Ultrasonic Welding

One of the newest and most interesting fields of joining technology is ultrasonic welding. In this process, high frequency vibrations are combined with pressure to join two materials together quickly and securely, without producing significant amounts of heat. These factors give it many advantages over traditional heat based welding techniques. These include the ability to weld metals of significantly dissimilar melting points; metals that normally form brittle alloys at the weld junction; and welds that are in close proximity to heat sensitive components, such as electronics or plastic components. Finally, ultrasonic welds are made without consumables, such as solder or filler that would ordinarily be used for the connection and with far less energy usage than traditional joining techniques. There are some restrictions on the types of joints that can be made with ultrasonic welding. One of these is that it is restricted primarily to nonferrous metals and plastics. Another is that at least one of the parts must be relatively light, as it would take a tremendous amount of energy to vibrate a heavy part at the necessary frequency. This restriction, unfortunately, limits the process to small components and wires.

How Ultrasonic Welding Works
The process of ultrasonic welding is fairly simple. It begins when the parts that are to be welded, such as two multi-strand copper wires for example, are placed together in the welding unit. The system then compresses the wires together with a force of between about 50 and several hundreds pounds per square inch to form a close connection between the two pieces. Next, the ultrasonic horn is used to vibrate the two pieces together at a rate of around 20,000 or 40,000 hertz, depending on the application.

The system that is used to scrub the pieces together consists of four major components. The first of these is the anvil. This is simply a piece of the machine, usually with a replaceable head, that holds one of the components still while the other is rubbed against it. The “business end” of the ultrasonic system consists of three major parts. The first of these is the ultrasonic transducer. This component takes an electrical signal from a power supply that is providing a 20khz AC signal and converts it to a mechanical motion at the same frequency. The vibration that results is at a frequency that is appreciably above the range of human hearing, hence the name ultrasonic.

Although this motion is very strong, it has a very low amplitude, or stroke length. This is not suitable for welding. The next part of the system, appropriately called the booster, increases the amplitude of the motion, at the cost of some of its force. This motion is then passed to the ultrasonic horn. This is the portion of the system that actually vibrates the work piece. In addition to providing the interface between the ultrasonic generator and the work piece, the horn also further amplifies the amplitude of the motion, again reducing its force. Like the anvil, the horn ends in a replaceable head.

Before the interaction of the pieces at the interface can be explained, some basic molecular physics must be reviewed. The first principal is that when two clean pieces of metal are placed in intimate contact, they will begin to share electrons, thus welding together. Second, at an atomic scale even surfaces those that look perfect and smooth are very rough and impure. The majority of this impurity is in the form of metal oxides that were produced when the bare metal was exposed to the atmosphere. The second part of the contamination is in the form of ordinary dirt and oils. These impurities form a layer that prevents the electrons in the two parts from passing between them, thus preventing them from welding together. In addition, the rough surface prevents the metals from being in intimate contact, which also prevents the exchange of electrons.

The ultrasonic welding process works by eliminating these factors that prevent the pieces from being in intimate contact. The vibration, combined with the pressure, first rubs the impurities and oxides off of the surfaces of the pieces where they are in contact. As the rubbing motion continues, the clean bare metal parts are rubbed together, polishing them to a molecularly smooth boundary. When the ultrasound is turned off there is no longer anything preventing the exchange of electrons between the two perfectly cleaned and polished pieces. Because of this the electrons begin to move between them forming a true metallurgical weld. This entire process usually takes about one quarter of a second.

Uses for Ultrasonic Welding of Metals
Due to its versatility, effectiveness and cost, there are a nearly infinite number of applications for ultrasonic welding. One of the most common is for the production of wiring harnesses. Ultrasonic welding is ideal for this application for a number of reasons. One of these is that it produces a nearly perfect electrical connection. Second, it requires no consumables, such as solder. Third, it uses far less energy than other forms of connecting, such as resistance welding, which is made difficult by the excellent electrical characteristics of copper wire. Finally, the ultrasonic welding process produces little heat, allowing it to be used near heat sensitive components. A subset of this field is the welding of enamel insulated magnet wire to other wires or connectors. In addition to the other advantages, the ultrasonic scrubbing action effectively removes the insulation in the normal process of making the weld, eliminating the extra step of removing it that would be necessary using other methods.

Another common use of ultrasonic welding, also in the electrical field, is joining aluminum wire, such as is used in some transformers, to copper wire. In this case, ultrasonic welding is the only practical method of performing the connection. Aluminum does not solder well, and attempting to thermally weld aluminum to copper produces alloys that are extremely brittle. Before the application of ultrasonic welding, the only way to perform these connections was to first plate the aluminum with nickel, then solder or weld to that. This process was complicated and uneconomical.

Other applications include the termination of both wires and pipes. Ultrasonic welding is well suited for attaching terminations, such as clips or other connectors to copper wires. Like joining wires together, this can be done much more economically with ultrasonic welding than with other techniques. Finally, ultrasonic welding is often used in HVAC industry, as well as others, for sealing the ends of copper tubes. In addition to the usual advantage of cost, ultrasonic welding is desirable for this application due to its speed and excellent reliability.

Ultrasonic Welding of Plastics
In addition to all of the uses of ultrasonics in the welding of metals, the technology can also be used to weld plastic parts together. In this application, the process is slightly different from that used to weld metals. One primary difference is that rather than scrubbing the parts together, as in metal welding, in plastic welding the parts are vibrated toward and away from each other. This heats and softens them. The continued motion then tends to mix the softened surfaces of the plastic together, forming both a physical, and if the plastics are selected correctly, a chemical bond between the parts.

Ultrasonic welding of plastics has many of the same advantages as using the method to weld metals, including lower cost, higher speed and greater reliability than most or all other methods of attaching the pieces. Unlike most other methods of plastics joining, ultrasonic welding uses no caustic chemicals and produces few fumes. The only restriction on welding plastics is that the parts must have relatively close melting points, or else one will melt completely before the other begins to soften.

Like ultrasonic metal welding, ultrasonic plastic welding has a wide variety of uses. These include sealing containers and bags, attaching plastic components and embedding metal components in plastics. This last example deserves special attention. When a metal part needs to be attached to a plastic part, convention methods include molding the metal component into the part when it is created, gluing the metal part to the plastic and locally melting the plastic component then inserting the piece into the molten area and allowing it to solidify. Ultrasonic welding of metal to plastic works by using the ultrasonic head to vibrate the metal component against the plastic one. This heats and softens the plastic part and allows the metal part to be inserted. As the plastic cools, a very strong bond is made. Although this process is similar to locally melting the area with another heat source, it has the advantage of heating a far smaller area to lower temperatures, reducing energy costs, decreasing cooling time and reducing the possibility of deformation due to thermal effects.

In conclusion, ultrasonic welding has many advantages over other welding techniques for many types of connections. These include speed, cost, reliability and many others. However, it should also be made clear that it is only ideal for a relatively small proportion of all of the possible joints. Ultrasonic welding should not be seen as a replacement for other techniques, such as GTAW, resistance, laser, etc. but rather as an option in situations for which it is well suited.