Investigation of fatigue strength of tool steels in sheet-bulk metal forming

To encounter trends regarding an efficient production of complex functional components in forming technology, the process class of sheet-bulk metal forming (SBMF) can be applied. SBMF is characterized by the application of bulk forming operations on sheet metal, often in combination with sheet forming operations [1]. The combination of these conventional process classes leads to locally varying load conditions. The resulting load conditions cause high tool loads, which lead to a reduced tool life, and an uncontrolled material flow. Several studies have shown that locally modified tool surfaces, so-called tailored surfaces, have the potential to control the material flow and thus to increase the die filling of functional elements [2]. A combination of these modified tool surfaces and high tool loads in SBMF is furthermore critical for the tool life and leads to fatigue. Tool fatigue is hardly predictable and due to a lack of data [3], a challenge in tool design. Thus, it is necessary to provide such data for tool steels used in SBMF. The aim of this study is the investigation of the influence of tailored surfaces on the fatigue strength of the powder metallurgical tool steel ASP2023 (1.3344, AISI M3:2), which is typically used in cold forging applications, with a hardness 60 HRC ± 1 HRC. To conduct this investigation, the rotating bending test is chosen. As tailored surfaces, a DLC-coating and a surface manufactured by a high-feed-milling process are chosen. As reference a polished surface which is typical for cold forging tools is used. Before the rotating bending test, the surface integrity is characterized by measuring topography and residual stresses. After testing, the determined values of the surface integrity are correlated with the reached fracture load cycle to derive functional relations. Based on the gained results the investigated tailored surfaces are evaluated regarding their feasibility to modify tool surfaces within SBMF.