Seminar: Metallography of casting alloys and metallurgical defects
Fundamentals of solidification
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Seminar: Metallography of casting alloys and metallurgical defects
Metalls
>
Metals have a crystalline structure in solid state body
>
A crystal is an anisotropic, homogeneous body. The atoms have a 3 dimensional
periodic structure.
>
The smallest unit of this structure
is a so called „Elementary cell“
>
Solid state bodies without this
structure are amorphous,
i.g. glas
Quartz crystal (trigonal)
Amorphous glas
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Seminar: Metallography of casting alloys and metallurgical defects
Crystal systems
>
There are 7 crystal systems with different angels and distances inside the
elementary cell
>
Metals are belonging to the cubic and hexagonal systems
Hexagonal elementary cell
(Magnesium)
Body centered cubic
elementary cell
(α-Fe)
Face centered cubic
elementary cell
(γ-Fe)
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Seminar: Metallography of casting alloys and metallurgical defects
Atomic structure in body centered and face centered
cubic lattice
Body centered
P = 68%
Face centered
P = 74 %
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Seminar: Metallography of casting alloys and metallurgical defects
Iron changes the crystalic structure with temperature
(Allotropy)
Temperature
[°C]
Modification
Crystal structure
Lattice constant
[nm]
At temperature
[°C]
bis 769
-Fe
Body centered
0,286
20
769 … 911
-Fe
Body centered
0,290
800
911… 1392
-Fe
Face centered
0,364
1100
1392 …1536
-Fe
Body centered
0,293
1425
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Seminar: Metallography of casting alloys and metallurgical defects
> The metallic atoms have a closed-packed structure
> But the closed-packed structure have no 100% filling
> There are gaps or holes between the metallic atoms, in which other atoms can be
located
In body centered
structures there are
12 tetraedric and
6 oktaedric gaps
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Seminar: Metallography of casting alloys and metallurgical defects
In face centered
structures there are
4 octaedric and
8 tetraedric gabs
This is the basic of crystalline solid solution (mixed crystals) and solid
solution alloys
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Seminar: Metallography of casting alloys and metallurgical defects
2-dimensional defects
Symmetric and unsymmetric small angel grain boundaries ( 10°)
(regular edge dislotations but one crystal)
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Seminar: Metallography of casting alloys and metallurgical defects
Crystals with an
orientation difference
> 10°  grain boundary
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Seminar: Metallography of casting alloys and metallurgical defects
Schematic grain boundary
Polycrystalline structure
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Seminar: Metallography of casting alloys and metallurgical defects
Morphology of the different solidification structures
> Solidification is the transition from liquid to solid state. The solidification is an
exogenous reaction.
> The transition starts at the liquids-temperature
and ends at solidus-temperature.
> There are two types of solidification morphology:
exogenous (nucleation at the moulding surface)
endogenous (nucleation in the melt)
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Seminar: Metallography of casting alloys and metallurgical defects
Type I: Exogenous solidification (small solidification period)
Melt
Quelle: Brunhuber (1984)
Melt
smooth bore
←
solidification
→
rough bore
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Seminar: Metallography of casting alloys and metallurgical defects
Type II: Endogenous solidification (wide solidification period)
>
>
pulpy type
spongy type
i.e. Ni-bronze, ductile iron, Cu-alloys
Quelle: Brunhuber (1984)
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Seminar: Metallography of casting alloys and metallurgical defects
Microporosity caused by an endogenous spongy solidification morphology
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Seminar: Metallography of casting alloys and metallurgical defects
In technical alloys there are mixed types of solidification morphology
(i.g. copper alloys)
Melt
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Seminar: Metallography of casting alloys and metallurgical defects
Solidification morphology of Ferrous-alloys
A)
Steel: exogenous-rough bore
B)
Cast iron (dendritic solidification): endogenous-pulpy or spongy
C)
Grey iron (eutectic solidification). Endogenous – shell-shaped
D)
Ductile iron (eutectic solidification): endogenous - pulpy
E)
White cast iron: exogenous – rough bore
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Seminar: Metallography of casting alloys and metallurgical defects
Sand casting
Chill casting
GJL
GJV
Schematic solidification
morphology of cast iron
(influenced by heat flow).
GJS
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Seminar: Metallography of casting alloys and metallurgical defects
Typical macrostructure of a thickwalled casting
1)
Globulitic, finely crystalline shell
zone (high local undercooling
caused by heat flow)
2)
Orientated radial crystallization
inverse to heat flow
3)
Coarse crystalline centre zone
(endogenous solidification)
Quelle: S. Engler (1981)
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Seminar: Metallography of casting alloys and metallurgical defects
Homogenious and hertogenious nucleation
>
Technical melts normally solidifies with heterogeneous
nucleation (wall surface, innoculants, oxidic particals etc.)
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Seminar: Metallography of casting alloys and metallurgical defects
Coarse crystalline
finely crystalline
> Nucleus formation and nucleus
growth running parallel in the
melt
Velocity v, w
> Nucleus formation rate v and
growth rate w are influenced by
the undercooling of the melt
> The undercooling of the melt is
influenced by the cooling rate
dT/dt and the chemistry of the
melt (nucleus formation
conditions)
Undercooling
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Seminar: Metallography of casting alloys and metallurgical defects
Dendrites
directional solidification
non-directional
solidification
dendrite
arm
dendrite center distance, l1
axis
equiaxed crystal
(= crystalline grain)
cut
cut dendrite arms
dendrite arm
spacing (DAS,
l2)
columnar crystal
(= crystalline grain)
DAS, l2
(following Prof. S. Engler, Foundry Institute of the RWTH Aachen, Germany)
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Seminar: Metallography of casting alloys and metallurgical defects
Dendrite arm spacing (DAS) - quantitative image analysis
DAS  x  ( m  1)  k  3 t f
X = 270 mm
m = 10
DAS = 30 mm
freezing range Ts-l
alloy

T

T
t
sl
tf = local solidification time

f
T = local freezing rate
(following BDG-Richtlinie / VDG Merkblatt P220, July 2011, Germany)
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Seminar: Metallography of casting alloys and metallurgical defects
A phase diagram shows us the thermodynamic state of metals
and alloys in the thermodynamic equillibrium
> It is a quantitative representation of the alloy as a function of temperature, chemical
composition (and pressure)
> Phase diagrams shows us the transition temperatures, the chemical composition of
the phases and the metallurgical structure of phases
> The phases diagrams are calculated for the thermodynamic equillibrium, real cooling
or heating rates influence the transition temperatures and the solubility (composition)
of the phases
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Seminar: Metallography of casting alloys and metallurgical defects
Phase diagrams
Holding point at phase
transition
Cooling / heating curve of pure iron
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Seminar: Metallography of casting alloys and metallurgical defects
Phase diagrams
Cooling curves
Phase diagram
Melt
(Solid solution,
mixed crystal)
Time
B (mass percentage)
Development of a phase diagram Gießerei-Lexikon, 1997
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Seminar: Metallography of casting alloys and metallurgical defects
Liquid state
Unlimited or
limited solubility
in solid state
Different types of binary phase diagrams [Gießerei-Lexikon, 1997]
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Seminar: Metallography of casting alloys and metallurgical defects
Phase diagrams
Monophase
Binary phase
Phase boundary
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Seminar: Metallography of casting alloys and metallurgical defects
The Fe-C-phase diagram
Cast iron
Steel
•Stabiles System
•Metastabiles System
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Seminar: Metallography of casting alloys and metallurgical defects
Solidification of cast iron
Fe-C-phase diagram with 2,4 % Si
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Seminar: Metallography of casting alloys and metallurgical defects
Solidification of primary austenite
At the liquids temperature the solidification starts with the nucleation of
austenite dendrites in the melt
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Seminar: Metallography of casting alloys and metallurgical defects
Thermodynamic non equilibrium:
Shell-type chemical composition of the dendrites
The local chemical composition of the austenite dendrites are
influenced by the solidification temperature and the solubility
of carbon in the austenite.
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Seminar: Metallography of casting alloys and metallurgical defects
Micrographs of grey iron: original primary austenite dendrites between the
Fe-C-eutectic phase
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Seminar: Metallography of casting alloys and metallurgical defects
Solidification of the eutectic phase
At the eutectic temperature (1160° C – 1130° C) the residual melt solidifies in an eutectic
phase between the primary dendrites.
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Seminar: Metallography of casting alloys and metallurgical defects
Solid-solid-transformation (eutectoide transformation)
>
At the eutectoid temperature (820° C – 770° C) the austenite transformed to pearlite.
>
The solid-solid transformation of the austenite to a lamellar ferrite-cementiteeutectoide starts at the grain boundary.
- 34 -
Seminar: Metallography of casting alloys and metallurgical defects
Solid-solid transformation caused by diffusion of carbon
>
>
Directly after the eutectorid transformation exists 100 % pearlite
Caused by low cooling rates their is enough time for a diffusion of the carbon to the
graphite phase. During the cooling period we have a formation of ferrite around the
graphite phase and growth of the graphite phase.
The diffusion of carbon is influenced by temperature, alloying elements like Cu, Mn, Sn
and the distance to the next carbon particle.
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Seminar: Metallography of casting alloys and metallurgical defects
Binary phase diagram of Al-Si-alloys
Melt
Melt+Al
Melt+Si
Si-content in mass %
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Seminar: Metallography of casting alloys and metallurgical defects
Solidification of an AlSi7 – alloy
a) cooling down the melt
C1
S + 
S+

+
Time
A
Masse-%
B
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Seminar: Metallography of casting alloys and metallurgical defects
Solidification of a hypoeutectic AlSi7-alloy
b) primary solidification
C1
primary  - dendrite
Melt
S + 
S+

+
Time
A
Masse-%
SEM-micrograph of an
Al-dendrite
B
- 38 -
Seminar: Metallography of casting alloys and metallurgical defects
Solidification of a hypoeutectic AlSi7-alloy
c) start of the eutectic solidification on the surface of the dendrites
C1
S + 
S+

Melt

+

A
primary
Masse-%
B
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Seminar: Metallography of casting alloys and metallurgical defects
Solidification of a hypoeutectic AlSi7-alloy
d) eutectic solidification of the retained melt
C1
 - primary
Eut (+)
S + 
S+

+
Time
A
Masse-%
B
- 40 -
Seminar: Metallography of casting alloys and metallurgical defects
Solidification of a hypoeutectic AlSi7-alloy
e) segregation of -phase (Silicon) out of the eutectic phase
decreasing solubility of Silicon in 
C1
S + 
S+

+
 - primary
Eut (+)
Time
A
Segregation
Masse-%
B
- 41 -
Seminar: Metallography of casting alloys and metallurgical defects
Solidification of an eutectic AlSi12,5 – alloy
a) cooling down the melt to the eutectic temperature
b) liquid-solid transformation (eutectic solidification at 577° C)
C2
S + 
S+

+
Eut (+)
A
Masse-%
B
- 42 -
Seminar: Metallography of casting alloys and metallurgical defects
Solidification of an eutectic AlSi 12,5 – alloy
b) segregation of -phase (Silicon) out of the eutectic phase
(decreasing solubility of Silicon in )
C2
S + 
S+

+
Eut (+)
A
Segregation
Masse-%
B
- 43 -
Seminar: Metallography of casting alloys and metallurgical defects
Solidification of a hypereutectic AlSi17 – alloy
a) primary solidification of -phase (Silicon)
b) eutectic solidification of the residual melt
c) segregation of -phase
ß - primary
Melt
C3
S + 
S+

+
ß - primary
Eut (+)
A
Masse-%
Segregation
B
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Seminar: Metallography of casting alloys and metallurgical defects
Microstructures of AlSi - alloys
Hypoeutectic
ALSi9
Eutectic
AlSi12
Hypereutectic
AlSi17
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