Springback compensation by stress superposition in forming of ultra high-strength steel sheets

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Today, the automotive industry is challenged by increasing environmental regulations as well as the continuous enhancement of the standard requirements regarding passengers safety. In order to achieve these requirements in car industry, the focus has been put on the reduction of car body weight through the use of lightweight materials as well as sheet metals with smaller thicknesses. Such tendencies lead to the increased use of ultra-high-strength steels (UHSS) for manufacturing of car body components. The structural car body components usually are manufactured by stretching or deep drawing. When the part is taken out of the die cavity after the forming process, a stress relaxation happens to reach a state of appropriate stress balance in the part. As a result of this stress relaxation, the springback or certain deviation between the formed part and the reference geometry occurs. Different kinds of springback can arise in this case: angle change, sidewall curl, radii change, and torsion or twisting. When forming car body structural parts from UHSS, an extremely high amount of springback can be expected. In state of the art exist some mechanically and geometrically based approaches which have shown certain possibilities for springback reduction. However, a methodology for springback compensation by parts manufactured from UHSS has not been developed until now.The motivation of this research results from the lack of such a methodology to compensate the springback when forming the parts from UHSS. For a successful development of such a kind of methodology for springback compensation, firstly, a dependency between the acting stresses after the forming and consequently caused part shape deviations or springback must be defined. The mentioned dependency should be described by a mathematical function. With the knowledge of this dependency between the stresses caused in the part and, therefore, occurred springback, a methodology for stress based springback compensation will be defined. Firstly, the compensation methodology to be developed will be based on dedicated stress superposition during the novel forming process with an alternating blank draw-in, and afterwards, if necessary, by changing the part cross-section size in certain part areas in the same tool. The required modification of the parts cross-sections will be provided by corresponding stretching without blank draw-in or by reduction of the drawing depth using back forming.