Why use sheet hydroforming dies?

It's a simple fact that the automobile industry constantly strives to reduce vehicle weight, while maintaining or increasing performance and fuel efficiency. Despite major advances in engine and powertrain technology, the need to reduce vehicle weight still exists. Most automobile manufacturers have looked for alternatives to the steel traditionally used in car production; hence, the introduction of high-strength steel.

High-strength steel is considerably thinner than conventional lower-strength materials, yet strong enough to meet industry standards. Using this new steel reduces vehicle weight, increases performance, and reduces fuel consumption. Unfortunately, stamping high-strength steel doesn't come without cost or problems. This article focuses on why sheet hydroforming operations are being researched heavily and most likely will become more common in the future.

Figure 1

High-strength Steel

High-strength steel doesn't have the stretchability of more conventional steel grades, which creates a problem when stretching the material is necessary to produce the required part geometry. High-strength steel also has relatively poor stretch distribution characteristics. In other words, its ability to stretch evenly over a large surface area is limited. This phenomenon often results in a fractured part when using conventional tooling is used, because very little stretching of the material occurs during forming except for the point of fracture. Higher-grade (low-strength) material distributes the stretch more evenly over the surface area (see Figure 1).

Forcing the Metal to Stretch and Flow

There are two basic ways to create a part's geometry are by stretch and flow.

Stretching the Material

The key to obtaining good stretch distribution when using high-strength material is to use a punch geometry that forcesthe material to stretch as evenlyas possible. Most of the time, forming punches with large radii or even dome-shaped punches are utilized to force the metal to stretch evenly. Unfortunately, parts are not always designed with large liberal radii, and even if they were, the stretch distribution result most likely would be poor. Why? Because higher friction created at the forming punch radii will result in thinning of the material. In other words, most parts created using high-strength materials with conventional dies exhibit a few areas of high stretch near a radius and numerous areas of little or no stretch in the flat areas. Again, poor stretch distribution.

Figure 2

Metal Flow

Metal flow is controlled largely by several basic factors: draw ratio, blank holder pressure, the die addendum, and the part geometry. One of the most influential factors, especially when using high-strength material, is the draw ratio. The draw ratio is defined as the direct relationship between the forming punch and the blank. If the blank is too far from the edge of the forming punch, very little or no metal flow occurs. This will most likely result in stretching and fracturing of the blank. If the blank is close enough to the punch contact area, the metal flows inward toward the punch, resulting in much less material stretching (see Figure 2).

Figure 3

Sheet Hydroforming Dies

So far, we have discussed conventional tooling principles. You must understand these basic principles clearly before you can understand how and why sheet hydroforming works. First of all, water, oil, and any other liquids have no distinct shape. These liquids will take the shape of any conceivable geometry that we desire. They also have the ability to change their shape continually during a metal forming operation.

Figure 3illustrates an attempt at forming a cone-shaped part in a single operation with a conventional drawing die. As you can see, the contact point of the punch is very far from the blank edge. Very little or no metal flow will take place, and fracture most likely will occur. Secondly, even if the metal does flow inward, most of the stretching occurs at or near the punch point rather than uniformly across the sheet. Achieving this particular part geometry using a conventional tool would take several steps or reductions.

Figure 4

Figure 4shows the same part being made in a sheet hydroforming die. The liquid is making contact with the entiresheet surface as opposed to a small contact area. This allows for an acceptable draw ratio as well as more even stretch distribution during forming. Sheet hydroformed parts exhibit more even thin-out throughout the entire part geometry.

The advantages of sheet hydroforming are as follows:

1. Requires fewer operations to make certain part geometries
2. Does not require lower or upper draw punch or cavity
3. Uses water, a widely available resource
4. Forces material to distribute stretch or strain more evenly
5. Reduces springback
6. Reduces material consumption
7. Forms higher strength materials

The disadvantages of sheet hydroforming are as follows:

1. Requires expensive equipment
2. Cycle times are generally poor
3. Operators often get wet

The Wave of the Future

This article touches on only a few basic principles of sheet hydroforming. Currently the aerospace industry uses sheet hydroforming the most, primarily because aircraft requires high-strength materials. As the automobile industry transitions from low-strength to high-strength materials, it may be necessary to change the process to match the material. Although sheet hydroforming currently is not cost-effective, I believe the technology eventually will evolve into a competitive means of metal forming.