Metallurgical Challenges in Joining Lightweight … Challenges in Joining Lightweight Dissimilar Materials ... Energy efficient welding processes ... Cu backing bar

Metallurgical Challenges in Joining

Lightweight Dissimilar Materials

Phil Prangnell & Joe Robson

School of Materials, University of Manchester, UK

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)

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

Fusion techniques

Resistance spot welding, GMAW, GTAW

Laser etc.

Intermetallic reaction

Qiu et al. Mater. Characterization 61 (2010)

Resistance spot welding

Al to Steel

10 μm

Al –Fe Phase diagram

IMC

Layer

Control of Interface Reaction

Thin as possible

< 200 nm

Ikeuchi, Yamamoto, Tikahashi, Aritoshi

Trans JWRI 34 (2005)

(Direct drive rotational friction welding)

Interface

cleavage Full Pullout

Al to Al (6111)

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4

Lo

ad

(kN

)

Displacement (mm)

0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4

Lo

ad

(kN

)

Displacement (mm)

Al – Steel

Target - Match Al to Al Joints (6111)

Low Joint

failure energy

Focus

Energy Efficient

Industrially viable - Spot Joining Methods

for aluminium – steel / magnesium

Current Spot Joining Methods for Al

Electrode

Component surface

Resistance Spot Welding (RSW)

+ Fast

- Electrode maintenance needed

- High energy costs ~ 50 kJ

~ 0.3 sec

Self Pierce Riveting (SPR)

+ Good mechanical properties

+ Join through adhesive

- High consumable costs

- Hard metals?

Low

< 0.5

sec

Energy/

Weld time

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

Ultrasonic Spot Welding (USW)

+ Very low energy

- Surface damage

< 1 kJ

< 0.5 sec

~ 2 - 4 kJ

2 – 5 sec

Laser Conduction Spot Welding

Uncoated Steel

AA6111T4 - DC04 steel

Supriyo Ganguly, Gonçalo Pardal, Sónia Meco

Ring

Clamp

Anvil

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

Specific point energy [kJ]

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

Inte

rme

talli

c la

ye

r th

ickn

ess [

m]

0

5

10

15

20

25

30

35

40

Cu backing bar

Water cooled plate

Steel

Al

Fe2Al5

FeAl3

Power density vs. Intermetallic layer thickness(spot size = 13 mm, interaction time = 3 s)

Power density [MW/cm2]

0.0022 0.0023 0.0024 0.0025 0.0026 0.0027 0.0028 0.0029

Inte

rme

talli

c la

ye

r th

ickn

ess [

m]

0

5

10

15

20

25

30

35

40

Cu backing bar

Water cooled plate

Laser Conduction Spot Welding

With – Thermal management

Water cooled plate

Copper

backing plate

IMC layer thickness

Laser Conduction Spot Welding

With – Thermal management Specific point energy vs. Shear strength

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

Specific point energy [kJ]

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

Shear

stre

ngth

[M

Pa]

0

20

40

60

80

100

120

140

Cu backing bar

Water cooled plate

Water cooled

plate

Copper

backing

plate

Steel

Al

IMC = 5 µm IMC = 35 µm

Cu Backing Bar Water cooled plate

Steel

Al

Lap Shear Strength

Temperature vs time - SP4

Time [s]

0 1 2 3 4 5 6

Tem

pera

ture

[°C

]

0

200

400

600

800

1000

Cu backing bar - Sample O9

Water cooled plate - Sample O15

Temperature

Laser Conduction Spot Welding

With – Thermal management Specific point energy vs. Shear strength

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

Specific point energy [kJ]

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

Shear

stre

ngth

[M

Pa]

0

20

40

60

80

100

120

140

Cu backing bar

Water cooled plate

Water cooled

plate

Copper

backing

plate

Steel

Al

IMC = 5 µm IMC = 35 µm

Cu Backing Bar Water cooled plate

Steel

Al

Lap Shear Strength

Laser Conduction Spot Welding

With – Thermal management Specific point energy vs. Shear strength

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

Specific point energy [kJ]

8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0

Shear

stre

ngth

[M

Pa]

0

20

40

60

80

100

120

140

Cu backing bar

Water cooled plate

Water cooled

plate

Copper

backing

plate

Cu Backing Bar Water cooled plate

Relative Lap Shear Strength

St

Al

St

Al

friction spot welded Al deck lid to the galvanized steel bolt retainer on the Mazda

MX-5. [Mazda Motor Corporation]

Solid State Welding Al-Steel

FSSW

Objective

Produce successful joints in short weld times

~ 1 second

In thin sheet ~ 1 mm thick

FSSW Al-Steel

Friction Stir Spot Joining Methods

3. Stitch, or Sweep

1. Standard Pin tool

2. Pinless tool

4. Refil

Pin displaces bottom sheet

Mechanical locking /hook

Exposure of fresh surface

Flow confined to soft top sheet

– abrades bottom?

or is this pure diffusion bonding?

Pin abrades bottom sheet

Currently not used with hard

materials?

Al

Fe

Standard 10 mm

diameter tool

Standard Pin Tool

WC tool Al

Fe

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

Uncoated Steel

Standard Pin Tool

Small Pullout Area -> longer times expands to include area

Under shoulder

Max Failure load ~ 80% Al-Al

Failure Energy ~ 40%

weld time 7 seconds!

0 2 4 6 8 10

1

2

3

1

2

3

Fra

ctu

re e

ne

rgy,

kN

.mm

Fa

ilure

lo

ad,

kN

Dwell time, s

Failure load

Fracture energy

Fe Al

Extent of mechanical locking is

limited in thin sheet

Standard Pin Tool

Interface temperature

bottom

top

200 µm

top

2 µm

bottom

1sec 5 sec 9 sec

IMC layer 1 µm

IMC layer 4 µm

Reaction Layer Thickness

Flat tool or tool with features

e.g. Wiper tool

Pinless Tool

6111T4 - DC04 steel

-0.8

-0.7

-0.6

-0.5

1.0

1.5

2.0

2.5

3.0

800

1200

1600

2000

Rotation speed, rpm

Plunging depth, mm

Failure load, kN 1.000

1.250

1.500

1.750

2.000

2.250

2.500

2.750

3.000

Not fully bonded

Reaction layer too

thick/ Top sheet

thinning

Optimum

Lap Shear Test Failure Loads

Max Failure load

80% Al-Al

Failure energy

~ 25% Al-Al

1 Sec Dwell time

1 Second dwell time

Top sheet

thinning

Reaction Behaviour

2 sec 5 sec

Wiper Tool 500 nm

Dwell time

1 sec

0

100

200

300

400

500

600

700

0 1 2 3 4 5

IMC

Laye

r Th

ickn

ess

(nm

)

Radial Distance From Center (mm)

Wiper tool 5 sec.

Flat tool 5 sec.

IMC Layer Thickness (nm)

Model: Courtesy of Aidan Reilly & H. Shercliff

Hot Dipped Zinc (DX56-Z )

Fe2Al5-xZnx

η(Zn-Fe)

5 µm

FSSW Zinc Coated Steels

Pinless tool

(a)

(b)

(c)

Plunge depth of 0.2 mm

Plunge depth of 0.4 mm

Plunge depth of 0.8 mm

FSW tool

Wiper tool (tool steel)

Flat shoulder profile (WC tool with coating)

Impossible to weld

1 second dwell

time

Very difficult

6111-T4/DX54Z

Zinc Coated Steels: Weld Envelope

Aluminium

Steel

Defect free

BUT

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

Defects

Shear Cracks

Disc pull out

7501000

12501500

17502000-0.2

-0.3

-0.4

-0.5

-0.61.5

2.0

2.5

3.0

3.5

Plunging depth, mm

Rotation speed, rpm

Failure load, kN

Aluminium to Zinc coated steel

Max Failure load 90% Al-Al

Failure energy 75% Al-Al

But too slow!

Total weld time > 6 seconds

Al Fe

Plunge

1 sec

Withdraw

Welding cycle

Lap Shear Test Failure Loads

6111-T4/DX54Z

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

1 Sec dwell time

6111-T4/DX54Z

6111-T4/DC04

1 sec,

1600 rpm

Effect of Zinc on flow Behaviour

Un-coated

Stick condition

Al

Steel

1600 rpm,-0.1mm,1s

1600 rpm,-0.3mm,1s

Al

Steel

Zn Coated

Slip condition

6111-T4/DX54Z

800 rpm ,-0.5mm, 1s

1600 rpm ,-0.5mm, 1s

0

100

200

300

400

500

0 10 20 30

Tem

pe

ratu

re (C

)

Time (sec.)

Peak Interface Temperatures

Al-Zn eutectic 381 °C

Weld Temperatures

2000 rpm ,-0.5mm, 1s

Centre = 406 °C

r/2 = 395 °C

Dispersion of Zn Coat

Zinc detected

Al-Zn eutectic

“Friction brazing”?

Fe

Al

IMC Reaction Layer

Fe2Al5-xZnx

Fe

Al

Fe2Al5-xZnx

200 nm

50 nm Fe2Al5-xZnx ~ 80 nm

Little change from Zn bath

Hot Dipped Zinc (DX56-Z )

Fe2Al5-xZnx

η(Zn-Fe)

5 µm

Abrasion Circle FSSW

ABC-FSSW

Pin abrades bottom sheet

Start

End

WC probes Tool steel

shoulder

10 mm Tool diameter 10 mm

Probe diameter 5 mm

Full width 15 mm

Pin trace area ~8 mm

Abrasion Circle” FSSW

5754H24

18.2s 3.64s 1.82s

60mm/min 300mm/min 600mm/min Travel speed

Rotation rate

1.1s 0.73s 0.55s 1.82s

1.1s 0.73s 0.55s 1.82s Travel speed

Rotation rate

2 mm

thick

1 mm 6111T4

to DC04 steel

800 rpm

Abrasion Circle” FSSW

0123456789

10

0 500 1000 1500 2000

Failu

re E

ner

gy (

kN.m

m)

Travel Speed (mm/min.)

6111T4 to DC04 steel

0

1

2

3

4

5

0 500 1000 1500 2000

Failu

re L

oa

d (

kN)

Travel Speed (mm/min.)

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

Fe

Al

“Dwell Time”

800 rpm

3.6 1.8 1.1 0.7 0.6 sec. “Dwell Time”

3.6 1.8 1.1 0.7 0.6 sec.

For 1 sec. Dwell Time

Max Failure load 100% Al-Al

Failure energy ~100% Al-Al

Failure Load Failure Energy

Abrasion Circle” FSSW

1 sec

Failure Comparison

FSSW Pinless tool

Optimised 1 Sec Dwell FSAW Circle Weld

1 Sec Dwell

Al-Steel

Interface Layer

Al Deformed

grains

Fe

parent

grains

interface

Fe

Fe 200 nm

200 nm

60mm/min

Fe

Al

USW Al to Steel Automotive Sheet

Al- Uncoated Steel

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 1000 2000 3000 4000 5000

Energy (J)

Lo

ad

(k

N)

1.4 kN

1.9 kN

10 um 10 um 10 um

2.4 kJ

3 Seconds

6111 - DC04

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 0.5 1 1.5 2 2.5 3 3.5 4

Time (Sec)

Lo

ad

(k

N)

1.4 kN

1.9 kN

Fracture Path

IMC

Modelling Interfacial Reaction

in Dissimilar Welds

1 um 3 Seconds

Al5Fe2

AlFe

AlFe3

Al

Fe

Al -Steel

‘Simple’ Predictions of IMC Layer Thickness

Al - Steel AA6111- DC04

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

500 C

10 µm 4 Hours 1 Hour 0.5 Sec

0 1 2 3 4 5 60

10

20

30

40

Expt.

Fit

d (

m)

t (hrs)1/2

k = 0.0397 m s-1/2 a

1.0 1.2 1.4 1.6 1.8-12

-10

-8

-6

-4

-2

0 Q = 117 KJ / mole

Expt.

Fit

ln (

k)

m s

-1/2

1000 / T(K)

b

500 C

Static Isothermal Kinetic data

Predictions of IMC Layer Thickness

450 C

Peak Temperature

500 C

400 C

6111 - Formable Steel DC04

500 C

450 C

400 C

Peak Temperature

IMC Layer thickness very

sensitive to peak temperature

Application to FSSW

0

20

40

60

80

100

120

140

160

180

200

0 1 2 3 4 5 6

IMC

Lay

er T

hic

knes

s (n

m)

Radial Distance From Center (mm)

500 nm

Dwell time

Predicted Max. Interface Temp.

Predicted Layer Thickness

Flat tool 3 sec.

Flat tool 1 sec.

2 sec 5 sec 1 sec

Inte

rfac

e Te

mp

erat

ure

C

300

320

340

360

380

400

420

440

460

480

500

-6 -4 -2 0 2 4 6

Peak T

emp.

at Int

erface

(C)

Radial Distance from Centre (mm)

340

360

380

400

420

440

460

480

500

Radial Distance from Centre (mm) 0 2 4 6

1 sec.

2 sec.

3 sec.

Application to FSSW

Advanced Model - IMC Layer growth

Al – Mg AA6111 –AZ31

Al-Mg

Mg-Mg

Al-Al

Application to Ultrasonic Spot Welding

2 mm

2 mm

Al Mg

Mg Mg

Al- Mg IMC Layer growth

5µm

5µm

5µm

5µm

10µm

10µm

10µm

10µm

Al

Mg

Al

M

g IMC

Al

Mg

Al12Mg17

Al3Mg2

0.3 s

0.5 s

0.7 s

1.0 s.

Welding

time

Advanced Model - IMC Layer Growth

Weld first forms

at asperities

Nucleation of of

first IMC island

Interface diffusion

controlled growth

1D diffusion

controlled growth

Al

Mg

Al

Fe

Inter diffusion

occurs

10µm

Al

Mg

Al12Mg17

Al3Mg2

mic

ron

Advanced Model – Example Predictions

Temperature in

weld cycle

Predicted vs

measured

layer thickness

Layer growth

in weld cycle

Preventing IMC Reaction in Dissimilar Welds

• Coatings

- Separate weld from dissimilar joint

- Diffusion Barrier coatings

• Inhibitors

Process

• Control of heat input

• Avoid liquid phase

Metallurgical

Mg Al

Coating

50µm

Cold Spray Al

Mg

Y-C Chen; Beijing Aeronatical

Joint failure energy

E.G. Cold Spray Coatings in Al-Mg USWs

Thick Al- Pre Coating- Separate Weld from Dissimilar Joint

Mg Al

Coating

50µm

Cold Spray Al

Mg 10µm

Thick Al- Pre Coating- Separate Weld from Dissimilar Joint

10µm 10µm

Coated Un-coated

Effect on Joint failure energy

E.G. Cold Spray Coatings in Al-Mg USWs

Thin Barrier Coating

2µm Mg

John Nicholls - Cranfield

PVD Mn coating ~ 0.9µm

Effect on Joint failure energy

Fa

ilure

En

erg

y k

N.m

m

Al Mg

Al

Mg

USW – 0.4 Sec

Mn Coatings in Al-Mg USWs

USW – 0.4 Sec

Effect on Joint failure energy

Fa

ilure

En

erg

y k

N.m

m

Thin Barrier Coating

Mn Coatings in Al-Mg USWs

2µm Mg

John Nicholls - Cranfield

PVD Mn coating ~ 0.9µm

USW – 0.4 Sec

2µm

PVD Mn coating ~ 0.9µm

Mg

10µm Al

Mg

10µm Al

Mg

USW – 0.4 Sec

Uncoated Coated

Al

Mg

5 µm

Break up

Effect on Joint failure energy

Fa

ilure

En

erg

y k

N.m

m

John Nicholls - Cranfield

Thin Barrier Coating

Mn Coatings in Al-Mg USWs

1.5 sec weld time

1 µm 1 µm

3 sec weld time

Fe2Al5-xZnx Passivated steel

1 µm 1 µm

Uncoated Steel

1.5 sec weld time 3 sec weld time

Al-Steel USWs

Inhibition of IMC Reaction

Can You Fix Interfacial Reaction in Fusion Welding??

Ranfeng Qiua, et al.

Mat. Sci Charact. 2010

P. Peyre et al. / Materials Science

and Engineering A 444 (2007)

327–338

Laser welding RSW welding

10 µm

e.g Out bursts occur by

diffusion through

passivation layer on

grain boundaries.

Potential for Improving Passivation Layers on Zn

Coated Steel for Fusion Welding to Aluminium

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

0.03–0.10 % 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.

LIGHT ALLOYS TOWARDS ENVIRNMENTALLY

SUSTAINABLE TRANSPORT