April 24, 2003
The last article in this series noted that variable periphery design, or cross-section expansion, often is thought to be the most important aspect of tube hydroforming design flexibility. Expansion in the hydroforming die commonly is assumed to be the most efficient and most effective method, implying the lowest part, tool, and capital cost. However, alternate methods of expanding a tubular blank before hydroforming can provide a more favorable result in one or more areas. This article focuses on the advantages and disadvantages of these alternate methods.
Expansion in the hydroforming die (hydroexpansion) requires fluid at high pressures to stretch the tube into the corners of the die cavity, which causes the tube wall to thin where end feeding is not effective. Alternatively, the tube can be expanded before insertion in the hydroforming die. Three methods are mechanical preform expansion, hydraulic preform expansion, and elastomeric bulge forming. The benefits each offers are little recognized and discussed later, but first a brief review of hydroexpansion.
Hydroexpansion seems simpler than pre-expansion followed by hydroforming. While this can be true, it's important to confirm process efficiency, because several conditions can reduce the benefits. These conditions are:
End feeding into the expanded area minimizes wall thinning. End feeding is opposed by friction between the die and the tube wall, which means that the process is effective for a limited distance from the end. When the friction force equals or exceeds the column strength of the formed tube, buckling occurs. This distance is determined by the amount of friction caused by bends, cross-section forming severity, lubrication, and material used.
Pre-expansion offers the possibility to avoid any restrictions these conditions may impose.
Mechanical expansion is accomplished by forcing a normally round forming mandrel of the desired diameter in the end of a round tube (see Figure 1) along a desired length, generating a frictional force between the mandrel and the inner tube wall. This force resolves into a radial component that expands the tube and an axial component that acts in the direction of mandrel travel to move material into the expanded region. This passive end feed counteracts much of the wall thinning that would otherwise occur, and the round section makes any thinning that does occur consistent around the periphery.
Friction works with this process to feed more material into the die in a passive manner. In contrast, friction between the hydroforming die surface and the tube outer surface opposes end feeding during hydroexpansion and must be controlled or minimized. If the passive end feed allows too much wall thinning, it may be supplemented by also pushing on the tube end (an active end feed), but cannot exceed the tube buckling strength.
This technique has been practiced for many years and may be dismissed by some as old technology. However, if a process contributes to a better product or reduces cost, it is worthy of consideration. Equipment to clamp the tube and advance the expanding mandrel is simple and relatively inexpensive.
Mechanical preform expansion can be done at the part end only, which is a common practice for hydroexpanded parts that require significant expansion (more than 10 percent). Large expansion is possible in one or more steps. Up to 67 percent expansion has been prototyped in one step in mild steel. Even larger expansion is possible but requires annealing to restore material formability. In most automotive production parts, expansion ranges from 0 to 25 percent. Aluminum, because of its lower stretchability, can achieve lower expansion levels before annealing is needed.
Hydraulic Preform Expansion
Hydraulic preform expansion uses fluid pressure to expand a portion of the tube by enclosing a straight round tube in a die. Sufficient internal fluid pressure is exerted to fill the die (Figure 2). Expanding in the round provides the best end feeding conditions and even wall thickness distribution. Initially the expanded portion of the tube is not in contact with the die and the remainder of the tube is undeformed, so friction plays a lesser role than in hydroexpansion. Hydraulic perform expansion is a useful technique for expanding the middle of a part, where end feeding during hydroexpansion is ineffective, but it can also be used elsewhere on the part. It leads to a more robust process for a given expansion, or alternatively, it provides the option of larger expansion.
The pressure required to expand a round tube is low compared to that required to expand a part with cross-section corners [that is, rectangular] according to Equation 1. For instance, for a 3-in.-dia. X 1.5-mm wall mild steel tube (YS = 35 KSI), only 1,500 PSI is required to begin tube expansion. Applying lower pressure further reduces friction and resistance to end feeding.
The hydraulic expansion die must be held closed, which probably is accomplished most efficiently in a hydroforming press, and it must be sized properly. Increasing the press-tonnage or bed size or using a separate small press will increase equipment costs, but particularly in the first case, the difference may be acceptable considering the benefits. The tonnage and bed increases are likely to be small because internal pressure is low, and the die will be small, because the tube is straight. Another cost is for the simple expansion die.
Because hydraulic expansion is used on a straight tube and most parts require bending, the effect of one on the other must be given strong consideration during part design to balance cost and part function to reach the most economical solution.
Applications for elastomeric bulge forming and hydraulic preform expansion are similar. Performed on straight tube only, elastomeric bulge forming uses an elastomeric plug slightly smaller than the minimum ID of the tube that is placed inside in the appropriate axial position. The plug then is compressed axially, causing it to expand radially, forcing the tube to expand (see Figure 3). Friction between the plug and tube causes material to be pulled into the expanded section, another example of passive end feeding. Additional end push can be applied if needed.
Elastomeric bulge forming can be performed without containing the part in a die to reduce tooling and equipment costs, but results will be less repeatable. Experimentation will indicate the most suitable alternative. In either case, since only the expanded section needs to be contained, equipment and tool cost should be reduced.
Once it has been determined that expansion provides sufficient benefit, the most advantageous method must be selected. These three methods are alternatives to hydroexpansion and can be used with any hydroforming process. They should be used when they provide enough advantages, including a favorable wall thickness pattern or a reduction in overall part cost, but they are not suitable or cost-effective for all parts. For such parts, hydroexpansion may be the best option.
Pressure Sequence Hydroforming
The automotive industry's aggressive cost-reduction measures require metal formers to choose the lowest-cost method to perform each operation. Normally the attributes of the expansion methods are not compared to arrive an an accurate cost for each. The direct costs, as detailed in this and the previous article, must be assessed, but perhaps of even greater importance are the secondary costs.
Secondary costs can include the need to use lasers to cut holes rather than cutting with the hydroforming die, because cutting away die material to fit punching assemblies weakens the die too much. Other examples of secondary costs are additional lubricant and special material for improving end feeding or stretching to form, as well as reducing corner radius to resist localized elongation.
Preform expansion may allow greater expansion without annealing. It also provides the ability to form more complex cross-section shapes for pressure sequence hydroforming because the corners are formed by compression bending (Figure 4) rather than stretching as occurs during hydroexpansion (Figure 5). Forming more complex shapes when hydroexpanding increases friction forces, which opposes end feeding. This reduces the amount that can be fed in and thus potential expansion. Complex shapes include sharp corners (for example, 2X wall thickness), as well as other features shown in Figure 6. These are all formed with less than 10,000 PSI and represent shapes that can be formed with pre-expansion using the process in Figure 4, but are not actually expanded.
High Pressure Hydroforming
Since the wall thinning is reduced and evenly distributed about the periphery of the tube in preform expansion, less localized strain occurs. Because of this, higher-strength and lower-elongation material, such as HSLA steel or aluminum, can be expanded without risk of rupture in the hydroforming die, (although less than mild steel). Near wall-to-wall sections (Figure 6 left) also can be achieved with pre-expansion.
The ability to use in-die hole punching is much improved with one of the preform methods. Because of the inherent economic and dimensional stability benefits of cutting holes before compared to cutting holes after, this is an important consideration. To accomplish this, holes in the cavity surface accompanied by large openings behind it to accommodate punch-driving mechanisms are required. Depending on position, quantity, and proximity to each other, the openings can dramatically weaken the die's ability to contain high internal forming pressure, potentially leading to die cracking and failure. Additionally, holes in the cavity surface may impede end feeding or increase the chance of rupture when hydroexpanding.
Production & Prototype Shapes
A preform expansion adds the cost of an additional operation, but may, in fact, decrease the overall cost per part. Expanding the ends of parts using hydroexpansion with end feeding increases cycle time, which perform expansion prevents.
Using hydraulic preform expansion or elastomeric bulge forming to expand the part in the middle facilitates expansion where end feeding is not effective while largely preserving wall thickness. If a similar part were to be hydroexpanded, a thicker wall would be required to accommodate wall thinning in the central portion of the part. Therefore, a thinner tube can be used in preforming, which decreases material cost and weight.
Hydroexpansion can be a more cost-effective way to extend expansion through a bend if the amount of material that can be fed in is enough to support the expansion amount. Doing this as a perform operation would result in bending difficulties and increased expense.
Expanding sections of a part adds cost, but is worthwhile if sufficient value is added. Questions to ask when considering expansion are:
When designing a part to fulfill a function, it is important to consider several different manufacturing approaches. The method of expansion used for a part to be hydroformed can dramatically affect part cost and effectiveness. Eliminating options too early can severely limit the design flexibility.