A Novel Macro-Micro Integrated Incremental Sheet Forming Process for Fabricating Parts with Functional Surface Microfeatures

Abstract

Large-area functional surface microfeatures (e.g., microgrooves and dimples) are of technological importance for metallic components requiring special functions such as drag reduction and energy efficiency enhancement. However, it is still challenging for small batch production of such microfeatures over large curved surfaces. Here, we demonstrated a novel macro-micro integrated incremental sheet forming (MMI-ISF) process to manufacture both the macroscopic geometric shape and surface microfeatures, simultaneously. First, we developed an experimental platform for the MMI-ISF process, using which we successfully fabricated the microgroove arrays on the deformed surface of AA2024-T6. We found that increasing the press-in depth (h) plays a dominant role in improving the microgroove transcription ratio (TR) in the MMI-ISF process, and the increase of the step size (∆z), the feed rate (f) and the spindle speed (n) contribute to a larger thickness reduction which benefits the embossing of the microgroove. Furthermore, the MMI-ISF process facilitates the uniform thickness distribution of the formed parts and, consequently, improves the surface quality. Compared with the as-received sheet, the hardness and yield strength of the formed parts are improved by around 10% and 31%, respectively. Nevertheless, the size and orientation of microgrooves have limited influence on the strengthening effect of the formed parts. Moreover, we performed electron backscatter diffraction (EBSD) to investigate the microstructure evolution of the formed parts. We observed a sandwich grain size distribution along the thickness direction, showing the characteristics of ‘coarse in the middle and fine at both ends’. We found that partial contact between the sheet and the embossed die is conducive to improving material flow and inhibiting grain fracture. Furthermore, the dual compression stress state between them induces a transformation of the <111> fiber texture to the <100> fiber texture in the inner and middle zones, resulting in a more uniform deformation. This work creates a pathway for low-cost and high-quality fabrication of functional surface microfeatures over curved metal sheets and unravels the forming mechanism of the MMI-ISF process.

Keywords: Incremental sheet forming, hybrid sheet forming, microfeatures, surface morphology