Bus bars are popular with EV manufacturers for several reasons. For one thing, they save space. Bus bars do not sit as high as cables do. They can also be formed at angles more tightly and more precisely than a cable can bend, fitting closely to vehicle profiles.

Another factor is automation. Even though many automotive assembly tasks have become automated, installing wiring continues to be a largely manual task. Robots have difficulty handling and positioning flexible cables, but they can easily pick and place a rigid bus bar.

Finally, bus bars carry more current. A bus bar can support up to 15 percent more power than a cable with the same cross-sectional area. In addition, the larger surface area of a bus bar dissipates heat more efficiently than a cable over its entire length.

One of the leading manufacturers of bus bars is the Kern-Liebers Group. Based in Schramberg, Germany, Kern-Liebers specializes in the production of complex strip and wire parts and assemblies.

Kern-Liebers has pioneered an inline technique to coat aluminum bus bars with silver. This obviates the need to send bus bars to an outside supplier for coating, which saves time and money. 

The silver coating enhances the conductivity of the bus bars, but it poses a challenge for secure attachment using screws, due to the relaxation properties of aluminum. To solve that problem, engineers integrated copper sleeves into the bus bar, providing a durable and electrically conductive connection that allows for assembly with screws.

The copper insert is essentially a contact washer. It eliminates concerns about compression set, which can occur in softer materials, such as aluminum over time. The screw goes through the insert, which allows for easy assembly and disassembly.

The challenge then became how to attach the sleeves to the bus bar. That problem was solved by Telsonic Ultrasonics Inc., a manufacturer of ultrasonic welding equipment for plastic and metal parts. 

For assembly, the copper sleeve is placed in a pre-punched hole in the bus bar. The sleeve is joined to the silver layer by ultrasonic welding, which preserves the integrity of the silver coating. The sleeve is designed with an oversized collar that enables the transmission of torsional ultrasonic vibrations through the sonotrode. 

The silver coating extends the compatibility of ultrasonic welding to new aluminum alloys. Ultrasonic welding enables the reliable and long-term joining of nonferrous metals with minimal electrical resistance.

This application was successfully executed using Telsonic’s torsional Soniqtwist technology. Traditionally, ultrasonic metal welding involves the application of linear, or longitudinal vibrations. However, such vibrations have limitations in terms of the size, shape and orientation of parts that can be welded. These limitations are particularly evident in the EV industry, where bus bars, thick cables, large connectors and thin battery tabs must be joined.

Torsional ultrasonic welding overcomes these limitations. With Soniqtwist, strong forces can be applied in a short time, which is critical for joining thick aluminum or copper parts.

On the other hand, torsional energy can also be gently exerted at the joint interface, making the process suitable for sensitive parts. The low-impact process will not damage nearby electronic components. The vibration stress induced on the lower workpiece is only a tenth of the stress induced by conventional ultrasonic welding. 

Strictly speaking, Soniqtwist is a high-frequency friction welding process that can be categorized somewhere between vibration welding and ultrasonic welding. The sonotrode oscillates torsionally around its longitudinal axis in both directions. In a short time—between 0.1 and 0.4 second—a lot of energy is applied to the boundary surface of both parts, with a frequency of 20 kilohertz and an amplitude of up to 80 microns.

The design of the Soniqtwist’s “ultrasonic stack” is like that of conventional ultrasonic welding equipment, with a generator, converter and sonotrode. However, the sonotrode moves torsionally rather than longitudinally, which reduces the overall load on the parts to be joined. The process can apply 14.4 kilowatts of energy to the parts. Even with difficult materials, this creates joint strengths that are not obtainable with conventional ultrasonic welding equipment.