Hydroforming High Strength Steel Tube for Automotive Structural Applications Using Ex
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31-12-2010, 02:48 PM

Gary Morphy

The need for lighter, stronger, more rigid vehicle structures will increasingly require complex hydroformed structural tubes to increase strength, and decrease weight, cost and part count. This effort will increase the use of high strength, low alloy (HSLA) steel, in place of SAE 1006/1008 or 1010 steel. Traditional hydroforming techniques require the higher elongation of the latter materials. An alternative tube hydroforming process has been developed to successfully use these, and HSLA grades from 310 (945XF) to 552 (980XF) MPa minimum yield stress. This paper concentrates on hydroforming steel with a focus on HSLA. It will demonstrate to automotive designers available features such as local section expansion and reduction, hole piercing, achievable cross sectional shapes and the relationship between tube size, corner radii, and wall thickness.

It is the purpose of this paper to illuminate how automotive structural steel parts can be hydroformed into complex cross sectional shapes that can vary dramatically along the part length, focusing on several HSLA grades. Available part design features, insight into how the process works, as well as the required internal pressure and press size are discussed. Box section structures have long been an integral part of designing and constructing a motor vehicle. This need has been satisfied to some degree by assembling several stampings into closed sections, with the requisite joint flanges and overlaps. Extra section size or wall thickness are usually used to compensate for the weakness inherent in the assembly joints. In the past there has been no viable way to mechanically form a tube into the complex part shapes required with the quality levels demanded.
Tubular hydroforming removes this constraint and extends tube use into these applications. Benefits such as increased strength, reduced weight, reduced tool cost, variability reduction, and part consolidation with all their attendant advantages are often realized in the same part. Overall tool and part cost is reduced, and running design changes, such as altering material thickness, are often easier to accommodate.

As crash, weight reduction, as well as, bending and torsional rigidity requirements become more stringent, hydroforming’s predominant virtue of making more efficient use of metal is paramount. Manufacturing scrap is routinely below 10%, and part weight savings compared to stamped assemblies result from eliminating joint overlap which also inherently improves structural properties. This in turn allows reduced section size and/or wall thickness.

Most hydroformed parts have used SAE1006/1008, 1010, or other low strength, high elongation steel. Extending the principle of more efficient metal use logically leads to using HSLA grades of steel. Weight saved by using HSLA results in a significantly lower net material cost despite the higher unit weight cost. When cost and weight can be reduced simultaneously it makes sense to maximize HSLA use. A natural extension is to use a process that gives maximum design flexibility for high strength, lower elongation material. A tube hydroforming process that offers these features, while increasing achievable part complexity, has been developed and presently produces a number of high volume parts. These include an engine cradle with relatively complex surfaces using 310 (945XF) MPa min. yield HSLA steel. Experimental work has been done using 552 (980XF) MPa min. yield, 621 MPa ultimate tensile strength steel.
Mechanical expansion prior to the hydroforming operation, and hydroexpansion during the hydroforming operation are two methods that will be discussed. Notable characteristics of these parts will be discussed, as well as other desirable features that can be designed in to satisfy particular needs.

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