Project Details
Projekt
Selective induction and stabilization of residual stresses in austenitic disc springs by incremental forming and integrated surface layer treatment
Subject Area
Primary Shaping and Reshaping Technology, Additive Manufacturing
Mechanical Properties of Metallic Materials and their Microstructural Origins
Mechanical Properties of Metallic Materials and their Microstructural Origins
Term
since 2017
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 372823079
The technological objective of the project is to investigate incremental sheet forming (ISF) for the production of disc springs and the integrated setting of residual stresses in order to achieve improved spring properties and high cyclic performance. The targeted adjustment of residual stresses during the forming process leads to better spring properties, the elimination of shot peening operations, and enables the flexible production of specialized springs or miniaturization of springs. According to DIN 2092, the tangential stresses determine the spring force of disc springs in operation. Through the specific adjustment of residual compressive stresses, the radial tensile stresses that limit the spring force occur much later in operation. In the case of metastable austenitic spring steels, the local contact also leads to martensite formation in the surface zone, which induces further residual compressive stresses.So far, the experimental setups and fixtures, micromagnetic methods and simulation methods have been successfully developed and validated. A wide range of process parameters have been experimentally characterized with respect to targeted residual stress generation and property improvement. An increase in compressive residual stresses and stabilization with surface layer influence and further significant improvements were demonstrated via direct manufacturing with incremental forming compared to conventional springs. A disadvantage of ISF is the high process time. Faster ISF forming leads to an increase in sheet temperature to above 100°C, which has been identified as a disturbance and affects residual stress and martensite development.High-performance disc springs are used under different mechanical load cycles and temperatures from -60°C to 250°C. The technological goal in the third phase is to systematically compensate for material fluctuations through temperature-controlled, rapid incremental forming processes and to set a desired spring characteristic curve that exhibits optimized stability under oscillating load. In terms of the program goals, the third application phase, therefore, aims to 1) design the IBU process isothermally (minimization of the disturbance variable "temperature") despite high forming speeds and to design the associated tool technology, as well as to optimally adjust the forming parameters based on process force and martensite content to material fluctuations using a prognosis model. It aims 2) to develop an in-situ residual stress measurement with the aid of a multi-sensor using micromagnetic measuring methods and to use this as condition monitoring for the change in properties under specific operating conditions. In addition 3) it aims to create a black prediction model for the improvement or development of properties through the targeted introduction of residual stresses in spring-hard, austenitic materials for the operation of disc springs at low and elevated temperatures.
DFG Programme
Priority Programmes
