WERKSTOFFWOCHE, Dresden 14.09.2015 Understanding of Inclusions - Characterization, Interactions and Boundaries of Removability with Special Focus on Aluminium melts Bernd Friedrich IME Process Metallurgy and Metal, RWTH Aachen University Prof. Dr.-Ing. Dr. h.c. Bernd Friedrich
Motivation Inclusion content is one of the most *Jaguar important quality requirements They effect the mechanical properties and formability *Constellium They must be removed to reach required product qualities
Product Defects: Straches on the Surface Example of stringers (scratches on a rolled surface) *Constellium
Motivation: Requirements of Metal Purity 3 ppm Alloys without filtration Extrusion 99,7 1 ppm / 1000 ppb 100 ppb Alloys with filtration Computer Discs Foil with filtration 10 ppb 1 ppb / 1000 ppt 100 ppt Non-metallic inclusions Peter Waite, Light Metals 2002 10 ppt
Aluminium Production carbides oxides carbides nitrides oxides borides intermetallics furnance treatment electrolysis channel treatment melting furnance oxides billet casting rolling ingot casting *Trimet Aluminium SE oxides oxides
Inclusions as a Part of Potential Impurities in Al-Melts Impurities in Al-melts Dissolved elements Inclusions Dissolved gas (H) Dissolved metals Primary: Na, Ca, Li, Mg Secondary: Fe, Si, Cu, Mn Oxides Al 2 O 3 MgO MgAl 2 O 4 SiO 2 Carbides Al 4 C 3 TiC SiC Nitrides AlN Borides TiB 2 AlB 2
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 endoand 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
Inclusion Formation by Interactions with Solids with pigments (TiO 2 /Fe 2 O 3 ) 2 Al + Fe 2 O 3 Al 2 O 3 + 2 Fe (Al) 2 Al + TiO 2 Al 2 O 3 + Ti (Al) used aluminum beverage cans (UBC scrap) with refractory materials 4Al + 3C = Al 4 C 3 Al + SiO 2 = Si + Al 2 O 3 graphite crucible
Type of Inclusions in Al-melts (1) - Oxides Type Morphology Density g/cm 3 Dimensions µm Oxides MgAl 2 O 4 Spinel Particles, skins, flakes 3.60 Dispersoids Oxide skins 0.1-100 10-5000 Dispersoids Oxide skins Al 2 O 3 (Corundum) Particles, skins 3.97 0.2-30 10-5000 MgO Particles, skins 3.58 Dispersoids Oxide skins 0.1-5 10-5000 SiO 2 Particles 2.66 0.5-30 CaO Particles 3.37 <5
Type of Inclusions in Al-melts (2) Non-oxides Type Morphology Density g/cm 3 Dimensions µm Carbides Al 4 C 3 Particles, clusters 2.36 0.5-25 SiC Particles 3.22 0.5-5 TiC Particles, clusters 4.7 <5 Borides TiB 2 Particles, clusters 4.5 1-30 AlB 2 Particles 3.19 0.1-3 Nitrides AlN Particles, skins 3.26 10-50 Chlorides CaCl 2, NaCl, MgCl 2 Liquid droplets 1.9-2.2 0.5-1
Formation Mechanisms of Inclusions (1) Simple Oxides 2Al + 3 x O x = Al 2 O 3 Origin: Refractory materials, atmosphere contact with solid or liquid aluminium Mg + 1 x O x = MgO Origin: Reaction between magnesium and oxygen in the melt when the alloy contains more than 2% Mg W. Schneider, Filtration of Aluminum Melts
Formation Mechanisms of Inclusions (2) Spinell Oxide 2Al (l) + Mg [Al] + 2O 2(g) Al 2 MgO 4(s) 2Al (l) + Mg [Al] + 2SiO 2(s) Al2MgO 4(s) + 2Si [Al] 3Mg [Al] + 4Al 2 O 3(s) 3Al 2 MgO 4(s) + 2Al (l) Origin: Spinel oxides usually form in alloys with <2% Mg content W. Schneider, Filtration of Aluminum Melts
Oxidation of Al-Mg Alloys 1 2 O 2 + Mg 2+ MgO + 2L + Crucible MgO Mg 2+ + O Al + 2e L + Mg 2+ e - MgO Metal channels O 2 Al + 3O Al 2 O 3 Aluminium alloy film Alumina Spinel Bulk aluminum alloy *Venugopalan
Continuous Inclusion Generation by Oxidation Inclusion concentration (k/kg) 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 Al 2 O 3 layers cause continuous oxidation of melt because of its instability. 100 90 80 70 60 50 40 30 20 10 0 Day 1 Day 2 Day 3 *M. Gökelma, Master Thesis, IME-RWTH Aachen Increasing inclusion concentration of the Mg-containing Al-alloy melt during three experiment days.
Formation Mechanisms of Inclusions (3) Al-carbide 4Al + 3SiC 3Si + Al 4 C 3 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
Formation Mechanisms of Inclusions (4) Ti-boride/carbide [Ti] + 2[B] TiB 2(s) Origin: Excess of boron can react with titanium during grain refining process 25 µm [Ti] + [C] TiC (s) [Ti] + C (s) TiC (s) [Ti] + Al 4 C 3(s) TiC (s) + Al (l) Origin: grain refining process W. Schneider, Filtration of Aluminum Melts
Off-line Inclusion Detection (1) PoDFA Principle off-line Principle: ABB PoDFA (Porous Disc Filtration Apparatus) method is based on optical evaluation of a filter cake which contains the concentrated inclusions to characterise
Off-line Inclusion Detection (2) PoDFA Results Al 2 O 3 thin films Al 2 O 3 thin films + Al 2 O 3 and SiC particles *M.Gökelma et al., TMS Light Metals 2016 *ABB
On-line Inclusion Detection (1) LiMCA Principle DV= f (particle volume) Principle: *LiMCA CM Brochure, ABB 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.
Inclusion concentration N20 in k/kg On-line Inclusion Detection (2) LiMCA Results Inclusion concentration in k/kg 5 4 3 2 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 20-25 25-30 30-35 35-40 40-45 45-50 50-60 60-70 Inclusion size in μm 1 0 0 10 20 30 40 50 60 70 80 Time in min
Interactions and Movement of Inclusions in Al-melts Heat radiation O 2 2Al + 3O = Al 2 O 3 Oxide layer break-off Oxide layer Magnetic field due to inductive heating Convection Brownian motion Resistance heating Refractory lining break off Settling Al-melt Clustering *M.Gökelma et al., Int. Al. Journal 04.2015
Clusters SiC Al 2 O 3 M.Gökelma et al. IMMC 17 International Metallurgy & Materials Congress, 2014
Settling - Forces acting on particles The most of the non-metallic inclusions tend to settle in light metals 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. Free settling is just possible in ideal case. In real, many parameters are present such as: surface tension (Al 2 O 3 thin films), melt movement, particle concentration V = 1 (ρ p ρ f ) gd 2 Stoke s equation 18 μ
Terminal velocity in mm/s Settling & Floating & Suspending Gravity Force (F G ) - Buoyancy Force (F v ) = Drag Force (F D ) F G = m s.g = ρ particle.v particle.g F v = m f.g = ρ fluid.v particle.g F D = 1/2.ρ fluid.ʋ 2 particle.c D.A 8 7 6 5 4 3 2 1 0 Al2O3 spherical particles Al2O3 thin disks with 1:10 thickness to diameter ratio 15 35 55 75 95 Diameter in µm *M.Badowski et al., TMS Light Metals 2015 Type Density g/cm 3 TiC 4.70 TiB 2 4.50 Al 2 O 3 3.97 MgAl 2 O 4 3.60 MgO 3.58 SiC 3.22 Al 4 C 3 2.36 Al-molten 2.35 (700 C) NaCl 2.17 LiCl 2.07
Velocity (mm/s) Terminal Velocity under Gravity Influence (settling) 6 5 MgO clusters 4 3 Al 2 O 3 thin discs 2 MgO.Al 2 O 3 thin discs 1 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Diameter and edge length (µm) *M. Gökelma, Master Thesis, IME-RWTH Aachen Al 4 C 3 cubic particles Settling velocities of non-spherical particles with different chemistries are shown Shape has also an effect on settling velocity Densities of particles: Al 2 O 3 > MgO.Al 2 O 3 > MgO > Al 4 C 3
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 (F G ) Floating particles Agglomerate *S. P. Mokkapati,
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).
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
Brownian motion - Simulation of Particles Movement Brownian agglomeration is due to disorganized random movement of small inclusions in liquid and lead to 10 21 collisions per second! Brownian Motion defined by average displacement-squared: *Tian, Irons, Wilkinson < r 2 > = 6 D t D=0.16 micron 2 /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
Forced Convection in Aluminium Melts Inductive heating Gas lancing Flotation Mammoth pump 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 EMP
Forced Convection Clustering due to Turbulent Flow Turbulent agglomeration can be understood by two processes which are turbulent inertial agglomeration and turbulent shear agglomeration Two particles in different flow patlines direct collide because of their different travelling velocities *S. P. Mokkapati,
Clustering: Attractive Forces *H. Yin,
Inclusion Removal Devices in Production-line Casting Unit BP-filter Degasser CF-filter CF-filter Launder Casting Furnace *Hydro Aluminium Rolled Products GmbH
LiMCA N20 Counts (k/kg) Limits of Settling in a Casting Furnace 100 Settling Phenomenon of Inclusions in Casting Furnaces 90 80 70 60 50 40 30 20 10 0 0 50 100 150 200 250 300 *LiMCA CM Brochure, ABB Cast Time (min)
Inclusion Removal Methods Type of Refining Effect Melting under salt Salt Refining Removing of Oxides Removal of Li, Na, Ca, Sr, oxides Purging gas treatment Removal of H 2, Li, Na, Ca, Sr, Zn, oxides, nitrides, carbides Chlorination Filtration Vacuum distillation Addition of primary aluminium Removal of Mg Removal of solid particles Removal of Mg, Zn Dilution of all impurity elements
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
Bubble Formation (growth conditions) p g > p atm + p h + p P g : internal gas pressure P atm : atmospheric pressure P h : metallostatic pressure P : pressure forced by surface tension gas/liquid p h p p g p atm 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
Principle of Inclusion Flotation by Gas Purging Al Inclusions Ar Argon bubbles Ar Inclusions Ar Porous plug
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 Molten Aluminium Gas bubble F G
Inclusion Flotation by Rotor Injection Fluxing gas mixture Metal in Metal out stirring gas dispersion
Technology Effect on Specific Bubble Surface Area spec. gas bubble surface. (m -1 14 12 10 8 6 4 2 0 rotor nozzle (high speed) porous plug 0 2 4 6 8 10 12 gas bubble diameter (mm)
Gas Purging of Aluminium in Operation 130t gas fired holding furnace with 16 Plug Al-Clean System
Gas Purging Results *Zhang
Principle of Inclusion Filtration To caster Al melt flow from degassing unit Clean melt Filter Separation of suspended particles: Oxide particles, oxide skin Refractory particles from trough Impurities of grain refiners (TiC, TiB 2 )
Filtration Mechanisms Cake filtration Deep bed filtration
Filter Systems Deep bed Filter BF Pore size: ~1000µm Ceramic Foam Filter CFF Pore size: ~ 2000µm (30ppi) Röhrenfilter BPF Pore size: ~ 450µm (24grit)
Filtrtion Efficiency Influence of Particle Density on Filtration Efficiency Analytical calculation by J. P. Desmoulins et al. 1,0 0,8 0,6 0,4 0,2 Density 4,5 g/cm³ Density 3,3 g/cm³ Density 2,36 g/cm³ 1,0 1,0 0,8 0,8 0,6 0,6 0,4 0,4 0,2 0,2 0 0 0 0 10 20 30 0 10 20 30 0 10 20 30 Inclusion size [µm] Inclusion size [µm] Inclusion size [µm]
Filtration Efficiency (%) Filtration Efficiency of Different Filter Systems 100 90 80 93 96 87 97 97 94 94 95 82 97 93 88 80 85 70 60 50 55 52 64 70 76 68 59 65 40 30 30 20 21 10 0 CFF 15" 30ppi CFF 15" 50ppi Ceramic Foam Filter CFF 17" 30ppi CFF 17" 50ppi CFF 17" 65ppi CFF 17" 80ppi Bonded Particle Filter BPF 16grit BPF 16grit + CFF 15" 30ppi Non-Connected Filter BPF 24grit + CFF 15" 30ppi NCF CFF 17" 30/50 ppi Bed Filter BF
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
WERKSTOFFWOCHE, Dresden 14.09.2015 Thank you for your attention! www.ime-aachen.de IME Process Metallurgy and Metal, RWTH Aachen University Prof. Dr.-Ing. Dr. h.c. Bernd Friedrich