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Evaluation of Post-Weld Heat Treatments to Restore the Corrosion Resistance of Friction Stir Welded Aluminum Alloy 7075-T73 vs. 7075-T6 C.A. Widener 1, a , D.A. Burford 1,b , B. Kumar 3,c , J.E. Talia 2,d , and B. Tweedy 1,e 1 National

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Evaluation of Post-Weld Heat Treatments to Restore the Corrosion Resistance of Friction Stir Welded Aluminum Alloy 7075-T73 vs. 7075-T6 –––

C.A. Widener1, a, D.A. Burford1,b, B. Kumar3,c, J.E. Talia2,d, and B. Tweedy1,e

1National Institute for Aviation Research, Wichita State University, 1845 Fairmount St., Box 93, Wichita, KS 67260, USA 2Department of Mechanical Engineering, Wichita State University, 1845 Fairmount St., Box 133, Wichita, KS 67260, USA 3Formerly of the National Institute for Aviation Research, Wichita State University, 1845 Fairmount St., Box 93, Wichita, KS 67260, USA achristian.widener@wichita.edu, bdwight.burford@wichita.edu, cbrijesh.kumar@boeing.com, dgeorge.talia@wichita.edu, ebryan.tweedy

@wichita.edu

Christian Widener, PhD July 5th, 2006 THERMEC ’06 – Session G4: Friction Stir Processing

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Acknowledgements

This work was funded in part by grants from:

State of Kansas – NIS Program Federal Aviation Administration

Conducted at the Advanced Joining Technology Lab in the National Institute for Aviation Research at Wichita State University.

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OUTLINE

Background

Literature Review Experimental Procedures

Results

Exfoliation Tension Electrical Conductivity Microhardness Fatigue Crack Propagation

Conclusions

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Literature Review

Post-weld artificial aging (PWAA) treatments for 7075 after FSW

W condition

PWAA to T6 or T73

T6 condition

PWAA to T6 ,T73, or RRA

T7 condition

PWAA to T6 ,T73, or RRA

Solution Heat Treatment

FSW in W,T6, T73, or O condition Plus, PWAA to T6 or T73 Tends to cause abnormal grain growth

Attention has primarily been focused on peak and under aged tempers, since it has generally be felt that the FSW heat cycle imparts an unknown additional age to already over aged material.

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Literature Review – FSW of 7075 and 7XXX Al Alloys

As-welded 7XXX series alloys – Susceptible to Exfoliation (EXCO) & Stress Corrosion Cracking (SCC) [Multiple authors]. As-welded 7075 will continue to age at room temperature (similar to W condition 7XXX material) [Nelson, T.W., et al. (2001)]. Stress corrosion resistance can be restored to 7075-T6 by post- weld aging to the T73 temper, at the expense of exfoliation resistance [Leonard, A.J. (2000)]. PWAA for 100 hrs @ 225°F can restore the SCC resistance to 7050-T7451 [Lumsden et al. (2003)]. SCC resistance can also be restored to 7249-T7451 by re-aging to T73 [Arbegast et al. (2002)]. SCC resistance was also restored to 7475-T73 using a multi-step (e.g. RRA) localized heat treatment [Merati, et al. (2003)]. It was reported that in unpublished research it was found that PWAA can restore exfoliation resistance to 7075-T73 [Lumsden et

al. (2005)].

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C-curves for Precipition for 7075-T6 and 7075-T73

150 250 350 450 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 Critical Time (s) Temperature (°C)

7075-T6 7075-T73

Literature Review - Quench Factor Analysis

Precipitation kinetics [Staley, 1987] Precipitates influence strength and corrosion response Over aged alloys

Less solute in solution More stable = Less sensitive to thermal cycles TMAZ 200s <1s

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Experimental Procedures

Materials

Thickness: 0.125-inch (3.2 mm) Alloys: 7075-T6 and 7075-T73

Pin Tool

Pin diameter: 0.188-inch (4.8 mm) Pin length: 0.115-inch (2.9 mm) Shoulder diameter: 0.375-inch (9.5 mm)

Welding Parameters

600 RPM 8 IPM (200 mm/min) 1800 lbs forge load 5-axis MTS ISTIR™ PDS (40” x 120” x 25”)

Test Methods

Exfoliation (ASTM G-34) Nugget Electrical Conductivity (%IACS) Vickers Microhardness Tension (ASTM E-8) Fatigue Crack Propagation (ASTM E-647)

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Results

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Exfoliation Results of Post Weld Treatments - 7075-T6

7075 7075-

T6 N.A. N.A. 7075 7075-

T6 + RRA + RRA – – 3hrs at 3hrs at 355 355° °F + F + 24 hrs at 24 hrs at 250 250° °F F Pitting in HAZ Pitting in HAZ 7075 7075-

T6 + PWAA + PWAA to T73 to T73 Reduced Reduced Pitting in HAZ Pitting in HAZ

Note: All pictures taken at the same magnification.

7075 7075-

T6 Parent Parent Material Material

RRA developed for 7075-T6 Base Material [Ferrer, C.P. et al. 2003].

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Exfoliation Results of Post Weld Treatments - 7075-T73

7075 7075-

T73 N.A. N.A. 7075 7075-

T73 100 hrs 100 hrs at 225 at 225° °F F 7075 7075-

T73 4 hrs at 4 hrs at 325 325° °F F Pitting in HAZ Pitting in HAZ 7075 7075-

T73 Parent Parent Material Material

Note: All pictures taken at the same magnification.

No Pitting in No Pitting in HAZ HAZ Trace Pitting Trace Pitting in HAZ in HAZ

Allowable for base material per AMS 2770 rev G. 7050-T74 [Lumsden et al. (2003)].

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7075 Exfoliation Macrographs

No pitting in 7075- T73 with PWAA PWAA did not prevent pitting in the HAZ of 7075-T6 7075-T73 plus PWAA had higher tensile strengths

Exfoliation Testing – ASTM G-34

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Summary of 7075 Exco/Tensile Results The exfoliation resistance was not fully restored to 7075-T6.

When either aged to T73 or treated with RRA, tensile

strength was lower than 7075-T73 treated for 4 hours @ 325°F (163°C).

The exfoliation resistance of 7075-T73 can be enhanced by PWAA, with only a minor reduction of mechanical properties.

100 hours at 225°F (107°C) – OR –

4 hours at 325°F

Joint efficiencies as high as 94% of –T73 parent

material strengths were achieved.

Yield strength was not significantly affected.

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Tensile Results of 7075 Post Weld Treatments

7075-T73 + 4hrs @ 325F 68.67 ksi 7075-T6 (Aged to T73) 64.12 ksi 7075-T6 plus RRA 62.28 ksi 7075-T6 Naturally Aged 74.93 ksi 7075-T73 Naturally Aged 69.16 ksi 7075-T73 + 4hrs @ 325F 68.67 ksi

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Ultimate Stress [Ksi]

7075-T73 + 100 hrs @ 225F 70.1 ksi

7075-T6 7075-T73

477 MPa 483 MPa 473 MPa 442 MPa 429 MPa 517 MPa Note: Standard deviations were typically 0.4 ksi (2.5 MPa), when 3 or more samples were tested.

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Electrical Conductivity

Electrical conductivity (plus hardness) is a reliable indicator of weld temper in aluminum alloys. Electrical Conductivity has been shown to correlate closely with stress corrosion cracking resistance in 7XXX series alloys, by a number of researchers. Increases in the overall electrical conductivity of an FSW joint have also been shown to reflect improvements in stress corrosion cracking resistance in a friction stir welded 7XXX alloy [Arbegast, et al. (2002)]. Measurements taken using a Staveley Nortec 2000S Eddy Current tester.

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Effect of PWAA on Electrical Conductivity in 7075-T6

7075-T6 FSW Conductivity Charts

(Note: All Specimens Naturally Aged Prior to Treatment for a minimum of 100 hrs)

28 30 32 34 36 38 40 42

1
0.75
0.5
0.25

0.25 0.5 0.75 1

(Advancing) Distance from Weld Center (in.) (Retreating) Conductivity (%IACS) 7075-T6 - Naturally Aged 7075-T6 - 24hrs @ 225F 7075-T6 - 100hrs @ 225F 7075-T6 - 26hrs @ 325F 7075-T6 - 9hrs @ 355F 7075-T6 - RRA - 8hrs @ 320F 7075-T6 - RRA - 11hrs @ 320F 7075-T6 - RRA - 2hrs @ 355F 7075-T6 - RRA - 3hrs @ 355F 7075-T6 - Parent Matl. 7075-T73 - Parent Matl.

Probe width

SCC Threshold

T73

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Effect of PWAA on Electrical Conductivity in 7075-T73

7075-T73 FSW Conductivity Charts

(Note: All Specimens Naturally Aged Prior to Treatment for a minimum of 1 year) 30 32 34 36 38 40 42

1
0.75
0.5
0.25

0.25 0.5 0.75 1

(Advancing) Distance from Weld Center (in.) (Retreating)

Conductivity (%IACS)

7075-T73 - Naturally Aged 7075-T73 - 100hrs @ 225F 7075-T73 - 2hrs @ 325F 7075-T73 - 4hrs @ 325F 7075-T73 - 8hrs @ 325F 7075-T73 - 24hrs @ 325F 7075-T73 - Parent Matl. 7075-T6 - Parent Matl. 0.125" - 7075-T73 Naturally Aged - FSW Conductivity Chart

28 30 32 34 36 38 40 42 44

1
0.75
0.5
0.25

0.25 0.5 0.75 1 (Advancing) Distance from Weld Center (in.) (Retreating)

Conductivity (%IACS)

4hrs @ 325F - N.A. 100 hrs 4hrs @ 325F - N.A. 12,000+ hrs 4hrs @ 325F - N.A. 100 hrs - 2σ 4hrs @ 325F - N.A. 100 hrs +2σ 4hrs @ 325F - N.A. 12,000+ hrs -2σ 4hrs @ 325F - N.A. 12,000+ hrs +2σ

SCC Threshold

NOTE: Natural aging time prior to PWAA at 325F can be significant.

Over 1 year N.A. prior to PWAA for 4hrs at 325F 100 hrs N.A. prior to PWAA for 4hrs at 325F

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7075 Microhardness Data

7075-T6 Vickers Microhardness vs. PWAA

100 110 120 130 140 150 160 170 180

0.45
0.25
0.05

0.15 0.35

Distance from Weldcenter (in.) H V200

Naturally Aged 1000 hrs 26 hrs - 325F 7075-T6 Parent 7075-T73 Parent 7075-T73 + 4hrs @ 325F

Retreating Advancing

7075-T73 Vickers Microhardness vs. PWAA

100 110 120 130 140 150 160 170 180

0.35
0.25
0.15
0.05

0.05 0.15 0.25 0.35

Distance from Weldcenter (in.) HV200

Naturally Aged 100 hrs - 225F 4 hrs - 325F 7075-T73 Parent 7075-T6 Parent

Reduction of Vickers microhardness in the HAZ with higher temperature PWAA treatments Minimal reduction of Vickers microhardness in the HAZ due to PWAA

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Fatigue Crack Propagation Testing

Background

For friction stir welding, research has

shown that the weld nugget (against the welding direction) generally has the lowest crack growth resistance of the weldment.

Due to its fine grain structure, it is also the

area of greatest concern for stress corrosion cracking.

Also, with the application of PWAA,

residual stresses can in some cases be dramatically reduced [Dawes et al. 2000].

Residual stress effects are also reportedly

reduced at higher load ratios, ≥R=0.5 [Dalle Donne et al. 2000] .

Procedure

ESE(t) coupon, R=0.5, 0.1 Hz Crack growth – in the nugget - against

welding direction

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Crack Length vs. Number of Cycles – 7075-T73

[a vs. N] for Beneficial PWAA Treatments in Al-7075-T73 (0.125" thick) - (Notch Placed in the Nugget Against the Welding Direction)

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1,000 10,000 100,000 1,000,000 Number of Cycles Crack Length (in) 7075-T73_100 hrs@225F 7075_325_2hrs 7075_325_4Hrs 7075-T73 Parent Material Crack Length Vs No. of Cycles Al-7075-T73 (0.125" thick) - Against Welding Direction 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1,000 10,000 100,000 1,000,000 Number of Cycles Crack Length (in) 7075-T73- Naturally Aged 7075_T73_24Hrs@225F 7075_T73_48Hrs@225F 7075-T73_100 hrs@225F 7075-T73_2hrs@325F 7075-T73_4Hrs@325F 7075-T73_8Hrs@325F 7075-T73_24Hrs@325F 7075-T73 Parent Material

Naturally aged specimens

This PWAA effect was also observed for 2024-T3 [Bussu and Irving, 2003].

PWAA specimens and parent specimens exhibited similar crack growth behavior

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Effect of PWAA on Fatigue Crack Growth – 7075-T73

da/dN vs. Kmax for Beneficial PWAA Treatments (7075-T73) [Notch placed in the nugget against the weld direction]

1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00 10.00 100.00 Kmax [Ksi-in0.5] da/dn [in/cycle] 100 hrs @ 225F 2 Hrs @ 325 4 hrs @ 325 Parent_01 Parent_02 Parent_03

Naturally Aged Specimens

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7075-T6 Conclusions

7075-T6 (0.125-inch)

As-welded plus PWAA (≈225°F)

Highest strength (74.9 ksi, 517 MPa) Poor exfoliation and stress corrosion cracking resistance

As-welded plus PWAA to –T73 (325°F ≥ T ≤ T355°F)

Marked reduction in tensile strength (64.1 ksi, 442 MPa) Poor exfoliation resistance (HAZ pitting), but good SCC

resistance has been reported

Retrogression and re-aging treatments did not improve tensile

strength, but did partially improve exfoliation resistance and increased electrical conductivity.

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7075-T73 Conclusions

7075-T73 (0.125-inch)

As-welded plus PWAA (≈225°F for 100 hours)

High strength (69.2 ksi, 477 MPa) Good exfoliation resistance, and good SCC resistance (reported) Crack growth rates comparable to parent material However, 4 days at 225°F is not a production friendly heat treatment

As-welded plus PWAA (325°F for 4 hours)

High strength (68.7 ksi, 473 MPa) Excellent exfoliation resistance, and high nugget electrical

conductivity (suggesting good SCC resistance)

May not invalidate the bulk material properties of 7075-T73 (per AMS

2770 rev G)

May also benefit from natural aging or a low temperature RRA

treatment step prior to PWAA at 325°F (163°C)

Appears to relieve residual stresses allowing for more predictable

crack growth behavior in the weldment

Similar results also observed for 0.040-inch (1 mm) material

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Questions?

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References

[1] Thomas, W.M., et al., “Friction Stir Welding,” U.S. Patent No. 5,460,317, October, 24, 1995. [2] Klæstrup Kristensen, J. et al. “Properties of Friction Stir Welded Joints in Aluminum Alloys 2024, 5083, 6082/6060 and 7075,” 5th Intl. Friction Stir Welding Symposium, September, 2004. [3] Heat Treatment of Wrought Aluminum Alloy Parts, Aerospace Material Specification, AMS 2770G, April, 2003. [4] Nelson, T.W., Steel, R.J., and Arbegast, W.J., “Investigation of Heat Treatment on the Properties of Friction Stir Welds,” ASM International Aeromat Conference Presentation, 2001. [5] Leonard, A.J., “Corrosion Resistance of Friction Stir Welds in Aluminum Alloys 2014A-T651 and 7075-T651,” 2nd International Friction Stir Welding Symposium, 26-28 June, 2000. [6]

Anonymous. “Corrective Measures to Restore Corrosion Resistance Following Friction Stir Welding,” Office of Naval

Research, Report no. A580234, Rockwell Scientific, 2004. [7] Kumar, B. et al. “Applicability of FSW for Aircraft Applications,” 46th AIAA SDM Conf., 2005. [8] Paglia, C.S. et al. “Corrosion and Environmentally Assisted Cracking Behavior of High Strength Al Alloys FSW: 7075- T651vs.7050-T7451,” FSW and Processing II, 2003, pp. 65-75. [9] Lumsden, J., Mahoney, M., and Pollock, G., “Stress Corrosion Susceptibility in 7050-T7451 Aluminum Following FSW,” 1st International Friction Stir Welding Symposium, June, 1999. [10] Merati, A., Sarda, K., Raizenne, D., Dalle Done, C., “Improving Corrosion Properties of Friction Stir Welded Al Alloys by Localized Heat Treatment,” FSW and Processing II, 2003, pp. 77-90. [11] Li, Z.X., Arbegast, W.J., Wilson, A.L., Moran, J., and Liu, J., “Post-Weld Aging of Friction Stir Welded 7249 Extrusions,” Trends in Welding Research Conference, 15-19 April, 2002, pp. 312-317. [12] Pao, P.S., Gill, S.J., Feng, C.R., and Sankaran, K.K., “Effects of Weld Microstructure on Fatigue Crack Growth in Friction Stir Welded Al 7050,” TMS Aluminum 2001, 2001, pp. 265-279. [13] Sankaran, K.K., Smith, H.L., and Jata, K., “Pitting Corrosion Behavior of Friction Stir Welded 7050-T74 Aluminum Alloy,” Trends in Welding Research, April, 2002, pp. 284-286. [14] Paglia, C.S. et al. “Investigating Post-weld Heat Treatments to Increase the Corrosion and Environmental Cracking Behavior of 7075-T6 FSW,” Trends in Welding Research, 2002, p. 279-283. [15] Lumsden, J., Pollock, G., and Mahoney, M., “Effect of Post Weld Heat Treatments on the Corrosion Properties of FSW AA7050,” FSW and Processing II, TMS, March, 2003, pp. 99-106. [16] Dunlavy, M., and Jata, K.V., “High-Cycle Corrosion Fatigue of Friction Stir Welded 7050-T7451,” Friction Stir Welding and Processing II, 2-6 March, 2003, pp. 91-98. [17] Dawes, M.G., Karger, S.A., Dickerson, T.L., and Pryzdatek, J., “Strength and Fracture Toughness of FSW in Al Alloys,” 2nd Intl. Friction Stir Welding Symposium, 26-28 June, 2000. [18] James, M., Mahoney, M., and Waldron, D., “Residual Stress Measurements in Friction Stir Welded Aluminum Alloys,” 1st International Friction Stir Welding Symposium, 14-16 June, 1999. [19] Dalle Donne, C., Biallas, G., Ghindini, T., Raimbeaux, G., “Effect of Weld Imperfections and Residual Stresses on FCP in FSW Joints,” 2nd International Symposium on FSW, 8 June, 2000. [20] John, R., and Jata, K.V., “Residual Stress Effects on Near-Threshold Fatigue Crack Growth in Friction Stir Welds,” Friction Stir Welding and Processing, 4-8 November, 2001, pp. 57-69. [21] Bussu, G., and Irving, P.E., “The Role of Residual Stress and HAZ Properties on FCP in Friction Stir Welded 2024-T351 Al Joints,” Intl. Jrnl. Fatigue, v. 25, n. 1, Jan., 2003, pp. 93-104. [22] Ferrer, C.P. et al. “Improvements in Strength and SCC Properties in Al Alloy 7075 via Low-Temperature RRA Heat Treatments,” Corrosion, vol. 59, no. 6, June, 2003, pp. 520-528.