Integrated Forming and Surface Engineering of Disc Springs by Inducing Residual Stresses by Incremental Sheet Forming

Abstract

Disc springs are conical annular discs, which are characterized by a high spring force with a small spring travel and good space utilization. In operation, they must meet high demands on the stability of the spring characteristic and the fatigue strength. Under loading, tensile stresses occur which limit the possible applications of disc springs. Compressive stresses can be generated in the stressed areas by means of shot-peening in order to extend the operating limits for a given yield and fatigue strength. Since the spring geometry and characteristics change during shot-peening, the design of the shot-peening treatment is iterative and cumbersome. The present research proposes an incremental forming process for forming and integrated targeted adjustment of residual stresses in disc springs from metastable austenitic stainless steel (MASS), to achieve improved spring properties and high cyclic strength. The main mechanism of residual stress generation is the transformation of metastable austenite into martensite under the action of the forming tool. Different experimental characterization techniques like the hole drilling method, X-ray diffraction, disc compression tests, optical microscopy and cyclic tests are used to correlate the residual stresses and disc spring properties. A numerical model is developed for simulating the martensite transformation in disc springs manufacturing. The results prove that incremental forming enables process-integrated engineering of the desired compressive residual stresses, entailing a higher spring force of metastable austenitic disc springs in comparison to conventional disc springs. Due to martensite formation, the generated residual stresses are stable under cyclic loading, which is not the case for conventionally manufactured springs.

Influence of process parameters on the residual stress state and properties in disc springs made by incremental sheet forming (ISF)

“…During the ISF operation, the sheet blanks are deformed locally by a small hemispherical tool along the designated path. Hajavifard et al [23] and Maqbool et al [22] concluded in their work that TPIF with a negative die is the best-suited variant for the forming of disc springs.…”

“…Two strategies for the targeted generation of residual stresses in MASS disc springs by ISF were proposed in previous studies [12,22,23]. The effect of the ISF process parameters, i.e., the forming tool diameter, feed rate, and step-down, on the generated residual stresses are investigated in detail using X-ray diffraction.…”

Targeted residual stress generation in single and two point incremental sheet forming (ISF)

“…Although residual stresses have an impact on geometric accuracy of formed components and their service life, only a limited amount of studies have been reported in the literature that deals with the ISF process and residual stresses. Furthermore, to the knowledge of authors, no study has been reported in the literature that focuses on the targeted utilization of the residual stresses of the ISF process other than the recent studies reported by the authors [6][7][8][9]. In the next section, a brief review of the literature dealing with the SPIF process and the residual stresses is presented.…”

Residual Stresses and Surface Roughness Analysis of Truncated Cones of Steel Sheet Made by Single Point Incremental Forming

“…Hajavifard et al [30] used ISF to improve properties of conical annular discs by introduction residual stresses in material. The main mechanism of residual stress generation was the transformation of metastable austenite into martensite under the action of the forming tool.…”