The evolution of tube hydroforming

The growth in hydroforming use has slowed as automakers and part suppliers, particularly in the automotive industry, are taking a step back to examine manufacturing options in an effort to determine the most efficient, cost-effective process. Some even have reverted to stamping and welding formerly hydroformed parts. Rationalization such as this is good and healthy for both customers and suppliers to ensure the technology is applied sensibly. By looking at how the industry got to this point, we can get a sense of where it's headed.

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

Because of these and other benefits, the implementation and application of tube hydroforming for making structural automotive parts have followed an interesting path.

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 economically construct a tubular part with sufficient design flexibility, dimensional stability, and hole-making ability. To compensate, the structural part supply industry manufactured tubelike parts from several stampings that were 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.

In 1990 TI Vari-Form began producing the first high-volume structural part, an instrument panel beam, 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 peripheries of the start tube and the die cavity being equal. This approach 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 other parts.

Around the same time, equipment companies in Germany recognized the benefits and business opportunities that tube hydroforming represented. Using a process originally adapted from a long-successful method of making plumbing T's, these companies began making parts using a process that eventually became known as high-pressure hydroforming, or HPH, which expanded the cross section 2 to 5 percent to make the process work. 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 hydroforming for locally increased rigidity.

An interesting dichotomy emerged: LPH technology in North America and HPH in Europe. LPH was used by the first parts-makers (Vari-Form, GM, Hydrodynamic Technologies); HPH was embraced and promoted by German press manufacturers (Wilhelm Schäfer Maschinenbau GmbH, Siempelkamp Pressen Systeme, 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. This increased once the benefits were understood and were too compelling to ignore. Controls technology was also much improved by this point. Both were key ingredients for adoption.

Second Stage

Equipment suppliers were quick to seize the opportunity to enter the hydroforming market, with the lead company being the aforementioned Schäfer. It was later 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 and HPH in particular. The latter soon became the 'conventional process' since it was openly available and marketed, and often became 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 had to add hydroforming to their portfolios to maintain current business and to grow. Some companies that implemented the technology felt forced to do so for these strategic reasons.

Figure 1 Hydroforming benefits

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 companies could adopt was fairly obvious. This was because of such business reasons mostly. Comparative process efficiency was not judged objectively.

For many companies that adopted hydroforming technology, detailed knowledge about the process was limited. In most cases, the pitfalls, challenges, and limitations of the HPH process and the benefits of LPH took a distant backseat to HPH's marketed advantages. This led to two significant results.

First, many adopted HPH when it was neither technically nor economically beneficial, resulting in a lack of profitability and advantage for both the supplier and the customer. Second, part designers endeavored to benefit from the advantages without giving the limitations or potential consequences enough consideration. This led to surprise design changes and costs that were substantial in some cases and a dramatically increased need to learn how to better deal with such issues. In some instances, hydroforming became a trial by fire and revealed some big challenges:

• Many holes could not be punched in the hydroforming die, leading to a need for laser cutting systems and robots.
• Many of the automation systems were not sufficiently beneficial or were too complex to achieve production efficiency.
• Some of the hydroforming tools lacked sufficient durability or were overstressed and cracked prematurely.
• Often the material wasn't formable enough for the more commonly used HPH process, so manufacturers had to resort to higher-formability material, annealing, and other measures to improve formability.

Designers initially understood that expansion below some maximum (for example, 20 percent) could be accomplished simply without increasing costs. While this might seem to be the case when using HPH (because nothing additional seems to be necessary for larger expansion), in fact many extra measures are necessary to make the process work. Some of the measures that are frequently used to increase formability or reduce the effects of concentrated wall thinning can increase cost substantially. Examples are:

• Switching to a higher-formability material
• Annealing before or during the process
• Substantial preforming
• Using a special forming process at the tube mill
• Applying lubricant (and later removing it from the hydroformed part)
• Postforming, laser trimming, and hole-cutting
• Generating and containing high-pressure fluid
• Enhancing the hydroforming tool (adding inserts and using other measures to reduce wear and increase die strength)
• Adhering to special requirements for the tube blank (burst test, length tolerance diameter, and specialized seam welding)
• Using alternative equipment and processes (larger-tonnage presses and longer cycle times)

Third Stage

In the past few years a new belief emerged that hydroforming does not deliver as many benefits as users desire. This opinion, largely rooted in capabilities being less than reported and surprises mentioned above, 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 part design intent. Some manufacturers found themselves in situations in which the investment or part cost was found to be too high after making the financial commitment.

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. Obviously, this is 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.

Designers and process engineers are right to be wary. HPH 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.

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. Several alternative ways can be used to achieve such features; working with an innovative hydroformer helps to strike a balance between part function and cost.

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.

Continued evolution in hydroforming is leading to better process designs and a wider range of part features that can be provided economically. Since these innovations are recent they are not widely known, but they can be crucial in determining whether it makes sense to hydroform. One certainty is that technology continues to grow and evolve, so that the notion of what made sense to hydroform yesterday will differ from what makes sense today.