Properties and Forces of Immersed Friction Stir Welded AA6061-T6

Properties and Forces of Immersed Friction Stir Welded AA6061-T6 Thomas Bloodworth George Cook Al Strauss

Outline • Introduction • Theory and Objective • VWAL Test Bed • Experimental Setup • Materials Testing • Results and Conclusions

Introduction • Immersed FSW for repair/construction • Rivet repair (Arbegast) • All prior advantages of conventional FSW • Determine trends for increased power input for ideal IFSW • Similar weld strengths as conventional with increased processed nugget hardness (Hofmann and Vecchio)

IFSW • Submerged / Immersed FSW (SFSW / IFSW) • Joining 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 • other beneficial properties

Theory • High quench rate • Power required increases • RPM dependent • Power (kW) = torque*angular velocity • Greater heat dissipation • Lower limit heat addition measured • DH = mwcpDTw • Thermocouple implantation

Theory • Hofmann and Vecchio show decrease in grain size by an order of magnitude • Increase in weld quality in IFSW 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

VWAL 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 • Experimental Matrix: • Rotational Speeds: 1000 – 2000 rpm • Travel Speeds: 5 – 14 ipm

Modifications • Anvil modified for a submerged welding environment • Water initially at room temperature (measured) • Equivalent welds run in air and water for mechanical comparison (i.e. Tensile testing, Cross Sectioning)

Experimental Setup • Weld speeds: 1000 – 2000 rpm, travel speeds 5 – 14 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” Trivex™ tool pin (probe) of length .235” • Clockwise rotation • Single pass welding

Experimental Setup • Shoulder plunge and lead angle: .009” , 10 • 80% Shoulder contact condition • Fine adjustments in plunge depth have been noted to create significant changes in weld quality • Therefore, significant care and effort was put forth to ensure constant plunge depth of .009” • Vertical encoder accurate to 10 microns • Tool creeps into material from the side and run at constant velocity off the weld sample

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

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

UTS vs IPM • FSW • General trend toward declining strength with travel speed increase • Constant RPM

Materials Results • IFSW • Largely Independent weld quality to travel speed at these rotational speeds

Materials Testing • IFSW • Largely RPM dependent at these travel speeds • Logarithmic regressions are similar at all travel speeds

Results • Submerged welds maintained 75-80% of parent UTS • Parent material UTS of 44.88 ksi compared well to the welded plate averaging UTS of ~30-35 ksi • Worm hole defect welds failed at 50-65% of parent UTS • Optimal welds for IFSW required a weld pitch increase of 38% • Weld pitch of dry to wet optimal welds • Dry welds: wp = 2000/11 = 182 rev/inch • Wet welds: wp = 2000/8 = 250 rev/inch

Axial Force • Axial Force independent of process or RPM

Axial Force • Axial Force independent of process or IPM

Moment • Moment has discernible increase for IFSW vs. FSW • Increase is from 2-5 Nm • Weld pitch dependent

Power • Linear power increase required from FSW to IFSW • Average increase of .5 kW required for similar parameters

Heat Addition • Heat input is assumed as lower limit • General linear trend; parameter dependent • Other mechanisms for heat loss and abnormalities • Conduction into anvil • Convection to air • Non-uniform heating

Conclusions • Average axial force independent of IFSW for the range explored • Average torque and therefore power increased from FSW to IFSW • FSW: 13.6 - 22.1 Nm; 2.8 – 3.4 kW • SFSW: 15.7 - 24.8 Nm; 3.3 – 3.7 kW

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 • Minimum increase in moment and power • Other process forces (i.e. Fz) not dependent

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 • UTSI for cross sectioning and microscopy

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.

Properties and Forces of Immersed Friction Stir Welded AA6061-T6