Written by Joe Stacy
National Sales Manager for Ultrasonic Metal Welding
Emerson Automation Solutions
For nearly 30 years, ultrasonic metal welding has offered manufacturers a uniquely reliable solution for joining a variety of soft, conductive non-ferrous metals ranging from copper, aluminum, and nickel, to lithium, brass, silver, and gold. The process is particularly useful for joining dissimilar combinations of materials in applications that use batteries, power-storage devices, wire harnesses and assemblies, electrical breakers and switches, consumer electronics and cell phones, and implantable medical devices.
Unlike resistance and laser welding, ultrasonic metal welding bonds metals without melting them, so the process never creates intermetallic compounds or particulates, or causes corrosion. This low-energy, solid-state process bonds metals in several configurations — including thin foils or sheets to stranded wires and bus bars (up to 2 mm in thickness), producing connections with a high electrical conductance for maximum electrical efficiency.
One of the technology’s most important applications involves transportation, where nearly all batteries used in electric vehicles rely on ultrasonic metal welding to join their smallest and most basic components. These include the thin nickel, copper, and alloy films used in anodes and the aluminum foils typically used in cathodes, as well as the anode and cathode tabs that bind their power-generating chemistries together.
A variant of the process, ultrasonic metal splicing, ensures the integrity of wire harnesses and terminations essential to delivering electric power, sensor inputs, or control signals in a host of other applications, worldwide.
Ultrasonic metal welding at work
As seen in the Converting electrical energy into ultrasonic welding energy diagram above, 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 or more delicate parts and 20 kHz for larger, thicker parts).
The electrical energy is sent through an RF cable to the converter. The converter uses piezoelectric ceramics to convert the electrical energy to mechanical oscillations at the operating frequency of the power supply. These oscillations are either increased or decreased, depending on the configuration of the booster and horn. The proper degree of oscillation, known as amplitude, is typically determined by an applications engineer. Precise control of amplitude is essential for repeatable metal welding.
The bonding is accomplished by using high-frequency vibration to the metals that are held under pressure, and applied by the actuator. The lower metal part is held stationary in a piece of tooling called an anvil, and 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, producing a shear force that “scrubs” surface oxidation and contaminants away and creates a smooth, metal-to-metal contact.
As oscillation continues, the metal surfaces heat up, plasticize, and co-mingle at their interface — and bond at the molecular level. The result is a continuous weld with a finely grained structure similar to the structure of cold-worked metals.
The entire process is very rapid, with welds typically completed in a fraction of a second.
Multiple methods for control
In addition to its ability to bond non-ferrous metals without melting or damaging them, ultrasonic metal welding offers multiple methods of control, which can meet several production challenges.
Among the most important of these control modes are:
• Weld energy control, which allows welding for a fixed length of time (time mode), to a particular finished weld height (height mode), or to a fixed level of energy input (energy mode). In energy mode, the ultrasonic metal welder automatically compensates, varying the duration of the weld to adapt to commonly occurring differences in the surface conditions (e.g. the degree of oxidation and contamination) of the metals being joined.
• Weld amplitude control, which regulates the length of oscillation (amplitude) delivered to the weld zone of each joint, and uses the capabilities of the weld power supply, converter, and sonotrode and horn assembly.
• Weld downforce control regulates the pressure applied to the joint being welded. Advances in automation for downforce control cut both ways, enabling the application of greater and lesser pressure to be applied with exceptional precision.
One example is Branson GSX, an ultrasonic welder from Emerson that incorporates a new “feather-light” electromechanical actuation system, which measures downforce so precisely that it can repeatably initiate ultrasonic welding with a fraction of the downforce required by older equipment.
Another new ultrasonic welding technique developed in Emerson’s Application Laboratory uses higher actuation pressure on stacked metal films so that a durable weld can be created more “gently,” using less total weld amplitude.
Ultrasonic metal welding and splicing: The process benefits
• Works with a wide range of non-ferrous materials
• Creates a permanent, metallurgical bond between dissimilar metals
• Requires no melting — there’s zero change to the chemistry or metallurgy of materials
• Joins highly conductive alloys — the reactivity of materials doesn’t matter
• Creates no intermetallic compounds, particulates, or corrosion-causing reactions
• Bonds thin, fragile metal films and structures
• Offers multiple methods of control, enabling process customization, repeatability, and SPC
• Provides a low-energy input (up to 30 times lower energy use than with fusion or resistance welding), and without consumables
• Offers the lowest total cost per weld of any welding technology