‘The shape of the structural element is at least as important as the material strength.’
It’s been almost two decades since the C5 Corvette hit the streets with its groundbreaking chassis built around hydroformed steel bumper-to-bumper frame rails. The technology gave engineers a chance to create components that were both lighter and stiffer than traditional stamped and welded assemblies.
“As revolutionary as the C5 project was, it was the first step leading to spectacular results in the 2014 C7,” explained project engineer Jeff Beaudoin of Vari-Form (Livonia, MI). Vari-Form produces the frame rails for GM. “Light and stiff is a difficult target to achieve when you consider all the demands a frame must meet. Fortunately, a smart design enabled by hydroforming makes it easier.”
A Little History
Corvette frames have evolved over the years from an early ’50s heavy boxed steel structure to a steel ladder design in the ’60s, and a unitized welded steel frame with cradle-mounted drivetrain components in the ’80s and early ’90s C4 Corvettes. Engineers working on the C5 platform discovered that the welds in the C4’s side frame rails allowed the structure to flex under high loads and began exploring alternative technologies to eliminate the welds.
What they developed was the first hydroformed Corvette frame component, a 14′ (4.27-m) long, bumper-to-bumper tubular steel rail that proved to be extremely stiff. So much so that it didn’t require additional bracing when used in the C5 convertible introduced in 1998.
The initial C6 Corvettes used the same steel frame rails as the C5. The high-performance Z06 models, however, replaced the steel with an aluminum alloy that cut the weight from 502 lb (226 kg) to 392 (176 kg), a nearly 22% reduction.
Vari-Form began making both steel and aluminum compo-nents for the C6 in 2005. They also produce a hydroformed instrument panel beam for both the C6 and C7 Corvettes.
The basic principle of hydroforming is quite simple. Place a seamless tube inside a die and fill the tube with high-pressure liquid until it deforms plastically and conforms to the shape of the die. The reality, of course, is somewhat different.
“The traditional process,” Beaudoin explained, “hits the tube with a single shot of very high-pressure liquid. That has a number of negative consequences, the most serious of which is stretching the material rather than moving it. Stretching can produce a part with uneven thickness.
“Our solution is a process called Pressure-Sequence Hydroforming (PSH), where we tailor the forming process to operate at much lower pressures, yet produce a complex part with quick cycle times.”
PSH begins with a bent tube that roughly matches the final design shape of the finished part. It is then placed in a die which is partially closed to apply pressure to the outside of the tube at the same time the tube is internally pressurized. The die is then closed progressively as the internal pressure builds until the part is completely formed.
“Because the PSH process better supports the tube as it forms, thinning and tool friction are minimized,” Beaudoin noted. “This is especially important with aluminum.
“We also can perform secondary operations like hole piercing while the part is fully pressurized and constrained in the die. This gives us very precise control over the finished geometry of the part, and that is a key factor in the success of the C7 frame.”
Substituting Geometry for Mass
While the weight difference between the C5/6 steel and aluminum frames was primarily due to the different materials used, the C7 frame represents an entirely new design approach. It is, in effect, a hybrid that makes use of three different metalforming technologies to optimize the performance of each element and the structure as a whole. GM describes it as an “open architecture frame with cast joint interfaces to maximize torsional rigidity at an affordable cost and mass.” What that means is that the C7 frame combines hydroformed, extruded and hollow-cast aluminum components to produce an extremely light and stiff structure.
It is, in fact, 99 lb (44.5 kg) lighter and 57% stiffer than the C6 steel frame it replaces. Even more remarkable is that it conforms to the patented topology first used in the C5 Corvette, a full perimeter frame with a center tunnel structure linking the engine and final drive cradles. In the C7, of course, all of these elements are aluminum.
While some of the weight reduction comes from substituting aluminum for steel, much of it does not. For example, where the steel frame rails had a uniform 2-mm wall thickness, the thickness of the frame rail sections on the C7 range from 2–11 mm to provide strength and stiffness exactly where it is needed and nowhere else.
“One thing we have learned is that the shape of the structural element is at least as important as the material strength,” Dean Gericke, Vari-Form engineering director said. “And because the PSH process lets us precisely control both internal and external part geometry, it’s an ideal solution for the major side frame rails on the C7.”
Building the C7 Frame
Each side rail is made up of five distinct segments. At the front and rear of the rail is an extruded “crush can” made of 7003-T6 aluminum. The extrusion is drawn into a figure-eight shaped section (two closed cells stacked on top of each other) that collapses in a controlled manner to absorb energy during a crash.
The front “crush cans” are attached to a hollow-cast “node” of A356 aluminum to which the suspension cradle is fastened. These precision cast components are accurate to ±1.0 mm to precisely position the attached components.
In the center is a hydroformed tubular rail made from 6063-T6 aluminum. It starts out as a 6” (152.4-mm) diameter tube that is 7′ (2.13-m) long, which makes it the largest hydroformed aluminum tube currently in production. The finished rail has two compound bends and weighs 23 lb (10.35 kg).
After forming, both ends of the center rail are milled to conform to the tight tolerances specified by the Corvette design team. The rail also has 41 holes pierced in the die during the hydroforming process. It is supplied to Chevrolet with 38 fasteners per set, including 24 collapsible nuts, eight manually installed rivets and six clinch nuts.
Next comes a hollow cast rear suspension node of A356 aluminum that integrates all of the rear chassis attachments. And, finally the rear “crush can” which completes the rail assembly. There are also four high-pressure die cast nodes that attach the extruded shear walls of the center tunnel to the side rails. Additional castings include the hinge pillar mounting points that integrate the cockpit frame and hinge pillar with the center rail and windshield upper corners. The engine and rear suspension cradles are also hollow castings.
The center tunnel is a closed structure incorporating eight extruded sheer walls, each of which incorporates 16 different custom thicknesses. The sheer walls are attached to the hydroformed center side rails with a casting. Inside the tunnel is a torque tube connecting the engine to the rear-mounted transaxle, and a pair of exhaust tubes.
The C7 frames are assembled in the body shop of Chevrolet’s Bowling Green, KY, Corvette plant using a special high-precision laser welding system capable of holding tolerances of 0.001″ (0.03 mm). This capability is the result of a $131 million upgrade of the plant, which include a $52 million investment in the new, high-tech body shop.
Looking To the Future
Vari-Form has sponsored an ongoing study into Hydroform Intensive Body Structure (HIBS). The three-phase HIBS study has essentially confirmed the lessons of the C6/C7 frame experience.
“The original HIBS study indicated that hydroformed tubes can be successfully used as drop-in replacements for conventional stamped and welded components,” Gericke said. “HIBS II put more reach into hydroform designs utilizing high-strength variable gage tubes and modern single-sided welding techniques.
“HIBS III gives us solid proof that hydroform geometry works. The obvious next steps are to replace compression-dominated structures like roof rails, B-pillars and body side structures with hydroformed components.”
Edited by Motorized Vehicle Manufacturing Yearbook Editor James D. Sawyer from information provided by Vari-Form.