September 14, 2004Tube hydroforming reshapes a tube from a normally round cross section to a desired shape. The final shape, usually rectangular, develops along the part length. The cross-sectional periphery may be consistent throughout the part and equal to the original tube, or it can be expanded in localized areas—usually at the ends. The decision to expand the tube or not plays a significant role in part design and in material, tool, and lubricant selection.
In expansion, the cross-section corner radius is an important consideration. As increased pressure stretches material into the corners, it also stretches the metal that is not yet in contact with the die surface, as shown in Figure 1. The stretched material decreases in width and becomes progressively thinner near where the wall is tangent to the die. This effect is more noticeable in sharper corners. Making the cross-sectional corner radius larger decreases the likelihood of rupture.
When the part is not being expanded, such as with pressure sequence hydroforming (PSH), the cross section forms as shown in Figure 2. This is a fundamentally different forming mechanism from that shown in Figure 1. Since water pressure is in the tube while the die is closing, it is the action of the die that causes the corners to form rather than the forming fluid, as is the case in Figure 1. Figure 2 shows compressive forces pushing the wall into the cross-sectional corner. As a result, the radius formed is not dependent on the water pressure being used.
Corner sharpness often is expressed as a multiple of the wall thickness— a common practice in the stamping industry. As can be seen in Figure 3, sharp corners relative to wall thickness can be achieved in a production part. The wall thickness shown is approximately 1.2 mm, and the outside corner radius is as small as 3.5 mm.
This factor is a key determinant of the maximum pressure required to completely form a part using high-pressure hydroforming (HPH) after the structural demands of material yield strength (YS) and thickness have been satisfied (Equation 1).
Using HPH to produce sharper corners, which increases internal pressure in cross sections, can be expensive. The pressure-generation equipment, the hydraulic process, and the hydroforming tool all must be proportionately stronger to achieve the desired corner. Increased die wear and selecting the right lubricant and material formable enough to prevent rupture also can increase cost.
Because corner sharpness does not depend on pressure with PSH, using this process to design sharper corners normally does not change the forming pressure or inflate costs. Furthermore, because PSH does not thin material in the corners and because the residual stresses are compressive rather than tensile, rupturing is not a concern.
Several design considerations favor sharper corners. One is extending flat surfaces for welding or placing a hole near a corner. Structurally, sharper corners improve a section's rigidity. Sharpening the corners is a technique that can be used to make parts more compact to accommodate packaging restrictions in the vehicle. A less tangible reason for making the corners sharper is that the part looks sharper and better defined—of higher quality.
Figures 4 and 5 and Table 1 illustrate that internal pressure used in the PSH process is not sensitive to corner sharpness. Corner sharpness can decrease 30 percent with no effect at all. Even a decrease of 70 percent generates only a 10 percent pressure increase. It is likely that even this P increase was unnecessary, but the experimental forming did not confirm this.
HPH techniques are well-suited to bulge forming as evidenced by many prototype parts that have been publicized. Examples are plumbing fittings, in particular T's, bicycle frame nodes, and camshafts.
Protrusions like those found on these parts would seem to be beneficial on automotive structural parts. It is unclear if their inclusion is technically or economically feasible in larger, normally steel structural parts. Several designs that included them evolved into production designs, in which they disappeared.
Indents that cannot be formed with a solid portion of the die can be accomplished by moving a portion of the cavity wall after the die is closed. Features that would create a die lock condition are an example. It probably is best to form such features when internal pressure is highest, but because this increases the size of the actuator needed, it is best to determine the lowest pressure needed for proper forming. Actuation units must be proportionately larger with HPH (because max. pressure is much higher than with PSH), with possible limitations on in-die hole punching or die integrity. Similarly, cross-section corners can be made sharper with such a mechanism.
PSH is better-suited to forming sharp corners, punching large numbers of holes, and creating dramatic indents, because the pressure used is much lower. High-strength materials as well as those with lower elongation can be formed with the same fluid pressure and press size used for lower-strength metals.