Investigation of Macro- and Grain-Scale Residual Stresses with an Emphasis on Spring-Back Behavior in Preheated Incrementally Formed AA 1050 H14 Components

Single point incremental sheet forming (SPIF) is a dieless forming operation that facilitates the production of complex and customized sheet metal components. The major shortcoming of the process is poor geometrical accuracy resulting from the bending and spring-back phenomenon. Residual stresses induced during the forming operation dominantly govern the spring-back behavior and mechanical properties of the formed parts. The present investigation involves preheating the sheet samples at different temperatures before forming to enable stress relief and associated microstructural changes. Geometrical accuracy, spring-back, and macro-residual stresses resulting in SPIF components with different preheating conditions have been evaluated by experimental investigations. The x-ray diffraction (XRD) technique is employed for the characterization of the macro-residual stresses on the inner and outer formed surfaces. The current work attempts to correlate the macro-residual stresses and the strain-induced misorientation evolution along with the orientation plots obtained from inverse pole figures (IPFs) by the electron backscattered diffraction (EBSD) technique. The strain gradient approach is utilized to calculate the geometrically necessary dislocations (GNDs) from kernel average misorientation (KAM) data for differently preheated SPIF samples. SPIF samples subjected to the preheating temperature as low as 230 °C resulted in fragmented grains with a higher density of pinned dislocations, and KAM values are transformed into larger equiaxed grains with apparent dislocation recovery in SPIF samples concerning higher preheating temperature. Results revealed that the preheating temperature of 330 °C promoted simultaneous static recovery and recrystallization behavior. This also resulted in the relaxation of the residual stresses by the elimination of low-angle grain boundaries which in turn resulted in enhanced geometrical accuracy by lowering spring-back in SPIF parts. The results conclude that the preheating of sheet samples is an effective solution for residual stress relaxation for enhanced geometrical accuracy in SPIF.