Joining Issues for Vehicle Batteries: Ultrasonic, Laser, Resistance Spot Welding

1. Battery Assembly: Joining Dissimilar Materials September 14, 2011 David Speth, Senior Engineer-Materials Email: dspeth@ewi.org Phone: 614.688.5162

2. 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

3. 2011 Commercial EV and PHEV Nissan Leaf Chevrolet Volt GM Plans 50,000+ Volts Nissan plans 200,000+ EVs Tesla working on Model S Tesla Roadster

4. EVs 2011-2014

5. 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

6. 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

7. 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

8. Substrate Comparison Property mismatch makes direct welding difficult

9. 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)

10. Static Force Sonotrode Vibration Weld Zone Workpieces Anvil Ultrasonic Metal Welding (UMW) • 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 Oxides, Contaminants Asperities

11. USMW Previous Results Aluminum 1100 Al 1100-0 Ni-plated Cu 110 • No Cu-Cu bonding observed • Ni-plating broken or thinned in some areas, but never removed • Profile of the horn and anvil are important 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

12. 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

13. USMW Tab to Tab Shear Peel

14. Laser Welding • Lasers use a focused beam of light to create welds Keyhole Mode Welding Generic Set-Up for Direct Beam Laser Welding Conduction Mode Welding

15. LW Advantages/Disadvantages 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 Advantages • Precise location of small welds • Low heat input • Minimal distortion • High speed • Non-contact • Can weld “shapes” Nickel Plated Copper on Copper-Shaped Weld

16. LW Test Specimens Laser Welds

17. LW Sample Cross Sections Aluminum on Nickel-Electroplated Copper-Voids Nickel-Electroplated Copper on Aluminum • Aluminum welded to other metals produced the weakest welds • Incomplete mixing of metals Copper on Nickel

18. 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.

19. 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

20. Expulsion Acceptable Nuggets Minimum Nugget Diameter Nugget Diameter Time A Small Nuggets Weld Current Lobe Curve Time A Weld Time Smaller “Brittle” Nuggets Expulsion Level Acceptable Nuggets Weld Current RSW Process Development • Produce a weld matrix to determine process limits • Current • Time • Force • Acceptance requirements • Application defined • Weld strength • Weld size

21. 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

22. RSW Peel Test Results • Force, current, and time equally important for Al and Cu • Weld time becomes more important for Ni and Ni plate

23. Non Destructive Evaluation Can excite welds with external source.

24. NDE X-Ray vs Thermal Signature Bad Weld Good Weld X-ray image showing weld nuggets (controlled specimen)

25. 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

26. Buckeye Bullet 2.5 August 2010 Buckeye Bullet “Hood Up” New international land-speed record for battery-powered vehicles of 307.66 mph Over 1500 Batteries http://blog.buckeyebullet.com/ EWI Laser Work Cell Assembled Battery Packs Battery

27. 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

28. Questions? Dr. David Speth Senior Engineer-Materials Email: dspeth@ewi.org Phone: 614.688.5162