Casting Flashcards
Class notes
what is casting?
heating up/melting any material to mold it into something. The shape of the mold determies the shape of the casting
what is the importance of casting
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
casting is a _____ state process
single/ net shape
near net shape is like a finish or an extra step
what are the 2 types of casting?
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
define foundry
a factory equipped for making molds and handling all steps of the casting process
what are the steps of casting
- make mold
- melt metal
- pour into mold
- let it freeze
- remove from mold
- expandable : break mold
- permanent: bend it and be gentle
what are the benefits of casting?
- complex parts can be manufactures
- makes both external and internal shapes
- no size limit
- less waste
- versatiles
what are some cons
- limits on mechanical properties ( the mech properties can change as it cools)
- some (expandable casting) and finish have poor dimensional accuracy
- shrinkage/porosity/cracks
(going from liquid to solid) - bad for workers and environments cuz of fumes and melting metal
- expensive for permanent tool casting
- microstructures are difficult to control
what are the things that make up the mold?
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
What is shrinkage
shrinkage means the mold has to be a little bigger than required
what is a pattern?
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
what are the 2 kinds of molds?
open molds: a container like shape
closed mold: to create complex shapes
why do mold cavities have a draft?
draft is a taper that is meant to remove the pattern
what is the pouring temp?
highers than the melting point of metal so it doesnt immediately solidify
when does solidification/freezing begin?
at melting point. this is when freezing begins
describe the TST cooling curve for Pure metal
total solidification time = time taken between pouring and complete solidification
what happens during solidification
a skin is formed, whihc slowly inscreases untile heat is transferred
grain structure for pure metals
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
which element will be the first to freeze for alloys?
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
grain structure for alloys
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
effects of cooling rate
slow : big dendritic structures - not as strong
high: small dentrites
- stronger, ductiltiy increases, decreases porosity
cost is higher
micro segregation alloy
segregation of alloy in micro scale
low melting point alloy will be stuck in between the spines dendrites microstructure of high melting point metal
describe how columnar dendrites look like
in line with eachother
describe how eqiaxed dentrites look like
evenly spaced and congruent shape, more circular and random formation
what is normal segregation
it is when the lower melting point metal in an alloy moves to the centre
what is inverse segregation (macro)
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)
gravity segregation( alloy macro segregation)
due to gravity high density compounds sinks and lighter elements float
convection
how to control cooling rate as we go from one state to another
promotes formation of chill zone and refines the grain size
what happens with increased convection
dentrites arm seperate i.e dentrities multiplication. increasing convention can make grains stronger
what happens with reducing/eliminating convection
it will result in coarse and longer columnar dentritic grains
how to prevent macro segregation
nucleation agent: we do not want segregation so it minimizes it by adding intermetalic compounds
create more grains through vibration and electromagnetic stirring
is micro or macro segregation easier to control
macro segregation is easier to control then micro
what is fluidity
A measure of the capability
of a metal to flow into and fill the
mold before freezing.
what is viscosity
Fluidity Decreases as viscosity (resistance to flow) and the
viscosity index (its sensitivity to
temperature) increases
surface tension
High surface tension reduce fluidity
inclusion
Inclusion (Insoluble
particles) have adverse effect on
fluidity
freezing rage( mostly impacts alloys)
shorter the freezing range higher the fluidity (alloys with long freezing range have low fluidity)
fixed freezing range
alloy changing freezing range
how to control the fluidity
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)
solidifcication time
chvorinovs rule:
TST = Cm(V/A)^N
Cm= mold constant
n= 2 unless otherwise specified
A = surface area of casting
molding constant is determined by…..
mold material casting metal pouring temp
what part should be the last to solidify in a casting system, in order to prevent shrinkage
the riser
from mr. C rule, we know:
(V/A)Riser> (V/A)Casting
how do you prevent errors during casting(directional solidification)
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
chills
internal or external heat sinks that casue rapid freezing in the regions you want to freeze first
porosity
trapped air molecules(will look like shperical shapes) and shrinkage(rough voids)
controlling porosity
- 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
Bernoulli theorm
v= sqrt(2gh)
Q= Volumetric flow rate
Q=v1A1=v2A2
TMF= Tie required to fill mold
TMF=V/Q
V=mold cavity volume
aspiration
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)
gates
- 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
Ingot Casting(expendable)
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
Continuous casting process(expendable)
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
continuous casting give you three basic shapes
slab, billet and bloom
sand casting
- 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
shell/vacuum casting(molding)
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
expendable pattern
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)
plaster
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.
ceramics
can withstand high temps, and good finish
sand binders
held together but 90% sand and 3%water and binding agent of clay
other bonding agents:
- organic resins
- inorganic binders
sand molds
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
material of pattern
- 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
types of pattern
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
chaplets..
hold to core together,
sand casting requires
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
surface cleaning
air blasting to fix the surface, tumbling
heat treatment
rate of cooling defines mechanical properties of metal. so heat treating can help bring out desired properties of metal and subsequent mahine process
machining allowance
- additional material left over on the casting where machining is necessary, this is different than shrinkage allowance, typically 1.5 to 3mm left over.
sand blow(at/below casting surface
balloon shaped gas cavity caused by release of mold gases during pouring
- low permeability, poor ventilation
pin holes (at or below casting surface)
release of gases during pouring
penetration
due to strength of molten material whihc can penetrate through sand
mold shift
no alignment pins and the cope and drag shift
porosity
macro porosity: due to shrinkage ( design for directional cooling)
micro porosity: ytrapped moleules, or dentrite shapes/shrinkage
easily differentiate between sand, plaster and ceramic casting
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
Permanent mold casting
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.
three kinds of permanent molds
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
slush casting
used to make hallow structures. Let the outside solidify and then pour out the rest
true/semi centrifugal
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)
