Conventional sheet hydroforming uses water with additives as pressure medium. However, viscous pressure forming (VPF), using a viscous material can also be used in the hydroforming process and offers the advantages of improved sealing to reduce leakage and easier handling in prototype production of a small number of parts. This paper describes the application of VPF to the forming of a non-symmetric part from steel, aluminum and a nickel alloy. FEA simulation and blank holder force control are utilized to optimize the process conditions. The effect of process variables upon the achievable part geometry is discussed. Comparison of FEA predictions with experiments illustrated that metal flow and thinning can be estimated with good accuracy.

Tube hydroforming is a relatively new technology compared to conventional stamping. Thus, there is no large “knowledge base” that can be utilized for process and die design. To remedy this situation, considerable research is now being conducted by various institutions on significant aspects of tube hydroforming technology, including material selection, friction, pre-form design, hydroforming process and tool design, die materials and coatings. ERC/NSM is also conducting R&D in tube hydroforming in association with its industrial partners. This paper summarizes some of the early results in hydroforming of low carbon steel and aluminum alloy 6061-T9 tubes.

Tube material properties are required for accurate design of the parts and tooling for the hydroforming process. Tensile test properties which are obtained using specimens taken from flat sheet prior to roll forming and welding change during the tube manufacturing process. This paper describes a practical test method for determining the flow stress of tube materials. This tool set, a simple stand alone hydraulic bulging fixture, was developed to expand the tubes under bi-axial loading conditions. With the use of this tool, along with analytical methods and finite element simulation, material properties can be determined for tubular workpieces. These material properties are used for subsequent finite element computer simulation of complex components manufactured from the same material. The tooling, designed for this test, may also be used to evaluate the formability and quality of the tubes.

Viscous pressure forming (VPF) offers a versatile approach to low volume stamping of difficult-to-form sheet metal alloys. The process entails the use of a highly viscous medium as a forming tool. Compared with solid metal punch forming, the necking in the sheet metal can be postponed in the VPF process, i.e. the VPF can improve the formability of sheet metal due to low friction effect at sheet/medium interface. In this study, the process of sheet stretching with VPF has been simulated by the finite element-based code DEFORM. The predictions were compared with the experimental measurements conducted at the ERC/NSM. An approach to determine the formability of sheet metal in stretching using a critical damage value concept has been proposed. It has been found that the formability of the sheet stretched with VPF is higher than that of the sheet stretched with a hemispherical solid metal punch.

With the availability of advanced machine designs and controls, tube hydroforming has become an economic alternative to various stamping processes. The technology is relatively new so that there is no large “knowledge base” to assist the product and process designers. This paper reviews the fundamentals of tube hydroforming technology and discusses how various parameters, such as tube material properties, pre-form geometry, lubrication and process control affect product design and quality. In addition, relations between process variables and achievable part geometry are discussed. Finally, using examples, the status of the current technology and critical issues for future development are reviewed.

The predominant failure modes in stamping of sheet metal parts (deep drawing and stretch forming) are wrinkling and fracture. In many cases these defects may be eliminated by appropriate control of the Blank Holding Force (BHF). This paper summarizes the results of a recent study on the formability of rectangular parts from aluminum alloy 2008-T4. Wrinkling and fracture limits have been determined and BHF control methods have been developed to eliminate defects, improve part quality, and increase the draw depth.

In the current study, a system for computer control of the variable blank-holder force (BHF) was implemented for a deep-drawing process, using a multi-action, computer-controlled press. The press was developed at the Engineering Research Center for Net Shape Manufacturing for simulation research in the cold forming of complex parts and for sheet-metal forming. The controlled blank-holder forces (BHF) system was programmed and tested. It was found that the system has a quick response and enables the user to preset the desired BHF path over the entire punch stroke. An acceptable BHF range was predicted experimentally by performing a series of deep-drawing tests with different constant BHF. The acceptable BHF range obtained is bounded by two critical loci, the rupture locus and the wrinkling locus. Various experiments were performed with different constant BHFs located within an acceptable zone. Higher punch forces were recorded and a thinner product was obtained for the higher BHF test. From the experimental results, it is expected that the minimum acceptable BHF locus (slightly above the wrinkling locus) is the optimal path of BHF variation during the drawing stroke.