Introduction to Aluminum Sheet The Aluminum Transportation Group Presenter Debdutta Roy, Ph.D. Novelis Senior Scientist, R&D Automotive 2019 Aluminum Transportation Group Presenter Russ Long Arconic Chief Engineer – Ground Transportation Products 2019 Aluminum Transportation Group Day 1 Agenda • Sheet production – flowpath • Aluminum alloys commonly used for BIW and closures • Sheet properties • Yield, ultimate and elongations as received • Properties after paint bake • Natural aging • Formability measures • Joining – spot welding, SPR, adhesives, flow drill screws, etc. • Design example – aluminum hood, aluminum door 2019 Aluminum Transportation Group Automotive Aluminum Sheet: Production Flow Path and General Metallurgy The Aluminum Transportation Group Overview • Application • Requirements guide ❑ Heat Treatable • Alloy selection Inner Panels Inner (Precipitation Attachments Hardenable) 6xxx ❑ Non Heat Treatable 5xxx BodyStructure Body in White in Body Typical Process Flow Hot Rolling Cold Rolling Annealing 2019 Aluminum Transportation Group Cast House Process Flow (DC Casting) Melting + skimming Furnace treatment In line treatment Batch Degassing preparation Melting furnace Casting furnace Ingots Casting Grain refiner Filtration 2019 Aluminum Transportation Group Cast House Process Flow (DC Casting) • Heavy gauge scrap + major alloying additions to reach target compositions is loaded into large melting furnaces. • If necessary, final alloying additions are made in the holder. This is the last chance to control chemical composition. 2019 Aluminum Transportation Group Cast House Process Flow (DC Casting) TREATMENT GAS (Ar + Cl2) Degassing: removal of hydrogen from molten metal by bubbling a mixture of gases through the melt. Fluxing: causes impurities, such as alkaline, sodium, and lithium to rise to the surface of the bath. Skimming is done to remove the dross from the surface of the molten metal. 2019 Aluminum Transportation Group Cast House Process Flow (DC Casting) Filtration: removal of inclusions from molten metal by passing through a filter media, typically different types of ceramic filters. Inclusions are retained at the surface of the filter media. Ingot Casting in the Aluminum Industry, Treatise on Materials Science & Technology, Volume 31, 1989, D.A. GRANGER 2019 Aluminum Transportation Group Cast House Process Flow (DC Casting) • Direct chill (DC) casting developed in 1930s – made possible higher quality, larger ingots, more alloys. • As the metal fills the mold and begins to solidify, the bottom block is lowered at a controlled rate. Water directly chills the solid Al shell. • 4 to 6 ingots can be cast at a time and can weigh 10-15 tons each. R. Nadella et al, Progress in Materials Science, 53 (3), 2008 2019 Aluminum Transportation Group As-Cast Microstructures As Cast As Cast Mg2Si Fe phases 2019 Aluminum Transportation Group -12- R&T Center Sierre / ©2015 Novelis Inc. Scalping of Ingots True Edge Edge scalper 2019 Aluminum Transportation Group Homogenization - Purpose Breakdown Tandem Casting Homogenization Hot Rolling Cold Rolling CASH Forming Purpose • Improve workability • Eliminate coring – i.e. segregation of alloying elements • Dissolve Mg2Si particles • Modify Fe phases • Form dispersoid particles • But cannot alter inclusions and inverse segregation (which must be removed by scalping) AA6xxx As-Cast Microstructure 2019 Aluminum Transportation Group Homogenization Practice: Temperature, Time 700 T [°C] 600 500 ℓ 400 Heat-up Phase Soak Cool to Hot- 300 Rolling T Temperature(°C) α+ℓ 200 100 Standard Eutectic 0 0 5 10 15 20 25 30 35 40 α Time (hours) solid state diffusion α+β • As diffusion is strongly T dependent, shorter homo. times can be achieved at higher T • T must be kept below the solidus (don’t melt!) and watch C [%] the eutectics • With increasing solute content (e.g., Mg), the allowable temperature decreases 2019 Aluminum Transportation Group Redistribution of Solute (6xxx) As cast After homogenization 550°C 10h • In the “as cast’’ state, a lot of cast precipitates/particles are present in the microstructure, especially at grain boundaries • Homogenization can dissolve soluble phases • Homogenization produces a more uniform solute distribution Reduction of micro-segregation 2019 Aluminum Transportation Group Homogenization: Temperature Effect on Particle Structure Breakdown Tandem Casting Homogenization Hot Rolling Cold Rolling CASH Forming AA6016 24 hr at 450°C 24 hr at 520°C 24 hr at 560°C 100µm 2019 Aluminum Transportation Group Hot Rolling Purpose Breakdown Tandem Casting Homogenization Hot Rolling Cold Rolling CASH Forming Purpose • Reduce the ingot thickness • Break up large second phase particles Fe phase • Recrystallize grains to produce a randomly oriented crystal structure Exit Temperature = 358°C 2019 Aluminum Transportation Group Hot Rolling – Parameters To Control Microstructure Preheat Roughing mill Finishing Mill Coiling • Temperature – controlled via input temperature, roll speed and reduction schedule • Strain – controlled via reduction schedule • Strain rate – controlled via roll speed • Interpass times – controlled via roll speed; in reversing mill also by reduction schedule (slab length), can affect temperature Temperature almost always wins! 2019 Aluminum Transportation Group Hot Rolling – Parameters To Control Microstructure Coiling temperature influences: • Precipitation of particles: Mg2Si, Si, Q-phase (for Cu containing alloys) → strength, formability, bendability at final gauge • Grain structure and size, isotropy of grain → texture → essential for bendability at final gauge, formability → roping tendency at final gauge High FHRT Low FHRT Mg2Si particles 2019 Aluminum Transportation Group Cold Rolling Purpose Breakdown Tandem Casting Homogenization Hot Rolling Cold Rolling CASH Forming Cold Rolling defined as rolling below temperature for recrystallization (< 150°C). Purpose • Reduce down to final gauge • Give the final surface finish to the metal (surface texture) Undeformed Grains Deformed Grains Most of heat of deformation contained within strip Mill Finish EDT (and hence coil) 2019 Aluminum Transportation Group Cold Rolling Cold reduction of 80% Elements of Metallurgy and Engineering Alloys F.C. Campbell, editor, p 487-508 DOI: 10.1361/emea2008p487 2019 Aluminum Transportation Group Cold Rolling - Effect of Cold Work on Recrystallization The level of cold work affects the 25% Cold Work 50% Cold Work driving force for recrystallization, and the resultant final gauge grain size and aspect ratio. 75% Cold Work 80% Cold Work Increase of cold work → finer grain size after annealing 2019 Aluminum Transportation Group Annealing Hot Rolling Cold Rolling Annealing After cold rolling • High strength • Low ductility Annealing • Soluble phases are precipitated • Heavily deformed microstructure with high density of dislocations and vacancies -> Highly Unstable 2019 Aluminum Transportation Group Annealing Hot Rolling Cold Rolling Annealing Schematic Showing the Process of Recovery and Recrystallization Deformed Microstructure Recovered Microstructure Partial Recrystallization Full Recrystallization 2019 Aluminum Transportation Group Annealing Hot Rolling Cold Rolling Annealing Microstructural changes during heating after cold rolling 2019 Aluminum Transportation Group Annealing Hot Rolling Cold Rolling Annealing Type Description Carried out at intermediate stage of a fabricating process so that material may Interannealing be worked to a further degree. Full annealing Annealing to give a fully recrystallized, soft material (O-temper). Partial softening of a material which has been cold rolled to a temper harder Partial annealing than that required (some recrystallization may occur). Recovery annealing Annealing carried out such that no recrystallization occurs. Annealing which occurs after hot rolling (re-roll gauge) without the application of Self annealing a separate heat treatment. 2019 Aluminum Transportation Group Annealing Hot Rolling Cold Rolling Annealing Batch annealing: • Processing of whole coils or blanks • Relatively slow heating and long thermal exposure at PMT • Also used for inter annealing Continuous annealing: • Solution heat treat or anneal • Coil pre-treatment • CASH line: • Annealing of AA5xxx and AA6xxx • SHT of AA6xxx 2019 Aluminum Transportation Group Solution Heat Treatment (SHT) Solution heat treatment consists of heating the strip up to ~ 500°C to 570°C followed by rapid cooling to room temperature – used in precipitation hardenable alloys. After rolling • High strength • Low ductility • Soluble phases are precipitated S Recrystallize SHT H Solutionize T T4 material • Soft • Formable • Hardening potential 2019 Aluminum Transportation Group Solution Heat Treatment Schematic figure of solution heat treatment heating up soaking cooling • Solutionising involves heating an alloy into the single phase (-Al) region of the phase diagram above its solvus temperature • Fast cooling to retain solute in solid solution 2019 Aluminum Transportation Group Microstructures of AA 6xxx Alloys As cast As rolled final gauge As solutionised (c) (a) (b) Coarse primary particles High density of intermetallic Most of the Mg2Si particles are (Mg2Si, -Al5FeSi, particles: dissolved AlCuSi in Cu containing alloys) Mg2Si and AlFeSi phases at the as cast cell boundaries 2019 Aluminum Transportation Group Precipitation Hardening 100 • Precipitation hardening is the primary strengthening 80 mechanism for heat treatable alloys 60 • Natural aging/T4 temper — spontaneous aging