Properties and Forces of Immersed Friction Stir Welded

AA6061-T6Thomas Bloodworth

George Cook

Al Strauss

Outline

1. Introduction

2. Theory and Objective

3. VWAL Test Bed

4. Experimental Setup

5. Materials Testing

6. Results and Conclusions

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

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

UTS vs IPM

0.000

5.000

10.000

15.000

20.000

25.000

30.000

35.000

B1 B2 B3 B4

1500 RPM

1000 RPM

2000 RPM

Materials Results

• IFSW• Largely Independent weld quality to travel speed

at these rotational speeds

UTS vs IPM

0.000

5.000

10.000

15.000

20.000

25.000

30.000

35.000

WA12 WA2 WA3

1000 RPM

1500 RPM

2000 RPM

Materials Testing

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

speedsUTS vs RPM

0.000

5.000

10.000

15.000

20.000

25.000

30.000

35.000

WA3 WB3 WC31 WC32

11 IPM

8 IPM

5 IPM

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 RPM5 IPM: Fz vs RPM

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1000 1500 2000

SFSW

FSW

8 IPM: Fz vs RPM

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1000 1500 2000

SFSW

FSW

11 IPM: Fz vs RPM

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1000 1500 2000

SFSW

FSW

Axial Force

• Axial Force independent of process or IPM1000 RPM: Fz vs IPM

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

5 8 11 14

SFSW

FSW

1500 RPM: Fz vs IPM

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

5 8 11 14

SFSW

FSW

2000 RPM: Fz vs IPM

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

5 8 11 14

SFSW

FSW

Moment

• Moment has discernible increase for IFSW vs. FSW

• Increase is from 2-5 Nm

• Weld pitch dependent2000 RPM : Mz vs IPM

0

5

10

15

20

25

30

5 8 11 14

SFSW

FSW

1500 RPM: Mz vs IPM

0

5

10

15

20

25

30

5 8 11 14

SFSW

FSW

Power

• Linear power increase required from FSW to IFSW

• Average increase of .5 kW required for similar parameters

Power (kW) vs IPM

0

0.5

1

1.5

2

2.5

3

3.5

4

5 8 11 14

IFSW

FSW

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

Heat Input vs IPM

0

50

100

150

200

250

5 8 11 14

IPM

Hea

t In

pu

t (k

J)

1000 RPM

1500 RPM

2000 RPM

Heat Input vs RPM

0

20

40

60

80

100

120

140

160

1000 1500 2000

5 IPM

8 IPM

11 IPM

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.