Understanding ultrasonic welding

CLICK ON IMAGE FOR LARGER VIEWFigure 1Although the means by which the vibrations are produced in the wedge-reed system differ from the lateral drive system, the results are the same.

Ultrasonic vibrations have been used for welding metals and plastics since the 1950s. For ultrasonic metal welding, the solid-state nature of the process, as well as other advantages, has led to widespread applications in the electronic, automotive, aerospace, appliance, and medical industries. Various features of ultrasonic metal welding, as well as recent trends in process development, are leading the way to expanded use of the process across a number of industry sectors.

In ultrasonic welding, ultrasonic vibrations create a friction-like relative motion between two surfaces that are held together under pressure. The motion deforms, shears, and flattens local surface asperities, dispersing interface oxides and contaminants, to bring metal-to-metal contact and bonding between the surfaces.1, 2 The process is solid-state, which means it occurs without melting or fusion of the base metals.

Figure 1 depicts the two main types of systems used for ultrasonic metal welding and also shows details of the local behavior in the weld zone. The lateral drive system comprises an ultrasonic transducer, a booster, and a horn/sonotrode. The power supply provides high-frequency electrical power to the piezoelectric-based transducer, creating a high-frequency mechanical vibration at the end of the transducer. A typical operating frequency is 20 kHz, but 30 kHz or higher is possible. This vibration is transmitted through the booster section, which may be designed to amplify the vibration, and is then transmitted to the horn/sonotrode, which transmits the vibrations to the workpieces.

CLICK ON IMAGE FOR LARGER VIEWFigure 2aThe lateral drive welding system shown here provides high-frequency electrical power to the piezoelectric-based transducer, creating a high-frequency mechanical vibration at the end of the transducer. Photo courtesy of EWI.

The workpieces, usually two thin sheets of metal in a simple lap joint, are firmly clamped between the sonotrode and a rigid anvil by a static force. The top workpiece is gripped against the moving sonotrode by a knurled pattern on the sonotrode surface. Likewise, the bottom workpiece is gripped against the anvil by a knurled pattern on the anvil. The ultrasonic vibrations of the sonotrode, which are parallel to the workpiece surfaces, create the relative friction-like motion between the interface of the workpieces, causing the deformation, shearing, and flattening of asperities previously noted.

Welding system components are housed in an enclosure case that grips the welding assembly at critical locations so as not to dampen the ultrasonic vibrations, and to provide a means of applying a force to and moving the assembly to bring the sonotrode into contact with the workpieces and apply the static force. An example of a lateral drive welder is shown in Figure 2A.

A second type of ultrasonic metal welding system is known as the wedge-reed. The key elements of this system are the piezoelectric-based transducer that drives a booster, which is called a wedge because of its distinctive shape (but otherwise plays the same role as the previously described booster). The wedge then drives a vertical rod (reed) into a bending vibration. The vibration at the end of the reed is then transmitted via the sonotrode on the reed to the workpieces (See Figure 2B).

The workpiece arrangement is similar to the lateral drive system—it's clamped between the sonotrode and anvil by a static force. The anvil of the wedge-reed system is not rigid (as with the lateral drive), but is designed to flex slightly under the action of the ultrasonic vibrations. Although the manner in which the vibrations are produced in the wedge-reed differ from the lateral drive, the results are the same: a vibrational motion of the sonotrode that is parallel to the workpiece surfaces and creates the relative friction-like motion at the workpieces' interface.

This is brought out in the more detailed look at the weld zone (Figure 1), which shows that the two systems produce the same effect in the weld bonding zone of a thin region of plastically deformed material where a solid-state bond has occurred between the workpieces, without melting of the materials.

Ultrasonic welding systems are similar to spot welding devices because they produce a bond over a small area of the parts (typically on the order of 40 mm2). It is also possible to produce an ultrasonic seam weld by continuously rolling an ultrasonically vibrated solid disk over the workpieces. Other types of ultrasonic bonding systems include torsion vibration and ultrasonic microbonding, used widely in the electronics industry to join fine wires to circuits and microchips and where the sizes of the welds are on the order of 0.150 mm2.

CLICK ON IMAGE FOR LARGER VIEWFigure 3aPhoto courtesy of Sonobond Corp.

A number of parameters can affect the welding process, such as ultrasonic frequency, vibration amplitude, static force, power, energy, time, materials, part geometry, and tooling.

Ultrasonic Frequency. Ultrasonic welding transducers are designed to operate at a specific frequency from 15 to 300 kHz for different systems and applications. Most metal welding systems operate at 20 to 40 kHz, with 20 kHz being the most common frequency.

Vibration Amplitude. The vibration amplitude of the welding tip is tied directly to the energy delivered to the weld. Ultrasonic vibration amplitudes are quite small—10, 30, or 50 microns at the weld, and seldom exceed 100 microns (approximately 0.004 inch). In some welding systems, the amplitude is a dependent variable; that is, it is related to the power applied to the system. In other systems the amplitude is an independent variable capable of being set and controlled at the power supply through a feedback control system.

Static Force. The force exerted on the workpieces via the welding tip and anvil creates intimate contact between the opposing surfaces as the weld vibrations begin. The magnitude of the force, which depends on the materials and thicknesses, as well as the size of the weld produced, may be from tens to thousands of newtons. For example, producing a weld of 40 mm2 in a 6000 series aluminum may use force of 1,500 N, while 10 mm2 welds in 0.5-mm-thick soft copper sheet may require only 400 N.

Power, Energy, and Time. While listed as separate weld parameters, power, energy, and time are best examined together since they are all closely related. When a weld is made, the voltage and current from the power supply result in electric power that flows to the transducer during the weld cycle. The energy delivered is the area under the weld power curve. Most welding power sources are rated by the peak power they can deliver, with this varying from a few hundred watts to several kilowatts. Most weld times are found to be less than one second. Based on constant power output, a 0.4-second weld from a 2-kW welder would deliver 800 joules of energy.

Materials. This encompasses a wide range of issues and parameters relating to ultrasonic metal welding. First is the type of material or material combination. Most materials and material combinations have been found to be weldable in some fashion, although specific weld parameter and performance data is generally lacking for most of them. The properties of the material, including modulus, yield strength, and hardness, are a key consideration.

Generally speaking, soft alloys like aluminum, copper, nickel, magnesium, gold, silver, and platinum are most easily welded ultrasonically. Harder alloys such as titanium, irons and steels, and nickel-based aerospace alloys and refractory metals (molybdenum and tungsten) are more difficult.

Material surface characteristics is another parameter, with these including finish, oxides, coatings, and contaminants.

Part Geometry. The shapes of the welded parts play an important role, the dominant factor being part thickness. Generally speaking, thin parts have a better chance of achieving a successful ultrasonic weld. Increasing the part thickness, in particular the part contacting the welding tip, requires a larger welding tip area, more static force, and higher weld power. Maximum achievable thicknesses will depend on the material and the welding power source's available power.

Tooling. Composed of the sonotrode/welding tip and the anvil, tooling serves to support the parts and to transmit ultrasonic energy and static force. In most cases, the tool tip is machined as an integral part of a solid sonotrode (see Figure 2A), but in some cases detachable tool tips are used. The tooling contact surfaces typically have machined knurled patterns of grooves and lands or other surface roughening to improve workpiece gripping.

While the weld tip and anvil contact surfaces are usually flat, the weld tip may be designed with a slight convex curvature in order to change the contact stresses.

CLICK ON IMAGE FOR LARGER VIEWFigure 3ePhoto courtesy of Dukane Corp.

Applications for ultrasonic welding are found in the electrical/electronic, automotive, aerospace, appliance, and medical industries. Currently the widest applications in these industries involve alloys of copper, aluminum, magnesium, and related softer metals, including gold and silver. Some examples are:

A future trend in the use of ultrasonic welding is in structural automotive and aerospace applications, joining thin-gauge sheet aluminum and other lightweight metals. The feasibility of the process has been demonstrated for closure panels on both helicopters and aircraft.

More powerful welding systems operating at 5 kW and more are being developed. This will permit welding the more challenging materials and thicker joints.

Research leading to better understanding of this welding process is under way in several industrial and university laboratories to determine the total range of materials and applications that realistically can be joined. Some progress is being made on joint configurations besides the most common lap joint; achievement of butt welds has been reported as an example. The increased use of process sensors may permit monitoring of and better joint quality.

A recently emerged application for ultrasonic welding is additive manufacturing, in which thin metal tapes are welded together, with interspersed machining operations, to produce solid metal parts. This application may have special use in the field of rapid prototyping.

Dr. Karl Graff is the technology leader in ultrasonics and Matt Bloss is project engineer at the Edison Welding Institute, 1250 Arthur E. Adams Drive, Columbus, OH 43221, 614-688-5000, [email protected].

Notes

1. K.F. Graff (chapter ed.), Chapter 8, "Ultrasonic Welding of Metals," Welding Handbook, 9th ed., Vol. 4 (Miami: American Welding Society, 2007).

2. K.F. Graff (chapter ed.), Chapter 9, "Ultrasonic Metal Welding," New Developments in Advanced Welding, ed. N. Ahmed (Cambridge, England: Woodhead Publishing, 2005).