Karl Graff, Ph.D., is a leading expert in the field of high power ultrasonics, including transducers, applications and systems. He is a senior engineer at the Edison Welding Institute (Columbus, OH) and previously served as the organization’s executive director. He also is the former head of the Sonic Power Laboratory at Ohio State University. ASSEMBLY recently asked Dr. Graff to explain the difference between metal and plastic ultrasonic welding.

Is the process of joining aluminum parts with ultrasonics any different than it is with other materials, such as thermoplastics?

The ultrasonic metal welding process (whether aluminum or other metals) is fundamentally different from welding plastics in how the ultrasonic energy (or vibrations) is delivered to the weld, and in how the actual weld is created.

First, ultrasonic metal welding delivers vibrations to the zone via transverse vibrations that are parallel to the weld surfaces. Ultrasonic plastic welding delivers vibrations to the zone via longitudinal vibrations that are normal (i.e., at right angles) to the weld surfaces.

Second, ultrasonic metal welding creates the weld via a frictional action of the surfaces that creates a solid-state bond without any melting of the material. Ultrasonic plastic welding is based on melting and fusion of the material (in a sense, like many metal welding processes, such as arc, resistance or laser), but at much, much lower temperatures than experienced in metal fusion processes.

There are two slightly different ways of delivering the ultrasonic vibrations to the weld zone: "lateral drive" and "wedge-reed." While each has selected special advantages, they both end up getting the vibrations to the weld zone in a transverse vibration action.

With this fundamental distinction between ultrasonic metal and plastic welding in mind, then one has ultrasonic welding of all metals done in the same way. Weld Arial in all cases are very fast-- on the order of 0.2 to 0.5 seconds.

Is there a difference in the tooling used to ultrasonically weld metal and plastic components?

The shape of the horns that transmit the ultrasonic vibrations into metal and plastic welds are quite different, although they are designed from the same principles of acoustics. The fundamental driving transducers for both ultrasonic metal and plastic welding are quite similar, as are some of the coupling horns (often called boosters) between the transducer and welding tool.

Most ultrasonic metal welding companies are in both plastic and metal welding, since much of the underlying technology is similar, including power supplies. Nevertheless, as ultrasonic metal welding tries to meet new challenges, especially in welding thicker materials, there will be increasing differentiation between metal and plastic welding apparatus.

Are high or low frequency levels typically used for metal and plastic welding applications?

Most ultrasonic metal welders work at 20 kilohertz, similar to plastic welders (40 kilohertz metal welders are available as well). In moving to higher power, it would not be surprising to see some drop in frequency, to say 15 kilohertz, as has occurred in plastic welders.

What's the biggest mistake end users make when using ultrasonics to weld parts?

Ultrasonic welding is based on acoustic and solid-state phenomena, and not fusion phenomena, so users may not make sufficient effort to acquaint themselves with this "animal." Assuring set-up of a good, robust set of welding procedures is very important. This becomes especially so for first-time users, since they may not have a backlog of past experience to guide them in setting up the process.

Static or clamping force happens to be a very important ultrasonic metal welding parameter. One must be aware of overall part vibrations induced by the ultrasonic welding action. Control of these matters lies in clamping and placement of welds so that resonant conditions are not encountered.

A user may set up a process that works at as low a force as possible (maybe to avoid material deformation), not realizing the process is at the "ragged edge" of that parameter. Then, a slight variation of incoming material in surface finish, hardness or cleanliness may throw it "over the edge" resulting in no, or poor welds. When that happens, the user is faced with a new process that was never used before (and that was not trusted too much to begin with) that isn’t working. This, however, is not untypical of many other processes.