Bending Corvette Iron With Water - Technologue - American Car

Tube hydroforming is a not-so-common technical term that describes a relatively new automotive metal-forming process in which small (garden hose-size) closed cross-section structural members or even large (six inches in diameter) frame rails are ballooned into proper shape from steel tubing filled with water pressurized to anywhere from 5000 to 100,000 psi.

Perhaps the most impressive place hydroformed pieces have shown up so far has been in the two main frame rails on the 1997 C-5 Chevrolet Corvette. Traditionally, these 16-foot-long parallel rails would have been made using 14 individually stamped and welded pieces of steel. At the plant, that would have meant many blanking, drawing, trimming, flanging, and piercing operations, several different fixtures, and numerous complicated welding operations. Instead, the new process involves a single steel tube, a monster tubing bender, a relatively simple die set, a pressurizing or "intensifier" system, and the water-oil pressurizing fluid. Estimates from GM claim the one die that makes the two Corvette rails replaced something like 100 stamping dies.

Charles Bruggemann, an engineering Ph.D. and manager at the body in white center at General Motors' Manufacturing Center in Warren, Michigan, proudly points to the increasing number of applications in virtually every new GM platform. Given the many new places hydroformed parts appear, it would seem this new body-manufacturing process has significant future potential. Bruggemann says that GM has been successfully using hydroformed parts in its vehicles for about six years.

Our expert says prime targets for hydroformed pieces are parts that must go through territory in the vehicle structure that have a variety of different package spaces or shapes. In the case of the Corvette, there were many localized features that the two-seater engineers wanted to accommodate. Conventional technology would have expensively built a rail out of many different pieces to fit those different required shapes and spaces.

At this time, GM is using hydroformed steel (and is thinking about using aluminum) pieces for instrument panel supports, powertrain cradles, suspension system subframes, and the A-post roof pillar in the all-new Buick Park Avenue. Other makers, including Ford and Chrysler-but particularly the German brands-are also taking advantage of the new manufacturing process.

As mentioned, one of the big advantages to carmakers is a reduction in tool and equipment complexity. But other plusses include less scrap, reduced labor costs, reduced part mass, increased strength, and greater part dimensional precision.

GM considers the fine points of its hydroforming processes to be dark, highly competitive internal secrets. So our expert would only speak in broad terms concerning hydroforming. Still, here's how it generally works at GM's plant in Pontiac, Michigan:1. A straight, 16-foot-long, round tube of galvanized steel is precisely bent on what looks like a large, exhaust-tube bending machine with a mandrel and a radius tool.

2. The pre-bent tube and its parallel sister are then moved to and dropped into a titanic female die, next to each other.

3. The ends of the tubes are clamped and sealed and the fluid, composed mostly of water with a small percentage of lubricant and an anti-rust agent, is pumped into the tubes' closed interior.

4. The other half of the die closes and locks over the partially filled steel tubes. (Final total fill for a single Corvette rail is 27 gallons of fluid.)

5. First one tube, and then the other tube's walls are then fully pressurized by a special pump or hydraulic ram (to approximately 6000 psi) ballooning them into the separate forms of the closed die, where they take their shapes. (At this point, the dies are resisting 8000 tons of opening force.)

The whole process is really quite speedy and takes somewhere between 1.5 to 2 minutes.