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 Institute for Aviation Research, Wichita State University, 1845 Fairmount St., Box 93, Wichita, KS 67260, USA 2 Department of Mechanical Engineering, Wichita State University, 1845 Fairmount St., Box 133, Wichita, KS 67260, USA 3 Formerly of the National Institute for Aviation Research, Wichita State University, 1845 Fairmount St., Box 93, Wichita, KS 67260, USA a christian.widener@wichita.edu, b dwight.burford@wichita.edu, c brijesh.kumar@boeing.com, d george.talia@wichita.edu, e bryan.tweedy @wichita.edu Christian Widener, PhD July 5th, 2006 THERMEC ’06 – Session G4: Friction Stir Processing 07-05-06 Slide 1
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. 07-05-06 Slide 2
OUTLINE � Background � Literature Review � Experimental Procedures � Results � Exfoliation � Tension � Electrical Conductivity � Microhardness � Fatigue Crack Propagation � Conclusions 07-05-06 Slide 3
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. 07-05-06 Slide 4
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)]. 07-05-06 Slide 5
Literature Review - Quench Factor Analysis � Precipitation kinetics [Staley, 1987] C-curves for Precipition for 7075-T6 and 7075-T73 <1s 200s 450 Temperature (°C) TMAZ 350 7075-T6 250 7075-T73 150 ` 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) � Precipitates influence strength and corrosion response � Over aged alloys � Less solute in solution � More stable = Less sensitive to thermal cycles 07-05-06 Slide 6
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) 07-05-06 Slide 7
Results 07-05-06 Slide 8
Exfoliation Results of Post Weld Treatments - 7075-T6 Note: All pictures taken at the same magnification. 7075- 7075 -T6 T6 Parent Parent Material Material Reduced Reduced Pitting in HAZ Pitting in HAZ Pitting in HAZ Pitting in HAZ 7075- -T6 T6 7075 + RRA – – + RRA 3hrs at 3hrs at 7075- -T6 T6 7075 355° °F + F + 355 7075- -T6 T6 + PWAA 7075 + PWAA 24 hrs at 24 hrs at N.A. to T73 N.A. to T73 250° °F F 250 07-05-06 RRA developed for 7075-T6 Base Material [Ferrer, C.P. et al. 2003]. Slide 9
Exfoliation Results of Post Weld Treatments - 7075-T73 Note: All pictures taken at the same magnification. 7075- 7075 -T73 T73 Parent Parent Material Material No Pitting in No Pitting in Pitting in HAZ Pitting in HAZ Trace Pitting Trace Pitting HAZ HAZ in HAZ in HAZ 7075- 7075 -T73 T73 7075- 7075 -T73 T73 7075- 7075 -T73 T73 100 hrs 100 hrs 4 hrs at 4 hrs at N.A. N.A. at 225° at 225 °F F 325° 325 °F F 07-05-06 7050-T74 [Lumsden et al. (2003)]. Allowable for base material per AMS 2770 rev G. Slide 10
7075 Exfoliation Macrographs Exfoliation Testing – ASTM G-34 � 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 07-05-06 Slide 11
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. 07-05-06 Slide 12
Tensile Results of 7075 Post Weld Treatments 7075-T73 7075-T6 80 75 70 65 60 7075-T73 + 100 hrs @ 225F 7075-T6 Naturally Aged Ultimate Stress [Ksi] 55 7075-T73 Naturally Aged 7075-T73 + 4hrs @ 325F 7075-T73 + 4hrs @ 325F 50 7075-T6 (Aged to T73) 7075-T6 plus RRA 45 74.93 ksi 69.16 ksi 70.1 ksi 68.67 ksi 68.67 ksi 40 64.12 ksi 62.28 ksi 35 30 25 20 15 10 5 0 517 MPa 477 MPa 483 MPa 473 MPa 442 MPa 429 MPa Note: Standard deviations were typically 0.4 ksi (2.5 MPa), when 3 or more samples were tested. 07-05-06 Slide 13
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. 07-05-06 Slide 14
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) 42 7075-T6 - Naturally Aged 40 7075-T6 - 24hrs @ 225F 7075-T6 - 100hrs @ 225F Conductivity (%IACS) 38 7075-T6 - 26hrs @ 325F SCC Threshold T73 7075-T6 - 9hrs @ 355F 36 7075-T6 - RRA - 8hrs @ 320F 34 7075-T6 - RRA - 11hrs @ 320F 7075-T6 - RRA - 2hrs @ 355F 32 7075-T6 - RRA - 3hrs @ 355F 7075-T6 - Parent Matl. 30 Probe width 7075-T73 - Parent Matl. 28 -1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1 (Advancing) Distance from Weld Center (in.) (Retreating) 07-05-06 Slide 15
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) 42 7075-T73 - Naturally Aged Conductivity (%IACS) 40 7075-T73 - 100hrs @ 225F 38 7075-T73 - 2hrs @ 325F SCC Threshold 7075-T73 - 4hrs @ 325F 36 7075-T73 - 8hrs @ 325F 34 7075-T73 - 24hrs @ 325F 7075-T73 - Parent Matl. 32 7075-T6 - Parent Matl. NOTE: Natural 30 aging time prior -1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1 to PWAA at (Advancing) Distance from Weld Center (in.) (Retreating) 325F can be 0.125" - 7075-T73 Naturally Aged - FSW Conductivity Chart significant. 44 Over 1 year 42 N.A. prior to Conductivity (%IACS) 40 4hrs @ 325F - N.A. 100 hrs PWAA for 100 hrs N.A. 4hrs @ 325F - N.A. 12,000+ hrs 38 4hrs at 325F 4hrs @ 325F - N.A. 100 hrs - 2 σ prior to 36 4hrs @ 325F - N.A. 100 hrs +2 σ 34 PWAA for 4hrs @ 325F - N.A. 12,000+ hrs -2 σ 4hrs @ 325F - N.A. 12,000+ hrs +2 σ 32 4hrs at 325F 30 28 -1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1 07-05-06 Slide 16 (Advancing) Distance from Weld Center (in.) (Retreating)
7075 Microhardness Data 7075-T6 Reduction of Vickers Vickers Microhardness vs. PWAA microhardness in the HAZ with higher temperature 180 PWAA treatments 170 160 Naturally Aged 1000 hrs 150 26 hrs - 325F H V200 140 7075-T6 Parent 7075-T73 Parent 130 7075-T73 + 4hrs @ 325F 120 110 7075-T73 100 Vickers Microhardness vs. PWAA -0.45 -0.25 -0.05 0.15 0.35 Distance from Weldcenter (in.) Advancing 180 Retreating 170 160 Naturally Aged 150 100 hrs - 225F HV200 Minimal reduction of 140 4 hrs - 325F Vickers microhardness in 7075-T73 Parent 130 7075-T6 Parent the HAZ due to PWAA 120 110 100 -0.35 -0.25 -0.15 -0.05 0.05 0.15 0.25 0.35 07-05-06 Distance from Weldcenter (in.) Slide 17
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