Module 2- Casting of Metals (2) and their use

casting of materals and their use in technology

1. Lecture 2: Casting of Metals
MFG 120

2. What is Casting?
•Casting is the process of pouring molten
metal into a near net shape.

3. Casting vs Cast Iron
•Many types of materials can be cast
–Steel
–Stainless Steel
–Cast Irons
–Copper
–Nickel
–Aluminum
•We typically refer to iron-carbon alloys as steel.
However, when the carbon exceeds 2%, the material
is classified as “Cast Iron”

4. Casting from Steelmaking

5. Ingots
•After Steel Making Two Options
–Ingots
–Continuous Casting
•An ingot is a large volume of steel that is poured
into a cast iron mold and then solidified.
–Think of it as a rectangular casting of steel

6. Ingots
•Liquid Steel contains dissolved oxygen
•As steel cools, less oxygen can be dissolved and can outgas
causing bubbles or create Iron-Oxide
1)Killed 2) Semi Killed 3)Rimmed 4)Capped
1)Killed
1)Completely deoxidized
2)Suffers from shrinkage
2)Semi Killed
1)Mostly deoxizied
2)Suitable for drawing
3)Rimmed
1)No deoxidizers
2)Used for cold bending
3)“rim” of pure iron on
outside
4)Capped
1)Rimmed steel covered
(capped) during
solidification
2)High yield, good for
sheets

7. Ingots
•The solidification process for
ingots results in a poor grain
structure that is weak.
–Large grains, columnar
shape
•Before we can use this steel,
we must change the grain
structure through additional
processing and/or heat
treatments

8. Continuous Casting vs Shaped Casting
•During Steel Making, after a BOF
or EAF furnace, continuous casting
may occur which cools the steel
into a wrought product shape (billet,
bloom, slab)
•In shape casting, metal is poured
into a near-net or final shape
of the desired product

9. Steel Solidification

10. Steel Solidification
•When a metal solidifies from a liquid to a solid, atoms nucleate a form
crystals.
–With the exception of specialized cases and intentionally slow solidification,
multiple nucleation sites form during solidification
•Crystals continue to grow in all directions until the come in contact with
other crystals. The process continues until the entire mass becomes a solid
•The crystals are called grains.
•The optical formation the grains is called the microstructure.
–Unique for the metal and processing
•Most metals from a dendritic structure during solidification.

11. Steel Solidification

12. Metal Solidification
•When liquid metal cools, up to 3 different
“zones” can form:
–Chill Zone
–Equiaxed Zone
–Columnar Zone

13. Chill Zone
•The mold is much cooler than the liquid
metal, causing the metal to rapidly solidify
into small grains

•As the liquid metal cools, the grains grow
(continue to cool to a solid) towards the
center of the mold

14. Columnar Zone
•The solid grains continue to cool. They
“compete” with one another and grow
outwards from the mold.

15. Equiaxed Zone
•This zone does not always occur
•As the liquid metal is cooling, the long
columns can breakdown and start to form
new grains towards the center.
•The cooling depends on the alloy and
temperature of the liquid and the mold

16. Shaped Casting Process Types
•Five Primary Types Will be Covered:
–Sand Casting
–Investment Casting
–Permanent Mold
–Die Casting
–Centrifugal Casting

17. Sand Casting

18. Sand Casting
•A solid and re-usable pattern of the item is
made. The item can be made of wood,
aluminum, or other convenient materials
•The molding board is also called the Flask.
The Flask contains the Drag and Cope
•The pattern (typically with a releasing
agent) has the sand packed all around it.
This bottom portion where the sand and
pattern are is called the: Drag

19. Sand Casting
•The Drag is flipped upside down
and the pattern removed to
create a cavity or “negative
patter”.
•The top portion of the mold is
called the Cope.

20. Sand Casting
•Gates and Risers are added.
–Gates: An opening for liquid metal
to be poured into the mold
–Riser: Overflow areas for the liquid
metal to help ensure complete
solidification

21. Sand Casting
•Gates and Risers are added.
–Gates: An opening for liquid metal to
be poured into the mold
–Riser: Overflow areas for the liquid
metal to help ensure complete
solidification
•If the part has a cavity, a Core is
placed inside the mold

22. Sand Casting
•Mold is put together and metal poured in.
•After cooling, the mold is removed and
any cores are broken up and brushed out
•The gates and risers will need to be cut.
There may also be a parting line that
needs to be ground

24. Sand Casting
•Sand: Silica, Chromite, or Zirconia + Clay,
Oatmeal, or other “stuff”
•There are two main types of molds:
–Green Sand Molds: Sand with 2-4% moisture or in
some cases oil bonded sands.
–Dry Sand Molds: Sand with resin mixed in. May have
to be baked.

25. Sand Casting Pros & Cons
•Pros:
–Versatile: They can be made “near net shape” which minimizes machining time
and material cost.
–Low Equipment Cost
–Design flexibility
–Can be economical in small volumes or can be scaled up for high volume
production.
•Cons:
–Cannot achieve close tolerances (dimensional accuracy)
–Surface finishes are rough
–Lower mechanical properties (slower cooling rate resulting in larger grains). Not
good for thin sections
–High labor costs

26. Sand Casting Defects
•Gas Porosity
•Shrinkage Porosity
•Hot Tears
•Misrun
•Cold Shut
•Scabs
•Veins
•“Inclusions”

27. Gas Porosity
•Created by gases that cannot escape prior to
solidification
•May be trapped gases during steel making, vacuum
degassing and improvements to the molten metal can
eliminate casting defects.
•User a coarser sand
•Ensure molds are dryer
•Reduce metal
temperature to increase
the speed to cool to solid

28. Shrinkage
•Occurs when there is
inadequate feed metal to fill
a section.
•Mold design and add risers
to ensure a reservoir of
liquid metal

29. Hot Tears
•Cracks that occur because hot metal is weak
(metal has lower strength at high temperature).
Usually seen at areas of high stress
•Potentially a result of mishandling a hot
casting.
•May also be caused by severe cooling rates in
intricate geometries (improved with chills)

30. Misrun
•A misrun is when metal does not fully fill the mold cavity (unfilled region)
–Insufficient liquid flow
–Low pouring temperature
–Too Slow pouring

31. Cold Shut
•Occurs when two metal fronts meet but are
too cold to fuse together (premature
freezing). Similar reasons as mis-runs

32. Scabs
•Scabs occur when the mold is too weak to resist erosion
by the molten metal. The Mold sand flakes off the
surface and metal fills in the void resulting in a scab.

33. Veins
•Veins occur when a core is not strong
enough and develops cracks.

34. Inclusions
•Inclusions can be mold sand or slag from the
melt that get into the part.

35. Investment Casting
(Lost Wax)
•A hybrid process: A permanent mold makes an expendable pattern.
•The pattern is made from a wax or foam, coated with a ceramic, and the mold
is built around the pattern
•The pattern remains in the mold until the ceramic is hardened or cured
•Wax is then heated and removed. Typically collected for reuse.
•Metal is poured into the void created by the removed wax
•Used for parts that have high detail
•Excellent accordance and surface finish (No parting line in cast part).
However, limited to small parts and more expensive.

36. Investment Casting
•Play Video

37. Special Case: Investment Casting
•Turbine Engine Blades
–Investment casting is used to create turbine blades for
jet engines
–The design of the mold and cooling process can allow for
the creation of a single crystal structure: all of the
material is in the same orientation
–The solidification process is designed to selectively allow
only one grain of material to grow

38. Permanent Molds
•A metal mold, usually made from cast iron, is used
–Typically reserved for high production due to tooling costs
•Castings are usually small and simple compared to Sand
Casting. But, they are better dimensional tolerance and better
surface finish.
•For high temperature applications, the mold is sprayed with a
refractory prior to casting.
•Molds wear out after a few thousand cycles and require
frequent replacement

39. Permanent Molds
•Advantages (compared to sand casting)
–Better dimensional accuracy
–More uniform 
–Better mechanical properties
–Improved surface finish

40. Die Casting Machines

41. Die Casting
•Die casting is similar to permanent molding, however the metal is
injected at high pressures (1000 to 100,000 psi)
–Allows for very high production rates
–Allows for intricate part geometries
–High pressure can cause turbulence and porosity. Possible to perform
casting under vacuum
•Typically zinc, aluminum, magnesium, and sometime coppers are die
cast
•Hot and Cold Chamber Machines
–Hot chamber machines are used for metals that have a low melting point.
•Good surface finish and tight dimensional tolerances. Typically not for
parts which required high strength

42. Centrifugal Casting
•Centrifugal casting is used to cast parts that
have a round symmetry (pipes)
•The mold is spun while metal is poured in.

43. Cast Part Properties

44. Properties of Cast Parts
•All three will affect the performance and machinability
of the part.
–Hardness
–Residual Stress State
–Microstructure
•Shake Out Time
–High Shakeout time --> Slowly removed from mold --> Slow cooling --> Lower mechanical
properties but lower residual stresses
–Low (short) Shakeout time --> Quickly removed from mold --> Fast cooling --> Higher
mechanical properties but higher residual stresses

45. Hardness
•Hardness will vary with chemistry and cooling
rate.
•Thin sections can cool too quickly resulting in a
higher hardness.
•In extreme cases, carbides can form which has
very detrimental effects on machinability.

46. Residual Stress
•Residual Stresses are stresses that remain in the material after the
applied external forces are removed.
•In some cases, the residual stress can be severe enough to cause
premature failure.
•The metal shrinks as it solidifies and cools. Residual stresses
develop when the mold or another section of the casting applies a
force to the part and restricts movement.

47. Microstructures
G
F
C
P
White Iron (1.6-3.6% C)
Gray Iron
(3.1-3.4% C)
Ductile Iron (3.1-3.4% C)
# Shakeout
time
Cooling
rate
Phases
formed
Residual
Stress
Tensile
Strength
1 Low High Fine
Pearlite
High High
2 Medium Mediu
m
Coarse
Pearlite
Medium Medium
3 High Low Coarse
Pearlite +
Ferrite
Low Low