- SPECIAL REPORTS
Ultrasonic welding is a proven method for assembling metal parts. Typical applications include wire harnesses, automotive parts, medical devices, rechargeable batteries, and copper tubing for heating, ventilating and cooling equipment.
But, alternative joining techniques, such as crimping, laser welding, resistance welding, riveting, soldering and spin welding, offer tempting solutions to manufacturing engineers. Each assembly process has pros and cons that must be weighed against cost factors.
Ultrasonic welding is a form of friction welding that joins parts together by vibrating them against each other. The parts are clamped together while a vibrating tool called a horn transfers ultrasonic energy to the joint interface. The vibrations are parallel to the weld surfaces.
Heat is generated through a combination of friction and hysteresis. The ultrasonic vibration breaks up oxides and films on the part surfaces, permitting metal-to-metal contact. When the ultrasonic vibrations stop, a solid-state bond is achieved.
Ultrasonic welding offers many advantages to assemblers. For instance, it is a fast, efficient process. Weld times are typically 0.2 to 0.5 second. No warm up or cool down time is necessary, unlike other joining processes, such as hot-plate welding. This solid-state welding process can join a wide variety of dissimilar alloys, thin-to-thin sections and thin-to-thick sections. Also, most oxides and surface oils do not inhibit joint weldability.
“Ultrasonics makes an otherwise very difficult joining situation doable,” says Karl Graff, Ph.D., senior engineer at the Edison Welding Institute (Columbus, OH). “It is more expensive than resistance welding machinery, but [less expensive] than lasers.”
Ultrasonics is much more cost-effective in the long run due to many factors, such as tooling, energy consumption and joint quality.
Key components of an ultrasonic welding system include a generator and a resonant stack. The stack has three components: a converter, a booster and a horn.
The generator converts electrical voltage into a high-frequency signal—usually 20 or 40 kilohertz. High power levels necessitate lower frequencies.
The converter, or transducer, consists of piezoelectric crystals attached to a base of aluminum or titanium. It changes high-frequency electrical signals into high-frequency mechanical vibrations. The booster is a mechanical transformer used to transmit vibration from the converter to the horn.
Process variables are very important to ultrasonic welding. Controlling time, force, depth, amplitude and power is critical to making good welds. Microprocessor- and software-based controls allow ultrasonic welders to automatically monitor a wide range of variables to ensure quality and repeatability.
Ultrasonic welders cost anywhere from $18,000 to $45,000, depending on power capacity and bells and whistles, such as statistical process control packages. A basic 20-kilohertz, 2.5- to 3.5-kilowatt ultrasonic metal welding system can be acquired for around $30,000. And, that cost has been coming down in recent years. For instance, machines that used to be priced at more than $40,000 are now available for $30,000 or less.
By comparison, spin welders and thermal stakers are very price competitive with ultrasonics. Resistance welders typically cost between $8,000 and $30,000. Laser welders are $50,000 and up. But, even a high-end resistance welder may not provide the same quality control features as an ultrasonic welder. Sonic weld times are frequently much faster than alternative joining processes.
Mechanical fastening equipment is relatively inexpensive, but the fasteners represent an extra cost that must be factored into any economic analysis. “Riveting requires the highest skill level and longest time and adds weight to a product,” notes Janet Devine, president of Sonobond Ultrasonics (West Chester, PA).
Ultrasonic welding requires energy to make the weld, which varies depending on the application. It also requires compressed air to actuate the pressure cylinders and provide some cooling. Weld energy ranges from 100 joules per weld to several hundred joules per weld.
A typical ultrasonic controller has a 3-kilowatt power supply, which is much smaller than a resistance welder. As a result, most applications require very little energy.
“Ultrasonic welding is a nonfusion joining method,” says Devine. “It uses only about 5 percent of the energy of resistance welding and about 20 percent of the energy required for making a riveted panel. Resistance welding often requires costly water cooling with its additional recycling and purifying costs.
“The greatest economic advantage may be the lower energy requirement,” adds 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.”
Ultrasonic welding requires the use of acoustical tools called horns. They transfer mechanical vibrations to the parts to be welded. Horns come in many different shapes and sizes, including cones, bars and hollow circles, squares and rectangles.
Horns are typically made from aluminum, titanium or hardened tool steel. Aluminum is inexpensive and be machined quickly and easily. It’s often used for large horns and prototypes. However, it’s not as durable as other materials. Titanium is more durable for long-term use, but it’s more expensive and difficult to machine.
As a rule of thumb, tools with amplitudes of less than 3 mils are made of aluminum; tools with amplitudes of 3 mils or higher are made of titanium. Hardened tool steel is used for welding metal and glass-filled plastics and for inserting metal fasteners into plastic.
“Ultrasonic tooling is more expensive to purchase, but has better value than other tooling,” notes Tony DiFinizio, engineering manager at Stapla Ultrasonics Corp. (Wilmington, MA). “The tooling will last several hundred thousand cycles. Resistance welding tooling will typically need to be changed daily in an automated environment. This will require down time on the line. The electrode will need to be removed, dressed by a machinist and then realigned in the fixture, which is a time-consuming process.”
Devine says her company’s welder uses a heat-treated steel tip that can be replaced for less than $400. “Typically, we get about 30,000 welds in copper and the tip can be redressed two or three times before replacement,” she points out. “Redressing cost is about $50 to $100. In other words, you can get about 120,000 welds per tip for about $600.
“In aluminum, the number of welds is reduced to about 10,000 spot welds,” adds Devine. “Care must be taken in selecting the tip geometry and welding parameters to minimize the tendency of the tip to weld to the aluminum sheet. Tooling costs are comparable to resistance welding. The tips are cheaper.”
In some ultrasonic welding systems, the horn must be replaced rather than just the tip. That can lead to substantially higher tooling costs.
“Tooling cost is not an insignificant factor, especially if frequent replacement is needed due to tool wear issues,” says Graff. A welding tool can range from $200 to $400 for small replaceable tools. A more complex welding horn can cost $1,500 to $2,000.
“If replacement is seldom needed, this may not be an issue,” Graff points out. “However, if wear is an issue because of welding a hard material, these costs can be a factor. If resistance welding were a viable option for making a weld, then one would find much lower tool costs for this joining process.”
Ultrasonic welding is a cost-effective assembly method when joining nonferrous materials, such as aluminum, brass, copper and nickel. “The advantages for 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. “Ultrasonics is also very useful for joining a thin material to a thick material. If minimal disturbance of material properties due to heat is a requirement, ultrasonics is often the best welding process.
“Issues of cost-effectiveness tend to come down to being able to make a given weld or not being able to, or having to go through extensive materials and part redesign,” adds Graff.
Wire attachment is a common application for ultrasonic welding. It is a popular alternative to mechanical crimping, which raises reliability issues.
“Ultrasonic welding is very cost-effective for wire splicing applications,” claims DiFinizio. “The machine cost is very low and the tooling is available at low prices due to high volume manufacturing of standard spare parts.
“Often, the price of an ultrasonic welder can be justified by the removal of another process such as a tinning station or by the removal of fasteners or clips. For instance, an ultrasonic tube sealer can reduce labor by at least one worker and eliminate stress injuries due to crimping and brazing methods.”
In some applications, ultrasonics may be the only economical alternative. For instance, in many cases, aluminum parts cannot be joined by resistance welding or soldering. According to Graff, “aluminum is very weldable ultrasonically, whereas it proves challenging to several conventional welding processes.”
Devine says ultrasonic welds are up to twice as strong as resistance welds for aluminum sheet welding. In addition, “resistance welding produces an undesirable heat-affected zone, an area in which the material structure is changed and sometimes degraded,” Devine points out. “The heat-affected zone in steel may not have an adverse effect on the overall joint quality, but in aluminum the material is definitely degraded and weakened.”
The most common use of ultrasonic metal welding is in wire-to-wire and wire-to-terminal assembly applications. According to Devine, ultrasonic welding produces greatly superior joints compared with crimping, soldering or resistance welding. She says wire harness manufacturers report a great improvement in quality at a cost reduction of 50 percent or more when switching from the crimp-and-solder method to ultrasonic welding of copper wires.
“The ultrasonically welded joints are stronger and have better electrical conductivity,” says Devine. “Weight is reduced by eliminating the mechanical crimp. Resistance welding for wire harness applications requires 20 times the energy than that used for ultrasonic welding.”
Welding copper tubing for HVAC and instrumentation is another common ultrasonic application. “This is a quicker and better method than the crimp-to-close and braze-to-seal method,” claims Devine. “The tube can be ultrasonically welded while under coolant charge.”
“Each application is different and has some unique advantages,” says DiFinizio. “For tube sealing, the labor cost reductions are the most important. In some large splices, the power savings can be the driving factor. Quality, consistency, environmental and safety factors hold true for all applications.”
According to DiFinizio, the quality control aspects of ultrasonic welding are an important cost savings that is often overlooked. “The machine gives quality feedback for every part made including total energy and power as well as part thickness,” argues DiFinizio. “The welder can detect problems, such as using the wrong number of wires, missing strands, changes in material hardness and thickness, and the absence of parts in the tooling. All of these features combine to prevent bad parts from entering production, which saves rework time, scrap parts, money and reputation.”
But, ultrasonics is not ideal for every assembly application. DiFinizio says ultrasonic welding is typically used in medium- to high-volume applications where initial costs can be amortized over the life of the tooling. In extremely high-speed applications, ultrasonics may not efficient. For example, it may not be able to keep pace with a crimp press.
“Welding ferrous alloys would be a poor application for ultrasonic welding,” says Devine. “The weld quality is poor and tip wear is a major problem. Some tinned products do not weld well.”
It’s important to factor in intangible costs associated with ultrasonic welding. For instance, because an ultrasonic welder requires very little air consumption and does not use water cooling or hydraulic lines, manufacturers can reduce operating costs. Extra ventilation is not needed, because no fumes or gases are produced during welding. In addition, ultrasonic welding does not require additional consumable materials, such as clips, solder, flux or rivets.
Parts cleaning is not usually an issue for ultrasonic welding, although it may be for resistance welding.
“The need to clean parts requires additional labor and time and is a definite disadvantage to the resistance welding process,” says Devine. “The presence of oil or surface contaminants on materials requires an additional cleaning process for resistance welding to be consistent, but ultrasonic welding is forgiving of most surface contaminants, including mill oil or oxides.”
With most ultrasonic welding equipment, training is relatively straightforward. “An operator can be trained in a day,” claims DiFinizio. Today’s microprocessor-controlled equipment can be set up to store and recall parameters for different weld combinations, reducing the chance of operator error due to incorrect machine settings. Also, welds can be made to a preselected energy level or a preselected weld height. Alarms are activated if the weld does not meet the selected criteria.
Major Research Project
The National Institute of Standards and Technology (NIST, Gaithersburg, MD) has just awarded a $4.4 million grant to develop ultrasonic welding equipment and processes suitable for the high-volume manufacture of all-aluminum vehicles. The research project aims to examine the inherent benefits of ultrasonic welding over alternative joining processes.
“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.”
Four organizations have formed a joint venture to develop ultrasonic metal welding technology to address automotive industry requirements. The four partners are contributing $4.5 million to the research effort, so a total of $8.9 million will be spent on developing new tools and processes.
Ford Motor Co. (Dearborn, MI) is heading the ambitious project and intends to implement ultrasonic welding technology in mass-produced vehicles. Other partners in the NIST advanced technology program include American Technology Inc. (AmTech, Danbury, CT), the Edison Welding Institute and Sonobond Ultrasonics.
In addition, three universities have been engaged as subcontractors in the project. Ohio State University (Columbus, OH) will assist with transducer characterization, welding system design, and sensor device development and testing. Researchers at the University of Michigan (Ann Arbor, MI) will develop fracture models. Wayne State University (Detroit) will perform numerical simulation and model validation.
“This program, if successful, will be one factor in reducing the cost of manufacturing aluminum vehicles, allowing them to be more widely produced and available,” says Susan Ward, Ph.D., leader of the joining methods team at Ford’s research and advanced engineering department. “Aluminum vehicle bodies weigh less than steel, fostering better fuel economy and lower emissions.”
“Ultrasonic metal welding is ideally suited to address the challenge facing the high-volume manufacture of such a fuel-efficient vehicle,” adds Richard Gehrin, president of AmTech. “It 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.”
However, ultrasonic metal welding has not been applied to structural welding of automobile bodies. Technical challenges include overcoming tool adhesion to the workpiece, extending tool life, and designing new ceramic piezoelectric materials for high-power transducers needed for joining thick sections. Weld performance and associated operating costs must prove comparable to that achieved in conventional steel-body construction.
“Ford plans on developing the technology and using it first in the production of a low-volume product,” notes Ward. “After that, it is expected to see wider manufacturing applications.”
According to Ward, areas where ultrasonic metal welding of aluminum could be used include assembly of hoods, deck lids, or an aluminum body-in-white. She believes ultrasonic metal welding will yield costs savings over other types of welding or self-piercing rivets.
“The advantage over other types of welding is energy savings,” Ward points out. “About eight times less energy is consumed utilizing ultrasonic metal welding vs. resistance spot welding.” And, because ultrasonic spot welding does not require consumables, the variable cost of an ultrasonic weld is much lower than for a riveted joint.