Casting Processes I
Dermot Brabazon
Ref: Kalpakjian, Serope, Manufacturing engineering and technology. - 3rd ed.
Reading, Mass : Addison-Wesley, 1995. - 0201538466 MAIN LENDING 670.42/KAL
(Also: MAIN LENDING 670/KAL )
The Material Transformation Process
SLS
Powders
Special
Pressing
Ingot
casting
Molten
Material
= Casting Processes
Increasing level of detail
Rolling
Forging/
Press forming
Extruding
Casting
Shapes
Single crystal
pulling
Blow
moulding
Machining
Continuous
Casting/Rolling
Sheet metal
forming
Finishing
Assembly
Stamping
Products
Raw Material
Injection
moulding
Firing/
Sintering
Casting
• The conversion of raw materials into useful
shapes using phase transformations
• One of the first steps in converting raw
materials into useful products
• Applicable to most materials
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Metals
Ceramics
Plastics
Glass
• Also includes mixtures
– Ceramic slips and slurries
Casting
– a phase change forming process
• Form the shape from a fluid material state in
a mould or container. These materials include
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Molten metals
Monomer solutions
Slips
Slurries
• Change the liquid into a solid by
• Removing heat
• Removing suspending liquid
• Initiating a reaction
– apply heat
– inject reactants
– irradiate with photons
Casting Fundamentals
• Casting advantages
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High shape complexity with internal cavities
Large shape size and variety
Wide variety of materials
Ease of production
Variety of materials that can be cast
Close tolerances (some processes)
High surface finish (some processes)
Excellent mechanical properties (some
processes)
– Economic (for some lot sizes)
Kalpakjian pp 262-263
Casting Fundamentals
• Casting disadvantages
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High setup costs (some processes)
Low tolerances (some processes)
Low surface finish (some processes)
Porosity (some processes)
Inhomogeneities (some processes)
Poor mechanical properties (some processes)
Casting Fundamentals
• Overall Process:
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Make mould
Pour in fluid to solidify
Cool/solidify
Remove shape from mould
Types of Casting
• Molten materials which solidify on cooling
– Metals, ceramics, glasses
• Liquids which solidify by reactions with light,
activators/hardeners or moisture
– Plastics
• Slurries which solidify by the extraction of the
suspending medium (usually water)
– Ceramics
Casting Fundamentals for Molten
Material
• Factors affecting solidification characteristics
from the molten state:
– Fluidity
• Flow of molten material into the cavity
– Heat transfer effects
• During solidification and cooling
– Solidification effects
– Influence of the type of mould material
Kalpakjian p 265
Fluidity of Molten Metal
• Fluidity is dependent on:
– Characteristics of the fluid
– Casting parameters
Fluidity - Characteristics of the
fluid
• Basically, Fluidity is the ability of the liquid to
flow into the mould
If these increase:
• Temperature sensitivity of
viscosity
• Surface tension
• Inclusions
• Freezing range
Kalpakjian pp 274-275
then
Fluidity Reduces
Theory of Fluid Flow
• Theory has three components:
– Bernoulli's theorem
– Continuity law
– Laminar vs turbulent flow
Kalpakjian pp 272-275
Bernoulli's theorem
2
v
p
h + ---- + ---- = constant
g
2g
where h is the elevation above a reference plane, p is the pressure at
that elevation, v is the velocity of the liquid at that elevation,  is the
density of the liquid and g is the gravitational constant
Continuity Law
For an incompressible liquid:
• Av = constant, called the flow rate
where A is the cross sectional area and v is the velocity
Laminar vs. Turbulent Flow
• Laminar flow is preferred
– Reynolds number (Re) less than 2000
• Turbulent flow (Re >4,000) can cause air
entrapment and dross (oxide) formation
– results in defects
Re = VD/
where V and D are a fluid characteristic velocity
and distance;  is density and  is viscosity.
Parts of casting mould to be
included in fluid flow analysis
• Pouring basin
– where the molten metal enters the mould
• Gating system
– connects the pouring basin to the rest of the
mould through
• Sprue (connects the pouring basin to the runners)
• Runners (carry the molten metal to the mould)
• Risers
– act as reservoirs to supply molten material as it
solidifies and shrinks
Fluidity - Casting parameters
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Mould design
Mould material
Mould surface characteristics
Degree of superheat
Rate of pouring
Heat transfer
Kalpakjian p 275
Casting Fundamentals for Molten
Material
• Factors affecting solidification characteristics
from the molten state
– Fluidity
• Flow of molten material into the cavity
– Heat transfer effects
• During solidification and cooling
– Solidification effects
– Influence of the type of mould material
Kalpakjian p 265
Heat Transfer
• Very complex phenomenon
• Very simple process
– A cold mould extracts heat from the melt causing it
to solidify
• Critical to design of mould
• Can compute a relative time for solidification
Kalpakjian p 275
Heat Transfer - solidification time
• Solidification time is proportional to the
square of the volume divided by the surface
area
– A sphere will have a much longer solidification
time than a complex shape of the same volume
Effect of Cooling Rate
– Rate of cooling is critical for the structure of
the material and hence its properties
• Slow cooling (~0.1K/s) gives large grain sizes
• Fast cooling (~10 K/s) gives small grain sizes
• Very fast cooling rates (>10 K/s) produce amorphous
materials
– Implications:• Should design artifact to be thin and not massive
• Require "chills" to control cooling rate
Casting Fundamentals for Molten
Material
• Factors affecting solidification characteristics
from the molten state
– Fluidity
• Flow of molten material into the cavity
– Heat transfer effects
• During solidification and cooling
– Solidification effects
– Influence of the type of mould material
Kalpakjian p 265
Solidification Effects
• This is where the material becomes important
– Plastics
• Not as critical as for metals
– Semiconductors
• Specialty crystal growing
• Single crystal so no microstructures
– Glass
• No microstructure (amorphous)
Kalpakjian pp 263-277
Solidification Effects - Metals
• Molten metal solidification events depend on
the type of material
– Pure metals
– Alloy
Solidification of Pure Metals
• Solidification occurs at one temperature
• Solidification occurs from the mould walls to
the center in a plane front
• Grains tend to be equiaxed in the centre of
the casting, but grow outward from the mould
wall in a columnar structure
• Nucleation agents can cause a more
equiaxed structure (more uniform grains and
size distribution)
Solidification of Alloys
• Eutectics behave similarly to pure metals
• Cast grain structure depends on phase
diagram
Copper-Nickel Phase Diagram
Kalpakjian p 120
Solidification of Alloys
• Alloys with liquidus and solidus temperatures have a
physical “mushy zone”
• “Mushy zone” has solid particles and liquid co-existing
• Solid particles tend to be dendritic (tree like) in nature
that grow from the mould wall
• Microstructure highly dependent on cooling rate
• Freezing range is the temperature difference between
the liquidus and solidus temperatures
• Ferrous alloys tend to have small freezing ranges
• Aluminium and magnesium alloys tend to have wide
freezing ranges
Crystallization Phenomena
Kalpakjian p 267
Why is solidification so important
for metals?
• The solidification events determine the
microstructure of the product:
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Grain size
Grain distribution
Grain morphology
Grain boundaries
Grain composition
Porosity content and type
Influence of grain size and
microporosity
If these decrease:
• Grain size
• Microporosity
Kalpakjian p 269
then
• Strength and
Ductility increase
• Cracking tendency
decreases
Structure-Property Relationships
• Slow cooling- uniform composition
• Normal cooling- micro and macro segregation
• Microsegregation
– Segregation of alloying elements within the grains
or dendrites
– Dendrite surface has higher concentration of
alloying elements than core
• Macrosegregation
– Segregation of alloying elements across the
casting itself
Structure-Property Relationships
• Types of macrosegregration
– Normal
• Constituents with lower melting temperature are driven
away from the mould wall to leave a higher concentration at
casting center
– Inverse
• Melt enters the cavities among the dendrites formed at the
surface
– Gravity
• Heavy elements sink to the bottom
• Macrosegregation gives rise to inhomogeneous
microstructures and therefore bad mechanical
properties
Avoidance of Macrosegregation
• Use
– Nucleation agents
– Create more grains and better chemical
homogeneity by mechanical means
• Rheocasting - stir the metal while it is in the mushy zone
• Vibration
• Electromagnetic stirring
Solidification Effects - Shrinkage
• The metal shrinks as it cools
– in the melt
– as it solidifies as a solid (largest)
Volume Solid Contraction of some metals:
Aluminium
Carbon Steel
Copper
Gray iron
Kalpakjian p 279
6.6%
2.5-3%
4.9%
-2.55
Impact of Shrinkage on mould
Design
• Dimensions of mould
• moulds must be constructed to be larger than
the final product because the metal shrinks
as it cools
– Patternmakers ruler
• Warpage due to differential shrinkage
• Defects due to induced stresses
• Porosity
Part Porosity
• Caused by shrinkage or gases
• Detrimental to the strength and ductility of the
metal, the surface finish and pressure
integrity of the part
Shrinkage Porosity
• Caused by differential cooling
• Thin sections cool faster than thick sections
leading to too little material in the thick
sections
• When the thick sections begin to solidify,
porosity develops due to the lack of feed
metal which is often cut off by already
solidified thin sections
• mould designers avoid this by the use of
chills and proper flow channels and riser
placement
Gas Porosity
• Liquid metals have greater solubility for gases
than solid metals
• Any gas in the melt appears as spherical
cavities
• Melt treatment must include various
degassification processes
• Can also have gases arising from reactions
(melt - mould)
List of other defect classes
• Projections
– fins, flash, swells (massive), rough surfaces
• Cavities
– internal, exposed, blowholes, pinholes
• Discontinuities
– cracks, cold and hot tearing, cold shuts
• Defective surface
– folds, laps, scars, adhering sand, oxide scale
• Incomplete casts
– insufficient metal, leaky moulds
• Incorrect dimensions or shape
– improper shrinkage allowance, warping, etc
• Porosity
See Kalpakjian pp 279-281
Casting Fundamentals for Molten
Material
• Factors affecting solidification characteristics
from the molten state
– Fluidity
• Flow of molten material into the cavity
– Heat transfer effects
• During solidification and cooling
– Solidification effects
– Influence of the type of mould material
Kalpakjian p 265
Influence of the mould material
• Mould material impacts on:
– the heat transfer rate
– the surface finish
– the number of and hence grain size of the
microstructure
• Selection of the mould material is strongly
influenced by the process