The increasing need for high strength complex hollow bodies for automotive application is pushing towards the use of sheet hydroforming techniques in conjunction with high strength steels. Unfortunately high strength steels are characterised by high springback levels. In this paper the springback and residual stresses have been analysed by means of laboratory trials carried out using the double sheet hydroforming technique. The attention has been focused on the upper blank of TRIP800 steel. The analysis has been performed using different approaches: i) characterisation of sample accuracy by means of a 3D coordinate measuring machine using a new proposed method based on the standard deviation calculation; ii) FEanalysis of both hydroforming and springback stages using an implicit FEM code; iii) residual stress evaluation by means of X-ray diffraction and laser cutting techniques. The effect of pressure, die insert geometry and friction at the blank holder on springback and residual stresses have been analysed and discussed in detail.

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The shortening of process chains and the combination of innovative manufacturing technologies offer the possibility to enhance the complexity of technical products. The presented work describes the development of a process chain for the manufacturing of complex hollow parts out of double sheets and tubes with a big variation of the cross section, e.g. for A-pillars in the automotive sector. Two blanks and a tube are inserted in a hydroforming tool by a handling robot. The processes forming of blanks and tubes as well as joining by clinching are integrated in this tool. After these steps, the handling robot places the part in a laser station, where integrated laser welding and cutting processes take place. This paper focuses on special solutions developed both in the hydroforming and in the laser processing. In particular, the connection between blanks and tube is extremely problematic with regard to the sealing in the hydroforming process and the welding operations due to an existing gap. The requested tightness is reached by reinforcing the tube in the connection area with a ring insert, by placing sealing profiles on the outer surface of the tube and particularly by adding a sealing medium to the hydroforming fluid. In laser operations, special challenges result from complex welding and cutting paths, contaminations of the blanks with hydroforming media and lubricants, gaps between the blanks that occur due to residual stresses after forming and burr formation.

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