Fundamentals of solidification

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Seminar: Metallography of casting alloys and metallurgical defects

Fundamentals of solidification

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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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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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Atomic structure in body centered and face centered cubic lattice

Body centeredP = 68%

Face centeredP = 74 %

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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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> 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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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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Symmetric and unsymmetric small angel grain boundaries ( 10°)(regular edge dislotations but one crystal)

2-dimensional defects

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Crystals with an orientation difference > 10° grain boundary

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Schematic grain boundary Polycrystalline structure

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> 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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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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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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Microporosity caused by an endogenous spongy solidification morphology

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In technical alloys there are mixed types of solidification morphology (i.g. copper alloys)

Melt

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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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Schematic solidification morphology of cast iron (influenced by heat flow).

Sand casting Chill casting

GJL

GJV

GJS

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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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Homogenious and hertogenious nucleation

> Technical melts normally solidifies with heterogeneous nucleation (wall surface, innoculants, oxidic particals etc.)

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> 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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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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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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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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Phase diagrams

Cooling / heating curve of pure iron

Holding point at phase transition

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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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Different types of binary phase diagrams [Gießerei-Lexikon, 1997]

Liquid state

Unlimited or limited solubility

in solid state

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Phase diagrams

Monophase

Binary phase

Phase boundary

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Cast ironSteel

•Stabiles System

•Metastabiles System

The Fe-C-phase diagram

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Solidification of cast iron

Fe-C-phase diagram with 2,4 % Si

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Solidification of primary austenite

At the liquids temperature the solidification starts with the nucleation of austenite dendrites in the melt

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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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Micrographs of grey iron: original primary austenite dendrites between the Fe-C-eutectic phase

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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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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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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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Binary phase diagram of Al-Si-alloys

Melt

Melt+Al

Melt+Si

Si-content in mass %

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Solidification of an AlSi7 – alloya) cooling down the melt

A Masse-% B

C 1

S + S +

+

Time

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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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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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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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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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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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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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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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Microstructures of AlSi - alloys

HypoeutecticALSi9

EutecticAlSi12

HypereutecticAlSi17

Fundamentals of solidification

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