AN INVESTIGATION OF SIZE EFFECTS ON THIN SHEET FORMABILITY FOR MICROFORMING APPLICATIONS

The increasing demand for powerful miniaturized products for all industrial applications has prompted the industry to develop new and innovative manufacturing processes to fabricate miniature parts. One of the major challenges facing the industry is the dynamic market which requires continuous improvements in design and fabrication techniques. This means providing products with complex features while sustaining high functionality. As a result, microfabrication has gained a wide interest as the technology of the future, where tabletop machine systems exist. Microforming processes have the capability of achieving mass production while minimizing material waste. Microforming techniques can produce net-shape products with intricacy in fewer steps than most conventional microfabrication processes. Despite the potential advantages, the industrial utilization of microforming technology is limited. The deformation and failure modes of materials during microforming is not yet well understood and varies significantly from the behavior of materials in conventional forming operations. In order to advance the microforming technology and enable the effective fabrication of microparts, more studies on the deformation and failure of materials during microforming are needed. In this research work, an effort to advance the current status of microforming processes for technologies of modern day essentials, is presented. The main contribution from this research is the development of a novel method for characterizing thin sheet formability by introducing a micro-mechanical bulge-forming setup. Various aspects of analyzing microscale formability, in the form of limiting strains and applied forces, along with addressing the well known size effects on miniaturization, were considered through the newly developed method. A high temperature testing method of microformed thin sheets was also developed. The aim of high temperature microforming is to study the material behavior of microformed thin sheets at elevated temperatures and to explore the capability of the known enhancement in formability at the macroscale level. The focus of this work was to develop a better understanding of tool-sheet metal interactions in microforming applications. This new knowledge would provide a predictive capability that will eliminate the current time-consuming and empirical techniques that, and this in turn would be expected to significantly lower the overall manufacturing cost and improve product quality

Microscale Metal Forming: Mesoscopic Size Effect, Extrusion and Molding

The continuing trend of metallic device and product miniaturization has motivated studies on microscale metal forming technologies. A better understanding of materials’ mechanical response and deformation behavior is of importance for the design and operation of micro metal forming processes. In this dissertation, uniaxial compression testing was conducted on Al ring and pillar specimens with characteristic dimensions at meso to micro scales. The experimental data reveal inadequacies of the existing surface layer model and provides a baseline for delineating deformation mechanisms in micro metal forming operations. Microscale reverse extrusion experiment was carried out on Cu and Al rod specimens with varying average grain sizes. Texture assessment on extruded Cu parts showed the texture components formed at tens of microns scale were consistent with those observed in macro scale extrusion. The grain size effect on both the mechanical response and deformation inhomogeneity was demonstrated and was further elucidated by a detailed comparison between the experimental results and the output of crystal plasticity finite element simulations. Another promising micro metal forming operation, namely microscale compression molding, was conducted on single crystal Al, using a series of rectangular double-punch sets with varying punch width and spacing in between. The characteristic molding pressure was observed to exhibit a significant dependence on both the spacing itself and the ratio of the spacing to the punch width. The molded features were characterized and the phenomenon of incomplete filling was observed and discussed. All these experimental results furnish new and basic knowledge for meso/micro scale metal forming technologies, as well as supplying data against which small scale plasticity theories/models can be tested

Metal Micro-forming

The miniaturization of industrial products is a global trend. Metal forming technology is not only suitable for mass production and excellent in productivity and cost reduction, but it is also a key processing method that is essential for products that utilize advantage of the mechanical and functional properties of metals. However, it is not easy to realize the processing even if the conventional metal forming technology is directly scaled down. This is because the characteristics of materials, processing methods, die and tools, etc., vary greatly with miniaturization. In metal micro forming technology, the size effect of major issues for micro forming have also been clarified academically. New processing methods for metal micro forming have also been developed by introducing new special processing techniques, and it is a new wave of innovation toward high precision, high degree of processing, and high flexibility. To date, several special issues and books have been published on micro-forming technology. This book contains 11 of the latest research results on metal micro forming technology. The editor believes that it will be very useful for understanding the state-of-the-art of metal micro forming technology and for understanding future trends

Crystal plasticity and phenomenological approaches for the simulation of deformation behavior in thin copper alloy sheets

In the expanding context of device miniaturization, forming processes of ultra thin sheet metals are gaining importance. Numerical simulation of these processes requires accurate material modeling. In this study, both the phenomenological modeling approach and the crystal plasticity finite element method (CPFEM) are considered. Theoretical definitions of both models, numerical implementation as well as their parameter identification procedures are outlined. Subsequently they are compared on a one to one basis, mainly with regards to their ability to predict mechanical responses for a variety of strain loading paths.Agence Nationale de la Recherche, ANR-12-RMNP-0009-0

Micro/Nano Manufacturing

Micro- and nano-scale manufacturing has been the subject of ever more research and industrial focus over the past 10 years. Traditional lithography-based technology forms the basis of micro-electro-mechanical systems (MEMS) manufacturing, but also precision manufacturing technologies have been developed to cover micro-scale dimensions and accuracies. Furthermore, these fundamentally different technology platforms are currently combined in order to exploit the strengths of both platforms. One example is the use of lithography-based technologies to establish nanostructures that are subsequently transferred to 3D geometries via injection molding. Manufacturing processes at the micro-scale are the key-enabling technologies to bridge the gap between the nano- and the macro-worlds to increase the accuracy of micro/nano-precision production technologies, and to integrate different dimensional scales in mass-manufacturing processes. Accordingly, this Special Issue seeks to showcase research papers, short communications, and review articles that focus on novel methodological developments in micro- and nano-scale manufacturing, i.e., on novel process chains including process optimization, quality assurance approaches and metrology

Designing a Tool System for Lowering Friction during the Ejection of In-Die Sintered Micro Gears

The continuous improvements in micro-forging technologies generally involve process, material, and tool design. The field assisted sintering technique (FAST) is a process that makes possible the manufacture of near-net-shape components in a closed-die setup. However, the final part quality is affected by the influence of friction during the ejection phase, caused by radial expansion of the compacted and sintered powder. This paper presents the development of a pre-stressed tool system for the manufacture of micro gears made of aluminum. By using the hot isostatic pressing (HIP) sintering process and different combinations of process parameters, the designed tool system was compared to a similar tool system designed without a pre-stressing strategy. The comparison between the two tool systems was based on the ejection force and part fidelity. The ejection force was measured during the tests, while the part fidelity was documented using an optical microscope and computed tomography in order to obtain a multi-scale characterization. The results showed that the use of pre-stress reduced the porosity in the gear by 40% and improved the dimensional fidelity by more than 75% compared to gears produced without pre-stress

Deformation Mechanics and Microstructure Evolution During Microforming of Metals

Deformation mechanics including dynamic strain, strain-rate and rotation of material elements and its spatio-temporal scaling behavior was studied using in situ characterization of prototypical microforming operations- Equal Channel Angular Pressing (ECAP), Indirect Extrusion (IE) and Deep Drawing (DD) across length scales (sub-millimeter and micron). Microforming devices including ECAP, IE and DD dies in plane strain condition were designed and fabricated to manifest process outcomes/anomalies in small length-scale deformations for a range of imposed strains: severe (ECAP), moderate (IE) and low (DD). This was captured by conducting in situ experiments on commercially pure metals: Ni 200, Oxygen Free High Conductivity (OFHC) Cu, Al 1100 and Pb. Set up microforming stages capable of in-situ observation in various length scales were implemented to employ Digital Image Correlation (DIC) technique in order to quantify the mechanics of deformation particularly in deformation zone where the temperature filed captured by in situ Infra-Red (IR) thermography completed the detailed understanding of thermomechanical phenomena prevail in microforming operations. To do this, ECAP and IE devices were designed with a transparent viewing window made of Sapphire block enables the imaging of the material flow during deformation using high-speed (CCD) and IR cameras. While DD of metallic sheets was performed in a microforming setup that sits inside the chamber of Scanning Electron Microscope (SEM) enables in situ characterization of material flow behavior using SEM based DIC. Pre and post–Mortem Microstructure analysis was carried out by performing Orientation Imaging Microscopy (OIM) across the microformed machine elements aiming to correlate spatially evolved microstructures/textures across the deformation zone with the mechanics of deformation obtained by in situ observations for the given materials system. In microforming, variables such as initial microstructure, process configuration and tooling design along with the deformation process parameters are known as crucial factors that determine the deformation behavior of material and therefore the process consequences including failure characteristics and quality of microparts surface finish. In the present dissertation, the effect of process parameters and scaling was studied and the role of characteristics of prior microstructures such as grain size and its distribution, grain morphology, twin size and density, pre-existing textures, etc. and their contribution in improving or disproving the formability was delineated for different deformation geometries and material systems. These studies revealed the strong dependence of the morphology and characteristics of plastic deformation zone (PDZ) to the process outcomes e.g. microstructure evolution, surface roughening, sudden failure, etc. which are the results of the mechanical/microscopical responses of material to the geometric confinements and strain gradients. The systematic studies of the effect of microscopic/macroscopic boundary conditions allows to determine the presence of any spatial confinement switchover in the mechanism of microscopic material response that will be eventually appeared in the quality of micro-machined components

A virtual crystal plasticity simulation tool for micro-forming

AbstractThe trend of increasing miniaturization of varied products and devices with a wide range of applications necessitates the forming of metallic parts with dimensions at the micron scale. In micro-forming, the stress and deformation are highly anisotropic. Hence, conventional macro-mechanics models fail to capture the important features, such as necking and bending resulting from strain localization. In this paper, a virtual integrated micro-mechanics simulation tool is presented, that was developed within the framework of Crystal Plasticity (CP) theory. With this tool, a polycrystalline Finite Element (FE) model was produced by introducing grain size, orientations and distribution patterns using VGRAIN software. ABAQUS software was used and the CP constitutive equations were implemented through a user-defined material subroutine, VUMAT. Typical micro-forming processes simulated include tension, extrusion and hydro-forming to demonstrate the effectiveness of the integrated simulation system. Finally, a map is proposed that establishes bounds of appropriate usage for different modeling techniques, namely a macromechanics plasticity model and a micro-mechanics crystal plasticity model, which will be useful to engineers in the metal forming industry in choosing suitable simulation tools

Bending Of Metallic Thin Foil Via High Energy Pulsed Laser Peening

Laser Peen Forming (LPF), a novel method used to form thin metallic engineering foils,is a manufacturing process utilizing strong shockwaves induced by high pressuresintroduced to the surface of a target by high energy, pulsed laser beams. It is a non-thermal,non-contact approach that improves the hardness and fatigue life of components byimparting beneficial residual stresses on the surface of the material. First, a review of theparameters of LPF processing, the forming mechanisms, process modeling techniques, andalternative LPF methods is discussed, to understand the procedures of industrialmanufacturing standards. Next, a simplification of metallic thin foils is proposed to developa novel low-cost and simplified LPF process design.The first segment pursues the general principles of LPF by reviewing its recentprogress, and that of comparable techniques. It discusses the process design, mechanismsattributed to forming, laser-material interactions, simulation methods, alternativeapproaches, and limitations thereof. The effect of laser intensity, material thickness,overlapping ratio, and number of scan tracks on bend angle, is discussed. Mechanisms ofLPF are reviewed by elucidating the mechanics of convex and concave curvature.Particularly, the Stress Gradient Mechanism (SGM) and the Stress Bending Mechanism(SBM), are modeled. In addition, the effect that drag bending has on the net bending of thesample piece is considered. The Fabbro model is followed for laser-material interactions.Advances in femtosecond and heat assisted LPF are considered with a focus on theadvantages and disadvantages over nanosecond methods.In the second segment, a streamlined LPF process design that eliminates ablativecoating and confining media is developed and explored to determine the feasibility ofbending of thin metallic foils. Experiments were conducted in a fashion similar totraditional LPF processing, with additional equipment unique to this new method, todetermine the final bend angle. Evidence has shown that laser intensity, sample thickness,and material strength are primary parameters in determining bending direction, amplitudeand process efficiency. In this part of the study, the effect of the number of passes and laserintensity on bending angle was examined. The results show that with increasing number ofpasses, there is an increase in bending angle, and, likewise, with an increase in laserintensity, there is an increase in bending angle. Then, the surface profile of the sample wasinspected, and the laser-processed area was characterized with respect to surfaceroughness. It was found that, relative to the original sample, the surface roughness of thelaser-processed area was not significantly impacted. Finally, the bend angle of LPF withand without an ablative coating, using equivalent experimental parameters, was compared.This portion of the study indicates that using an ablative coating under this new LPFprocess design is counterproductive