Sound Welds in Aluminum: The Ultrasonic Approach
Ford researchers are working on a project that could allow aluminum body structures to be assembled comparatively economically through the use of ultrasonic energy.
Will those flying sparks from spot welding robots that are so characteristic of body shops (and especially typical in photos of body shops) become a thing of the past? If work being done by Dr. Susan Ward, Manufacturing Systems Dept., Ford Research Laboratory, and her colleagues continues on the path that it’s going, then this is not inconceivable. If fact, the National Institute of Standards and Technology (NIST has recently awarded Ford and its three partners in this undertaking—American Technology (Shelton, CT) and Sonobond Ultrasonics (West Chester, PA), both ultrasonic joining equipment suppliers, and the Edison Welding Institute (Columbus, OH), a leading nonprofit welding research center—$4.4-million to develop ultrasonic welding capabilities for processing aluminum in automotive assembly applications. These parties are adding $4.5-million, which brings the total to $8.9-million on the project, which can have some rather significant paybacks in terms of the joining process in vehicle assembly and could help increase the use of aluminum for automotive body structures.
The sparks will give way to sound waves that are well above what people can ordinarily hear (~18,000 Hz is the top end of typical human hearing); ultrasonic welding is performed at frequencies on the order of 20,000 Hz. Essentially, what happens is that the pieces to be welded are placed between a set of tools. One is static, called the “anvil,” and the other, sometimes called the “horn,” vibrates a short distance at a high frequency: say 20 microns at 20 kHz. There is some pressure that holds the parts together, but it is the back and forth vibration that actually causes the bonding. This is not a case where the friction causes the interface of the materials to melt as is the case in resistance welding but, rather, one in which as the two pieces of material go back and forth rapidly the surface oxides are removed and the materials bond together at the atomic level. (In ultrasonic welding of plastics, the materials do reach their melting point.) Under magnification a joint between two pieces of aluminum almost appears as though it isn’t there, as though it is one piece of metal, not two. The cycle time for the process is as fast as a conventional spot welding operation.
Not only are there no sparks, but also there is significantly less energy consumed compared with spot welding: According to Dr. Ward, about eight times less energy. One of the inhibitors to the use of aluminum body panels is the fact that it takes more energy to weld aluminum than steel because of the thermal diffusion characteristics of aluminum. Consequently, they’re looking at the potential for processing closure panels (hoods, deck lids, doors) and fenders with the ultrasonic process because this isn’t a joining process that’s based on melting the materials to be joined, so thermal diffusion is not an issue. Larry Reatherford, an engineer who works in the development lab with Ward, points out that whereas water cooling is required for spot welding—which represents costs both in terms of material and energy—it isn’t necessary for ultrasonic welding, which is another benefit of the process. And speaking of cooling, because ultrasonic welding is a comparatively low-heat process, there isn’t a heat-affected zone that affects the mechanical properties of the joined materials.
Just as conventional spot welding robots typically have a C-frame design wherein the two pieces to be joined are clamped and welded, Ward says that this same arrangement can be set up for ultrasonic welding. (Essentially, there’s a power supply that starts with electrical energy that then goes into a transducer, which transforms it into mechanical energy in the form of vibrations. Then there is a booster to amplify the vibrations, and then it is sent into the horn that vibrates the materials to be welded, while the anvil remains stationary.)
Aluminum spot welding is performed with copper-tipped tools. During spot welding operations, aluminum begins to transfer to the tips. Consequently, it becomes necessary to dress the tips, to remove the build-up, so that welding can proceed. This is another instance of a cost that isn’t associated with ultrasonic welding, which doesn’t have this material transfer issue.
In some instances, such as when using rivets to join aluminum, there is a tendency to use adhesive bonding, as well. The adhesive helps increase the stiffness of the joined pieces. According to Ward, adhesives can be used when using ultrasonic welding. (The adhesive material would migrate away from the point where the weld is being made, of course.)
Although they are optimistic about the development of ultrasonic welding, there is still work to be done. They are getting a better understanding of the fundamentals of the process, such as the appropriate control parameters for good welds. Although the process is well understood for plastics joining, and although it has been used for welding metal, this metal welding has been pretty much limited to wire bonding, not joining the 0.9- to 3-mm sheet that’s typical of automotive assembly operations. They need to work on developing design guidelines that can be used by engineers to determine, say, how close to flanges the welding can be done and how strong the joints are. In addition to which, they must do work on industrialization of the equipment. While ultrasonic welding is certainly not unfamiliar in some automotive operations, those tend to be plastics-related, not car- or truck-building sites. So they need to work on equipment and fixtures and the like.
Susan Ward notes of welding processes, “We always use what we know. So we’re looking at other technologies to enlarge the base of what we know.” Perhaps before too very long, there will be a solid knowledge base of ultrasonic welding in auto assembly plants and deployment of the process.
Larry Reatherford, Ford engineer, ultrasonically welding aluminum coupons as part of a development program that is being undertaken with the goal of developing aluminum welding technology that’s suitable—both from the standpoints of the process and the equipment sufficiently robust for use in automotive assembly plants. Joining is done with high frequency mechanical vibratory energy, not heat as is the case with conventional welding processes.
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