R&D Update: Sheet hydroforming

Sheet hydroforming (SHF) was not considered for automotive manufacturing until recently because of its high cycle time. However, recent advancements in hydraulics, press design, and controls have reduced cycle time considerably, making SHF attractive to automakers. For example, General Motors recently purchased an SHF press from Amino Corporation in Japan for the commercial production of the Pontiac Solstice®.

SHF can provide:

• Low tooling cost.

• Improved part properties after forming, such as dent resistance and energy absorption.

• The excellent surface finish because there is no metal-to-metal concat on the exterior panels.

SHF uses a punch and a pressure pot (SHF-P) or a female die (SHF-D), depending on the male or the female die that has the shape to be formed.

SHF-P

In SHF-P, the sheet is deep-drawn against a counterpressure in a pot container rather than a female die in a regular stamping operation (see Figure 1 ). The medium in the pressure pot can be either passive (pressure generated by the incompressibility of the medium during the forward stroke of the punch) or active (pressure generated by an external pump or accumulator).

During SHF-P, fluid pressure acting acting on the sheet increases the frictional force at the sheet/punch interface, which leads to a draw-in of the material from the flange and less thinning in the formed part. The counter pressure also help to suppress the formation of secondary wrinkles in the side wall.

SHF-D

In SHF-D, the sheet is formed against a female die by the hydraulic pressure of the fluid (see Figure 2 ). The forming operation can be divided into two phrases:

1. Free forming, during which the sheet bulges freely in the die cavity until it initiates contact with the die. This introduces uniform strain distribution through the sheet, which uses the material’s formability effectively as compared to the conventional stamping process, in which deformation is localized in the sheet at the punch corner radius. In addition, the dent resistance of the hydroformed part is increased compared to a conventionally stamped part.
2. Calibration of the sheet against the die cavity to obtain the desired geometry and corner radii.

The ERC/NSM designed and fabricated tooling for the SHF-D process in a conventional 160-ton hydraulic stamping press with a numerically controlled hydraulic cushion to form a variety of parts (see Figure 3 ).

The stationary punch is mounted on the press bed, while the blank holder rests on the cushion pins, and the die is mounted on the press ram. The pressurizing medium required to generate the pressure is placed in the cavity formed by the punch and the blank holder at the topmost position.

Initially the sheet is placed on the blank holder. The upper die moves down and clamps the sheet against the blank holder.After the sheet is clamped, further movement of the upper die results in movement of the blank holder relative to the stationary punch. This relative motion results in compression of the pressurizing medium between the sheet and punch

Because the pressurizing medium does not compress, pressure is generated to act on the sheet. Thus, by pure mechanical motion in a conventional press, sheet hydroforming is achieved in the designed tooling.