Rivet bonding aluminum

Sept. 19, 2016
Techs must know how to use a rivet gun, which rivet to use and the required adhesives for the job.

Car and truck designers are utilizing more and more aluminum as a substrate due to its lightweight, strong properties that make it ideal for boosting performance and fuel economy. Aluminum-bodied vehicles are also built with a combination of other materials such as steel, plastics and carbon fiber.

By now, many technicians should not only be seeing damaged aluminum panels on vehicles, but some structural components also, such as strut towers and inner reinforcements on steel vehicles. In some areas, technicians will also be seeing aluminum-intensive vehicles, such as the Ford F-150, besides high-end European vehicles and Tesla models, which have been out in limited areas for 15 to 20 years. However, the usual repair techniques utilized on steel vehicles, such as metal welding (MAG) and Squeeze Type Resistance Welding (STRSW), may not be ideal or even available to all facilities. Steel is MAG welded (Metal Active Gas) and aluminum is MIG welded (Metal Inert Gas). Many OEs that produce aluminum-intensive vehicles will use a combination of MIG welding, resistance welding (extremely limited), clinches, friction-stir welding, rivets, adhesives and rivet bonding to build their vehicles. Conversely, in the repair field we are really left with only four choices: MIG weld, bond, rivet or rivet bond. While rivet bonding is a relatively simple procedure, it is important for the repair technician to understand how to use a rivet gun, which rivet must be used for the application and the required adhesive(s) to perform a correct repair.

Rivet bonding, a combination of rivets and adhesive, provides immediate mechanical fixturing of panels, resulting in a quick repair and excellent joint strength when the adhesive has cured.

Welding vs. rivet bonding
At the OEM level, spot welding of aluminum is possible, but it has been challenging. Power requirements for aluminum welding are about three times the power needed for steel. Aluminum conducts heat much faster than steel, so the heat must be concentrated in the weld zone to maintain the heat and the background current. Because of the concentration of heat in the weld puddle, the weld will always try to suck back, undercut and melt the aluminum being welded. Additionally, an oxide layer forms on bare aluminum rapidly and this oxide must be removed prior to the application of body fillers, adhesives, welding or primers.

Welding aluminum properly requires lots of practice and good technique. Two to four hours practice per week is required to maintain good habits and techniques. Unfortunately, not every technician will be able to perform proper aluminum welding.

Aluminum welding requires highly specialized weld units, inverter pulse type, which have a cold start feature (pre-heating) and a crater fill (cool down) at the end of what is now a hot weld. To perform proper welds, it may be unfeasible for most of the average repair shop. But the OEMs understand this, and many of their repair procedures are including mechanical fastening (rivets or rivet bonding with adhesives). This process is quicker, more cost effective and an easier process for most shops. Additionally, it is fairly easy to learn and master with little training and practice required. With rivet bonding, welding heat is eliminated from the joining procedure, and the rivets deliver such a tight fit that in conjunction with the adhesive material, corrosion is unlikely to occur in the mating joint.

“New” rivet bonding?
With the advent of aluminum and composites as substrates in vehicle manufacture, rivet bonding will be a new procedure for many repair shops. As a fastening method, though, rivets are not new, as they have been in use for more than 60 years in the aerospace industry. Aircraft, which are mostly made of aluminum, feature rivet fasteners everywhere — inside and outside.

To perform a successful repair, the technician must know the rivet style specified by the OEM and the adhesive recommended by the OEM, along with using the proper tooling for installing the different rivets. Follow OEM guidelines for choosing which rivets and adhesives to use when making vehicle repairs. Any deviation from the OEM standards can result in a failed repair, rework and customer dissatisfaction.

Structural rivets are available in various materials, strengths and coatings. Always consult OEM technical resources to determine the correct part number and locations, as well as the suggested adhesive or sealant usage within the joint.

Rivets and tooling
Repair technicians should familiarize themselves with the three basic types of rivets used on most aluminum vehicles — pull-style, self-piercing and flow-form rivets.

Pull-style rivets are referred to as pop rivets™. These structural rivets, such as the Hemlock® model, are strong, feature a mechanical lock and break close-to-flush with the rivet head. To install a pull-style rivet, drill a hole in the panel, insert the rivet, and with the appropriate tooling, pull the metal shank to crush the rivet.

Self-piercing rivets (SPRs) are pushed into the metal panel with a specialized tool; no pre-drilling is needed. An SPR cuts its own hole and creates a mechanical lock within the material. As an example, Henrob® SPRs fasten two or more sheets of metal by driving a rivet through the top sheet(s) and securing the rivet into the lower sheet without piercing it.

Flow-form rivets are used extensively in the construction industry, especially for building high-rise structures. They are now available in a small-scale version for vehicle repair. To install a flow-form rivet, punch a hole in the panel, apply adhesive, insert the rivet and press the panels together.

For the most part, each type of rivet requires its own installation tooling. If the proper tooling is not used, the workpiece could be distorted or enough clamping force will not be delivered to successfully install the rivet. There are some specialized tooling systems available that can install all three types of rivets. For example, Wielander+Shill’s Xpress 800® is a modular system that can press-out existing rivets and install different types of rivets.

Specific installation tools for each type of rivet are available. As an example, Chief Automotive Technologies’ HD Rivet Gun is a heavy-duty riveting tool designed for riveting thin sheet metal, such as aluminum, where high insertion force is necessary. This pneumatic-hydraulic rivet gun presses-out and rivets SPRs.

Rivets and adhesives
When beginning a repair procedure on a damaged vehicle, it’s important to make sure you are using the right rivet for the repair. First, check the OEM recommendation for the rivet part number, and then order the rivets per repair. Only specialized repair shops, for a particular vehicle, will be able to keep in stock the rivets needed for repairing that vehicle. Each type of rivet is classified by the OEM for grip range, coating and strength, with part numbers indicated for repair rivets.

In the rivet bonding process, adhesives provide strength, corrosion protection and sealing. As with rivets, it’s important to use the adhesive recommended by the OEM for the repair procedure. Though OEMs might use several different types of adhesives in vehicle manufacture, they will usually specify only one type of repair adhesive. The choice of adhesive depends on the function of the joint in the vehicle. 

In any case, the OEM will be specifying a structural adhesive or the next-level-up, a crash-durable adhesive or impact-resistant adhesive, for rivet bonding. Be sure to follow OEM requirements for this process. These “impact-toughened” adhesives offer the strength of structural adhesives combined with excellent flexibility. This property is especially important when joining thinner substrates, such as aluminum, where the extra flexibility helps to hold the joints together in crash mode.

Simplicity with complexity
Rivet bonding is a relatively simple process; the complexity results from having to fix multiple car models. Each damaged vehicle coming into a repair shop will have different requirements for the type of rivets and adhesives to be used. Follow OEM specifications for rivet type and adhesive brand to ensure that a proper repair will be made.

Using adhesives in the rivet bonding process is not complicated. First, prep the panels per OEM requirements. Apply adhesive to the panels that will be bonded, position them properly and lock them in place with temporary clamping. You are now ready to install the rivets. The rivets will be installed through the adhesive and the substrate panel. If you are using a pull-style rivet, drill the holes first, deburr all of the holes, apply the adhesives and join the panels. With SPRs, install the rivets once the panels have been joined when using flow-form rivets, punch the holes after the panels have been joined, deburr the holes and then install the rivets.

Here are a few fastening guidelines to help with the rivet bonding process:

  • Follow OEM repair recommendations to determine if a bond-only installation is allowed.
  • If a bond-only recommendation is specified, replacement is complete when the panels are clamped.
  • Mechanical fastening should be completed when the replacement panel is clamped and before the adhesive begins to cure.
  • Clamps can be removed as the mechanical fastening is completed, since the fasteners will hold the panel in position until the adhesive cures.

Choosing an adhesive
Adhesives are specified by the OEM according to supplier name; this means that the OEM has tested the adhesive material and knows that it will work for the repair. Don’t just reach for “any adhesive” off the shop shelves to do a repair. While a shop might have some adhesives in stock, it is preferable not to use adhesive that is past its expiration date. As with rivets, order the OEM-recommended adhesive to execute the repair work.

BMW and Mercedes recommend their branded products. Cadillac recommends Fuso 2098 Crash Durable and 3M 07333 Impact Resistant adhesives. Ford recommends their Motorcraft TA-1-B adhesive and also Fusor 108B and 3M 08115. Chrysler recommends Fuso 112B and 3M 08116 adhesives.

Some OEMs will recommend a specific brand of adhesive or “an equivalent.” But this is where a problem can occur. How does a technician know what the OEM means by an “equivalent” product? In most instances, the OEM does not provide enough information to help the technician make an informed decision about equivalency.

At the OEM level, there are standards used to define the various chemicals that are employed to build a vehicle, and the properties that these chemicals need to provide. But the statement “or equivalent” does not tell the end user (repair shop) what those standards are and how to determine if the equivalent adhesive would meet those standards.  

To ensure that a vehicle is fixed properly, use the OEM-specified adhesive. If for some reason that material is unavailable and the OEM says to use an “equivalent” and they do not list an equivalent, then the recommendation would be to contact I-CAR, Lord Fusor or 3M to determine the equivalent. Failure to use the proper procedures, materials and products could change the designed intent of the vehicle to some degree, and in a subsequent collision event, cause a failure in the vehicle.

Mixing and preparation
The adhesives you will be using for rivet bonding are two-component formulations and it is important that the cartridge is equalized before applying the adhesive. When using two-component adhesives, “Part A” must be mixed with “Part B” to obtain the proper adhesive blend. But most important, the cartridge must be equalized before use. A small amount of the adhesive must be pre-extruded to ensure that both parts will be dispensed when pressure is applied to the cartridge through the applicator. The adhesive will be weakened if the correct mix is not attained. To prepare the cartridge after equalization, attach the mixer, expend a small portion of the mixed material and discard it; the adhesive will then be on ratio.

Surface preparation is another important element of the rivet bonding procedure. Follow the adhesive manufacturer’s instructions and OEM recommendations for proper surface preparation. This should include removing any existing adhesive and/or corrosion on the substrate’s surface. When replacing panels, remove the e-coating on the service part so that you are working with a metal-to-metal surface. After the adhesive has cured, the panel is ready for finishing processes and painting.

As OEMs continue to use lighter substrates (aluminum, carbon fiber, plastics) in vehicle design, rivet bonding (cold-joining) will become a standard repair procedure. Rivet bonding is not a difficult process, but it is crucial to use OEM-recommended rivets and adhesives. If the appropriate steps are followed, a successful repair will have been performed.

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