Tube hydroforming of steel for automotive applications.

Tube hydroforming has the potential to produce large structural automotive components which may be utilised for weight reduction in future generation vehicles, by replacing stamped and spot-welded steel assemblies. However, limited implementation of this technology has taken place for Body-In-White (B-I-W) components, due to the complexity of the process and low levels of confidence and knowledge of the technology. This is coupled with assembly issues that this technology presents for B-I-W construction. In contrast the application of this technology for sub-frame and chassis component applications has been successful, principally due to the less stringent assembly requirements and proven cost and performance related benefits. The tube hydroforming process utilises forming fluid, under high pressure, to stretch a tube blank into the shape of a die cavity. The application of the internal pressure may be accompanied by axial feeding of the tube ends to push additional tube material into the die cavity. Close control of process parameters and the die design are essential to produce successful, defect-free components. However, the behaviour and response of steel and the influence of friction under these forming conditions are unknown entities. On the basis of a critical review of literature, a research programme was initiated to engage some of the key forming issues inhibiting wide-scale implementation of steel tube hydroforming for BIW automotive applications. The principal aims of the project were to identify the fundamental influences of steel properties on the tube hydroforming process and to develop a mathematical model of the process for steel tube. The research programme entailed small-scale formability tests and large-scale experimental trials, accompanied by the development of analytical and finite element (FE) models of the tube hydroforming process for various steel grades. The analytical and FE models could be used as design aids in the development of automotive BIW hydroformed components. The research project identified significant changes in both mechanical properties and surface characteristics as a result of the Electric Resistance Welding (ERW) tube manufacturing process. This in turn had a significant impact upon the hydroforming behaviour of the steel tubes. An analytical forming limit curve (FLC) model evaluated in this thesis was deemed to provided a robust means of predicting splitting or excessive thinning of a tube hydroformed component as a result of die geometry, tube material or processing conditions. The FE models developed, which incorporated the analytical FLCs, illustrated that the tube hydroforming process could be predicted with a high level of confidence for simple components