Casting

Question 1

what is casting?

Answer 1

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

Question 2

what is the importance of casting

Answer 2

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

Question 3

casting is a _____ state process

Answer 3

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

Question 4

what are the 2 types of casting?

Answer 4

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

Question 5

define foundry

Answer 5

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

Question 6

what are the steps of casting

Answer 6

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

Question 7

what are the benefits of casting?

Answer 7

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

Question 8

what are some cons

Answer 8

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

Question 9

what are the things that make up the mold?

Answer 9

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

Question 10

What is shrinkage

Answer 10

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

Question 11

what is a pattern?

Answer 11

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

Question 12

what are the 2 kinds of molds?

Answer 12

open molds: a container like shape

closed mold: to create complex shapes

Question 13

why do mold cavities have a draft?

Answer 13

draft is a taper that is meant to remove the pattern

Question 14

what is the pouring temp?

Answer 14

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

Question 15

when does solidification/freezing begin?

Answer 15

at melting point. this is when freezing begins

Question 16

describe the TST cooling curve for Pure metal

Answer 16

total solidification time = time taken between pouring and complete solidification

Question 17

what happens during solidification

Answer 17

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

Question 18

grain structure for pure metals

Answer 18

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

Question 19

which element will be the first to freeze for alloys?

Answer 19

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

Question 20

grain structure for alloys

Answer 20

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

Question 21

effects of cooling rate

Answer 21

slow : big dendritic structures - not as strong

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

Question 22

micro segregation alloy

Answer 22

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

Question 23

describe how columnar dendrites look like

Answer 23

in line with eachother

Question 24

describe how eqiaxed dentrites look like

Answer 24

evenly spaced and congruent shape, more circular and random formation

Question 25

what is normal segregation

Answer 25

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

Question 26

what is inverse segregation (macro)

Answer 26

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)

Question 27

gravity segregation( alloy macro segregation)

Answer 27

due to gravity high density compounds sinks and lighter elements float

Question 28

convection

Answer 28

how to control cooling rate as we go from one state to another promotes formation of chill zone and refines the grain size

Question 29

what happens with increased convection

Answer 29

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

Question 30

what happens with reducing/eliminating convection

Answer 30

it will result in coarse and longer columnar dentritic grains

Question 31

how to prevent macro segregation

Answer 31

nucleation agent: we do not want segregation so it minimizes it by adding intermetalic compounds create more grains through vibration and electromagnetic stirring

Question 32

is micro or macro segregation easier to control

Answer 32

macro segregation is easier to control then micro

Question 33

what is fluidity

Answer 33

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

Question 34

what is viscosity

Answer 34

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

Question 35

surface tension

Answer 35

High surface tension reduce fluidity

Question 36

inclusion

Answer 36

Inclusion (Insoluble particles) have adverse effect on fluidity

Question 37

freezing rage( mostly impacts alloys)

Answer 37

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

Question 38

how to control the fluidity

Answer 38

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)

Question 39

solidifcication time

Answer 39

chvorinovs rule: TST = Cm(V/A)^N Cm= mold constant n= 2 unless otherwise specified A = surface area of casting

Question 40

molding constant is determined by.....

Answer 40

mold material casting metal pouring temp

Question 41

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

Answer 41

the riser from mr. C rule, we know: (V/A)Riser> (V/A)Casting

Question 42

how do you prevent errors during casting(directional solidification)

Answer 42

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

Question 43

chills

Answer 43

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

Question 44

porosity

Answer 44

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

Question 45

controlling porosity

Answer 45

- 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

Question 46

Bernoulli theorm

Answer 46

v= sqrt(2gh) Q= Volumetric flow rate Q=v1A1=v2A2 TMF= Tie required to fill mold TMF=V/Q V=mold cavity volume

Question 47

aspiration

Answer 47

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)

Question 48

gates

Answer 48

- 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

Question 49

Ingot Casting(expendable)

Answer 49

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

Question 50

Continuous casting process(expendable)

Answer 50

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

Question 51

continuous casting give you three basic shapes

Answer 51

slab, billet and bloom

Question 52

sand casting

Answer 52

- 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

Question 53

shell/vacuum casting(molding)

Answer 53

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

Question 54

expendable pattern

Answer 54

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)

Question 55

plaster

Answer 55

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.

Question 56

ceramics

Answer 56

can withstand high temps, and good finish

Question 57

sand binders

Answer 57

held together but 90% sand and 3%water and binding agent of clay other bonding agents: - organic resins - inorganic binders

Question 58

sand molds

Answer 58

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

Question 59

material of pattern

Answer 59

- 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

Question 60

types of pattern

Answer 60

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

Question 61

chaplets..

Answer 61

hold to core together,

Question 62

sand casting requires

Answer 62

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

Question 63

surface cleaning

Answer 63

air blasting to fix the surface, tumbling

Question 64

heat treatment

Answer 64

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

Question 65

machining allowance

Answer 65

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

Question 66

sand blow(at/below casting surface

Answer 66

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

Question 67

pin holes (at or below casting surface)

Answer 67

release of gases during pouring

Question 68

penetration

Answer 68

due to strength of molten material whihc can penetrate through sand

Question 69

mold shift

Answer 69

no alignment pins and the cope and drag shift

Question 70

porosity

Answer 70

macro porosity: due to shrinkage ( design for directional cooling) micro porosity: ytrapped moleules, or dentrite shapes/shrinkage

Question 71

easily differentiate between sand, plaster and ceramic casting

Answer 71

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

Question 72

Permanent mold casting

Answer 72

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.

Question 73

three kinds of permanent molds

Answer 73

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

Question 74

slush casting

Answer 74

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

Question 75

true/semi centrifugal

Answer 75

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)