Characterisation of phase transformation and induced residual stresses in incrementally formed disc springs: an experimental and numerical study

Disc springs are shallow, conical components suitable for a broad application spectrum in the industry. They need to have a reliable and long service life. However, operational tensile stresses can limit their lifetime. Traditionally, their performance can be improved by inducing compressive residual stresses using shot peening which causes additional time and cost in the production process. In this study, the incremental sheet metal forming (ISF) approach was employed for an integrated and targeted induction of residual compressive stresses in the disc springs manufacturing process. The springs were produced using AISI 301 stainless steel sheet blanks. The roles of various process parameters such as forming tool diameter, tool step down, feed rate and initial sheet blank condition on the characteristics of induced residual stresses were studied experimentally and numerically. Residual stress measurements were carried out using the X-ray diffraction method. With the help of the ISF method, the compressive residual stress induction could be integrated into the forming process of disc springs. As a result, additional postforming treatments could be omitted. Forming tool diameter and forming temperature significantly influenced the induced residual stress characteristics. In addition, the martensite formation of metastable austenitic steels showed an important contribution to the residual stress properties. Furthermore, the developed numerical approach could precisely predict the formation of the residual stresses and can be applied for the design of the disc springs with the targeted residual stress properties.