A New Look at an Old Technology

June 1, 2004
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Hybrid joining processes combine the best of both worlds.

This month, gardens across the United States are bursting with color. Many flowers are hybrid varieties, a popular process pioneered by a French botanist in the mid-19th century. By experimenting with hybridization, Jean-Baptiste Guillot developed entirely new varieties of roses.

Today, manufacturing engineers are experimenting with hybrid joining processes to assemble dissimilar materials. By combining the best of both worlds, such as adhesive bonding and self-piercing rivets, hybrid riveting offers new opportunities.

Although hybrid riveting is not as widespread as other joining techniques, it is gaining popularity. The technology has existed for more than 10 years, but it has only been taken seriously the last few years as real-world applications slowly emerge. European manufacturers, such as German automakers, have been more receptive to hybrid techniques, especially when it comes to joining aluminium.



Two for One

Hybrid riveting is becoming a more common joining process for assembling structural components. "Improved efficiency and durability can be achieved without compromising the joint integrity," claims Scott Tremblay, application engineering manager at Henkel Loctite Corp. (Rocky Hill, CT). "Traditional fastening methods, such as riveting and welding, are often inappropriate for joining dissimilar materials. The use of hybrid-joining, combining the use of two or more joining methods, such as a riveted joint and adhesive, takes advantage of both methods while reducing the stress concentration and propagation within the joint."

Hybrid joining comes in many different forms, but it typically is defined as "the combination of two or more joining techniques to produce joints with properties additional to those obtained from a single technique," says Ewen Kellar, principal project leader of the polymers group at The Welding Institute (TWI, Cambridge, England).

"The most common types of hybrid joints are used for joining sheet materials and involve an adhesive in conjunction with a single-point joint technique, such as a rivet," explains Kellar. Techniques such as riv-bonding have been successfully used to join aluminum sheet in aerospace and automotive applications. Typically, adhesive is applied to the surfaces to be joined prior to assembly. Then the parts are joined with rivets.

Compared to joints made only with rivets, riv-bonding offers additional benefits, such as:

• Continuous, leak-tight joints.

• Higher-strength joints.

• Increased joint stiffness.

• Improved peel and impact resistance, because crack growth away from the joint is arrested by the adhesive bond.

Engineers who want to harness the benefits of hybrid riveting must have an open mind and adopt a new way of thinking. "Although purists may argue that a high quality, all-bonded structure is aesthetically and structurally superior, or that a bolted structure is the simplest and most effective solution, both approaches have their shortcomings," notes Kellar.

"The hybrid joint offers the benefits of both systems in that the number of fasteners can be reduced significantly, thereby reducing the effect of hole damage and increasing the fatigue resistance of the structure," adds Kellar. "The fasteners that remain can act as a backup mechanism in the event of significant adhesive failure."

Whenever two different joining technologies are used, engineers must carefully consider how the processes will react with or against each other. "When using welding in combination with an adhesive, the generation of fumes from locally burnt adhesive, as well as inclusions into the weld, can be problematic," notes Michael LaPensee, vice president of applications and marketing at Henrob Corp. (Novi, MI).

"Rivets combined with adhesive removes these issues. However, the adhesive can have a lubricating effect on the riveting process. This variable needs to be taken into account during rivet and die selection."



Applications Abound

Hybrid riveting is typically used when the required joint properties cannot be achieved from a single joining technology. Most applications involve dissimilar structures, such as joining steel to aluminum or composites to magnesium.

"Riv-bonding is not a high-volume, mainstream process," LaPensee points out. "It is usually applied on a case-by-case basis, often using existing rivets and existing adhesives."

Adhesive and rivet combination bonding is primarily used in aircraft assembly and automotive body panel fabrication, where the bonds are subjected to a wide range of dynamic and impact loads. It is often used for attaching dampening materials to structural members.

"[Most riv-bonding applications attempt to] capitalize on the load-spreading nature of an adhesive joint and its ability to maintain a smooth, seamless finish," says Henkel Loctite's Tremblay. "In some applications, the adhesive itself is the primary joining agent."

Mechanical techniques, such as rivets, act as a "tool" in the fixturing process and relieve the adhesive interface from stress and impact caused by peel, cleavage and long-term static and dynamic loads. The rivet acts as a fixture applying uniform clamp load while the adhesive cures. Another benefit of using adhesives is that they serve as a sealant-a protective boundary against corrosion-or damping material.

Self-drilling screws are also used with adhesive bonding. "The extruded aluminum frames on several European sports cars are assembled by this method," says Martin Schnurr, executive vice president of Textron Fastening Systems (Troy, MI). "Aluminum panels and extrusions are assembled with adhesives in fixtures, and then supported by drill screws that form their own holes and self tap. These types of screws avoid swarf, thus preventing debris that can inhibit bond strength."

Rivets also are applied in bonded joints in the aluminum frames of these vehicles, and break-stem fasteners are bonded in steel and aluminum joints. Schnurr says rivet bonding is used to assemble heavy-duty truck cabs where two-sided bonding tape joins the aluminum I-beam to its fiberglass cap. Rivets locate, reinforce and provide fail-safe backup. Similar processes are used to assemble recreational vehicles, buses and truck trailers.

Internal aerospace structures are another common application for riv-bonding. However, rivet bonding processes are not generally used on exterior structural assemblies of business jets and commercial aircraft.



Riv-Bonding Benefits

With riv-bonding, an adhesive in film or paste form is applied to the joint and then the rivet is set. The presence of adhesive does not change the basic setting operation, although it may have some influence on the setting parameters required and the load-displacement characteristics monitored during setting. Curing of the adhesive normally follows setting of the rivet.

"The rivet maintains the assembly location prior to adhesive cure, and later serves as a fail-safe backup to prevent catastrophic failure should debonding occur during service," says Schnurr. "The adhesive is the belt and the rivet is the suspenders."

Schnurr sees riv-bonding increasingly used in the automotive industry. Because it is a cold joining technology, riv-bonding can be applied to a wide range of substrates, including prepainted materials.

"The chief advantages of rivet bonding are control and flexibility," claims Schnurr. "Control is achieved by precisely fixturing and locating the component with the rivets prior to curing or during final assembly. Flexibility comes from the ability to create a joint clamp without costly, complex clamping fixtures."

The rivet-bonding combination reduces stress concentration in high-strength joints. The goal is synergy: Exceeding the individual properties of two processes by combining them in a single joining system.

"One of the key attributes of combining adhesives and mechanical assembly technologies is that a variety of benefits can be achieved which exceed the sum of the individual properties of the components in the holding process," adds Henkel Loctite's Tremblay. "The use of hybrid joining takes advantage of the individual components, resulting in improved joint quality and durability while minimizing production costs."

Most riv-bonding applications use self-piercing rivets. They do not require predrilled holes or hole alignment, and have high fatigue properties. However, blind rivets or clinching can also be used with adhesive.

"Self-pierce riveting is the best mechanical joining method to be combined with adhesive bonding," says Joe Butvin, director of sales and marketing at Bollhoff-Rivnut Inc. (Kendallville, IN). "It gives the best all-around ease of use and strength as a hybrid joint. The rivet pierces the top sheet and forms within the lower sheet and does not penetrate it. Therefore, the adhesive cannot leak through the joint."

If adhesive is used with solid rivets that are installed into drilled holes, it creates extra work for assemblers. For instance, the holes need to be made and cleaned before the adhesive can be placed in the joint. "On most structures, this would require fitting up of the parts, drilling the holes, then separating the parts again to clean the joint area before the adhesive can be applied," notes Henrob's LaPensee.

In most cases, it is relatively easy to combine riveting with adhesive bonding. "As long as the self-piercing operation occurs while the adhesive is in a liquid form, there should not be any problems," notes Tremblay. He says two-part epoxies and two-part acrylics work best for riv-bonding.

According to LaPensee, the most appropriate adhesives to use with self-piercing rivets are pumpable, heat-cured products. "These adhesives move relatively easy under the clamping and rivet insertion forces, allowing the joint to be brought fully together," explains LaPensee. "The heat cure prevents the adhesive characteristics changing over time, between the first and last joint, for example, or if production should pause for some reason part way through assembly.

"However, heat needs to be applied to the finished assembly to make the cure," adds LaPensee. "This, of course, can add cost if a paint bake or similar facility is not already in place."

There are other potential drawbacks to adding an adhesive to rivets that engineers may need to take into consideration. For instance:

• There are longer processing times compared with self-piercing riveting alone.

• There can be problems with contamination of the riveting tooling, such as blocking the die.

• Component surface preparation procedures may be required to ensure consistently acceptable bond quality and durability.

• Anisotropic adhesive distribution results from localized pressure caused by the riveting operation.

"I would not say there are distinct disadvantages [to riv-bonding], but there are challenges and considerations," says Mike Shirkey, president of Orbitform Inc. (Jackson, MI). "For example, when combining riveting with adhesive bonding, typically the adhesive bonding is done first.

"Therefore, there tends to be less alignment forgiveness when designing machines and fixtures to do the riveting," warns Shirkey. "Once parts are placed together with adhesive, there is little opportunity to move or adjust. This requires a little different thinking process when designing the tools."

Another obvious disadvantage to a hybrid joint is cost. "You are typically buying two separate joints for each location," says LaPensee. "There is also a cycle time and assembly complexity penalty in having two processes in place on the assembly line. Therefore, riv-bonding is normally only used where the benefits can justify the added costs."



New and Improved Process

To address some of those shortcomings, engineers at TWI have developed a new hybrid riveting process called AdhFAST. "This system is unique in that components can be assembled dry, checked for tolerance and then injected with adhesive at the final stage," says Steve Westgate, technical specialist in TWI's laser and sheet processes group. "This ensures a simplified process with increased quality control, maximizing the benefits of hybrid joint technology. Adhesive dispensing can be metered, with minimal wastage, making the process cleaner, with resultant health, safety and economic benefits."

According to Westgate, AdhFAST differs from traditional riv-bonding in that the adhesive is introduced into the joint after the structure has been assembled using fasteners. Adhesive is injected into the joint through specially designed fasteners.

The fasteners incorporate a means of controlling the spacing between the top and bottom substrates. "This gives greater control of bond-line thickness and, therefore, improved process reliability and joint quality," claims TWI's Kellar. "The AdhFAST fastener can be either in the form of a specially designed component or, in some cases, a standard commercial fastener to which some modification has been made."

Kellar says AdhFAST is a three-in-one fastening system "that allows adhesive to be injected through the middle of the device while retaining the joint and controlling the gap between the substrates or the bond-line thickness."

The AdhFAST system was developed to improve the performance and cost savings of traditional riv-bonding. "The use of such fasteners enables the advantages of adhesives to be combined with the benefits and confidence associated with fastener technology," Kellar points out.

Prototype AdhFAST fasteners have been made in aluminium alloy and a commercial rivet system has been adapted to work using a modified washer as the spacing element. But, Kellar claims that any material with appropriate structural properties, including other metals, plastics and composites, could be used.

The fastener design is very flexible and enables the three functions of retention, spacing and injection to be achieved. Injection of the adhesive can be carried out manually or by an automated process, making it ideal for high-volume production.



Automotive Applications

Automakers are constantly looking for new ways to reduce the weight and improve the performance of their vehicles. Aluminum offers tremendous potential, but also raises numerous challenges when it comes to cost-effective production processes. For instance, aluminum is typically more difficult to weld than steel and behaves differently when stressed.

As automakers continue to experiment with lightweight materials, most observers predict that hybrid riveting applications will grow. "The concept and use of hybrid bonding of dissimilar substrates is fairly recent," says Henkel Loctite's Tremblay. "It is becoming more popular in manufacturing body panels where [engineers] are looking for improved quality, reliability and long-term structural durability."

"Automobiles have historically been assembled using multiple spot welds and, in more recent years, self-piercing rivets," adds Henrob's LaPensee. "However, as the requirements for stiffer and quieter vehicles become more demanding, the use of adhesives is becoming more widespread. [But], adhesives typically have very poor peel strength, which compromises vehicle crash performance.

"The solution is to use a hybrid riv-bonded joint where the adhesive is the primary joining mechanism providing the stiffness and noise, vibration and harshness characteristics," adds LaPensee. "The rivets get free ride, but become the primary mechanism during a crash event when the adhesive would normally peel."

As automakers seek new ways to build aluminum vehicles economically, especially sports coupes and sports sedans, they're experimenting with new alloys and assembly technologies. Aluminum applications range from side-impact beams, bumper beams and crash boxes to structural modules and complete car bodies. And, there have been significant efforts to develop better adhesives, surface preparations and application systems to give engineers another assembly option.

"In automotive assembly, rivet bonding is used most frequently on extruded aluminum parts, where adhesives eliminate the need to weld," says Textron Fastening Systems' Schnurr. "Welding in aluminum can create significant stress by increasing the rigidity of a joint section and by reducing mechanical properties baked in by heat-treating processes. Bonding tends to be more specific in process controls, and it maintains consistent joint spacing."

Several European manufacturers have demonstrated that aluminum bodies assembled using riv-bonding technology can equal or exceed chassis stiffness requirements. Successful aluminum-intensive vehicles that use riv-bonding technology include the Audi A8, the BMW 5 Series and the Jaguar XJ.

"The new BMW 5 Series uses aluminum and steel from A pillar to A pillar," says Bollhoff-Rivnut's Butvin. "[BMW uses] 600 self-piercing rivets to assemble the components and most have adhesive involved."

The Jaguar XJ features an aluminum-sheet-intensive monocoque construction. It is assembled using lightweight aluminum joined by self-piercing rivets and epoxy adhesive. Jaguar engineers claim they adopted state-of-the-art techniques from the aerospace industry.

"The XJ's body is 60 percent stiffer yet 40 percent lighter than before," boasts Mike Richardson, research manager at Jaguar Cars Ltd. (Coventry, England). "In the XJ8, that translates into an overall weight savings of almost 200 pounds."

The new XJ's body structure comprises pressings of sheet aluminum combined with aluminum alloy extrusions and aluminum castings. "[It] represents an industry-first use of rivet-bonding construction for an aluminum-intensive monocoque body," says David Scholes, XJ chief program engineer. "Rivet-bonding uses self-piercing rivets and epoxy adhesives for strength, robustness and durability when joining aluminum pressings."

At Jaguar, adhesive is applied robotically during assembly. It heat-cures to optimal strength during the vehicle painting process. Approximately 3,200 rivets are used in the construction of the new body.



Sidebar: When Rivets Don't Fly, Hybrid Joints Do

Traditional aircraft assembly is based on structures of riveted aluminum and composites. Riveted structures typically have good peel and shear resistance, but they have limited fatigue strength, are labor-intensive and degrade the exterior appearance of aircraft. Composites are gaining popularity, but they can be several times more expensive than aluminum.

To address those issues, some aerospace manufacturers are experimenting with weld-bond processes. Weld-bond joints are a desirable alternative to rivets because welds overcome the shortcomings of bonds, and vice versa. Weld bonding combines the best properties of adhesives and welding, and is much faster than riveting.

"A paste adhesive is normally applied to one sheet and the joint closed," explains Steve Westgate, technical specialist for resistance welding in the laser and sheet processes group at The Welding Institute (TWI, Cambridge, England). "A spot weld is then made through the adhesive. The electrode force displaces the adhesive to obtain electrical contact between the sheets and a weld is made in the normal way.

"As the heating of the weld is very localized, little damage occurs in the adhesive around the weld," adds Westgate. "The adhesive is cured to complete the assembly. Heat curing paste-type adhesives are normally used as these are stable and have a consistent viscosity at room temperature. Typically, such adhesives are cured in an oven at up to 180 C for 30 minutes."

Although welds can replace rivets with good peel and good shear resistance, they show poor fatigue resistance. "Adhesive bonds, unlike welds, provide exceptionally good fatigue resistance," notes George Ritter, principal research engineer at the Edison Welding Institute (EWI, Columbus, OH). "Their lower peel strength makes them questionable for terminal joints, yet they perform well on intermediate joints."

Ritter and his colleagues have been experimenting with laser welding. They claim that the laser weld-bond process could reduce airframe manufacturing costs by 25 percent.

"One important advantage of laser welding is that the weld could be made on the inside of the structure under construction, using fiber optic delivery from the outside," says Ritter. "Conductive heat resistance welding [tests] did not provide the surface finish that laser welding did."

According to TWI's Westgate, resistance spot welding in combination with adhesives is one of the most common hybrid joining technologies used in automotive applications. "Use with structural adhesives, rather than low-strength adhesives or sealants, is on the increase," Westgate points out. "The weld bonding process is often fully automated and utilizes robotic dispensing systems."

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