GM is using ultrasonic welding in the 16.5 kWh battery pack in the new Cadillac ELR extended-range electric luxury coupe that goes on sale in North America in early 2014 (earlier post). The battery-specific welding process is a result of collaboration among GM’s Manufacturing Systems Research Lab, Advanced Propulsion Center, and Brownstown Battery Assembly plant near Detroit. GM first applied the process on the Chevrolet Volt and further refined it for ELR.
Ultrasonic welding uses specialized tools called an anvil and horn to apply high-frequency vibrations to the battery’s copper and aluminum electrodes. This introduces oscillating shear forces at the interface between two metals, causing elastoplastic deformation at the interface. This ultimately results in a structure similar to that of a diffusion weld that does not require melting-point temperatures or joining material such as adhesives, soldering or fasteners.
The basic components of an ultrasonic welding system include:
A transducer which transforms high frequency electrical energy to vibratory energy and is incorporated in the welding head;
A power supply which converts line power to the high frequency and high voltage needed by the transducer;
The welding head which also provides the means—pneumatic, hydraulic, or mechanical—to clamp the workpieces; and
The components or waveguides to transmit the energy to the desired weld area.
The ultrasonic welding of high conductivity materials such as copper and aluminum requires substantially less energy than does resistance welding, notes Sonobond, a leading provider of ultrasonic welding equipment. (In 1960, Sonobond (then known as Aeroprojects) received the first patent for ultrasonic metal welding.)
In a paper published earlier this year in the Journal of Manufacturing Science and Engineering a team from The University of Michigan, General Motors R&D Center, and the University of Hawaii noted that “Although ultrasonic metal welding is well-qualified for battery manufacturing, there is a lack of scientific quality guidelines for implementing ultrasonic welding in volume production.
In order to establish such quality guidelines, this paper first identifies a number of critical weld attributes that determine the quality of welds by experimentally characterizing the weld formation over time using copper-to-copper welding as an example. Samples of different weld quality were cross-sectioned and characterized with optical microscopy, scanning electronic microscopy (SEM), and hardness measurements in order to identify the relationship between physical weld attributes and weld performance. A novel microstructural classification method for the weld region of an ultrasonic metal weld is introduced to complete the weld quality characterization. The methodology provided in this paper links process parameters to weld performance through physical weld attributes.—Lee et al.
For the production system at Brownstown, GM uses an integrated camera vision system is used to shoot a reference image of the weld area prior to the operation to achieve pinpoint accuracy. Quality operators check electrode tabs before and after welding, and the system monitors dozens of signal processing features during each weld.
Every ELR battery, for example, has close to 200 ultrasonic welds. Each is required to meet stringent quality requirements, enabling Cadillac to offer an eight-year/100,000-mile battery system warranty.
Short cycle times, low capital costs and manufacturing flexibility through the use of automation are other advantages of ultrasonic welding.
Ultrasonic welding is used in many industries including electrical and computer, automotive, aerospace, medical, and packaging. The technique is popular for bonding thermoplastics, as well as wires, microcircuit connections, sheet metal, foils, ribbons and meshes.
Ultrasonic welding is a far superior joining technology in applications where it can be deployed. Cadillac’s innovative process will produce batteries with superior quality compared with traditional methods, and do it more efficiently. This is one example of technology development that is becoming pervasive in today’s world-class vehicles.—Jay Baron, president and CEO of the Center for Automotive Research in Ann Arbor
The ELR’s T-shaped battery pack is located along the centerline of the vehicle, between the front and rear wheels for optimal weight distribution. The 5.5-foot-long (1.6 m), 435-pound (198 kg) pack supplies energy to an advanced electric drive unit capable of 295 lb-ft of torque (400 N·m). Using only the energy stored in the battery, the ELR will deliver a GM-estimated range of about 35 miles (56 km) of pure electric driving, depending on terrain, driving techniques and temperature.
Charging the ELR’s battery can be done with a 120V electrical outlet or a dedicated 240V charging station. The vehicle can be completely recharged in about 4.5 hours using a 240V outlet, depending on the outside temperature.
The Cadillac ELR is built at GM’s Detroit-Hamtramck Assembly Plant. Its battery pack is built from cell to pack at Brownstown and shipped to Detroit-Hamtramck for assembly into the vehicle.