Performance of high strength AlZnMg(Cu) aluminium

alloys after W-temper and warm forming

Paul Oberhauser*, Nikolay Sotirov**, Torsten Grohmann*, Peter Schulz* * AMAG Rolling GmbH / ** AIT Leichtmetallkompetenzzentrum GmbH

TTP 2013 - Tools and Technologies for Processing Ultra High Strength Materials, Graz, 19./20.09.2013

Outline

AMAG (Austria Metall AG)

Forming Processes

Strength Development

Bending and Corrosion Performance

3 3

AMAG value chain

Gießen

Primary

aluminium

Alumina

Sept-Îles, Canada

Ranshofen, Österreich

Petroleum coke,

pitch

Electric power

Scrap,

Primary

aluminium

Rolling slabs

Energy

(gas,

electric power)

Foundry

alloys

Flat rolled

products

4 4

Quality supplier of primary aluminium, foundry alloys

and special flat rolled products of aluminium

Overview

Casting Division Rolling Division Metal Division

Alouette

20 %

Business activities

Leading Producer of aluminium cast- and

flat-rolled products

Metal Division: Interface to raw

material market, stake in the smelter

Alouette

Casting Division: high quality recycling

foundry alloys

Rolling Division: high quality flat-rolled

products

Employees: 1,490 employees1)

Sales

814 mEUR

Key figures 2012

EBITDA

134 mEUR

External shipments

327,800 tons 1) Full time equivalent including leasing personnel, without apprentices, including the

percentage personnel share out of the 20% participation in smelter Alouette (average)

5 5

Motivation Use of aluminium alloys for automotive applications

5xxx AlMg(Mn)

Low to medium strength in soft temper

Good formability, e.g. for door inner

Not suitable for outer body – strain marks

6xxx AlMgSi

Medium to high strength

Standard for outer body in aluminium

Cold forming in T4 naturally aged

Strength nearly doubles during paint bake

7xxx AlZnMg(Cu)

High to ultra high strength

Transfer of adapted aerospace grades to the automotive business

Limited cold forming ability – heat supported forming procedures available

Corrosion performance inferior – coating and certain temper needed

Chance for substitution of press hardened steel – weight specific properties comparable

Application areas for aluminum sheet

Source: ATZ / Audi

6 6

Material Properties High strength 7xxx AlZnMg(Cu) alloys

AlZnMg(Cu) alloy family

Copper and non Cu-containing

Precipitation hardening alloy

MgZn2 (h-phase) respectively Al2CuMg (S-phase) TEM: Hardening particles

Rm [MPa] Rp0,2 [MPa) A50 [%]

AMAG TopForm© UHS

AA7075 (AlZn5,5MgCu)

≥ 540 ≥ 470 ≥ 7

guideline values for temper T6 575 510 12

AA7021 (AlZn5,5Mg1) ≥ 400 ≥ 350 ≥ 7

guideline values for temper T6 425 390 11

AA7020 (AlZn4,5Mg1) ≥ 350 ≥ 280 ≥ 8

guideline values for temper T6 380 335 12

7 7

Motivation Specific strength of 7xxx vs. high strength steels

P. Oberhauser, TTP 2013, 19 September 2013, Graz

(Austria)

0

10

20

30

40

50

60

0 50 100 150 200 250 300

Specific Tensile Strength Rm/r

Elo

ng

atio

n [%

]

Advanced High Strength Steels (AHSS)

conventional High Strength Steels

(HSS)

Ultra High Strength Steels (UHSS)

7xxx Aluminium suitable for light-weight

crash-resistant components

- high specific strength

- high residual elongation

Source: voestalpine,

ThyssenKrupp

8 8

Forming Processes Warm forming – Blank in maximum strength T6 heated to about 200°C

Temperature Time Diagram (schematic): Rolling Mill and Press Shop

Warm

Forming

Solution

Heat Treatment

200°C

400°C

minutes hours minutes

20°C

Aluminium Rolling Mill Press

Shop

Quench

(water)

Artificial Ageing

9 9

Forming Processes Forming behaviour AA7075 T6 at room temperature vs. warm forming

Warm forming at 170°C-230°C drastically improves the formability of peak-temper AA7075 T6

Defined mechanical properties after forming process

formed at RT formed at 230°C

AA7075-T6, 2 mm

Sotirov et al., Forming in Car Body Engineering, Bad Nauheim, 26.-27. September 2012

10 10

Forming Processes W-temper forming: Blanks in T4 or T6 receive a solution heat treatment

Temperature time diagram (schematic): Press and Paint Shop

200°C

400°C

minutes minutes

20°C

Press Shop Paint Shop

hours

Solution

Heat Treatment

Quench

(water)

Cold

Forming

5-step

Paint Bake Cycle*

*5-step PB:

15 min/125°C

+25 min/185°C

+15 min/160°C

+15 min/150°C

+30 min/140°C

11 11

100

150

200

250

300

350

400

0 10 20 30 40 50 60S

tre

ng

th

[MP

a]

Time between quench and tensile testing [min]

Mechanical Properties of AA7075 and AlZnMg in W-Temper5 min solution heat treatment at 485°C, water quench

Rm AA7075 Rm AlZnMg Rp0,2 AA7075 Rp0,2 AlZnMg

Forming Processes Comparison W-temper forming AA7075 vs. AlZnMg

W-Temper Forming test alloy AMAG AlZnMg vs. AlZnMgCu

Solution heat treatment @ ~470°C and water quenching in the press shop

Pressing of the parts within ~ 30 min after quenching (fast natural ageing)

Cu-free test alloy shows similar formability as standard AA5182 in soft temper

AA7075 considerably less formable

AlZnMg AA7075

13 13

Solution heat treatment

400°C 470°C

Experimental Process windows for W-temper forming and warm forming

Forming at

170°C 210°C

Water

Quench

Cooling by

forced air

W-temper forming

Water

Quench

Cooling by

forced air

AA7075 T6 , AlZnMg T79 AA7075 T6, AlZnMg T4

Forming at RT

(3 min after quench)

Warm forming

Sheets, 2 mm

16 16

0

100

200

300

400

500

600

700

TopForm UHS T6 (as

delivered)

Warmforming Topform UHS

170°C / WQ

Warmforming Topform UHS

170°C / AQ

Warmforming Topform UHS

210°C / WQ

AlZnMg T79 (as delivered)

Warmforming AlZnMg 170°C

/ WQ

Warmforming AlZnMg 170°C

/ AQ

Str

en

gth

[M

Pa

]

Strength of AMAG Topform UHS vs. AlZnMg after warm forming

Rp0,2

Rm

0

100

200

300

400

500

600

700

TopForm UHS T6 (as

delivered)

Topform UHS 170°C /

WQ + PB

Topform UHS 170°C /

AQ + PB

Topform UHS 210°C /

WQ + PB

Topform UHS 210°C /

AQ + PB

7921 T79 (as

delivered)

AlZnMg 170°C / WQ

+ PB

AlZnMg 170°C / AQ

+ PB

Str

en

gth

[M

Pa

]

Strength of AMAG Topform UHS vs. AlZnMg after warm forming and 5-step Paint Bake

Rp0,2 + PB

Rm + PB

Strength Development Warm forming – strength development

After warm forming at 170°C, both alloys moderately loose strength during 5-step paint bake cycle

Warm forming at >200°C enables re-aging during paint bake steps (RRA-response)

Stable properties, independent of water quench or cooling by air ventilation after warm forming

TopForm UHS AlZnMg TopForm UHS AlZnMg

+ Paintbake

19 19

0

100

200

300

400

500

600

700

W-Temper AA7075

470°C / WQ

W-Temper AA7075

470°C / AQ

W-Temper AA7075

400°C / WQ

W-Temper AA7075

400°C / AQ

W-Temper AlZnMg

470°C / WQ

W-Temper AlZnMg

470°C / AQ

W-Temper AlZnMg

400°C / WQ

W-Temper AlZnMg

400°C / AQ

Str

en

gth

[M

Pa

]

Strength of AA7075 vs. AlZnMg after W-temper forming and RT storage

Rp0,2

Rm

Strength Development W-temper forming – strength development

0

100

200

300

400

500

600

700

AA7075 470°C / WQ

+ PB

AA7075 470°C / AQ +

PB

AA7075 400°C / WQ

+ PB

AA7075 400°C / AQ +

PB

AlZnMg 470°C / WQ

+ PB

AlZnMg 470°C / AQ +

PB

AlZnMg 400°C / WQ

+ PB

AlZnMg 400°C / AQ +

PBS

tre

ng

th [M

Pa

]

Strength of AA7075 vs. AlZnMg after W-temper forming and 5-step paint bake cycle

Rp0,2 + PB

Rm + PB

AA7075 has to be solution heat treated in a very close process window

AlZnMg very tolerant in terms of SHT temperature and quenching speed

AlZnMg regains strength after 5-step paint bake cycle (“aging response”)

AA7075 AlZnMg AlZnMg AA7075

+ Paintbake

20 20

Strength Development AMAG TopForm© UHS vs. W-Temper AlZnMg: Comparison of strength

Warm Forming at

temperatures <200°C

moderately decreases

strength at 5 step paint

bake cycle (5xPB) due

to overaging

Warm Forming at >200°C

enables re-aging at

5xPB (so called RRA)

W-Temper Forming

benefits from 5xPB

almost independent of

forced air quench (AQ)

or water quench (WQ)

Stable conditions for both

variants after 5xPB

Warm forming of AMAG TopForm© UHS vs. W-Temper Forming of AMAG AlZnMg

TopForm UHS AlZnMg

21 21

Crash Performance Bending test (ISO 7438, PAPP PWT 4101)

Material / forming process Bending angle b

AA7075 W-Temper 120-138

TopForm UHS warm forming 142-153

AlZnMg W-Temper 110-142

AlZnMg warm forming 109-133

b

a

Samples 60 mm x 25 mm from side-wall of “Smiley” part

Bending punch radius 0,4 mm, roll distance 2*a

AlZnMg shows better bending behaviour than AA7075

22 22

100

105

110

115

120

125

130

135

140

145

150

300 350 400 450 500 550 600

Be

nd

ing

An

gle

b

Yield Strength [MPa]

Bending Angle as a Function of Yield Strength(Bending test after forming, 2 weeks storage and 5-step paint bake simulation)

Bending Angle AA7075

Bending Angle AlZnMg

Crash Performance Bending test

AA7075 W-Temper 400°C

+ 5-step paint bake:

L = 23,1 MS/m

b1= 120°; b2 = 136°

AlZnMg Warm Forming

+ 5-step paint bake:

L = 23,1 MS/m

b1= 109°; b2 = 133°

b1 b2

23 23

Corrosion Performance Stress corrosion cracking: AMAG TopForm© UHS vs. W-Temper AlZnMg

C-Ring and tensile bar

Load 300 MPa

AMAG TopForm© UHS

AA7075 (AlZn5,5MgCu)

AlZnMg

(in development)

Forming & Temper ASTM G47 DIN 50908 ASTM G47 DIN 50908

W-temper

+ 5-step paintbake passed passed failed 2 d failed 1 d

+ 15 h / 150°C passed passed passed failed 3 d

+ 40 h / 150°C passed passed passed passed

Warm forming

+ 5-step paintbake --- passed --- failed 1 d

AMAG TopForm© UHS W-Temper

Material has to withstand

SCC (Stress Corrosion

Cracking) test 30 days in

order to pass

24 24

Corrosion Performance

AlZnMg W-Temper:

Ageing of parts necessary for proper SCC resistance

AMAG TopForm© UHS:

Cu alloy for SCC resistance

Source: Sarkar et al., Metal Trans. A,

Vol. 12, Issue 11, p.1939; New York 1981

Stress corrosion cracking: AMAG TopForm© UHS vs. W-Temper AlZnMg

25 25

Summary

Warm forming

Material heat treatment at rolling mill (peak strength T6, T79)

Forming at about 200°C for short times preserves strength

Final properties with or without 5-step paint bake well defined (overaged)

W-temper forming

Material receives solution heat treatment immediately before forming

Thermal treatment needed after forming in order to reach required strength

levels

Very narrow process window for Cu-containing AA7075

Cu-free AlZnMg needs additional heat treatment to get immune against

stress corrosion cracking

Contact:

paul.oberhauser@amag.at

torsten.grohmann@amag.at