Behind the remarkable success of the latest automotive and mobile communication and computing technologies, there is another behind-the-scenes success story about the ever smaller but ever more powerful power sources that keep them moving: batteries and power storage devices. Surging interest in green and mobile technologies drives global manufacturers to seek continued innovations in battery technology to further minimize space, maximize power density and storage capacity, and extend operating time, range, and capability.
For more than 25 years, all of these battery-related innovations have been assisted by a joining technology — ultrasonic metal welding — that enabled battery makers to hurdle a major barrier in advanced battery design. When this technology came into broader use around 1990, it gave battery manufacturers a highly reliable way to bond the numerous fragile, dissimilar, and reactive metal components essential to advanced battery design without degrading their performance, reliability, or operating life.
Ultrasonic metal welding technology lends itself extremely well to joining the often thin, fragile, and dissimilar nonferrous materials essential to advanced battery designs. These soft, conductive materials include copper, aluminum, nickel, brass, titanium, silver, and even gold. Today, ultrasonically welded connections perform in batteries that power cellphones, laptops, implantable medical devices, electric and hybrid vehicles, military drones, remote control aircraft and vehicles, and even NASA’s Mars rovers Spirit and Opportunity.
One of the critical differences between ultrasonic welding and other metal welding processes like resistance or laser welding is that ultrasonic welding is a comparatively low-energy process — it never melts the materials that are being welded. The melting caused by high-energy welding processes can be essential to joining high-strength ferrous metals; however, melting becomes a problem when it comes to joining softer, nonferrous metals because it leads to the formation of intermetallic compounds that can cause premature material, connection, and battery failures. Melting copper and aluminum together, for example, would produce a connection — at least for a time — but also result in a galvanic corrosion reaction that would cause that connection to fail. Weld-related melting also complicates the successful assembly of the multiple thin foils that must be linked to tabs in modern battery designs.
How Ultrasonic Metal Welding Works
As seen in Figure 1, the power supply takes a standard electrical line voltage (typically 50 or 60 Hz) and converts it to the frequency required for metal welding (40 kHz for smaller/more delicate parts or 20 kHz for larger, thicker parts). This electrical energy is sent through an RF cable to the converter. The converter utilizes piezoelectric ceramics to convert the electrical energy to mechanical oscillations at the operating frequency of the power supply. These oscillations are increased or decreased based on the configuration of the booster and horn. The proper degree of oscillation, known as amplitude, typically is determined by an applications engineer. Precise control of amplitude is essential for repeatable metal welding.
The bonding is accomplished by applying high-frequency vibration to two metal parts that are held under pressure, applied vertically by an air-powered cylinder. The lower metal part is held stationary in a piece of tooling called an anvil, while the upper part is pressed against it while subject to the motion of an oscillating horn or “sonotrode.” The sonotrode extends horizontally from the power supply of the welder and is the source of the ultrasonic energy that creates the metal-to-metal bond.
When the weld process begins, the upper part is oscillated by the horn against the lower part. Initially, this rapid, oscillating shear force disrupts surface oxidation and contaminants on the adjoining metal surfaces, creating a wide area of metal-to-metal contact. As the oscillation continues, the force breaks down surface asperities (rough areas) on the metal surfaces until a continuous weld area is produced. When the oscillation ceases, the now-welded area is characterized by atomic diffusion across the interface of the joined parts, as the metal surfaces of the welded parts recrystallize into finely grained structures, similar to the structures of cold-worked metals. The entire process is very rapid, with welds typically completed in a fraction of a second.
The friction generated by the oscillating shear force of the upper part against the lower creates heat but not enough heat to melt either of the metals. (Typically, the heat generated equals about 1/3 to 1/2 the absolute melting point of either material.) This solid-state process creates a strong welded bond yet avoids burning through thin or fragile foils or melting the materials and forming intermetallic compounds.
Critical Factors in Weld Process Control
Ultrasonic metal welding is a very specialized process that demands very precise control over several critical factors: weld energy control, weld amplitude control, and weld tooling quality.
Weld energy control. Ultrasonic metal welding systems offer multiple modes of control over weld quality. They can weld for a fixed length of time (time mode) or weld to a particular finished weld height (height mode); however, welding in energy mode generally provides the best results. This mode, which assures that each weld receives an identical amount of joining energy, is calculated as follows:
where E equals energy in joules, P equals power in watts, and T equals time in seconds. The watts of power (P) that are consumed in the weld are further broken down as P = F × A, where F equals the downward force exerted on the weld (typically by a pneumatic actuator) and A equals the amplitude of the vibrating shear force directed into the weld.
Thus, when welding in energy mode, an ultrasonic metal welder automatically compensates (using time) for commonly occurring differences in the surface conditions of the metals being joined. Put another way, adjoining metal surfaces that have a greater degree of oxidation or contamination will require a longer initial period of “scrubbing” before conductivity is established and effective metal-to-metal joining takes place. Energy mode welding readily compensates for these differences.
Weld amplitude control. In an ultrasonic metal weld, amplitude refers to the length of the oscillation delivered to the weld zone by the upper moving metal part. Successful and repeatable ultrasonic metal weld production demands that this amplitude be carefully calculated based on the materials being assembled and then precisely controlled throughout the production process using the capabilities of the weld power supply, converter, and sonotrode/horn assembly.
Weld tooling. Another major contributing factor in ultrasonic metal welding success is the design and material composition of weld tooling, particularly the anvil and the sonotrode or horn. The anvil is essential for holding the stationary metal part firmly in place, while the horn must effectively “grip” the moving metal part, precisely delivering the oscillating shear forces through it that will establish the bond. The ability of the horn to grip the upper part is created through a specialized machining process that produces a pattern of spherical, diamond, or serrated “knurls” on the tip of the horn. These knurls, together with the overall design of the horn, are another essential factor in the ultrasonic metal welding process.
Weld process documentation. In a growing number of industries, device assembly or manufacturing data may be needed to comply with regulations (e.g., Unique Device Identifier requirements for medical devices), meet quality requirements, or support product warranty or service operations. Branson ultrasonic welding equipment has the capability to record weld parameters and process data as needed to comply with diverse global requirements.
Making Critical Connections in Batteries
Whether individual batteries are Li-ion cylindrical or prismatic types, or newer Li-polymer pouch designs, ultrasonic welds provide the most proven and reliable solution for interconnecting the nickel and copper tabs and foils that typically comprise cathodes, as well as the aluminum foils and tabs used in many anode structures. Welding of these or other small battery structures is typically done at an amplitude of 40 kHz, due to the low number of very thin foil structures being joined. In prismatic cells, 40-kHz ultrasonic welding can join up to 25 thin copper or aluminum foils, simplifying the assembly of multilayer battery designs.
When more than one battery is needed as part of a larger battery module or pack joined by a bus bar structure, ultrasonic metal welds also scale up to meet these requirements. Depending on the application, ultrasonic metal welds can join conductive metal components up to 2 mm in thickness in applications such as battery packs, wire harnesses, and battery cables. For these larger welds, an amplitude of 20 kHz is often used. At this amplitude, the welding process can use up to 5000 W of available power and up to 80 microns of motion (amplitude). 20-KHz ultrasonic metal welding applications include large battery packs for electric cars, battery packs for special vehicles (specialized mining vehicles, large drones, etc.). Applications like these might use prismatic batteries that can join foils from up to 100 layers onto a single tab.
This article was written by Joe Stacy, National Sales Manager for Ultrasonic Metal Welding, Assembly Technologies, at Emerson (St. Louis, MO). For more information, visit here .