Upload
others
View
7
Download
0
Embed Size (px)
Citation preview
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
Product Defects: Straches on the Surface
Example of stringers (scratches on a rolled surface) *Constellium
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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.
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
Clusters
SiC
Al2O3
M.Gökelma et al. IMMC´17 International
Metallurgy & Materials Congress, 2014
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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“
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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,
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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).
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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,
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
Clustering: Attractive Forces
*H. Yin,
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
Inclusion Removal Devices in Production-line
Casting Unit
BP-filter
CF-filter
Degasser
Launder
Casting Furnace
CF-filter
*Hydro Aluminium
Rolled Products GmbH
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
Principle of Inclusion Flotation by Gas Purging
Porous plug
Al
Ar
Ar
Ar
Argon
bubbles
Inclusions
Inclusions
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
FG
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
Inclusion Flotation by Rotor Injection
Metal in Metal
out
stirring gas dispersion
Fluxing
gas
mixture
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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)
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
Gas Purging of Aluminium in Operation
130t – gas fired holding furnace
with 16 Plug Al-Clean System
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
Gas Purging Results
*Zhang
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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)
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
Cake filtration Deep bed filtration
Filtration Mechanisms
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
Filter Systems
Ceramic Foam Filter CFF
Pore size: ~ 2000µm (30ppi)
Röhrenfilter BPF
Pore size: ~ 450µm (24grit)
Deep bed Filter BF
Pore size: ~1000µm
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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]
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
Me
tall
urg
ie
Pro
ze
ss
tec
hn
ik
Recycling
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
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