Papers by Keyword: Micro Forming

Abstract: Based on robust numerical formulations and various material models, finite element (FE) analysis becomes a powerful tool in conventional sheet metal forming process. Unfortunately, the present constitutive equations irrelevant to thickness that describe well conventional sheet deformation modes have difficulties being applied directly to ultra-thin sheet deformation modes. In the present study, a constitutive equation considering size effect is established by introducing a scale factor that represents size effects through thickness and width directions. Uniaxial tensile tests were used to evaluate the scale factor of different thicknesses together with the parameter identification. The developed constitutive equation reveals that thickness is the most important factor effecting on the constitutive relation of ultra-thin sheet. 2D draw forming process of C7035 ultra-thin sheet is analyzed using JSTAMP/NV introducing the developed constitutive equation. The analysis results show that there are obvious differences in the punch forces and loading geometries according to the size effect through thickness direction. Specimen width has slight effect on the flow stress although specimen thickness has strong effect on the flow stress. It is expected that the proposed constitutive equation gives good applicability to FE analysis of micro-scale forming.

Abstract: Ultrasonic-assisted metal forming have been studied numerously in conventional macro scale. However, ultrasonic dynamic impact effect, occurring in micro scale, has never been studied thoroughly, which makes the characteristics of material deformation more unpredictable in ultrasonic-assisted micro forming. The purpose of this study is to confirm the critical condition for occurrence of ultrasonic dynamic impact effect and to investigate the dimensional height dependency of ultrasonic dynamic impact effect on material deformation. In this paper, commercially pure aluminum 1100 with varying height (φ2×2mm, φ2×1.5mm, φ2×1mm) were selected for conventional static (without ultrasonic vibration) and ultrasonic-assisted compression tests. Ultrasonic-induced stress reduction was evaluated and the contour shape of deformed specimens was compared to investigate the ultrasonic dynamic impact effect on material deformation. The results showed that, as dimensional height of specimen decreased, ultrasonic vibration can reduce forming stress more effectively. In addition, a surprising anti-barreling shape and a significant contact surface area expansion were observed near contact surfaces in every specimen compressed with ultrasonic-assistance, indicating that additional plastic deformation can be produced by ultrasonic dynamic impact effect. An ultrasonic dynamic impact factor (y) is proposed and estimated by an exponential type trend line as y = 2.42e^-1.48x for different dimensional specimen height (x) to quantify the ultrasonic dynamic impact effect. The promising prospect of ultrasonic vibration in micro-forming was demonstrated by the findings above, which helped to provide a basis to understand the underlying mechanism of ultrasonic-assisted micro forming and design the process in the future.

Abstract: For a better process understanding of micro deep drawing processes and reliable prediction of component failure in FE simulations, it requires the most accurate knowledge of actual material behaviour. However, it is not sufficient to describe material failure for a multi axial stress state in deep drawing using a mechanical parameter as the elongation from tensile test. A forming limit diagram and a forming limit curve are more suited to describe the limit of formability under deep drawing stress state conditions. Methods like hydraulic or pneumatic bulge tests are available to determine forming limit curves even for thin metal foil materials. Nevertheless, using these methods only positive minor strains can be achieved. Especially for a deep drawing process negative minor strains and the left side of a forming limit diagram are more important. Therefore, in this study, experiments based on scaled Nakazima tests were performed to determine complete forming limit diagrams for different foil materials with a thickness range of 20 µm to 25 µm. Scaling the test setup improves the handling of thin specimens. Results with a higher local resolution and the specimens’ size is much closer to the actual size of a micro deep drawn component. Using this testing method forming limit diagrams for the materials Al99.5, E-Cu58, stainless austenitic nickel-chromium steel X5CrNi18-10 (1.4301 / AISI 304), all produced by rolling, and an Al-Zr-foil, produced by a PVD sputtering process, were determined for the micro range.

Abstract: This article aims at the discussion of deformation behavior considering size effect on curl forming process of sheet metal. In this study, the test specimens were made by phosphor bronze sheets for curl forming test. The specimens with different thickness were firstly heated at different temperatures for obtaining the objective grain sizes. And the mechanical properties of specimen were acquired by using tensile test. Through the curl forming test with a curl forming machine, the curled angles, springback and curling load were measured and analyzed for investigating the grain size effect of the chamfer and carbon lubricant during the curl forming process.

Abstract: Many parts of today´s world are manufactured by cold forming operations or more precisely by mechanical upsetting such as wheelmounts or bevel gears because cold forming has several advantages compared to other machining operations. Due to size effects, the maximum achievable upset ratio in mechanical upsetting decreases with decreasing size of the work piece, so that upsetting becomes inefficient in micro range. Here, one promising approach is the laser rod end melting process generating a so called preform which allows upset ratios of several hundred within one stage. This preform is subsequently calibrated by a mechanical upsetting process. A numerical simulation showing the open die upsetting process of preforms, modeled with Abaqus, is presented in this work. Results of numerically determined natural strain are compared with an analytical model showing that the analytical model is a reasonable approximation. The maximum achievable average natural strain of preforms in upsetting is experimentally determined to be at least φ*=2.75.

Abstract: The miniaturization of products has driven the demand for better realization in the micro forming process. Micro sheetmetal forming becomes more susceptible to friction attributed to the decrease in the weighting of body force and the increase in surface force, as the size of workpiece decreases. In this work, micro cupping tests were utilized to test the influence of workpiece thickness and surface texturing both on stainless steel and copper sheets under an oil-lubricated condition. Three thickness gauges of workpiece, 0.05, 0.1 and 0.2 mm, were selected in order to represent the micro, meso and macro forming conditions, respectively. Longitudinal and isotropic lay conditions were obtained through imprinting the textured rolls produced by grinding and sand blasting methods. The result of the micro cupping tests show that the cup heights are comparable for both longitudinal and isotropic lays in forming the sheets of 0.1 and 0.2 mm thickness. However, isotropic lay has a greater cup height than that of longitudinal lay in forming the 0.05 mm sheets. This indicates that surface texturing becomes influential in the formability of micro sheetmetal forming.