Performance of high strength AlZnMg(Cu) aluminium alloys after W-temper and warm forming

Transcription

1 Performance of high strength (Cu) aluminium alloys after W-temper and warm forming Paul Oberhauser*, Nikolay Sotirov**, Torsten Grohmann*, Peter Schulz* * AMAG Rolling GmbH / ** AIT Leichtmetallkompetenzzentrum GmbH TTP Tools and Technologies for Processing Ultra High Strength Materials, Graz, 19./

2 Outline AMAG (Austria Metall AG) Forming Processes Strength Development Bending and Corrosion Performance

3 AMAG value chain Alumina Petroleum coke, pitch Gießen Primary aluminium Electric power Sept-Îles, Canada Scrap, Primary aluminium Foundry alloys Rolling slabs Energy (gas, electric power) Ranshofen, Österreich Flat rolled products 3

4 Quality supplier of primary aluminium, foundry alloys and special flat rolled products of aluminium Overview Business activities Leading Producer of aluminium cast- and flat-rolled products 4 Metal Division Alouette 20 % Key figures 2012 Sales 814 meur Casting Division External shipments 327,800 tons EBITDA 134 meur Rolling Division 1) Full time equivalent including leasing personnel, without apprentices, including the percentage personnel share out of the 20% participation in smelter Alouette (average) 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 employees 1)

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 (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 Source: ATZ / Audi Application areas for aluminum sheet Chance for substitution of press hardened steel weight specific properties comparable 5

6 Material Properties High strength 7xxx (Cu) alloys (Cu) alloy family Copper and non Cu-containing Precipitation hardening alloy MgZn 2 (h-phase) respectively Al 2 CuMg (S-phase) TEM: Hardening particles Rm [MPa] Rp0,2 [MPa) A50 [%] AMAG TopForm UHS AA7075 (AlZn5,5MgCu) guideline values for temper T AA7021 (AlZn5,5Mg1) guideline values for temper T AA7020 (AlZn4,5Mg1) guideline values for temper T

7 Elongation [%] Motivation Specific strength of 7xxx vs. high strength steels xxx Aluminium suitable for light-weight crash-resistant components - high specific strength - high residual elongation conventional High Strength Steels (HSS) Advanced High Strength Steels (AHSS) Ultra High Strength Steels (UHSS) P. Oberhauser, TTP 2013, 19 September 2013, Graz (Austria) Specific Tensile Strength Rm/r Source: voestalpine, ThyssenKrupp 7

8 Forming Processes W-temper forming: Blanks in T4 or T6 receive a solution heat treatment Press Shop Solution Heat Treatment Paint Shop 400 C 200 C 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 20 C minutes hours minutes Temperature time diagram (schematic): Press and Paint Shop 8

9 Strength [MPa] Forming Processes Comparison W-temper forming AA7075 vs. W-Temper Forming test alloy AMAG vs. Cu Solution heat ~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 Mechanical Properties of AA7075 and in W-Temper 5 min solution heat treatment at 485 C, water quench 400 Rm AA7075 Rm Rp0,2 AA7075 Rp0, AA Time between quench and tensile testing [min] 9

10 Forming Processes Warm forming Blank in maximum strength T6 heated to about 200 C Solution Heat Treatment Aluminium Rolling Mill Press Shop 400 C 200 C Quench (water) Artificial Ageing Warm Forming 20 C minutes hours minutes Temperature Time Diagram (schematic): Rolling Mill and Press Shop 10

11 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 AA7075-T6, 2 mm formed at RT formed at 230 C 11 Sotirov et al., Forming in Car Body Engineering, Bad Nauheim, September 2012

12 Experimental Process windows for W-temper forming and warm forming Warm forming AA7075 T6, T79 Forming at 170 C 210 C Sheets, 2 mm W-temper forming AA7075 T6, T4 Solution heat treatment 400 C 470 C Water Quench Cooling by forced air Water Quench Cooling by forced air Forming at RT (3 min after quench) 13

13 Strength [MPa] Strength [MPa] 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 Strength of AMAG Topform UHS vs. after warm forming Rp0,2 Rm Strength of AMAG Topform UHS vs. after warm forming and 5-step Paint Bake + Paintbake Rp0,2 + PB Rm + PB TopForm UHS Warmforming Warmforming Warmforming T6 (as Topform UHS Topform UHS Topform UHS delivered) 170 C / WQ 170 C / AQ 210 C / WQ T79 Warmforming Warmforming (as delivered) 170 C 170 C / WQ / AQ 100 TopForm UHS TopForm UHS 0 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) 170 C / WQ + PB 170 C / AQ + PB 16

14 Strength [MPa] Strength [MPa] Strength Development W-temper forming strength development AA7075 has to be solution heat treated in a very close process window very tolerant in terms of SHT temperature and quenching speed regains strength after 5-step paint bake cycle ( aging response ) Strength of AA7075 vs. after W-temper forming and RT storage Rp0,2 Rm Strength of AA7075 vs. after W-temper forming and 5-step paint bake cycle + Paintbake Rp0,2 + PB Rm + PB W-Temper AA C / WQ AA7075 AA7075 W-Temper AA C / AQ W-Temper AA C / WQ W-Temper AA C / AQ W-Temper 470 C / WQ W-Temper 470 C / AQ W-Temper 400 C / WQ W-Temper 400 C / AQ AA C / WQ + PB AA C / AQ + PB AA C / WQ + PB AA C / AQ + PB 470 C / WQ + PB 470 C / AQ + PB 400 C / WQ + PB 400 C / AQ + PB 19

15 Strength Development AMAG TopForm UHS vs. W-Temper : Comparison of strength TopForm UHS 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 20

16 Crash Performance Bending test (ISO 7438, PAPP PWT 4101) Samples 60 mm x 25 mm from side-wall of Smiley part Bending punch radius 0,4 mm, roll distance 2*a shows better bending behaviour than AA7075 Material / forming process Bending angle b AA7075 W-Temper TopForm UHS warm forming W-Temper b warm forming a 21

17 Bending Angle b Crash Performance Bending test Bending Angle as a Function of Yield Strength (Bending test after forming, 2 weeks storage and 5-step paint bake simulation) b1 b Bending Angle AA7075 Bending Angle Yield Strength [MPa] AA7075 W-Temper 400 C + 5-step paint bake: L = 23,1 MS/m b1= 120 ; b2 = 136 Warm Forming + 5-step paint bake: L = 23,1 MS/m b1= 109 ; b2 =

18 Corrosion Performance Stress corrosion cracking: AMAG TopForm UHS vs. W-Temper C-Ring and tensile bar Load 300 MPa AMAG TopForm UHS AA7075 (AlZn5,5MgCu) (in development) Forming & Temper ASTM G47 DIN ASTM G47 DIN 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 Material has to withstand SCC (Stress Corrosion Cracking) test 30 days in order to pass AMAG TopForm UHS W-Temper 23

19 Corrosion Performance Stress corrosion cracking: AMAG TopForm UHS vs. W-Temper AMAG TopForm UHS: Cu alloy for SCC resistance W-Temper: Ageing of parts necessary for proper SCC resistance Source: Sarkar et al., Metal Trans. A, Vol. 12, Issue 11, p.1939; New York

20 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 needs additional heat treatment to get immune against stress corrosion cracking 25

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