Bonding Metal Parts

Structural adhesive is being used for more and more assembly applications.

In the musical "Fiddler on the Roof," the main character, Tevye, sings about "tradition." He laments how the age-old ways of the past are slowly giving way to new customs. If Tevye was an engineer, he'd probably be singing about new processes being used to join metal components.

Traditionally, aluminum, steel and other metal parts are joined together with mechanical or thermal methods, such as rivets or resistance welding. But, structural adhesive is now an alternative that engineers must seriously consider.

Manufacturers ranging from Deere & Co. (Moline, IL) to Renault S.A. (Boulogne-Billancourt Cedex, France) are using acrylics, epoxies, urethanes and other adhesives to replace traditional metal joining processes. Adhesives offer numerous benefits, such as their ability to join dissimilar substrates and sensitive materials with a continuous joint. Other advantages include:

n Cost savings. Adhesive joining methods typically offer significant material and labor cost savings over welding and mechanical joining methods.

n Design flexibility. Engineers are able to create more varied designs, because structural adhesives eliminate the need to allocate space for mechanical fasteners.

n Lower weight. Adhesives significantly lower the weight of assembled products.

n Structural integrity. Adhesives can minimize problems associated with joints failing due to vibration. Also, advances in adhesive additives improve quality control by helping assemblers determine if enough adhesive has been placed in the joint to make a good bond.

"There are a number of benefits of using an adhesive to join metal parts," says Jim Burger, senior technical service engineer in the industrial adhesives and tapes division of 3M Co. (St. Paul, MN). He believes stress distribution is a key benefit. "An adhesive bond distributes the stress across the entire bond vs. concentrating it at the point of mechanical attachment," explains Burger. "Adhesives can also provide a seal as well as a bond. Adhesives are known to reduce squeaks and rattles and improve noise, vibration and harshness."

Adhesive bonding offers aesthetic advantages as well. For instance, unsightly rivets, protruding bolts, screw heads and surface markings, such as spot welds, are eliminated. A smooth, bonded surface can help reduce drag because the contour of the part surface is not interrupted by a fastener. Time-consuming operations required to remove welding slag and prepare the surface for finishing can be avoided with adhesives.

Adhesives allow assemblers to bond dissimilar metals, without promoting galvanic corrosion. They allow the bonding of coated, galvanized or painted metals without negatively affecting the surface finish. And, they can act as a dielectric insulator between components.

Improved design versatility is another benefit that is sometimes overlooked. "Adhesives can be used on very small assemblies because they do not require drilling holes in parts to facilitate mechanical joining methods," says Pat Courtney, engineering project manager at Henkel Loctite Corp. (Rocky Hill, CT). "In addition, adhesives can aid in bonding metals to other substrates, such as glass and plastic. Thinner sections of metal can also be used because the load is distributed over a wider area."

Courtney says adhesives are also compatible with lean manufacturing initiatives. "The expansion in the variety of adhesives available, coupled with advances in adhesive dispensing, curing and monitoring technology, has made the use of adhesives in single-part-flow, high-speed manufacturing operations commonplace," he points out.

"When you attach a fastener or spot weld a joint, your assembled joint is at full strength, while an adhesive will require time to develop strength," says Courtney. "However, in operations with a large number of fasteners or applications that have large areas that need to be welded, the total processing time in the application and assembly of the joint with the adhesive is much faster.

"Even when the fixture time of the adhesive is taken into account, the bonding process can be faster," claims Courtney. "If one considers the time associated with drilling holes in the assembly to prepare it for mechanical fastening, or the time associated with the secondary operations required by welding, the adhesive option is even more attractive."



New Technology, New Applications

Traditionally, adhesive has been primarily used to bond plastic parts. "Ten years ago, people said it would never work for metal bonding applications," recalls Dave Bongiorni, market development manager at Devcon (Danvers, MA). But, recent technological advancements and new chemistries have made adhesives more compatible with aluminum, steel and titanium.

"Users have an unprecedented choice of materials and performance characteristics," says Bob Kruger, marketing manager at Huntsman Advanced Materials (Los Angeles). "Today's adhesives, including epoxies, polyurethanes and methacrylates, exhibit greater strength and a broader range of physical characteristics, including varied work lives, lap shear strengths and resilience. The adhesives also have versatile cure cycles ranging from seconds to days."

In the past, two-part no-mix acrylics were somewhat brittle and offered slower cure speeds. "Subsequent developments in this area have led to the introduction of products that offer much faster cure schedules, improved flexibility and impact resistance, and more versatile cure systems," notes Courtney.

"Some even can be cured by ultraviolet light in addition to the traditional activator cure, thus further simplifying their use in in-line manufacturing processes," adds Courtney. "A similar trend in developing faster curing products with simplified processing requirements and better performance properties can be seen with epoxies, urethanes and two-part static mix acrylics."

"Today, engineers are relying less on hot-melt structural adhesives and are more frequently specifying two-component and heat-cure adhesives," says Curtis Partridge, marketing manager at Hernon Manufacturing Inc. (Sanford, FL). "These improvements offer more control over the fixture time of the product. This allows more precise placement of components."

Because of new technology, there is increasing demand for structural adhesive in metal joining applications. "Adhesive bonding is often the attachment method of choice in order to minimize galvanic corrosion, increase joint strength and reduce weight," says Burger. "As new materials are introduced, the need to bond dissimilar materials is also increasing." For instance, recent developments have allowed the structural bonding of metals to low surface energy substrates, such as polyolefins.

Automotive and aerospace manufacturers have achieved the largest benefits from structural adhesives because they are constantly seeking to reduce the weight of their products. "For every pound that an aircraft designer can remove from the assembly, that is a pound that can be added to payload or fuel," Partridge points out. "Automotive engineers are seeking lower weight because of the increased pressure to improve the fuel economy of vehicles. The aesthetics of adhesives also lend themselves well to the flowing line trends of today's automobile and truck designs." In addition, automakers are using adhesives to address noise and vibration issues.

Because adhesive works well with aluminum, aircraft manufacturers bond stringers to fuselages. Adhesive is also used in applications involving ailerons, landing gear doors, wing flaps and other control surfaces, and it's ideal for attaching composite materials to metal components.

Some automakers are using adhesive for body assembly applications, such as joining outer skin to frames, bonding hem flanges, stiffening window posts and attaching reinforcement ribs to bulkheads. A leading European automaker has just started to use adhesive instead of welding on some underbody components, such as the engine compartment and rear frame.

According to Curtis, truck, bus and recreational vehicle manufacuturers that have traditionally used fasteners and welding are heavy users of adhesives. For instance, many truck trailer assemblers are now bonding side panels.

Demand for structural adhesives is increasing in other market sectors. Many different industries are using adhesives to join sheet stock, structural frames and other metal components. "Just about any industry can find benefits in using structural adhesives, but the hand-tool, toy, power tool and appliance makers are discovering structural adhesives are the best way to lower costs and increase reliability," says Partridge. "Any seam that traditionally called for a weld is now a perfect candidate for structural adhesives."

Typical applications include:

  • Advertising-attaching nameplates and signage.
  • Appliances-attaching reinforcement ribs in the corner of sheet metal assemblies; bonding shelf assemblies; bonding dryer drum assemblies; and attaching retaining gears to shafts for washing machines.
  • Hard disk drives-bonding magnets to base plates; bonding shafts to bushings in motors and armatures; and coil bonding.
  • Medical devices-assembling needles and prosthetics.
  • Office furniture-assembling desks and file cabinets.
  • Sports equipment-assembling bicycle frames and golf clubs.



Drawbacks

Despite numerous advantages, metal bonding is not ideal for all applications. For instance, some metals simply aren't compatible with adhesive. "Magnesium and nickel-based alloys are difficult to bond with adhesive," says George Ritter, Ph.D., principal engineer for adhesives and composites at the Edison Welding Institute (EWI, Columbus, OH).

"Some soft metals, such as pewter and lead, can be difficult to bond successfully because the adhesive will tend to pull off a thin surface layer of the material when it is subjected to a heavy load," adds Courtney.

In addition, adhesives need to cure after application, which can add extra time to the assembly process. And, adhesive creates a relatively permanent bond that can be difficult to disassemble if repairs or alternations are needed.

Another drawback to adhesive is surface preparation. Many adhesives will not bond to oily metals. Surface preparation can take anywhere from 30 seconds to 2 minutes.

"Adhesive bonding requires more surface preparation than traditional joining methods, such as fastening and welding," says Ritter. "When bonding metal, the oxide layer must be stabilized."

"Degreasing is required, followed by abrading and then cleaning," explains Kruger. "Metal substrates may also require chemical or electrolytic pretreatment to improve bond strength and long-term durability."

"Surface preparation is very important because the adhesive only sees the surface," warns Ritter. "Good surface preparation can range from simple cleaning to extensive anodizing and priming. The degree of surface preparation depends on the needed performance and service lifetime."

Because of the expense and time involved in surface preparation, manufacturers have worked hard to improve this step of the bonding process. A new, much easier form of surface preparation called sol-gel has been developed by engineers at the Boeing Co. (Chicago). "The sol-gel process is used in place of anodizing and has been applied successfully to aluminum and stainless steel," says Ritter. "It has merit on titanium. The process is water-based. It eliminates the need for using hazardous materials and successive processing tanks." Sol-gel can be applied by spraying, brushing or dip-coating.

Some adhesives are more tolerant of surface oils, such as two-part no-mix acrylics and two-part static-mix acrylics. Two-component methacrylate adhesives also require a minimal amount of surface preparation. "However, in general, a clean dry surface is important for achieving maximum bond strength and environmental durability," says Courtney.



Joint Design

When switching from mechanical joining to adhesive bonding, designs must be reviewed and possibly altered. "Fastened joints are different in terms of their stress-load bearing potential than adhesive bonded joints," warns Kruger.

Adhesives depend on surface contact to create a bond to the substrate. Joints must be designed to give the adhesive as much surface contact as possible. Lap joints are the most commonly used adhesive joint and work best with metal applications. They are simple to make and load predominantly in tension.

"With any adhesive joint, the goal is to achieve as uniform a stress distribution as possible," adds Henkel Loctite's Courtney. "Part designs that place the joint under tensile or compressive loading when the joint is loaded are ideal. Typically, it can be difficult to design a joint to achieve this, so shear loading is also used. Fortunately, the rigid nature of most metal substrates helps achieve a more uniform stress distribution when shear loading is used.

"At all costs, it is best to minimize or eliminate any peel or cleavage forces on the joint when the assembly is loaded," warns Courtney. "These forces concentrate the entire load on a small portion of the joint and can dramatically increase your adhesive performance requirements, thus limiting your choice of adhesives and your ability to minimize cost."

"A joint that provides several thousand psi joint strength in tension may provide only a few pounds of strength in peel," adds Ritter.

Joint integrity depends on surface preparation, the type of loading and environmental exposure. "Adhesives fail by creep mechanisms that are time-dependent, while mechanical fasteners usually fail by fatigue mechanisms that are cycle-dependent," Ritter points out. "Bond reliability is difficult to assess from testing, and field results play heavily in determining the effectiveness of the overall method."

According to Ritter, adhesives are "more affected by harsh environments because of the possibility of chemical attack on the adhesive or the possibility of corrosive attack at the bond line, which weakens the bond. Mechanical fasteners can be affected by corrosion, which weakens the mechanical linkage."



Bonding vs. Fastening

Many engineers believe mechanical fastening is the only way to join metal parts. However, most adhesive experts claim that the ability of an adhesive to distribute a load over its entire bond area rather than in a limited area, as with a mechanical fastener, can lead to improved assembly durability and fatigue resistance. They also point out that adhesives are generally stronger and have less chance of failing than fasteners.

Threaded fasteners rely on friction between the parts and the threads to hold an assembly together, explains Partridge. Stress-points can be created because mechanical fasteners only join the assembly at specific points. At each point of contact, mechanical fasteners exert force in just one small area of the substrate surface. Structural adhesives produce a molecular bond along the entire length of the joint, which results in more strength and less failures.

Adhesive processes generally offer cost benefits over mechanical fasteners because total material costs are lower. In addition to eliminating the cost of the fasteners, adhesives eliminate the cost of drilling and tapping holes, as well as sealing devices such as gaskets. "Adhesive prevents the loosening and leaking that can occur when fastener holes expand due to vibration or flexing," notes Devcon's Bongiorni.

"Tooling costs associated with the joining operation are eliminated. Costs of inventorying and stocking a wide variety of components-bolts, nuts, washers, rivets and other parts-are eliminated," explains Courtney. "Minimizing the number of parts used in the assembly also reduces quality costs. The ability to automate the adhesive assembly process frequently lowers labor costs as well."

At very high temperatures, mechanical fasteners perform better than adhesives. "Once temperatures exceed 400 F, your structural adhesive options diminish rapidly," warns Courtney.



Bonding vs. Welding

For many years, engineers have been taught that welding is the best way to achieve a quick, high-strength, permanent joint. But, adhesives are encroaching on many traditional thermal applications.

"Traditional automated welding systems can require a large capital investment for equipment and continued high maintenance and operating costs," says 3M's Burger. "Smaller, manual welding systems are less expensive, but can require a more highly skilled labor force. Both automated and manual welding systems typically require labor-intensive surface finishing processes." In contrast, adhesives can be applied automatically or with unskilled labor.

According to Hernon's Partridge, welding equipment is more expensive to operate than adhesive systems. "Welding requires more energy and the removal of hazardous fumes," he explains. "Eye protection and fire hazards are additional cost considerations."

Some experts also claim that welding will promote corrosion, while adhesives can eliminate corrosion. In addition, Bongiorni says adhesive "is particularly useful for bonding thin metal panels in applications where welding would produce aesthetically unacceptable dimpling or scorching."

However, welded joints withstand very high temperatures better than bonded joints.



Material Matters

Adhesives work well with both aluminum and steel. "On average, higher bond strengths are achieved on steel than aluminum," says Courtney. "This is likely due to the presence of a weak oxide layer on the aluminum that can detach easily. When aluminum is prepared by acid etching, much higher strengths are achieved."

Typically, epoxies and methacrylates work best with steel. "Because of the high ionic activity of steel, anaerobic structural adhesives have been a consistent option for this substrate," notes Partridge. "Two-component methacrylate adhesives have [recently] become a more popular choice for bonding steel."

According to Huntsman's Kruger, epoxies are the most widely used adhesive for aluminum applications. However, he says some methacrylates are also well-suited for bonding aluminum.

The best adhesive for a given application is generally a function of part design, processing needs and performance requirements. "For example, if you had a part with a large gap, such as 0.1 inch, and you could tolerate a slow fixture time and longer cure schedule, such as 24 hours, and you wanted the highest shear strength possible, an epoxy or two-part static-mix acrylic would be your best choice," says Courtney. "Epoxies cure more slowly than the two-part static-mix acrylics, but offer better high-temperature resistance.

"On the other hand, if you had tightly fitting parts, needed the adhesive to fixture in less than 30 seconds and wanted very high impact strength, a two-part no-mix acrylic adhesive or rubber-toughened cyanoacrylate may be a better choice," adds Courtney. "In this case, the two-part no-mix acrylics require the application of an activator, so you might prefer the simpler processing of a one-part cyanoacrylate.

"However, if your use temperature is greater than 250 F, the acrylic will be a better choice. If the assembly will see a great deal of flexing and repeated impact, the toughness of a urethane would be worth considering. At the end of the day, there isn't a clear winner; just a variety of different options with various processing and performance trade-offs."



Myths and Fallacies

Some engineers shy away from using adhesive technology. "Many don't want to change what they've been doing," says Kruger. Some feel that adhesives are "messy" or take too long to fully cure. Others simply believe that you can't beat a good old-fashioned fastener or weld.

"One of the biggest misconceptions with adhesives is that they are messy, slow and extremely difficult to use in a high-speed manufacturing environment," notes Courtney. "However, dispensing systems on the market can repeatably dispense adhesives. Whether it's a workcell or inline manufacturing environment, there are adhesive handling options available that improve workflow, reduce inconsistency and minimize waste."

Using adhesive to join metal parts is different than traditional joining processes, such as fastening or welding. As a result, it requires some learning and adjustment.

"It takes work to make an adhesive work," warns EWI's Ritter. "Adhesive has different strength characteristics depending on what you do with it. You must do an incredible amount of testing." Ritter says a bolt or a weld is much more predictable.

"The design information associated with joining assemblies with mechanical fasteners and welding are widely taught, while the information required to make good design decisions concerning adhesive selection and implementation is harder to come by," Courtney points out. "As a result, people tend to shy away from the technology until its benefits force them to climb the learning curve."

Courtney admits that the wide variety of adhesives on the market can make selecting the correct adhesive for an application appear to be a more daunting task than going with a "traditional" joining process. "Many folks have had some training in traditional joining methods that gives them a comfort level they may not have with adhesive joining methods," says Courtney. "Fortunately, the benefits of adhesives generally yield a high payoff for those who invest the time to learn about them."

"Often, adhesives are not considered due to a historical educational gap in adhesive bonding," adds Burger. "This is now changing as textbooks are starting to include chapters on adhesive bonding and universities are adding the subject to their curriculum."

Indeed, Bongiorni says there is a distinct age-related knowledge gap. "Younger engineers-anyone under 45 years old-are much more likely to use adhesives for metal joining applications," he explains. "They are more exposed to adhesives as a viable replacement for fasteners and welds." Bongiorni believes it's only a matter of time before tradition gives way to new ideas.

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