Mustafa A. Ahmetoglu

Tube Hydroforming - State-of-the-Art and Future Trends

Abstract 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.

Evaluation of tube formability and material characteristics: hydraulic bulge testing of tubes

Abstract 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.

Tube hydroforming: current research, applications and need for training

Abstract 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.

Simulation of roll forming process with the 3-D FEM code PAM-STAMP

Abstract Roll forming is a well known process used to manufacture long sheet metal products with constant cross section. Despite the long history of the process, the design procedure for the roll formed products, forming rolls, and roll pass sequences still remains more an art than a science. Thus, to avoid forming defects and to reduce the process development efforts, finite element analysis can be used to predict strain distributions and sheet geometry during and after the process. This paper summarizes the results of roll forming simulations made with a commercial FEM code. Deformed geometry and strain distributions predicted by simulations are compared with the results from previously conducted experiments. This study provided considerable experience how and when it is best to apply a commercial 3-D FEM code to the design of a roll forming process.

Deep drawing of round cups from tailor-welded blanks

Recent developments in the welding technology and the considerations regarding material cost, dimensional accuracy and weight, force automakers and appliance manufacturers to use tailor-welded blanks in their stamping operations. The welding of blanks with different thicknesses, materials and/or surface conditions introduces many challenging formability problems for process development and tool design. In cooperation with industrial partners, projects are being conducted at the ERC for Net Shape Manufacturing to study the formability of tailor-welded blanks and develop guidelines for tool design. This paper gives a summary of the preliminary work done on the deep drawing of round cups from tailor-welded blanks.

Improvement of hem quality by optimizing flanging and pre-hemming operations using computer aided die design

Abstract Flanging and hemming of straight edge-flat surface (plane strain) aluminum killed-draw quality steel was investigated restricting the study to the radius flat hem with an inner sheet. The major process parameters in straight edge hemming were determined and relations between them and some forming defects were established. Among these parameters were flanging die corner radius, pre-hem path, pre-hem stroke and final hemming force. Possible process and tool design modifications, which may lead to quality improvements in hemming were tested experimentally and using FEM. The commercial finite element program DEFORM was used to simulate the flanging and hemming operations including springback, and the predictions were compared with limited experimental results. Results of the experiments and simulations were summarized in the form of trend-lines for the use of process and die designers.

Application of viscous pressure forming (VPF) to low volume stamping of difficult-to-form alloys —results of preliminary FEM simulations

Abstract Viscous Pressure Forming (VPF), a recently developed process, offers a versatile approach to forming of sheet metal parts in small quantities. The process entails the use of a highly viscous medium typically applied to both sides of the sheet. In its simplest implementation, the viscous medium is pumped under pressure into the cavity on one side of the sheet to be formed and controllably released through a single or multiple ports from the other side. Thus, the VPF process is a combination of the bulk (viscous media) and sheet metal forming process. The finite element-based code, DEFORM, which was specifically designed for analysis of the bulk forming operations, has been chosen to simulate the axisymmetrical viscous pressure forming of sheet metals, i.e., stretching and drawing. The simulation results show that (a) properties of the viscous medium, especially, viscosity, has a great effect on the process; (b) the sheet can be stretched more uniformly when the viscous medium is introduced on both sides of the sheet; (c) the location of the outlet port in the die influences the forming processes and can be used to control the deformation of the sheet metal.

Bending, flanging, and hemming of aluminum sheet—an experimental study

Abstract Hemming is often used as the last stage of stamping operations. It is used either to improve appearance (to create a smooth edge rather than a razor edge with burrs) or to attach one sheet metal part to another. Hemming is defined as folding of bent or flanged sheet 180° or more. However, the deformation mechanism, and hence the process, is different and more complex than simple bending and flanging process. Therefore, very little information is available in research literature, and most of the hemming dies are designed based on trial and error. This paper gives the results obtained from the hemming experiments that were conducted at the Engineering Research Center for Net Shape Manufacturing (ERC/NSM) by using Aluminum Alloy 1050.

Blank holder force (BHF) control in viscous pressure forming (VPF) of sheet metal

Abstract An eight-point BHF control system with a flexible blank holder is designed and built as part of an experimental viscous pressure forming (VPF) machine. This paper describes results of VPF experiments, and addresses several blank holding issues specific to the VPF process. FEM simulations of hydroforming with a multi-point BHF control and an elastic blank holder are conducted to fine-tune the control system as well as to predict the forming loads. The FEM results are compared with experimental VPF results. The viscous medium “blow through” phenomenon between the blank holder and the sheet during VPF is described and quantified.

Hydroforming of sheet metal using a viscous pressure medium

Abstract 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.