Acura TLX Is Honda’s New Body-Build Benchmark

All-new for 2021, Acura’s midsize sport sedan gets an exclusive platform to go with its structural, materials and dynamic advancements.

Honda R&D increased advanced high-strength steel content from 4.2% on the outgoing TLX to 17% on the 2021 model. (Acura)

Honda body-engineering presentations have become a much-anticipated highlight of the annual Great Designs in Steel (GDIS) conference in Michigan. Honda’s ‘ACE’ vehicle structures are considered by competitors to be benchmarks in overall integrity, occupant protection and mass efficiency. The newest iteration – that of the all-new 2021 Acura TLX – adds dynamic behavior to the other ACE attributes.

Acura's first use of laser brazing was employed to join the 2021 TLX's roof panel to the bodysides along a 2.9-meter (9.5-ft.) run. (Acura)

The TLX’s body development leader, Jeremy Lucas, deems the new sport sedan to be “the most exciting project I’ve been involved with in my 23 years at Honda.” In a recent interview with SAE Media, he also praised the GDIS event, as “an environment in which we in the body-engineering and materials communities can learn from each other.”

For the first time in its history, the TLX is on an exclusive platform that uses both higher percentages of high-strength steel alloys (980 and 1500 MPa in the underbody structure) and aluminum including the front fenders. The unique architecture gave Honda’s body stylists and Lucas’ body team more freedom to achieve their goals, including meeting the new angle-oblique and narrow-offset impact modes.

Body development boss Jeremy Lucas, shown here in a Zoom call with SAE, is a 23-year Honda engineering vet. (Lindsay Brooke)
New-for-2021 cast-AL front shock towers join to the steel substructure with self-piercing rivets and adhesive, the lightweight castings saving 10 kg versus steel. (Acura)
TLX cross-car load path and center tunnel is AHSS-intensive. (Acura)
Detail of the TLX’s multi-material joint construction featuring SPRs. (Acura)

It also gave the powertrain engineers space and structure to accommodate two turbocharged engines (including a new V6) with both front- and ‘super handling’ AWD drivelines. Overall body width increased 50 mm (1.9 in.) versus the previous generation car, and the new model is dramatically styled with deeper rear-quarter “hips.” Rear fender angle is 30.3° from horizontal.

Executing the new car’s more aggressive, sophisticated styling required Honda Development & Manufacturing, which includes the company’s in-house tooling group, to develop a new stamping sequence. A second draw process was added to the traditional Honda ‘draw, trim, bend’ cadence to achieve the sharp character lines on the aluminum hood.

“Often you’re hamstrung by whatever commonality you’re trying to share,” he noted. “Having the all-new V6 turbo engine meant we didn’t have an existing package that we had to clear.” Keeping the TLX’s hoodline as low as possible was a key program goal, enabled by changes that could be made to the new V6 which was in parallel development. “We usually don’t get that opportunity because we’re working with engines that already exist,” Lucas added.

The cabin-to-trunk bulkhead, previously bolted on gussets in the ’20 vehicle, is now a new all-welded ring structure on the ’21 car that improves rear torsional rigidity by 90% and saves 8 kg (17.6 lb.) versus previous methods for that level of rigidity. Carryover technologies include 1-piece hot-stamped door rings, aluminum hood, and a magnesium cross-car beam.

Ditching the ditch for brazing

Laser brazing, a first for Acura, was employed to join the roof panel to the bodysides along a 2.9-meter (9.5-ft.) run. The robotic construction technique helps increase body stiffness, Lucas said, while giving the TLX a premium look by eliminating the ditch joint and its typically fussy plastic trim strips.

“We feel that having a laser-brazed roof was simply a requirement for TLX – and it’s not the easiest thing for Manufacturing to execute,” Lucas observed. “Ditch joints are simpler and easier to do from a manufacturing standpoint. But once it was clear we were moving to laser brazing, we didn’t just take an off-the-shelf technology and apply it.”

The system used on TLX was uniquely refined in-house to be much more efficient than what was available off the shelf, he said. “We have proprietary know-how for how the panels need to be made, their geometries and rigidity, and how we make the laser braze. We’re doing the lasered seams in one continuous run, which was important for our factory efficiency. Not everybody does it that way, because you’re putting a lot of heat into the panels which could cause warpage. Our process definitely required an increase in tolerance control between panels,” he noted. “But it makes for a quick run with repeatable quality.”

AL castings, SPR joining

Popping open the TLX’s hood reveals a pair of two large aluminum castings that comprise the tops of the front shock towers. The high-pressure die castings, in AlSi10MgMn alloy, provide a trifecta of engineering benefits: high rigidity (an 80% increase in local damper fitting point stiffness), mass reduction (10 kg per vehicle versus steel), and component integration – the AL castings replace an 8-part fabrication with a single piece.

Engineering the TLX’s mixed-material front body structure required new joining solutions from Lucas’ team and their key suppliers that leverage Honda’s vanguard experience with rivet bonding. “The aluminum-to-steel joining process is done primarily using self-piercing rivets (SPR),” Lucas explained. “But SPRs are a potential damage point in terms of corrosion. For TLX, we use coated rivets, which add an extra level of protection. We also e-coat the aluminum castings before they’re joined. So, there’s already a layer of e-coat between the steel and aluminum before the joint happens.”

The new TLX features Honda’s most extensive use of structural adhesives among its sedans – about 30 meters (98 linear feet) of it. “It’s hard to deny the benefits of adhesives in vehicle body engineering. I’d love to use it everywhere,” Lucas exclaimed. “It does take a lot of cycle time to apply – and cycle time ultimately drives the cost of the vehicle. But it’s a very robust solution for integrating these structures into the TLX body.”

Additionally, in what he calls Honda’s “belt-and-suspenders approach,” dust sealer is robotically applied between the joints and the panels, covering the SPR areas, to prohibit water entry between the panels. “We may have a bit of overkill here, but our internal standards for corrosion protection are very high; we march to our own drummer.”

In some locations of the front structure there are joints with three-layer material stack-ups. Handling all the combinations of panel thicknesses required three different SPR lengths to optimize each joint. “A lot of work was put into this because we were not only optimizing the joint strengths, using the different SPR lengths, but also optimizing the tooling process to do this.” To efficiently integrate it into Honda’s overall white-body activity, the SPR process was developed in house but is handled by a Tier-1 supplier trained by Honda.

“The entire process allows us to integrate aluminum castings on a model-specific basis without interrupting the body shop in the plant,” Lucas explained. “We don’t want to drive cost into other models built on the same line unnecessarily.” Perhaps most meaningful to the TLX customer, the design engineering and materials changes – and moving the 23.7-kg (53.2-lb) battery to the trunk – improved the car’s front/rear mass balance: from 60/40% on the 2020 model, to a more sporting 57/43% balance in the 2021 model.