Damage-controlled forming processes, influence on damage development during deep drawing

 

In the manufacture of semi-finished steel products, it is unavoidable that damage in the form of material discontinuities (pores, inclusions, segregations) is introduced into the material. In subsequent forming processes, the material is exposed to stress conditions which can cause the damage to grow and new damage to occur. Cracks can form at these damage points during operation, which reduces the service life of the workpieces.

Deep drawing is a forming manufacturing process in which three-dimensional workpieces are produced from sheet metal. It is used in particular in the production of car body parts in automotive engineering. The state of damage in the deep-drawn workpiece can be specifically changed through adapted process control, thus increasing the service life of a given part. In addition, the knowledge of the damage condition in the workpiece enables a reduction of the required material and thus of the vehicle weight.

During the first funding phase of the SFB TRR 188, subproject A06 will investigate the basics of damage control in deep drawing using the example of a DP800 dual-phase steel. First, the influence of defined stress states on the damage development in the sheet metal is examined on the basis of analogy experiments. Then deep-drawing tests are carried out on rotationally symmetrical cups in which both the process parameters (punch speed, holding-down force, drawing ring radius, lubricant) and the process control (simple deep-drawing, intermittent deep-drawing, deep-drawing in return, reverse drawing, deep-drawing with counterpressure) are varied. The cups produced in this way are analysed with regard to the existing damage conditions. In parallel, numerical simulations of deep drawing are carried out using the finite element method in order to determine the temporal development of the stress states during deep drawing. The combination of the resulting damage states with the stress states during deep drawing allows a targeted modification of the deep drawing process to reduce the damage present in the workpiece.

In the second phase of the SFB, the findings will be transferred to more complex geometries and the influence of friction on the damage development will be investigated. The aim of the third phase is the synthesis of a design model for deep drawing processes to reduce the damage in the workpiece.