range have low fluidity)

fixed freezing range

alloy changing freezing range

38
Q

how to control the fluidity

A

design sprue, runner, risers to influence it

higher the thermal conductivity of the mold and its surface roughness, lower the fluidity( heating the mold improves fluidity

superheat: pouring the material much higher than melting point

pouring rate: lower the rate of porur, lower the fluidity(metal cools faster)

39
Q

solidifcication time

A

chvorinovs rule:

TST = Cm(V/A)^N

Cm= mold constant
n= 2 unless otherwise specified
A = surface area of casting

40
Q

molding constant is determined by…..

A

mold material casting metal pouring temp

41
Q

what part should be the last to solidify in a casting system, in order to prevent shrinkage

A

the riser
from mr. C rule, we know:

(V/A)Riser> (V/A)Casting

42
Q

how do you prevent errors during casting(directional solidification)

A

solidification must occur towards the risers, meaning that the area most distant from liquid metal supply must freeze first. I.E area with lowest V/A ratio

43
Q

chills

A

internal or external heat sinks that casue rapid freezing in the regions you want to freeze first

44
Q

porosity

A

trapped air molecules(will look like shperical shapes) and shrinkage(rough voids)

45
Q

controlling porosity

A
  • vaccum - expensice
  • design of runners and gates to avoid turbulence
  • degassing agent - elemts used to react with gas to move it towards the surface
  • ventilation - having proper ventilation
46
Q

Bernoulli theorm

A

v= sqrt(2gh)

Q= Volumetric flow rate

Q=v1A1=v2A2
TMF= Tie required to fill mold

TMF=V/Q
V=mold cavity volume

47
Q

aspiration

A

depending is trapping of air molecule. To avoid aspirations, engineering wil design sprues that taper down towards the runner

this eqn can be used to find the cross sectional area needed for the taper:

A1/A2= SQRT(h2/h1)

48
Q

gates

A
  • multiple gates
  • the gate closest to the sprue should be placed far away so that it can be easily removed
  • the minimum gate ;length is 3-5 times the gate diameter
  • curved gates should be avoided
49
Q

Ingot Casting(expendable)

A

like open casting, it is not a final finished process, meanth for very simple shape

  • there is a ladle, can be poured from top or bottom
    top: more air molecules trapped
    bottom pouring: minimizing porosity
50
Q

Continuous casting process(expendable)

A

an automated process:

dummy starter at the bottom to start the process

molten material held in a tundish, and enters chamber, it then has pinch rolls to pull the casting down ( it determines speed

and then once a solidiifcatinons skin is formed, it is cut to the shape required using a sheering or torch cutting

51
Q

continuous casting give you three basic shapes

A

slab, billet and bloom

52
Q

sand casting

A
  • needs a binding element (clay)
  • lowest accuracy and precision for molding
  • near net shape
  • clay can create more porosity due to its moisture
  • nearly all alloys can be sand casted

properties

  • cohesiveness ( ability of sand to retain shape)
  • strength ( maintain shape even with turbulence
  • permeability (the ability of air molecules to escape through grains)(bigger grain sizes, more permeable but has a poor surface finish)(small grain sizes, trapped molecules, but better finish)
  • refractoriness/ thermal stability: ability to withstand high temps and cracking
  • collapsibility : sand should be reusable
53
Q

shell/vacuum casting(molding)

A

designed to prevent problems from sand casting make shell of casting dry to minimize porosity and improve surface finish, however it is expensive since metal is used as a pattern

shell: a heated metal pattern that come into contact with thermosetting resin binder, this creates a shell, which you will clamp together and put into flask
thermosetting beinder: as the temperature increases the binder become activated and holds the sand together.

Vacccum Molding: completely eliminates porosity since there is no binding agent being used and gives ou good surface finish, however very time consuming

54
Q

expendable pattern

A

you loose the pattern not the mold
- lost foam (expanded polystyrene): foam immediately vaporizes and molten metal in poured into it. raw foam beads re placed in die and takes the shape of die cavity . once die is opened pattern is removed. The foam is sprayed with multiple layers of refractory coated material
- wax (investment casting)

55
Q

plaster

A

better finish and accuracy but a lot of porosity, and can not handle high temps
wood can not be used as a pattern due to moisture.

56
Q

ceramics

A

can withstand high temps, and good finish

57
Q

sand binders

A

held together but 90% sand and 3%water and binding agent of clay

other bonding agents:
- organic resins
- inorganic binders

58
Q

sand molds

A

Green sands : contains moisture (binder is clay)

dry sands: binder is organic, can be baked to make it stronger

skin dried: same as green sand except you dry the skin of the mold cavity that comes in contact with the mold. don’t dry too much or you lose strength

59
Q

material of pattern

A
  • wood: when you come into contact with moisture is bad for wood and causes warping
  • metal: expensive, but moisture issues are resolved

-plastic : compromise between wood and metal

60
Q

types of pattern

A

solid pattern: you csnt tell the partition line, making it difficult to assemble mold

slip mold: clear partition line, predetermined, but can cause misaslignment

match plate pattern: no misaslignment and clear partition line

cope and drag patter: split pattern

61
Q

chaplets..

A

hold to core together,

62
Q

sand casting requires

A

trimming since it is near net shape

removing the core: automatically fall out, sometimes use chemical dissolving component that will dissolve the binding agent
solid cores must be hammered out

63
Q

surface cleaning

A

air blasting to fix the surface, tumbling

64
Q

heat treatment

A

rate of cooling defines mechanical properties of metal. so heat treating can help bring out desired properties of metal and subsequent mahine process

65
Q

machining allowance

A
  • additional material left over on the casting where machining is necessary, this is different than shrinkage allowance, typically 1.5 to 3mm left over.
66
Q

sand blow(at/below casting surface

A

balloon shaped gas cavity caused by release of mold gases during pouring
- low permeability, poor ventilation

67
Q

pin holes (at or below casting surface)

A

release of gases during pouring

68
Q

penetration

A

due to strength of molten material whihc can penetrate through sand

69
Q

mold shift

A

no alignment pins and the cope and drag shift

70
Q

porosity

A

macro porosity: due to shrinkage ( design for directional cooling)

micro porosity: ytrapped moleules, or dentrite shapes/shrinkage

71
Q

easily differentiate between sand, plaster and ceramic casting

A

sand casting - poor surface finish, accuracy, resolution, but can withstand higher materials, porosity

plaster, good finish, high porosity

ceramic, better surface finish, low porosity, and can withstand high temperature

72
Q

Permanent mold casting

A

molds are made out of metals
- the melting point of the casting material is limited as it must be lower than the melting point of the mold metal. it also provides a stronger cast, as there is higher rate of heat escaping, meaning there are smaller grains in the chill zone, and smaller, stronger dentrities.

73
Q

three kinds of permanent molds

A

basic permanent mold casting
- tungsten is good to die cast iron and cast steel
- ejetor pins makes removal easier

die casting - using pressure
- hot chamber: continuous supply of molten material. plunger applies pressure through gooseneck into die cast

-cold chamber: no continuous source of molten metal, but can handle metal of high melting points. a ladle is used to pour into chamber and plunger pushes it into cavity

centrifugal casting - mold is going to rotate/spin

74
Q

slush casting

A

used to make hallow structures. Let the outside solidify and then pour out the rest

75
Q

true/semi centrifugal

A

radially symmetrical structures. the internal shape will always be circular

true: surface finish will be stronger outside diamter than inside diamter

semi: desnity of metal is greater at ourside then centre of rotation(used for wheels and pulleys)

76
Q
A

MEC 322 (4 decks)

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