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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
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 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