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Corresponding author: K. J. Kim, Department of Automobile Engineer-ing, Seojeong College University, Gyeonggi-do, Yangjoo-si, 482-777, Korea
1 Introduction
The hydro-forming technology has been drastically developed for the 10 years in automotive industries. During the initial time before this decade, it has not been thoroughly used by automo-tive industries. The hydro-forming parts were mainly of simple manifold ring shape and it was applied to sanitary parts or musi-cal instruments [1]. In recent years, the hydro-forming technol-ogy has obtained a competitive position in the automotive market because of high pressure hydraulic systems, the development of precision computer control systems and the wide range of press systems.
Tube hydroforming is a manufacturing process in which an internal hydraulic pressure is applied to transform either a straight or a pre-bent tubular blank into a structural component with different or varying cross-sectional shapes along its length [2]. The blank is placed in a closed die and the internal hydraulic pressure forces the tube to conform to the shape of the die cavity. Punches may be mounted in the die to pierce holes in the tube wall during the forming operation. The process is being used or being considered for making a wide range of components for automotive applications. Automotive parts currently under development or in production include seat frames, engine cra-dles, rails, exhaust manifolds, and space frame components [3]. Interest in the tube hydroforming process by the automotive industry is due to the possibility of replacing many multi-piece stamped and welded assemblies in body, frame or chassis com-ponents with one-piece hydroformed components. Thus, there is a great potential for not only weight saving, but also for tooling and labor cost saving that may occur due to the elimination of multi-stage stamping and assembly processes through part con-solidation [4]. Additional benefits of tube hydroforming over stamping are improved dimensional accuracy, improved struc-tural strength and stiffness, and consistent dimensional repeat-ability [2 – 4].
A tubular blank is expanded by high-pressurized hydraulic fluid in the die cavity, which is designed to yield the geometry of the final product. The incoming tube, which is cut to proper length, needs to be bent to have near shape of the final product. In some cases, the tube requires a pre-bending process as a pre-forming process. The tube must be bent to the approximate cen-terline of the finished part prior to hydroforming to enable the tube to be placed in the die cavity. During the bending process, the tube undergoes considerable deformation including thin-ning. In most of the cases, the preformed geometry significantly influences on the success of hydroforming process. Tube pre-forming may be required prior to hydroforming complex cross-sections in order to avoid localized pinching of the tube as the die closes. The pre-forming operation often involves localized flat-tening of sections of the tube (i. e. changing the initial cross sec-tion prior to hydroforming) [3 – 8]. The preformed tube is then delivered into the hydroforming die and pressurized from both ends by the internal fluid. A hydraulic liquid fills the tube with two side cylinders then closing the ends of the tube. Simultane-ously, the liquid is pressurized and the cylinders are pushed in from the side. The material of the tube yields and flows into the die cavity and the part is formed [1]. In order to meet the require-ments of light-weight of automobile parts, the strength grade of
Figure 1. Shape of instrument panel beam part.
Figure 2. Simulation model for U-forming.
the tubular material for hydroforming is also increasing. 300~400 MPa grade of tensile strength steel has been widely used for hydroforming. In this study, the whole process of instru-ment panel (IP) beam development by conical tube hydroform-ing using a 370 MPa grade of tensile strength of steel material is presented. In addition, the comparison of the quality is examined between simulated and experimental IP-beam from the point of geometry and thinning. In the case of IP-beam, since its diame-ter difference between each end side is nearly doubled, the con-ventional tube could not be used. Therefore, a conical tube whose diameter is varied according to the IP-beam following as IP-beam location should be used in the present study.