Properties and Forces of Immersed Friction Stir Welded AA6061-T6

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Properties and Forces of Immersed Friction Stir
Welded AA6061-T6

• Thomas Bloodworth
• George Cook
• Al Strauss

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Outline

1. Introduction
2. Theory and Objective
3. VWAL Test Bed
4. Experimental Setup
5. Materials Testing
6. Results and Conclusions

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Methodology Provide a operational parameterization of IFSW weld forces Temperatures via thermocouple implantation Cross sectioning for visual fault detection Use a standard FSW tool in a modified backing plate Perform butt welds of AA6061-T6
Capability Examines forces and faults characteristic to the IFSW process and addresses fixes FSW, having a solid foot as an industrial joining technique, may have further untapped benefits in welding in a water environment Benefits Increase in weld nugget hardness Increase in UTS FSW UTS 281.5 MPa IFSW UTS 296.1 MPa Decrease in grain size by order of magnitude
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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)

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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

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Theory

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

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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

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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

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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)

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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

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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

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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

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Materials Testing

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

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UTS vs IPM

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

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Materials Results

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

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Materials Testing

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

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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

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Axial Force

• Axial Force independent of process or RPM

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Axial Force

• Axial Force independent of process or IPM

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Moment

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

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Power

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

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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

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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

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Conclusions

• Optimal submerged (wet) FSWs 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

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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

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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. MSE
  2005.
• Arbegast W. et al. Friction Stir Spot Welding.
  6th International Symposium on FSW. 2006.