Tool- and Preheat Coil- Offset in Dissimilar Friction Stir Welding

Adonyi, Y., Rutledge, J., Wang, P.LeTourneau University, Longview, TX

DEFINITIONS and CLARIFICATIONS

•LeTourneau University is in Texas, not Canada

FSW setup:•Offset: distance from the joint centerline•Tool tilt angle used: 1° forward

•Dissimilar: Copper OFHC + Aluminum Al 6061-T6 0.25-in thick plates in butt-joint configuration

Project History•Industry request for wide plate Cu/Al transition pieces for electrical power transmission (round pieces Friction Welded for decades)

•Undergraduate Senior Design funded by the School of Engineering at LeTourneau University

•Graduate R&D components added later•Thanks to: Nathan Dix, Kevin Peluso, Collin Overstreet, Zach Danko, and others

Definitions: Tool Pin Offset from Centerline (towards the Al side)

Al Cu

Advancing sideRetreating side

Setup: HF 50 kVA 3 kHz Coil – 1.0” on the leading side of the H 13 tool

Weld Travel Cu Al

H-13 Steel FSW Tool Geometry

Shank: 1.0’’ Dia, scrolled shoulder

Pin: 5.6 mm, Dia. 5.0 mm height, helical

Setup: Coil Offset towards Cu side, 3 kHz, 85% of 50 kVA

Cu Al

Problem Definition, Cu+Al welds

•Brittle Intermetallics•Possible Solutions: pin offset, tool geometry

•Different Flow Stresses•Different melting points and conductivities

•Possible solutions: HFI preheating, pin offset

•Liquation Cracking•Segregation and localized melting

•Possible solution: pin offset

Definitions: Cu-Al intermetallics: Likely θ vs. δ (Al-rich vs Cu-rich)

“Al-rich” CuAl intermetallic

“Cu-rich” CuAl intermetallic

Initial Findings•Optimized parameters established for Cu-Al

•Range: 1,700-2,580 lbs force, 800-1,400 RPM, 2-8 IPM travel speed, H 13 tool steel

•1° Tool Forward Tilt angle

-> Best parameters: 1,250 RPM, 2,500 lbs, 3 IPM•Decided to offset the pin and use HF preheat to further optimize the FSW weld quality

•Initiated FEA modeling using COMSOL to better understand process fundamentals and further optimize weld quality

OBJECTIVES•Predict FSW thermal cycles in Cu+Al dissimilar joints, with pin and coil offset•Produce thermal symmetry between the parts by offsetting the tool pin and HF preheat•Optimize Al+Cu Friction Stir Weld quality based on destructive testing

FEA Assumptions: Frictional Coefficient Change with Temperature

Steady State Assumption: COMSOL Predicted Temperatures

Steady-state

Steady State: Experimental Data, Downforce, Temperature

0 50 100 150 200 250 3000

200

400

600

800

1000

1200

1400

1600

1800

2000

Time (s)

Downforce (lbs)

Plunge

Dwell

Steady- State

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

T_CuT_Al

Time (seconds)

Temperature (degC)

Plunge

Dwell

Steady- State

COMSOL predictions – Reciprocal of Thermal resistance

Predicted Isotherms

Heat Transfer Coefficient as a function of position from the tool center.

COMSOL predictions: Thermal insulation from steel anvil

Steel block Concrete block

Roles: 1) limit the heat transfer to the anvil, 2) increase the average weld temperature

New Fixture built to Constrain Concrete Block

Experimental Setup

Data Acquisition and Controls

Design of Experiments for FEA validation only

• Force: 1,400 – 2,600 lbs• Rotational speed: 529 – 1,250 RPM• Travel speed: 1.63 – 5.55 IMP• Weld success or failure was predicted based on maximum in-weld temperatures

• The welds predicted to fail based on insufficient heat-input to plasticize the copper, failed destructive testing, and deviated significantly from the FEA model.

RESULTS• FEA Predictions vs. actual temperatures• Pin offset effects on weld temperature• Coil offset effects on microstructure• Metallography and destructive testing

Thermocouple validations of IR camera data (in addition to Gleeble)

Al side, TC 1 visible, TC 2 underneath tool, Al side

Weld time, sec

Temper

atur

e, d

eg

C

FEA validation using Thermocouples

Al side

CM- COMSOL predicted temp

TC – Thermocouple temps

Cu side: Predicted vs. Actual Temps

+/- 11°C error

(excluding the intentionally out of range welds)

ACTUAL TEMPERATURE, deg C

PREDICTE

D TE

MPER

ATUR

E, d

eg C

AL side: Predicted vs. Actual Temps+/- 5.25°C error

(except for the intentionally out of range welds)

ACTUAL TEMPERATURE, deg C

PREDICTE

D TEMP

ERAT

URE,

deg

C

FEA Analysis: Discussion•Current model is accurate to within a couple of percent with frictional coefficient correction factor.

•Model proved accurate enough for predicting weld conditions to validate implementation of ceramic anvil.•Limiting the contact resistance influence.

•Increasing maximum temperatures.•Model should be sophisticated by adding metal flow predictions

Pin offset – Predicted power

• Pin offset can be used to control the ratio of mixing between the copper and the aluminum in a dissimilar FSW.

• Offsetting towards the aluminum results in lower heat inputs into the copper and vice-versa, but the total heat input remained relative constant

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

q_Cu (W)q_Al(W)q_Tot (W)

Cu side % Pin Offset Al

side

Pow

er,

W

Total

FLIR Validation – symmetrical temperature distribution•Designed and installed a camera mount for the FLIR IR camera

•FLIR Tools allows recording and playback of all welds including IR data

Isotherms at the weld end

HF Coil Offset Welds•Preheat: 100°C•Baseline: “Ideal” weld parameters

•Weld #2: 6 ipm•Hi recorded with thermal camera

•Three setups:•Completely over Cu

•2/3 over Cu•½ Cu, ½ Al

Coil offset effect, Cu vs Al side

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

400

CopperAluminum

0 20 40 60 80 100 120 140 160 180 2000

50

100

150

200

250

300

350

400

CopperAluminum

85% on Cu side

100% on Cu sideWeld time, sec

HFI Coil Offset Welds: Analysis•Identical analysis procedures as pin offset weld•#1 – temp difference•#2 – failed•#3 – minimal temperature differential

•Good mixing throughout the entirety of the welds

Cu-rich intermetallics

Al-rich intermetallics morphology

EDS spectra, Al-rich intermetallics

Al-rich intermetallics•High RPMs also resulted in Al- rich intermetallics.

5x 100x

Destructive Testing, Longitudinal Bend•FSW with Al-rich intermetallics

•FSW with Cu-rich intermetallics

FSW Weld Hardness, HV 300g load

Cu BM Cu side FSW Al side FSW Al BM0

50

100

150

200

250Average Hardness, HV

Al-rich

NDE: Eddy Current Testing, 500 kHzElectrical conductivity map (proportional with hardness)

•Cu base metal: 100 IACS%•Al base metal: 44.8 IACS%•Al-rich intermetallic: 29 IACS%(Cu-rich intermetallics: 26 IACS%)

FSW weld containingAl-richintermetallics

Al

Cu

Conclusions I•The FEA model accurately predicted heat generation even w/o mass flow

•Better mixing between Cu and Al reduce the amount of Cu-rich brittle intermetallics via pin and coil offset.

•Higher temperatures (RPMs) also produced ductile Al-rich intermetallics.

Conclusions II•The roles of temperature- and metal mixing- on Cu/Al FSW microstructure and properties were separated using a combination of FEA simulations and experiments

•New FSW microstructure with improved conductivity and better ductility (containing Al-rich intermetallics) was identified and consistently reproduced

•Tool pin- and preheat coil- offset proved essential in controlling the above, hence the long title

Future Research

•Improve pin-placement/offset control

•Monitor tool torque and temperature•Improve FEA model power input•Quantify intermetallic composition (X ray Diffraction)

•Couple mixed mass model with temperature model

•Funding needed!