September 14, 2011David Speth, Senior Engineer-MaterialsEmail: dspeth@ewi.orgPhone: 614.688.5162

Battery Assembly: Joining Dissimilar Materials

Outline

Developing EV Market Joining Issues for Vehicle Batteries

─ Project with OSU Center for Automotive Research Ultrasonic Metal Welding Laser Welding Resistance Spot Welding Nondestructive Evaluation Summary and Acknowledgements

2011 Commercial EV and PHEV

Chevrolet VoltNissan Leaf

Tesla Roadster

GM Plans 50,000+ Volts

Nissan plans 200,000+ EVs

Tesla working on Model S

EVs 2011-2014Manufacturer Vehicle

Audi eTron EV (2012); PHEV (2014)

BMW MiniE EV (2012); City Car (2013)

BYD E6 EV (2012); F3DM PHEV (2012)

Coda Sedan EV (2011)

Chrysler/Fiat Fiat 500 EV (2012)

Fisker Karma EV (2011)

Ford Fusion HEV (commercial); Transit Connect EV (commercial); Escape HEV (commercial); Focus EV (2011); CMax PHEV (2013)

GM Volt PHEV (commercial); Ampera PHEV (2011); Cadillac SRX HEV (2012)

Honda Insight HEV (commercial); Civic HEV (commercial); Fit EV (2012)

Mazda Mazda 2 EV (2012)

Mitsubishi iMEV EV (2011)

Nissan Leaf EV (commercial); other platforms

Tesla Model S EV (2012);

Toyota Prius HEV (commercial); Prius PHEV (2012); RAV4 HEV (2012)

Volkswagen Eup EV (2013); Gold EV (2013); Jetta EV (2013)

Vehicle Electrification Challenge

Scale factor (size, capacity)─ Cell phone 4 W─ Laptop 80 W─ HEV 1,500 W─ PHEV 10,000 W─ EV 45,000 W

Design Life/Life Cycle Cost─ Cell phone 12-24

months─ Laptop 12-18 months─ HEV, PHEV, EV >120 months

New demands require new manufacturing industry

─ Working environment─ State-of-charge window─ Rapid charge and discharge

Relative Power

1

10

100

1000

10000

100000

Cell phone Laptop HEV PHEV EV

Application

Po

wer

(W

)

Cells to Modules to Packs Can be 100s to 1000s of electrical joints

per pack─ Bus bars─ Interconnects─ Collectors

Pouch/cell seal Voltage sensor leads Balance of plant

─ Motor connections─ Thermal management─ Battery management

Joining Issues No single process dominates

─ Ultrasonic─ Laser─ Resistance─ Soldering─ Adhesives

Complex material combinations─ Copper (native, plated)─ Aluminum─ Nickel─ Steel─ Dissimilar combinations

Need─ Speed─ High reliability─ Durability─ Low heat input─ NDE approach

Substrate Comparison

Property Cu Al NiThermal conductivity (W/m-°K) 390 229 70

Melting point (°C) 1080 652 1430

Thermal expansion coefficient (ppm/°C) 17.3 24.1 12

Heat capacity (J/kg-°C) 386 900 456

Absorption (at 1064 nm%) 2-5 8 32

Conductivity (106 S/m) 57 34 18

Resistivity (10-6 -cm) 2.11 2.87 9.5

Specific heat (J/kg/°K) 386 238 455

Latent heat of fusion (J/g) 205 388 298

Electrochemical potential (V) 0.34 -1.66 -0.257

Thermal Diffusivity (cm2/s) 1.14 0.91 0.11

Property mismatch makes direct welding difficult

OSU CAR EWI Welding Study

Process screening study for module/pack assembly─ Laser, resistance and ultrasonic metal welding─ Copper, aluminum, nickel, nickel-plated copper (electro- and

electroless-)─ Foil (0.001 in.); tab (0.005 in.); bus (0.032 in.)

Mechanical and electrical properties─ Shear strength─ Peel strength (T peel)─ Resistance/conductivity/thermal profile─ Metallography

Non-destructive evaluation/process monitoring Electrical cycling (OSU CAR) Mechanical fatigue (Phase 2)

Ultrasonic Metal Welding (UMW)

Oxides, Contaminants

Asperities

Advantages─ Solid-state, low heat input─ Welds through contaminants─ Low power─ No filler or cover gas─ Fast─ Excellent for Al, Ni, Cu

Disadvantages─ Unfamiliar process─ Lap joints, thin sheet only─ Deforms parts─ Large weld size─ Requires open access─ Noise─ Substrate-horn adhesion

Static Force

Sonotrode

Vibration

Workpieces

Anvil

Weld Zone

USMW Previous Results

No Cu-Cu bonding observed Ni-plating broken or thinned in

some areas, but never removed Profile of the horn and anvil are

important

Al 1100-0

Ni-plated Cu 110

Ni-plated Cu 110

Ni-plated Cu 110

Ni-plated Cu 110

Ni-plated Cu 110

Ni-plated Cu 110

Ni-plated Cu 110

Aluminum 1100

Ni-plated Cu 110

USMW OSU Preliminary Results

Tab to Bus─ Aluminum tabs to all bus materials (Al, Cu, and Ni-plated Cu) result

in weld joints with similar mechanical strength─ Ni-plated copper tabs to all bus materials-lower than expected peel

strength─ Copper tab to aluminum bus shows low peel but high tensile

strength Tab to Tab

─ Aluminum and copper join well─ Aluminum to other substrates less successful

Foil to tab─ USW can easily join multiple thin layers in a single step

USMW Tab to Tab

Peel Shear

Laser Welding

Lasers use a focused beam of light to create welds

Generic Set-Up for Direct Beam Laser Welding

Keyhole Mode Welding

Conduction Mode Welding

LW Advantages/Disadvantages

Advantages Precise location of small welds Low heat input Minimal distortion High speed Non-contact Can weld “shapes”

Disadvantages Laser cost $$ Need line-of-sight access Requires good fit-up, tooling Heating starts on the surface Limited weld penetration

especially on copper Makes fusion welds Welds very narrow Eye safety hazard

Nickel Plated Copper on Copper-Shaped Weld

LW Test Specimens

Laser Welds

LW Sample Cross Sections

Nickel-Electroplated Copper on Aluminum

Copper on Nickel

Aluminum on Nickel-Electroplated Copper-Voids

Aluminum welded to other metals produced the weakest welds

Incomplete mixing of metals

Resistance Spot Welding

Resistive heating of workpieces or electrodes

─ Common─ Adaptable─ Low cycle time and heat input─ Self-fixturing─ Self-monitoring equipment

Block diagram of AC welding system.

RSW Variants Solid state is preferred for

battery assembly Advantages

─ Rapid cycle time─ Low heat input─ Multiple welds easy─ Process monitoring possible

Disadvantages for batteries

─ Dissimilar metals ─ Low resistance─ High conductivity─ Current path can limit geometry─ Access can be limited

Electrodes or Welding Tips

Spot Weld

RSW Process Development

Produce a weld matrix to determine process limits

─ Current─ Time─ Force

Acceptance requirements─ Application defined─ Weld strength─ Weld size

ExpulsionAcceptableNuggets

Small Nuggets

Minimum Nugget Diameter

Weld Current

Weld Current

ExpulsionLevel

TimeA

Smaller“Brittle” Nuggets

AcceptableNuggets

LobeCurve

Nu

gg

et

Dia

me

ter

We

ld T

ime

Time A

RSW Tensile Shear Results

Weld force and current important for Al and Cu

Force and current become less important for Ni and Ni-plate

Weld time less important for al and cu becomes important for Ni plate

RSW Peel Test Results

Force, current, and time equally important for Al and Cu

Weld time becomes more important for Ni and Ni plate

Non Destructive Evaluation

ActivationEnergySource

Good Fusion

ActivationEnergy

ActivationEnergySource

Poor Fusion

ActivationEnergy

Can excite welds with external source.

NDE X-Ray vs Thermal Signature

Good WeldBad Weld

X-ray image showing weld nuggets

(controlled specimen)

Summary Batteries for motive power have numerous joints

─ Material combinations increase complexity Electrical testing is not sufficient to determine if a weld is good

─ Conductivity/resistance good even if weld is weak Several processes are used

─ Ultrasonic metal welding─ Excellent for Al, Cu, Ni─ Good for multiple layers─ Need to complete metallurgy and data analysis

─ Laser welding─ Flexible─ May be limited to like-to-like welds─ Need to look for intermetallic compound formation

─ Resistance Welding─ Most combinations can be welded─ Parameter selection can be based on like-to-like results─ Need to finish metallurgical analysis

Nondestructive evaluation approaches can be used for process development and perhaps production

Buckeye Bullet 2.5 August 2010

EWI Laser Work Cell Assembled Battery Packs

Buckeye Bullet “Hood Up”

Battery

New international land-speed record for battery-powered vehicles of 307.66 mph

http://blog.buckeyebullet.com/

Over 1500 Batteries

Acknowledgements

Support of the Department of Energy through the Ohio State University Center for Automotive Research─ DOE Award DE-EE0004188 ─ National Center of Excellence for Energy Storage 168.10

Team effort─ Tim Frech─ Mitch Matheny─ Jay Eastman─ Sam Lewis─ Warren Peterson─ Barb Christel─ Nancy Porter─ Mike Ryan

Questions?Dr. David SpethSenior Engineer-MaterialsEmail: dspeth@ewi.orgPhone: 614.688.5162