Failure behavior of different sheet metals after passing a drawbead

Abstract

Nowadays, light weight design leads to more complex geometries and the use of high-strength materials in metal forming processes. This fact generates a need of accurate material flow control during deep drawing processes. Commonly, this is realized and supported by the use of drawbeads. They are also one common possibility to regulate serial forming processes, like they are used in car body manufacturing. While retention or opening forces of different drawbead geometries are already well investigated, there are only a few examinations dealing with the forming limit of sheet metal after passing a drawbead. This investigation analyses the influence of a commonly used drawbead geometry on the formability of a mild steel DC04, an advanced high-strength steel DP800 and an aluminum alloy AA6014. First of all, the forming limit of all initial materials is analyzed by Nakajima tests to generate a normed Forming Limit Curve (FLC). Afterwards, metal sheets are preloaded in a modified strip drawing test with a drawbead geometry. As those strips are too narrow for conventional Nakajima tests, tensile test specimens A50 are used to determine one characteristic point in the forming limit curve. Therefore, A50 specimens in the initial and preloaded state are tested with a conventional tensile test setup and an optical measurement system. This is done to analyze the changes in the formability in the uniaxial stress state after a drawbead passage. Here, the evaluation method according to DIN EN ISO 12004-2:2009-02 is applied. Following, those results are discussed and the impact of drawbeads on different sheet metal materials is evaluated to specify the limitations in forming processes using drawbeads.