A Beneficial Bulge

Tubular sheet metal components get a lot tougher to make when they need to incorporate branches. Sure, they can always be fabricated in pieces and welded together, but this approach adds a manufacturing process and can risk the creation of failure points at the joint. These kinds of complex tubular shapes are no battle with bulge forming.

This variant of hydroforming uses a fluid under hydrostatic pressure to expand the sheet metal within a die cavity. "The fluid pounds the metal into the female die," explains John Fritskey, vice president of engineering for Voss Industries (www.vossind.com), which runs one of a handful of high-precision bulge forming operations in the U.S.

Unlike pure hydroforming, the bulge forming process features an important provision for maintaining a constant wall thickness. As the part expands, additional sheet metal is fed into the die cavity to prevent thinning. According to Fritskey, there is some art to this science of axial feeding: Knowing how much extra material to feed into the die and how quickly to feed it requires some experience. Too much extra material in the die, and the part could buckle under the compressive stresses. Too little, and the walls would thin.

In terms of process capabilities, "bulge forming essentially produces a net shape," Fritskey says, citing dimensional tolerances within ±0.005 inches. But its ability to resist thinning and maintain precise wall thicknesses is what sets it apart. As a rough guideline, Fritskey says, a 0.060-inch-thick sheet would typically thin no more than 0.003 inches during forming. "The thinning really is minimal," he says.

Precise control of thinning can obviously benefit components designed to stringent strength specifications. It can also have weight implications. Fritskey points out that one strategy to combat uncontrolled wall thinning, which can cause cracking, is adding wall stock that the component doesn't really need from a strength standpoint. "With pure hydroforming, if you want a minimum wall of 0.030 inches, you might have to start a 0.045-inch sheet," he says, noting that some of that extra wall stock would remain in some regions of the finished part. "We could start with 0.032-inches."

Because bulge forming has strength and weight implications and because it works well with superalloys, Voss has mostly applied the process to aerospace ducting components. One of the best examples of what the process can do is a 6-inch tall wye that goes into the engine cooling system of a GE jet engine. "People are still dumbfounded when I show them this part," says Fritskey. "They can never figure out how we make it in one piece."

And how do they? First, Voss forms the sheet metal into a cup shape. This cup then goes into the bulge-forming machine, which blows out two "Mickey Mouse ears." A trimming operation finally opens up the ears, creating the finished tubes. This bulge-formed component replaced a previous design in which two stampings were welded together to make the wye. "We got the job because there were problems with weld failures in the valley between the two ears," Fritskey recalls.

Bulge forming isn't for every job. Simpler processes, like spinning, can be more cost effective at extremely low production volumes, especially when parts lack branching geometries. Pure hydroforming, without axial feeding, can work just fine when parts lack tight wall thickness requirements. And properly designed weldments won't necessarily fail and can be cost effective at low production volumes. Bulge forming really comes into its own when part complexity and the need for wall thickness control intersect with production volumes high enough to justify tooling that typically costs $15,000 to $30,000. "One thousand parts is an ideal volume for us," says Fritskey. But he adds that much higher volumes are possible given the speed of the forming process—typically under 30 seconds. And sometimes, an inability to make the part any other way justifies lower volumes too.

Voss's bulge forming equipment can currently handle parts up to 24 inches in diameter and 24 inches long, but it will soon start up new presses that will boost part size to 36 inches across and long. The process works with a variety of metals, including aluminum, stainless steels, titanium, nickel alloys, and other high-temperature alloys.