Metallurgical Influences on Anodising Qualities

Transcription

1 Brighter ideas with aluminum Metallurgical Influences on Anodising Qualities ESTAL Congress 21./ Dr. mont. Andreas Tesch

2 Content 1. Who is Novelis? 2. General Influences on the anodising result 3. Influences of the alloying elements 4. Influences of the casting process 5. Influences of the rolling process 6. Influences of intermetallic precipitates (particles) 7. Summary 2

3 Novelis is a Global Industry Leader The world s leading producer of aluminum rolled products The world s leading recycler of used aluminum beverage cans 3

4 A Member of the Aditya Birla Group International Operations 13 Countries JV Sunlife Financial inc. The Aditya Birla Group at a Glance Revenues of $28 billion Market cap of $31.5 billion 100,000 employees 25 nationalities 4

5 Novelis Worldwide Footprint North America Employees: 3,200 Revenues: $4.1 Bn Shipments: 1,166 kt Europe Employees: 6,000 Revenues: $4.3 Bn Shipments: 1,106 kt Novelis Total Employees 12,700 Revenues $11.2 billion 1 Shipments 3,150 kt 2 Locations 32 Countries 11 1 Earnings based on FY 08 results; reported in US dollars 2 Shipments include rolled products and ingots South America Employees: 2,000 Revenues: $1.0 Bn Shipments: 348 kt Employees: Revenues: Shipments: Asia 1,500 $1.8 Bn 530 kt 5

6 Novelis Europe Portfolio Overview FY2009 Foil & Technical Products 16% Painted 11% Litho 9% Others 1 5% Foilstock 3rd 5% Automotive 8% Specialties & Bare 14% Beverage & Food Can 32% 1 Platestock, casting alloys, industrial goods Göttingen 6

7 Novelis Supplies World Leaders Automotive and Transportation Cans & Closures Building & Industrial Printing & Lithographic Flexible & Semi-rigid Packaging High-precision products Long term relationships with blue-chip customers 7

8 Influences on the result of the anodising Aluminium & alloying elements Etching Casting Anodising Hot & Cold rolling (Annealing) Sealing Levelling & Cut to length Processstability Project: Sabic,Sittard NL, Company: Sorba Projects B. V., Winterswijk NL Architect: Group A, Rotterdam NL 8

9 Metallurgical influences The most critical aspect for architectural applications is the uniformity of the surface appearance (gloss, colour etc.) Of the various steps in the surface treatment process at the Anodisers, the alkaline etch step dominates Microstructural features known to influence etch response Intermetallic phases Solid solution elements (alloying elements) Grain structure / texture In order to control these features we need to understand how both alloy composition and thermo-mechanical processing influence their evolution through the process route 9

10 Influence of the alloying elements 10

11 Alloying elements and appearance after anodizing Magnesium (Mg > 5%) Mg < 3% clear and colourless coatings / >5% Reduces gloss of high purity aluminium Iron (Fe > 0.5%) Reduces gloss of high purity aluminium Dark grey or black streaks when Fe:Si ~ 7:1 Silicon (Si > 0.5%) Cloudiness when out of solution, grey / when Si > 5% brown /violet Copper (Cu > 1%) Discoloration (stained grey till brown) / anodising layer softer Manganese (Mn > 0.1%) Discoloration yellow / > 0,3% Brown/grey thick anodic films Zinc (Zn > 2%) Slight cloudiness Chromium (Cr > 0.1%) Sligthly yellow / > 0.3% dark yellow 11

12 Influence of the casting process

13 Influences of the casting process Non-metallic inclusions from refractory degradation Clusters of titanium boride grain-refiner particles Macro segregation (Solidification causes shrinkage of material, Solute (e.g. Mg) enriched liquid moves to compensate for this shrinkage) Micro segregation (Lack of diffusion in the solid state compositional variation within a grain) Formation of Dendrites (Tannenbaumgefüge) due to under cooled liquid during solidification (interface solid/liquid unstable negative thermal gradient) addition of small amounts of Ni, Co, V, Cr, Mn to avoid / delay formation risk of discoloration due to additional elements (area of conflict!) Dendrite coarsening (Dendrite arm spacing increases with solidification time) Formation of intermetallic phases (depend on solubility of metallic atoms in the aluminium crystal and cooling rate) Homogenisation reduces micro segregation, nucleate and grow small dispersoids, modify coarse intermetallic phases Proper casting process is a key driver for a proper anodising quality! 13

14 Influence of rolling process 14

15 Hot & Cold Rolling (Annealing) Strain hardening (Deformation of the grains Stresses in the grains due dislocation accumulation at the grain boundaries) Annealing (Time & Temperature) Recrystallisation (Nucleation and growth of new grains consuming the deformed grains grain fining) 15

16 Microstructure of anodising qualities Microstructure EN-AW 5005 H14 - AQ Microstructure EN-AW H14 - NQ A homogeneous grain size distribution ensures a very uniform decorative appearance of the anodised surface. This special microstructure can only be reached by controlling each process step (casting, hot & cold rolling, annealing) carefully. The fine grains provide very good bending and edging properties 16

17 Intermetallic particle distribution at the surface Anodising Quality Commercial Quality 10µm 10µm Mg 2 Si Mg 2 Si There are differences in the microstructure: - coarse particles of Mg 2 Si (up to 10 µm) only in commercial quality - Fe-containing phases are bigger and less homogeneous in commercial quality 17

18 Influences of intermetallic precipitates (particles) Chemical composition, type, number, shape and size of the intermetallic precipitates influence the homogeneity of the gloss, the colour and the brightness of the anodised surface - e.g.: Al 6 (Fe,Mn) will be implemented in the Oxide (Al 2 O 3 ) during anodising grey coloration - e.g.: small addition of Si α-phase (Al 12 (Fe, Mn) 3 Si)) forms Oxide layer colourless and brilliant metallic since α-phase dissolves in the electrolyte during anodising - e.g. Mg 2 Si dissolves during anodising, leaving pits of same size as particles significant effect on scattering of light - e.g. Fe-containing intermetallics (Al-Fe, Al-Fe-Si, Al-Fe-Mn-Si) influence the recrystallisation grain size Less disturbance: The smaller The homogeneously distributed Chemical composition, grain size of the casted ingot and heat treatment influence the type, number, shape and size of the intermetallic precipitates 18

19 Summary Aluminium & alloying elements Colour Gloss Type of intermetallic phases Etching Casting Hot & Cold rolling (Annealing) Segregation Grain size Anodising Formation & distribution of intermetallic particles Microstructure Distribution of intermetallic particles Sealing Shape and size of intermetallic particles Levelling & Cut to length Processstability Project: Sabic,Sittard NL, Company: Sorba Projects B. V., Winterswijk NL Architect: Group A, Rotterdam NL 19

20 Thank you for your attention! 20

21 Action list J57S for VOA 1. Metallurgical Investigations further basic investigations concerning the influence of alloying elements on the anodising result Literature research Definition of lab test program and conduction Evaluation, discussion and summary of the lab test results 2. Anodising tests Evaluation of test done in the past at Novelis Evaluation of test done in the past together with Anodisers (ring test 1 and 2) Meeting with Anodisers and suppliers of chemicals Detailed investigations concerning the build up of the smut and material loss during etching in the Innovation Center Detailed investigations concerning the build up of the smut and material loss during etching at selected Anodisers Comparison and evaluation of the results of all tests 21

22 Etch response of AQ model alloys: Background Assumption: Final appearance of AQ products is predominantly controlled by the scalloped surface morphology developed during alkaline etching This etching process is micro galvanically driven and is controlled by: - Cathodic intermetallics (mainly Fe-containing phases) - Anodic intermetallics (e.g. Mg 2 Si) - Aluminium matrix (elements in solid solution) Source: Enabling Science Review, Göttingen µm 22

23 Etch response of AQ model alloys: Background Alkaline solution Al (met) Al e - Aluminium oxide Aluminium matrix ph 2 H e - H 2 (g) + 2 OH - Iron containing intermetallic Al (met) Al e - Role of cathodically active intermetallics: Controls the rate of reduction reactions. Locally enhanced aluminium dissolution due to alkalinisation Causes formation of hemispherical pits 23

24 Etch response of AQ model alloys: Background Alkaline solution Smut: Mg & Si rich hydroxides Al 3+ Al 3+ 2 H e - H 2 (g) + 2 OH - Aluminium matrix Iron containing intermetallic Activity of microgalvanic couple driven by potential difference between Al matrix and particles Nobility of intermetallic phase: (Al-Fe > Al-Fe-Si > Al-Fe-Mn-Si) Low content of noble elements in solid solution (Cu, Mn) 24

25 Etch response of AQ model alloys: Background Role of anodic intermetallics: Dissolve preferentially to matrix, leaving pits of same size as particles. Alkaline solution Mg Mg e - Mg Mg e - Aluminium oxide Aluminium matrix Mg 2 Si particles Particles smaller than wavelength of visible light not expected to affect appearance Formation of sufficiently large pits can have a significant effect on scattering of light 25