Shape Casting Flashcards
How old does shape casting date back to?
5,000 BCE
What are the 3 main benefits of Shape Casting?
- Large form freedom
- Retain properties at elevated temp
- Distinct character and Texture
In casting, Liquid metal is poured into a mold that defines the
______________of the product
Negative
When casting, it is best to fill the mold “_________” to prevent
splashing (and thus _________ and loss of material
properties) and helps prevents gas entrapment
bottom up, oxidation
Castings generally have _________ alloy content
o e.g.: Steel (Fe< 2% carbon) is not typically cast,
but cast iron (Fe with ______ carbon is)
higher, 2-4%
What are the two reasons for higher alloy content in castings?
Lower viscosity (Better mold filling), Lower melting temperature (Lower energy, Faster throughput)
Want casting metals to be near ______________
* Homogeneous composition and lowest possible
melting temp and typically lower viscosity
eutectic composition
Higher alloy content comes at expense of
______________ such as Toughness, ductility and fatigue strength being reduced by presence of alloy
o In cast iron, stiffness is also __________relative to low carbon steel
material properties, lowered
What is the major trade off in casting?
Castability for material properties
Why do you want slow filling speeds in casting?
Laminar flow minimizes surface area and oxidation
What is the transition from laminar to turbulent flow predicted with?
Reynolds Number
For flow over a flat plate, the transition is typically Re=_______________
500,000
The downside of filling slowly is ____________ and ________________
slow production and cold running
_____________ is when parts of the mold solidify before all of the mold is filled
Cold Running
You can prevent cold running by increasing ______________ and ___________________ but those require more Finishing after and increased cost and weight.
gates and fill time,
Casting is usually ________ at ________ pressures and _____ at ________ pressures
Slow, low, fast, high
When casting cools it _________ in volume
reduces
Volume reduction is highes in _____________ range
solidification
What are the three phases that cooling occurs in?
Tpouring to Tliquidus, Liquid to solid phase transition (Tliquidus to Tsolidus), Tsolidus to RT
Tpouring to Tliquidus
specific heat - liquid phase,
Liquid to solid phase transition (Tliquidus to Tsolidus)
Latent heat of solidification
Tsolidus to RT
Specific Heat, solid phase (J/(kg-k))
What can be done about the
solidification shrinkage?
Add extra material during filling (known as “feeding”)
What about the shrinkage that occurs after solidification?
Make the mold bigger (typically 1%-2%)
_____________________ determines how long it takes before part is cool enough
and strong enough to be handled?
Chvorinov’s Rule
Chvorinov’s rule works well for large solid castings, but it
_________________time for hollow and thin-walled parts
underestimates
For thin walled parts (< _____mm thick), chvorinov’s rule very much
_______________ solidification and cooling time.
* A better model is proportional to_____
* Because the mold expands while the casting contracts
creating an air gap that acts like an _________ limiting
4, underestimates, V/A, insulator
What are the three main defects that affect casting?
porosity, internal stresses, distortion
What are the two main causes of porosity?
Shrinkage porosity, Gas Bubbles
_______________”: Not enough extra material
is fed into the casting to compensate for
solidification shrinkage
* Can happen when a thin section of a mold or a
poorly designed runner solidifies thus ________ flow into the thicker section
* Can be avoided with good mold and filling
design
Shrinkage Porosity, blocking
Think about cooling time being related to V/A
* Which cools more quickly, thinner sections or
thicker sections?
Thinner
Different sections with different thicknesses will cool at different rates which leads to _________________ within the casting
temperature gradients
________________ is the amount of thermal expansion or
contraction
Thermal Strain
Local differences in temperature will therefore cause
differences in contraction, these differences cause
stresses by __________ Law
Hookes law
If the casting is free to deform and the stresses exceed
the________ stress, permanent deformation can occur
yield
Predicting internal stresses and/or deformation often
requires ________________because of complex
geometries involved
Simulations
The issues of porosity, internal stress and distortion can
typically be mitigated through careful selection of _______
and design of part to avoid large ___________________
Alloy, Thickness Changes
________________ is the process of choosing the best alloy for the job
* Large field with a lot of “tricks” and rules of thumb
developed over the years
Casting Metallurgy
Inoculants are high melting point __________ added before pouring
o As liquid cools, solids form around the inoculant,
promoting ________ grains and less __________
powders, smaller, segregation
Sometimes ________ additions of the right element
can dramatically improve the tensile strength of a cast
metal (e.g., trace amounts of sodium in Al castings)
o Called “___________”
small, poisoning
What casting method is this?
Very cheap
o Can be made to a wide range of sizes
o Rather fragile and only support low filling
speeds and pressures
o Long solidification times = larger grains
o Rough surface properties
o Poor tolerances
Sand Casting
Plane where drag and cope meet is
called the “__________”
Parting Plane
What is the down ward channel is a casting mold called?
Sprue
Sand cores allow manufacture of
________ products
hollow
Riser (aka “_______”) shows when filling is complete and helps allow
for shrinkage
feeder
What method is of casting is this?
Shorter cycle times = Finer microstructure
o Smoother products and good control over tolerances
o Filling speeds are low
o Product size more limited than sand casting
o Aluminum and magnesium are commonly cast using this method
Gravity Die Casting
What method is of casting is this?
Also uses reusable steel dies
o Pressures from 2 to 5 bar
o Good reproduction of die details
o Dies often placed on top of furnace and fed through riser tube using gas pressure
o Aluminum & magnesium are also commonly cast using this method
Low Pressure Die Casting
Uses very expensive steel dies
o Pressures up to 2000 bar
o Cannot use Sand cores
o But can use sliding elements to
make undercuts
o Injection speed are high
o Fast throughput
o Low quality
o Thinner products
o Limited to Zink, aluminum,
and magnesium
High Pressure Die Casting
Relatively rare method
o Pattern made of wax and attached to the sprue
o Dipped into a ceramic slurry that cures while the
wax melts away
o The hollow ceramic shell is used as the mold
o After casting, shell is destroyed to remove part
o Very time consuming process
o Good for very complex geometries
Investment Casting
Metals have a __________ grain structure
o Grains grow within solid and along mold wall and where
they impinge on each other is the _____________
crystalline, grain boundary
Cast grains are typically _______ to _____ mm large
0.05, 5
o Smaller grain size = _____________
Better Properties
Grain size is largely dictated by cooling rate
o ________ cooling leads to smaller grains
fast
That are the 3 main reasons alloying elements are used?
- Improve processability (e.g., low viscosity or
melt temp) - Improve material properties (e.g., strength)
- Minimize effects of impurities or improve
microstructure
Alloying elements can introduce impurities that can be
detrimental
Detrimental
___________________ is the name given to the non-uniform
distribution of dissolved elements during solidification
segregation
In the liquid state, alloy elements are dissolved in an
“____________”
Atomic Soup
During cooling, the first solids to form will be _________than average, so the remainder is
relatively _________ which will lead to impurities
purer, enriched
The last part to solidify will be the grain boundaries
so the impurities tend to be ___________near the
grain boundaries
concentrated
