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Joining Dissimilar Materials

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1 Joining Dissimilar Materials
Battery Assembly: Joining Dissimilar Materials September 14, 2011 David Speth, Senior Engineer-Materials Phone:

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 GM Plans 50,000+ Volts
Nissan Leaf Chevrolet Volt GM Plans 50,000+ Volts Nissan plans 200,000+ EVs Tesla working on Model S Volt is PHEV with 40 mile nominal electric range. Market price is about $40,000 with numerous state and Federal incentives for early adopters. GM has low lease rates to encourage potential buyers to lease instead of buy. Leaf is electric vehicle with 100 mile nominal range. Market price is about $32,500. Tesla has more than 6800 cells because it was designed when the only lithium cells commercially available were cylindrical cells designed for laptop computers. The range of the Tesla EV is more than 250 miles. Cost is over $100,000. GM and Nissan are committed to the EV market. Both have announced plans to manufacture more than 50,000 vehicles in 2012 even though sales in 2011 have been less than 1,000 vehicles per month due mostly to limited battery pack manufacturing capacity. Tesla Roadster

4 EVs 2011-2014 Manufacturer 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) Almost every major automotive manufacturer has plans to introduce a hybrid, plug-in hybrid or electric vehicle between now and Several start ups are also planning to bring vehicles to market. HEV have about cells/pack. Operational demand on the battery is not high for most systems. Nickel-metal hydride batteries can compete for these vehicles, but lithium batteries will gain market share. PHEV have cells per pack. EV have up to 300 cells per pack (range dependent). Cell manufacturers have capacity coming on line in 2011 and 2012 supported by the Federal government grants as part of the ARRA program in 2010. To get a return on investment, manufacturers will need to develop lower cost manufacturing and higher reliability processes. (Note that some estimates are that 30-40% of cells do not pass the first quality check.) Pike research anticipates cumulative PHEV + EV sales 5.2M by 2017 (up from 114,000 in 2011) with an additional 8.7M HEV.

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 months Laptop months HEV, PHEV, EV >120 months New demands require new manufacturing industry Working environment State-of-charge window Rapid charge and discharge Lithium cells have been manufactured for cell phones, music players and laptops for several years. These cells are much smaller than those required for automotive applications and the performance requirements are not as stringent as those for automotive. For example, a cell phone or laptop battery is expected to last about months (max). Replacement may cost about $200 for a laptop. Automotive packs cost about $10,000 in Consumers will not be willing to replace packs every months. Cost is a major issue. To achieve a reasonable payback period for vehicle electrification, battery cost will need to be less than half what it is today. The service life of the pack must also be extended to at least 10 years in the vehicle for PHEV and EV to be successful in the broader market. HEV all NiMH batteries today. Economically these batteries have performance window that is a good fit for HEV requirements. The market will shift to Li as costs decline. Market predictions anticipate about 35% of HEV will be lithium by 2020. Battery companies have done a good job inventing new chemistry to improve cell performance. They have been less successful at designing cells and modules for manufacturing.

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 There are 100s or 1000s of electrical joints per pack. This include joints in the cell between the foil (+/-0.001” thick) current collectors and the tabs (0.005” thick), tab to tab connections and tab to bus (>0.025” thick) connections. Since many of these are in series, all must be good for the cell, module, and pack to function properly. This project focused on foil to tab, tab to tab and tab to bus connections. Most of the results to be presented will be on tab-to-tab joints.

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 EWI has been working with consumer battery and automotive companies for many years. As the automotive industry began to develop electrified vehicles, we noted that large scale manufacturing of lithium batteries will require the development of fast, inexpensive and reliable joining processes. As part of a Symposium on battery manufacturing EWI did a survey of attendees about their concerns related to battery manufacturing. The results of the survey are captured here. Lithium batteries are complex assemblies. There are several joining processes involved and these processes must be capable of joining a wide range of materials. This includes making joints between dissimilar materials like nickel, aluminum, and copper. Reliability (at 6σ % yield) 1/2000 packs will fail. Modules and packs have a wide range of designs so joints have varying geometries. Packs must have 10+ year service life to be economically viable. Joints must not impede current flow either in operation or during fast charging. Most companies do not admit to any NDE beyond electrical continuity testing. Is this sufficient? 7

8 Substrate Comparison Property Cu Al Ni
Thermal 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) 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 These substrates also present challenges because their physical properties are very different (particularly the low melting point of aluminum). This makes dissimilar combinations difficult to assemble. Copper and Aluminum form a range of intermetallic compounds Problem for electronics industry Shows up in wire bonding/encapsulation Major problem for fusion welds Has been observed in solid state welds Composition variable CuAl2; CuAl; Cu4Al3; Cu3Al2; Cu9Al4 These compounds are brittle and low electrical conductivity Evidence shows phases can grow over hours to days at 150°C Reports of growth at temperatures as low as 120°C Reports that growth is influenced by electric current Nickel plating may prevent intermetallic formation and aging Adds process step and cost 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) The study here was designed in conjunction with The Center for Automotive Research at The Ohio State University. Working with faculty members Giorgio Rizzoni and Yann Guezenec, we devised a program to evaluate welding of copper, aluminum, nickel, and nickel-plated copper combinations appropriate for the assembly of battery cells, modules and packs. The mechanical and electrical properties of each weld were determined and the results were analyzed to help manufacturers select the best welding process and substrate for their product. EWI also explored a non destructive technique for monitoring weld quality. OSUCAR designed and is testing the cycling performance of small battery packs to determine how much impact the welded connection has on pack performance. Ultimately we would like to characterize the fatigue performance of these joints, but that was beyond the scope of the present study. Let’s look at each process in turn starting with ultrasonic metal welding and proceeding to laser and resistance welding. I will finish with a brief description of our efforts to develop NDE and a summary.

10 Ultrasonic Metal Welding (UMW)
Static Force Sonotrode Vibration Workpieces Anvil Weld Zone 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 Ultrasonic metal welding is becoming very popular in several assembly industries. It has several advantages. Since it is a solid state process, it can be adapted to dissimilar materials combinations and avoids most concerns about formation of intermetallic compounds. It is ideally suited to welding the highly conductive materials used in batteries including plated copper. On the other hand it has limitations. Since high forces are involved, it creates large deformations in the weld zone. This looks bad and can create residual stresses. It is also a relatively new process that manufacturers do not have confidence in. The process is relatively simple. US energy (10,000 Hz+) is used to rub two parts together. This scrubbing breaks off oxide and contamination on the surface and rubs off surface asperities creating two ‘smooth’ clean metal surfaces. Once these contact, a weld is formed. This understanding is common in the adhesion community. Adhesive scientists always mention that the best bond would be between two clean flat metal surfaces. US welding creates something approaching that situation.

11 USMW Previous Results No Cu-Cu bonding observed
Ni-plated Cu 110 Aluminum 1100 Al Ni-plated Cu 110 Ni-plated Cu 110 Ni-plated Cu 110 Ni-plated Cu 110 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 Cross sections show ultrasonic metal welds done as part of a senior research Capstone project at Ohio State completed about 2 years ago. Welds are between plated copper substrates and it is interesting to note that even when the base substrate is highly deformed, the Ni plating is not disrupted. 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 Preliminary results For tab to bus welds aluminum tab welds have similar strength regardless of what the bus material is (Al, Cu, Ni, Ni plated Cu). Nickel plated copper tab peel strength was lower than expected Copper tab to aluminum bus has acceptable tensile shear strength but low peel. This may require careful design if aluminum bus is required for weight or cost control. For tab to tab joints, US metal welding works well for aluminum to copper welds. It is less successful for welds between aluminum and nickel surfaces. One reason for this is that aluminum tends to stick to the horn and anvil face during welding. This would be a detriment during production. For foil to tab joints, US metal welding is a good technique for joining thin (0.001) foils to tabs in multiple layers. Welds with 10 foils were made in a single step for both copper and aluminum.

13 USMW Tab to Tab Shear Peel
Aluminum controls the weld behavior in dissimilar welds. Aluminum has lowest properties so this makes sense. Nickel plated copper shows low strength. This may be due to non optimized welding parameters for nickel containing samples. May also reflect fact that harder materials are more difficult to weld. Better welds are being evaluated. Copper to dissimilar substrates higher strength reflecting better properties than aluminum. Not so hard that USMW struggles to make the weld. Peel Shear

14 Generic Set-Up for Direct Beam Laser Welding Conduction Mode 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 Nickel Plated Copper on Copper-Shaped Weld
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. Resistance welding is a classic technique for joining metal sheets.

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 RSW Process Development
Produce a weld matrix to determine process limits Current Time Force Acceptance requirements Application defined Weld strength Weld size Expulsion Acceptable Nuggets Small Nuggets Minimum Nugget Diameter Weld Current Level Time A Smaller “Brittle” Lobe Curve Nugget Diameter Weld Time Time A Process development usually done to generate weld lobes that define parameters that product acceptable weld strength without burn through or metal expulsion. Strength evaluation is commonly done by a hand peel test to determine if the weld persists after the substrate fails. This is pulling a nugget. Need OSU project specific results to insert showing what works and what does not work. Strength results and cross sections would be good.

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 Welding aluminum to copper works best at average settings between aluminum-aluminum and copper to copper. Welding to nickel plated copper requires use of parameters that work best for nickel plated copper.

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 “Hood Up” Assembled Battery Packs
Buckeye Bullet 2.5 August 2010 Buckeye Bullet “Hood Up” New international land-speed record for battery-powered vehicles of mph Over 1500 Batteries EWI Laser Work Cell Assembled Battery Packs EWI is about to start efforts with OSU CAR to assemble the Buckeye Bullet 3.0. this vehicle will have a new frame, a new drive train and new batteries. Goal is 350 mph+. Look for records in 2012 or 2013! Battery

27 Acknowledgements Support of the Department of Energy through the Ohio State University Center for Automotive Research DOE Award DE-EE National Center of Excellence for Energy Storage 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
Phone:


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