Solidification Continued

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Solidification Continued
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Cast Structure

• Cast structure depends on the rate of heat
  transfer and flow of liquid metal during
  solidification
• Pure metal
• Grains grow opposite to heat transfer
• Grain development is called homogenous nucleation
  grains grow upon themselves.
• Alloys
• Solidification begins when temperature drops
  below Liquidus.
• Columnar dendrites are formed.

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Growth of Crystal
Metal
Mold
Preferred direction of growth
Small random orientation crystals
Columnar crystals
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Schematic illustration of three Cast Structures
Equiaxed Structure
Chill zone
Equiaxed zone
Columnar Zone
Heterogeneous nucleation of grains using
nucleating agents
Solid Solution alloys
Pure metals
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Freezing Range

• Freezing Range
• TL -TS
• Ferrous
• Short Range (90 F)
• Aluminum,Magnesium
• Long Range (200 F)

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Center Line Feeding Resistance
Freezing Diagram of ordinary sand mold

• is the time between start and end of freezing at
  center line.
• CFR AC/OC
• ( Ratio of freezing time at center line to total
  solidification time)
• CFR gt 70 is not desirable

Freezing Diagram for a chilled mold
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Solidification Pattern in gray cast iron

• Solidification patterns for gray cast iron in a
  180 mm(7 inch)4 square casting
• Note after 11 min of cooling the dendrites reach
  each other, but the casting is still mushy
  throughout.
• Takes about 2 hrs to solidify completely

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Solidification Pattern in Carbon steel
0.05-0.10 C steel
0.25-0.30 C steel
0.55-0.60 C steel

• Solidification of carbon steels in sand and chill
  (metal) molds
• Note the difference in the solidification pattern
  as the carbon content increases

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Types of grain formed

• Alloys having symmetrical phase diagram
  generally lamellar
• When volume fraction of minor phase of alloy is
  less than about 25 fibrous (cast iron)
• Metals remain mushy during the process of
  solidification
• Short freezing range (lt 90 F)
• Long freezing range (gt 200 F)

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Effects of cooling rate

• Slow cooling rate 102 K/s
• coarse dendrite structure
• Fast cooling rate 104 K/s
• structure is finer and dendrites are smaller
• Rapid cooling 106 - 108 K/s
• amorphous
• Structure of grain affects the property of grain

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Effect of Grain Size

• When grain size decreases
• Strength and ductility increases
• Micro property decrease (inter dendrite voids)
• Tendency for hot tearing reduces
• Lack of uniformity causes anisotropic properties
  ( properties vary in different directions)

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Hall-Petch Equation (section 3.4.1 of text)
Y Yi kd-1/2 Yi Basic yield stress k
indicating dislocation is piled up d grain
diameter
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Structure Property Relationship

• Slow cooling
• dendrites form uniform composition
• Normal Cooling
• Cored dendrites are formed, Surface composition
  varies, Concentration gradient is present

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Micro Segregation

• The process by which solute in an alloy is
  rejected towards the surface during
  solidification of dendrites
• Concentration in the root of dendrites

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• Macro segregation
• Various composition throughout the casting
• Normal Segregation
• constituents move towards center.
• In plane front movement, lower melting point
• Higher concentrations of alloying element in the
  center.

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Inverse Segregation

• In dendritic structure however center has lower
  concentration of alloy( inverse segregation)
• Metals with higher alloying enter space between
  dendrites.

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Gravity Segregation

• Alloying elements with higher density sinks to
  the bottom (Antimony in Antimony-Lead alloy)

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Kinetics of liquid solid interface
G Thermal gradient R Rate at which interface
moves
G/R 105 to 107
Schematic illustration of cast structures plane
front single phase
G/R 1010 to 1012
Schematic illustration of cast structures plane
front two phase
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Inoculants

• Are nucleating agents added to the alloy to
  create heterogeneous nucleation and create
  equiaxed grains.

Equiaxed grains
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Other Effects On Dendrite formation

• Convection
• in the molten metal affects dendrite formation.
• Can help dendrites to separate and form equiaxed
  grains
• Agitation
• Dendrites can also be broken by agitation
  (mechanical vibration)
• We can obtain a fine grained equiaxed casting
  also known as rheocasting
• Experiments in space to obtain, gravity free
  casting

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Three methods of casting turbine blades

• Three methods of casting turbine blades
• A conventional casting with ceramic mold,
  Regular polycrystal
• B directional solidification
• C Method to produce single crystal blade
• Nickel Alloys

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Melting Furnaces
Induction
Cupola
Gas fired crucible
Electric Arc
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Summary of Solidification

• Grain formation
• Rate of cooling and effect on solidification
• Dendrite, equiaxed, amorphous etc
• Segregation micro, macro, gravity
• Single crystal and directional solidification
• Types of furnaces