Tube Hydroforming Design Flexibility—Part I
Design flexibility is something that all automotive designers want, but too often they lack a thorough understanding of what that means—what aspects of design flexibility apply to a certain part and their effect on cost.
A methodology often is adopted when (or even before) a part development program begins, and before an informed decision is possible. This can introduce design compromises and extra costs that those involved either do not recognize or have to live with because they are committed to a system, and it is too late to change.
Ideally, it will help you to identify the best way to make a part, considering all the options and thereby minimizing cost.
The subject is reviewed relative to the two dominant hydroforming techniques being used today because they provide a framework for discussion. These are pressure sequence hydroforming (PSH) and high-pressure hydroforming (HPH). A brief explanation of each process is given later for those unfamiliar with how they work.
The main design flexibility categories are:
- Variable versus constant periphery design.
- Material type, properties, and thickness (overall and process-induced variation).
- Finished part cross-sectional corner sharpness.
- Part features (forming severity).
Other categories include dimensional stability, process conditions, and future ability to adjust to design changes (seeFigure 1).
When examining this design flexibility, it is paramount to keep your options open and know the full meaning of the term when you are deciding how to make parts economically and ensure continued competitiveness.
Tube hydroforming technology in prototype, low-volume, and plumbing applications has been used for many years. Over the past 10 years, designers have found that many different part types can be made more economically or with a greater range of features when hydroformed.
The technology is relatively new for automotive structural parts, and the depth of knowledge is often limited. This can lead to acceptance of partial or misleading information as complete fact, which can be detrimental and dangerous.
Part complexity tends to increase as designers try to minimize part count and assembly operations and maximize economy and functionality. As complexity increases, containing the tubular blank inside the die cavity becomes more difficult. Failing to contain it leads to pinching of part of the tube periphery between the die halves, and the tube doesn't fill the cavity properly. It also can rupture.
All of this makes the scrap pile swell.
Consequently, PSH and HPH—two distinctly different techniques—were developed to facilitate more complex forming and prevent problems with pinching, forming, and rupturing. The reference to pressure in each name can be misleading. The real difference lies in how each captures the starting blank in the die cavity to form a part successfully. The final or maximum pressure used is simply what is needed to complete forming the part. There is no direct correlation between pressure level and potential part design flexibility, although it is a common misconception.
Most automotive structural parts need to be bent, and some are preformed before hydroforming.
In Figure 2, the blank is placed in the cavity and the die begins to close. When the die is partially closed, the tube is partly crushed. The ends are sealed, and low-pressure fluid fills the blank, making it relatively incompressible and like a formable solid. The die starts to close again with the desired low pressure maintained while the part volume reduces.
Low internal pressure during closure discourages pinching between the die halves in the same way that it is difficult to pinch a balloon. This liquid mandrel also resists or prevents unwanted inward deformation.
Closing the die halves generates mechanical forces that deform the starting tube. In addition, the cavity periphery essentially is the same as the starting tube. Combined with the benefits of low internal pressure, the forces are guided in a more useful direction. They act through the tube wall and compressively around the cross sections to force material into the corners.
When properly designed and managed, these factors cooperate to force the tube to take on a complex shape with far less pressure (typically one-fifth to one-third) than is needed with other techniques. Normally, the maximum pressure needed is less than 48 Megapascals (Mpa) (7,000 pounds per square inch [PSI]).
The tube wall is guided to the desired location and shape with no wall thinning, because the cavity and blank are the same periphery, leaving no room to expand.
This process offers superior design flexibility in several respects.
Another way to prevent pinching of the blank between the die halves is to use a tube whose circumference is 5 percent to 10 percent smaller than the desired final part periphery. When the die closes, there is no pressurized fluid in the starting tube, and it takes on the cavity shape to a limited degree (see Figure 3).
With HPH, some undesired inward deformation occurs, and cross-sectional corners are not filled when the die is closed completely. Water is injected and pressurized until these corners form completely, and any unwanted deformation is forced out against the die. Maximum pressure usually exceeds 140 MPa (20,000 PSI) and can go up to 690 MPa (100,000 PSI).
Required pressure depends on material yield strength, wall thickness, and the inside radius of the sharpest cross sectional corner. Another consideration that usually increases the maximum pressure required is calibration, which is used to reduce springback resulting from this process.
The tube wall is ballooned, or blown out, against the cavity wall. As a result, material thickness can vary significantly throughout the part. Much of the ongoing development efforts in the hydroforming industry involving special materials, tube making, lubrication, simulation, and other areas strives to minimize wall thinning and these types of variation.
Earlier development efforts resulted in the use of end feeding, lubrication, annealing, surface texturing, and lower-strength and high-elongation materials to address wall thinning. Hydroformers should be aware of the issues that arise from these techniques to minimize their detrimental effects. Some of these effects are material formability, friction, tool design and wear—all of which can impact part design flexibility.
As with PSH, this process offers superior design flexibility, despite certain drawbacks.
Design flexibility is a general term that has different meanings for each industry and for each type of product. Everyone knows its importance, but defining what it means and how it should be maximized are difficult, particularly because it often changes, depending on part requirements and design.
You must get to the heart of what design flexibility means for your industry and part types to decide the most appropriate technique to use. Cutting corners here leads to unpleasant surprises later.
Design flexibility as it applies to your products must be broken down into categories (see Figure 1) to judge whether the best result is being achieved. To obtain flexibility of one sort, other types should not be easily sacrificed, or at least sacrificed only after discovering what is being given up.
In other words, to understand how design flexibility applies generally, you must understand how it applies in detail to several specific parts.