Metal bonding offers a practical alternative to welding 

Many of the major OEMs now recognize adhesive bonding as an important alternative to conventional welding for many repairs. Specific repair standards may vary by OEM so refer to TSBs and other repair guidelines as well as bonding material instructions. The example below covers common procedures.

Remove all paint and rust from surfaces to be bonded using a 36- or 50-grit disc. Straighten all metal and then “dry-fit” the part. Make adjustments to the part until you have a good fit. Clean all areas with general purpose adhesive cleaner.  Then apply adhesive to all areas to be bonded following the adhesive manufacturer recommendations. A plastic spreader should be used to tool out the adhesive to cover all bare metal surfaces. This will set up the corrosion protection necessary. Apply a second adhesive bead approximately 1/4 in. from the inside edge of the replacement panel. Clamp the panel in its proper position. Tool any adhesive “squeeze out” to seal the outside edge of the seam along the bonded edge of the panel. Welding is still required along areas such as the vertical edge of the panel and where overlap or butt-ups of panels occur. Check with the OEM for specific guidelines on all repairs. Heat lamps may be applied to accelerate the cure of the adhesive. Consult manufacturers’ recommendations for specific requirements.

By Jim Perritt, Contributing Editor

At the automobile manufacturing factories, welding has been the main method of sheet metal joining for more than 80 years. However, in the past decade, car manufacturers have been looking for alternative methods of joining metals. Adhesive bonding technology is rapidly gaining acceptance as an alternative to spot welding because it offers many benefits to the automotive design engineer. These benefits include the ability to join dissimilar materials, greater flexibility in manufacturing and large cost savings to the automotive industry. 

Today, car manufacturers often use adhesives in combination with spot welds, or weld bonding, and for panel stiffening and vibration dampening. The average mid-sized car contains 4,800 spot welds. The average cost of a spot weld is about a nickel. According to a speech given at a recent Auto Interiors Conference by George Hamilton, CEO of Dow Automotive, it is estimated that as many as one-half of all spot welds could be replaced in the vehicle assembly process using adhesive bonding, resulting in savings to the OEM of more than $80 per vehicle. Additionally, vehicles produced using adhesive bonding techniques benefit from improved crash performance, corrosion durability, and greater stiffness resulting in improved NVH. 

With increasing use of adhesives at the factory level as well as in aftermarket repair, OEMs are working with adhesive companies to evaluate products and repair techniques to ensure quality and durable repairs in the aftermarket. Since General Motors Corp. (GM) published GM Technical Service Bulletin No. 02-08-98-001A, which provides detailed guidelines for repair shops looking to use adhesive bonding of exterior panels as an alternative to MIG welding, body shops have begun to recognize the importance of using an adhesive that meets OEM specifications for performance and durability. Recently, DaimlerChrysler expanded its recommendations for adhesive use to include structural weld bonding in its brochure No. 81-170-03005. 

Choosing an Adhesive
The principle of adhesion starts with the concept of “wetting,” defined as “the intimate contact of two panels to be joined.” Adhesives need to achieve complete contact with both of the panels being joined for bonding to occur. Wetting is the first essential step for all adhesives, whether they are simple glues, cements, sealants or structural adhesives. The importance of this concept is in the need to choose the appropriate adhesive for each size job. The selected adhesive must stay wet during the entire assembly process, from the time the adhesive is dispensed until the two panels being bonded are joined and clamped to ensure full contact of the bonded panels. Because the size of the panels to be bonded has a direct impact on the amount of wetting time needed to properly install the panels, it is important to choose an adhesive that has a “work time” greater than the longest possible time required to complete the repair. For this reason, most adhesive companies offer two or more adhesives for application of small, medium and large panels. 

Once good wetting takes place, an adhesive needs to become solid and not flow at all. The technical term is setting, or cure. The ideal adhesive will have sufficient wetting or open time to install the panel but will set quickly and cure so that other repairs can be made on the vehicle without disturbing the bonded panels. The faster the set and subsequent cure, the sooner the car can be moved for frame straightening or other procedures. Typically, acrylic adhesives set and cure the fastest, followed by epoxies and urethanes. 

Finally, a good metal bonding adhesive must be strong and durable. A high-quality metal bonding adhesive must not only withstand laboratory-strength tests but should also be proven through destructive testing such as Federal Motor Vehicle Safety Standard (FMVSS) 301, which measures fuel system integrity in a crash and FMVSS 208, which measures roof integrity, and/or offset barrier crash tests such those performed by OEM’s to determine crashworthiness. In addition, to ensure a lasting repair, the adhesive should be subjected to various aging tests, heat and cold cycles, and salt-water spray tests. These tests typically simulate the stress and durability that an adhesive must be able to stand beyond the life of a vehicle. 

Why and Where to Use Adhesives
As seen in the automotive factories, the use of adhesives for bonding metal body panels is becoming more commonplace in the aftermarket. Unlike welding, adhesives distribute stress uniformly, provide sealing characteristics and sound dampening, and provide a moisture barrier, minimizing the chance for corrosion. In fact, when using a full bonding method for attaching secondary metal body panels, some adhesive manufacturers provide a lifetime warranty against corrosion as well as a standard lifetime adhesive performance guarantee. Adhesive bonding can improve repair cycle time and, as compared to welding, is relatively easy to learn and master. 

Adhesive manufacturers are recommending the bonding of outer panels, including door skins, quarter panels, rear body panels and roof panels. Many adhesive manufacturers have conducted testing to validate the bonding of these parts and secondary panels. There are some adhesive manufacturers who are currently working with several OEMs to develop repair procedures incorporating the use of adhesives for repair of structural components. However, until these procedures are finalized, it is recommended to use adhesives only for the bonding of secondary, non-structural panels only. 

To learn more about adhesive applications and repair techniques, some adhesive manufacturers offer clinics. Many manufacturers will come to the shop to provide instruction on the use of their products. In addition, I-CAR offers an adhesive bonding course. 

What’s Next?
With increased use of adhesives in the factories and in aftermarket repair, OEMs are working with adhesive companies to evaluate products and repair techniques to ensure quality, durable and dependable repairs in the aftermarket. Many OEMs and other manufacturers are recommending adhesives for replacement of welding in numerous technical service bulletins, and it’s apparent that the OEMs believe adhesive bonding is an important repair alternative to conventional welding techniques. Meanwhile, all of the major manufacturers continue to investigate the use of adhesives for repair of structural components as well as secondary body panels as they work to provide alternative welding and varied repair procedures.