- 1 - seminar: metallography of casting alloys and metallurgical defects fundamentals of...
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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 structureis 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 centeredP = 68%
Face centeredP = 74 %
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Seminar: Metallography of casting alloys and metallurgical defects
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
Iron changes the crystalic structure with temperature (Allotropy)
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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 are4 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
Symmetric and unsymmetric small angel grain boundaries ( 10°)(regular edge dislotations but one crystal)
2-dimensional defects
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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
> Solidification is the transition from liquid to solid state. The solidification is an exogenous reaction.
> The transition starts at the liquids-temperatureand ends at solidus-temperature.
> There are two types of solidification morphology:exogenous (nucleation at the moulding surface)endogenous (nucleation in the melt)
Morphology of the different solidification structures
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Seminar: Metallography of casting alloys and metallurgical defects
Type I: Exogenous solidification (small solidification period)
Que
lle: B
runh
uber
(19
84)
smooth bore ← solidification → rough bore
Melt
Melt
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Seminar: Metallography of casting alloys and metallurgical defects
Type II: Endogenous solidification (wide solidification period)
> pulpy type> spongy type
Quelle: Brunhuber (1984)
i.e. Ni-bronze, ductile iron, Cu-alloys
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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
Schematic solidification morphology of cast iron (influenced by heat flow).
Sand casting Chill casting
GJL
GJV
GJS
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Seminar: Metallography of casting alloys and metallurgical defects
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)
Typical macrostructure of a thickwalled casting
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
> Nucleus formation and nucleus growth running parallel in the melt
> 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)
Coarse crystalline finely crystalline
Undercooling
Vel
ocity
v, w
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Seminar: Metallography of casting alloys and metallurgical defects
directional solidification non-directional solidification
dendritearm
dendrite axis
center distance,1
dendrite arm spacing (DAS,
2)
cutcut dendrite arms
columnar crystal (= crystalline grain)
equiaxed crystal(= crystalline grain)
DAS, 2
(following Prof. S. Engler, Foundry Institute of the RWTH Aachen, Germany)
Dendrites
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Seminar: Metallography of casting alloys and metallurgical defects
X = 270 m m = 10 DAS = 30 m
3)1( ftkmxDAS
tTT
f
ls
Dendrite arm spacing (DAS) - quantitative image analysis
freezing range Ts-lalloy
tf = local solidification time
= local freezing rate T
(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
Cooling / heating curve of pure iron
Holding point at phase transition
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Seminar: Metallography of casting alloys and metallurgical defects
Development of a phase diagram Gießerei-Lexikon, 1997
Phase diagrams
Cooling curves Phase diagram
Time
B (mass percentage)
Melt
(Solid solution,mixed crystal)
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Seminar: Metallography of casting alloys and metallurgical defects
Different types of binary phase diagrams [Gießerei-Lexikon, 1997]
Liquid state
Unlimited or limited solubility
in solid state
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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
Cast ironSteel
•Stabiles System
•Metastabiles System
The Fe-C-phase diagram
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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
The local chemical composition of the austenite dendrites are influenced by the solidification temperature and the solubility of carbon in the austenite.
Thermodynamic non equilibrium: Shell-type chemical composition of the dendrites
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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-cementite-eutectoide starts at the grain boundary.
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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 – alloya) cooling down the melt
A Masse-% B
C 1
S + S +
+
Time
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Seminar: Metallography of casting alloys and metallurgical defects
Solidification of a hypoeutectic AlSi7-alloyb) primary solidification
A Masse-% B
C 1
S + S +
+
Melt
primary - dendrite
SEM-micrograph of an Al-dendrite
Time
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Seminar: Metallography of casting alloys and metallurgical defects
A Masse-% B
Solidification of a hypoeutectic AlSi7-alloyc) start of the eutectic solidification on the surface of the dendrites
C 1
S + S +
+
Melt
- primary
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Seminar: Metallography of casting alloys and metallurgical defects
A Masse-% B
C 1
S + S +
+
- primary
Eut (+)
Solidification of a hypoeutectic AlSi7-alloyd) eutectic solidification of the retained melt
Time
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Seminar: Metallography of casting alloys and metallurgical defects
A Masse-% B
C 1
S + S +
+
-Segregation
- primary
Eut (+)
Solidification of a hypoeutectic AlSi7-alloye) segregation of -phase (Silicon) out of the eutectic phase decreasing solubility of Silicon in
Time
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Seminar: Metallography of casting alloys and metallurgical defects
A Masse-% B
Solidification of an eutectic AlSi12,5 – alloya) cooling down the melt to the eutectic temperature
b) liquid-solid transformation (eutectic solidification at 577° C)
C 2
S + S +
+ Eut (+)
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Seminar: Metallography of casting alloys and metallurgical defects
A Masse-% B
Solidification of an eutectic AlSi 12,5 – alloyb) segregation of -phase (Silicon) out of the eutectic phase (decreasing solubility of Silicon in )
C 2
S + S +
+
-Segregation
Eut (+)
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Seminar: Metallography of casting alloys and metallurgical defects
A Masse-% B
Solidification of a hypereutectic AlSi17 – alloya) primary solidification of -phase (Silicon)b) eutectic solidification of the residual meltc) segregation of -phase
C 3
S + S +
+
Melt
ß - primary
-Segregation
Eut (+)
ß - primary
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Seminar: Metallography of casting alloys and metallurgical defects
Microstructures of AlSi - alloys
HypoeutecticALSi9
EutecticAlSi12
HypereutecticAlSi17