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Strategic developments compress timelines, reduce costs for hydroforming

Die build strategies, advanced forming knowledge move market forward

With each passing year, it seems that the automotive industry offers consumers more choices and more features than ever before, while still maintaining or improving fuel efficiency and safety. These trends are nothing new, but have been motivating to try new design concepts for decades.

Automobile design teams work toward four broad goals: meet or exceed federal safety standards, meet or exceed fuel efficiency requirements, increase market share, and control costs. For their part, vehicle structure designers can employ a variety of processes and materials to achieve these goals, and in recent decades, they have relied increasingly on the hydroforming process and alternatives to mild steel, such as advanced high-strength steel (AHSS) and aluminum alloys.

Although hydroforming isn’t spreading through the automotive industry as fast as it did in the 1990s, when it was an emerging and disruptive technology, it continues to be a viable option for many parts. It’s no exaggeration to say that hydroforming facilitates forward-thinking designs for the vehicles of the future.

However, hydroforming does come under fire from time to time. It’s a capital-intensive process that requires a careful, thorough analysis to justify the necessary investment. The global nature of the automotive industry complicates matters.

To that end, recent developments in business strategies target the most pressing concerns, and they are driving down the costs associated with the development and processing of hydroformed parts. The new cost model allows body-in-white designers to reconsider hydroforming as an economically attractive alternative for lightweight designs.

The three areas of progress concern die development, die standardization, and metallurgical research.

  1. The part development and die construction processes are more economical than ever before.
  2. Standardization permits die use on nearly any hydroforming press.
  3. Research has proven that AHSS can be hydroformed reliably, reducing part weight while improving both structural rigidity and safety.

Upgradable Dies

An automotive industry axiom is that hydroforming dies are too expensive. This is true if the OEM follows the conventional path, using different die designs for various geographic regions (using separate suppliers). This is also true if the OEM orders prototype tooling before considering the impact of part design on tooling design and the production process. A big-picture, holistic view can help minimize the die development cycle time and cost.

While the capital investment of a hydroforming die is higher than that of a stamping die, it is important to remember that hydroforming reduces associated variable and material costs. The process reduces the number of parts in an assembly and eliminates the variable costs of welding, joining, and related logistics associated with corresponding stamped-and-welded assemblies. Additionally, hydroforming has a greater rate of material utilization than stamping, often 98 to 100 percent versus 68 percent. This reduction is normally more than $5 per vehicle.

A typical part development cycle requires one die for research and development work, a second die for prototyping, and a third die for production. Using upgradable dies can eliminate one of these. This requires the die supplier to build the prototyping die so it can be upgraded to a production die. This strategy, applied to a theoretical vehicle that requires six hydroforming dies, can yield a cost reduction of $300,000 per die for a total savings of $1.8 million.

Best practices for early collaboration through transparent communication and goal alignment create strategic relationships that support the implementation of innovative designs. A good start for a hydroforming program involves collaborative training so that part designers understand the potentials and limitations of formability for various advanced materials, and the hydroforming process experts understand the final part requirements. Early collaboration helps to streamline the development and prevent unexpected failures, such as wrinkles caused by compressing too much material or bursting caused by stretching too little material. It also provides better insight in developing springback compensation strategies.

Figure 1
Designing hydroforming dies for any system, anywhere in the world, helps to improve process consistency, which improves part consistency.

Predicting the forming capabilities early in the part design process can go a long way in reducing reworks and project delays.

Die Standardization for Simple, Reliable Manufacturing

As the costs of process development and die construction fall, hydroforming is transitioning from black box technology to a commodity process. OEM designers and hydroforming experts with global launch experience can collaborate early on the part design and take advantage of feasibility studies with standardization in mind.

The conventional part development practice is centered on the press; the part and the die are designed to fit a specific press model. If several manufacturers are tasked to supply the same or similar part, each makes a unique die and develops its own process. Inevitably, the part varies from supplier to supplier, and any lessons learned stay in-house. This decentralized approach also reduces production flexibility in two ways. First, the make-or-buy production decision, as well as supplier selection, must be made earlier in the development

process. Second, if the production needs to be moved to another location, it may come with the expense of new dies because the dies may not have been designed and produced to be moved easily.

Designing the part and die as universal items—that is, building a die that can make the part consistently in nearly any press—increases decision-making flexibility and production flexibility. It also reduces the die development cost, frees up more time to make decisions about where and how to produce the part, and reduces the costs associated with moving production if the need to do so arises in the future (see Figure 1).

While it may sound easy to design a standard hydroformed part for production flexibility, it is a complex undertaking because of the variety of hydroforming press systems operating in the automotive supplier market. The successful approach to accomplish this is to standardize the tool for forming and processing first, then design the connections with flexibility to adapt to various hydraulic, water, and electrical connections. Lessons learned can be shared quickly so that all of the suppliers can adapt processes and overcome any issues that may arise.

Advanced High-strength Materials Can Be Hydroformed Reliably

While the stamping industry has broader, deeper experience with AHSS, the hydroforming industry is catching up quickly. For example, the use of dual-phase (DP) 1000 hydroformed tubes in body-in-white structures is expected to increase over the next decade. Hydroformed roof rails and B-pillars made from AHSS appear in many vehicle platforms throughout the world. DP 1000 allows good strain hardening to deliver structural rigidity and high tensile strengths that make the material suitable for energy absorption during impact.

The Ford Fusion® uses a hydroformed DP 1000 roof rail that includes the A-pillar and B-pillar (see Figure 2). This design cuts 22 lbs. per car compared to a design using hot-stamped components. Likewise, the use of DP steel stampings and a hydroformed A -and B-pillar resulted in significant improvements in the torsional and bending stiffness of the 2013 Ford Fusion.1

However, the characteristics of AHSS that make it desirable for structural performance, specifically its high minimum yield strength (MYS) and ultimate tensile strength (UTS), come with challenging springback characteristics. In other words, these materials have less formability and more springback than conventional steels. As the part’s geometric complexity increases, so does the difficulty in making the part from AHSS.

If the production process involves several steps, such as bending, preforming, and hydroforming, the material’s springback characteristics create significant challenges to designing a robust, repeatable process. The various effects springback has on part geometry can be categorized three ways:

  1. Global springback occurs when the cross section of the tubular part is correct, but the overall geometric shape of the part is incorrect. In this case, the tube tries to return to its original straight geometry. Longer parts usually have more springback (see Figure 3).
  2. Crowning is a local occurrence on the part geometry, where the hydroformed cross section tries to return to its original round geometry. This is often seen in long, flat sections. After hydroforming, the surface bows outward.
  3. Twisting occurs as the cross section of the tubular part rotates against the targeted geometry. This is the most complicated type of springback to resolve.

Springback of a hydroformed tube often consists of a combination of these three effects. The presence and severity of each type is a result of the material’s MYS, material property variations, part geometry, tube thickness, and the number of process steps. Finite element analysis (FEA) simulation often predicts the degree of formability fairly well, but the springback prediction accuracy is limited. However, springback prediction methods are continuously improving, benefiting from the latest advances in simulations, laser scanning, and custom CAD modules.

Figure 2
The hydroformed DP 1000 roof rail and B-pillar are critical structural features of the 2013 Ford Fusion.

Stamping die manufacturers have reported the need to recut the die up to six or eight times. Early projects in hydroforming springback compensation required similar amounts. Through experience, feedback from real programs, and better simulation, the amount of recuts to produce the given geometry has now been cut down to three. The long-term goal would be to reduce this further.

Note

1. Shawn Morgans, “Advanced High-Strength Steel Technologies in the 2013 Ford Fusion,” in proceedings from Great Designs in Steel, sponsored by the Steel Market Development Institute, May 16, 2012.

This article was adapted from a white paper, “3 New Things About Vehicle Structures and Costs that OEMS Should Know About Hydroforming,” May 2014, Schuler Inc.

Figure 3
The DP1000 roof rail with a 6.5-ft. length has about 0.4 in. of springback at each end after forming.

About the Author

Klaus Hertell

Director of Engineering

7145 Commerce Blvd.

Canton, MI 48187

734-207-7200