Evaluation of Friction Stir Weld Process and Properties for Aircraft Application

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1 Evaluation of Friction Stir Weld Process and Properties for Aircraft Application Christian Widener

2 Evaluation of Friction Stir Weld Process and Properties for Aircraft Application Motivation and Key Issues 1.1 Friction stir welding (FSW) is a solid state joining process which is being increasing used on commercial aircraft. Lower cost - Reduced weight, manufacturing time, and parts count Environmentally friendly reduced ergonomic impact, lower energy consumption, no emissions 1.2 Significant interest has been shown by aircraft manufacturers, but there is a need to better understand: Crack initiation and growth in FSW structures Fatigue Corrosion 1.3 So far, has proved difficult to implement Design parameters Specifications Certification 2

3 Evaluation of Friction Stir Weld Process and Properties for Aircraft Application Objective To develop standards and specifications for friction stir welded 2XXX and 7XXX series alloys, and to evaluate the performance of FSW stiffened panels Approach 3.1 Material Testing Coupon Level Strength, crack growth, corrosion, etc. 3.2 Structures Testing Integrally stiffened structure Static strength; tension, compression, and shear 3.3 Fixed Shoulder Tool Design Potential improvement for fatigue Still in development equipment challenges 3

4 FAA Sponsored Project Information Principal Investigators & Researchers Dwight Burford, PhD Bryan Tweedy, MS Christian Widener, PhD FAA Technical Monitor Curtis Davies Industry Participation David Ogan, Boeing Military - Wichita Jennifer Graham-Rateliff, Bombardier Aerospace Ron Weddle, Cessna John Barnes, Lockheed Martin Byron Colcher, Raytheon Mike Cumming, Spirit AeroSystems 4

5 3.1 Material Test Plan Objectives Identify key process variables for regulation and certification in friction stir welded aluminum alloys. Develop methodologies for guiding the development of process and performance specifications and allowables. First step, Identify optimized initial temper selection and post weld aging treatments for the enhancement of the corrosion resistance of 2024 and 7075 aluminum alloys, while maintaining favorable mechanical properties Determine if the potential exists for the proper combination of initial temper and post-weld artificial aging (PWAA) to also enhance the corrosion resistance of dissimilar FSW joints of 2024 and

7075 Exfoliation Macrographs Exfoliation Testing ASTM

prevent pitting in the HAZ of 7075-T6 7075-T73 plus

PWAA (2-4 hours @ 325F) Ultimate Tensile Strength (ksi)

6 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-T T73 plus PWAA had higher tensile strengths 7075-T73 Parent FSW + PWAA ( F) Ultimate Tensile Strength (ksi) Standard Deviation (ksi) Joint Efficiency 90.5% 6

7 Effect of PWAA on Fatigue Crack Growth FSW vs. Parent Material T73 da/dn vs. Kmax for Beneficial PWAA Treatments (7075-T73) [Notch placed in the nugget against the weld direction] 1.00E E-04 da/dn [in/cycle] 1.00E E E E Kmax [Ksi-in0.5] F Parent_01 Parent_02 Parent_03 7

and temperature variations on the exfoliation

8 PWAA Design of Experiments Analysis 7075-T73 3-D response surface and cube showing the effect of time and temperature variations on the exfoliation resistance of 7075-T73 with PWAA. 5 hrs 4 hrs 3 hrs 310 F 325 F 340 F 8

2024 Exfoliation Macrographs Change in Ultimate Tensile Strength with PWAA for Friction Stir Welded 0.125" Al 2024-T3 2024-T3 vs. 2024-T81 2024-T3 Highest as-welded tensile strength 92.

9 2024 Exfoliation Macrographs Change in Ultimate Tensile Strength with PWAA for Friction Stir Welded 0.125" Al 2024-T T3 vs T T3 Highest as-welded tensile strength 92.5% joint efficiency Exfoliation sensitive 2024-T81 Exhibited superior exfoliation resistance 82.5% joint efficiency T81 - [Base Material] 72.1 ksi T3 - [Base Material] 68.5 ksi Naturally Aged 63.5 ksi F 63.5 ksi F 60.4 ksi F 58.5 ksi F 58.0 ksi 365F 55.0 ksi F 53.0 ksi 2024-T81 - Naturally Aged 59.5 ksi 40.0 UTS (ksi) EXCO ASTM G-34 9

10 Fatigue Crack Growth FSW vs. Parent Material T3 1.00E-03 da/dn Vs Kmax (2024-T3) With PWAA Beneficial for Exfoliation [Notch Place in the Nugget Against the Weld Direction] 1.00E-04 da/dn 1.00E E E Kmax [Ksi-in0.5] 4.5 Hrs@ 365F 9 365F 2024-T3_Parent_ T3_Parent_ T3_Parent_ T3_Parent_04 10

Dissimilar Alloy Exfoliation Micrographs EXCO ASTM G-34 Reduction in exfoliation pit depth, as a result of PWAA. The mixing of the two alloys is clearly visible.

11 Dissimilar Alloy Exfoliation Micrographs EXCO ASTM G-34 Reduction in exfoliation pit depth, as a result of PWAA. The mixing of the two alloys is clearly visible. No preferential attack at the dissimilar interface PWAA F in Reduces residual stresses F Develops precipitates in the nugget and TMAZ Raises electrical conductivity Naturally Aged Over aged tempers Less solute in solution = Less potential difference 11

12 3.1 Material Test Plan Conclusions T3 as-welded is exfoliation sensitive Can only be improved by PWAA to T81, which greatly impacts tensile strength T81 as-welded has excellent exfoliation resistance Stronger than 2024-T3 plus PWAA to T T6 and 7075-T73 are exfoliation sensitive 7075-T6 as-welded achieves greatest tensile strengths 7075-T73 plus PWAA has excellent exfoliation resistance 7075-T73 plus PWAA is stronger than 7075-T6 plus PWAA to T73 temper 12

13 Material Test Plan Conclusions cont. 4. PWAA treatments including retrogression and re-aging treatments do not restore exfoliation resistance to 7075-T6 material after FSW. 5. For 7075-T73, a PWAA treatment, like 4 hours at 325 F, can enhance corrosion resistance, with only a slight reduction in tensile strength May not invalidate the bulk material properties of 7075-T73 per AMS 2770 rev G [8]. 6. Dissimilar joints of 2XXX and 7XXX alloys with good corrosion resistance can be possible Exfoliation resistance largely restored to a 2024-T81 to T73 dissimilar joint using PWAA treatments 13

14 FSW Performance Specification Similar to an AMS material specification 3.2 Initial Temper 3.3 Heat Treatment Heat to 250 F±10 F for 11 to 12 hours, followed by 325 F±10 F for 2 to 4 hours, etc. 3.4 Longitudinal tensile properties Min 67.0 ksi 3.5 Electrical Conductivity Minimum 37 %IACS in the weld nugget 3.6 Acceptance Tests Etc. 14

15 3.2 Structural Test Plan Objectives Evaluate the static response of friction stir welded flat stiffened panels Compare to riveted panels to evaluate the static strength performance of the FSW panels Compare test results with finite element models Evaluate predictive capability of models regarding load carrying capability of unitized FSW structures Fatigue and Damage Tolerance 15

16 Structures Approach Configurations of interest Hat Section lap-welded Stiffener in T6 stiffeners Spaced 8.0 on-center Mimics KC-135 architecture T3 sheet Followed similar USAF testing procedures and fixture design Testing Tension Compression Shear FSW Lap-Joint Test Mode FSW Riveted Tension 3 3 Compression 3 3 Shear Sheet 7075 Hat Stiffeners 16

17 Structures Lap Weld Development MTS PDS 5 axis FSW machine Tapered pin tool with 3 flats diameter pin at base, shoulder diameter 1200 rpm, 6 ipm 1.5 o lead angle 850 lbs load control 17

Structures Lap Weld Micrograph Weld 7075-T6 Stiffener 2024-T3 Sheet Sheet thinning 7075-T6 0.

18 Structures Lap Weld Micrograph Weld 7075-T6 Stiffener 2024-T3 Sheet Sheet thinning 7075-T T3 No Post-weld Aging All welds received a minimum of 10 days natural aging Tensile Testing Coupons were machined in 1-inch increments from the plates. 18

19 Weld Fixturing and Setup Modular fixturing designed for FSW setup Holds stiffener and skin to anvil for welding Used for fabricating panels for all three testing modes 19

Good correlation between finite element model predictions and actual test results FSW or

20 Tension Panel Test Configuration 7 strain gages 3 on stiffener side, 4 on back side 1 strain gage on each edge to ensure symmetric loading MTS hydraulic 100 kip load frame Good correlation between finite element model predictions and actual test results FSW or Rivet Zones #1 #4 1.0 TYP # #7 # #3 1.0 TYP # Top View Bottom View 20

21 Tension Panel Stress-Strain Comparison Stiffened Panel Tension Test FSW vs. Riveted 7075-T6 to 2024-T Edge of Panel 41% increase in total elongation Stress (psi) Middle of Panel FSW ksi Riveted ksi 10% increase in UTS Percent Elongation 21

Compression Panel Test Configuration Photogrammetry used on

strain gages 5 on each side 1 strain gage on each stiffener

Top View Bottom View 15.50 TYP 8.75 TYP 3.75 TYP 7.

22 Compression Panel Test Configuration Photogrammetry used on skin side to capture out of plane strain and displacements 10 strain gages 5 on each side 1 strain gage on each stiffener to ensure symmetric loading MTS hydraulic 100 kip load frame Top View Bottom View TYP 8.75 TYP 3.75 TYP 7.0 TYP #1 #4 #5 #2 #3 FSW or Rivet Zones #7 #10 #8 #11 #9 #4 #5 Edge View 22

Compression Panel Load- Displacement Curve FSW vs Riveted 25000 20000

(lbs) 15000 10000 5000 FSW Actual ultimate loads: Riveted: 20,752 lbs FSW:

23 Compression Panel Load- Displacement Curve FSW vs Riveted Predicted ultimate loads: Riveted: 18,900 lbs FSW: 20,890 lbs Riveted Load (lbs) FSW Actual ultimate loads: Riveted: 20,752 lbs FSW: 20,862 lbs Displacement (in.) 23

24 Shear Panel Test Configuration Photogrammetry used on skin side to capture out of plane strain and displacements 12 strain gages used with 6 on each side MTS hydraulic 100 kip load frame 24

Shear Panel Load-Displacement Curve Comparison of Shear Testing Results for FSW and Riveted Panels FSW 40000 35000 8%

lbs) 33% Increase in Axial Displacement 5000 FSW: Avg Max Load (35,157 lbs) 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.

25 Shear Panel Load-Displacement Curve Comparison of Shear Testing Results for FSW and Riveted Panels FSW % Increases in Load Carrying Capability Load (lbs) FSW Panels Riveted Riveted: Max Load (32,360 lbs) 33% Increase in Axial Displacement 5000 FSW: Avg Max Load (35,157 lbs) Displacement (in.) FSW panels were able to dissipate strain energy less abruptly Able to sustain stiffener damage and continue to carry a load 25

Shear Panel Load-Displacement Curve 40000 35000 168% increase in energy absorbed 30000 25000 Load (lbs) 20000

3 0.4 0.5 0.6 0.7 0.8 0.9 1 Displacement (in.

26 Shear Panel Load-Displacement Curve % increase in energy absorbed Load (lbs) A (12,300 in-lbs) A+B (20,700 in-lbs) B = increased area under FSW panel curve Displacement (in.) The predicted ultimate load for the riveted panel was within 2% and within 10% for the FSW panel (using Bruhn methods) Post-buckling analysis not performed using FEM methods 26

27 3.2 Structural Test Plan Conclusions FSW statically superior to riveted structure Tension 10% increase in UTS 41% increase in total elongation Compression Equivalent to riveted stress level Increased displacement in FSW panels after stiffener collapse Stiffeners carried a majority of the load with skin buckling at lower loads Shear 8% increase in ultimate strength compared to riveted panels 33% increase in total axial displacement 68% increase in energy absorbed prior to failure 27

3.3 Fixed Shoulder Tool Design Objectives

forging Status Encountered delays due to

has been redesigned to include a surface

28 3.3 Fixed Shoulder Tool Design Objectives - Improve surface finish eliminate weld flash Improve fatigue and corrosion response Increase weld strength better forging Status Encountered delays due to equipment limitations Fixed shoulder tool has been redesigned to include a surface adaptable pressure shoulder Fabrication is underway 28

29 Additional NIS Cost Match Research Tool design Several patentable tool designs have been developed for both butt and lap welds Prototype development Actual aerospace parts have been fabricated Vacuum fixturing Rapid fixturing solutions have been demonstrated Refill spot welding Alternate fit-up and joining technique Weld fit-up tolerances Plate gap, weld path misalignment and voids, and dissimilar thickness tolerances investigated Friction Stir Spot Welding Increased joint strength over riveted and resistance spot welded joints Manufacturing Assist Technology Machining tabs Increasing length/width of aluminum sheet 29

30 A Look Forward Benefit to Aviation Lean & Green Technology Cost Savings Low energy use Reduced manufacturing time Part count reduction Reduced weight Environmentally Friendly No sparks No fumes No hearing protection required Low Ergonomic Impact Design Parameters and Process Guides Process & Performance Specifications A & B basis vs. S basis Mil HDBK 5 type data Testing and Certification Friction Stir Spot Welds vs. Resistance spot welds and Rivets 30

Friction Stir Spot Weld Table Friction Stir Spot Welds (FSSW) Stronger than riveted values More repeatable than resistance spot welding Table X.X.XX.

31 Friction Stir Spot Weld Table Friction Stir Spot Welds (FSSW) Stronger than riveted values More repeatable than resistance spot welding Table X.X.XX.X(a) Static Strength of Friction Stir Spot Welds MATERIAL Thickness (in.) 2024-T T3 Clad Dissimilar T6/2024-T3 Clad 7075-T lbs # lbs* lbs^ * FSW , 5651 with pin, 800 rpm, in plunge ^ FSW , Turbo tool with pin, 1500 rpm, in. plunge # FSW , Concave three flat tool, 2000 rpm, in. plunge 31

32 FSSW Specifications FSSW Static Strength Tables: Welding tools treated as analogous to rivet types Demonstrate that different tools can produce similar results Shear area analogous to rivet diameter Sheet thickness Orientation of thinnest sheet Order of alloy sheets in joint 32

33 FSSW Specifications FSSW Static Strength Tables: Populate Range in sheet thicknesses Range in alloys/tempers Confirmation Second source S-basis (initially) 33

34 FSSW Specifications FSSW Static Strength Tables: Extend to other dissimilar metal joint pairs (e.g T43 to 2024-T3) 34

35 FSSW Specifications FSSW Placement: Some specifications may be transferable Investigate edge margins Range in sheet thicknesses Tooling Optimum Spacing 35

36 FSSW Specifications FSSW Fatigue Strength: Comparison Resistance Spot weld Blind Fasteners Evaluate Round-robin testing 36

37 Conclusions FSW Specifications Specifications are critical to implementation & regulation of FSW and related technologies Developing design allowables for FSW joints and spot welds is a natural extension to standards reference tables (e.g. MMPDS) Compare directly to point joining methods, including resistance spot welding and fastener joining Analysis methods have been incorporated for resistance spot welding Development will take place in a phased approach Demonstrate approach Tables Statistical basis Confirm with Round-Robin testing 37

AWS New Welding Technologies

AWS New Welding Technologies Friction Stir Welding - Recent Developments and Process Enhancements Presentation Contents: FSW Market Review Challenges for FSW Users Process Enhancements Summary Mike Russell