Introduction to Solidification 4. Microstructure

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Introduction to Solidification4. Microstructure

• Rongshan QIN
• http//cml.postech.ac.kr/rqin

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Casting ductile iron cylinders http//www.mse.mtu
.edu/casting_iron/casting_iron.html
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Microstructure of ingots
T. F. Brower and M.C. Flemings, Trans. AIME, 239,
1620 (1967)
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Phase-field simulation of casting
H.B. Dong and P.D. Lee, Acta Mater. 53 (2005) 659
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Outer chilled zones
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Outer chilled zones
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Outer chilled zones
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Outer chilled zones
Pure metals Formation of shell because
temperature gradient is the key factor in grain
growth.
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Outer chilled zones
Alloy solutions Growth is determined by
temperature gradient as well as constitutional
supercooling.
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Outer chilled zones
T
liquidus
solidus
c
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Outer chilled zones
Pouring temperature
re-melted?
survived?
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Microstructure of ingot

• Chilled zone
• Fine equiaxed grains.
• Pure substance Continuous shell.
• Solution Particles
• Particles flushed away from wall into the central
• Re-melted
• Survived nucleus

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Intermediate columnar zone
The grain is overtaken by neighbors.
Columnar grains grows
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Intermediate columnar zone
Growth and overtaken
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Intermediate columnar zone
Columnar growth blocked
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Intermediate columnar zone
Supercooling and interfacial energy Curvature and
interface kinetic coefficient.
Formation of dendrite
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Intermediate columnar zone
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Intermediate columnar zone
Bridging of secondary arms
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Intermediate columnar zone
Root melting and dendrite fragments
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Central equiaxed zone

• Equiaxed grain
• Nucleation
• Supercooling
• Falling particles
• Dendrite fragments

• Elevated pouring temperature
• Larger equiaxed grains

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Structure and properties

• More columnar zone
• Anisotropic properties
• Magnetic materials
• Turbo blade.
• More equiaxed zone
• Isotropic properties
• Less segregation

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Microstructure and processing

• materials
• composition
• nucleation ability
• mold
• thermal conductivity, specific heat
• casting condition
• pouring temperature
• convection.

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In summary

• Three microstructure zones
• Outer chilled zone
• Intermediate columnar zone
• Central equiaxed zones

• Microstructure control

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References

• T. F. Brower and M.C. Flemings, Trans. AIME, 239,
  1620 (1967).
• H.B. Dong and P.D. Lee, Acta Mater. 53 (2005)
  659.
• W.J. Boettinger, et al., Annual Review of
  Materials Research 32 (2002) 163.