An Investigation into Submerged Friction Stir Welding

1. An Investigation into Submerged Friction Stir Welding Vanderbilt University Welding Automation Laboratory: Nashville, TN Thomas S. Bloodworth III Paul A. Fleming David H. Lammlein Tracie J. Prater Dr. George E. Cook Dr. Alvin M. Strauss Dr. Mitch Wilkes Los Alamos National Laboratory: Los Alamos, NM. Dr. Thomas Lienert Dr. Matthew Bement

2. Overview • Introduction • Objective • VUWAL Test Bed • Experimental Setup • Materials Testing • Results and Conclusions • Future Work • Acknowledgements

3. Introduction • Friction Stir Welding (FSW) • Frictional heat with sufficient stirring plasticizes weld-piece (Thomas et al) • Advantageous to conventional welding techniques • No Fumes • Solid State • Non-consumable Tool • Welds maintain up to 95% of UTS compared to parent material

4. Introduction • Light weight materials used in production (e.g. Aluminum) • FSW is used primarily to weld Aluminum Alloys (AA) • Process currently becoming more prevalent: • Aerospace (e.g. Boeing, Airbus) • Automotive (e.g. Audi) • Marine (SFSW / IFSW)

5. Objective • Submerged / Immersed FSW (SFSW / IFSW) • Processing of the weld piece completely submerged in a fluid (i.e. water) • Greater heat dissipation reduces grain size in the weld nugget (Hofmann and Vecchio) • Increases material hardness • Theoretically increases tensile strength

6. Objective • Hofmann and Vecchio show decrease in grain size by an order of magnitude • Increase in weld quality in SFSW may lead to prevalent use in underwater repair and/or construction (Arbegast et al) • Friction Stir Spot Welds (FSSW) • Repair of faulty MIG welds (TWI) • Process must be quantitatively verified and understood before reliable uses may be attained

7. VUWAL Test Bed

8. VUWAL Capabilities • VUWAL Test Bed: Milwaukee #2K Universal Milling Machine utilizing a Kearney and Treker Heavy Duty Vertical Head Attachment modified to accommodate high spindle speeds. • 4 – axis position controlled automation • Experimental force and torque data recorded using a Kistler 4 – axis dynamometer (RCD) Type 9124 B • Rotational Speeds: 0 – 5000 rpm • Travel Speeds: 0 – 100 ipm

9. VUWAL Test Bed • Anvil modified for a submerged welding environment • Water initially at room temperature • Equivalent welds run in air and water for mechanical comparison (i.e. Tensile testing)

10. Experimental Setup • Optimal dry welds run 2000 rpm, 16 ipm • Wet welds speeds: 2000 – 3000 rpm, travel speeds 10 – 20 ipm • Weld samples • AA 6061-T6: 3 x 8 x ¼” (butt weld configuration) • Tool • 01PH Steel (Rockwell C38) • 5/8” non – profiled shoulder • ¼” – 20 tpi LH tool pin (probe) of length .235” • Clockwise rotation • Single pass welding

11. Experimental Procedure • Shoulder plunge and lead angle: .004” , 20 • Fine adjustments in plunge depth have been noted to create significant changes in force data as well as excess flash buildup • Therefore, significant care and effort was put forth to ensure constant plunge depth of .004” • Vertical encoder accurate to 10 microns • Tool creeps into material from the side and run at constant velocity off the weld sample

12. Materials Testing • Tensile testing done using standards set using the AWS handbook • Samples milled for tensile testing • Three tensile specimens were milled from each weld run • ½ “ wide x ¼ “ thick specimens were used for the testing

13. Materials Testing • Tensile specimens tested using an Instron Universal Tester • Recorded values included UTS and UYS in lbf

14. Results • Stress – Strain curves were generated from the data gathered from the tensile test • Weld pitch “rule” is not followed in IFSW (Revolutions / Inch)

15. Results • IFSW run with weld parameters 2000 rpm, 10 ipm • Developed optimal tensile properties • Wet parameter set 3000 rpm, 15 ipm developed worm hole defect

16. Results

17. Results

18. Results

19. Results

20. Results • Submerged welds maintained 90-95% of parent UTS • Parent material UTS of 44.88 ksi compared well to the welded plate averaging UTS of ~41 ksi • Worm hole defect welds failed at 65% of parent UTS • effective dry weld equivalent tests not run • Optimal welds for IFSW required a weld pitch increase of 60% • Weld pitch of dry to wet optimal welds • Dry welds: wp = 2000/16 = 125 rev/inch • Wet welds: wp = 2000/10 = 200 rev/inch

21. Results • Average torque increased from FSW to IFSW • FSW: 16 Nm • SFSW: 18.5 Nm • Elastic Modulus also increases for IFSW when compared to FSW • FSW: 1250 ksi • SFSW: 1450 ksi

22. Summary and Conclusions • Optimal submerged (wet) FSW’s were compared to conventional dry FSW • Decrease in grain growth in the weld nugget due to inhibition by the fluid (water) • Water welds performed as well if not better than dry welds in tensile tests • Elastic Modulus of the SFSW’s were considerably higher than that of traditional FSW • Leading to a less elastic and therefore less workable material • Dry FSW: E = ~1200 ksi • SFSW: E = ~1400 ksi

23. Future Work • Fracture Surface Microscopy • Cross section work for electron microscopy • TEM • SEM • Hardness Testing for comparison • Further Mechanical testing • e.g. bend tests

24. Acknowledgements • This work was supported in part by: • Los Alamos National Laboratory • NASA (GSRP and MSFC) • The American Welding Society • Robin Midgett for materials testing capabilities

25. References • Thomas M.W., Nicholas E.D., Needham J.C., Murch M.G., Templesmith P., Dawes C.J.:G.B. patent application No. 9125978.8, 1991. • Crawford R., Cook G.E. et al. “Robotic Friction Stir Welding”. Industrial Robot 2004 31 (1) 55-63. • Hofmann D.C. and Vecchio K.S. “Submerged friction stir processing (SFSP): An improved method for creating ultra-fine-grained bulk materials”. MS&E 2005. • Arbegast W. et al. “Friction Stir Spot Welding”. 6th International Symposium on FSW. 2006.