introduction to aluminum sheet...may 01, 2019 · 2019 aluminum transportation group annealing hot...

Introduction to Aluminum Sheet

The Aluminum Transportation Group

2019 Aluminum Transportation Group

Debdutta Roy, Ph.D.

Novelis

Senior Scientist, R&D Automotive

Presenter


Russ Long

Arconic

Chief Engineer – Ground Transportation Products

Presenter


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

Automotive Aluminum Sheet: Production

Flow Path and General Metallurgy

The Aluminum Transportation Group


OverviewB

od

y St

ruct

ure

Inn

er

Pan

els

Att

ach

me

nts

Bo

dy

in W

hit

e

• Application• Requirements

guide• Alloy selection

❑ Heat Treatable (Precipitation Hardenable) 6xxx

❑ Non Heat Treatable 5xxx

AnnealingCold RollingHot Rolling

Typical Process Flow


Cast House Process Flow (DC Casting)

Batch

preparation

Melting furnace

Melting + skimming

Casting furnace

Furnace treatment In line treatment

Degassing

Grain refiner

Filtration

CastingIngots



• 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.



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.

TREATMENT GAS (Ar + Cl2)



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



R. Nadella et al, Progress in Materials Science, 53 (3), 2008

• 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.

2019 Aluminum Transportation Group-12- R&T Center Sierre / ©2015 Novelis Inc.

Fe phasesMg2Si

As CastAs Cast

As-Cast Microstructures


Scalping of Ingots

True Edge

Edge scalper


Homogenization - Purpose

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)

Casting HomogenizationBreakdown Tandem

Hot Rolling Cold Rolling CASH Forming

AA6xxx As-Cast Microstructure


0

100

200

300

400

500

600

700

0 5 10 15 20 25 30 35 40

Time (hours)

Te

mp

era

ture

(°C

)

Heat-up

Phase Soak Cool to Hot-

Rolling T

Standard

Homogenization Practice: Temperature, TimeT [°C]

α+ℓ

α

C [%]

ℓ

α+β

Eutectic

• 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 the eutectics

• With increasing solute content (e.g., Mg), the allowable temperature decreases

solid state diffusion


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

Redistribution of Solute (6xxx)

Reduction of micro-segregation


24 hr at 560°C

Homogenization: Temperature Effect on Particle Structure



100µm24 hr at 450°C 24 hr at 520°C

AA6016


Hot Rolling Purpose

Exit Temperature = 358°C



Purpose

• Reduce the ingot thickness

• Break up large second phase particles

• Recrystallize grains to produce a randomly oriented crystal structure

Fe phase


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!


Hot Rolling – Parameters To Control Microstructure

• 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

Coiling temperature influences:

Mg2Siparticles

High FHRT Low FHRT


Cold Rolling Purpose

Purpose • Reduce down to final gauge

• Give the final surface finish to the metal (surface texture)

Mill Finish EDT



Deformed GrainsUndeformed Grains

Cold Rolling defined as rolling below temperature for recrystallization (< 150°C).

Most of heat of deformation contained within strip (and hence coil)


Cold Rolling

Elements of Metallurgy and Engineering Alloys

F.C. Campbell, editor, p 487-508

DOI: 10.1361/emea2008p487

Cold reduction of 80%


Cold Rolling - Effect of Cold Work on

Recrystallization

The level of cold work affects the

driving force for recrystallization,

and the resultant final gauge grain

size and aspect ratio.

25% Cold Work 50% Cold Work

75% Cold Work

Increase of cold work → finer grain size after annealing

80% Cold Work


Annealing


After cold rolling• High strength

• Low ductility

• Soluble phases are precipitated

• Heavily deformed

microstructure with high

density of dislocations and

vacancies

-> Highly Unstable

Annealing


Annealing


Schematic Showing the Process of Recovery and Recrystallization

Deformed Microstructure Recovered Microstructure Partial Recrystallization Full Recrystallization


Annealing


Microstructural changes during heating after cold rolling


Annealing


Type Description

InterannealingCarried out at intermediate stage of a fabricating process so that material may

be worked to a further degree.

Full annealing Annealing to give a fully recrystallized, soft material (O-temper).

Partial annealingPartial softening of a material which has been cold rolled to a temper harder

than that required (some recrystallization may occur).

Recovery annealing Annealing carried out such that no recrystallization occurs.

Self annealingAnnealing which occurs after hot rolling (re-roll gauge) without the application of

a separate heat treatment.


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



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

RecrystallizeSolutionize

SHT

T4 material

• Soft

• Formable

• Hardening potential

SHT


• 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

heating up soaking cooling

Schematic figure of solution heat treatment

Solution Heat Treatment


As cast

(a)

(c)

As solutionisedAs rolled final gauge

(b)

High density of intermetallic particles:Mg2Si and AlFeSi phases

Most of the Mg2Si particles aredissolved

Coarse primary particles(Mg2Si, -Al5FeSi, AlCuSi in Cu containing alloys)at the as cast cell boundaries

Microstructures of AA 6xxx Alloys


Precipitation Hardening

• Precipitation hardening is the primary strengthening mechanism for heat treatable alloys

• Natural aging/T4 temper — spontaneous aging of W

temper alloy at room temperature

• T6 Temper — Artificially aged and contains a uniform

dispersion of fine precipitates

0

20

40

60

80

100

0 20 40 60

Rp

0.2

[M

Pa]

Time after SHT [h]Strongest increase of strength during the firsthours after SHT


GP zones

’’

’

clusters coherent

semi-coherent

Tem

pe

ratu

re

ssss

Precipitation Mechanism

Sequence

Mg2Si

Mg2Si’’ (needles) Mg5Si6

incoherent

Precipitates form by nucleation and growth from supersaturated solid solution

Maximum Strength Contribution


Artificial Aging

T4 (ssss)

Rp

0.2

Time in temperature (log)

’’

Peak age

Underaged

’

Overaged


Paint Drying at Customer Provides Bake Hardening

Typical cycles:

• 185°C – 20 min

• 170°C – 20 min


Impact of Paint Bake Cycles


Effects of Texture - Roping

Roping:

surface

roughening that

occurs when sheet

is deformed

Formed panel: example of bad roping performance


Effects of Texture - RopingCause of roping: spatial alignment of grains of similar orientation along RD

(“grain colonies“)

texture in an intensively roping sheet

Any processing that shortens the texture

alignment, for example, inter annealing, reduces

the tendency to roping

texture in a roping-free sheet

Intensive roping Limited roping


Roping – Texture Evolution During Processing

No Inter annealing – Roping Intensive Presence of Inter annealing – Roping Free


Typical Mechanical Properties – Exterior with Flat Hemming

As received After paint bake1 Comments

Grade Typical

Yield

(MPa)

Typical

Ultimate

(MPa)

Typical

T. Elong

(%)

Minimum

U. Elong

(%)

Minimum

rave

Typical

r45

Typical

Yield

(MPa)

Typical

Ultimate

(MPa)

Typical

T. Elong

(%)

Alloy

6EH 95-135 195-260 27 19 0.50 0.30 220 280 23 6022, 6016

12%PS + 20 minutes at 185°C


Typical Mechanical Properties – Exterior Without Flat Hemming

As received After paint bake1 Comments

Grade Typical

Yield

(MPa)

Typical

Ultimate

(MPa)

Typical

T. Elong

(%)

Minimum

U. Elong

(%)

Minimum

rave

Typical

r45

Typical

Yield

(MPa)

Typical

Ultimate

(MPa)

Typical

T. Elong

(%)

Alloy

6DR1 105-145 200-270 27 19 0.50 0.30 230 300 22 6022, 6016

12%PS + 20 minutes at 185°C


Typical Mechanical Properties – Interior Reinforcements

As received After paint bake Comments

Grade Typical

Yield

(MPa)

Typical

Ultimate

(MPa)

Minimum

T. Elong

(%)

Typical

U. Elong

(%)

Minimum

rave

Typical

r45

Minimum

Yield

(MPa)

Typical

Ultimate

(MPa)

Typical

T. Elong

(%)

Alloy

6HS2 125-185 220-300 22 19 0.50 0.30 2351 302 22 6111

6HS2 125-185 220-300 22 19 0.50 0.30 2602 320 13 6111


Flat Hem Testing


5xxx Automotive SheetAA Chemical Composition Limits

Alloy Si Fe Cu Mn Mg Cr

5754 0.40 0.40 0.10 0.50 2.6-3.6 0.30

5182 0.20 0.35 0.15 0.20-0.50 4.0-5.0 0.10

5052 0.25 0.40 0.10 0.10 2.2-2.8 0.15-0.35

• The main alloys used for automotive body sheet are 5754 and 5182. Some 5052 also used in Europe.

• Used in the fully recrystallized condition – for maximum formability

• O-temper – no OEM requirements for surface quality after forming. Used in totally hidden locations, such as

unexposed door inners

• RSS-temper (reduced stretchers strain) – has OEM requirements for surface quality after forming. Used for

exposed or partially exposed applications, such as some door, hood, deck lid inner applications, etc.

• Mg contents greater than 3.5 are not recommended for extended elevated temperature exposure > 150̊ F to

avoid developing sensitivity to stress corrosion cracking


Typical Mechanical Properties – 5754-O and 5182-O

As received Comments

Grade Typical

Yield

(MPa)

Typical

Ultimate

(MPa)

Minimum

T. Elong

(%)

Minimum

U. Elong

(%)

Minimum

rave

Alloy

5HF 105-155 250-300 27 20 0.60 5182-O

5ST 100-140 215-270 25 18 0.60 5754-O


5182-O Stress Strain Curve

• In 5xxx alloys, Mg

atoms interact very

strongly with

dislocations as they try

to move through the

aluminum crystal

lattice

• Two types of surface

features can be

created in 5xxx (and

some other Mg

containing alloys)

Because of these features, 5xxx is not used for outer panels (Use 6xxx for outer panels)


Work Hardening and Bake Softening of 5xxx

Alloys


Conclusions• Automotive aluminum alloys can be broadly classified into 2 major groups:

• Heat treatable (or precipitation hardenable) 6xxx series

• Processing of alloys can be modified to optimize strength, formability and

crash properties depending on final application

• Aging behavior of these alloys provide strengthening during paint bake cycles

at customer

• Non heat treatable 5xxx series

• Strengthening component - mainly through solid solution strengthening

caused by Mg atoms

• Interaction of Mg atoms with dislocations -> dynamic strain ageing -> stretcher

strain marks or Ludering -> not suitable for exterior applications


Questions?


Arc Length Calculation

K factor for calculating arc

length during bending

Steel K = normally 0.38

Aluminum K = normally

0.43

For bend radius of T to

bend radius of 3T


Hood Example

Ref. A2MAC1


Hood Example – Cadillac ATS

Part Material Gauge

(mm)

Weight (kg)

Hood outer 6000-IH-90 0.9 2.06

Hood inner 6000-IBR-100 0.8 1.3

Palm reinforcement 6000-IBR-100 1.25 0.2

Hinge reinforcement Al-S-6000-R-110-U 1.65 0.12 x 2 = 0.24

Latch reinforcement Al-S-6000-R-110-U 1.25 0.18

Latch ring assembly steel 0.05

Totals 3.8


Typical Hood Gauges


Affordable Aluminum Door Concept


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introduction to aluminum sheet...may 01, 2019 · 2019 aluminum transportation group annealing hot...

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