Growing use of lightweight materials in the auto industry is forcing engineers to rethink how they splice wires and cables. While there are many technologies for directly splicing two or more wires together, including laser welding, resistance welding and soldering, ultrasonic metal welding has become a popular alternative for assembling wire harnesses.

Ultrasonic welding creates solid-state metallurgical bonds that have high conductivity. It does this without producing arcs, sparks or fumes, and without melting the metals.

Wire strands are vibrated together under pressure at a frequency from 15 to 40 kilohertz. The vibrational energy disperses surface oxides to create galling on the strands to form a metallurgical bond.

“The ultrasonic process provides the lowest resistance weld available,” claims Joe Stacy, national sales manager at Branson Ultrasonics Corp. “Having a low resistance weld allows the use of smaller cables, saving [manufacturers] money and space.”

Ultrasonic welding also lets engineers carefully monitor the entire weld process. Controllers monitor the time and power it takes to make the weld. They also automatically compensate for part variations and cleanliness.

“The ultrasonic process is 97 percent efficient from an electrical aspect,” Stacy points out. “It also allows for sequential welding, and the tooling can last up to 1 million cycles.”

“Repeatability, quality data collection and robustness of the equipment make ultrasonic welding ideal for splicing wires,” adds John Corsi, manager of welding sales development at Sonics & Materials Inc. “Extensive studies conducted by the auto industry have documented its superiority to all other technologies.

“This has driven the existence of standards such as USCAR 38, which has created a uniform and accepted approach to interconnections within the automotive industry,” notes Corsi.

Splicing Alternatives

Despite numerous benefits, ultrasonic welding may not be ideal for all wire processing applications. For example, wire bundles smaller than 1 square millimeter can be difficult to weld without damage. In addition, wires with high strand count and extremely fine individual wires may encounter wire breakage from ultrasonic welding.

And, because ultrasonic welding requires an overlap, butt joints are not always possible. Overlap joints typically work better for multiple wire materials or sizes. Butt joints work well for solid wires.

Many engineers are comfortable with older assembly techniques and are reluctant to switch from resistance welding or soldering to ultrasonic welding, despite the advantages.

Resistance welding is typically used for splicing stranded wires, and is better for less-conductive materials like stainless steel and nickel. However, the heat generated during resistance welding can anneal the wire, which can reduce the tensile strength of the wire strands adjacent to the weld. Electrode wear in resistance welders is also high, due to the current density.

Conductive materials, such as aluminum, brass, copper, gold, nickel and silver, are typically ultrasonically welded. Hard, ferrous materials, such as carbon steel and stainless steel, are better candidates for resistance welding.

Soldering is another old-school splicing alternative for some wire processing applications, such as terminating tin-plated wires. However, it’s typically used for low-volume production or for applications where field repair is expected, such as audio cabling and multipin connectors.

Soldering is often used to seal a wire into a housing, where the solder joint creates both a moisture-tight seal and an electrical connection. But, soldering cannot be used if the joint temperature in service will approach the melting point of the solder.

Aerospace manufacturers typically prefer soldering. “Ultrasonic welding can only be used in nonmilitary aerospace applications, due to soldering requirements,” notes Corsi. “As most aerospace manufacturers have interests in both military and commercial products, they do not universally embrace ultrasonic technology.

“Their standards are based around tinned wires that are optimized for the requirements of the soldering process, which in turn, reduce the quality of ultrasonic welding,” adds Corsi. “There are a few exceptions. But, due to soldering being the principle joining method, ultrasonics are usually only considered when soldering’s shortcomings become too detrimental for the project to succeed.”

An additional wire splicing alternative for some applications is laser welding. However, the technology is still expensive, and it can be difficult to use when attaching round wire to flat terminals. Laser welding is primarily used for sensors that require tiny wire that can only be accessed from one side, making it difficult to use resistance or ultrasonic welding.

Laser welding is more difficult to use for wire splicing applications if dissimilar materials are to be welded, such as copper to steel or nickel to aluminum. In these cases, a solid-state process, such as resistance welding or ultrasonic welding, should be used to avoid formation of brittle alloys in the weld.

Since ultrasonics is a much less violent process, the nugget that is produced is far superior when it comes to data collection, ease of use and preventing wire from becoming brittle. “Resistance welding is a structural weld of usually ferrous and highly resistive metals,” says Corsi. “Because copper is so conductive, it requires an explosive amount of energy to heat up the metals.

“Laser welding also has the issue of reflectivity, but it does have a niche with complicated geometries when there is no opportunity to properly anvil the substrates,” Corsi points out.

“Ultrasonics does not utilize heat,” adds Corsi. “Instead, the relative motion of the substrates achieves a true metallurgical bond through galling where there is an exchange of materials during the process.”

Aluminum Challenges

As automotive engineers seek new ways to reduce vehicle weight, they’re bullish on aluminum wire and cable. Aluminum offers a lower cost per amp and provides up to 48 percent mass reduction over copper, trimming an average of 2 kilograms from a typical vehicle’s wiring harness.

However, aluminum is only about 60 percent as conductive as copper. So, to replace a copper wire, engineers need to upsize the aluminum conductor about two gauge sizes. For instance, a 20-gauge copper wire would be replaced with an 18-gauge aluminum wire.

That change in wire size creates some splicing challenges. “It’s not so much the gauge of the wire, but the cross-section of the splice,” says Stacy. “The bigger the cross-section, the more power is required to make the weld.

“On a smaller cross-section, like when you get down to 0.6 square millimeter it is hard to control your process,” warns Stacy. “A 4,000-watt power supply is only using 40 watts to make the weld, or 1 percent of the available power.

“Because the force needed to make the weld is very [small], most actuators require a different weld cylinder or cam block to reduce the force in order to control the process,” explains Stacy.

“[With ultrasonic welding], a quality 20-kilohertz splice can be created from less than 1 square millimeter to 40 square millimeters,” adds Corsi. “With the introduction of 15-kilohertz wire splicers, more than 100 square millimeters can be spliced utilizing the same basic methods that operators have grown used to.”

“As wire size increases, the energy required to ultrasonically weld also increases,” notes Janet Devine, president of Sonobond Ultrasonics. “Manufacturers are looking for fast, dependable, cost-efficient [equipment to splice] heavy-duty cables used in wire harnesses for cars, trucks, trains and industrial machinery.”

Sonobond recently answered the call with its Dual Head SpliceRite ultrasonic wire splicer. It features two ultrasonic transducers and couplers—one above and one below the wire bundle. In addition, there’s a set of pneumatically driven jaws that gather the wires tightly to a preset width.

The upper tip then descends to complete the compressing of the bundle, and ultrasonic power is applied. When the cycle has been completed, the jaws open so the wires can be removed.

The dual-head system provides 3,500 watts of ultrasonic power from both sides of the bundle for stranded copper wire bundles with cross sections from 40 to 100 square millimeters. “This is the only standard wire splicer that can also weld tinned wire bundles up to 60 square millimeters in cross section,” claims Devine.

Devine and other experts predict there will be a growing need for larger wiring harnesses with more complex splicing challenges, as automakers ramp up production of hybrid and electric vehicles.

Many automotive suppliers are turning to aluminum wire to save weight and reduce cost, especially in big, load-carrying cables that connect batteries and other critical under-the-hood components. Aluminum is typically a good candidate for anything requiring high current.

The ongoing electrification of the auto industry is forcing engineers to use larger wiring splices than in the past. “Splices are growing in size by leaps and bounds,” notes Saeed Mogadam, president of Telsonic Solutions Inc. “We have seen applications [as large as] 124 square millimeters, for both aluminum and copper wiring.”

“Electric and hybrid vehicles require copper splicing of 50 square millimeters,” Stacy points out. “I have seen aluminum splices [larger than] 80 square millimeters.

“But, the bigger the splice, the more difficult it becomes to transfer sonic energy throughout the entire splice,” warns Stacy. “Aluminum strands become soft and no longer move, which is a requirement to achieving a sonic bond.”

“The bell curve is being challenged by the addition of splices at both the small end of the spectrum and the large end,” adds Corsi. “Instrumentation and monitoring has a growing need for wires in the 32 AWG to 36 AWG range.  On the other hand, a full electric vehicle requires cabling up to 120 square millimeters.

“With these larger cables comes the additional burden of weight, which is being addressed with aluminum wire that has a better weight-to-current carrying capability ratio,” explains Corsi. “This has driven the development of next-generation 15-kilohertz welders and tooling.”

According to Mogadam, resistance welding cannot be used with aluminum wiring, because it gets melted faster than a nugget is formed. “With ultrasonic welding, there is no melting, but aluminum can easily stick to tooling,” he warns.

Tooling must be designed to prevent sticking. This can be achieved through the design of the tooling’s surface pattern and the coatings applied to them. 

 Mogadam also says aluminum produces less voids in the weld nugget, due to the softness of the material. “Not all aluminum wires work perfect with ultrasonic welding,” he adds. “Most aluminum wire manufacturers are still learning.”