February 26, 2004Combined with the information in Part III of this series that focused on cross-section expansion before hydroforming, this article discusses the most common options used in preparing tube for hydroforming and achieving the designer-intended part. Properly executing bending and cross-section preforming prevents unpleasant surprises. Knowing what can and can't be done is important.
Bending is a necessary preforming operation in producing almost all automotive structural parts. Several different tube bending techniques are used in various industries. However, because of its many advantages— speed, accuracy, repeatability, and relatively good control of wall thickness variation from bending (thinning on the outside of the bend, thickening inside)—CNC rotary draw mandrel bending is the most commonly used technique for hydroformed parts.
Product design focuses on bend radius and angle, as well as the amount of straight length between the tangent points of adjacent bends, as shown in Figure 1. Decreasing radius and increasing bend angle increase bend severity, thus using a greater proportion of the material elongation or formability.
Subsequent operations, such as hydroforming, also may require significant elongation to form the part successfully. This need increases substantially as the material is stretched more. Ensuring there is enough formability for all operations in a process sequence is critical for stable and robust production. Other, related issues are accounting for material property variability during the product's life and, conversely, incurring higher tube cost for specifying chemical restriction properties.
Elongation requirements rise substantially when material is stretched, such as when the whole tube is expanded to avoid pinching or a portion of it is expanded to satisfy part requirements. Both conditions are normal when high-pressure hydroforming (HPH) parts, but not with pressure sequence hydroforming (PSH). These challenging requirements can be addressed several ways.
Annealing returns the material to a high-elongation, lower-strength state, such as it was before work hardening from operations like sheet rolling, tubemaking, bending, and preforming. Although the whole part can be annealed, it seems that local annealing (probably induction) usually is employed on the bent regions where cold working is concentrated.
After annealing, 20-plus minutes of cooling is necessary to complete the softening process. Annealing must be done in an oxygen-free environment to prevent the formation of iron oxide or black scale on the part surface. Annealing generally is viewed as an expensive option.
A second approach is limiting bend angle and radius to preserve elongation for hydroforming. Doing this can prevent the need to anneal, but may lead to a multipart assembly when one component could have done the job. Alternatively, avoiding useful features that could be designed in, such as abrupt indents and local sharp cross-sectional corners, to reduce formability requirements may limit the part's usefulness.
A third approach is to develop special material with a higher starting elongation that maintains a suitable residual formability after the interceding operations are complete. Although using material with higher yield strength (YS) may cost more, it may be the best alternative.
Preforming the tube cross section over some or all of the part length may be required to avoid pinching a portion of the tube between the die sections. There are at least two ways to accomplish this process.
The first method squeezes the round perpendicular (to make an oval shape) to the direction of die travel, because a section in the die cavity is too narrow for the start tube OD. This preform method, which has little effect on elongation, requires a hydraulic cylinder to compress the section in the direction desired between plates that are flat perpendicular to the tube centerline, allowing it to form freely in the other directions (Figure 2).
Alternatively, the cross section can be formed closer to the desired final shape with an enclosing die cavity-probably accompanied by some unwanted deformation-then hydroformed to complete forming and remove any deformations (Figure 3). This is done in the context of HPH when the difference between blank and final part peripheries is inadequate to prevent pinching along the part length when the die is closed. When the blank is compressed in one direction, it also is constrained in the others. The forces required to accomplish this performing process are substantially higher. Therefore, this process must be done in a press.
Bending and cross-section preforming directly affect hydroformed part quality. Make sure you're using the appropriate methods and materials.
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