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IME Process Metallurgy and Metal Recycling, RWTH Aachen University Prof. Dr.-Ing. Dr. h.c. Bernd Friedrich Understanding of Inclusions - Characterization, Interactions and Boundaries of Removability with Special Focus on Aluminium melts Bernd Friedrich WERKSTOFFWOCHE, Dresden 14.09.2015

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Page 1: Understanding of Inclusions - Characterization ...werkstoffwoche.de/fileadmin/user_upload/Understanding_of_Inclusio… · ppt ppt ppb ppb Alloys without filtration Extrusion Alloys

IME Process Metallurgy and

Metal Recycling, RWTH Aachen University

Prof. Dr.-Ing. Dr. h.c. Bernd Friedrich

Understanding of Inclusions - Characterization,

Interactions and Boundaries of Removability with

Special Focus on Aluminium melts

Bernd Friedrich

WERKSTOFFWOCHE, Dresden 14.09.2015

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Motivation

• Inclusion content is one of the most

important quality requirements

• They effect the mechanical properties

and formability

• They must be removed to reach

required product qualities

*Constellium

*Jaguar

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Product Defects: Straches on the Surface

Example of stringers (scratches on a rolled surface) *Constellium

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Motivation: Requirements of Metal Purity

1 ppb / 1000 ppt

10

100

10

100

1 ppm / 1000 ppb

3 ppm

ppt

ppt

ppb

ppb

Alloys

without

filtration

Extrusion

Alloys with

filtration Computer

Discs

Foil

with

filtration

Peter Waite, Light Metals 2002

Non-metallic inclusions

99,7

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Aluminium Production

furnance treatment channel treatment

electrolysis

billet casting rolling ingot casting

melting furnance

oxides

carbides

nitrides

carbides

oxides

borides

intermetallics oxides

oxides oxides

*Trimet Aluminium SE

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Inclusions as a Part of Potential Impurities in Al-Melts

Impurities in Al-melts

Dissolved elements Inclusions

Dissolved metals

Primary: Na, Ca, Li, Mg…

Secondary: Fe, Si, Cu, Mn…

Dissolved gas

(H)

Oxides

Al2O3

MgO

MgAl2O4

SiO2

Carbides

Al4C3

TiC

SiC

Nitrides

AlN

Borides

TiB2

AlB2

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Non-metallic Inclusions

• The non-metallic inclusions can

vary from <1-500 µm

• They influence mechanical

properties and surface quality

• They can be devided into endo-

and exogeneous inclusions

Example of inclusions in a thin wall product

(wall thickness about 100 µm)

If not removed, they will appear as hole in foil,

surface defects on sheets, edge cracking in slabs

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Inclusion Formation by Interactions with Solids

used aluminum beverage cans

(UBC scrap)

graphite crucible

with pigments (TiO2/Fe2O3)

2 Al + Fe2O3 → Al2O3 + 2 Fe(Al)

2 Al + TiO2 → Al2O3 + Ti(Al)

with refractory materials

4Al + 3C = Al4C3

Al + SiO2 = Si + Al2O3

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Type of Inclusions in Al-melts (1) - Oxides

Type Morphology Density g/cm3 Dimensions µm

Oxides

MgAl2O4 Spinel Particles, skins, flakes 3.60

Dispersoids Oxide skins

0.1-100 10-5000

Al2O3 (Corundum) Particles, skins 3.97

Dispersoids Oxide skins

0.2-30 10-5000

MgO Particles, skins 3.58

Dispersoids Oxide skins

0.1-5 10-5000

SiO2 Particles 2.66 0.5-30

CaO Particles 3.37 <5

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Type of Inclusions in Al-melts (2) – Non-oxides

Type Morphology Density g/cm3 Dimensions µm

Carbides

Al4C3 Particles, clusters 2.36 0.5-25

SiC Particles 3.22 0.5-5

TiC Particles, clusters 4.7 <5

Borides

TiB2 Particles, clusters 4.5 1-30

AlB2 Particles 3.19 0.1-3

Nitrides

AlN Particles, skins 3.26 10-50

Chlorides

CaCl2, NaCl, MgCl2 Liquid droplets 1.9-2.2 0.5-1

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Mg + 𝟏

𝒙Ox = MgO

Formation Mechanisms of Inclusions (1) – Simple Oxides

2Al + 𝟑

𝒙Ox = Al2O3

Origin: Refractory materials, atmosphere

contact with solid or liquid aluminium

Origin: Reaction between magnesium

and oxygen in the melt when the alloy

contains more than 2% Mg

W. Schneider, Filtration of Aluminum Melts

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Formation Mechanisms of Inclusions (2) – Spinell Oxide

2Al(l) + Mg[Al] + 2O2(g) → Al2MgO4(s)

2Al(l) + Mg[Al] + 2SiO2(s) → Al2MgO4(s) + 2Si[Al]

3Mg[Al] + 4Al2O3(s) → 3Al2MgO4(s) + 2Al(l)

Origin: Spinel oxides usually form in

alloys with <2% Mg content

W. Schneider, Filtration of Aluminum Melts

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Oxidation of Al-Mg Alloys

Crucible 1

2 O2 +

Mg2+ → MgO + 2L+

L+ Mg2+

e-

MgO MgO → Mg2+ + OAl + 2e

Metal channels O

Aluminium alloy

film

2 Al + 3O → Al2O3

Bulk aluminum

alloy

*Venugopalan

Alumina

Spinel

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Continuous Inclusion Generation by Oxidation

Oxidation behaviour of aluminium melts differs from each other due to

different alloying elements.

The oxide layer of pure aluminium is stable, but Mg-containing Al2O3 layers

cause continuous oxidation of melt because of its instability.

Increasing inclusion concentration of the Mg-containing Al-alloy melt

during three experiment days.

0

10

20

30

40

50

60

70

80

90

100

Inc

lus

ion

co

nc

en

tra

tio

n

(k/k

g)

Day 1 Day 2 Day 3

*M. Gökelma, Master Thesis, IME-RWTH Aachen

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Formation Mechanisms of Inclusions (3) – Al-carbide

4Al + 3SiC → 3Si + Al4C3

Origin: They are formed if the solubility of

carbon is above the limit

Generation by reactions between

- melt and cathode-anode in cells

- molten metal and tools

- melt and refractory

- melt carbon from alloying elements

W. Schneider, Filtration of Aluminum Melts

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Formation Mechanisms of Inclusions (4) – Ti-boride/carbide

[Ti] + 2[B] TiB2(s)

25 µm

[Ti] + [C] TiC(s)

[Ti] + C(s) TiC(s)

[Ti] + Al4C3(s) TiC(s) + Al(l)

Origin: Excess of boron can react with

titanium during grain refining process

Origin: grain refining process

W. Schneider, Filtration of Aluminum Melts

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Off-line Inclusion Detection (1) – PoDFA Principle

Principle:

PoDFA (Porous Disc Filtration Apparatus) method is based on optical evaluation of

a filter cake which contains the concentrated inclusions to characterise

„ABB“

off-line

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Off-line Inclusion Detection (2) – PoDFA Results

Al2O3 thin films Al2O3 thin films

+

Al2O3 and SiC particles

*ABB

*M.Gökelma et al., TMS Light Metals 2016

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On-line Inclusion Detection (1) – LiMCA Principle

DV= f(particle volume)

*LiMCA CM Brochure, ABB

Principle:

LiMCA (Liquid Metal Clenliness Analyzer) method is based on measuring the

voltage difference (which was caused by passing particles) between electrodes

and post process the obtained data as particle concentration and size.

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On-line Inclusion Detection (2) – LiMCA Results

0

1

2

3

4

5

0 10 20 30 40 50 60 70 80

Incl

usi

on

co

nce

ntr

atio

n N

20

in k

/kg

Time in min

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

20-25 25-30 30-35 35-40 40-45 45-50 50-60 60-70

Incl

usi

on

co

nce

ntr

atio

n in

k/k

g

Inclusion size in μm

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Interactions and Movement of Inclusions in Al-melts

Convection

Settling

Clustering

Brownian

motion

Oxide layer

Heat radiation

Resistance

heating

Refractory

lining break off

Al-melt

O2

2Al + 3O = Al2O3

Magnetic field

due to inductive

heating

Oxide layer

break-off

*M.Gökelma et al., Int. Al. Journal 04.2015

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Clusters

SiC

Al2O3

M.Gökelma et al. IMMC´17 International

Metallurgy & Materials Congress, 2014

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Settling - Forces acting on particles

This phenomenon is valid for a spherical

particle with higher density than the melt.

Melt flow due to stirring or natural

convection can easily impact the settling

behaviour of a particle.

The most of the non-metallic inclusions

tend to settle in light metals

Free settling is just possible in ideal case.

In real, many parameters are present such

as: surface tension (Al2O3 thin films), melt

movement, particle concentration V = 1

18

(𝜌𝑝−𝜌𝑓)

𝜇𝑔𝑑2 „Stoke‘s equation“

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Settling & Floating & Suspending

Type Density g/cm3

TiC 4.70

TiB2 4.50

Al2O3 3.97

MgAl2O4 3.60

MgO 3.58

SiC 3.22

Al4C3 2.36

Al-molten 2.35 (700°C)

NaCl 2.17

LiCl 2.07

Gravity Force (FG) - Buoyancy Force (Fv) = Drag Force (FD)

FG = ms.g = ρparticle.Vparticle.g

Fv = mf.g = ρfluid.Vparticle.g

FD = 1/2.ρfluid.ʋ2

particle.CD.A

0

1

2

3

4

5

6

7

8

15 35 55 75 95

Te

rmin

al ve

loc

ity i

n m

m/s

Diameter in µm

Al2O3 spherical particles

Al2O3 thin disks with 1:10thickness to diameter ratio

*M.Badowski et al., TMS Light Metals 2015

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Terminal Velocity under Gravity Influence (settling)

0

1

2

3

4

5

6

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Ve

loc

ity (

mm

/s)

Diameter and edge length (µm)

MgO clusters

MgO.Al2O3 thin discs

Al2O3 thin discs

Al4C3 cubic particles

Settling velocities of non-spherical particles with different chemistries are shown

Shape has also an effect on settling velocity

Densities of particles: Al2O3 > MgO.Al2O3 > MgO > Al4C3

*M. Gökelma, Master Thesis, IME-RWTH Aachen

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Clustering by settling

Gravitational Gradient: In a fluid, the small particles slowly settle

down in the solution and settling velocity increases with increasing size

of particle due to higher gravitational forces (FG)

Agglomerate Floating

particles *S. P. Mokkapati,

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Natural Convection by Temperature Gradient (1)

In casting and holding furnaces in Aluminium industry, big amount of

heat is lost from the surface by radiation which directly affects the melt

flow direction and velocity (hydrodynamics).

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Natural Convection by Temperature Gradient (2)

Wall Heating generates natural convection in different directions

depending on the location of heating elements. This directly affects the

motion of entraining of the suspended particles in melt.

*M.Badowski et al., TMS Light Metals 2015

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Brownian motion - Simulation of Particles Movement

Brownian agglomeration is due to disorganized random movement of

small inclusions in liquid and lead to 1021 collisions per second!

Brownian Motion defined by average displacement-squared:

< |r|2 > = 6 D t D=0.16 micron2/second for this particle

D is diffusion constant of the particle depends on the size and shape of

the particle, and on the viscosity and temperature of the fluid

*Tian, Irons, Wilkinson

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Forced Convection in Aluminium Melts

Inductive heating Gas lancing Flotation Mammoth pump

EMP

Melts are moved by external forces which

promotes the inclusion movement. This causes:

• Collision of particles (clustering)

• Generation of new inclusions (oxidation)

• Dragged particles by the melt flow

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Forced Convection – Clustering due to Turbulent Flow

Two particles in

different flow patlines

direct collide because

of their different

travelling velocities

Turbulent agglomeration can be understood by two processes which

are turbulent inertial agglomeration and turbulent shear agglomeration

*S. P. Mokkapati,

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Clustering: Attractive Forces

*H. Yin,

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Inclusion Removal Devices in Production-line

Casting Unit

BP-filter

CF-filter

Degasser

Launder

Casting Furnace

CF-filter

*Hydro Aluminium

Rolled Products GmbH

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Limits of Settling in a Casting Furnace

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300

LiM

CA

N2

0 C

ou

nts

(k

/kg

)

Cast Time (min)

Settling Phenomenon of Inclusions in Casting Furnaces

*LiMCA CM Brochure, ABB

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Inclusion Removal Methods

Type of Refining Effect

Melting under salt Removing of Oxides

Salt Refining Removal of Li, Na, Ca, Sr, oxides

Purging gas treatment Removal of H2, Li, Na, Ca, Sr, Zn, oxides,

nitrides, carbides

Chlorination Removal of Mg

Filtration Removal of solid particles

Vacuum distillation Removal of Mg, Zn

Addition of primary aluminium Dilution of all impurity elements

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Inclusion Flotation by Gas Purging

Gas Purging is a well-known process for purification of Al-melts.

However it has big influence on heat convection and hydrodynamics of

melt. This bath movement caused by gas purging increases the

frequency of collision which results clustering.

purge gas

purge gas

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Bubble Formation (growth conditions)

pg > patm + ph + p

Pg: internal gas pressure

Patm: atmospheric pressure

Ph: metallostatic pressure

P: pressure forced by surface tension gas/liquid

patm ph

p

pg

individual gas bubbles grow until they reach a

specific size and separate

bubble size at separation, depends on the

diameter of the nozzle and the Reynold number

of the nozzle

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Principle of Inclusion Flotation by Gas Purging

Porous plug

Al

Ar

Ar

Ar

Argon

bubbles

Inclusions

Inclusions

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Molten Aluminium

Gas bubble

Mechanism of Particle Removal by Gas Bubbles

Use of interfacial tension of solid liquid (physiochemical technique)

Particles to be separated are adhered to fluid

Wet particles sink down to the bottom

Non-wetted particles are transported to the surface by air bubbles

Emerging froth (foam) is separated

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FG

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Inclusion Flotation by Rotor Injection

Metal in Metal

out

stirring gas dispersion

Fluxing

gas

mixture

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Technology Effect on Specific Bubble Surface Area

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12

porous plug

gas bubble diameter (mm)

sp

ec. g

as b

ub

ble

su

rfa

ce

. (m

-1

rotor nozzle (high speed)

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Gas Purging of Aluminium in Operation

130t – gas fired holding furnace

with 16 Plug Al-Clean System

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Gas Purging Results

*Zhang

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Principle of Inclusion Filtration

Filter

Al melt flow from

degassing unit

Clean melt

To

caster

Separation of suspended particles:

Oxide particles, oxide skin

Refractory particles from trough

Impurities of grain refiners (TiC, TiB2)

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Cake filtration Deep bed filtration

Filtration Mechanisms

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Filter Systems

Ceramic Foam Filter CFF

Pore size: ~ 2000µm (30ppi)

Röhrenfilter BPF

Pore size: ~ 450µm (24grit)

Deep bed Filter BF

Pore size: ~1000µm

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1,0 1,0

0,8 0,8

0,6 0,6

0,4 0,4

0,2 0,2

0 0

Density 4,5 g/cm³ Density 3,3 g/cm³ Density 2,36 g/cm³

1,0

0,8

0,6

0,4

0,2

0

0 0 0 10 10 10 20 20 20

Inclusion size [µm]

30 30 30

Fil

trti

on

Eff

icie

nc

y

Influence of Particle Density on Filtration Efficiency

Analytical calculation by J. P. Desmoulins et al.

Inclusion size [µm] Inclusion size [µm]

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93

87

94 94 95

88

80

85

97979796

65

30

59

93

68

76

82

70

64

5255

21

0

10

20

30

40

50

60

70

80

90

100

CFF

15" 30ppi

CFF

15" 50ppi

CFF

17" 30ppi

CFF

17" 50ppi

CFF

17" 65ppi

CFF

17" 80ppi

BPF

16grit

BPF

16grit

+ CFF

15" 30ppi

BPF

24grit

+ CFF

15" 30ppi

NCF CFF

17"

30/50 ppi

BF

Filtr

ati

on

Eff

icie

ncy (

%)

Filtration Efficiency of Different Filter Systems

Bonded Particle Filter

Non-Connected Filter

Bed Filter Ceramic Foam Filter

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Summary

Inclusions exist in all aluminium melts with different

morphologies and chemistries which impact the product quality

pigments, refractory materials, atmosphere, alloying elements,

input materials are the main inclusion generation mechanisms

Inclusion movement in melts effect the removal and detection

efficiency

Future trends:

The quality requirements will be higher and melts must be

cleaner

Particle behaviour in melts must be better understood

Detection and removal methods must be improved

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IME Process Metallurgy and

Metal Recycling, RWTH Aachen University

Prof. Dr.-Ing. Dr. h.c. Bernd Friedrich

WERKSTOFFWOCHE, Dresden 14.09.2015

Thank you for your attention!

www.ime-aachen.de