Application of Hydroformed Aluminum Extrusions to Vehicle Sub-Frame with Varied Wall Thickness

In a typical hydroforming operation, a round tube of constant wall thickness is bent into the overall shape desired for the final part, then placed between a pair of dies. Despite some small percentage of stretch that may occur as the tube expands, the wall thickness in the original tube is therefore substantially constant at all points. In some circumstances, a part is locally thickened or reinforced for extra strength. Normally, this is achieved by using a separate piece of reinforcement at selected location. In this paper, it is intended to present a unique method to achieve an optimal structural design allowing thin or thick gages where required along its cross-section. This is done via hydroforming an aluminum extrusion tube to an optimal frame structure having varied wall thickness to satisfy various loading requirements at a minimum weight. The engine cradle is used as an example to demonstrate this methodology. The finite element model in conjunction with an optimization software has been used to determine the variable gage thickness for various locations of the structure under all the required loading conditions. The advantages of using varied wall thickness design over the constant wall design was proven out in this example. Approximately 37% of mass reduction is achieved with this unique design.