Novel strategies to enhance industrial acceptability of incremental sheet forming process

Abstract

The aerospace industry (impellers and pressurized bulkheads) and biomedical implants (cranial plate, dental plate and bone joint support) deal with high precision forming of sheets with intricate designs where high geometrical accuracy and formability is required. Incremental Sheet Forming (ISF) has evolved as a rapid prototyping-based die-less generative manufacturing process to make customized products for these industries. In this process, the sheet's localized plastic deformation is achieved using a curved end-based forming tool that follows the user-defined numerical toolpath. ISF gives excellent formability when paralleled with traditional forming processes, like deep drawing and spinning. Apart from certain advantages, this process has few limitations like: low process speed, non-uniform sheet thickness reduction, and difficulty in forming high wall angle-based curvilinear geometries. These limitations limit its industrial acceptability and it must be improved to make this process's unique features available to a wide range of products. The present work focuses on developing novel effective strategies to improve the industrial acceptability of ISF process. The summary of the developed strategies is listed below: Strategy 1: Adaptive increment based uniform strain distribution in ISF This strategy's concept is the distribution of the deformation among various predefined increments such that uniform stretching of the sheet with each increment in the axial direction could be ensured. The sheet deformation is higher for the steeper wall angles and lesser for the shallow wall angles based geometries. Further, during the first few increments of the forming tool, the sheet deformation is mainly caused by the sheet bending and after few increments, sheet stretching increases with the forming depth. Hence, by adopting this strategy, improvement in the thickness distribution is expected in the region of steep wall angles. The experiments have been carried out on a precision hybrid three-axis CNC machine (DT-110 Hybrid μEDM machine, Mikrotools®). Hemispherical end based forming tool (Tool steel, Dia. 8 mm) has been used to deform 0.91 mm Aluminium sheet (Al 6063-T6). Experiments have shown a percentage improvement in the sheet thickness of 7.9% in the critical region, while simulations have shown a percentage improvement of 4.35%. Lower values of selected uniform stretching can exhibit better percentage improvement in the sheet thickness distribution, but simultaneously it increases the forming time. In this study, the constant value of sheet stretching is so selected that it does not impact the process's forming time. Finite element analysis has been attempted to predict the sheet thickness results; however, it takes high computational time, of the order of a few days to weeks. Thus, leading to the requirement of a simplified approach has been desired to predict the sheet thickness of the deformed curvilinear profile It was inferred from the literature review that the researchers were utilizing the traditional incremental toolpaths from the last two decades, which have their inherent limitations. Hence it was realized to develop a novel incremental toolpath to improve this process Strategy 2: Fractal Geometry Rooted Incremental Toolpath (FGBIT) for ISF Fractals are the mathematical objects that have attracted artists' imagination since the 1960s. FGBIT is a space-filling curve which uses Hilbert pattern-based fractal toolpath to deform a square cup incrementally. The hypothesis behind FGBIT is that it improves the formability and stress distribution of the ISF process. Further, it also induces residual stresses in the formed region, improving the strength-to-weight ratio and fatigue life of the formed components. While conducting ISF using FGBIT, the inclusion of significant compressive stresses on the surface of the base region of the geometry has been observed. Hence, by using FGBIT, metal components with high fatigue life and improved strength-to-weight ratio can be formed without using any pre or post-processing of the sheets. FGBIT is bit slower than conventional toolpaths however; the process's speed can be improved by using high feed rates. FGBIT based incremental toolpath is recommended for the industries with highly customized fabrication. While developing the novel strategies to improve the ISF process, it had been observed that FGBIT was increasing the forming time. Hence, to compensate for this limitation, a novel modification in the clamping mechanism was realized in the form of electromagnetic clamping. Strategy 3: A novel Electromagnetic Fixture (EF) for ISF The clamping area for ISF is generally the periphery of the sheet blank. Electromagnetic Fixture (EF) technique is based on the electromagnetic force to clamp the sheet metal blank from its periphery. EF is fabricated with magnetic actuators located along the sheet blank's clamping area. The proposed design of EF abides by the principles of electromagnetic induction. The EF enables the sheet metal to be clamped on a magnetic surface, i.e. EF's top surface. The designed inductors (electromagnetic actuators) enable uniform magnetic force over the entire contact surface without deforming the sheet metal. The electromagnetic fixture is designed so that the inductors nearly cover the forming area uniformly. All four inductors are made of the same material, have the same number of Cu coil turns, and the same input DC. Hence, it generates nearly uniform and strong magnetic field over the clamping surface. In conventional fixture, clamping force is localized near the bolted region. More bolts are required to make it close to the uniform clamping (if the same tightening torque has been used). Hence, electromagnetic clamping allows uniform sheet flow near the clamping region, unlike conventional fixture. This results in lesser forming forces in EF case due to less resistance to the sheet flow near the periphery of the clamping region. It has been observed that the EF was able to withstand the maximum forming forces of 700 N during experiments The proposed design could be a stepping stone towards the industrial acceptance of this process as it has reduced clamping time from 10 minutes to 20 seconds. Electromagnetic clamping has been proposed as a one-click clamping solution to various metal forming processes. This fixturing concept will improve the ISF process's speed and effectiveness and offer future possibilities of the process window enhancement.