The evolution of tube hydroforming

Tube hydroforming has evolved over the last 20 years and now is being used to make many different parts more efficiently. The most common and highest-profile applications have been in the automotive industry. Structural components perhaps are the most touted hydroformed parts, because the benefits derived from hydroforming them, such as increased performance and weight and cost reductions, are very important.

Because of these and other benefits (Figure 1), the implementation and application of tube hydroforming for making structural automotive parts have followed an interesting path in the past 16 years.

Figure 1 Hydroforming Benefits

In the Beginning

For many years certain vehicle functions were best accomplished with tubelike structures. The limitation was that until the late 1980s there was no way to construct a tubular part with sufficient design flexibility, dimensional stability, and hole-making ability. To compensate, the structural part supply industry developed the ability to make tubelike parts from several stampings welded together. Tube hydroforming, a technique that uses a fluid either to form or aid in forming a part from ductile metal, filled an overdue need in the industry, which may explain its relatively rapid acceptance in manufacturing automotive structures.

TI Vari-Form in Ontario began producing the first high-volume structural part, an instrument panel beam, in 1990 using a low-pressure hydroforming (LPH) process that came to be known as pressure sequence hydroforming. The tube was formed by closing the die with low-pressure water inside, with the tube periphery and the die cavity equal. This mechanically formed or bent the final cross-section corners completely. A constant periphery was required along the part length for best process economy, although the cross-section shape could change substantially. This forming approach was developed to improve mechanical tube forming practices and did not require optimal tube quality to work well. The company patented this technique and soon began using it to produce parts besides the instrument panel beam.

Around the same time, equipment companies in Germany recognized the benefits and the business opportunities that tube hydroforming represented. Using a process originally adapted from a successful method of making plumbing T's, these companies began making parts using a process that eventually was commonly referred to as high-pressure hydroforming, or HPH, which expanded the cross section 2 percent to 5 percent. Stretching material into cross-section corners with fluid after the die was closed required high fluid pressure. This process led naturally to expanding cross sections even more during the hydroforming operation.

An interesting dichotomy was in place from hydroforming's beginning—LPH technology in North America; HPH in Europe. LPH was used by the first parts-makers (Vari-Form, GM, HydroDynamic); HPH was embraced and promoted by German press manufacturers (Schafer, Siempelkamp, Huber & Bauer, and Hydrap).

Both camps also found a small (at first) receptive audience for hydroforming technology at some automotive OEMs and Tier 1 suppliers, once the benefits were understood and were too compelling to ignore. This was another key ingredient for adoption.

Second Stage

Equipment suppliers were quick to seize the opportunity to enter the hydroforming market, with the lead company being Schafer Machinenbau, which later was bought by Schuler to become Schuler Hydroforming. The company began promoting the technology in both Europe and North America, highlighting the benefits of tube hydroforming in general and HPH in particular. The latter soon became the conventional approach, and often the only one with which many engineers and companies were familiar.

An increasing number of automotive OEMs and Tier 1 structural parts suppliers began to believe strongly that they hadto add hydroforming to their portfolios to maintain current business and grow. Some companies that implemented the technology felt forced to do so for these strategic reasons.

Given the fact that HPH was the only readily available technique—LPH use was constrained by patents and not offered by equipment-makers—the choice of which hydroforming process suppliers could adopt was fairly obvious.

For many companies that adopted hydroforming technology, detailed knowledge about the process was limited, and in most cases, knowing the pitfalls, challenges, limitations, and the benefits of the low-pressure approach took a distant back seat to HPH's advantages. This led to two significant results.

First, using HPH when it was neither technically nor economically beneficial resulted in a lack of profitability and advantage for both the supplier and the customer. Second, the part designers' haste to benefit from the advantages without giving the limitations or potential consequences enough consideration spawned design changes, surprise costs, and a dramatically increased interest in learning how to deal with these issues better. In some instances, using the process became a trial by fire that resulted in:

• The inability to punch most holes in the hydroforming die, which resulted in a boom in costly laser cutting systems and robots.
• Automation that was not sufficiently beneficial or added to production complexity.
• Tools that lacked the durability required or were overstressed and cracked.
• Adding annealing and various other measures to improve formability.

Designers understood that expansion below some maximum (for example, 20 percent) could be accomplished simply without increasing costs. This can seem true when using HPH, since nothing additional is needed from the HPH press for larger expansion. However, a number of external measures need to be taken to make the process work. Some of the measures that are almost always used to increase formability or reduce the effects of concentrated wall thinning can increase cost substantially. They include:

• High-Formability Material
• Special Shape Forming at Tube Mill
• Lubricant Application
• Postforming/LaserTrimming/Hole Cutting
• Generation/Containment of High-Pressure Fluid
• Tube Blank (Burst Test, Length Tolerance Diameter, Seam Weld)
• Substantial Preforming
• Hydroforming Tool, Cycle Time
• Press Cost, Operating Cost
• Lubricant Cleanup

Third Stage

In the past few years the belief that hydroforming does not deliver as many benefits as users desire has grown. This opinion, largely rooted in capabilities being less than reported and unexpected surprises, has slowed the fast-paced expansion seen in the second stage.

The slowdown is particularly evident in Europe, where cost has, until recently, been secondary to fulfilling the design intent for the part. Some manufacturers found themselves in situations in which the investment or part cost was found to be too high, after the fact.

Also, several North American products that were hydroformed have reverted to being stamped and welded for economic reasons. In these cases, the replaced technology was HPH with axial feeding. Whereas people in the automotive industry believed that hydroforming made structural parts cheaper, they now realized that the benefits were potential, not guaranteed, and depended on how the parts and process were designed and the overall cost of producing the part.

Tube Hydroforming Today

Today tube hydroforming has plateaued in popularity, predominantly because it is widely perceived as being expensive, an unfavorable quality, particularly in the hyper-price-sensitive automotive industry. Too often this perception is true. Putting the equipment in place to produce parts can be costly, but an even more significant concern is the piece cost. As the industry has developed, it has become standard practice that tube hydroforming requires an intense focus on process simulation to predict as many difficulties as possible beforehand. HPH also can require special material, special tube manufacturing methods, lubrication, preforming, end feeding, annealing, and a number of other measures to improve formability.

LPH, which does not have the same concerns as HPH, follows a developed methodology to form parts and, with suitable tooling and production, usually proceeds smoothly. The number of low-pressure presses being introduced to the marketplace continues to grow as applications increase.

While most parts that can be hydroformed may be produced by both HPH and LPH, certain part features that are achieved easily with one process may be difficult or impossible to achieve economically with the other process. An example is sharp cross-section corners at one or more locations. LPH can handle this quite easily, while HPH has a tough time achieving it. On the other hand, small expansions that can be done only in the hydroforming die can easily be done with HPH, but LPH has trouble with them.

Many minds now are focused on making hydroforming less expensive. However, the question arises: Are we considering all options and basing further developments on the right technological foundation? Doing so is imperative.

It is not the biggest or strongest that survive, nor the most intelligent, but the one most responsive to change.
—Charles Darwin