Ultrasonic welding—an established technique for assembling metal parts—is drawing a lot of attention these days. In fact, in the push to mass-produce all-aluminum vehicles, ultrasonic welding is front and center as a highly promising low-cost joining method. The technique is currently being developed to meet automotive industry requirements in an $8.9 million project funded by The National Institute of Standards and Technology (NIST) and four partners—Ford Motor Co., American Technology Inc. (AmTech), the Edison Welding Institute, and Sonobond Ultrasonics.

And while its role in the assembly of these fuel-efficient all-aluminum cars is still under development, ultrasonic welding has already proven its effectiveness in a wide range of applications, including wire harnesses, automotive parts, medical devices, rechargeable batteries and copper tubing for HVAC equipment.

The Mechanics

Ultrasonic welding joins parts together by causing them to vibrate against each other. A form of friction welding, this method uses a vibrating tool called a horn, which transmits ultrasonic energy to the area where the parts to be joined are interfacing. The process produces heat through the combined forces of friction and hysteresis. The ultrasonic vibrations, which run parallel to the weld surfaces, split up oxides and films on the part surfaces, allowing direct contact between metals. When the ultrasonic vibrations cease, a solid-state bond is formed.

Richard Gehrin, president of AmTech, says, ultrasonic welding is "a solid-state process used for dissimilar metals of both thin and thick cross section. It can weld through oxides and oils and creates negligible odors and fumes. Energy consumption is low and weld times are short."

Advantages

In fact, weld times are usually only 0.2 to 0.5 second. Assemblers who use this process don't have to worry about allotting time for warm up or cool down, which is a must in other joining processes, such as hot-plate welding. Additionally, the ultrasonic technique can weld together thin to thin sections and thin to thick parts quickly and efficiently. "Ultrasonics makes an otherwise very difficult joining situation doable," says Karl Graff, Ph.D., senior engineer at the Edison Welding Institute, Ohio.

The technique excels in welding nonferrous materials, such as aluminum, brass, copper and nickel. "The advantages of ultrasonic welding appear for high thermally conductive materials, such as aluminum, copper and magnesium, which can be troublesome to weld for resistance welders and lasers," says Graff.

In addition, ultrasonic welding is more cost-effective than most other joining techniques, including crimping, laser welding, resistance welding, riveting, soldering and spin welding.

The Cost-Effective Question

Among the factors making ultrasonic welding economical in the long run are tooling, energy consumption and joint quality.

"Ultrasonic tooling is more expensive to purchase, but has better value than other tooling," says Tony DiFinizio, engineering manager at Stapla Ultrasonics Corp. in Massachusetts. "The tooling will last several hundred thousand cycles."

This compares favorably to resistance welding, he notes, which is an alternative joining process. "Resistance welding tooling will typically need to be changed daily in an automated environment. This will require down time on the line," explains DiFinizio. "The electrode will need to be removed, dressed by a machinist and then realigned in the fixture, which is a time-consuming process."

Energy consumption is another area where assemblers can enjoy cost savings from using ultrasonic welding "Ultrasonic welding is a nonfusion joining method," says Janet Devine, president of Pennsylvania-based Sonobond Ultrasonics. "It uses only about 5% of the energy of resistance welding and about 20% of the energy required for making a riveted panel. Resistance welding often requires costly water cooling with its additional recycling and purifying costs."

The average ultrasonic controller relies on a small, 3-kilowatt power supply. This means that most applications need low amounts of energy. "The greatest economic advantage may be the lower energy requirement," continues Devine. "For instance, a plant with multiple resistance welders may require a new electric substation to handle the energy. However, the same number of ultrasonic welders can be handled with the normal power capacity of an industrial plant."

The third factor contributing to the cost-effectiveness of ultrasonic welding is joint quality. Many ultrasonic welders incorporate quality control features that ensure the integrity of welds. And according to Devine, in wire-to-wire and wire-to-terminal assembly applications, the technique creates much stronger joints than those formed by crimping, soldering and resistance welding. As a result, the higher price of ultrasonic welders, which can range from $18,000-$45,000, can be offset over time. What's more, the price of these machines has been dropping in recent years. Assemblers can buy a standard 20-kilohertz, 2.5 to 3.5-kilowatt ultrasonic metal welding system for about $30,000.

Resistance welders, which range from $8,000-$30,000, may be cheaper, but even a high-priced resistance welder doesn't necessarily come with quality control features that can compare with those of an ultrasonic welder. Another alternative—mechanical fastening equipment—is also initially less expensive but these machines require fasteners, which add to the cost.

Conclusion

Fast and efficient, ultrasonic welding offers assemblers many advantages. And the technique could soon add the mass production of all-aluminum cars to its long list of applications. "High-volume manufacturing of aluminum automobiles requires a low-cost metal joining technology in order to be viable," says H. Felix Wu, NIST advanced technology program project manager. "Ultrasonic metal welding overcomes weaknesses inherent in available joining methods for aluminum automotive body structures and is cost-efficient and environmentally friendly."

Source: The Economics of Ultrasonics
Austin Weber
Assembly Magazine, Aug. 1, 2003
http://www.assemblymag.com