Honda Sticking with Structural Adhesives

We query a 2022 Acura MDX principal engineer on the company’s expanding use of adhesives, and their role as a dynamic performance enhancer.

The 2022 Acura MDX uses more than two meters of structural adhesives in its front-damper housings. (Acura)

For the all-new 2021 Acura TLX sedan, Honda revealed a new design for its front shock towers (aka “damper housings”) that leveraged dissimilar materials, self-piercing rivets (SPRs) and adhesives. Carrying that experience forward to the 2022 Acura MDX SUV, which for the first time would feature a performance Type-S trim (top), the MDX would net a similar design for its front-damper housings, each of which feature more than two meters (6.6 ft) of structural adhesives.

The new front-end architecture of the 2022 Acura MDX was engineered to provide a more connected feeling between the driver, the vehicle, and the road. (Honda)

The adhesive, developed by DuPont to Honda’s stringent specification requirements (but now available commercially), is the same structural compound used on the TLX  and on other portions of the MDX body structure. Applied by robot at room temperatures, the adhesive cures during the normal e-coat heating-oven cycle. In the damper housings, the adhesive is used to bond steel components to e-coated aluminum castings.

According to Patrick Shafer, principal engineer and body design leader for the 2022 Acura MDX, growing expertise in adhesives has been key in helping meet the program’s performance targets. The materials play a key role, he noted, in the relationship between body rigidity and the chassis’ dynamic response, with the goal of providing a more connected feeling between the driver, the vehicle, and the road.

“Adhesive was one of the tools we used to achieve significant increases in both global body rigidity and local stiffness in the MDX, especially when you consider our high dynamic targets for the first-ever MDX Type-S,” Shafer said. “Utilizing adhesives in key areas of the vehicle structure to further improve joint efficiency enabled us to efficiently achieve our targets while managing the overall body weight. Ultimately, the driver experiences a smoother ride with less handling delay.” An edited subset of Shafer’s enlightening responses to our queries follows:

What learnings were carried forward from the 2021 TLX to the 2022 MDX?

Many of the same technical team members were involved in both the TLX and the MDX developments. This consistent oversight allowed any key learnings during the TLX development to be immediately fed into the MDX design. The vast majority of those items were in the details of casting features, CAE method improvements, or joining details. The most visible example of learnings being carried forward is the expansion of self-tapping bolts to structural areas on the MDX. The TLX applies M6 self-tapping bolts in non-structural locations where brackets are assembled to the castings. With the history and detailed understanding we gained during the TLX development, we expanded the application on the MDX to encompass M8 structural bolts between the casting and the dash enclosure, further improving the packaging. The dash enclosure includes an integrated tower bar structure and lateral suspension loads are directly carried by this joint.

Do the TLX and MDX share the same front architecture?

The TLX and the MDX have very different front-end architectures and are produced at separate Honda plants in Ohio. While TLX and MDX are unique platforms at these facilities, their white bodies are built on the same high-volume, steel resistance spot-welding lines as the other vehicles produced. For example, the TLX shares the same weld line that produces the Honda Accord among other models at the Marysville Auto Plant, and the MDX shares the same weld line that produces the Honda CR-V and Acura RDX at the East Liberty Auto Plant.

Early in the development planning phase, we made the strategic decision to consolidate both damper housing assemblies to a single new multi-material joining line at one of our nearby core Tier-1 suppliers. This multi-material joining line was concurrently developed to support both vehicles’ differing architectures, and to supply both Honda plants. This enabled us to deploy the cast aluminum damper housings directly into our high-volume, resistance spot-welding steel weld lines without the need for additional joining equipment at the two Honda plants.

While very different in architecture and construction, the materials in use within the TLX and MDX front-damper housings are the same. The casting alloy is the same, as are the steel sheet grades joined to it, and the structural adhesive. Not all of the rivet lengths are shared due to differences in joint stacks between the models.

How does the adhesive help increase structural rigidity?
The new front-damper housing is engineered with dissimilar materials including e-coated aluminum casting and hot-dipped galvanneal-steel stampings. (Honda)
In the front-damper housings of the 2022 Acura MDX, the adhesive path follows the same connection areas as the self-piercing rivets (SPRs) around the perimeter of the casting where it joins the steel structure. (Honda)

In a traditional point joining method, such as a spot weld, SPR, or bolt, the force applied to the joint is localized around the weld or the fastener. The areas between the connections still have freedom of movement and the potential to deform separately. These micro deflections reduce the efficiency of the joint. Adhesive bonds the two surfaces in a larger area than the fastener or weld itself, more effectively distributing the loads across the entire adhesive length and utilizing the entirety of the joined surfaces more cohesively. This improves the joined area’s efficiency and leads to improvement in deflection, thereby increasing body rigidity.

Does the adhesive follow the same connection path of the other joining methods?

In the case of the TLX and MDX damper housings, the adhesive follows the same connection areas as the SPRs around the perimeter of the casting where it connects to the steel structure. In both damper housing designs, we minimized the contact area to the SPR joining locations, therefore we do not have areas where [adhesive] bonding was the sole joining method.

Are there benefits to using adhesive with SPRs?

There are advantages in using adhesives with SPRs. Aside from the joint deflection improvements, the durability performance of the SPR joint is also improved. The adhesive distributes the forces across the adhesive area. Depending on the type, direction, and magnitude of the cyclic loading, the adhesive bonding can significantly delay the onset of localized fatigue cracking or loosening of the SPR connection.

The adhesive’s primary purpose in the TLX and MDX damper housings is for body rigidity, but there is a secondary benefit for corrosion toughness. The TLX and MDX apply a multi-layer approach to galvanic corrosion protection to the dissimilar material joining areas. The primary isolation between the steel and aluminum is the e-coat applied to the casting prior to the joining operation. We also utilize dust seal on both sides of the joint edges and in high water-splash areas to the SPR heads, to prevent water intrusion into the joint. The adhesive applied within the joint also aids to isolate the materials, but it is a side benefit and not its primary purpose within the joint.

Can the adhesive be used across dissimilar materials?

The adhesive was formulated to join both the steel and the aluminum components and their associated coatings. In the damper housing assembly, the aluminum casting is e-coated prior to the SPR joining process. In this application, the adhesive joins the previously e-coated aluminum casting to the surrounding hot-dipped galvanneal-steel stampings. At the Honda plant, the completed damper assembly’s steel components are resistance spot welded into the rest of the body structure on our weld line. The completed white body is e-coated, and the adhesive cures in the e-coat oven prior to body paint.

Can the adhesive be used within multi-layer material stack-ups?

There is the possibility adhesive could be applied between all layers of a 3-sheet stack, depending on the vehicle’s targets and the specific loading condition of the joint. In the case of the TLX damper housing, the optimized solution was achieved through application between the casting and the first sheet only. During our developments, every adhesive application is thoroughly vetted through CAE optimization to determine the adhesive locations that achieve the maximum benefit to meet the vehicle’s performance targets. In this way we can ensure we are efficient in providing the maximum benefit while limiting equipment and consumable cost.