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

Abstract

The mechanical properties and the operating life of a formed component are highly dependent on the residual stress state. There is always a high magnitude of residual stresses in the components formed by incremental sheet forming (ISF) due to the localized deformation mechanism. Hence, a thorough understanding of the generation of the residual stresses by ISF is necessary. This study investigates the residual stress generation mechanism for two process variants of ISF, i.e., Single Point Incremental Forming (SPIF) and Two Point Incremental Forming (TPIF). This understanding is used to control and targetedly generate the residual stresses to improve the part performance. In this regard, the residual stress state in a truncated cone geometry manufactured using SPIF and disc springs manufactured using TPIF was experimentally analyzed. Validated numerical models for both process variants were developed to study the residual stresses in detail. The residual stress state in SPIF is such that the tool contact side develops tensile residual stresses and the non-contact side undergoes compressive residual stresses. The tool step-down variation was used to control residual stresses and improve the fatigue strength of truncated cones manufactured using SPIF. For TPIF, two different forming strategies were used to analyze the residual stress generation mechanism and the role of major process parameters. The residual stresses for TPIF are pre-dominantly compressive in both directions of forming tool motion. For both process variants of the ISF process, it is shown that the residual stresses can be beneficially utilized to improve mechanical properties of the components.

Setting Residual Stresses in Tensile Stress-Superposed Incremental Sheet Forming

“…A control of residual stresses can lead to optimized component properties such as geometric accuracy and fatigue strength. In recent publications the possibility of controlling residual stresses through incremental forming processes was analyzed and proved [7]. Due to the incremental process character, the process parameters can be adjusted locally during the entire process, in contrast to conventional forming processes.…”

Numerical Study to Promote the Residual Stresses Development during ISF Process with Improvement in Two Point Incremental Die Forming

“…AISI 301 shows the TRIP behavior when subjected to the deformation during ISF process, characterized by a continuous cyclic bending/unbending deformation mechanism. Following [17,19], a non-linear isotropic/kinematic hardening law with an integrated Oslen-Cohen model [18] was created to give a comprehensive knowledge of phase transition during deformation and to aid in the analysis of sub-surface residual stresses. The constitutive model takes into account the austenite (γ) of volume (1 − 𝑓𝑓 𝛼𝛼 ′ ) and the martensitic (𝛼𝛼 ′ ) that is formed out of the parent γ-phase having volume of 𝑓𝑓 𝛼𝛼 ′ .…”