Tubular hydroforming is an inside-out process. That is, while most pressworking operations—including sheet hydroforming—externally force a material into shape, with tubular hydroforming, the tubular workpiece (e.g., aluminum, low-carbon steel, copper) is placed in a negative mold and is then filled under high pressure with an emulsion consisting of water and fluids for lubricity and rust protection that forces the tube to take the shape of the mold. Inside out.
While the process has been used for the past several years in the auto industry, having had its start in the production of exhaust pipes, one of the earlier applications was in the production of musical instruments. When you go to the manufacturing floor at Schuler Inc.’s (schulergroup.com) North American Headquarters in Canton, MI, you can get the sense of industrial music with the amps cranked to 11, with the floor even shaking, as the company’s Hydroforming Div. not only designs and engineers hydroforming systems for customers, but actually runs production parts for them—on the order of 600,000 per year.
For example, right now they’re producing the front crush tip for the Jeep Wrangler. The cycle time is 30 seconds, which, compared with conventional stamping, is a long time. But, Bob Rich, vice president of the Hydroforming Div., points out that the time required to hydroform the frame component is actually shorter in the long run: “If you were to stamp the part in halves you’d have to go to a welding station, do the full-length weld, and do a check to make sure you haven’t deformed the part—so the overall process time from a starting product to a finished product is relatively competitive.”
While frame components have been a mainstay in hydroforming for a while, Rich says that some OEMs are starting to use tubular structures—high-strength steel and aluminum—in their bodies-in-white as they work to provide structures that are both light and strong. “With between 2% and 5% expansion of the material,” he says, “you actually work-harden the material, so you gain some additional strength.”
There Are Limitations
While he’s a proponent of the process, Rich says that it’s not a “cure-all.” He explains: “There are limitations. You have to be careful that you’re not thinning material too much or bending it in a manner where you create a stress fracture and split the material.” To spot possible problems prior to production, Schuler spends a lot of time doing simulation work. They start with a nonlinear simulation where they digitally form the part. This helps them discover if the part can be made, if it will split, or if it is too thin or thick in certain areas. If there are problems, the team will work with the customers to come up with solutions.
After any problems are addressed, they move to linear simulation where they design the tooling. It’s at this point that they make sure the part is not going to wrinkle or crack and that it will be robust enough to do what it needs to do.
Depending on the shape being formed, some pre-processing may also be required. When a part needs to be curved in order to take the contour of the envelope in which it has to fit, some tube bending and pre-forming using a CNC machine, push-roll bending, or press bending may be required. Slight bends can be done using hydroforming; more-severe shapes have to be performed prior to the process.
Speed is also a consideration when it comes to different materials used. Hydroforming aluminum is slower than steel because there’s a lot of sensitivity in forming it. It can wrinkle and scratch easily, and it requires more post-processing. Still, Rich and his colleagues believe that there is going to be a significant increase in the amount of hydroformed aluminum in the months ahead so they’re working on the ways and means to optimize the process.