Ultrasonic welding of battery components

Advanced Battery Technology, Sep 1999 by DiFinizio, Tony

Ultrasonic metal welding is a process with unique advantages. Manufacturers of wire harnesses, electronic components and appliances use it as the joining technology of choice. This exciting method is especially suited for use in welding battery components.

Are you interested in welding multiple layers of foil onto a terminal? Is your current process damaging delicate mesh? Would you like to weld combinations of metals such as copper, aluminum, lithium, nickel, and more? Do environmental concerns have you thinking about the future? And finally, does your current process weld your parts but never tell you if the quality of the weld was within your process parameters? Ultrasonic metal welding is the answer to all of these problems.

How Does It Work?

Ultrasonic metal welding is a low power type of friction welding that generates little heat and can be controlled to assure process quality. The principle is based on the idea of scrubbing two materials across each other while clamped under pressure to create a bond. (See illustration above.)

The first material to be welded (called the substrate) is set upon a stationary tool called an anvil. A blacksmith uses an anvil as a solid backing for the pounding of metal into various shapes. An ultrasonic anvil also forms a solid foundation from which to create a weld. The anvil is knurled with fine ridges that bite into the material, holding it in place during the weld process. The second material to be welded (the superstrate) is placed on top of the substrate, overlapping where a lap joint is to be made. The vibrating tool, the sonotrode, or horn, is then lowered onto the layers, clamping them under pressure. Fine knurls on the horn surface grip the superstrate and move it in time with the vibrating tool. This creates a scrubbing action between layers of material.

This scrubbing motion disperses the layer of oxides and impurities at the boundary until the base metals are in direct contact. The correct atoms from each material then diffuse into each other creating a pure metallurgical bond. Materials can be joined with discreet spots or continuous seam welds.

The horn vibrates at a frequency of 20kHz for most applications and 35 or 40kHz for some low power applications. The amplitude or stroke of vibrations typically varies from about 6 to 36 microns (half wavelength).

Ultrasonic Metal Welding Advantages

The primary advantage in connecting battery components is the ability to weld dissimilar materials. Materials such as copper, aluminum, and noble metals can be welded in combinations of foil, mesh, and wire. Many layers of foil or mesh can be welded to a tab in one operation, thus eliminating extra steps and costs.

Since the oxides have been dispersed and the base metals have been joined together, the resulting junction has a lower resistance than other bonding techniques. The contact area can be customized to provide the ideal current path for your application.

Ultrasonic metal welding is not a melting process. The small amount of heat generated as a by-product of the scrubbing action is not enough to change the properties of the parent metal. The electrical conductivity and temper of the original material are maintained. Ultrasonic metal welding can easily be adapted for use in inert environments.

This unique joining process uses no consumable materials, thereby cutting costs by eliminating process steps and the headaches associated with materials hazardous to the environment. The ultrasonic vibrations dissipate surface impurities, and in many cases you can avoid the use of chemical cleaners required for many other processes. Combine these advantages with the low power consumption of the controller and ultrasonic welding is clearly the most environmentally friendly welding process. Also, ultrasonic metal welding equipment quickly pays for itself by decreasing production time, eliminating consumable materials and increasing the quality of assemblies.

The Machine

The ultrasonic welding system consists of an ultrasonic controller designed specifically for metal welding, a pneumatically actuated weld head, the oscillating system including the horn, and the anvil with special tooling.

The controller generates the power used by the vibrating tools, sets and monitors the precise weld parameters, and provides the intelligence for the process. The controller takes in 220V, 60Hz power and creates the 20kHz signal with enough power to drive the system at a given amplitude.

The heart of the oscillating system, the converter, uses this power to produce mechanical vibrations. The converter comprises piezoelectric crystals, sandwiched between contact plates. The high frequency energy from the controller is applied to the plates, which transform the energy into mechanical vibrations.

The sonotrode, or horn, receives these mechanical vibrations and applies the forces into the workpiece. The horn is a specialized tool made from a solid piece of tool steel, shaped to fit each application geometry, and tuned to the exact frequency created by the controller and converter. By matching these frequencies, the smooth transition of oscillating energy is assured. Sometimes a cylinder of different diameters is added between the horn and converter. This part, called a booster, is used to increase or reduce the amplitude of the vibrations. When you put the ultrasonic converter, booster and horn together you create this oscillating system.