Blazer EV Is a Steel-Intensive Showcase

August 26, 2024

Lindsay Brooke

Chevrolet Blazer EV customers are likely to cite the Ultium battery and dual-motor driveline as their vehicle’s most advanced technologies. But in doing so, they’d miss an equally key aspect of GM’s sporty midsize electric SUV: its steel-intensive body structure featuring a broad mix of alloys and an impact-defying battery enclosure.

The words ‘steel’ and ‘BEV’ seem incongruent, given the widely forecasted trend toward aluminum architectures based on two or three large castings. GM itself has invested heavily in the Tesla-pioneered ‘gigacasting’ tech and will deploy it first in Cadillac’s new, ultra-exclusive (and much-delayed) Celestiq flagship  . But GM’s electrification strategy depends on a variety of material and body-structure solutions. It’s a horses-for-courses approach aimed at balancing performance and product affordability, explained Blazer EV’s chief engineer Hoda Eiliat.

“In GM’s materials position, steel is obviously the most cost-effective approach,” Eiliat declared in her keynote at the 2024 Great Designs in Steel conference. Innovations in new alloys, metal forming and joining technologies continue to drive steel’s competitiveness, she told SAE Media following her presentation, adding that “obviously we have to look elsewhere when we have mass and parts-consolidation priorities.”

The breakdown of Blazer EV’s body materials shows increased adoption of tougher, more mass-efficient steels. Forty-five percent of the 5,337-lb (2,421 kg) all-wheel drive vehicle’s total materials composition is in medium-high-strength alloys boasting a yield strength of 180-580 MPa. Thirty percent of the total is split evenly between mild steel (mainly body outers) and ultra-high-strength alloys, the latter including multi-phase, Martinsitic and Gen-3 products rated at 980 MPa or above. Dual-phase advanced high-strength steels (590-780 MPa) constitute 9% of the total.

The most prominent use of aluminum is in the extruded side rockers in 6082 alloy with a T6 heat treatment.

Managing mass and impact load

Beyond steel’s cost superiority, GM considers the ferrous metal to be vital for optimizing product diversity, rapid refreshes, and achieving production scale. All are essential for democratizing EVs, Eiliat said. The Blazer EV is one of eight vehicle programs that are currently on GM’s so-called BEV Crossover platform, with more in development. The basic architecture accommodates two major variants serving Buick, Cadillac, Chevy, Honda and Acura nameplates. They’re differentiated by long and short distances measured from the driver’s ball-of-foot to the front axle centerline. Blazer’s all-new steel upper body structure also has two variants: base roof and sunroof. Much of the lower structure carries over parts from the Cadillac Lyriq.

Blazer’s underbody structure is engineered to accommodate front- or all-wheel drive and two different battery enclosures, depending on whether the vehicle uses a 10-module or 12-module Ultium battery pack (what GM calls the RESS, or rechargeable energy storage system). The all-steel RESS is unusual in an industry that has thus far favored aluminum battery enclosures. It is carried over entirely from the Lyriq and employs press-hardened steels (PHS), which are key to its structural role in the vehicle and its impact performance. PHS makes up 11% of the total.

The EV’s small-or-large battery sizing required a new approach to managing the 500 kg (1,102 lb) mass. Special stamped-steel front rails were developed “to bring the total mass variation of the two RESS packages into one front subsystem,” Eiliat said. “And we still managed to commonize parts between the two.”

Blazer EV’s frontal- and side-impact load management strategies use the body and RESS in unison. “Our strategy was to manage the load between battery and body in one-third and two-thirds proportions, with two-thirds of the load transmitted through the body and one-third through the battery,” Eiliat explained. “In the event of a full flat barrier engagement, the load transmits from the front rail then into the first crossbar in the RESS, then splits into the center tunnel and also into the rockers. The same thing happens in the rear, where forming the crossbars in press-hardened steel helps minimize intrusion.”

Enabling the body and RESS to “talk to each other” is what GM calls a Cradle Bolt Stabilizer, basically a shear plate that connects the front cradle with the body at one point and connects the body and the rails at two points. Frontal and side impact loads are uniformly transferred into the RESS’s transverse cross members that are bordered with longitudinal ‘ski rails.’ Eiliat said the extruded aluminum rockers enable a uniform transfer of about 30% side-impact loads into the ski rails.

Bolstering the B-pillars

In her presentation, Eiliat revealed the Blazer EV’s body-in-white bending stiffness to be 25.4 Hz as measured by GM. The body’s torsional stiffness increases as a function of the RESS pack. The 10-module battery pack has five bays (space between crossmembers where the cell modules reside), while the 12-module RESS has six bays. “As your battery gets bigger and the roof structure gets stronger and stiffer, your stiffness numbers go up,” she said.

For the GDIS conference, GM brought a Blazer EV white body and RESS for display. Both properties were closely examined, photographed, and measured by attendees throughout the event. Of particular focus was the impact-critical B-pillar, constructed in Gen-3 AHSS.

“We decided to use Gen-3 to reduce mass with equivalent performance in IIHS 2.0 and roof crush [testing],” Eiliat noted. “We save on cost using Gen-3 steel. To facilitate this application, we created a unique material spec, GMW17627, which remains an industry benchmark. We use unique forming processes that compensate for springback while adding stiffness. We also have a specific welding schedule.”

The GMW17627 specification, titled “Retained Austenite Bearing Advanced High Strength Sheet Steel,” was published in 2018. It covers the requirements for continuous cast, cold rolled, retained austenite bearing sheet steel with specified minimum tensile strengths from 690 MPa to 1180 MPa.

This specification applies to uncoated and coated retained austenite-bearing advanced high-strength steels that rely on one or more induced plasticity mechanisms to achieve enhanced combinations of strength and ductility relative to high-strength steels. Typical applications include body panels, body structure members, and reinforcements for enhanced strength and vehicle impact performance.

Highlighting the joining strategies employed by GM at its Ramos, Mexico, plant, Eiliat said the Blazer EV body features roughly 6,000 resistance spot welds. There are over four linear meters (13.1 ft.) of laser brazing between the vehicle’s roof panel and bodyside outer. The lower body structure is MIG welded. Structural adhesives are used in the upper section. The aluminum side rails attach to the steel body structure using brackets, rivets, and are isolated with adhesives and tapes.

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