"Building mass-production cars out of aluminum presents significant challenges," explains Boddie. "Aluminum offers high strength-to-weight ratios yet isn’t as ductile as steel, nor as easily joined via spot welding or other traditional assembly techniques. Further refinement is needed to develop mass-production assembly techniques for aluminum vehicles. The next step is to develop the manufacturing methods that support the aluminum revolution."

Why Aluminum?

Automakers have been experimenting with aluminum in a wide range of components, such as engine blocks, cylinder heads, manifolds, wheels, closure panels and body structures. Ducker Worldwide (Bloomfield Hills, MI) claims that more than 50 percent of current aluminum content in vehicles is castings for drivetrain components, but there is growing application for hoods, trunks and other weight-intensive body panels.

With the body alone accounting for approximately 25 percent of the total mass of a typical vehicle, aluminum offers a great potential for mass savings. The typical car or light truck also contains a large amount of aluminum foil, largely for heat exchangers such as the radiator.

The Aluminum Association Inc. (Washington, DC) claims that the use of automotive aluminum doubled between 1991 and 1999, and is expected to double again by 2005. In fact, aluminum now surpasses plastic in average vehicle content, and ranks third behind iron and steel.

"This year’s average aluminum content for passenger cars and light trucks combined is 274 pounds," notes Dr. Richard Klimisch, vice president of the aluminum producers’ group. "This reflects a 23-pound increase over the average of 251 pounds of aluminum estimated for all vehicles produced in 1999."

According to Klimisch, "total auto industry usage of aluminum more than doubled for cars and tripled for sport utility vehicles, pickups and minivans over the past decade." He predicts aluminum content will continue growing at its current rate well into the foreseeable future.

The weight of aluminum is approximately one-third of an equivalent volume of steel. A component can be one-and-a-half Arial thicker than a steel version while retaining a 50 percent mass advantage. Aluminum also offers up to 60 percent weight reduction compared with steel and cast iron in vehicle applications. And Klimisch claims that, pound for pound, aluminum can be up to two-and-a-half Arial stronger than steel, absorbing up to twice as much energy in a crash.

"Aluminum is being used more often where weight is a factor," says Tony DiFinizio, engineering manager at Stapla Ultrasonics Corp. (Wilmington, MA). "We have seen applications that include engine control boards, fuel cells, heaters and heat exchangers, and structural parts."

Despite numerous benefits, some manufacturing engineers are not convinced that aluminum is better than steel. They point out that aluminum is more difficult to weld than steel, and behaves differently when stressed.

"Aluminum currently costs four Arial as much as steel, is expensive to weld and isn’t light enough by itself to reduce vehicle weight by 40 percent," says the Partnership for a New Generation of Vehicles (PNGV, Washington, DC), a consortium between U.S. automakers and the Dept. of Commerce. "However, aluminum might be used to reduce weight by up to 25 percent."

Before aluminum can be used in mass volume, there is a critical need for new sheet manufacturing processes that will significantly lower the cost compared with existing methods. According to the Automotive Lightweight Materials Program at the U.S. Dept. of Energy’s Office of Advanced Automotive Technologies (Washington, DC), "notable progress has been made in the development of continuously cast and hot-rolled aluminum alloy sheet."

Proponents claim that aluminum is easily recycled and is relatively affordable compared with more exotic materials such as magnesium and titanium. But, the ecological benefits of using aluminum must be balanced with production issues. For instance, researchers in Europe have estimated that an auto body made of pure aluminum would need to be driven up to 500,000 kilometers before the huge amount of energy used in producing the material is offset.

Aluminum is produced during an electrochemical reduction reaction in which aluminum oxide is dissolved in a bath of molten sodium aluminum fluoride at a temperature of 1,760 F. An electric current is passed through the cell causing the reduction of alumina into primary aluminum metal.

Ultrasonic Applications

Traditional automotive applications for ultrasonic metal welding include assembling wire harnesses, attaching wire-to-wire and wire-to-terminals, attaching nickel or aluminum tabs to the aluminum or copper mesh material inside battery packs, and welding copper to aluminum heat sinks for ignition modules and antilock brake systems. But, there is growing interest in using ultrasonics to weld large aluminum auto parts, such as body structures and panels.

"Any welding process that can replace mechanical fastening or adhesives is of interest in the aluminum area," explains Dr. Karl Graff, executive director of the Edison Welding Institute (EWI, Columbus, OH). "Aluminum is very weldable ultrasonically, whereas it proves challenging to several conventional welding processes, such as resistance welding. It is tolerant of fit and finish, especially needed for high volume applications.

"Resistance welding needs very high currents to work with aluminum," says Graff. "Arc welding may be too slow, and also dumps a lot of heat into the material. Laser welding needs excellent fit-up, and friction stir welding is slow. A new process, magnetic pulse welding, is showing very good promise for certain aluminum joining operations."

Graff says ultrasonics welds aluminum and other high thermal conductivity materials, such as copper and magnesium, very well. Because ultrasonic metal welding is a solid state bonding process, it has little effect on the properties of metals. For instance, there is no heat-affected zone, as with all of the fusion-based processes.

"Aluminum is one of the easiest metals to join ultrasonically, so it lends itself to seam welding, spot welding and other structural applications," adds Joe Stacy, national sales manager at American Technology Inc. (AmTech, Shelton, CT). However, he says there are not many large components within a vehicle that are ultrasonically welded.

"We are working on the inner structures of a lithium ion battery, which does require welded aluminum parts, for the electric vehicle program," notes Stacy. Other applications include welding brackets for sensors, fuel tanks and transmission parts.

Graff, Stacy and other observers point out that no large-scale structural parts are currently being produced with ultrasonic welding technology. But, development efforts are focusing on closures and covers, such as trunk lids.

"With enough work to prove sys-tem robustness, reduce tip sticking and learn more about the design needs for clamping large workpieces, it will be used in the near future," predicts Janet Devine, president of Sonobond Ultrasonics (West Chester, PA).

Ultrasonic Benefits

Using ultrasonics to weld aluminum offers numerous benefits, such as:

• Quality control. The welders monitor process values to guarantee the quality of each weld.
• True metallurgical bond. Ultrasonic welding creates a true metal bond with very low resistance and superior electrical integrity.
• Ultrasonic welders use very little power. A 3-kilowatt controller can be fed from a 220-volt line. Energy use is about one-tenth that for resistance welding.
• There are no consumable materials, such as solder, rivets, adhesives or clips.
• The process is environmentally friendly. Parts don’t need to be treated with chemicals, such as acids or solder flux.
• Parts can be welded without cleaning or worrying about contamination, because ultrasonics is a cleaning process within a welding process. With resistance welding, aluminum parts must be extremely clean before they can be successfully welded.
• Ultrasonic welders can be easily integrated into automation.

"Ultrasonics is particularly advantageous for welding delicate parts, joints where a seal is required or the parts will be exposed to harsh environments, and when excellent electrical integrity is desired," adds Stapla’s DiFinizio.

Because of the solid state nature of the bond, aluminum can be welded to a number of other metals, such as copper and magnesium. "Joints have been made, in one fashion or another, in a tremendous range of metals, including dissimilar metal combinations," says EWI’s Graff. "Very thin aluminum, copper and other metallic foils have been joined to nonmetals such as glass and ceramic."

It is not possible to weld aluminum directly to thermoplastic. "However, it is quite common to stake aluminum parts into a plastic material for structural integrity," notes AmTech’s Stacy.

Joint Design

Ultrasonic aluminum welding applications are limited to lap joints. "Most parts need to achieve some type of overlap, such as a lap joint or flange configuration," explains Dr. Graff. "Surfaces should be flat or near flat."

The ultrasonic welding process works by vibration at the interface scrubbing away the oxides, generating dislocation slip leading to transfer of material (atomic diffusion) from one material to the other, without melting. "This can not happen with a butt joint," says Devine.

"Some work has been done on the butt welding configuration, but it has not yet spread to production use," adds EWI’s Graff. "We will be looking at a small scale butt joint in the near future."

Most aluminum auto parts now being tested for ultrasonic welding are thin sheet metals. Those parts have maximum thicknesses of 2 to 3 millimeters. "R&D efforts are focused on more powerful systems, to permit increased thicknesses, and are looking at added joint configurations," Graff points out. "The goal is to get to 6 millimeter thicknesses. We hope to be welding at the thickness within 3 years." When that happens, automakers will be more likely to use aluminum in main load-bearing parts of body structures, such as pillars and posts.

In addition to joint design and part thickness issues, several problems still need to be resolved before ultrasonics will enjoy widespread application in joining large aluminum auto parts. For instance, "tip sticking has been alleviated but not totally eliminated," Devine points out. "The steel welding tip tends to stick to the aluminum sheet during the weld process.

"Access problems need to be addressed by redesigning welding guns to give better access on shaped parts," adds Devine. "Fixtures are needed to clamp the parts to be welded so that the vibration does not travel through the part. This vibration can cause noise and occasionally may damage a previous weld."

"Care must be taken to design the tooling for aluminum welding," warns Stapla’s DiFinizio. "The tooling cannot be too aggressive or the parts may be damaged." He says special coatings can be used to prevent parts from sticking to the tooling.