Strategic development of micro-incremental forming process with size-effect based fracture modeling for ultra-thin sheets

Abstract

Micro-forming is an advanced micro-manufacturing process aimed at producing high precision, small-scale components made of ultra-thin metallic sheets (foils) for a wide range of applications, such as in microelectronics, biomedical, avionics, and automotive industries. This process involves the deformation of materials with thicknesses in the micrometer range, which presents unique challenges due to the increased sensitivity of the foils to size-effect dependent external forces, such as plastic deformation, anisotropy, springback, and friction. As the thickness of the material decreases, the influence of material properties, surface characteristics, and process parameters becomes more pronounced. The ability to precisely control these variables is crucial for achieving the desired mechanical properties and dimensional accuracy in the final micro-part. The mechanical properties of the material, such as yield strength, ductility, and strain hardening behavior, can vary significantly with thickness. This makes it difficult to predict the response of foils to external forces and demands more sophisticated modeling and experimentation to ensure successful forming results. Recent advancements in micro-forming techniques, including micro-deep drawing, micro-extrusion, and micro-stamping, etc. are well established at micro-scale with developed specialized tools and equipment. Micro-incremental sheet forming (μISF) process is a recent development to tackle the issues arising due to size-effect and constraints in the fabrication of required tooling at the micro-scale. It is a die-less process with multifold formability compared to traditional processes, for the production of customized miniature/ micro parts at ultra-precision range with better energy optimization. In μISF, the forming tool navigates through the surface of the foil to precisely deform it to complex symmetric and non-symmetric 3D components as per the prescribed toolpath. In this work, a micro-forming set-up was designed and developed to conduct the µISF experiments. At micro-scale, studying the grain size of the material is crucial to understand the deformation behavior. The intrinsic anisotropy of the foils was minimized through controlled heat treatment, and varying grain sizes, having different microstructures, were generated to investigate their effect on the formability of CP-Ti-Gr2 foils. It was established that higher annealing temperature, increase in grain size and higher step depth assisted in improving the ductility of the foils, leading to enhanced forming depth of the micro-parts. This was explained by a decrease in the material resistance to dislocation motion caused by an increase in the volume fraction of surface grains (Vs>Vi) at an increasing step depth in the deformation zone (Ar). The microstructural analysis through EBSD also showed similar results with the presence of a higher fraction of LAGBs and larger KAM angle. This work also involves numerical simulations in ABAQUS®, incorporating the influence of size-effect by coupling it with a suitable toolpath strategy and an appropriate damage mechanics model, to accurately predict the nature of micro-scale deformation in the μISF process. A mixed material model with size-dependent parameters (grain size, thickness, etc.) was considered in the finite element analysis using the theory of surface layer model, to examine the flow stress behavior of the material. The FEA results showed a reasonable agreement with the experimental results to predict the failure of the micro-parts. The FGBIT strategy showed better stress and thickness distribution of the formed micro-part compared to the spiral toolpath. In micro-forming, friction and wear are critical factors that can significantly impact both the material and tooling. Lubrication plays an essential role in minimizing friction, improving the surface finish, and reducing the occurrence of defects. A sustainable lubrication approach, studying frictional size-effect was also explored through closed and open lubrication pocket models (LPM) theory to improve tribological performance of the formed micro-parts. A significant decrease in the forming load and energy consumption was observed with the environment-friendly MoS2 powder compared to liquid lubricant. Some innovative ways of fabricating the forming tool and increasing the stiffness of the foils, for better forming attributes, were explored in this work. The foils are susceptible to bending and distortion under self-weight as they lack stiffness and fail early during forming. The stacking of foils (SOF) approach showed a significant increase in the formability of the target parts, when multiple foils were formed simultaneously to increase the stiffness and plastic deformation of the material. A new approach of Reverse-µEDM technique was used to fabricate micro-forming tools for the µISF process. The study showed that with the correct parameter setting of the discharge energy, a precise stresses free hemispherical-end profile of the tool can be obtained with good surface finish using the Reverse-µEDM process.