Examining Hydroforming

Interest in hydroforming is growing, Jonathan Ball, hydroforming sales manager at Schuler Inc. (schulergroup.com), says, from a number of automotive perspectives, including both lightweighting and occupant safety. A closed-section hydroformed tube, whether dual-phase steel or aluminum, offers comparable protection to that of a stamped and welded section, but using lower-gauge material. Which means the part is light and strong.

Producing roof rails, frame sections, and A- and B-pillars, traditional hydroforming applications, entails loading a tube into a hydroform die and fixturing the die assembly into a hydroforming press. The tube is filled with water, sealed, and press pressure from the outside together with increased water pressure from the inside deforms the tube to the required shape. Ball describes a more detailed sequence Schuler calls “pressure sequence hydroforming,” where the press closes with nominal pressure already in the tube, then pressure is applied until the deformation point, contributing to tighter part specs.

Multi-Function Die Design
Schuler designs, simulates and builds production hydroforming dies in its facility in Canton, Michigan, for customers. Such dies not only form tubes into rails and pillars, they can also pierce or section parts. Prototype dies establish part feasibility, and die inserts customize component forming and features. “The inserts make the dies upgradeable, there’s never a need to throw away a prototype die,” Ball says.

Finite element analysis helps determine strain distribution and rates during deformation and can identify problem areas such as wrinkling, thinning or splits. Such analysis is conducted not only on the part, but to identify weak points in the die structure, as well.

Aluminum Versus Steel
Klaus Hertell, Schuler vice president, forming technologies, recently outlined and compared hydroforming applications on two Ford vehicles: the roof rail on the 2015 F150 (6000 series aluminum) and that on the 2015 Ford Edge (DP1000 high-strength steel). Both were designed and developed to improve body-in-white stiffness and Schuler helped design and build the production dies for each.
In the case of the F150, extruded aluminum tubes were the starting point of the frame members as opposed to welded tubes (steel or aluminum) produced on a tube mill. Where tube mills largely produce round or oval tubes, extrusion dies make a wide range of section shapes possible to begin with. And where weld seams can exhibit varying hardness versus the base material, extruded tubes have no weld seam to contend with.

Hertell stresses the point was not an either/or choice where aluminum or steel was concerned, but how hydroforming could benefit each, given the material grades and thicknesses. In the case of the F150 aluminum tubes, the 6000 series tubes were 50 mm in diameter, 2.8 mm thick and had a weight per foot result of 0.68 lb. By comparison, the DP1000 advanced high-strength steel tubes in the Ford Edge were 60 mm in diameter and weighed in at 1.74 pounds per foot, but were only 1.8 mm thick.

Steel or aluminum, robust hydroforming begins with tube bending, Hertell says. Factors to consider in achieving repeatable bending results include bending equipment settings (mandrel shape and position) and material specifications (forming limits and dimensional consistency of incoming tube). Such specs can be more challenging for aluminum than steel for a given part.

Hertell also referred to pressure sequence hydroforming as a process assist. Closing a press with a nominal 700 psi of water pressure in the tube helps support part dimensions, achieve tighter corner radii, and avoid wrinkles and splits. 

Another factor to consider in hydroforming is springback, more significant in high-strength steel, but still occurring in aluminum. With hydroforming tubes, the types of springback that take place include crowning, twisting and overall (global). Given springback is a metal forming fact of life, methods of dealing with it include addressing tool design and examining part requirements for which sections need to achieve critical dimensions for assembly or packaging and which may be not as critical. Schuler practices a closed-loop process during die development that entails laser-scanning tubes and feeding the information back into the CAD program for reverse engineering and recutting the hydroforming die as necessary.

Advances such as hot stamping are being examined in hydroforming at Schuler with a process Jon Ball calls “tube hot-forming.” A hot boron steel tube is placed in the hydroforming die, the tube is filled with gas, and deformation takes place as the press closes and the tube is quenched in the die.