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Metallurgical Challenges in Joining Lightweight Dissimilar Materials Phil Prangnell & Joe Robson Acknowledgements Ying–Chun Chen, Lexi Panteli, Farid Haddadi, (Manchester) Stewart Williams, Supriyo Ganguly, Gonçalo Pardal, Sónia Meco (Cranfield) Hugh Shercliff, Aidan Reilly et al. (Cambridge– Dept. Engineering) Nick Wright, Mike Shergold (JLR), Tim Wilkes, Bruce Davies (MEL), A. Smith (Tarta), Tym Burman (Novelis)

School of Materials, University of Manchester, UK


LATEST2 – Priority Areas  Energy efficient welding processes  Joining dissimilar metals  Composite to metal joining  Integrated modelling - material interactions and joint performance  Surface engineering for enhanced performance


Welding Multi-material Structures Al –Fe Phase diagram

Fusion techniques Resistance spot welding, GMAW, GTAW Laser etc.

Intermetallic reaction

Resistance spot welding Al to Steel

10 μm

IMC Layer

Qiu et al. Mater. Characterization 61 (2010)


Control of Interface Reaction Thin as possible < 200 nm

Ikeuchi, Yamamoto, Tikahashi, Aritoshi Trans JWRI 34 (2005)

(Direct drive rotational friction welding)


Target - Match Al to Al Joints (6111) Al â&#x20AC;&#x201C; Steel

3.5

3.5

3

3

2.5

2.5

Load (kN)

Load (kN)

Al to Al (6111)

2 1.5 1

1.5 1

0.5

0.5

0

0 0

1

2

3

Displacement (mm)

Full Pullout

4

Low Joint failure energy

2

0

1

2

3

Displacement (mm)

Interface cleavage

4


Focus

Energy Efficient Industrially viable - Spot Joining Methods for aluminium â&#x20AC;&#x201C; steel / magnesium


Current Spot Joining Methods for Al Resistance Spot Welding (RSW)

Energy/ Weld time

+ Fast - Electrode maintenance needed - High energy costs

~ 50 kJ Component surface

~ 0.3 sec Electrode

Self Pierce Riveting (SPR) + Good mechanical properties + Join through adhesive - High consumable costs - Hard metals?

Low < 0.5 sec


Alternative Spot Joining Methods Fusion: Rapid thermal Cycle Laser Conduction Spot Welding + Fast + low heat input + Excellent surface quality - Low efficiency

~ 40 kJ < 1sec Steel Al


Alternative Spot Joining Methods Solid state Friction Stir Spot Welding (FSSW) + Low energy - Keyhole - Slow ~ 2 - 4 kJ 2 â&#x20AC;&#x201C; 5 sec

Ultrasonic Spot Welding (USW) + Very low energy - Surface damage

< 1 kJ

< 0.5 sec


Laser Conduction Spot Welding Ring Clamp

Anvil

Uncoated Steel AA6111T4 - DC04 steel

Supriyo Ganguly, Gon莽alo Pardal, S贸nia Meco


Laser Conduction Spot Welding With â&#x20AC;&#x201C; Thermal management

Specific point energy vs. Intermetallic layer thickness (spot size = 13 mm, interaction time = 3 s)

IMC layer thickness 40

Intermetallic layer thickness [ď ­m]

35

Steel 30

Water cooled plate

25

Fe2Al5 20

FeAl3

Copper backing plate

15 10

Al

5 0 8.5

9.0

9.5

10.0

10.5

11.0

Specific point energy [kJ] Cu backing bar cooledvs. plateIntermetallic PowerWater density

layer thickness (spot size = 13 mm, interaction time = 3 s)

11.5

12.0


Laser Conduction Spot Welding With – Thermal management Specific point energy vs. Shear strength (spot size = 13 mm, interaction time = 3 s)

Lap Shear Strength

140

Water cooled plate

Cu Backing Bar Steel

Steel

Al

Al

100

80

60

Copper backing plate

Water cooled plate

40

20

IMC = 5 vsµm Temperature time - SP4

0 8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

Specific point energy [kJ]

IMC = 35 µm

1000

Cu backing bar Water cooled plate

Temperature

800 Temperature [°C]

Shear strength [MPa]

120

600 400 200 0 0

1

2

3 Time [s]

4

5

6


Laser Conduction Spot Welding With – Thermal management Specific point energy vs. Shear strength (spot size = 13 mm, interaction time = 3 s)

Lap Shear Strength

140

Cu Backing Bar

Water cooled plate

Shear strength [MPa]

120

Steel

Steel

Al

Al

100

80

60

Copper backing plate

Water cooled plate

40

20

0 8.5

9.0

9.5

10.0

10.5

11.0

Specific point energy [kJ] Cu backing bar Water cooled plate

11.5

12.0

IMC = 5 µm

IMC = 35 µm


Laser Conduction Spot Welding With â&#x20AC;&#x201C; Thermal management Specific point energy vs. Shear strength (spot size = 13 mm, interaction time = 3 s)

Relative Lap Shear Strength 140

Water cooled plate

Cu Backing Bar

Shear strength [MPa]

120

100

80

60

Copper backing plate

Water cooled plate

40

St

St

Al

Al

20

0 8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

Specific point energy [kJ] Cu backing bar Water cooled plate


Solid State Welding Al-Steel FSSW

friction spot welded Al deck lid to the galvanized steel bolt retainer on the Mazda MX-5. [Mazda Motor Corporation]


FSSW Al-Steel Objective Produce successful joints in short weld times ~ 1 second In thin sheet ~ 1 mm thick


Friction Stir Spot Joining Methods 1. Standard Pin tool Pin displaces bottom sheet Mechanical locking /hook Exposure of fresh surface

Al Fe

2. Pinless tool Flow confined to soft top sheet â&#x20AC;&#x201C; abrades bottom? or is this pure diffusion bonding?

3. Stitch, or Sweep Pin abrades bottom sheet

4. Refil Currently not used with hard materials?

Standard 10 mm diameter tool


Standard Pin Tool

Al Fe

Uncoated Steel

Standard 10 mm diameter tool 2000 rpm, plunge depth -1.6 mm, 1s, plunge rate 50 mm/min, withdraw rate 50 mm/min

AA6111T4 - DC04 steel

WC tool


3

3

2

2

1

0

Failure load Fracture energy

2

4

6

8

1

Fracture energy, kN.mm

Failure load, kN

Standard Pin Tool

Al Fe

Extent of mechanical locking is limited in thin sheet

10

Dwell time, s

Max Failure load ~ 80% Al-Al

Failure Energy ~ 40% weld time 7 seconds!

Small Pullout Area

-> longer times expands to include area Under shoulder


Standard Pin Tool Interface temperature top

bottom 200 µm

Reaction Layer Thickness top

1sec

5 sec

9 sec IMC layer 4 µm

bottom

IMC layer 1 µm

2 µm


Pinless Tool

Flat tool or tool with features

e.g. Wiper tool

6111T4 - DC04 steel


Lap Shear Test Failure Loads 1 Second dwell time Failure load, kN

1.000 1.250 1.500 1.750 2.000 2.250 2.500 2.750 3.000

3.0 2.5 2.0

ď ą Max Failure load 80% Al-Al

1.5

Top sheet thinning

Failure energy

1.0 -0.8

~ 25% Al-Al

-0.7 2000

Plunging depth, mm

1 Sec Dwell time

1600

-0.6

1200 Rotation

speed, rpm

-0.5 800

Optimum Not fully bonded

Reaction layer too thick/ Top sheet thinning


Reaction Behaviour Dwell time 1 sec

2 sec

5 sec

Wiper Tool

500 nm

IMC Layer Thickness (nm)

IMC Layer Thickness (nm)

700

600

Wiper tool 5 sec.

500 400

Flat tool 5 sec.

300 200 100 0 0

1

2

3

4

5

Radial Distance From Center (mm)

Model: Courtesy of Aidan Reilly & H. Shercliff


FSSW Zinc Coated Steels

Hot Dipped Zinc (DX56-Z ) Pinless tool

5 µm

η(Zn-Fe)

Fe2Al5-xZnx


Zinc Coated Steels: Weld Envelope 6111-T4/DX54Z

(a)

Plunge depth of 0.2 mm

Aluminium Steel

Defects 1 second dwell time

(b)

Plunge depthShear of 0.4 mm Cracks Disc pull out

(c)

Defect free of 0.8 mm Plunge depth

FSW tool

Impossible to weld

Wiper tool (tool steel) Flat shoulder profile (WC tool with coating) BUT Plunge rate 10 mm/min - retraction rate 5 mm/min

Very difficult


Lap Shear Test Failure Loads Aluminium to Zinc coated steel 6111-T4/DX54Z Failure load, kN

Al

3.5

Fe

3.0 2.5 2.0 1.5 -0.6 -0.5 -0.4

Plunging depth, mm -0.3 -0.2 2000

1750

1500

1250

1000

750

1 Sec dwell time

Rotation speed, rpm

ď ą Max Failure load 90% Al-Al

Welding cycle

Failure energy 75% Al-Al But too slow!

Plunge

Withdraw 1 sec

plunge rate 10 mm/min - retraction rate 5 mm/min

Total weld time > 6 seconds


Effect of Zinc on flow Behaviour 6111-T4/DC04

Un-coated 1600 rpm,-0.1mm,1s

Al Steel

6111-T4/DX54Z

1600 rpm,-0.3mm,1s

1 sec, 1600 rpm

Stick condition 6111-T4/DX54Z

Zn Coated 800 rpm ,-0.5mm, 1s

Al Steel 1600 rpm ,-0.5mm, 1s

Slip condition


Weld Temperatures

Peak Interface Temperatures 500

Temperature (C)

Centre = 406 °C 400

Al-Zn eutectic 381 °C

r/2 = 395 °C

300 200 100

0 0

10

Time (sec.) 2000 rpm ,-0.5mm, 1s

20

30


Dispersion of Zn Coat Zinc detected

Al-Zn eutectic

“Friction brazing”?


IMC Reaction Layer Hot Dipped Zinc (DX56-Z ) Al 5 µm

η(Zn-Fe)

Fe2Al5-xZnx

Fe2Al5-xZnx

Fe

Al 200 nm

Fe2Al5-xZnx

Fe2Al5-xZnx ~ 80 nm Little change from Zn bath

Fe

50 nm


Abrasion Circle FSSW ABC-FSSW

Pin abrades bottom sheet


Abrasion Circleâ&#x20AC;? FSSW Tool steel shoulder

WC probes

Start End

Full 10 width mm 15 mm Pin trace area ~8 mm

Tool diameter 10 mm Probe diameter 5 mm


Abrasion Circleâ&#x20AC;? FSSW Travel speed 60mm/min Rotation rate 18.2s

300mm/min

3.64s

600mm/min

1.82s

5754H24 2 mm thick

800 rpm

Travel speed Rotation rate 1 mm 6111T4 to DC04 steel

1.82s

1.82s

1.1s

1.1s

0.73s

0.73s

0.55s

0.55s


Abrasion Circle” FSSW “Dwell Time” 1.8

1.1

“Dwell Time” 0.6 sec.

Failure Load

4

Failure Load (kN)

0.7

Failure Energy (kN.mm)

5

3.6

3 2 1 0 0

800 rpm

500

1000

1500

2000

Travel Speed (mm/min.)

60 mm/min 300 mm/min 2000 mm/min

10 9 8 7 6 5 4 3 2 1 0

3.6

1.8

1.1

0.7

0.6 sec.

Failure Energy

1 sec 0

500

1000

1500

2000

Travel Speed (mm/min.)

6111T4 to DC04 steel

Fe For 1 sec. Dwell Time Max Failure load 100% Al-Al Al

Failure energy ~100% Al-Al


Failure Comparison FSSW Pinless tool Optimised 1 Sec Dwell

Al-Steel

FSAW Circle Weld 1 Sec Dwell


Interface Layer 60mm/min

Al Fe

interface Deformed grains

Al

Fe Fe

200 nm

Fe parent grains

200 nm


USW Al to Steel Automotive Sheet


Al- Uncoated Steel 0

Load (kN) Load(kN)

6111 - DC04

2.4 kJ

10 um

IMC

Fracture Path

10 um

1

4.5 4.5 4 3.5 3.5 33 2.5 2.5 2 2 1.5 1.5 1 1 0.5 0.50 0

3

3.5

4

1.4 kN kN 1.4 1.9 kN kN 1.9

0

3 Seconds

0.5

Time (Sec) 1.5 2 2.5

1000

2000 3000 Energy (J)

10 um

4000

5000


Modelling Interfacial Reaction in Dissimilar Welds

Al -Steel 1 um

3 Seconds

Al

Al5Fe2 AlFe AlFe3

Fe


‘Simple’ Predictions of IMC Layer Thickness Static Isothermal Kinetic data Al - Steel AA6111- DC04

1 Hour

0.5 Sec

4 Hours

40

10 µm

0 Q = 117 KJ / mole

a

-1/2

k = 0.0397 m s

b

-2 -1/2

500 C

ln (k) m s

500 C

d (m)

30

20

-4 -6 -8

10 -10 Expt. Fit

0 0

1

2

3

4

5

6

1/2

t (hrs)

Parabolic growth law d =C1 exp (-Q/RT) t1/2

-12 1.0

Expt. Fit

1.2

1.4

1000 / T(K)

1.6

1.8


Application to FSSW

6111 - Formable Steel DC04

Predictions of IMC Layer Thickness Peak Temperature 500 C 450 C 400 C

500 C Peak Temperature

IMC Layer450 thickness very C sensitive to peak temperature 400 C


Application to FSSW Dwell time 1 sec

5 sec

2 sec

500 nm

Predicted Layer Thickness

Interface Temperature ď&#x201A;°C

500 500 480 480 460 460

1 sec. 2 sec. 3 sec.

440 440 420 420 400 400 380 380 360 360 340 340 320 300

-2

IMC Layer Thickness (nm)

Predicted Max. Interface Temp.

0 0

2 2

4 4

Radial Distance from Centre (mm)

Distance from Centre (mm)

6

6

200 180 160 140 120 100 80 60 40 20 0

Flat tool 3 sec.

Flat tool 1 sec.

0

1

2

3

4

5

Radial Distance From Center (mm)

6


Advanced Model - IMC Layer growth Application to Ultrasonic Spot Welding

Al-Al

Al – Mg AA6111 –AZ31

Mg

Mg

Mg-Mg Al-Mg 2 mm

Al

Mg

2 mm


Al- Mg IMC Layer growth Welding time

Mg

0.3 s 5µm

Al 10µm

0.5 s

5µm

10µm

M g IMC

Al

0.7 s

5µm

Mg

10µm Al12Mg17

1.0 s. 10µm

Al3Mg2 5µm

Al


Advanced Model - IMC Layer Growth Weld first forms at asperities

Mg 10Âľm

Al

Inter diffusion occurs

Al

Fe

Mg

Nucleation of of first IMC island

Interface diffusion controlled growth

Al12Mg17

Al3Mg2 Al

1D diffusion controlled growth


Advanced Model â&#x20AC;&#x201C; Example Predictions

micron

Temperature in weld cycle

Layer growth in weld cycle

Predicted vs measured layer thickness


Preventing IMC Reaction in Dissimilar Welds Process • Control of heat input • Avoid liquid phase

Metallurgical • Coatings - Separate weld from dissimilar joint - Diffusion Barrier coatings • Inhibitors


Thick Al- Pre Coating- Separate Weld from Dissimilar Joint E.G. Cold Spray Coatings in Al-Mg USWs

Joint failure energy

Al Coating

Mg

Cold Spray Al 50Âľm

Mg

Y-C Chen; Beijing Aeronatical


Thick Al- Pre Coating- Separate Weld from Dissimilar Joint E.G. Cold Spray Coatings in Al-Mg USWs

Effect on Joint failure energy

Mg

Al Coating

Cold Spray Al 50µm

Un-coated

Coated

Mg 10µm

10µm

10µm


Thin Barrier Coating Mn Coatings in Al-Mg USWs

2µm

Mg

PVD Mn coating ~ 0.9µm

Mg

John Nicholls - Cranfield

Failure Energy kN.mm

Effect on Joint failure energy

Al

USW – 0.4 Sec Mg Al


Thin Barrier Coating Mn Coatings in Al-Mg USWs

2µm

Mg

PVD Mn coating ~ 0.9µm

Failure Energy kN.mm

Effect on Joint failure energy

John Nicholls - Cranfield

USW – 0.4 Sec USW – 0.4 Sec


Thin Barrier Coating Mn Coatings in Al-Mg USWs

2µm

Mg

PVD Mn coating ~ 0.9µm

Failure Energy kN.mm

Effect on Joint failure energy

John Nicholls - Cranfield

USW – 0.4 Sec Uncoated

Coated Mg

Mg

Al

10µm

Al

Mg

10µm

Break up

Al 5 µm


Inhibition of IMC Reaction Al-Steel USWs Uncoated Steel 1.5 sec weld time

1 µm

3 sec weld time

1 µm

Fe2Al5-xZnx Passivated steel 1.5 sec weld time

1 µm

3 sec weld time

1 µm


Can You Fix Interfacial Reaction in Fusion Welding?? Laser welding

P. Peyre et al. / Materials Science and Engineering A 444 (2007) 327–338

RSW welding

10 µm

Ranfeng Qiua, et al. Mat. Sci Charact. 2010


Potential for Improving Passivation Layers on Zn Coated Steel for Fusion Welding to Aluminium e.g Out bursts occur by diffusion through passivation layer on grain boundaries.

Doping of Zn bath with rare earth mischmetal is known to inhibit outbursts and reaction between Fe and liquid Zn. e.g. Galfan coated Steel 0.03â&#x20AC;&#x201C;0.10 % Cerium + lanthanum


Summary  The thickness of the intermeteallic reaction layer is a key issue in dissimilar metal welding even with solid state processes. => Poor joint failure energies  The reaction layer thickness can be reliably predicted with kinetic models.  Novel friction welding techniques can be used to successfully weld Al and dissimilar metal combinations with very high energy efficiency.  Rapid fusion processes, with careful thermal management, are viable.  There is potential to use pre-coatings and tailor galvanising on steel to inhibit reaction - allowing more flexible fusion welding.

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