HighPa-Shock - Increasing reproducibility of the spring-back angle of thin metal sheets by inducing residual compressive stress with laser shock

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Metallic micro components can be found in many products of daily life such as in smartphones, tablets or laptops. A majority of these micro parts are fabricated by cold forming operations as these allow low cycle times and generally a high reproducibility. However, when it comes to bending of thin metallic sheets, the reproducibility of the bending angle is rather low. This is due to the fact that the spring-back suffers large scatter. It is found that the scatter in spring-back is caused by a scatter in residual stress state of the work piece. The residual stresses are brought in the material for example by manufacturing processes prior to the bending process such as casting or rolling.Own experimental results show that the resulting bending angle can be influenced by a laser shock process prior to bending operation. Within this laser shock process, a laser shock wave is generated by an ultra-short-pulse laser and this shock wave induces residual compressive stress within the work piece. The specific mechanism behind this complex process interaction is not understood yet and therefore cannot be used industrially. It is the long term goal of this project to fully understand the influence of the laser shock process on the reproducibility of the bending process. In the first part of the priority programme it is investigated if a reproducible residual stress state can be generated by laser induced shock waves. For that, the residual stress state of commercially available steel sheets with a thickness of 0,2 mm are examined and mapped. Subsequently the stress state that can be induced by a single laser shock pulse in an annealed sheet is investigated by varying the pulse parameters. This knowledge is enhanced to evaluate if a maximum compressive stress state is achievable independent of number of applied pulses. Finally it is aimed to achieve a planar stress state by applying multiple allocated laser shock pulses.