Process Conditions, Equipment, and Conclusions
February 7, 2006
In this article Gary Morphy reviews high-pressure and pressure sequence hydroforming and discusses factors to consider when deciding which process is best for a particular application. The decision should be based in part on anticipating future needs.
A common saying is that all good things must come to an end. I am not really sure why this must be true, and I certainly hope that there are many exceptions. However, it is true for this series of tube hydroforming design flexibility articles that began in March 2002. This final installment focuses on process conditions and equipment.
In hydroforming, the process condition that receives the most attention is internal forming pressure. The hydroforming process used makes a dramatic difference in the amount of pressure needed to form the part successfully. How the pressure is used makes an even bigger difference.
Although an earlier article in this series discussed in detail the differences between two important hydroforming processes, here is a brief review.
High-pressure Hydroforming (HPH). In HPH (Figure 1), the tube is bent and preformed as needed and placed in the hydroforming die. The die is closed with the tube empty and then filled with water that is pressurized according to a pattern normally coordinated with the movement of cylinders located at the part ends to end-feed material.
To stretch the material to conform to the die cavity shape, pressure levels must be sufficient to overcome material work hardening and corner sharpening during forming. Maximum pressure normally is determined by the calibration pressure required to reduce the part's dimensional variability. This process causes wall thinning concentrated in the cross-section corners, which can be reduced by using several measures, including end feeding and lubrication.
Additionally, since forming is achieved by the pressurized water, part design features like the material yield strength (YS), thickness, and cross-sectional corner radius all have a direct effect on the pressure needed to hydroform the part completely.
Pressure Sequence Hydroforming (PSH). As with HPH, in PSH (Figure 2) the tube is bent, preformed, and put in the die. Although this procedure generally is the same for both processes, the details may differ.
Pressure Sequence Hydroforming
As the hydroforming die is partly closed, the ends are sealed and the tube is filled with very low-pressure water. The tube's periphery and the die cavity are equal to each other along the part's length. The die continues to close and reshapes the metal until it is fully closed.
The pressurized water inside the tube discourages material from crumpling into the inside of the part, and the die surface completely prevents it from forming outside the desired part shape, except at the die split line. Part and process design concentrates on making sure these undesired effects do not occur. The result is a reshaped part with no wall thinning. With no expansion or stretching present, there is no opportunity for thinning.
The maximum pressure needed to form a part with PSH is relatively constant despite substantially changing yield strength, wall thickness, sharpest inside cross-sectional corner radius, or even all three present at the same time. This is true because the die is forming the material, with water aiding the process. Therefore, forming the part is not pressure dependent. Also, the main criterion for determining the press clamping force size is the vertically projected area of the part multiplied by the internal pressure. The part size also determines the bed size.
HPH process conditions also can include end-feeding cylinder position, speed, and lubrication. Alternatively, PSH is affected by press closing speed, how much the die is closed when the part is filled with fluid, and bender repeatability, among other factors.
Process conditions often are consequences of the hydroforming method and part design decisions (i.e., expansion) made earlier or are sometimes dictated by the customer. Often these conditions, notably forming pressure, can present problems or elevate costs, but cannot be eliminated or reduced unless the part or process design is changed. Sometimes design change is a possibility; too often, it isn't.
Understanding process differences in the context of the real requirements of the part function is important, because process conditions directly affect production costs. Determining the necessary pressure level obviously is a crucial factor that greatly impacts capital, tool and part cost, and flexibility.
The energy necessary to generate pressure and contain it with a press is a major factor in process cost. Higher pressure means higher costs.
It now is understood that to maximize future design flexibility and equipment use beyond producing parts for the current program, HPH must be used with the largest press that can be justified. Of course, this is limited by press cost and slower cycle time that tend to come with larger presses. Changes in YS, wall thickness, and corner radii that happen as part design evolves can dictate equipment for substantially higher pressure and clamping force. The product that succeeds the one the equipment was bought for may require substantially higher pressure along with a larger press and pressure delivery system.
Another approach to consider is that PSH equipment designed to run an engine cradle part, for example, is suitable for making similar-sized parts in the future, regardless of design changes. The paradox is that using low pressure, commonly viewed as a limitation, becomes a substantial strength when combined with sequenced pressure levels, because the pressure and equipment size will not need to change. PSH also provides a more efficient process coupled with the design flexibility detailed above.
Design flexibility is a multifaceted concept that usually receives insufficient consideration when different manufacturing techniques are being evaluated. This probably is truer than normal for tube hydroforming because of the technology's relative recent rise to prominence. A conventional wisdom has not developed yet. Often the assessments that are made are only partly true. It always is better to seek opinions from different technological viewpoints to see if they agree or conflict.
To determine the best hydroforming process for your operation, consider all factors, including variable versus constant periphery design; material type, properties, and thickness (overall and process-caused variation); bending; finished part cross-sectional corner sharpness; part features (forming severity); holes; and dimensional stability. Also consider the process conditions, section expansion, and the future ability to adjust to design changes.
Both HPH and PSH have obvious strengths. PSH also has subtle strengths that must be understood before using. These include high degrees of flexibility with respect to material, part shape and severe shaping, dimensional stability, expansion, and continued equipment viability.
Asking questions based on process and equipment considerations will expand your hydroforming knowledge and lead to more efficient and effective component manufacturing.
For those who have read this series, thank you for your attention. I hope that the articles have helped you better understand tube hydroforming, a relatively young and developing part manufacturing technology. I welcome comments and questions about the articles in the series or hydroforming in general.