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.
Hydroforming 101
The basic principle of hydroforming is quite simple. Place a seamless tube inside a die and fi ll 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 com- plex part with quick cycle times.” PSH begins with a bent tube that roughly matches the
fi nal design shape of the fi nished 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 pressur- ized. The die is then closed pro- gressively as the internal pressure builds until the part is completely formed.
“Because the
PSH process bet- ter supports the tube as it forms,
Three metal forming/machining technologies contribute to the frame of the C7 Corvette:
hydroformed main rails, machined castings for connection nodes, and extrusions in the crush zones—all made of aluminum.
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 fi n- ished 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 ap- proach. It is, in effect, a hybrid that makes use of three differ- ent 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 hydro- formed, 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 fi rst used in the C5 Corvette, a full perimeter frame with a center tunnel struc- ture linking the engine and fi nal drive cradles. In the C7, of course, all of these elements are aluminum.
59 — Motorized Vehicle Manufacturing 2016
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