Material Aspects of Tube-Hydroforming 1999-01-3204

Tube hydroforming includes several aspects related to the manufacturing process in particular pre-bending, part design, press and tooling, and also naturally the material to be formed. This is regardless of whether the low pressure technique is applied resulting in a constant perimeter or the high-pressure technique aiming to achieve a suitable perimeter expansion by means of high internal pressure setting. For auto-body structures, thin-walled steel sheet sections are recommended as a replacement for mainly spot-welded box-shape components. Run-of-the-mill tube products receive their properties during traditional tube making processes, such as roll-forming, and widely used HF-welding. However, a new process has emerged using laser-beam energy as the welding source thus resulting in a very narrow weld seam. For this tube making method, typical automotive steel sheet grades which include all coated products can be used. This paper describes the materials used for the tube and section making process by means of laser-beam welding. It will concentrate on tube gages lower than 2.0 mm. Additionally, results obtained from high-pressure hydroforming T-pieces are shown. The results also indicate that the laser-beam weld is less sensitive to the placement of the weld with regard to the location in the forming tool. Hydroforming also needs the aid of simulation codes to speed-up development and to establish a robust manufacturing process from the early design phase on. Thus reliable material parameters are required, starting with biaxial test results describing the nature of yield loci for material used for the hydroforming process with and without end feeding. The paper will stress the fact that beside material properties, also, how to use the well known forming limit diagram to evaluate the strain inherent in hydroforming process determine the severity of the process. A proposal is made as how to cope with these issues.