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Materials & Advanced Manufacturing (M&AM)

Microstructural Analysis and Creep Behavior of 25mm Thick Friction Stir Welded AA2139-T8

Uchechi Okekea, b, Carl Boehlerta

a Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI b Tank Automotive Research, Development, and Engineering Center, Warren, MI

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Background – AA2139-T8 and FSW

AA2139-T8

• Al-Cu-Mg-Ag alloy
• T8 tempered

– Solution treating (525 – 544°C) – Cold worked (rolled) – Artificially aged (163 – 256°C)

• T8 yields the θ’ and the Ω precipitates

Friction Stir Welding (FSW)

• FSW is a solid-state joining technique which involves a frictional heat

flux and severe plastic deformation.

• Develops unique microstructures due to dynamic recrystallization
• This process refines the microstructure.
• The mechanical strength of FSW materials is determined by the

resulting microstructure.

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Research Motivation and Objective

• Research on FSW has been primarily performed on thin plates of Al alloys.
• Limited transferable knowledge on FSW effects on the microstructure for thick Al

alloys.

• Limited transferable knowledge on FSW effects on the mechanical properties for thick

Al alloys.

• Limited information about the effects of the SZ microstructure on the tensile-creep

behavior.

• Due to growing interests and the application of lightweight alloys for vehicles,

elevated temperature loading environments should be investigated.

• This study aims to provide information on the tensile-creep deformation behavior of

thick plate FSW Al-Cu-Mg-Ag alloys.

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FSW Parameters and Creep Test Conditions

• Temperatures: 225 to 275°C
• Applied Stress: 25 to 125 MPa
• Sample Prep.:

– Flat dog-bone shaped – Gage dimensions: width=12.7mm, thickness=1.78mm

• SEM/TEM analysis

Region Temp (°C) Stress (MPa) HAZ 250-275 50 Mid-Bottom-1 250-275 50 HAZ 225 50-75-100 Mid-Bottom-2 225 50-75 Bottom 250 25-35-50 Mid-Bottom-3 250 25-35-50

127 mm SZ samples HAZ samples

Top Mid Top Mid Mid Bottom Bottom Parameter Specification Shoulder Diameter 1.625” Pin Length 0.972” Plunge Depth 0.02-0.005” Spindle Speed 150-250 RPM Travel Speed 2 IPM Total Length 18”

FSW processing parameters

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Stress increase creep behavior – 225°C, 50-125 MPa

The SZ Mid Bottom was less creep resistant than the HAZ.

0.2 0.4 0.6 0.8 1 100 200 300 400 500 600 700 800 225oC Creep Strain (%) Time (hrs)

σ = 50MPa σ = 75MPa σ = 100MPa σ = 125MPa

HAZ SZ Mid Bottom-2

CR=5.53x10-10 s-1 CR=8.49x10-10 s-1 CR=8.78x10-10 s-1 CR=1.86x10-9 s-1 CR=1.23x10-9 s-1 CR=1.33x10-8 s-1

0.95% strain to failure; Vp=4.3% 0.75% strain to failure; Vp=3.5%

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HAZ Mid-Bottom

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0.5 1 1.5 2 2.5 50 100 150 200 250 300 350 σ=50 MPa Creep Strain (%) Time (hrs) HAZ SZ Mid Bottom 250oC 275oC

CR=7.62x10-9 s-1 CR=1.23x10-7 s-1 CR=1.5x10-10 s-1 CR=3.07x10-10 s-1

Temperature increase creep behavior – 250-275°C, 50 MPa

The SZ Mid Bottom was less creep resistant than the HAZ.

2.4% strain to failure; Vp=7.2% 0.25% strain to failure; Vp=3.8%

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HAZ Mid-Bottom

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The grain size & precipitate distribution varies through the depth of the weld.

• The grain size decreases from the top of the weld towards the bottom.
• The Vp increases from the top of the weld towards the bottom.

GS= avg. grain size Vp= volume percent

Top Mid Top Mid Mid Bot Bottom

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Stress increase creep behavior through the depth

• f the SZ – 250°C, 25-35-50 MPa
• The SZ Mid-Bottom failed before the SZ Bottom.
• SZ Bottom had a higher creep strain to failure.
• Creep failure varies through the depth of the SZ.
• Precipitates have coarsened  Unstable microstructure!

0.5 1 1.5 2 2.5 100 200 300 400 500 600 250oC Creep Strain (%) Time (hrs)

σ = 35 MPa σ = 50 MPa σ = 25 MPa CR=1.34x10-9 s-1 CR=2.72x10-9 s-1 CR=2.06x10-8 s-1 CR=7.03x10-10 s-1 CR=1.07x10-8 s-1 CR=1.06x10-7 s-1

SZ Mid Bottom-3 SZ Bottom

b)

1.6% strain to failure; Vp=11.3% 2.4% strain to failure; Vp=12.5%

As-Welded As-Welded

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Bottom Mid-Bottom

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TEM of the base metal

• Fine precipitate phases

are observed (θ’ and Ω).

• These precipitates

strengthen the alloy.

<100> <110>

110 nm 34 nm 123 nm 24 nm

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0.5 1 1.5 2 2.5 100 200 300 400 500 600 250oC Creep Strain (%) Time (hrs)

σ = 35 MPa σ = 50 MPa σ = 25 MPa CR=1.34x10-9 s-1 CR=2.72x10-9 s-1 CR=2.06x10-8 s-1 CR=7.03x10-10 s-1 CR=1.07x10-8 s-1 CR=1.06x10-7 s-1

SZ Mid Bottom-3 SZ Bottom

TEM of creep tested SZ through the depth of the weld at 250°C, 25-35-50 MPa

• The microstructure was unstable during testing.
• Precipitation and coarsening of the precipitates was observed in both areas.

<110> - Mid Bottom-3 <100> - Bottom

θ’

273 nm 177 nm

θ’

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Ω phase is not present

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Summary

• The SZ was less creep resistant than the HAZ.
• The refined microstructure of the SZ led to reduced creep resistance.
• The SZ microstructure was dynamic/unstable during testing.

– Precipitate coarsening and dissolution may have contributed to this.

• The microstructure in the SZ is a function of the thickness through the weld.
• The resulting creep properties were a function of the microstructure.

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Acknowledgments

• This work was supported by the National Science Foundation Division of Material Research

(Grant No. DMR1107117).

• Center for Nanoscale Materials at Argonne National Laboratory via the U.S. Department of

Energy (Contract No. DE-AC02-06CH11357).

– Special thanks to Dr. Yuzi Liu who was a great assistance in acquiring the as-welded and deformed TEM images.

• Institute of Materials Research, Tohoku University through the National Science Foundation

East Asia & Pacific Summer Institute Fellowship (Award No. 1515111).

– The authors are grateful to Dr. Makoto Nagasako and Mr. Shun Ito for acquiring the TEM images.

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Tensile Behavior at RT

100 200 300 400 500 1 2 3 4 5 6 7 8

Engineering Stress (MPa)

Strain (%) BM SZ

BM FSW UTS (MPa) 433 364 0.2% YS (MPa) 300 265

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Secondary Creep Mechanism Analysis

0.5 1 1.5 2 2.5 50 100 150 200

Creep Strain (%) Time (hrs) T=250C T=275C CR=1.24x10

• 7 s
• 1

CR = 1.45x10

• 8 s
• 1

FSW, 50 MPa

2 10-8 4 10-8 6 10-8 8 10-8 1 10-7 1.2 10-7 1.4 10-7 50 100 150 200

Creep Rate (s

• 1)

Time (hrs) T=250C CR = 1.45x10

• 8 s
• 1

FSW, 50 MPa

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Fractographs of 225°C, 50-125 MPa

225°C, 50-75 MPa SZ Mid Bottom 225°C, 50-125 MPa HAZ

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Fractographs of 250-275°C, 50 MPa

250-275°C, 50 MPa SZ Mid Bottom

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Fractographs of the SZ at 250°C, 25-35-50 MPa

250°C, 25-35-50 MPa SZ Mid Bottom 250°C, 25-35-50 MPa SZ Bottom

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